Urban drainage full_2011

sanitary engineering - ct3420

Urban Drainage

2

sanitary engineering - ct3420 urban drainage

Table of contents

1. Introduction 3 1.1 Urban drainage and sewerage 3 1.2 History of urban drainage and sewerage 3 1.3 Purpose of urban drainage systems 5

2. Approaches to urban drainage 5 2.1 Piped or natural systems 5 2.2 Combined and separate systems 6 2.3 Improved combined and improved separate sewer systems 7 2.4 Pressurised and vacuum systems 8 2.5 Components of sewer systems 10 2.6 Concepts,definitionsandabbreviations 11

3. Inflowsintourbandrainagesystems 12 3.1 Amount of wastewater 12 3.1.1 Domestic wastewater 3.1.2 Industrial wastewater 3.1.3 Infiltrationandexfiltration 3.1.4 Drainage water 3.1.5 Extraneouswater 3.2 Amount of stormwater 15 3.2.1 Amount of precipitation and runoff behaviour 3.2.2 Rainfall intensity duration frequency and rainfall mass curves 3.2.3 Using rainfall mass curves 3.2.4 Run-off processes 4. Environmental requirements for sewer design 20 4.1 Development of environmental guidelines for sewer systems 20 4.2 “Basic effort/Basisinspanning” 21 4.3 European Framework Directive 22 4.4 Storage and stormwater pumping capacity 23 4.5 Hydrodynamic evaluation of environmental impacts of sewer systems 24 4.6 References 26

5. Hydraulics for sewer design 27 5.1 Uniformchannelflowinpartiallyfilledsewerpipelines 27 5.2 Hydraulic resistance in components of sewer systems 31 5.2.1Seweroverflowweirs 5.2.2 Inlets, outlets and manholes

3


1. Introduction

1.1 Urban drainage and sewerageDrainage is defined in the Cambridge Online Dictionary as the process of water or waste liquids flowing away from somewhere into the groundor down pipes. In the case of urban drainage, “somewhere” is the urban environment, mainly constituted of roads, houses and green spaces. Urban drainage may be used to describe the process of collecting and transporting wastewater, rainwater/stormwater or a combination of both.

Sewage is water-carried wastes, in either solution orsuspension,thatisintendedtoflowawayfroma community. “Sewage” and “Sewerage” may be used interchangeably in the USA but elsewhere they retain separate and different meanings - sewage being the liquid material and sewerage being the pipes, pumps and infrastructure through whichsewageflows.Similar tourbandrainage,sewerage may refer to systems collecting wastewater or rainwater or a combination of both.

1.2 History of urban drainage and sewerageArtificial drainage systems were developed as soon as humans attempted to control their environment. Archaeological evidence reveals that drainage was provided to the buildings of many ancient civilisation such as the Mespotamians, the Minoans (Crete) and the Greeks (Athens).

RomansThe Romans are well known for their public health engineering achievements, particularly the impressive aquaducts bringing water into the city. Equallyvitalweretheartificialdrainstheybuilt,ofwhichthemostfamousisthecloacamaxima,builtto drain the Forum Romanum (and still in use today) (Butler and Davies, 2004). The Romans were proud of their “rooms of easement” (i.e., latrines). Public baths included such rooms -- adjacent to gardens.ThereRomanofficialswouldsometimescontinue discussions with visiting dignitaries while sitting on the latrines. Elongated rectangular platforms with several adjacent seats were utilized

(some with privacy partitions, but most without). These latrine rooms were often co-ed, as were the baths. Public latrines were used by many people, but for the most part, human wastes were thrown intothestreet.Romehadextensivestreetwashingprograms (with water supplied by aqueducts, the firstbeingbuiltin312BCE).Onlyafewhomeshadwater piped directly from the aqueducts; the vast majority of the people came to fountains to gather their water. Even though not many homes were directly plumbed into the sewers, when the wastes were thrown into the street, the street washing resulted in most of the human wastes ending up in the sewers anyway. Direct connection of homes to the sewers was not mandated until nearly 100 CE (cost was a factor; also mandating such a connection was then considered an invasion of privacy). Sewage resulting from the public baths and the included latrines was discharged into sewers and eventually to the Tiber River. It is worth noting that the Romans recognized the value of their water (which had been transported to the city via aqueducts, often over a distance of 20-30 miles); as such, any wastewater from the public bath facilities was often re-used, frequently as the flushingwaterthatflowedcontinuouslythroughthepubliclatrinefacilities.Fromthelatrines,itflowedto a point of discharge into the sewer system.

Middle Ages to 20th centuryThe Roman Empire fell in early CE along with the concepts of baths, basic sanitation, aqueducts, engineered water or sewage systems, etc. Sanitation reverted back to the basics: very primitive. During the so-called “Dark Ages,” there arose a brotherhood among men noted for skill in combat. There also evolved a creed that uncleanliness was next to godliness.As such, bathing/sanitation became quite uncommon; homes, towns, and streams became filthy.Diseaseswere commonplace; epidemicsdecimatedtownsandvillages.Twenty-fivepercent(or more) of the ancient European population died of disease (cholera, plague, etc.). The major transmitter of the plague was rats (actually bacteria conveyedfromratstopeopleviafleabites).Theratpopulation thrived amongst the mess and stench commonplace in medieval times.

4


Living conditions aggravated most seriously in large cities like London and Paris. In the beginning, natural streams were used as sewers. As cities developed, these natural drains were structurally covered. Early on, these sewers were used primarily for storm waters. For instance in Paris, theMenilmontantsewer,firstnoted in theearly1400s, was initially an open wash and later a closedconduit.ItinterceptedsurfaceflowsfromParis’ north slope area (i.e., that area lying on the right bank of the Seine River). It was called the “Great Drain” (grand ègout or ègout de ceinture).

Prior to the wide use of cesspools in Paris, cesspits (ones that percolate) were widely used. Their use in combination with the large growing population, however, resulted in the subsoil of Paris becoming putrid. Cesspools, instead, were then encouraged. However, they required periodic/routine cleaning, which the city couldn’t adequately provide. Another stinky mess arose. A “Nite Soil” program started to facilitate the collection and disposal (elsewhere) of the wastes (in community cesspools, rivers, vegetable gardens). The problem was that all of the people could not afford the service.

In the 1830s a series of cholera epidemics started in Paris. To combat the epidemics, new and bigger sewers (called “Les egouts” ) began to be constructed in the 1840s-1890s. They became the pride of Paris. The design father of the complex system of sewers under Pariswas Eugéne Belguard. The construction of this newer/larger system started in 1850, on borrowed money. By 1870, over 500 km of new sewers were either in service or under construction. By 1930, the entire system (a “combined” system) was finished:“Onesewerforeachstreet.”Fromthesetimes, “Sewerman” became a profession. Tours of the sewers were given by the “sewermen” on weekends. Some of the sludge found in the sewers was removed through manholes. Most of it was moved downstream via boats (with “wings”) to the discharge point of the sewer into the river -- where the sludge was pushed onto barges, from whence it was transported to various places of reuse or disposal.

London’s oldest “sewer,” known as the Ludgate Hill Sewer, was constructed in 1668. (Initially, it was an open channel fed by springs, big enough to be used by boats. It was covered in 1732.) Early sewers (initially, natural watercourses that had been covered) started in the London area in the 1730s -- primarily for storm water. Privies/cesspools were used to collect home wastes; some of these facilities also “collected” the methane generated by the decaying waste. The resultwasoftenexplosions/fires...anddeath.

1858-59 were the years of the “Big Stink” in London. The Thames River received wastes of thousands of people who lived upstream of Parliament. Many of the sewers tributary to the Thames River could only physically drain during low tide. The problem was that at low tide, the riverdidnothaveenoughflowtocarrythewastedownstream and out to sea. The incoming tide pushed the waste upstream. This cycle resulted in the river becoming virtually a wide-open-to-the-sunlightcesspoolfortheexcrementofnearlythree million people! Parliament had to shut down often in summer months. This situation created an even greater problem: the Thames was also the source of water for a large portion of London! During these years, various ways to minimize sewer odors were tried, including the addition to the sewers (especially in warm weather) of large quantities of lime or chloride of lime.

Sir Joseph William Bazalgette, CB (28 March 1819 – 15 March 1891) was chief engineer of London’s Metropolitan Board of Works and his major achievement was the creation in of a sewer network for central London which was instrumental in relieving the city from the Great Stink. Bazalgette’s solution was to construct 1,100 miles (1,800 km) of underground brick main sewersto interceptsewageoutflows,and1,100miles (1,800 km) of street sewers, to intercept therawsewagewhichupuntilthenflowedfreelythrough the streets and thoroughfares of London. Theoutflowswerediverteddownstreamwherethey were dumped, untreated, into the Thames. The scheme involved major pumping stations on the north and south sides of the Thames.

5


Bazalgette’s foresight may be seen in the diameter of the sewers. When planning the network he took the densest population, gave every person the most generous allowance of sewage production and came up with a diameter of pipe needed. He then said ‘Well, we’re only going to do this once and there’s always the unforeseen.’ and doubled the diameter to be used. As it is the sewers are still in use to this day.

The unintended consequence of the new sewer system was to eliminate cholera not only in places that no longer stank, but wherever water supplies ceased to be contaminated by sewage. This result was unintentional, since disease was believed to be transferred by bad odours. It was not until 1854 that Dr. Snow made the connection between human wastes (from over-loaded privies) and water supplies (wells) within the “Broad Street Neighborhood”: he found that a well at 40 Broad Street was found to be contaminated with sewagefromanearbyoverloaded/flowingprivy;the well was removed from service and the cholera outbreak ended. In the mid-1800s Louis Pasteur proved disease could be caused by germs. The link between bacteria and infectious diseases was beginning to be understood.

1.3 Purpose of urban drainage systemsThe four objectives of drain and sewer systems are (NEN-EN752:2008):• Public health and safety;• Environmental protection;• Sustainable development;• Occupational health and safety.

Drain and Sewer systems are provided in order to prevent spread of disease by contact with faecal and other waterborne waste, to protect drinking water sources from contamination by waterborne waste and to carry runoff and surface water away while minimising hazards to the public. Additionally, the impact of drain and sewer systems on the receiving waters shall meet the requirements of any national or local regulations or the relevant authority. Finally, sewer systems should be designed, constructed, operated, maintained and rehabilitated at the

best environmental, social and economical costs so that it uses materials that minimise the depletion of finite resources, can be operatedwith the minimum practicable use of energy and can be constructed, operated and, at the end of their life, decommissioned with the minimum practicable impact on the environment. To protect the health of sewer workers occupational health and safety risks likely to arise during installation, operation, maintenance, and rehabilitation should be minimised.

2. Approaches to urban drainage

2.1 Piped or natural systemsHistorical developments of urban drainage systems has been from natural towards piped systems: natural channels were used to collect rainwater and as cities developed these were structurally covered to create additional space for urban developments and to contain the bad smells arising from the waste collection channels. The urban drainage systems that were designed in the 19th century and onwards consisted mainly of underground pipes, because they had to be incorporatedintoexisting,denselybuiltcities.

The recent trend, that started in the 1970’s, has been to move towards a more natural means of drainage,usinginfiltrationandstoragepropertiesof semi-natural features such as constructed wetlands, ponds and permeable pavements. The movement towards increased use of natural drainage mechanisms has been termed differently in different countries. In the US, these techniques are usually called ‘best management practices’ (BMPs). In the UK they are called Sustainable Urban Drainage Solutions (SUDS). In Australia the term ‘water sensitive urban design’ is often used to refer to the incorporation of natural drainage mechanisms in urban areas.

This series of lectures is mainly dedicated to piped systems that constitute the majority of urban drainagesystemsinexistingurbanareasintheNetherlands and many other western European countries.

6


2.2 Combined and separate systemsUrban drainage systems collect and transport to types of flows:wastewater and stormwater.In combined systems these flows are drainedthroughoneandthesamesystem(seefigure2.1),in separate systems wastewater and stormwater are drained through separate pipes. In western Europe, most older systems are combined systems. In the Netherlands about 70% of the population is connected to combined systems, while a little over 25% is connected to separate systems. The same percentages apply to the UK, France and Germany.

Combined sewers systemsDuring dry weather, combined systems carry only wastewaterflow.Duringrainfall,theflowincreasesas a result of the inflow of stormwater. The combinedflowofwastewaterandstormwater istransported towards a wastewater treatment plant.

Thestormwaterflowexceedsthewastewaterfloweven under light rainfall conditions. During heavy rainfall, stormwater flow exceedswastewaterflowby a factor 100 to 1000ormore. It is noteconomically feasible to provide capacity for the total flowunder these conditions.Therefore, incombined systems so-called combined sewer overflowsareinstalledtodischargeexcesswaterthat cannot be transported towards the wastewater treatment plant, to surfacewater. Theoverflowwaterisamixtureofwastewaterandstormwaterand as a result the quality of the surface water, in mostcasestemporarily,isextremelyreduced.Thedyingoffoffish,foulodoursandvisualpollutionis often the result.

Separate systems do not have this drawback of mixing relatively clean stormwater withwastewater. A separate sewer system consists of two sewer pipelines, one for wastewater and one for thedrainageofstormwater (Seefigure2.2).Some cities, like the city of Amsterdam, started

Figure 2.1 – Principle combined system

Figure 2.2 – Principle separate system

7


constructing separate systems early on, yet in most cities in the Netherlands separate systems started to be constructed from the 1970’s onwards, as the drawbacks of combined systems become more and more apparent.

In theory, in separate systems stormwater does notbecomemixedwithwastewater. It happensfrom time to time, that domestic sewer connections accidentally become connected to stormwater pipes. This puts a constant strain on the quality of the receiving surface water. Also the opposite can happen; the stormwater pipes become connected to the wastewater system. This can result in overloading of the wastewater system. In both cases one speaks of faulty or illegal connections.

Initially, the assumption was that stormwater that was drained through stormwater systems was uncontaminated, since during drainage it did notgetmixedorcomeincontactwithdomesticwastewater or other wastewater. However, research has shown that this assumption does not hold true. During runoff over urban surfaces rainwater takes up all sorts of contaminated substances. These substances settle on the surface as a result of traffic, human activitiesand direct deposition from the atmosphere. Stormwater systems drain towards surface water more than 50 times per year on average, during each precipitation of importance, which implies that polluted stormwater is discharged to surface water frequently. On the other combined systemsoverflowlessthan10timesannually,onaverage. The result is that the annual pollution load to surface water from separate systems and combined systems can be equally large.

Separate systems have the additional disadvantage that construction and maintenance costs of separate sewer systems are in most cases higher than in combined systems. Only in in cases where a large amount of surface water is present to locate stormwater oulets, so transport distances to surface water are short, separate systems can be cheaper than a combined system.

2.3 Improved combined and improved separate sewer systems As the drawbacks of combined and separate systems became apparent, attempts have been made to overcome the disadvantages to both combined systems and separate systems.

Improved combined systems include so-c a l l ed s to rage - and - se t t l ement bas ins (bergbezinkbassins, in Dutch) that are installed at seweroverflowstoreducetheamountofwaterthatisspilledthroughthecombinedseweroverflows(Seefigure2.3).Duringheavyprecipitation theoverflowingwater is retained in the basin. Theamount of overflowing water decreases as a result of this and with that the contamination of the surface water is decreased as well. Once the basin is full the on-coming water is discharged to the surface water. Before the water reaches the (external)combinedseweroverflow,itispartiallystripped of pollutants, thanks to sedimentation in the basin.

A storage and settlement basin, therefore, reduces the amount of combined sewer overflowwaterand in addition the overflowing water is less contaminated. However, one condition to this is of course that the design of the basin must be such that it allows sedimentation to occur.

Combined sewer systems can also be improved throughinstallingvortexoverflowsorbyadjustingthe combined sewer overflow itself (‘improvedoverflowdrain’). The ideabehind these special

WWTP receivingwater

storage and settlement tank

supply pipe

1 = internal combined sewer overflow2 = extreme combined sewer overflow

1 2

Figure 2.3 – Improved combined system

8


structures is that they remove pollutants from the overflowwaterbycontainingsettleablematerialthat many of the pollutants adhere to.

Improved separate systems are devised to overcome the effect of faulty connections in separate sewer systems. This done by connecting the stormwater system with a wastewater sewer in asuitableplace(Seefigure2.4).Thewastewaterflowthatundesirablygoesthroughthestormwatersewer is in this way led to the wastewater system and eventually to the wastewater treatment plant. A weir is installed at the outlet of the stormwater system to contain stormwater inside the system during small precipitation and transport it towards the wastewater system. The installation of a weir makes sure that a part of the contaminated rainwater (because of faulty connections) also undergoes treatment in the wastewater treatment plant.

This solution does not entirely remove the harmful effects caused by connecting rainwater pipes to the wastewater system. However, due to the installationofextrapumpingcapacity forthe transport of stormwater from the stormwater system, this capacity is also available to transport stormwater that was directly (and erroneously) connected to the wastewater system. This is indeed taken into account in the design of the wastewater treatment plant that should allow the treatment of an limited amount of rainwater from the improved separate system. The amount of stormwater that ends up in the wastewater treatment plant this way depends on the distribution of rainfall over small

and large rainfall events: stormwater from most of the small events (up to about 5mm) are transported towards the wastewater treatment plant and only a part of the stormwater from larger events (> ±5mm) is discharged to surface water.

One other possibility to reduce the adverse effects of sewer overflows is the enlargement of the capacity of sewer pumping stations that pump sewer water towards the wastewater treatment plant. The disadvantage of this is, that a larger amount of relatively clean water must be treated in the wastewater treatment plant and that the hydraulic capacity of the wastewater treatment plantmust be enlarged to copewith the extrawater(unlesstheexistingcapacityissufficient).Theefficiencyofwastewatertreatmentplantsislowerforlessconcentratedinfluentwaters,sobytransporting more stormwater to the treatment plant,itsefficiencyisreducedandfewerpollutantsare removed during treatment. In the end, it comes downtofindingabalancebetweenthepreventionofcombinedseweroverflows,reductionofpollutedstormwaterflowsandmaintenanceoftreatmentefficiency at the wastewater treatment plant. This balance is studied in optimization studies for wastewater systems (in Dutch: Optimalisatie AfvalwaterSysteem, OAS).

Furthermore, the amount of overflow water can be reduced by minimising the amount of runoff water that is connected to the urban drainage system. The reduction of the amount of connected runoff-surface is called disconnection of impermeable surfaces (in Dutch: afkoppelen). Of course disconnection can only take place when the stormwater can be transported to alternative facilities such as infiltration systems or open drainage channels and ponds.

2.4 Pressurised and vacuum systemsIn conventional sewerage methods wastewater is collected throughagravity-flowsewer, usinggravity to transport wastewater. If the sewage cannot be transported under gravity, because ground level variations are small and transport distance are long, pressurized or vacuum

WWTP

rain water pump

receivingwater

combinedsewer overflow

storm watersewer

wastewatersewer

Figure 2.4 – Improved separate system

9


systems can be applied. These systems are general installed in areaswhere a gravity flowsewersystemistooexpensive,suchasdetachedbuildings that are spread far apart.

In general pressurized and vacuum system only transport wastewater and no stormwater, because the latter would necessitate a larger pipe diameter and pumping capacity and therefore reduce the financial advantages of a pressurized systemscompared to gravity systems.

Pressurised sewer systems consist of a few (small) pumping stations and pressurised mains. Wastewater from a few detached houses is led by agravity-flowlinetoapumpchamber.Fromtherethe water is pumped into the pressurized main tothenextpumpchamber.Oneormorehousescan be connected to these pump chambers. This method allows the wastewater from a few detached houses to be collected. Finally the wastewater is led to the gravity sewer systems that eventually transports it to the wastewater treatmentplant(Seefigure2.5).

Many pressurized sewers in the Netherlands were constructed with state subsidy. The intention behind distributing the subsidy was to get as many buildings as possible connected to a sewer system. Currently 3.6% of the population in the Netherlands is connected through pressurized sewers.

There are disadvantages to pressur ised sewer systems. The most obvious ones are, in

comparisontothegravity-flowsewersystem,highmaintenance and replacement costs. Foul odours occur as wastewater is transported over large distances as a result of anaerobic degradation of the transported wastewater. This results in the production of sulphuric gases (H2S) that lead to severe corrosion of concrete sewers where pressurised sewers are connected to gravity sewers.

In vacuum sewers residential wastewater is led through vacuum pipelines to a pump chamber. From this pump chamber the following transport of the wastewater takes place under pressure. (Seefigure2.6)

Vacuum sewer systems have been put to little use in the Netherlands. Although the operational safety is comparable to pressurised sewer systems, its higher installation costs prevent its widespread usage. One disadvantage of vacuum sewer systems in relation to pressurised sewer

Figure 2.5 – Example of a pressurized sewer system

Figure 2.6 – Principle diagram vacuum sewerage system for a house boat

10


systems is also that vacuum sewer systems can barely overcome slight vertical obstacles. This is because the negative pressure (vacuum) in the system itself is limited.

2.5 Components of sewer systemsA sewer system consists of a network of sewer pipelines that are connected through manholes. The sewer pipelines are made up of pipes, which can be circular shaped, rectangular or oval. In old sewers large rectangular pipes are sometimes covered with a barrel vaulting.

The pipes (Dutch: rioolleidingen) are made of concrete, cast iron, PVC, HDPE, PE, glazed stoneware (in Dutch: gres) or brickwork. Most sewer pipes in the Netherlands (72% of the total sewer length (RIONED, 2010) are made of concrete.

Manholes (Dutch: rioolputten) are constructed where more than two sewer connect and at regular distances to allow inspection of the sewer system. Manholes need to be accessible so that it is possible enter materials for inspection and maintenance of sewer systems. This means that themaximumdistancebetweenmanholes isofthe order of 40 to 60 m. Manholes in systems of concrete sewers are usually made of concrete, sometimes of brickwork. Manholes in systems of PVC sewers are made of PVC. The manholes are covered with a manhole cover. These are most often made of cast iron, as is the frame in which the cover is encased.

Sealed covers (Dutch: geknevelde putdeksels) canbeappliedatlocationswhereexcesspressuresometimes causes water to push off the manhole coveraswaterflowsoutofthemanhole,creatingadangeroustrafficsituation.

Internal weirs (Dutch: interne stuwen) are installed in combined sewer systems with the aim to optimise in-sewer storage, especially when different part of the sewer systems have different bottom levels. As a result of this more water is stored inside the sewer system before combined

seweroverflowstakeplace,thuscombinedseweroverflowsoccurlessfrequentlyandsurfacewateris less polluted.

Combined sewer overflows (Dutch: gemengde overstorten) are implemented in combined sewer systems to relieve the pressure on the system during precipitation.

Outlets (Dutch: regenwateruit laten) are implemented in separate stormwater systems to allowoutflowof stormwater towards surfacewater. Outlets in separate stormwater systems have no weirs; there is a constant open connection between surface water and the stormwater system.

Emergency outlets (Dutch: nooduitlaten) are installed in the pumping stations’ collection chambers. Emergency outlets are supposed to workexclusively incase thepumpingstation isbroken down for a long period.

Combined sewer over f lows, out fal ls and emergency outfalls are usually made of concrete, and occasionally of brickwork.

Gully pots or sewer inlets (Dutch: kolken) collect water from roads and transport it to the sewer system.

House connections (Dutch: huisaansluitingen) collect water from households and transport it to the sewer system. Combined house connections collect wastewater and stormwater and transport it to a combined sewer system; separate house connections collect wastewater or stormwater and transport it respectively to a separate wastewater or stormwater system. House connections are predominantly made of PVC. Old connections are usually made of glazened stoneware.

Pumping stations (Dutch: pompstations) are indispensable inflatareas like thewesternandnorthern parts of the Netherlands. Sewer pumping stations have capacities anywhere between a few m3/h to a few thousand m3/h, depending on the size of the system that drains to the pumping

11


station. The design, building and installation of bigger sewage pumping stations, is a matter for specialists.

Shutting down pumps creates shockwaves in the pressurised mains. Pumping stations with a large capacity that are connected to pressurised mains that are a few kilometres long may need to buffer the shockwaves by mounting so-called water hammer provisions. These are usually surge towers or pressure vessels. Even the dimensioning of water shock provisions is a matter for specialists.

Pressurised mains (Dutch: drukleidingen) are components of pressurised sewer systems. Small small pressurised systems (Dutch: drukriolering) connect detached houses at long distances from a gravity system to the gravity system. Large pressurised systems (Dutch: persleidingen) consist of pressurised pipelines of large capacity that transport water from a gravity system to a wastewater treatment plant or between subcatchments of a gravity system. The pressurised mains are predominantly made of HDPE or PE.

Storage and Settling Basins or Tanks (Dutch: bergbezinkbassins) are placed behind combined seweroverflowstoprovideextrastoragecapacity,

contain pollutants and limit the frequency and pollution content of combined sewer overflows.With careful design the tanks are able to contribute nicely towards improving the surface quality. Storage and Settling Basins are usually designed as closed basins, especially if they are installed close to buildings. The basins are usually made of concrete.

2.6 C o n c e p t s , d e f i n i t i o n s a n d abbreviations In this paragraph a list is given with the most important concepts and abbreviations in the seweragefield.Thetermsinboldaredefinedjustas in the NEN 3300: ‘Buitenriolering; termen en definities’(Drainsandsewersoutsidebuildings;termsanddefinitions’).Inaddition,alistofthesewith the technical terms found in English, French and German literature is provided.

Abbreviations WWTP Wastewater Treatment PlantBOD BiochemicalOxygenDemandCOD ChemicalOxygenDemandDWF Dry Weather Flow TKN Total Kjeldahl NitrogenOF OverflowFrequencyPOC Pump Over-Capacity WWF Wet Weather Flow

Nederlands Engels Duits Frans

afvalwater wastewater Schmutzwasser eauxusées

afvloeiingscoëfficiënt runoffcoefficient Abflußbeiwert coefficientderuisselement

afvoerend oppervlak catchment area Einzugsgebiet bassin versant

droogweerafvoer dryweatherflow Trockenwetterabfluß débit de te,ps sec

gemengd rioolstelsel combined system Mischsystem réseau unitaire

gescheiden rioolstelsel separate system Trennsystem réseau (de type) séparatif

grondwater groundwater Grundwasser eauxsouterrain

huisaansluiting house sewer system Grundstück entwässerung drainage domestique

huishoudelijk afvalwater domestic sewage Häusliches Schmutzwasser eauxuséesdomestiques

industrieel afvalwater tradeeffluent Betriebliches Schmutzwasser effluentindustriel

lekwater infiltration/exfiltration Infiltration/Exfiltration infiltration

afstromend hemelwater stormwater Regenwasser eauxderuissement

regenwaterafvoer rain discharge Regenabfluß écoulement de pluie

riolering sewer system Kanalisation réseau d’assainissement

riool sewer Abwasserkanal émissionaire

rioolwater sewage Abwasser effluent

12


3. Inflows into urban drainage systems

Sewer water consists of wastewater or rainwater oramixofboth.Wastewaterisbrokendownintodomestic wastewater and industrial wastewater, infiltration and drainagewater and extraneouswater.Thewastewaterflowisalsoknownasdryweatherflow;wetweatherflowreferstotheflowunder rainfall conditions.

3.1 Amount of wastewater 3.1.1 Domestic wastewaterThe amount of daily domestic wastewater depends on the drinking-water consumption per person, loss and the number of individuals consuming drinking water. When designing wastewater systems future developments must be taken into account,suchasanexpectedpopulationgrowthor changes in the drinking-water consumption per person. Additionally the amount of wastewater fluctuationsthroughoutthedaymustalsobetakeninto account.

Drinking water consumption per person Average drinking-water consumption depends on both prosperity and climate conditions. In manypoorareas,toiletflushingdoesnotoccur.In prosperous areas in the tropics, subtropics and arid areas, a lot of drinking water is consumed to

watergardenstofillpoolsandmorewaterisusedto bathe compared to colder climates.

In the Netherlands the average drinking water consumption is 125 to 135 liters per person per day. The drinking water consumption per person can be subdivided into various domestic activities, such as washing clothes, cooking bathing, toilet flushingandwashingdishes.Lessthancirca40%of the supplied drinking water is used for personal hygiene, drinking and cooking. The other 60% of the drinking water is used for purposes that do not require drinking water quality. In recent years drinking water companies have investigated possibilities to produce second, lower quality water for such purposes. Application of second quality water in households requires a second distribution network for this water, which in most casesdoesnotmakesuchsolutionscost-efficient.For industrial purposes, requiring large volumes of second quality water, the production of separate lowequalitywatercanbebeneficial.

LossesA part of the supplied drinking water is lost during the consumption process and does not reach the sewer system. This refers for instance to water used for watering gardens and water that evaporates, for example from laundry. In theNetherlands these losses amount to nearly 10% of the drinking water consumption, so that the amount of wastewater is around 115 to 120 liters per person.Where the drinking-water supply is less than 20 liters per inhabitant per day (only one faucet in the house) nothing gets drained to the sewer. With a drinking-water supply of circa 80 liters per inhabitant per day virtually all the supplied drinking water is drained through the sewer. Above that amount, increasing degrees of water usage does notreachthesewer.Seefigure3.1.

Leakage losses that occur in the distribution networkalsoplaya largepart inexplaining thedifference between drinking water consumption and wastewater production. These losses can make up 10 to 50% of drinking-water production in some countries. In the Netherlands leakage

Figure 3.1 – Relationship between drinking water supply and wastewater drainage

13


losses amount to less than 5% of production. In table 3.1 drinking-water supply (read: drinking water production) and the amount of wastewater for a few places is shown.

In many areas the amount of wastewater is substantially smaller than the amount of supplied drinking water. In Amsterdam and the Grand Rapidswastewater flow is however larger thandrinking water supply. In Amsterdam this can be attributedtotheinfiltrationofgroundwaterintothesewer network. Presumably something similar is going on in Grand Rapids.

The presented figures demonstrate thatwhendimensioning a wastewater system based on the amount of drinking-water production it can lead to either over or under designing of wastewater system capacity.

Distribution over the dayWastewater is not produced evenly throughout the day;atnightthereisbarelyanywastewaterflow.The largest amount of wastewater is produced during about 10 hours of the day. Wastewater production in most systems shows a peak in the morningandevening.Seefigure3.2.

In the Netherlands it is common practice to take into account a domesticwastewater flowequal to 12 l/(inhabitant.h) in the design of sewer systems. Hereby is assumed that the total amount of domestic wastewater of 120 l/(inh.day) is discharged in 10 hours. The peak factor employedintheNetherlandsforwastewaterflowthen becomes: • peak drainage is: 120/10 = 12 l/(inh.h)• average drainage is: 120/24 = 5 l/(inh.h) • peak factor is: 12/5 = 2,4

In France the following peak factor is applied:

P = peak factor (1,5 < P < 3) -qm = drinking water supply l/s

In the United States the following various formulas are used: For Desmoines, for example, thefollowing formula applies:

where I is the population times thousand.In table 3.2 the peak factor is given for varying populations.

The previous information shows that the magnitude of the applied peak factors from country to country differs.Thisisexplainedbytheempiricalcharacterof these peak factors.

city supply

(lpppd)

drainage

(lpppd)

Las Vegas (US) 1.560 760

Little Rock (US) 190 190

Wyoming (US) 570 300

Boston (US) 550 530

Caïro (Egypt) 800 150

Amsterdam

(part of the city)

130 209

Grand Rapids (US) 670 720

Table 3.1 – Drinking water supply and wastewater drainage

Figure 3.2 – Daily dry weather flow fluctuations

Number of

inhabitants

France USA The

Netherlands

100 3,0 4,2 2,4

1.000 3,0 3,8 2,4

100.000 1,7 2,0 2,4

Table 3.2 – Peak factors. Peak factors calculated for France are limited between 1.5 and 3.

Pqm

= +1 5 2 5. .(3.1)

P ll

= ++

184

(3.2)

14


When it comes to appropriate design of wastewater systems, the prevention of rainwater connections to a separate wastewater system is of much greater importance than the value of the applied peak factor. This is illustrated by the following:

The peak wastewater production per inhabitant is 12 l/h in the Netherlands. The amount of impervious surface connected to a sewer system is about 60 m2perperson.Thisamountstoawastewaterflowof 0.2 mm/hour (12l/60m2). Wastewater sewer systemsare designed for a filling rate of 50%.Thefillingrateisequaltothewaterdepthinthesewer divided by the diameter. The full capacity ofthesewersisforafillingrateof100%,doublethedesigninflow.Thismarginiskepttoallowforfuture changes, expected or unexpected, suchas the connection of new urban developments to the system and especially faulty connections of stormwater to the wastewater system. When the design capacity of a wastewater system equals 0.2 mm/h, faulty connections of stormwater from 1/100thofthetotalimpervioussurfacefillupthewastewater system to full capacity for a rainfall intensity of 20 mm/h. Larger rainfall intensities and larger amounts of impervious surfaces connected through faulty connections lead to overloading of the wastewater system. This may result in wastewater flowing back up into the houses through the house connections.

3.1.2 Industrial wastewaterThe amount of industrial drinking-water consumed varies per type of company. When a sewer system needs to be designed for a new neighbourhood an estimation of the industrial wastewater production is made based on average drinking waterconsumptionfiguresfortheexpectedtypeof industries. If it is not known which industry will establish itself an assumption of a load of 2 l/(s.ha of industrial area) is often made. This load is derived from the gross impervious surface, since the net impervious surface is not yet known at the moment the design is made.

Iftheplanisforanexistingneighbourhoodthenthe amount of wastewater can be calculated based on data from the drinking water company and an

estimation of losses during the industrial process. It must be realized however that some industries acting in their own interest supply their own water by abstracting ground water. In that case a permit for groundwater abstraction must be obtained from the local authorities (the province). The water consumption from such industry can be derived fromthetextonthepermit.

Sometimes certain industries (the soft drink industries, breweries) do not drain their water into the sewerage system. This must be taken into account in setting up the design for the sewer system.

3.1.3 Infiltration and exfiltrationSewer systems are constructed to be watertight. However, in older systems, particularly in areas with poor soil conditions, this is often not the case. Joints between sewer pipes become leaky as a result of degradation of the rubber sealing rings or because pipes move due to ground settlement.

Therefore,insewersystemdesignaleakflowrateof 0.2 m3/(km/h) sewer per hour is often taken into account for sewers below the groundwater table (in the United States up to 3 m3/(km/h)).

When sewers are constructed above the groundwater table, leakage of wastewater out of the system can give rise to groundwater contamination.

3.1.4 Drainage waterDrains are often laid in areas with high groundwater tables to keep the groundwater table at a minimum depth below roads and building foundations. When the drainage water cannot be discharges to surface water because there no water courses nearby, the drainage water is often discharged by connecting the drains to the sewer system. Water quality managers are generally not in favour of transporting comparatively clean drainage water to sewers and to a wastewater treatment plant. Therefore drainage water, if connected to sewers if preferably connected to separate stormwater sewers.

15


Iftheexpectedamountofdrainagewaterisknown,this can be taken into account in the design of the sewer system. If not, the drainage water is assumed to be incorporated in the leakage water.

3.1.5 Extraneous waterBrooks or covered waterways can also make up part of sewer systems, particularly in the eastern and southern parts of the Netherlands. The termextraneouswater applies to the flow thatis transported from a brook or waterway into the sewer system. In the dimensioning of a sewer system or in control calculations for existingsystems this flowmust be properly taken intoaccount.

3.2 Amount of stormwater One of the design requirements for urban drainage systems is that houses and buildings may not become flooded during precipitation of any intensityandvolume.Damagesof floodeventscaused by heavy rainfall are reported in the press at regular intervals. To prevent such damages it is of great importance to have good insight into the possible expectations of the amountsof precipitation and the runoff behavior over the lifetime of a system.

3.2.1 Amount of precipitation and runoff behaviour In the Hydrology course fundamental concepts of rainfalleventswereexplained,whicharebrieflyrepeated here: Every rainfall event or storm event can be described by its temporal and spatial characteristics:• Rainfallvolume,mostlyexpressedintermsof

rainfall depth (d) on the surface [mm]• Duration (t) of the rainfall event [hours]• Rainfall intensity (P or I): amount of rainfall per

unit of time [mm/h]• Frequency: frequency of occurrence, usually

expressed in terms of a frequency scale T(i.e. the parameter under consideration occurs once every T years)

• Area: geographical scale of rainfall [m2 or km2]

Rainfall depth, duration and intensity in one station (point measurement) have the following relationship:

And the average rainfall intensity is:

A mass curve presents the accumulation of rainfall with time. A hyetograph presents the variation of rainfall intensity with time.

Total rainfall depth (d), rainfall duration (t) and frequencyscale(T)arealsocalledtheexternalstatistics of a storm event and can be described by probabilistic distributions. The internal statistics of a storm event refer to the distribution of rainfall intensity with time, during the storm event. This distribution is often depicted in the form of a histogram. The distribution of rainfall intensity with time is important for urban drainage, because run-off processes in urban areas are fast and peak rainfall intensities within a storm event determine whether system capacity gets overloaded and floodingoccurs.

3.2.2 Rainfall intensity duration frequency and rainfall mass curvesUse of intensity duration frequency curves (IDF-curves) and mass curves is widespread in the design and analysis of urban drainage systems, sewer systems as well as open channels systems, retention basins and storage and settlement tanks. IDF-curves and mass curves provide information on the occurrence of rainfall intensities over a given period of time for certain return periods.

Mass curves and IDF-curves, based on available precipitation information, can be compiled in two ways:• partial series• extremevaluesseries

With partial series all the peaks that are above a certain threshold are taken into account in

d Pdtt

= ∫0

(3.3)[L]

P dt

= (3.4)[LT-1]

P dt

=

16


statisticalanalysis.Inextremevalueseriesonlythemaximumoccurrencesthattakeplacewithinacertainperiod(ayearforexample)aretakenintoaccount. In the latter case high precipitation that isneverthelesslowerthantheannualmaximumprecipitation is not included in statistical analysis. This precipitation can however be higher than the maximumthatoccursinoneoftheotheryearsinconsideration. As a result not all the information is processed and some information gets lost.

An example of amass curve that has beenfrequently used in urban drainage design in the Netherlands are Braak’s curves, dating from 1933. In recent year, these curves have been updated by

KNMI and Meteoconsult (Wijngaard et al., 2004; Malda et al., 2006).

Braak’s curves are compiled from 37 years of analysed rainfall values. Recent curves by KNMI are based on hourly rainfall data for one location, the Bilt, for a period of 97 years (1906-2003). Meteoconsult has extended this analysis byincluding rainfall data from 23 rainfall stations in the Netherlands, including the Bilt, for 10 years of rainfall data per station. The analysis for the Bilt hasbeenextendedbyBuishandandWijngaarden(2007) to include short duration rainfall down to a 5 minute time-step.

It is clear that the differences in the length of the examinedperiodsandthedatatimestepwillresultin variations between rainfall duration curves. The reliability of the various rainfall duration curves dependsontheperiodexaminedandtheappliedmethod.

Figure 3.3 is extracted from the publication byBuishand and Wijngaarden, KNMI technical report TR-295.

The return period and rainfall duration that is to be used in analysis and design depends on the type of system. Urban drainage systems typically require rainfall data of the order of several minutes up to hours. In rural areas like polders, run-off processes are slower and rainfall over a period of several days is critical to determine the required water system capacities. The return period that is used in design depends mainly on economic factorssuchasexpecteddamagetobuildingsoragricultural products.

Influence of area sizeIn the previous text rainfall was described asa point measurement. In reality, storms have spatial dimensions and rainfall intensities usually decrease with distance from the center of the storm. This implies that average rainfall over a certain area decreases with the size of the area (seefigure3.4).

Figure 3.3 - Return periods for exceedance frequencies of once per 1, 10, 100, 250 and 1000 years voor rainfall duration between 5 and 120 minutes (Buishand and Wijngaarden, 2007).

Figure 3.4 - Influence of area size on average rainfall depth over area

17


Urban drainage systems usually drain small areas compared to surface water systems in rural areas. This means that high intensity rainfall over short periods of time is of importance for the design of system capacity. It also means that system transport capacity must be large compared to that of rural water systems. This would lead to very large pipe dimensions if the same return periods were applied to rural and urban systems. In the Netherlands, relatively low return periods are applied for design calculations, mostly T=2 years. Urban drainage system are designed to get overloaded at a relatively high frequency, in which case streets take over a part of the transport and storage capacity of underground systems. This is not a problem as long as buildings are locatedsufficientlyhighabovestreetlevel.Whileinsomelocationsfloodingoccursmorefrequentlyas a result of increase in impervious areas that were connected to urban drainage systems over the past decades, the use of street storage to compensate for a lack of transport capacity in sewer systems is a debated topic. In the end, economic considerations are likely to end the discussion in favour of street storage instead ofhavingto increasepipecapacities inexistingsystems.

3.2.3 Using rainfall mass curves The recorded values in the rainfall duration curves refer to the amount of precipitation that falls within acertainperiodoftime,ortheexternalstormeventstatistics. The internal storm event statistics for a

rainfall depth of for instance 20 mm over a duration of 60 minutes can be composed in many different ways,forexample:1. 5mmofsteadyrainfallinthefirst50minutes,

followedbytheremaining15mminthenext10 minutes;

2. 15mmsteadyrainfallinthefirst10minutes,followedbytheremainingamountinthenext50 minutes;

3. steady precipitation over the whole period of 60 minutes.

Suppose that a storage settlement tank must be dimensioned for precipitation that falls in 60 minutes and for a return period of 5 years. From figure3.3itcanbereadthatthedesignerforT=5must take a rainfall volume of about 20 mm into account. The capacity of the pumping station that empties the tank is 0.2 mm per minute (12 mm/h). The required tank volume is determined by the greatest difference between the supply (rainfall) and drainage (pumping) that can occur at any moment.Figure3.5showsthat for thefirstandsecond storm characteristics the required storage capacity must be 13 mm, while a straightforward use of the rainfall mass curves yields a required storagecapacityof8mm.Thisexampleclearlyshows that using rainfall mass curves can cause under-designing in sewerage systems or its components.

The vast majority of rain showers do not fall linearly. This means that the amount of combined

Figure 3.5 – Relationship of distribution precipitation – necessary storage

18


stormwateroverflowsandstormwateroverflowfrequencies will be greater than what is derived by the calculation based on rainfall duration curves. It is also important to consider that a previous storm canleadtofillingof(apartof)availablesystemstorage, thus influencing starting conditions forthe following storm.

This means that under-designing of retention basins can occur. Other important factors can conversely lead to over-designing. The most important are:• Not taking rainfall losses into account

(evaporation, surface wetting, retention on the surface, interception by vegetation etc.);

• Overestimationof the runoff coefficient (seefollowing paraghraph; runoff coefficients in the Dutch Guidelines for Sewer Design (Leidraad Riolering) are on the high side, so the calculated runoff load to the stormwater system is higher than what occurs in practice.);

• Not taking storage in drains and house connections into account;

• Not taking runoff delay into account (in general thishaslittleinfluenceonthedesignofsewersystems, since runoff processes are fast).

3.2.4 Run-off processesPart of the precipitation does not run off to the sewer system, thusdoesnot add to inflow intothe system. Careful estimation of the amount of precipitation that actually does run off to the sewer system is important to prevent under-design or over-design of the sewer system. Runoff behaviour depends on the following processes:• interception• evapotranspiration • infiltration• retention through ponding

Interception is the process in which part of the precipitation on the surface is absorbed and does not run off. Evapotranspiration is constituted of rainwater that directly evaporates from the ground, plants and building as well as water that evaporates indirectly through plants. The term infiltrationreferstowaterthatsinksintothesoilthrough pervious surfaces. Retention through ponding occurs on uneven surfaces, where water doesnotflowtowardsasewerinlet.

Runoff behaviour is taken into account with the helpofarunoffcoefficientinthedesignofsewersystems. In table 3.3 a few typical values are shown.

In general in the Netherlands values used in sewerdesignasafirstapproximationare:runoffcoefficientC=1 for imperviousareas(roofsandroads) and C=0 for pervious areas (parks and gardens).

The runoff coefficient showswhich part of theprecipitation runs off the surface and comes into the sewer system. The record shows that the runoffcoefficientamountsto48%ofthetotalbuiltup area. This is 2% less than the percentage of impervious surfaces that is present in the built up area. This is nearly the case in each built up area. Thisiswhywhenexaminingtheworkingofsewersystemsthedrainageisassumedexclusivelyfor

Surface of the earth runoff coefficient

slate roofs 0,95

tiled roofs 0,90

flatroofs 0,50 - 0,70

asphalt roads 0,85 - 0,90

tile paths 0,75 - 0,85

cobblestone pavement 0,25 - 0,60

gravel roads 0,15 - 0,30

bare surfaces 0,10 - 0,20

parks, strips of land 0,05 - 0,10

Table 3.3 – Runoff coefficient

Prop

ortio

n

Run

off

coef

ficie

nt

Rel

ativ

e pr

opor

tion

Asphalt roads 22,5% 0,90 0,20

Cobblestone roads 7,5% 0,50 0,08

Pitched roofs 10,0% 0,90 0,09

Flat roofs 10,0% 0,70 0,07

Unpaved/pervious runoff 25,0% 0,15 0,04

Impervious runoff 25,0% 0,00 0,00

Total 100 0,48

Table 3.4 – Estimation compounded runoff coefficient

19


impervioussurfaces.Therunoffcoefficient,whichin that case is applied, naturally has the value 1!

Traditionally, based on years of experience,Dutch sewer systems were designed to be able to process a constant rainfall intensity of 60 l/s/ha. For hilly areas sometimes 90 l/s/ha was chosen inordertopromotebetterfloodprevention.Thehectares included relate to the total impervious surface. Most older sewer systems, up to the 1970’s were designed this way. In the 1980’s computers started to be used for sewer design and more advanced calculations could be made, taking rainfall intensity variations into account.

More details on rainfall characteristics and sewer design are explained in the Fundamentals ofUrban Drainage course (CT4491).

Question:

1. Given a residential urban area of 1 ha, 100 houses and 250 inhabitants.

a. Calculatetheaverageannualstormwaterflowand theaverageannualwastewater flow forthis area.

b. Calculate thepeak stormwater flowand thepeakwastewaterflowforthisarea

Answer:

a. Annualstormwaterflow:assumeannualrainfall:800 mm; annual stormwater f low: annual rainfall*area:0.8x104 = 8000 m3

Annualwastewaterflow:assumedailywastewaterproduction:120l/p/day;annualwastewaterflow:dailywwprod*nrofinhab*days/year:250x120x365= 10,950 m3

b. • Assumepeak stormwater flow: 60 l/s/ha (or

90or120l/s/ha);peakstormwaterflow:peakflow*area:60x104 /1000 = 600 l/s

• Assumepeakwastewater flow: 1.5x120 l/p/day; peakwastewater flow: peak flow*nr ofinhab*conversion days to seconds: 1.5*120*250 / 24/3600 = 0.5 l/s

20


4. Environmental requirements for sewer design

4.1 Development of environmental guidelines for sewer systemsIn the beginning of the 20th century, surface waters in many urban areas became heavily polluted as a result of increased, untreated overflows from sewer systems and industries.Wastewater treatment plants began to be built near large cities to collect and treat polluted water and control the pollution of urban surface waters.

In the Netherlands, in 1970, the Surface Water Pollution Act (Wet Verontreiniging Opper-vlaktewateren, WVO) was adopted, aiming at the reductionandpreventionofpollutedoverflowstosurface water. It enforced treatment of polluted overflowstosurfacewatersbasedonasystemof pollution permits. This Act has been replaced by the Dutch Water Law (Waterwet) in December 2009.

In the Netherlands, in 1970, the Surface Water Pollution Act (Wet Verontreiniging Opper-vlaktewateren, WVO) was adopted, aiming at the reductionandpreventionofpollutedoverflowstosurface water. It enforced treatment of polluted overflowstosurfacewatersbasedonasystemof pollution permits. This Act has been replaced by the Dutch Water Law (Waterwet) in December 2009.

In 1986 a fire at theSantoz chemical plant inSwitzerland had disastrous consequences for the Rhine.Thousandsofgallonsof toxicchemicalswerewashed into the river andmillions of fishand other wildlife were killed. There was a public outcry and politicians from all the Rhine countries agreed that action had to be taken. The result was the Rhine Action Programme of 1987.

The Rhine Action Programme stated that by 1995:• Discharge of themost important noxious

substances should be cut by 50% compared with 1985.

• Safety norms in industrial plants should be tightened.

• Weirsmust be fittedwith fish passages toallowthefishtotravelupstreamandspawninggrounds must be restored in the upper tributaries.

• The riverside environment should be restored to allow the return of plants and animals typical of the Rhine.

Thefirstpointhasbeenanimportanttriggerforreduction of (combined) overflows from urbandrainage systems.Still thereareapproximately13,000 over f low structures of (improved) combined sewer systems in the Netherlands. During heavy rainfall, sewage water diluted with stormwater is discharged, together with disturbed sewage sludge, from the combined

Figure 4.1 - Polluted spills to surface waters

21


sewer systems via the overflow structures intothe surface water. In separate systems, collected stormwater is discharged to surface water together with pollutions washed down from impervious surfaces.Manyoverflowstructuresaresituatedon small(semi-)stagnant watercourses. As a result,theinfluenceofdischargesfromoverflowstructures on surface water quality, sediments and the aquatic ecosystem is considerable.

Initially, requirements for granting discharge permits based on the Surface Water Pollution Act, mainly concerned the average frequency at which overflows from combined sewer systems canoccur. No permit was necessary for stormwater outlets of separate systems. Meanwhile research has shown that the over f low frequency is representative to a limited extent only for thepollution load and the effects on surface water. From 1983 to 1990 the National Sewerage and Water Quality Working Group (Nationale Werkgroep Riolering en Waterkwaliteit, NWRW) conducted thorough research on combined sewer overflowsfromsewersystemsanditseffectsonsurface water quality.

Intermezzo: Conclusions of the National Sewerage and Water Quality Working Group (Nationale Werkgroep Riolering en Waterkwaliteit, NWRW)The most important conclusions are:• The annual pollution loads from combined and

from separate sewer systems is of the same order of magnitude;

• Improved separate system produce the lowest pollution loads, compared to (improved) combined systems and (non-improved) separate systems;

• Combined sewer overflows and stormwateroutlets should preferably be located at large, non-stagnant waters. Discharge to small brooks, canals and isolated ponds should be avoided.

• Only stormwater that runs off from roads with lowtrafficintensitiescanbedirectlydrainedtosurface water;

• Themostefficientmethodtoreduceoverflowloads from combined sewer systems is to

add a storage and settling tank between the combinedseweroverflowandsurfacewater(this implies installing an improved combined system).

4.2 “Basic effort/Basisinspanning”Based on the results of NWRW research, the CUWVO(CommitteeforexecutionoftheSurfaceWater Pollution Act) recommended a basic effort (in Dutch: Basisinspanning) to reduce the adverse effects on surface water quality caused byoverflowsfromsewersystemsintheirreport(CUWVO, 1992). This basic effort in principle applies to all sewer systems. The recommended basic effort is based on the principle of best practicablemeansandhasbeendefinedsoasto act as a reference effort corresponding with acertainexpectedtypeandamountofpollutionemission.

Depending on local circumstances, a solution may be selected based on lowest costs , provided that theexpectedpollutionloadcorrespondswiththatof thedefinedbasiceffort.Thebasicefforthasbeendefinedintheformofreferencesystemsforthree different circumstances:• for sewer systems to be newly built: the

preferred system is an improved separate sewer system;

• forexistingcombinedsewersystems:systemsshould have an in-sewer storage capacity equivalent to 7 mm of rainfall, a stormwater pumping capacity (Dutch: pompovercapaciteit) of 0.7 mm/h and additional storage capacity equivalent to 2 mm in storage and settling tanks;

• forexistingseparatesewersystems:systemsshould be converted to improved separate sewer systems. Treatment of stormwater at stormwater outlets should only be implemented in cases of highly polluted impervious surfaces connected to the storm water system and large expected adverse effects on the receivingwater.

The recommended storage capacities and stormwater pumping capacity for combined

22


sewer systems were based on the NWRW results for annual pollution loads and an analysis of combined sewer overflow frequencies. Thedefined capacities would correspond with an overflow frequency of about 10 times per yearand an annual pollution load (according to NWRW results) of 350 kgCOD/ha/yr. It is furthermore of importance that the maintenance condition and lay-out of sewer systems are such that sedimentation is kept to a minimum in order to prevent pollutions adhering to sediments from being discharged to surface water. Discharges to sensitive surface waters should be especially prevented. In the CUWVO 1992 report, the year 1998was in principle takenas a final date forrealisation of the recommended basic effort. In theexecutionofimprovementmeasures,priorityshould be given to situations where adverse effects on surface water quality are most severe.

In 2001, the Commit tee for Integrated Watermanagement (CIW, successor of CUWVO), issued a new report (CIW, 2001) that provided a furtherelaborationofthe“basiceffort”definition,which upon practical application had proved to be open for different interpretations. This reportstatesthat“Thereferencesystemdefinedby CUWVO is taken as the starting point for establishing the criterion for average annual emissions of pollutants,which is expressed inkilograms of organic material (COD) per hectare of relevant impervious surface per year.” The report establishes the following criterion: the criterion for maximumannualpollutionisamaximumloadof50 kg of organic material (COD) per hectare per year, totaled over all combined systems within a municipality. The criterion applies to the total area of impervious surface connected to the sewerage system within the drainage area.

The evaluation is based on the following principles:• The average annual emission of pollutants from

existingcombinedsystemsmustnotexceedthe criterion.

• The average annual emission of pollutants from combined systems should be met across the entire municipality.

• Overflow volumes should be calculated in accordance with Module C2100 of the Sewerage Guidelines for a time series corresponding to the De Bilt weather centre’s 10-year rainfall t ime series 1955-1964 (hydrodynamic calculations for rainfall time serieswillbeexplainedlaterinthischapter).

• The average concentration of organic material (COD)duringoverflowsshouldbe250mg/l.

• Average storm water sedimentation tank performance should be 45%.

• The results of measurements may only be used in relation to the determination of the basiceffortinspecificcircumstances,followingconsultation with the water management authority.

4.3 European Framework DirectiveThe Water Framework Directive (more formally the Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing aframeworkforCommunityactioninthefieldofwater policy) is a European Union directive which commits European Union member states to achieve good qualitative and quantitative status of all water bodies (including marine waters up to kilometer from shore) by 2015. It is a framework in the sense that it prescribes steps to reach the common goal rather than adopting the more traditional limit value approach.

The directive defines ‘surfacewater status’ asthegeneral expressionof the status of a bodyof surface water, determined by the poorer of its ecological status and its chemical status. Thus, to achieve ‘good surface water status’ both the ecological status and the chemical status of a surface water body need to be at least ‘good’. Ecological status refers to the quality of the structure and functioning of aquatic ecosystems of the surface waters. Water is an important facet of all life and the water framework directive sets standards which ensure the safe access of this resource.

The Directive requires the production of a number ofkeydocumentsoversixyearplanningcycles.

23


Most important among these is the River Basin Management Plans, to be published in 2009, 2015 and 2021. Draft River Basin Management Plans are published for consultation at least one year prior.

Goodecologicalstatusisdefinedlocallyasbeinglower than a theoretical reference point of pristine conditions, i.e. in the absence of anthropogenic influence.Inareasunderheavyanthropologicalinfluence,suchasmostareasintheNetherlands,the theoretical reference point is replaced by a self-definedreferencepointthatwaterauthoritiescan decide upon, based on an analysis of current ecologicalstatusandanthropogenicinfluences.Aditionally,limitvalueshavebeendefinedfor33priority substances and groups of substances. Pollution control measures should be implemented to meet those limit values in all surface waters.

In March 2005 the risk analysis reports for the four river catchments that the Netherlands are involved in, Rhine, Meuse, Scheldt and Ems catchments, were delivered to the EU, reporting onexpectedachievementswithregardtomeetingthe Directive’s objectives in 2015.

4.4 Storage and stormwater pumping capacity: traditional approach to evaluate environmental impacts of combined sewer systemsThe following definitions are important in the understanding and evaluation of environmental impactsofcombinedseweroverflows:

Storage: in-sewer storage of combined sewer systemsisdefinedastheavailablestoragevolumein sewer pipelines that is situated below the lowest overflowweirinthesystem.Thisstoragevolumeis often referred to as “below-weir-storage” (Dutch: onderdrempelberging). Storage in manholes was traditionally not taken into account, nor was the volumeofdryweatherflow.

Dynamic storage: available storage volume above the lowest overflowweir in the system,water can be stored here as a result of the building

up of an energy gradient that is required for transporttowardstheoverflow.Dynamicstorageis estimated to be of the order of 0.2 to 0.3 mm.

Stormwater pumping capacity (Dutch: pomp-overcapaciteit, poc): average pumping capacity that is available after the end of a storm to empty stored stormwater in a sewer system. This capacity is usually calculated as the gros pumping capacityminusdryweatherflow.

Overflow frequency (Dutch: Overstortings-frequentie): average number of combined sewer overflows per unit of time, usually number of overflowsperyear.

Traditionally, evaluation of combined sewer overflows was based on expected overflowfrequencies.Calculationoftheoverflowfrequencywas based on available storage and stormwater pumping capacity. This method is based on the concept of reference systems: a reference system is defined that correspondswithanacceptableof combined sewer overflow frequency. The referencesystemisdefined in termsofstoragecapacity and stormwater pumping capacity. The method then compares the capacities of a sewer system to those of a reference system to decide whethersystemcapacitiesaresufficient.

The calculation method is called “Kuipers method” and provides an easy and straightforward way to evaluate expected pollution from combinedseweroverflows.Thesewersystemismodeledasabasinwithasingleoverflowweirlevelandasingle, continuous stormwater pumping capacity

Figure 4.2 - Schematization of combined sewer system for overflow frequency calculation

24


(figure4.2).Thestoragecapacityandstormwaterpumping capacity are calculated by dividing the total storage capacity in a sewer system and the total stormwater pumping capacity in a system by the impervious area connected to the sewer system.

The Kuipers method is based on a so-called dot-graph(Dutch:stippengrafiek),seefigure4.3.The dot-graph contains rainfall events events for the period 1926 to 1962. The Kuipers method evaluates the capacities of a system by a drawing a line in the graph that corresponds with the given storage and stormwater pumping capacities. The number of storm events that by volume would haveexceeded thegivenstorageandpumpingcapacities can directly be read from the graph. The fraction of this count and the length of the dot graph period (37 years) gives an estimate of the overflowfrequencyforthissystem.

The Veldkamp graph was developed based on theKuipers’dotgraphtofurtherfacilitateoverflowfrequency evaluation: in the Veldkamp graph, theexpectedoverflowfrequencycandirectlyberead from the graph, based on a storage capacity

expressed inmm and a stormwater pumpingcapacityexpressedinmm/u.

The disadvantage of the Kuipers method is that it neglects dynamic effects that occur in reality duringstormwaterflow.Especiallyinlargersewersystemsandsystemswithdifferentoverflowweirlevels, this approach is not realistic.

4.5 Hydrodynamic evaluat ion of environmental impacts of sewer systemsTheNWRWresearchresultsshowedthatoverflowfrequency is unsuitable as a criterion for evaluation of pollution loads from combined sewer systems. Still, the Kuipers’ method was applied to design and evaluate sewer systems until about 1995- 1998.

From 1998 onwards a new method was developed in the Netherlands to design and evaluate combinedseweroverflows.Thismethodisbasedon hydrodynamic calculations with a simulation modelforanextendedperiodof10to25years.Thisresultsinaseriesofoverflowquantitiesforallcombinedseweroverflowlocationsinthesewer

duration (minutes)

rain

fall

dept

h (m

m)

0

10

20

30

40

50

60

0 300 600 900 1200 1500 1800

Figure 4.3 - The Kuipers dot graph: every dot represents a storm event a given duration and rainfall volume or rainfall depth.

25


system. These results are used to evaluate the performance of the combined system based on:• Combined sewer overflow frequency per

overflowlocation• Totalcombinedseweroverflowfrequencyfor

the entire system• Combined sewer over f low volumes per

overflowandfortheentiresystem(m3)• Combinedseweroverflowquantities(m3/h)

Inthisway,dynamicpropertiesofflowinsewersystems are taken into account and a more completeevaluationofcombinedseweroverflows

and their potential effect on receiving waters can be conducted. It is important to include dry periods between storm events in the calculation, because these provide information on the starting conditions of the storm event: especially whether thestoragecapacityofthesystemstillpartlyfilledbythepreviousevent.Similarly,dryweatherflowvariations during the calculation period should be properly taken into account, because these influence the amount of storage available for stormwaterinflow.

1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980

1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980

1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980

70

56

42

28

14

0

1800

1440

1080

720

360

0

12000

9600

7200

4800

2400

0

590416

m3

minuten

mm

overflow volumes

overflow durations

rainfall volume

Figure 4.4 - Graphs depicting daily rainfall volume (top), overflow durations (middle) and overflow volumes (bottom) for the period 1955 to 1980 (25 years). Taken from: Module C2100 of the Sewerage Guidelines, RIONED, 2004.

26


Dynamic calculations for extended periods oftime, as described in this method, require a lot of computational power capacity, especially for larger sewer systems of several hundreds of kilometers of sewer length. Therefore large sewer systems areschematizedtoamoresimplifiedform,yetstillrepresentative of system characteristics, to reduce calculation time.

The following results of rainfall series calculations for combined sewer overflow evaluation are usually reported.Foreveryseweroverflowlocation:• Thetotalcombinedseweroverflowvolumefor

the calculation period;• Theaverageyearlyoverflowvolume;• Thetotalnumberofoverflowevents;• Theaveragenumberofoverfloweventsper

year;• The overflow volume at this location as a

percentageofthetotaloverflowvolumeofthesystem.

Foreveryseweroverflowevent(overfloweventisdefinedbyaseparationofatleast24hourswiththenextoverflowevent).• Start and end time of the event;• Total and net overflow event duration (net

duration = total duration – dry period of less than 24 hours)

• Eventoverflowvolume;• Maximumoverflowvolumeper5minutes• Maximumoverflowvolumeper15minutes• Maximumper30minutes

Figure4.4givesanexampleoftheresultsofancombined overflow series calculation. These results and the abovementioned reports are used to date to evaluate environmental impacts of sewer systems. In addition to this evaluation, theexpectedeffectsonsurfacewaterqualityareanalysed separately in a so-called “water quality track” (Dutch: waterkwaliteitsspoor) approach. The “water quality track” defines measures to obtain acceptable water quality conditions. Measures can apply to source control, reduction ofseweroverflowsandchangesinwatersystemcharacteristics (such as vegetated embankments,

minimumflow).Thewaterqualitytrackisimportantin those cases where conventional measures to reduce emissions from sewer systems are not sufficient. The impact of overflows on surfacewater quality are leading in the establishment of additional measures for water quality improvement. The targets as defined in theEuropeanWaterFramework serve as a reference. STOWA (the Dutch Organisation for applied research on water systems) recently released a document with guidelines on how to evaluate water quality conditions. (STOWA, 2010). The assessment of impactsofseweroverflowsonsurfacewaterqualityisoftendifficult,becausemanyinfluencingfactors play a role. The guideline document aims to help water authorities in this assessment. It also strongly recommended establishment of surface water quality monitoring programs, since in most cases insufficient data are available for properwater quality assessment.

4.6 References

• CUWVO, 1992. Coördinatiecommissie Uitvoering Wet Verontreiniging Opper-vlaktewateren, Werkgroep VI, Overstortingen uit rioolstelsels en regenwaterlozingen. Aanbevelingen voor het beleid en de vergunningverlening. April 1992.

• CIW, 2001, Riooloverstorten. Deel 2: Eenduidige basisinspanning. Nadere uitwerking van de definitievandebasisinspanning.

• EU, 2000. Directive 2000/60/EC of the European Parliament and of the Council of 23 october 2000 establishing a framework for Communityactioninthefieldofwaterpolicy.

• STOWA, 2010. Rappor t Knelpunten-beoordelingsmethode waterkwaliteitsspoor (STOWA 2010-17)

27


5. Hydraulics for sewer design

5.1 Uniformchannel flow in partiallyfilledsewerpipelinesIn sewer pipelines, flowexperiences significantfriction and is often turbulent (Re>2000), so the Bernoulli equation does not apply. Instead, friction effects lead to a continual reduction in the value of the Bernoulli constant (equation 5.1).

Instead, the De Saint-Venant equations ((5.2) and5.3))areusedtodescribetheflowofwaterthrough sewer systems. The equations consist of 2 components: the continuity equation (5.2) and the equation of motion (5.3), under the assumption that components of velocity in the y and z direction are negligible in comparison to the component of velocityinthexdirection(uy=uz<<ux).

Continuity equation (mass balance):

Equation of motion (momentum balance):

Where:

Q flowrate(m3/s)A flowareaorwetarea(m2)g gravity acceleration (~9.813 m/s2)z bottom elevation (m)y waterdepth (inpart-full flow)orpressure

head(inpipeflow)(m)t time (s)t wall shear stressr density (kg/m3)W wet perimeter (m)B surface width (m)

The equation of motion (eq.5.3) consists of 4 components:

: Advective acceleration

: Convective acceleration

: Gravity force

: Friction

Varioussimplificationsfromthecomplete5.1and5.2 equations are applied in practice. Depending on the type of hydraulic conditions, parts of the equationcanbeneglected.Simplifiedequationsare applied to either speed up the numerical simulation process or to enable analytical solutions. Term I and II (acceleration terms) can forexamplebeneglectedforstationary,uniformflowatnormaldepthinchannelsandpartiallyfilledpipes:inthosecasesflowdoesnotvarywithtimeand gravity forces are at equilibrium with friction forces. We will see later that the same applies for fullpipeflow.

Under stationary conditions and uniform flow at normal depth in partially filled pipes, gravityforces are at equilibrium with friction forces (see figure5.1)

pg

ug

zρ

+ + =2

2constant (5.1)

B yt

Qx

δ δδ

+ = 0 (5.2)

δδ

δδ

δδ

τρ

Qt x

QA

g A y zx

+

+ ⋅ ⋅

++ ⋅ =

2

0( )Ω

(5.3)

δδQt

δδx

QA

2

g A y zx

⋅ ⋅+δδ

( )

τρ⋅Ω

Figure 5.1 - Flow in partially filled pipeline, friction force due to shear stress along the pipe wall.

28


andthemomentumequation(5.3)simplifiesto:

Forflowatnormaldepth,thefrictionslopeequalsthe bottom slope:

(bottom slope):

Ratio A/W is the hydraulic radius Rh, so:

Wall shear stress (t)isapproximatelyproportionalto the velocity squared (u2);commonexpressionsfor t are based on a friction factor:

C:ChézycoefficientC[L1/2 T-1]

l: White-Colebrook friction factor l [-]

For a pipe section of length L, friction losses dHfr due to wall shear stress are given by:

formula of Darcy Weisbach

Where:

Dh: hydraulic diameter: or

A: wet area:

a: angle between the vertical and the centre-to-waterline radiusatagivenfilling rate (seealsolater in this chapter)

W: wet perimeter:

B: width of water surface: ; for full pipe sin a=0, B=0

For full pipes under stationary conditions, gravity forces and friction forces are at equilibrium and the sameequation(5.5)appliesasforpartiallyfilledpipelineflowatnormaldepth.Yetforfullpipeflow,the friction slope does not necessarily equal the bottom slope:

In the formula of Darcy Weisbach (equation 5.9), for full pipes, the hydraulic diameter Dh equals the pipe diameter D:

formula of Darcy Weisbach for full pipes

In full pipes the wet area A and hydraulic radius Rh are calculated as follows:

A: wet area: ;

for full pipe a=p, B=0 and A=p r2 or, since r=1/2D: A=0.25p D2

R: hydraulic radius: Rh=A/Ω=0.25D

The value of the friction coefficientl can be calculated according to White-Colebrook’s formula:

Where:Re Reynolds number [-]k wall roughness (mm)D diameter (m)l frictioncoefficient[-]

Examples of the value of k, the pipe wallroughness, are given in table 5.1 for various kinds of sewer pipe materials.

g A y zx

⋅ ⋅+

+ ⋅ =δδ

τρ

( )Ω 0 (5.4)

S S y zxfr b= =+δδ

ρ τ⋅ ⋅ ⋅ = ⋅g A Sb Ω (5.5)

τ ρ= ⋅ ⋅ ⋅g R Sh b (5.6)

τρ

=guC

2

2(5.7)

τλρ

=u2

8(5.8)

dH LD

ugfr

h

= ⋅λ2

2(5.9)

4 ⋅AΩ

D Rh h= 4

A r B r y= − −α 2

2( )

Ω = 2αR

B R= 2 sinα

S y zx

Sfr b=+

≠δδ

dH LDugfr = ⋅λ2

2

A r B r y= − −α 2

2( )

1 2 0 27 2 5λ λ= − +log( . .

Re)k

D(5.10)

29


The k-value that is applied for gravity sewers systems is an averaged k-value; the resistance of pipe joints, sediments and other rough elements is accounted for in the general k-values mentioned in table 5.1. In pressurised pipelines the k-value of the material itself is used, since added roughness of joints and sediments is assumed to be of little influence. Friction losses due to valves, pipe curves are accounted for separately.

The Reynolds number Re is calculated by:

u pipeflowvelocity(m/s)n kinematic viscosity of water (m²/s)

The value of (EQ) depends on the temperature and the type of wastewater to be transported. For wastewaterofnearly18˚C(EQ)isaround10-6 m²/s. Foraflowvelocityof1m/s,thatfrequentlyoccurs

in the design load of a sewer system, and 0.25 m < D < 1 m: 0.25·106 ≤Re≤106 and

Then, the value of the friction factor l based can be calculated by:

The value of l can also be read from the Moody diagram(figure5.2) thatchartsl (verticalaxis,left) as a function of the Reynolds number Re (horizontalaxis)andk/D(verticalaxis,right).Theleft part of the diagram shows a linear relation between l and Re that is applicable for laminar for flow conditions. For transitional turbulenceconditions, l depends on both Re and k/D, while for rough turbulence conditions l depends on k/D only.

Material k-value in mm

Brickwork 1 - 5

Cement 0.5 - 2

Plastic 0.2 - 0.5

Table 5.1 – k-value pipeline materials

Re = uDν

(5.11)

0 27 2 5. .Re

kD

>>λ

(5.12)

AA

yR

BD

yRf

= −

− −

−1 1 11

πcos (5.13)

Figure 5.2 - Moody Diagram for friction factor l

30


Partially filled versus full pipe geometric relationsThe relations between geometric formulae for partiallyfilledandfullpipelinesareexpressedintermsof thefilling ratey/R that iscalculatedatfollows:

The following relations apply for the ratios of partiallyfilledversusfullpipes(subscriptfappliesto full pipes):

Wet area ratio

Wetted perimeter ratio

Hydraulic radius ratio

In figure 5.4 the ratios for wet area, wetted perimeter, hydraulic ratio and width water surface/diameter (B/D) are depicted for varying fillingrates y/R. The figure shows thatwet area andwetted perimeter ratios are more or less linear, the hydraulic ratio radius increases towards a maximumabove1 at about 75%filling rate (y/R≈1.7),thendecreasesto1forfullpipes(y/R=2).

Thewidthatthewatersurfaceismaximumandequaltothepipediameterforhalffilledpipes(y/R=1); it is 0 for empty and for full pipes.

Figure 5.5 shows the ratios for the Reynolds number,dischargeandflowvelocitiesforvaryingfillingratesy/R(subscriptfappliestofullpipes).Themaximumpipedischargeoccursforafillingrate just below 1. In calculations for sewer pipes, full-pipe discharges are used, since these apply forfullpipeflowaswellasfornearlyfilledpipes(y/R≈1.9),whilehigherdischargesoccurforalimitedrangeoffillingrates.Thefigureshowsthatflowvelocities are equal for half full pipes (y/R=1) and for full pipes. The Reynold number ratios reaches amaximumforafillingratey/Rofabout1.6.

Figure 5.3 - Partially filled pipe with filling depth y, pipe radius R, angle a and wetted area A.

AA

yR

BD

yRf

= −

− −

−1 1 11

πcos (5.14)

ΩΩf

yR

= −

−1 11

πcos (5.15)

RR

A Ah

h f

f

f,

//

=Ω Ω

(5.16)

Figure 5.4 - Ratios for wet area, wetted perimeter, hydraulic ratio and width water surface/diameter (B/D) for varying filling rates y/R of pipelines (subscript f applies to full pipes)

Figure 5.5 - Ratios for the Reynolds number, discharge and flow velocities for varying filling rates y/R in pipelines (subscript f applies to full pipes)

31


5.2 Hydraulic resistance in components of sewer systemsIn this paragraph local friction losses for various sewer system components are discussed.

5.2.1 Sewer overflow weirsSewer overflow weirs are typically walls of concrete or brickwork situated in a manhole or at the end of a pipe. Therefore, in most cases, they canbeconsideredassharp-crestedandtheflowovertheseweirsasacaseofrapidlyvariedflow.Waterbacksupbeforetheweirsothatinflowingover the weir the water goes through critical depth.

Figure5.6 illustrates flowover a sharp crestedweir.Thefollowingequationappliesfortheflowover the weir crest if friction losses are neglected.

And:

Also:

Therefore:

Or:

Where:uc criticalflowvelocityaboveweirB weir crest width

Thestreamlinesinthewaterflowabovethecrestare not parallel or normal to the area in the plane, so in reality friction losses do occur. To account for thiseffect,theconstant1.7intheflowequationisreplacedbyaweircoefficientm(5.21).

The value of the weir crest m depends on the shape of the crest; the value of m varies roughly from 1.5 to 3. For a reliable calculation of the dischargeoveraweir,theweircoefficientshouldbe calibrated by in-situ measurements.

Figure 5.6 and equations 5.17 to 5.21 apply for free flowovertheweir:flowabovetheweiriscriticalandthedownstreamwaterleveldoesnotinfluencetheflow.Thisistruewhenthedownstreamwaterhd level is less than 2/3 of the upstream water level H1(fig.5.7).

Inpractice,weirparametersandflowconditionsdepend not only on the shape of the weir crest,

Figure 5.6 - Free flow over a sharp crested weir, with energy level H1 upstream of the weir, energy level H2 downstream of the weir and h2 the water level above the weir.

Figure 5.7 - Submerged flow over a sharp crested weir, with energy level H1 upstream of the weir, energy level H2 downstream of the weir and hd the downstream water level.

ughc

2

1= (critical flow above weir crest)(5.17)

H h ug

ug

ug

ug

c c c c1 2

2 2 2 2

2 232

= + = + = (5.18)

h H223 1= (5.19)

Q u h B BH gHc= =223 1

23 1 (5.20)

Q BH= 1 7 1

32.

Q mBH=32 (5.21)

32


butalsoonmaintenanceconditionsoftheinflowandoutflowpipesandtheweiritself,asfigure5.8illustrates.

5.2.2 Inlets, outlets and manholesFigure5.9givesaschematicrepresentationofflowthroughaweiropening.Aswaterflowsthroughthe opening, local deceleration losses occur. The following equation applies for the calculation of the headlossduetoflowthroughanarrowopening(5.22):

Where: hL local head lossA1 cross-section of weir openingA2 cross-section of downstream pipe; in case

of a reservoir A2→∞(infinity)u1 upstreamflowvelocity

Foroutflowofapipeintoareservoir,A1/A2 goes to zero and hL equals u2/2g.

Whenasewerdischargesunderwatertheoutflowlosses equal a the velocity head times a loss factor k (5.23). The value of k varies between 0 and 1.

The same equation (5.23) is used to calculate head losses in manholes. The value of k for local losses in manholes depends on the height of the water level in the manhole. Figure 5.10 gives values of k for varying water heights for two different manhole bottom shapes.

In the Netherlands, the bottom manhole shape in figure5.10–topisusuallyapplied.Measurementshave shown thatwhen the filling height of themanhole is at least 1 time the pipe diameter, the value of k lies between 0.7 and 0.9.

When more than two pipes meet in a manhole, the local head loss is usually calculated based on the flowvelocityoftheoutflowdischarges.Thetotalhead loss in the manhole is the sum of the head losses for the outgoing pipes. In practice, it is often assumed that the local losses in manholes are negligible compared to the friction losses along the pipe length.

h AA

ugL = −

1

21

2

12

(5.22)

Figure 5.9 – Flow through a weir opening. Situation on the left show flow through opening; situation on the right shows flow through an outlet pipe in the weir.

h k ugL =2

2(5.23)

Figure 5.8 - Flow over weir influenced by downstream maintenance condition.

33


Figure 5.10 – Local loss coefficient k for varying water level heights in manholes.

Urban drainage full_2011

Technology