atv-a-118-e

STANDARDATV - A 118EHydraulic Dimensioningand Verification of Drainage SystemsNovember 1999ISBN 3-924063-49-4

W A S T E W A T E R - W A S T EGERMAN ATV STANDARDS

Distribution: GFA - Verlag für Abwasser, Abfall und GewässerschutzTheodor-Heuss-Allee 17 � D-53773 Hennef � P. O. Box 11 65 � D-53758 HennefTel. 00 49 22 42 / 8 72-120 � Fax: 00 49 22 42 / 8 72-100E-Mail: [email protected] � Internet: www.atv.de

2 November 1999

Preparation

The following collaborated with the production of ATV Standard A-118 E: Prof. Dr.-Ing. Theo G, Schmitt, Kaiserslautern (Chairman) Dr.-Ing. Holger W. Bröker, Langenfeld Dipl. HTL Christian Eicher, Belp/Schweiz Dipl.-Ing. Rüdiger Heidebrecht, Hennef Ltd. BD Dipl.-Ing. Ulrich Keseling, Hagen Dipl.-Ing. (FH) Sybille Klotsche, Dresden RBM Dipl.-Ing. Wolfgang Königer, München BD Dipl.-Ing. Albert Kreil, Kassel Dipl.-Ing. Frank Männig, Dresden BD Dipl.-Ing. Dietmar Schaber, Karlsruhe Prof. Dr.-Ing. Friedhelm Sieker, Hannover Dr.-Ing. Wolfgang Verworn, Hannover Dipl.-Ing. G. Vogel, Potsdam ir. Jacob G Voorhoeve, Amersfoort/Netherlands All rights, in particular those of translation into other languages, are reserved. No part of this Standard may be reproduced in any form - by photocopy, microfilm or any other process - or transferred into a language usable in machines, in particular data processing machines, without the written approval of the publisher. GFA Gesellschaft zur Förderung der Abwassertechnik e.V., Hennef 1999

Original German version printed by: DCM, Meckenheim, Germany

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Contents Preparation...................................................................................................................... 2

Notes for Users ............................................................................................................... 5

Foreword ......................................................................................................................... 5

1 Scope........................................................................................................................ 5

2 Definitions ................................................................................................................ 6 2.1 Terms ................................................................................................................. 6

2.2 Symbols.............................................................................................................. 9

3 General ..................................................................................................................... 9 3.1 Fundamentals..................................................................................................... 9

3.2 Drainage Systems ............................................................................................ 10

3.3 Task.................................................................................................................. 10

3.4 Relevant Discharge Parameters and Cross-sections ....................................... 10

4 Wastewater and Infiltration Water Discharge...................................................... 11 4.1 Calculation Principles ....................................................................................... 11

4.1.1 Existing Drainage Systems........................................................................ 11

4.1.2 Planning of New Drainage Systems .......................................................... 12

4.1.2.1 Domestic Wastewater ........................................................................ 12 4.1.2.2 Industrial Wastewater......................................................................... 12 4.1.2.3 Infiltration Water ................................................................................. 12 4.2 Calculation of the Wastewater and Infiltration Water Discharge....................... 13

5 Precipitation Runoff .............................................................................................. 14 5.1 Target and Verification Parameters .................................................................. 14

5.2 Initial Details on Precipitation Loading.............................................................. 16

5.2.1 Rainfall Duration Frequency Curve and Block Rain .................................. 17

5.2.2 Intensity Variable Synthetic Rainfall .......................................................... 18

5.2.2.1 Individual Synthetic Rainfall ............................................................... 18 5.2.2.2 Synthetic Rainfall Groups................................................................... 18 5.2.3 Measured Heavy Rainfall Series ............................................................... 18

5.2.4 Precipitation Continuum ............................................................................ 19

5.2.5 Defining of Permitted Discharges .............................................................. 19

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5.3 Stormwater Runoff............................................................................................ 20

5.3.1 Run-off Formation ..................................................................................... 20

5.3.1.1 Individual Loss Approaches ............................................................... 20 5.3.1.2 Discharge Coefficient ......................................................................... 21 5.3.2 Run-off Concentration ............................................................................... 22

5.3.3 Taking Account of Measures for Decentralised Stormwater Management 22

5.4 Calculation Methods for Sewer Discharge........................................................ 22

5.4.1 Hydrological Methods................................................................................ 22

5.4.1.1 Time Coefficient Method .................................................................... 23 5.4.1.2 Hydrological Discharge Models .......................................................... 23 5.4.2 Hydrodynamic Calculation Methods .......................................................... 23

6 Hydraulic Calculation and Implementation of Verification ................................ 24 6.1 Application Criteria ........................................................................................... 24

6.1.1 Calculation Methods and Precipitation Loading......................................... 24

6.1.2 Catchment Area ........................................................................................ 25

6.2 Task.................................................................................................................. 25

6.2.1 Re-dimensioning of Drainage Networks .................................................... 25

6.2.2 Recalculation of Existing Systems............................................................. 26

6.2.3 Calculation of Rehabilitation Variants ........................................................ 26

6.2.4 Verification of Overdamming Frequency ................................................... 27

6.3 Safety against Flooding .................................................................................... 27

7 Applicable Standard Specifications and Rules and Standards......................... 28

7.1 ATV Standards Wastewater- Waste .............................................................. 28

European and DIN Standard Specifications ............................................................... 29

Literature ...................................................................................................................... 29

Appendix ....................................................................................................................... 32

A1 Creation of synthetic rainfall according to Euler Type II ............................ 32

A 2 Production of a synthetic rainfall group...................................................... 33

A 3 Compilation of heavy rainfall series ............................................................. 35

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Notes for Users

This ATV Standard is the result of honorary, technical-scientific/economic collaboration which has been achieved in accordance with the principles applicable for this activity (statutes, rules of procedure of the ATV and ATV Standard ATV-A 400). For this, according to precedents, there exists an actual presumption that it is textually and technically correct and also generally recognised. Everyone is at liberty to apply this Standard. However, an obligation for application can arise from legal or administrative regulations, a contract or other legal reason. This Standard is an important, however, not the sole source of information for correct solutions. With its application no one avoids responsibility for his own action or for the correct application in specific cases; this applies in particular for the correct handling of the margins described in the Standard.

Foreword

ATV Standard A-118 was first published in 1956 under the title “Standards for the calculation of stormwater and combined wastewater sewers”. In 1977 it was republished in a revised edition with the title “Standards for the hydraulic calculation of domestic and industrial wastewater, stormwater and combined wastewater sewers” (ATV 1977). A renewed revision was started in 1984. This was occasioned by European standardisation as common standard specifications for drainage systems had been developed. In addition to taking into account the specifications of the standard series DIN EN 752 on flooding protection, technical development, additional knowledge and the increasing employment of computer calculations were also included in the revision. In particular, a survey of towns and communities and of 340 engineer offices on the then current application practice of sewer calculations, carried out in 1994 (ATV, 1996), has also been included. Thanks to the collaboration of specialist colleagues, the experiences of neighbouring countries has also been included with the revision. In each application case the available local and project-specific characteristics are to be checked as to whether the rules listed below can be applied without limitation. They may be supplemented or replaced by other approaches if appropriate knowledge or experience is available. In general, there is an absolute necessity to consider the water management characteristics and requirements as a whole. The inclusion of the overall concept of the residential and municipal drainage with the hydraulic calculation of drainage systems is as an absolute must. This concerns both measures for the management of stormwater as well as the loading of surface waters with stormwater and combined water discharges.

1 Scope

This ATV Standard concerns the dimensioning and verification of drainage systems, which are mainly operated as gravity systems and which serve for the discharge of domestic and industrial wastewater, stormwater and combined wastewater. Its range of validity, in accordance with the standard specification series DIN EN 752 “Drain and sewer systems outside buildings”, ranges from the point where the wastewater leaves the building and/or roof drainage system or flows into road gullies up to the point where the wastewater is discharged into a treatment plant or into surface waters. Drains and sewers beneath buildings are excluded here so far as they are not part of the building drainage system1). Statements in Chaps. 3 to 6 refer, in the first instance, to public drainage systems. It applies, by extension, also for the drainage of larger private surface units (commercial/industrial concerns, housing developments).

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For the hydraulic calculation of sewer cross-sections and open profiles attention is drawn to the ATV-DVWK Standard ATV-DVWK-A 110E “Hydraulic dimensioning and performance verification of sewers and drains”. ATV-A 111 “Richtlinien für die hydraulische Dimensionierung und Leistungsnachweis von Abwasserkanälen und -leitungen” (Standards for the hydraulic dimensioning and performance verification of sewers and drains”) [Translators note: not yet (February 2003) available in English] applies for stormwater overflow discharge facilities; for other special structures ATV-A 112 “Richtlinien für die hydraulische Dimensionierung und den Leistungsnachweis von Sonderbauwerken in Abwasserkanälen und -leitungen” [Translators note: not yet (February 2003) available in English]. Special forms of drainage and their dimensioning are, for example, listed in DIN EN 1091, DIN EN 1671, in ATV Standard ATV-A 116E “Special sewer systems - Vacuum drainage service - pressure drainage service” as well as in ATV Standard ATV-138E “Construction and dimensioning of facilities for decentralised percolation of non-harmful polluted precipitation water”. The dimensioning of storage and retention facilities in drainage networks is part of ATV Standard ATV-A 117 [Translators note: not yet (February 2003) available in English], while the arrangement, dimensioning and design of stormwater overflow discharge structures in combined wastewater systems are described in ATV Standard A-128E.

2 Definitions

2.1 Terms Technical terms used in the following text are mainly contained in DIN EN 752-1 “Drain and sewer systems outside buildings; Generalities and definitions” and in DIN 4045 “Wastewater engineering vocabulary”. The most important are listed below. Coefficient of discharge Factor depending on the catchment area with which the quantity of stormwater per unit of time has to be multiplied in order to obtain the expected stormwater runoff which is to be discharged into the drainage system. Discharge damping Reduction of the peak discharge through temporary storage of the runoff. Discharge simulation Modelling of discharges in drainage systems. Wastewater Domestic and industrial wastewater and/or stormwater discharged in a drain or sewer. Sewer Usually a buried pipeline or other facility for the discharge of domestic and industrial wastewater and/or stormwater from drains. Drain Usually a buried pipe for the discharge of domestic and industrial wastewater and/or stormwater from the point of occurrence to the sewer. __________________ 1) Facilities for the drainage of buildings fall under the scope of the standard specification series DIN EN 12056 “Drainage systems

within buildings”. Rules for private property drainage facilities of larger surface units are taken up in DIN 1986.

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Commercial wastewater Effluent completely or partially from industrial or commercial concerns. Catchment area Area with discharge to a drain, a sewer or a form of surface water Flow time (DIN EN 752-4) Time which the stormwater requires to runoff from the hydraulically relevant point of the catchment area to a fixed point of the drainage system. Gravity system Drainage system with which the discharge takes place through gravity and with which the pipeline is normally operated partially filled. Infiltration water Undesirable discharge into a drainage system. Private property drainage System of pipes and additional buildings for the discharge of industrial or domestic wastewater and/or stormwater to a [public] sewer system or other disposal facility. Frequency Number of events which, in the long-term statistical mean, reach or exceed a certain value (reciprocal of the recurrence time). Domestic wastewater Wastewater from kitchens, washbasins, bathrooms, toilets and similar facilities. Sewer system Network of pipelines and additional buildings which discharges domestic and industrial wastewater and/or stormwater from drains to sewage treatment plants. Combined system Drainage system consisting of a single pipeline sewer system for the joint discharge of domestic and industrial wastewater and stormwater. Modified combined system (i.a.w. ATV A-105E, 1997) Special case of a combined system. Only domestic and industrial wastewater as well as precipitation water requiring treatment are fed to the combined sewer; this combined wastewater is discharged and treated. Precipitation water not requiring treatment is completely or partially percolated directly at the point of occurrence or discharged directly into a body of surface water. Trough loss Precipitation, retained in surface troughs, which does not contribute to stormwater run-off. Surface flooding Condition with which wastewater and/or stormwater escape from a drainage system or are unable to enter this system and either remain on the surface or penetrate buildings from the surface (comp. also “flooding”). Roughness Measurement of the frictional resistance of the surface of a pipe or channel with turbulent flow. Stormwater runoff Precipitation water which, on a surface, drains into a drainage system or into a receiving water.

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Rainfall intensity Average amount of precipitation in a catchment area which occurs within a certain time period. Level of backed-up water (DIN 4045) Height, within a private sewerage system, below which special measures are to be taken against backwater Backwater line Calculated or actually occurring wastewater levels within a drainage system above a certain control cross-section. Rehabilitation All measures for repair or improvement of existing drainage systems. Wastewater Water changed by usage and discharged in a drainage system. Storage tank Closed or open tank for the temporary storage of wastewater. Sewer with storage capacity Oversized sewer with the function of a storage tank. Separate system Drainage system normally consisting of two drain/sewer systems for the separate discharge of wastewater and stormwater. Modified separate system (i.a.w. ATV A-105E, 1997) Domestic and commercial wastewater is fed to the wastewater sewer. Precipitation water not requiring treatment is completely or partially percolated directly at the point of occurrence or discharged directly into a body of surface water. So far as precipitation water requiring treatment cannot be avoided, this is fed in a stormwater sewer to stormwater treatment. Dry weather flow Discharge into a drainage system with specified dry weather conditions. Flooding Condition with which wastewater and/or stormwater escape from a drainage system or are unable to enter this system and either remain on the surface or penetrate buildings from the surface (comp. also “surface flooding”). Additional impoundment (overdamming) Loading condition of the sewer system with which the water level exceeds a defined reference level. Recurrence time, annual Average period of time in which an event achieves or exceeds a value (reciprocal of the frequency of occurrence).

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2.2 Symbols Symbol Unit Designation

AC,s D ID n nai Q Qc Qcf Qd Qdw Qiw Qs Qs,S Qtot q qc qd qiw qs,S r r(D,n) r(D,T) r15 S Tn ψp

ha min I/ha 1/a 1/a l/s l/s l/s l/s l/s l/s l/s l/s l/s l/s l/(s.1000I) I/(s.ha) l/(s.ha) l/(s.ha) l/(s.ha) l/(s.ha) l/(s.ha) l/(s.ha) % a -

Catchment area with sewers Rainfall duration or duration step Population density in the catchment area Frequency of occurrence Calculated frequency of additional impoundment (overdamming) Volume flow of discharge, throughflow, inflow Discharge of commercial wastewater Calculated discharge capacity with complete filling Discharge of domestic wastewater Dry weather discharge Infiltration water discharge Stormwater run-off Unavoidable rainfall run-off in the wastewater sewer of a separate system (Infiltration water component with rainfall) Relevant measure of discharge for dimensioning Discharge Commercial wastewater discharge (referred to AC,s) Inhabitant specific domestic wastewater discharge Infiltration water discharge with dry weather (referred to AC,s) Stormwater discharge in the wastewater sewer (referred to AC,s; see Qs,S)) Rainfall intensity Rainfall intensity with a duration of D and frequency of occurrence of n Rainfall intensity with a duration of D and recurrence time Tn Rainfall intensity with a duration of 15 min Average ground slope Recurrence time Peak discharge coefficient (max. discharge capacity/associated rainfall intensity)

3 General

3.1 Fundamentals The discharge occurring in a drainage area consists of domestic and commercial wastewater, infiltration water and precipitation water. Accordingly drainage systems serve, inter alia (comp. ATV-M101), for - the maintenance of hygienic conditions in residential areas through the complete collection and

discharge to the sewage treatment plant of wastewater produced - the extensive prevention of damage due to flooding and saturation as a result of precipitation run-off

and - the greatest possible maintenance of the usability of residential areas independent of the weather

conditions (“ease of drainage”). The maintenance of hygienic conditions is a main concern of residential drainage systems and it is imperative that this be met by all wastewater disposal facilities. The two latter mentioned objectives have, up to now, been taken into account by the discharge of precipitation water in combined or stormwater sewers. The exclusive discharge principle, however, competes with and is in part inconsistent with the targets set by water management of damping discharge peaks, increasing low water in bodies of surface water and maintaining evaporation and the reestablishment of groundwater. Therefore, a reduction of the discharged infiltration and precipitation water is to be sought in co-ordination with the above named objectives.

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With the planning and dimensioning of new networks as well as with the rehabilitation of existing systems all possibilities are to be made use of to keep non-hazardous polluted precipitation water away from the sewer system and to reduce the discharge of precipitation. To this belong, in particular, measures for decentralised retention of stormwater and percolation as well as the delayed (open) discharge of slightly polluted precipitation water (ATV-A 105E, 1997). All procedures mentioned in this ATV Standard, also the precipitation discharge model, represent methods of approximation with which simplifying assumptions must still be made. The reliability of the results can be increased if precipitation, discharge and water level measurements in sewer networks are carried out and flow into the model application. This applies in particular for the verification calculation.

3.2 Drainage Systems The drainage of residential areas usually takes place using the combined system or the separate system. Taking into account more recent principles for the handling of stormwater there result mixed forms which are designated as modified systems. With the separate system domestic and process wastewater are discharged in one sewer and the stormwater as well as possibly land drain water in their own stormwater sewer. In the combined system the domestic and process wastewater are discharged together with the precipitation run-off in a common sewer (combined sewer). Modified drainage systems result from the requirement in future to move away from complete discharge with precipitation water and to differentiate this according to its properties. Non-hazardous polluted precipitation water is to be kept extensively away from the sewer system through decentralised retention, percolation and as far as possible separate (if necessary also open) discharge of the remaining share of the discharge. In particular, through this measure, existing sewers and the wastewater treatment plant are hydraulically relieved and combined wastewater overflow can be reduced.

3.3 Task Depending on the drainage procedure, normal, stormwater and combined sewers and drains as well as possibly open channels (separate stormwater discharge) are to be calculated in accordance with this ATV Standard. For this, the following tasks are to be differentiated (see Chap. 6): - new dimensioning of drainage networks - recalculation of existing systems - calculation of rehabilitation variants - verification of overdamming frequency - assessment of security against flooding.

3.4 Relevant Discharge Parameters and Cross-sections In separate and combined sewer systems the following applies for the determination of the total discharge Qtot: Separate system - normal (wastewater) sewer Qtot = Qd + Qr,T [l/s] (1)

Qd - dry weather discharge Qr,T - unavoidable stormwater discharge into the normal sewer of separate areas

- stormwater sewer Qtot = Qr [l/s] (2) Qr - wet weather discharge

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Combined system (combined wastewater sewer) Qtot = Qd + Qr [l/s] (3) Note is to be taken of ATV Standard ATV-A 110E “Standards for the Hydraulic Dimensioning and performance Verification of Sewers and Drains” for the hydraulic calculation of sewer cross-sections and open profiles. With the dimensioning of gravity sewers the mathematical discharge capacity QV is not to be fully exploited. It is recommended that the next largest cross-section is selected if the determined total discharge Qtot with stormwater and combined sewers is ca. 90 % of the discharge capacity QV. With the dimensioning of normal sewers, taking into account the cost effective aspects in the individual case, it is to be carefully considered to what extent additional reserves for non-foreseeable future developments (i.a. deliberate discharge of polluted precipitation run-off) or for operating conditions are required. For operational reasons (i.a. danger of blockage, flushing, TV inspection, later establishment of connections) it is recommended, independent of the calculated total discharge, in general not to undercut the following minimum nominal widths in public sewers with gravity discharge: normal sewers DN 250 stormwater, combined sewer DN 300 In justified cases (e.g. small discharge in rurally structured areas or in dispersed residential areas, main connecting sewers with good gradient conditions, steep stretches, implementation of measures for stormwater management) small cross-sections - however, as far as possible not below DN 250 - can also be chosen. In this case particular attention is to be paid to the operational aspects and, if required, suitable measures for the avoidance of deposits and blockages are to be taken. This also applies to the selection of the shape of the cross-section. The formulations in ATV Standard ATV-A 111 “Richtlinien für die hydraulische Dimensionierung und Leistungsnachweis von Abwasserkanälen und -leitungen” apply for pipe throttles.

4 Wastewater and Infiltration Water Discharge

The dry weather discharge in drainage systems of residential areas consists of the following components: - domestic wastewater - process wastewater (commercial, industrial) and - infiltration water whose discharge quantities are to be determined separately.

4.1 Calculation Principles With all named discharge quantities and discharges given below one is concerned with hourly peak values not with daily averages. The discharges are related to the sewered catchment area AE,k (not to the impermeable surface Au!). Separate approaches apply for the dimensioning of stormwater overflows and wastewater treatment plants.

4.1.1 Existing Drainage Systems The size of the dry weather discharge of existing drainage systems should fundamentally be determined and assured using sufficiently long measurement periods. If necessary, this also concerns stormwater discharged in normal sewers. Here the actual discharge relationships, including the infiltration water component of the dry weather discharge, are more correctly recorded than using global planning values. The discharge measurements are, as far as possible, to be carried out in different seasons in order to record seasonal influences, i.a. variations in infiltration water yields.

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With existing commercial and industrial areas as well as for larger hotels, rest homes, sanatoriums, barracks, camping sites and similar, separate assessments, if necessary also with measurements of discharge, should be carried out. With this, in particular, the water consumption from own extraction plants should also be included. So far as planning conditions are considered, residential areas and developments in consumption as well as possible effects on rehabilitation projects are to be taken into account.

4.1.2 Planning of New Drainage Systems

4.1.2.1 Domestic Wastewater The domestic wastewater discharge Qd is essentially determined from personal water consumption. It is influenced by the residential density and structure and, due to different habits, is subject to the style and living standards of the population. In addition, regional demands and the size of residential areas can also be of significance. This applies to a particular degree for communities in conurbations. The spectrum of associated residential densities normally lies between 20 I/ha (rural areas, open development) and 300 I/ha (city centres). The average daily water consumption of the population, including small commercial activities, currently lies between 80 and 200 l/I.d). It is recommended that, for the calculation of future wastewater discharge, the values are based on an assured water requirement forecast of the local water supplier and, in dimensioning, a wastewater yield of 150 l/I.d should not be undercut. This is based on the limited accuracy of water requirement forecasts for the longer forecast periods normal with sewer system planning (≥ 50 years). The daily variations with the determination of the specific peak discharge have to be taken into account for the dimensioning of sewers and drains. The hourly peak discharges [m3/h] from experience lie between 1/8 (rural areas) and 1/16 (large towns) of the daily value [m3/d]. When there are no specific local details a dimensioning value for sewers of qd = 4 l/(s.1000I) is recommended for the hourly peak value of the domestic wastewater discharge. This should not, even with the assessment of available consumption values, be significantly undercut. Resulting from the hourly peak value of the wastewater discharge value for qd greater than 5 l/(s.1000I), the calculation approaches should be examined in order to prevent overdimensioning. 4.1.2.2 Industrial Wastewater With planned commercial and industrial areas usually no precise details are given on the type and size of the concerns to be sited there. For the dimensioning of sewers in commercial and industrial areas an area-specific approach using the following operational wastewater discharge rates qc is recommended: - businesses with low water consumption

qc = 0.2 to 0.5 l/(s.ha)

- businesses with medium to high water consumption qc = 0.5 to 1.0 l/(s.ha)

Larger values are to be applied in operation-specifically justified individual cases. 4.1.2.3 Infiltration Water Infiltration water covers unwanted discharges which get into the sewer system and, with the penetration of groundwater and depending on the type of sewer, through which various false discharges can be caused (Table 1). To this also counts precipitation water flowing in wastewater sewers with rainfall. Due to the disadvantageous effects increased attention is always to be paid to keeping the infiltration water inflow as small as possible by using suitable measures. False discharges of wastewater into stormwater sewers are generally to be prevented.

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Combined sewers

Stormwater sewers

Wastewater sewers

Groundwater penetrating (leaks)

Groundwater penetrating (leaks)

Groundwater penetrating (leaks)

Inflowing drainage and spring water

Inflowing drainage, spring water and water from streams*)

Inflowing drainage and spring water

Inflowing wastewater (false discharges)

Inflowing wastewater (via manhole covers, false discharges)

*) The admissibility of the discharge of water from drainage, springs and streams into stormwater sewers is to be checked in individual cases.

a) Infiltration water discharge with dry weather Qf The size of the infiltration discharge with dry weather can be specified, specifically for the location, via an infiltration water discharge qiw. For new planning an infiltration water discharge giw of 0.05 to 0.15 l/(s.ha) appears sufficient. b) Unavoidable stormwater discharge in the normal sewer of separate areas Qr,t. While infiltration water for combined and stormed water sewers is not, as a rule, relevant for dimensioning, with the dimensioning of normal sewers, in addition to the infiltration water flowing off with dry weather an additional formulation for infiltrating stormwater (e.g. from the surface via manhole covers) should be made. This addition depends very much on local conditions and can be several times the wastewater discharge. It can - in addition to the infiltration water with dry weather - be taken into account through a stormwater discharge rate. qr,t = 0.2 to 0.7 l/(s.ha) (in justified cases even more) With existing normal sewers the component Qr,t an be determined through comparable discharge measurements with dry and wet weather. c) Global value as multiple m of the wastewater discharge Qww With insufficient information the infiltration water discharge in normal sewers can be estimated globally as multiple m of the wastewater discharge. m = 0.1 to 1.0 (in justified cases even > 1) Existing measurement results from comparable and/or neighbouring areas can be applied for the infiltration formulation with the dimensioning of new sewer, insofar as the location-specific circumstances allow this.

4.2 Calculation of the Wastewater and Infiltration Water Discharge The size of the dry weather discharge Qdw is derived from the sum of the individual components - domestic wastewater flow Qd - commercial wastewater flow Qc - infiltration water discharge Qiw as follows: Qdw = Qd + Qc + Qiw [l/s] (4)

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The domestic wastewater discharge Qd is calculated using the specific wastewater qd, the surface area of the sewered catchment area AC,s and the population density ID:

1000

ADqQ s,Cd

d

⋅⋅=

I [l/s] (5)

qd = specific daily amount of domestic wastewater per inhabitant (e.g. 4 l/(s.1000I)) AC,s,1 = surface area of the residential area covered by the sewer system (ha) ID = population density of the catchment area [I/ha] The commercial wastewater discharge Qc, using the catchment area AC,s with the discharge rate qc given in Sec. 4.1.2.2, is determined as follows: 2,s,Ccc AqQ ⋅= [l/s] (6) qc = commercial wastewater discharge rate [l/(s.ha)] AC,s,2 = surface area of the commercial and industrial area covered by the sewer system [ha] The infiltration discharge Qiw with dry weather is determined as follows in accordance with Sect. 4.1.2.3 (a) using a location-specific infiltration discharge rate qiw:

s,Ciwiw AqQ ⋅= [l/s] (7)

qiw = infiltration water discharge rate (with dry weather) [l/(s.ha)] AC,s = surface area of the catchment area (general) covered by the sewer system [ha] With the dimensioning of normal sewers the unavoidable stormwater run-off Qr,T due to the stormwater discharge rate qr,T is to be applied as additional infiltration water component: 3,s,CT,rT,r AqQ ⋅= [l/s] (8) qr,T = stormwater discharge rate in the normal sewer [l/(s.ha)] AC,s = surface area of the catchment area covered by the normal sewer system [ha] Alternatively the infiltration water discharge (with normal sewers consisting of dry and wet weather components) can be determined globally as multiple m of the normal wastewater discharge: )QQ(mQ cdiw +⋅= [l/s] (9)

5 Precipitation Runoff

5.1 Target and Verification Parameters Drainage systems of built-up areas are to be so conceived and dimensioned that the principles formulated in Chap. 3 are met as far as possible optimally. For economic reasons, however, they cannot be designed in such a manner that, with rainfall, an absolute protection against flooding or the ground becoming waterlogged is guaranteed. Therefore target values for a reasonable “drainage comfort” have to be defined whose observation is assured through the selected sewer cross-section and other drainage elements. From the European standard specification DIN EN 752 the flooding frequency is specified as the measure for flood protection of drainage systems. It corresponds with the frequency of occurrence of flooding, by which “wastewater and/or stormwater escapes from a drainage system or cannot enter this and either stays on the surface or penetrates buildings” (DIN EN 752-1). In German drainage practice, flooding is connected

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with damage occurring or functional disruption (e.g. with underpasses) due to the escape of water or the impossibility of water entering the drainage system as a result of overload. The requirements on flooding protection are to be selected dependent on the respective locality. Here, in the first instance, differentiation should take place according to type of structural utilisation (rural areas, residential areas, city centres, industrial and commercial areas) and to facilities which have to be specially drained (underground traffic systems, underpasses). In addition, the local conditions, precipitation events, locally different hazards with the occurrence of overloading of the drainage system, the topographic situation of the area (mountain or hillside position, low lying point, proximity to bodies of surface water), the situation with regard to receiving waters, the danger of flooding of the surface waters and possibilities for discharge via the road system or via areas which have not been built on as well as the respective potential for damage are to be taken into account. The European standard specification DIN EN 752-2 (1996) contains the requirements on the protection against flooding, printed in Table 2, which are recommended for the design of new systems as well as with existing systems so far as no appropriate specifications are laid down by the responsible office. Here attention is drawn to justified deviations which are possible in individual cases. Table 2: Frequencies recommended in DIN EN 752 for the design (from DIN EN 752-2, 1996)

Frequency of design rainfall1)

(once in “n” years)

Location Frequency of flooding

(once in “n” years)

1 in 1 Rural areas 1 in 10 1 in 2 Residential areas 1 in 20 1 in 2 1 in 5

City centres, industrial and commercial areas:

with examination for flooding,

without examination for flooding

1 in 30 -

1 in 10 Underground traffic facilities, underpasses

1 in 50

1) For design rainfall no overloading may occur For the dimensioning of drainage networks without the carrying out of verification (new planning), Din EN 752-2 gives frequencies of design rainfall which apply for the employment of flow time procedures. With this, the determined maximum discharges for the respective discharge capacity with complete filling may not be exceeded. For larger drainage systems and generally with the application of discharge simulation models, in particular where significant damage or hazards can occur, DIN EN 752 recommends that the measure of flooding protection is determined via the specifications on permitted flooding frequencies. The process of flooding is, however, to a high degree dependent on local conditions (e.g. low areas of individual properties in relation to the road level). The actual flooding frequency can thus be determined to a great extent only through observations and experience with existing sewers and, if required, improved through design measures (e.g. raising of curbs, drainage of low points using lifting devices). As technical modelling to represent flooding is not possible with the current state of the art the overdamming frequency is introduced below as additional target parameter for the mathematical

16 November 1999

verification of drainage systems. Overdamming is understood to be the exceeding of a certain reference level by the calculated maximum water level. In many cases the surface of the ground (e.g. height of the manhole cover) is selected as the reference level of the calculated maximum water level as, with the exceeding of this value, there is an escape of water on to the surface of the ground (surface of the road) and the possibility of flooding exists. This height, in many cases, corresponds with the level of backwater laid down in the municipal drainage bylaws, below which measures against backflow are to be taken within the drainage system. On the basis of the details in DIN EN 752-2 (see Table 2) and subject to the determination of other values by the responsible office, the values in accordance with Table 3 are recommended for the verification of overdamming frequency with new planning and/or after rehabilitation (reference level: “ground line”). With the selection of the overdamming frequency, the local conditions (hazard and damage potential, see above) are to be taken into account appropriately. Table 3: Recommended overdamming frequencies for the mathematical verification with new

planning and/or after rehabilitation (here: reference level: “ground line”).

Location Overdamming frequencies - new

planning and/or after rehabilitation

(once in “n” years) Rural areas 1 in 2 Residential areas 1 in 3 City centres, industrial and commercial areas

less often than 1 in 5

Underground traffic facilities, underpasses

less often than 1 in 10 1)

1) With underpasses notice is to be taken, that with overdamming above the ground, as a rule flooding follows so far as non-specific local safety measures exist. Here the overdamming and flooding frequency corresponds with the value “1 in 50” given in Table 2.

The carrying out of the mathematical verification using the overdamming frequency in accordance with Table 3 can basically take place with the various precipitation loads according to Sect. 5.2. Statements on the scope can be found in Chap. 6. It is recommended, in the first step, to carry out the mathematical verification according to the target parameter overdamming frequency and, in the second step, to examine and, if necessary, to secure through structural measures the respectively required flooding protection considering the local conditions (see Chap. 6). The efficiency of existing drainage systems should, in the first instance, be assessed on their actual discharge behaviour. Reason for a systematic examination of the efficiency, for example through a hydraulic recalculation in accordance with Sect. 6.2.2, could be flooding or other obvious system overloading occurring in the past (frequently), as well as planned discharge-relevant expansion and structural modifications within the catchment area. The assessment of the overdamming frequency determined with this, with regard to the necessity for rehabilitation, can orient itself on the target parameters given in ATV (1995b). These are seen as “mean efficiency” of existing systems designed according to previous dimensioning practice.

5.2 Initial Details on Precipitation Loading Up until now, almost exclusively rainfall data in the form of block rain (rainfall duration frequency curve, synthetic rainfall or historical heavy rainfall have been used as initial details for the dimensioning of the sewer cross-section. These initial dimensioning details assume that the stormwater discharge produced from the area of the residential surfaces have to be discharged. Depending on the calculation method, substantial

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discharges and or relevant water levels (hydrodynamic methods) are calculated from the initial rainfall details. With the introduction of concepts of stormwater management in the residential drainage system there is a possible additional initial target detail in that only so much precipitation run-off is to be discharged via the sewer system, as is desirable or permissible from the water management aspect and/or is absolutely necessary due to local conditions. The hereby resultant (permitted) inflows to the sewer system as a rule lie significantly below those of the bare discharge conception. Common target of both approaches is the guaranteeing of an adequate drainage comfort within the residential areas.

5.2.1 Rainfall Duration Frequency Curve and Block Rain The calculation of the stormwater run-off according to specified rainfall duration frequency curves proceeds from the knowledge that heavy rainfalls are of short duration, slight rainfall on the other hand continues for longer. The average rainfall intensity i [mm/min] or the rainfall intensity r [l/(s.ha)], with the same statistical frequency, reduce with increasing rainfall duration. The relationship between average rainfall intensity r, rainfall duration D and frequency n [1/a] is determined through the statistical evaluation of precipitation registrations in accordance with ATV Standard ATV-A 121. Full-coverage heavy rainfall assessments of the German Weather Service (DWD) are available for the whole of the Federal Republic of Germany (DWD, 1997). As an example, Fig. 1 shows the average rainfall intensity r of the precipitation duration of 15 min for various frequencies for four German cities and illustrates the considerable regional differences. For the dimensioning of sewer networks it is nevertheless insufficient to apply the rainfall intensity with a duration of 15 minutes only as, depending on the local conditions (ground slope, extent of hardened surfaces, flow times), shorter or longer rainfall durations can be relevant. The shortest rainfall duration to be considered should be selected, dependent on the ground slope and the extent of the hardened surfaces, according to Table 4.

Fig. 1: Average rainfall intensity r for the rainfall duration of 15 minutes for various frequencies n

as an example for four stations (DWD, 1997)

18 November 1999

Table 4: Relevant shortest rainfall duration in dependence on the average ground slope and the extent of hardened surfaces

Average ground slope

Hardened surface

Shortest rainfall duration

≤ 50 % 15 min < 1 % > 50 % 10 min

1 % to 4 % 10 min ≤ 50 % 10 min > 4 % > 50 % 5 min

5.2.2 Intensity Variable Synthetic Rainfall Synthetic rainfalls show a variable intensity over the selected rainfall duration, which is determined from an assessment in phases of the rainfall intensity duration curve. Synthetic rainfalls can be employed as individual rainfall or rainfall group for discharge models. 5.2.2.1 Individual Synthetic Rainfall With the creation and usage of individual synthetic rainfalls their duration and intensity progression with regard to time are to be so selected that the associated rainfall duration frequency curve completely covers the relevant area for the sewer network. That means in every maximum precipitation section of the synthetic rainfall the average rainfall intensity must be the same as that of the rainfall duration frequency curve with the appropriate duration. The rainfall should be at least twice the longest relevant flow time in the drainage network. The intensity distribution is carried out based on the statistical evaluation of heavy rainfall. With synthetic rainfall according to EULER (Type II) the point in time for the start of the rainfall intensity is determined with the highest precipitation intensity with 0.3 times the synthetic rainfall duration and rounded down to a multiple of five minutes. The next lower intervals are joined on to the left of the time axis until the point in time t = 0 is reached. Further rainfall intervals follow the time axis to the right after the peak interval and fill the time period up to the end of the synthetic rainfall. 5.2.2.2 Synthetic Rainfall Groups As critical discharge conditions, both from short-term heavy rainfall (by area) and also through long periods of continuous rainfall with large amounts of precipitation, can be brought about in large catchment areas, with longer flow times in the network, so-called synthetic rainfall groups should be employed for dimensioning and the overdamming verification in place of an individual synthetic rainfall event of different duration. The aim is that individual rainfalls of a synthetic rainfall group cover the rainfall frequency duration curve by phase respectively in the range of their rainfall duration. The characteristic intensity progression of the various rainfall durations is determined on the basis of statistic assessments of measured precipitation (comp. Otter and Königer, 1986; Schaardt, 1999). One possibility for the creation of synthetic rainfall groups is presented in Appx. A2.

5.2.3 Measured Heavy Rainfall Series The above-given initial precipitation details - block rain and intensity-variable synthetic rainfall or synthetic rainfall groups - are derived statistically from rainfall duration frequency curves and/or rainfall amount curves, whereby duration, frequency of excesses and typified time history are first determined. On the other hand, with the initial details from heavy rainfall series, direct rainfall events with actually occurring duration and time history are used, which are selected and extracted from existing rainfall records (Appx. A3). Procedures for the systematic selection of the heavy rainfall series are described by SARTOR (1994) and SIEKER (1997).

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For the production of heavy rainfall series a recording period of the precipitation of 30 years or more is desirable. However, today such durations are still not available. The minimum necessary recording period depends on the overdamming frequency to be verified, whereby the guidance values can be taken from Table 5. It is recommended that the statistical representation of the selected heavy rainfall events are assessed using precipitation statistics. The verification calculation using heavy rainfall series provides maximum water levels of the respective events for the individual shafts. Through counting there results the figure x the number of times the laiddown reference level, e.g. the surface of the ground, is exceeded. The annual frequency nai of these excesses (“overdamming frequency”) follows from this in the simplest way in that the number of excesses x is divided by the figure M, the number of years on which the heavy rainfall series is based: nai = x : M Table 5: Recommended guidance values for the minimum duration of precipitation records

Overdamming frequency

Minimum period for recordings

1 in 1 to 1 in 2 years 10 years 1 in 3 years 15 years 1 in 5 years 20 years 1 in 10 years 30 years

According to VERWORN (1999) this simple counting provides sufficiently reliable statements on the annual frequency of overdamming, if x ≥ 3. This boundary condition is always met for the recurrence time for which the verification is to be carried out, if the minimum duration of recording according to Table 5 is observed. An extensive differentiation of overdamming events is possible if, for each individual event, the duration of the overdamming process and the volume of the water escaping are given in the calculation.

5.2.4 Precipitation Continuum Fundamentally the precipitation continuum including the dry periods can be based, also directly, on the sewer network calculation - analogous to the pollutant load calculation, so that the pre-selection of relevant heavy rainfall can be dispensed with. Here, a large number of rainfall events, which are not relevant for dimensioning are, however, recorded. In view of the associated enormous calculation expense and the profusion of data to be assessed, the continuum consideration is currently to be seen as still being a special application (e.g. in connection with sewer network controls). Here the problem-matching limitation of individual overdamming events must be carried out for the calculation of associated frequencies according to common initial details.

5.2.5 Defining of Permitted Discharges The defining of permitted discharges and/or discharge intensities for a catchment area or an area thereof can be occasioned by network-related and surface water-related targets. It can take place fundamentally both for the complete area of a new development as well as for the new exploitation of larger plots of land with connection to existing systems. By limiting the precipitation run-off with the aid of decentralised measures of stormwater management the hydraulic overloading in the network can be avoided or reduced and the loading of surface waters can be reduced through stormwater overflows and stormwater outfall structures.

20 November 1999

It would be ideal if the defining of permitted discharge values for a construction area were oriented to the flooding discharge intensity of the original unbuilt-up area. These vary, depending on regional characteristics and overdamming frequency, mainly between 1 and 10 l/(s.ha). Such a strict limitation cannot, however, usually be observed for one building development so that a residential area-based “increase”, determined on local conditions and the structure of the development, is appropriate. With hydraulic bottlenecks in existing systems the permitted discharge density would have to be determined directly on the hydraulic efficiency of the drainage network.

5.3 Stormwater Runoff The precipitation discharge process in residential areas can be subdivided into the phases run-off formation, run-off concentration and sewer discharge. A detailed representation on this can be found, inter alia, in ATV Advisory Leaflet ATV-M 165.

5.3.1 Run-off Formation The run-off formation includes the physical processes which lead to a conversion of the precipitation, which has fallen, into a run-off from the surface (stormwater run-off): wetting, filling of depressions, evaporation and percolation into the ground. The parts of the precipitation which do not reach the discharge are designated as losses. The size of the precipitation with an effect on the discharge depends above all on the following influencing factors: - proportion of hardened surfaces, - type of hardened surface, - ground slope, - strength and duration of the rainfall, - type of soil and vegetation (permeable surfaces) The degree of hardening corresponds with the measurable element of the hardened surfaces of the area AC,s in the (partial) catchment area covered by the sewer system. The discharge coefficient designates the ratio of discharge to precipitation for the surface under consideration (see Sect. 5.3.1.2). As a rule it is dependent on the event. 5.3.1.1 Individual Loss Approaches Due to their different discharge behaviour hardened surfaces (roofs, roads) and unhardened surfaces are to be separated from each other as accurately as possible. Permeable hardened surfaces (e.g. paved covering, gravel paths) and surfaces deliberately decoupled from the sewer system are to be taken into account in a suitable fashion. A procedure for the detailed determination of the degree of hardening in residential areas is described by LAUBE and WILLEMS (1991). (a) Impermeable hardened surfaces With impermeable hardened surfaces there are always significant wetting and depression losses. The evaporation with the heavy rainfall events relevant for the sewer network calculation are negligible. (a1) Wetting loss The wetting loss is deducted as initial loss from the first precipitation intervals. It can be applied as 0.3 - 0.7 mm for dried surfaces depending on their properties. (a2) Depression loss The scale of the depression loss according to previous experience for hardened dried surfaces is 0.5 - 2.0 mm, depending on the type of hardening and ground slope. Distribution by time can take place according to various assumptions. The values given under (a1) and (a2) apply in form only for the consideration of individual rainfall. Separate consideration is to be given for continuum simulation.

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(b) Permeable and partially permeable surfaces With (partially) permeable surfaces percolation into the ground is additionally to be taken into account. The scale of individual losses is influenced by the ground slope, the surface properties and vegetation, percolation additionally from the type of soil and conditions at the start of rainfall. The possible amounts of loss vary due to the heterogeneous surfaces in significantly larger areas. The contribution to run-off from unhardened surfaces is difficult to quantify particularly as it is does not drain directly into the sewer system. Taking account of this in the sewer network calculation is to be examined in each individual case for the relevant recurrence times and local conditions and is to be agreed with the responsible office. 5.3.1.2 Discharge Coefficient While detailed discharge models for sewer network calculation normally calculate the stormwater run-off directly from the specification of the degree of hardening (and other model parameters), normal methods for sewer network calculation work with the specified discharge coefficients or those derived from the degree of hardening and other influencing parameters. For the sewer network calculation the peak discharge coefficient ψp is relevant, which describes the relationship between the resulting maximum discharge intensity and the associated rainfall intensity. ψp = max. discharge intensity /associated rainfall intensity = q [l/(s.ha)] / r [l/(s.ha)] For the employment of flow time methods the peak discharge coefficients ψp dependent on the hardened surface component, the ground slope group and the relevant reference rainfall intensity r15 in accordance with Table 6 are recommended. They are related to the area of the sewered catchment area (AC,s). Special local conditions with regard to the roof surface component and type of roof as well as slope of the drainage surfaces are to be taken into account appropriately. The peak discharge coefficients given apply only for flow lengths of from 40 - 70 m. Occasionally they provide incorrect discharge values, in particular with non- or slightly hardened (part) catchment areas. Here supplementary information with regard to local characteristics (geology, groundwater conditions, saturation behaviour of the ground etc.) are to be brought in. The combination of larger non-built-up external areas with sewered catchment areas generally requires a separate consideration due to the different discharge conditions and different relevant rainfall events. Table 6: Recommended peak discharge coefficients for various rainfall intensities with a rainfall

duration of 15 min (r15) dependent on the average ground slope SG and the degree of hardening (for flow time methods from ATV Standard ATV-A 118, 1977)

Degree of hardening

[%]

Group 1 SG < 1 %

Group 2 1 % ≤ SG ≤ 4 %

Group 2 4 % < SG ≤ 10 %

Group 2 SG > 10 %

for r15 [l/(s.ha)] of 100 130 180 225 100 130 180 225 100 130 180 225 100 130 180 2250*) 10*) 20 30 40 50 60 70 80 90 100

0.00 0.09 0.18 0.28 0.37 0.46 0.55 0.64 0.74 0.83 0.92

0.00 0.09 0.18 0.28 0.37 0.46 0.55 0.64 0.74 0.83 0.92

0.10 0.19 0.27 0.36 0.44 0.53 0.61 0.70 0.78 0.87 0.95

0.31 0.38 0.44 0.51 0.57 0.64 0.70 0.77 0.83 0.90 0.96

0.10 0.18 0.27 0.35 0.44 0.52 0.60 0.68 0.77 0.86 0.94

0.150.230.310.390.470.550.630.710.790.87095

0.300.370.430..500.560.630.700.760.830.890.96

(0.46

(0.51)

0.560.610.660.720.770.820.870.920.97

0.150.230.310.390.470.550.620.700.780.860.94

0.200.280.350.420.500.580.650.720.800.880.95

(0.45)

0.500.550.600.650.710.760.810.860.910.96

(0.60)

(0.64)

0.67 0.71 0.75 0.79 0.82 0.86 0.90 0.93 0.97

0.20 0.28 0.35 0.42 0.50 0.58 0.65 0.72 0.80 0.88 0.95

0.30 0.37 0.43 0.50 0.56 0.63 0.70 0.76 0.83 0-89 0.96

(0.55)

(0.59)

0.630.680.720.760.800.840.870.930.97

(0.75

(0.77)

0.800.820.840.870.890.910.930.960.98

) *) degrees of hardening ≤ 10 % as a rule require separate consideration

22 November 1999

5.3.2 Run-off Concentration The run-off concentration describes the conversion of the precipitation, distributed over the surface and which has an influence on the run-off, into the discharge hydrograph. With this, the flow procedures on the surface (translation) and the delaying effects (retention) play a role. These complex physical processes up to now could only be included approximately in the sewer network calculation. The stormwater run-off can be described according to various model approaches, inter alia: - unit hydrograph - storage models (individual linear storage, linear storage cascade, nonlinear models) The differences in discharge behaviour of hard and permeable partial surfaces are to be observed also with the discharge concentration, in particular the as-a-rule significantly larger delay in run-off with permeable surfaces. In many cases the same calculation approach using different model parameters is used for delay of run-off (translation and retention). Details on the selection and size of the model parameters can be made only in connection with specific model approaches and are to be taken from the relevant model descriptions or specialist literature (Keser, 1980; ATV Advisory Leaflet ATV-M165).

5.3.3 Taking Account of Measures for Decentralised Stormwater Management The above described approaches for the calculation of stormwater run-off refer to the effective run-off surfaces covered by the sewer system. With normally designed sewer networks these were usually all hard surfaces (roofs, courtyards, parking lots, roads, paths and squares) and, depending on the local conditions and - at least with heavy rainfall events - partially also unhardened surfaces. Through the realisation of decentralised stormwater management the discharge behaviour in the catchment area becomes more complex, as its effect, depending on the type of measure and depending on the amount of rainfall and chronological progress of an event, can be different. Thus a discharge throttling to retain a permitted discharge rate during the complete event effects a constant inflow to the public drainage system. Decentralised stormwater storage, roofs with vegetation and percolation facilities can contribute, following the exhaustion of storage and/or percolation capacity, to the stormwater discharge into the sewer system. Here, if required, the different design criteria in comparison with the public drainage system, are also to be observed The existence of this type of measure assumes a particularly high degree of care with the consideration of the drainage area and the mathematical reproduction of the discharge behaviour using calculation methods which can correctly describe the phenomena in their effect. This applies also for its transformation into existing systems.

5.4 Calculation Methods for Sewer Discharge Sewer network calculation methods determine discharges and water levels from the initial details on dry weather discharge and the determined run-off from the surface, depending on the calculation, as maximum values or with associated hydrographs. Corresponding with the calculation approach for the sewer discharge, they are characterised as hydrological or hydrodynamic. Hydrological methods use empirical approaches or transfer functions for discharge calculation. Hydrodynamic calculation approaches are based on the solution of the Saint Venant Equations (comp. ATV-M 165).

5.4.1 Hydrological Methods Hydrological methods first calculate the sewer discharge with the aid of defined transfer or storage functions. Associated water levels are determined separately - as a rule via normal discharge relationships. With normal procedures, e.g.: - time coefficient method - time-flow parameter method

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- hydrograph superposition - flood plan method the calculation of maximum values are at the front. They are also designated as flow-time methods, as the discharge calculation is built up substantially on the flow time. 5.4.1.1 Time Coefficient Method The conventional calculation method employed most frequently is the time coefficient method (ATV, 1996), which corresponds with the “rational method” of the English language regions. Using the time coefficient area the greatest stormwater discharge is determined under the assumption that the flow time in the sewer network is set to be the same as the standard rainfall duration. With this the peak discharge coefficient ψp according to Table 6 is applied. The leading stormwater discharge Qs is determined using the following formula:

s,Cs)n,D(s ArQ ⋅ψ⋅= [l/s] (8)

The rainfall intensity r(D,n) which was previously formed from the product of the reference rainfall intensity r15,1 and time coefficient ψ of a certain rainfall duration D and rainfall frequency n, can be obtained from the heavy precipitation data of the DWD (1997) or the locally available precipitation data and its evaluation in accordance with ATV Standard ATV-A 121. In the atlas of the DWD “Heavy precipitation for Germany - KOSTRA” (DWD, 1997) there is an EDP program for the determination of location-specific amounts of precipitation and rainfall intensities of various duration phases D and recurrence times Tn (Disk). More detailed information on the application of the time coefficient method and the other flow time methods are to be found in the relevant specialist literature (i.a. ATV, 1995a). 5.4.1.2 Hydrological Discharge Models Hydrological discharge models employ transfer functions in order to calculate the discharge hydrographs within the sewer network from the inflow waves to the sewer network, whereby the wave displacement (translation) and damping (retention) with the discharge process are taken into account. The associated water levels are determined in a separate calculation process. Therefore the hydrological discharge models are not in a position to take into account the influence of overload conditions. Therefore they should only be applied to sewer networks in which backwater and flow reversal play only a subordinate role for the load cases considered.

5.4.2 Hydrodynamic Calculation Methods Hydrodynamic calculation methods build directly on the physical-hydraulic legitimacy of the flow process in sewers, described mathematically through the Saint-Venant differential equations (ATV-A 110E). By solving the complete equation system the hydrodynamic calculation method, through the permanent linking of discharge and water level as well as via the channel geometry, the flow rate also keeps its validity for various flow and system conditions. Overload conditions such as pressure discharge and backwater up to flow reversal are immediately taken into account and reflected realistically. Even the direct inclusion of branching and special structures produces no principle difficulties. Simplifications in the movement equation in these cases, however, lead to a limitation of the validity and accuracy of the calculation results. As the differential equation system cannot be solved analytically, mathematical methods of approximation in the form of implicit and explicit differential procedures are used (i.a. Verworn 1980; Königer, 1991). Through this the continuous discharge activity is considered discretely in path and time intervals. As a rule, sewer sections are used as path increment. The time intervals can be selected as constant or load-dependent (Schmitt, 1985) and can vary over the range from seconds (explicit) up to several minutes (implicit). For the mathematical verification of overdamming frequency (Sect. 6.2.4) attention is drawn to the new definitions within the framework of the revision of ATV-A110E for the consideration of the influence of local losses (in particular shaft impounding).

24 November 1999

Further information on calculation methods can be found in ATV Advisory Leaflet ATV-M 165 and in DIN EN 752-4.

6 Hydraulic Calculation and Implementation of Verification

6.1 Application Criteria With the application of hydraulic calculations the following tasks, which are characterised in more detail in Sect. 6.2, are to be differentiated: - re-dimensioning, - recalculation of existing systems, - calculation of rehabilitation variants, - verification of overdamming frequency. Calculation methods and precipitation loading are to be selected dependent on the respective objective and task as well as on the characteristics and constraints of the drainage system, and to observe the linkages between the given criteria.

6.1.1 Calculation Methods and Precipitation Loading Basically the following calculation methods are available (comp. Sect. 5.4): - flow time methods - hydrological flow models - hydrodynamic flow models. They are linked with the definition of certain precipitation loading (Sect. 5.2). Thus flow time methods in general resort to the rainfall duration frequency curve or block rainfall. With the employment of discharge models, the use of individual synthetic rainfall, synthetic rainfall groups or heavy rainfall series are to be examined in the individual case, on the basis of the complexity of the system as well as existing questions. The meaningfulness for various combinations of calculation methods and precipitation loads are shown in Table 7. Water levels above the crown of the sewer can only be calculated correctly using hydrodynamic methods. Table 7: Arrangement and meaningfulness of calculation methods and precipitation loading

Flow time methods

Hydrological models Hydrodynamic models

Rainfall duration frequency curve, synthetic rainfall

Maximum discharge 1)

Application not recommended

Application not recommended

Synthetic rainfall Euler (Type II) Application not possible

Discharge (Max. value, hydrograph)

Discharge and water level (Max. value, hydrograph)

Synthetic rainfall groups Application not possible

Discharge (Max. value, hydrograph)

Discharge and water level (Max. value, hydrograph)

Measured heavy rainfall series Application not possible

Discharge (Max. values, hydrograph, statistics)

Discharge and water level(Max. value, hydrograph, statistic)

1) Diagrammatic discharge hydrographs (“flood curves”) can be given with flood plan and cumulative methods

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Comparative calculations have shown that the shaft overdamming, identified using the frequency determined using the Euler synthetic rainfall (Type II), occurs with heavy rainfall simulation with comparable recurrence times. The estimation of the overdamming frequency according to both methods thus agrees well, over a wide range (i.a. Schmitt, Thomas, 1997; Stecker, Reimers, 1997). It is therefore recommended with verification calculations to select the rainfall frequency of Euler synthetic rainfall according to the overdamming frequency required in the application case.

6.1.2 Catchment Area The conditions and requirements of the drainage system are decisive for the selection of the calculation methods and the precipitation loading. So far as nothing else is specified by the responsible office for catchment areas up to 200 ha (AC,s) or flow times up to 15 min (“small catchment areas”) in accordance with DIN EN 752-4 simple empirical methods are recommended. The flow time is approximated without taking into account the discharge process on the surface for flow rates with complete filling and here are related to the complete catchment area. For the calculation of larger developments (> 200 ha), and drainage systems which can have considerable effects on existing sewer systems, the employment of discharge models is advised (comp. Sect. 5.2). In the above-mentioned comparative calculations the extensively equal value of verification calculations by means of Euler synthetic rainfall (Type II) and heavy rainfall simulation has been determined for a broad spectrum of catchment areas. However, larger differences can occur in individual cases with large flow times and existing system peculiarities, in particular with drainage systems with increased storage volumes (stormwater tanks, sewers with storage capacity), with deliberate utilisation of existing sewer storage volumes or with other applications of the sewer network management. In these cases the preferred employment of synthetic rainfall groups or the long-term simulation by means of heavy rainfall series is recommended. If necessary, with enlarged systems, the effects of an uneven rainfall distribution over the catchment area with the relevant heavy rainfall events having limited area coverage, are to be taken into account.

6.2 Task

6.2.1 Re-dimensioning of Drainage Networks With the re-dimensioning of smaller (simpler) drainage networks the dimensioning using flow time methods (time coefficient method, flood planning method) and block rainfall is generally sufficient. The resultant dimensioning value for the stormwater or combined wastewater discharge is to be a max. of 90 % of the discharge capacity of the selected sewer profile (ATV-A 110E). With larger networks it is recommended to safeguard dimensioning through a verification calculation. If possible the initial dimensioning should be corrected for so long until the required verification parameter (overdamming frequency) in the complete drainage system is maintained with as far as possible economical measures and taking into account the operating interests. With the connecting up of new sewer networks with existing systems first a dimensioning of the planned network is required. In a subsequent recalculation it is to be examined whether and, if required, with what frequency the areas to be connected lead to a disadvantageous overloading of the existing network (s. Sect. 6.2.4). Table 8 provides recommendations for the selection of the calculation methods and precipitation loading for the re-dimensioning. Table 8: Recommendations on applications for the re-dimensioning of drainage systems Flow time methods Hydrological models Hydrodynamic modelsRainfall duration frequency curve, synthetic rainfall

Recommended

Synthetic rainfall Euler (Type II) Possible Possible Synthetic rainfall groups Not recommended Not recommended Measured heavy rainfall series Not recommended Not recommended

26 November 1999

6.2.2 Recalculation of Existing Systems The hydraulic recalculation should generally be a component of an overall consideration of the condition and function of existing drainage systems. It can be occasioned in particular through apparent system overloads or flooding which has occurred in the past or pending changes in the sewer system catchment area, which can influence the discharge. For this case there are recommended selection possibilities with regard to calculation methods and precipitation loading which are to be found in Table 9. Table 9: Recommendations on applications for the recalculation of existing systems

Flow time methods Hydrological models Hydrodynamic modelsRainfall duration frequency curve, synthetic rainfall

Possible

Synthetic rainfall Euler (Type II) Possible Recommended Synthetic rainfall groups Possible Recommended Measured heavy rainfall series Possible Recommended

Essential aims of the analysis could be: - determination of the rates of utilisation and the functional hydraulic capability, - if required, determination of the water level relationships (e.g. overloaded systems), - identification of weak points and reserves, - establishment of the requirement for rehabilitation (note planning limits). The hydraulic recalculation should be supplemented through systematic observations of system behaviour with the occurrence of heavy rainfall events and through possibly existing measurements. The level of the rehabilitation requirement of existing systems in this case, however, does not result directly from the specifications of DIN EN 752-2 (see Table 2); but primarily from the actual discharge behaviour of drainage systems, the frequency and the effects of overloading which occur and thus from the defined flooding protection. This can be determined through systematic observations (questioning of residents, deployments of the fire services, measurements of discharge and water levels) and the inspection of the locality.

6.2.3 Calculation of Rehabilitation Variants According to the established (hydraulic) rehabilitation requirement - taking into account of the planning limits (≥ 50 years for drainage systems ) - possible rehabilitation measures are to be elaborated and investigated in a consideration of variants with regard to the effects on the discharge behaviour. This should take place using the same calculation methods as for the recalculation of the actual status, if required with reduced scope of precipitation loading (Table 10). From the assessment of the necessary rehabilitation methods and, if required, different rehabilitation variants, the levels of priority for possible rehabilitation measures based on the determined frequencies and the scale of calculated overloading can be established. With the elaboration of rehabilitation concepts, in addition to the hydraulic requirements of rehabilitation, further criteria are to be taken into account, in particular - structural condition of the sewers, - necessity for/capacity of stormwater overflow structures in combined systems, - general demands on water pollution control.

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Table 10: Recommendations on applications for the calculation of rehabilitation variants Flow time methods Hydrological models Hydrodynamic models Rainfall duration frequency curve, synthetic rainfall

Possible

Synthetic rainfall Euler (Type II)

Possible Recommended

Synthetic rainfall groups Possible Recommended Measured heavy rainfall series

Possible Recommended

6.2.4 Verification of Overdamming Frequency The implementation of a verification calculation supplementary to re-dimensioning is recommended for large drainage networks. It is always to be recommended, when complicated hydraulic conditions or special requirements for flood protection exist in the drainage area. The verification of security against overdamming should, in addition, always be carried out where doubts exist about sufficient protection against flooding. In this case the statements in Sect. 5.1 with regard to recurrence times and overdamming frequencies apply (Table 3). The verification action starts from the calculated or defined cross-section and delivers, as result, the frequency or recurrence time with which the water levels occur above a certain level (e.g. surface of a road). Water levels above the crown of the sewer can be calculated accurately using hydrodynamic methods. The application recommendations expressed in Table 11 apply for the carrying out of the verification calculation. Table 11: Recommendations on applications for the verification of overdamming frequency Flow time methods Hydrological models Hydrodynamic models Rainfall duration frequency curve, synthetic rainfall

Possible

Synthetic rainfall Euler (Type II)

Possible Recommended

Synthetic rainfall groups Possible Recommended Measured heavy rainfall series

Possible Recommended

For complex system conditions, in particular in connection with pronounced storage effects, sufficiently comprehensive and locally accurate precipitation data have particular significance for the verification calculation with heavy rainfall series or synthetic rainfall groups. With extended systems, possible effects of an uneven rainfall distribution could require attention. For the employment of Euler synthetic rainfall (Type II) it is recommended that the rainfall frequency be selected the same as the specified overdamming frequency (comp. Table 3).

6.3 Safety against Flooding The European Standard Specification DIN EN 752-2 assumes the frequency of flooding as verification criteria (comp. Sect. 5.1). The flooding process on the surface and the flood protection are, to a high degree dependent on the local situation and therefore require an assessment of the conditions in the local area. The possibility and danger of flooding in the case of overdamming of the sewer system is also to be checked in the case of a simple dimensioning in accordance with Sect. 6.1.1 or at the end of the hydraulic recalculation. Due to difficulties in reflecting the surface flooding process technically in a model, an assessment of the conditions on site is absolutely necessary for the area with calculated overdamming. If required, available records on previous system behaviour are to be evaluated (fire service deployments and similar). Verification calculations should identify, for network points with overdamming across the ground, the volume of combined wastewater or stormwater which emerges or is not discharged as well as the duration of the calculated overdamming in order better to be able to assess the danger of flooding of neighbouring

28 November 1999

properties or of significant prejudicing of the function of important traffic facilities (underpasses). If a flooding of neighbouring properties is to be feared due to topographical and other local characteristics, suitable measures (design) with the layout of roads are to be implemented or the discharge of excess (waste-) water into no-endangered areas is to be examined. So far as the latter is discharged on to the road surface or is stored, the contour line of the road should be included in the examination and harmless discharge assured, if required using structural measures.

7 Applicable Standard Specifications and Rules and Standards

7.1 ATV Standards Wastewater- Waste ATV-A 105E: “Selection of the Drainage System” ATV-A 110E: “Standards for the Hydraulic Dimensioning and Performance Verification of Sewers and Drains” ATV-A 111E: “Standards for the Hydraulic Dimensioning and Performance Verification of Stormwater Overflow Installations in Sewers and Drains” ATV-A 112: “Richtlinien für die hydraulische Dimensionierung und den Leistungsnachweis von Sonderbauwerken in Abwasserkanälen und -leitungen” [Standards for the Hydraulic Dimensioning and Performance Verification of Special Structures in Sewers and Drains] ATV-A 116E: “Special Sewer Systems - Vacuum Drainage Service - Pressure Drainage Service” ATV-A 117: “Richtlinien für die Bemessung, die Gestaltung und den Betrieb von Regenrückhaltebecken” [Standards for the Dimensioning, Design and Operation of Stormwater Holding Tanks] ATV-A 121: “Niederschlag - Starkregenauswertung nach Wiederkehrzeit und Dauer, Niederschlagsmessungen Auswertung” [Precipitation - Heavy Rainfall Evaluation according to Recurrence Time and Duration, Evaluation of Precipitation Measurements] ATV-A 128E: “Standards for the Dimensioning and Design of Stormwater Overflows in Combined Wastewater Sewers” ATV-A 131E: “ Dimensioning of Single-Stage Activated Sludge Plants” ATV-A 138E: “Construction and Dimensioning of Facilities for Decentralised Percolation of Non-Harmful Polluted Precipitation Water” ATV-A 200E: “Principles for the Disposal of Wastewater in Rurally Structured Areas” ATV-A 400E: “Principles for the Preparation of German ATV Standards” ATV-M 101E:

ATV-A 118 E

November 1999 29

“Planning of Drain and Sewer systems; New Construction, Rehabilitation and Replacement” ATV-M 165: “Anforderungen an Niederschlag-Abfluß-Berechnungen in der Stadtentwässerung” [Requirements on Precipitation Run-off calculations in Municipal Drainage Systems]

European and DIN Standard Specifications DIN EN 752: “Drain and sewer systems outside buildings” Part 1: Generalities and definitions Part 2: Performance requirements Part 3: Planning Part 4: Hydraulic design and environmental considerations Part 5: Rehabilitation Part 6: Pumping installations Part 7: Maintenance and operations DIN EN 1091: “Vacuum sewerage systems outside buildings” DIN EN 1671: “Pressure sewerage systems outside buildings” DIN EN 12056: “Gravity drainage inside buildings” DIN 1986-2: “Drainage systems for buildings and private property, Part 2: Determination of nominal widths for sewers and drains”

Literature [Translator's note: known translations are give in English, otherwise a courtesy translation is provided in square brackets]

Althaus, J.: Vergleich von Modellregen zur Kanalnetzberechnung,

[Comparison of synthetic rainfall for sewer network calculation] Mitt. Inst. Wasserwirt., Hydr. u. landw. Wasserbau TU Hannover, Vol. 56 (1984).

ATV (1977):

„Richtlinien für die hydraulische Bemessung von Regen-, Schmutz- und Mischwasserkanälen“ [Standards for the hydraulic dimensioning of stormwater, normal and combined wastewater sewers] ATV Rules and Standards, ATV Standard ATV-A 118, St. Augustin

ATV (1995a):

„Planung der Kanalisation“ [Planning of the sewer system] ATV Manual , 4th Edition. Publisher W. Ernst und Sohn, Berlin/München

30 November 1999

ATV (1995b):

„Überstau und Überflutung - Definitionen und Anwendungsbereiche“ [Overdamming and flooding - definitions and areas of application] ATV Report ATV Working Group 1.2.6, Korrespondenz Abwasser, Vol. 9

ATV (1996):

„Umfrageergebnisses zum Stand der Kanalnetzberechnungsverfahren und der Bemessungskriterien“ [Results of a survey on the status of sewer network calculation methods and the dimensioning criteria], ATV Report ATV Working Group 1.2.1, Korrespondenz Abwasser, Vol. 5, May 1996

Cassar, A.; Dohm, H. (1997):

„Besonderheiten bei der hydraulischen Nachweisrechnung großer Kanalnetze“ [Peculiarities with the hydraulic verification of large sewer networks], Zeitschrift für Stadtentwässerung und Gewässerschutz (SuG) [Journal for Municipal Drainage and Water Pollution Control], Vol. 28

DWD (1997):

„Starkniederschlagshöhen für die Bundesrepublik Deutschland“ [Heavy precipitation amounts for the Federal republic of Germany], Parts 1 and 2; Selbstverlag des deutschen Wetterdienstes [Own publishing by the German Weather Service]; Offenbach/Main

Engel, N. (1994):

„Hydrologische Simulation der Ausflußtransformation in Kanalisationsnetzen“ [Hydrological simulation of discharge transformation in sewer networks] Technical reports on hydrological technology and hydraulics, Institut für Wasserbau [Institute of Hydraulic Engineering], University of Darmstadt, Vol. 52

Kesser, J. (1980):

„Beitrag zur Qualifizierung der hydrologischen Parameter für Siedlungsgebiete“ [Contribution on the quantification of hydrological parameters for residential areas] Mitteilung des Instituts für Wasserwirtschaft, Hydrologie und landwirtschaftlichen Wasserbau [Report of the Institute for Water Management, Hydrology and Agricultural Hydraulic Engineering], Vol. 47, University of Hannover

Königer, W. (1981):

„Die Anwendung der Extremal-3-Verteilung bei der Regenauswertung und der Niedrigwasseranalyse“ [The application of Extremal-3 distribution with rainfall assessment and low water analysis] gwf-wasser/Abwasser 122, Vol. 10, p. 46-466

Königer, W. (1991):

„Hydraulische Grundlagen von Niederschlagsabflußmodellen“ [Hydraulic bases of precipitation discharge models] Zeitschrift für Stadtentwässerung und Gewässerschutz (SuG) [Journal for Municipal Drainage and Water Pollution Control], Vol. 15, p. 37-62

Meißner, E. (1991):

„Bemessung von Misch- und Regenwasserkanälen“ [Dimensioning of combined wastewater and stormwater sewers] Advisory Leaflet No. 4.3-2 dated 01.06.91, Bayerisches Landesamt für Wasserwirtschaft, München 1991

Neumann, W. (1976):

„Der Oberflächenabfluß in städtischen Einzugsgebieten“ [The surface run-off in municipal catchment areas], Berichte aus Wasserwirtschaft und Gesundheitsingenieurwesen, Vol. 11, TU München.

Laube, F.W., Willems, G. (1991):

„Ermittlung der befestigten Fläche zur Bemessung von Regenbecken“ [Determination of the hard surfaces for the dimensioning of stormwater tanks], Korrespondenz Abwasser, Vol. 10, p. 1336 ff.

ATV-A 118 E

November 1999 31

Otter, J., Königer, W. (1986):

„Bemessungsregen für Kanalnetz, Regenüberläufe und Regenbecken“. [dimensioning rainfall for sewer networks, stormwater overflows and stormwater tanks], Gas-Wasser-Abwasser 66, Vol. 3, p. 124-128.

Pecker, R.(1995):

„Bemessung von Regen- und Mischwasserkanälen im europäischen Vergleich“ [Dimensioning of stormwater and combined sewers in European comparison], 3rd Saarland Wastewater Day/ATV Federal State Group Conference Hessen/Rheinland-Pfalz/Saarland

Pilgrim, D.H., Cordery, I. (1975):

“Rainfall temporal patterns for design flood”, Journal of the Hydraulics Division ASCE 101, HY 1, p. 81-95.

Sartor, J. (1994):

„Die Wahrscheinlichkeit des gleichzeitigen Auftretens maßgebender Abflußereignisse in Kanalisationsnetzen und natürlichen Gewässern“ [The probability of concurrent occurrence of relative discharge events in sewer system networks and natural surface waters], Berichte des Fachgebietes Wasserbau und Wasserwirtschaft, University of Kaiserslautern, Vol.3.

Schmitt, T.G. (1985):

„Der instationäre Kanalabfluss in der Schmutzfrachtmodellierung“ [The unsteady sewer discharge in pollution load modelling] Gas-Wasser-Abwasser 58 (1978), No. 11, p. 658-667.

Schmitt, T.G., Thomas, M. (1997):

„Untersuchungen zum rechnerischen Überstaunachweis auf der Basis von Modellregen und Regenserien“ [Investigations for the mathematical overdamming verification on the basis of synthetic rainfall and rainfall series], Final report of the ATV A 1.6 Project (unpublished).

Sieker, F. (1997):

„Bildung und Anwendung von Starkregenserien für den Überstaunachweis bei Misch- und Regenwasserkanälen“ [Formation and application of heavy rainfall series for the verification of overdamming with combined and stormwater sewers] gwf Wasser Abwasser, Vol. 5, p. 260-263.

Stecker, A., Reimers, M. (1997):

„Vergleichende Kanalnetzberechnung mit Modellregen und Langzeit-Serien-Simulation“ [Comparative sewer network calculation using synthetic rainfall and long-term series simulation] Zeitschrift für Stadtentwässerung und Gewässetschutz (SuG), Vol. 40.

Verworn, H.-R. (1999):

„Die Anwendung von Simulationsmodellen in der Stadtentwässerung“ [The application of simulation models in municipal drainage] Schriftenreihe für Zeitschrift und Gewässerschutz, Vol. 18, SuG-Verlag Hannover

Verworn, W. (1980):

„Hydrodynamische Kanalnetzberechnung und die Auswirkungen von Vereinfachung der Bewegungsgleichungen“ [Hydrodynamic sewer network calculation and the effects of simplification of the laws of motion] Mitteilungen des Instituts für Wasserwirtschaft, Hydrologie und Landwirtschaftlichen Wasserbau, Hannover, Vol. 47.

Werp, M. (1992):

„Dimensionierung von Kanalnetzen in der Rechtsprechung des Bundesgerichtshofs“ [Dimensioning of sewer networks in the jurisdiction of the (German) Federal Supreme Court] Korrespondenz Abwasser, Vol. 9.

32 November 1999

Appendix

A1 Creation of synthetic rainfall according to Euler Type II Given are, for example, the following rainfall amount curves for n = 1.0:

D (min) 5 10 15 20 30 45 60

h (mm) 6.1 9.5 11.4 12.8 14.7 16.6 18.0

The rainfall amounts of the individual time intervals result through subtraction from the rainfall sums (Fig. A1-1).

Fig. A1-1: Rainfall amount h in 5 minute intervals. The sum of all individual intervals up to

the time t corresponds with the value of the rainfall amount curve With synthetic rainfall according to EULER (Type II) the point in time for the start of the rainfall intensity is determined with the highest precipitation intensity with 0.3 times the synthetic rainfall duration and rounded down to a multiple of five minutes. The next lower intervals are joined on to the left of the time axis until the point in time t = 0 is reached. Further rainfall intervals follow the time axis to the right after the peak interval and fill the time period up to the end of the synthetic rainfall. Fig. A1-2 shows the synthetic rainfall obtained through transposition of the intervals. The duration of the synthetic rainfall peaks should, as a rule, be 5 minutes. This corresponds, in general, with the flow time on the surface. The damping of the discharge wave with longer flow paths on the surface, for example with outside areas, takes place within the framework of the calculation of the stormwater run-off. This synthetic rainfall, as a rule, delivers discharges and water levels which lie on the safe side.

ATV-A 118 E

November 1999 33

Fig. A1-2: Individual synthetic rainfall according to Euler (Type 2) through transposition of

the 5 minute intervals from Fig. A1-1

A 2 Production of a synthetic rainfall group The compilation of the individual rainfall of different duration within a recurrence time determined for a synthetic rainfall group is based on the statistic evaluation of measured precipitation series. The progression of the precipitation characteristic for the respective rainfall duration is obtained by means of standardisation of the measured natural rainfall of the same rainfall duration, which can take place through the chronological centring of the crucial point or the 5 minute peak intervals of precipitation. While the centring of the crucial point with increasing rainfall duration effects a certain flattening of the rainfall progression, there result pronounced precipitation peaks through the maximum value centring (Schaardt, 1999). The method using the centring of precipitation critical points (Otter, Königer, 1986) is illustrated below as an example. Table A2-1: Measured heavy rainfall events (h15 > 4.0 mm) in 5 minute intervals, centred on the

critical point of the maximum 15 minute stage.

Critical point

Date 1 2 3 4 5 6 7 8 9 27.08.1956 0.185 2.590 3.358 0.172 0.857 0.834 0.913 0.450 0.201 10.09.1956 0.149 0.149 3.636 2.546 0.260 0.260 0.158 0 0 08.06.1957 0 0.900 3.184 1.013 0.795 0.944 0.531 0.480 0.564 etc.

First, as with the frequency analysis of heavy rainfall (ATV, 1985), maximum precipitation stages for various duration phases D are sought from a continuous rainfall series. With this, the threshold values should be so selected that, on one hand, at least 30 events per continuous period and, on the other, not more than two or three events per year are taken out. That means, that a minimum duration of the precipitation series of 10 to 20 years is required. Rainfall events with which less than 2/3 of the maximum phase are filled by rain are excluded. The critical points of the maximum phases (bold figures in Table A2-1) are determined and are rounded to an integral column number. All rainfall events taken from the series are centred with regard to this critical point (Table A2-1). The intensity progression of the rainfall event (including antecedent and successive rainfall) is standardised in that the individual intervals are so multiplied by a factor that their sum in the maximum stage of the corresponding duration gives the value 1 (table A2-2).

34 November 1999

Table A2-2: Standardised intensity progressions of the measured events (Σh15 = 1.0)

1 2 3 4 5 6 7 8 9 27.08.1956 0.03 0.42 0.55 0.03 0.14 0.13 0.15 0.07 0.03 10.09.1956 0.02 0.02 0.56 0.40 0.04 0.02 0.158 0 0 08.06.1957 0 0.18 0.62 0.20 0.16 0.19 0.10 0.09 0.11 etc.

Now all intervals in a column are sorted according to size and the value determined which is equally exceeded or undercut (median, 50 % value). The thus obtained intensity progression is multiplied by a factor (see Table A2-3) so that the amount of rainfall of the maximum phase corresponds with the desired frequency (e.g. 12 mm). The start of the antecedent rainfall is achieved if the rainfall intensity undercuts a boundary intensity of some 0.1 - 0.2 mm/5 min. Table A2-3: Median values per column of the standardised intensities (2nd line) and multiplication

of all values by the factor 12.0 / (0.18 + 0.56 + 0.20) (3rd line)

1 2 3 3 4 5 6 7 8 Median value 0.02 0.18 0.56 0.20 0.14 0.13 0.10 0.07 0.03 Synthetic rainfall h15 = 12.0 mm

0.26 2.30 7.15 2.55 0.260 1.66 1.28 0.89 0.38

With that, the synthetic rainfall is present with a typical intensity progression corresponding with the rainfall duration.

Fig. A2-1: Synthetic rainfall D = 15 minute (example) For other duration phases the synthetic rainfall is derived statistically in the same way. Through plotting all synthetic rainfalls of the rainfall duration frequency curve a frequency can be examined as to whether the synthetic rainfall covers the rainfall duration frequency curve from the statistical precipitation evaluation (e.g. DWD, 1997) well in the area of their respective rainfall duration, and does not exceed these in any interval Fig. A2-2).

ATV-A 118 E

November 1999 35

Fig. A2-2: Rainfall duration frequency curves of the synthetic rainfalls D = 15 and 30 minutes

and the rainfall duration frequency curve for the same frequency Further literature: Otter, J., Königer, W. (1986):

„Bemessungsregen für Kanalnetz, Regenüberläufe und Regenbecken [Dimensioning rainfall for sewer networks, stormwater overflows and stormwater tanks]“, Gas-Wasser-Abwasser 66, Vol. 3, p. 124-128

Schaardt, V.(1999:

„Belastungsannahmen bei der Kanalnetzberechnung größerer Einzugsgebiete [Loading assumptions with the calculation of sewer networks in larger catchment areas]“, gwf wasser Abwasser 140, Vol. 1, p. 27-35

A 3 Compilation of heavy rainfall series With the defining of heavy rainfall series in accordance with Sect. 5.2.3, rainfall events are applied with their actual duration and chronological progression, which are selected according to certain criteria and taken from the available rainfall registers. The series must at least contain all events which potentially can lead, within the sewer network, to an overdamming of the laiddown reference level. For security, however, as a rule further pronounced events are also included in the series which would have only a low probability of leading to overdamming. The employment of the complete series of all registered rainfall events would make a selection process superfluous, however, due to the required calculation times with the current state-of-the-art with software and hardware, it is too expensive and also not necessary. For the production of heavy rainfall series a recording period of precipitation of 30 years or more is desirable. Frequently data of this length of time are, however, still not available today. The as a minimum necessary registration period depends on the overdamming frequency to be verified. For this, the guidance values given in Table 4 (Sect. 5.2.3) can be adopted. Smallest chronological separation between individual events Independent rainfall events are separated from subsequent events by precipitation-free periods. The effects of the rainfall events with regard to discharges and water levels within the sewer network, however, go on beyond the respective end of rainfall so that this can overlap the effects of a following event if the separation of the two events is very short. The separation between the events of a heavy rainfall series must take this into account. The minimum chronological separation between individual events results from the emptying time of the drainage system. The gap in rainfall should, however, be at least four hours.

36 November 1999

Lowest intensity of precipitation With the limitation of rainfall events compared with dry periods there is the question of which intensity or amount of precipitation a rain shower has to have within a certain period in order to apply, both with regard to time and amount, as part of the event. In this respect the following can serve as approximate value: hN,min = 0.1 mm in 5 min. or 0.5 mm in 1 hr. Procedure for the selection of relevant events A possible criterion for the selection of the events is the introduction of a limiting value for the sum of precipitation of the natural events, which are independent according to the above given criteria. As guidance value the following can, for example, apply as approximate value: Nmin = 10 mm. As here the duration of the precipitation event is not taken into account and precipitation of 10 mm, distributed over, for example, 1 hr most probably will not cause an overdamming, according to these criteria alone an unnecessarily large number of events are included in the series. There is therefore interest to restrict further the selection of events. FUCHS (1994) recommends, in accordance with ATV Standard ATV-A 121, proceeding from the series of individual continuous periods. With these series not only the natural closed events are taken into account but also all extreme rainfall periods within the closed events. These events are arranged according to the continuous period of the amount of the precipitation. The number of events taken up in the series are then calculated for each continuous period as follows: nT/M71.2N ⋅= with: N = number of the events to be taken into account, proceeding from the largest event M = duration of the precipitation series based on the evaluations (in years) Tn = recurrence time of the overdamming frequency to be verified (in years) It is to be estimated which continuous periods are relevant for the respective network. In practice, the selection of 15 min as the shortest and of 6 hr as the longest continuous period has proved itself. Overall it suffices to carry out the calculation for some 6 continuous periods. For all events selected within the different continuous periods with which, in general, one is concerned with periods of rainfall and not self-contained rain showers, it is determined via the specification of the date and time of the associated self-contained rainfall event from which the rainfall period originates and are identified as relevant events for the verification calculation. In this case it results, in general, that several rainfall periods determined previously from various continuous periods belong respectively to the same closed event. With this, the final number of selected events compared with the product (N x no. of duration levels) is reduced considerably. VERWORN (1995) specifies a procedure which further limits the number of relevant events and, at the same time, via the determination of so-called frequency or recurrence time profiles allows a detailed preliminary estimate of the effects of the individual rainfall events on the network to be investigated. The frequency or recurrence time profiles of a natural rainfall event is determined as follows: Proceeding from the data sequence of the digitalised event given in 5 minute steps (comp. Fig. A3-1) the maximum interval of the various continuous periods of 5, 10, 15, ... minutes up to the boundary duration of the event are selected in turn according the amount of rainfall. For each so determined amount of rainfall the associated recurrence time is determined according to the statistical method of ATV Standard ATV-A 121, supported by the German Weather Service (DWD) evaluation “Heavy rainfall amounts for the Federal Republic of Germany” (KOSTRA). These recurrence times are entered as ordinate values above which the given continuous periods are entered as abscissa values. It should be noted that the abscissa in this diagram does not reflect the time axis of the natural event but the succession of the various continuous periods.

ATV-A 118 E

November 1999 37

Fig. A3-1 shows the amount of rainfall of a natural event of overall 5 hours duration in 5 minute steps and in cumulative form. The profile of the recurrence times for the continuous periods of 5 minutes to 360 minutes is presented in Fig. A3-2 (the continuous periods extended up to 6 hours compared with the natural rainfall duration, result from addition of 5 minute intervals of rainfall amount 0). The selection and limitation of the relevant heavy rainfall can now be carried out very simply in that a limiting value Tn,lim is determined which a natural event at any possible point has to achieve or exceed its recurrence profile in order to be taken into the selection. This limiting value is made dependent on the permitted recurrence time Tn,perm upon which the sewer network is to be dimensioned, in order to limit the event further. In general, all natural events, which at any point in the sewer network lead to an overloading, are recorded with the specification of Tn,lim ≥ Tn,perm : 2. If, for example, a sewer network is to be dimensioned on a recurrence time of overloading of once in 5 years, a limiting value of Tn,perm = 2.5 results. This value, in the above example, is exceeded in the continuous periods between 75 and 360 minutes, i.e. the above event is to be taken up as relevant event in the selection series.

Fig. A3-1: Amount of rainfall of a natural event

Fig. A3-2: Profile of the recurrence times

38 November 1999

Carrying out verification using heavy rainfall series In the carrying out of the verification of a sufficient hydraulic efficiency of new or existing sewer networks it is to be confirmed mathematically, using defined heavy rainfall series (comp. Sect. 5.2.3), that the specified overdamming frequency or recurrence time of the overdamming event is not exceeded at any shaft in the sewer network. In particular with new planning the objective of carrying out verification can also be for reasons of cost or general water management and to orient the layout to the required overdamming frequency. With mathematical verification, using a hydrodynamic precipitation run-off model, it is calculated for each individual precipitation event at which shafts of the sewer network the water level exceeds the defined reference level of the overdamming frequency (in general the road or ground surface). The volume of exiting water and the duration of the overdamming procedure can also serve as possible additional result of the calculation to be taken into account, in that this can be included for the assessment of the effects of the overdamming event in the vicinity of the shaft concerned with regard to the required maintenance of the target parameter flooding frequency. Following implementation of the hydrodynamic calculation for all events of the defined heavy rainfall series the number of incidents of exceeding the overdamming level are counted for each shaft. The time-related relative frequency of the exceeding then results very simply in that the number of incidents of exceeding is divided by the period of registration of the heavy rainfall series. If the number of calculated incidents of exceeding at one shaft is, for example, 5 and if the heavy rainfall series is based on a period of registration of Tn = 20 years, there results the relative frequency of exceeding of nai = 5/20 = 0.25 or the recurrence time for exceeding is Tn = 1/nai = 4 years. A further statistic evaluation in the sense of matching a statistic distribution function, which also allows an extrapolation beyond the time period of the registration period is, with regard to the start of overdamming, not possible as here only the alternative statement “undercutting or exceeding the reference level” is available. The possibility of the employment of a statistical distribution function, however, exists with regard to the simulation dimensions “exiting volume of water” and “duration of the overdamming procedure”, which can be useful for the evaluation of the flooding danger if a sufficiently large number of overdamming event is available. Further information on the application of heavy rainfall series simulation in the steps actual status analysis, rehabilitation calculations, verification calculation can be found in the further literature. Further literature: Broll-Bickhardt, J., and Verworn, H.-R. (1995):

„Bewertung der Häufigkeit extremer Regenereignisse in Bremen im Jahre 1993 [Evaluation of the frequency of rainfall events in Bremen in 1993]“, Zeitschrift für Stadtentwässerung und Gewässerschutz (SuG), Vol. 32

Fuchs, L. (1994): „Integriertes System Bauwesen - ISYBAU - Fachinformationssystem Abwasser - Hydraulisches Konzept (Entwurf) [Integrated System for Civil Engineering - ISYBAU - Specialist Information Systems - Hydraulic Concept (Draft)]“, gwf wasser Abwasser 140, Vol. 1, p. 27-35

Kolbinger, A. and Meggeneder, M. (1995)

„Iterative Sanierungsberechnung eines städtischen Kanalnetzes [Iterative rehabilitation of a municipal serer network]“, Zeitschrift für Stadtentwässerung und Gewässerschutz (SuG), Vol 32