Storm Sewers, Page 1
Storm Sewers storm sewer systems
are dendritic systems used to collect and direct stormwater runoff
storm sewer systems are integral components of any urban infrastructure
curbs, gutters and storm inlets are an equally important component of the drainage system
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Storm Sewer Design urban development can create potentially severe problems the construction of houses, buildings and paved roads significantly increases
the impervious fraction of a basin with urbanization, the direction and timing of runoff is dramatically changed the storm sewer system is a network of pipes used to transport storm water
runoff within urbanized areas the layout of the network requires experience and sound engineering judgment the design of a storm sewer system involves 2 components
• runoff prediction• rational method
• hydraulic analysis of pipe flows• spreadsheet approaches
storm drainage is provided on a “major” and “minor” system• the minor storm drainage system of local storm sewers shall be designed
for flows resulting from a 5-year storm• the major storm drainage system shall permit continuous overland flow
along roads and easements to the SWM pond without flooding property during the 100-year storm
Storm Sewers, Page 5
Storm Sewer Software traditionally, strom sewer design is completed using design spreadsheets over the past decade, numerous computer programs have been developed to aid
in the design of urban drainage systems:• StormCAD, CulvertMaster, CivilStorm, FlowMaster, PondPack, etc. (Haestad)• PC-SWMM, EPA-SWMM• Hydraflow (inteliSOLVE)• Storm Sewers (Scientific Software Group)• GWN-Storm (Scientific Software Group)• Splash (Ripple-Thru)• InletMaster• PipeMate• Visual Drainage• Visual Hydro• Hydra (Pizer)• MIDUSS
Storm Sewers, Page 6
Region of Waterloo Design Guidelines for Municipal Services
Design Flows• the quantity of storm water shall be computed using the Rational Method
where Q is the peak runoff rate (m3/s)C is the runoff coefficienti is the rainfall intensity (mm/hr) and A is the contributing drainage area (ha.)
Assumptions: the peak rate of runoff at any point is a direct function of the average rainfall
intensity during the time of concentration (the entire catchment is contributing) the time of concentration is the time required for runoff to be established and
flow to the outlet the runoff coefficient is constant over the catchment, during the progress of
the storm (does not change with time or between storms)
Q CiAp 360
Storm Sewers, Page 7
all storm sewers shall be designed to a 5 year storm event
Rainfall Intensity• The values of rainfall intensity shall be determined using
• where a, b and c are defined as follows:
ccavg bt
ai
a b c2-year 582 4.6 0.756
5-year 1395 12.7 0.839
25 year 3509 22.6 0.925
100-year 5886 28.6 0.969
Storm Sewers, Page 8
Time of Concentration (tc) the time of concentration is defined as the time it takes for runoff to travel
from the hydraulically most distant part of the catchment to the point of reference downstream. Mathematically, the time of concentration is given by:
where Tc is the time of concentration (min);Ti is the inlet time (min); andTp is the pipe travel time (min).
the inlet time is the time for the overland flow to reach the storm sewer inlet. inlet times for urban drainage systems generally vary between 5 and 20 minutes. there are various approaches to estimating the inlet times.
• inlet times vary according to the ground slope, land use, length of flow path and other factors.
• in some municipalities, the maximum inlet times are specified under a drainage policy.
• alternatively, inlet times can be calculated using empirical equations or nomographs.
pic TTT
Storm Sewers, Page 9
for the Region of Waterloo, the following inlet times are specified:
Runoff Coefficient Inlet Timeless than 0.5 15 minutes0.50 ≤ R ≤ 0.75 10 minutesgreater than 0.75 5 minutes
at the various points along the storm sewer, the time of concentration will consist of the inlet time to the most upstream inlet plus the travel time along the sewer.
the travel time (min) through the pipe (Tp) is given by:
where Lp is the length of the pipe segment (m)V is the mean velocity of the flow (m/min)
where two of more sewer branches meet at a junction, the time of concentration for the combined sewer is taken to be the longest Tc.
TLVp
p
Storm Sewers, Page 10
Runoff Coefficient the runoff coefficient accounts for all catchment losses the coefficient is a subjective parameter and is a function of land use
in reality, catchment losses should be a function of various parameters such as• infiltration rate• slope• soil compaction• soil porosity, etc.
for multiple land use catchments an area weighted average is used
typical published runoff coefficients are applicable for a 5 to 10-year frequency design
AiQ
C p volumerainfall volumerunoff
description lower bound upper bound
Commercialdowntownneighbourhood
0.900.50
1.000.70
Residentialsingle-familydetached multi-unitsattached multi-unitsapartments
0.400.450.600.60
0.450.660.750.80
IndustrialDowntownSuburban
0.900.60
1.000.90
Parks, open space 0.15 0.35
Storm Sewers, Page 11
minimum pipe size the minimum pipe diameter for main lines shall be 300 mm available pipe sizes vary slightly with each manufacturer but for this course, assume
that following pipe sizes are available (mm) – 300, 375, 450, 525, 600, 675, 750, 825, 900, 1050, 1200, 1350, 1500, 1650, 1800, 1950, 2100, etc.
Manning’s n for concrete, PVC and HDPE pipes, a Manning’s n of 0.013 shall be used
pipe gradient for the first reach of permanent dead end sewers, the minimum pipe gradient shall
be 1% for all other pipes, the flow velocity criteria shall be used to govern the pipe
gradient
flow velocities the minimum velocity allowed for storm sewers is 0.80 m/s and the maximum
allowable velocity is 6.0 m/s under peak theoretical flows in the last reach, before the outlet, the maximum allowable velocity shall be 4.0 m/s.
Storm Sewers, Page 12
pipe depth the obvert shall be a minimum of 1.5 m below the final road grade
headwalls head walls shall be used for 525 mm diameter or larger sewers, permanent pool or
submerged conditions
maintenance holes maintenance holes 3000 mm and smaller shall be pre-cast concrete the minimum maintenance hole diameter is 1200 mm the maximum spacing for maintenance holes shall be based on the sewer diameter
Sewer Diameter Maintenance Hole Spacingless than 900mm 90 m900mm ≤ Dia < 1350mm 120 m≥ 1350 mm requires the approval of the
Chief Municipal Engineer
Storm Sewers, Page 13
location of maintenance holes maintenance holes shall be located at all
• junctions• changes in grade• changes in material• changes in alignment• changes in pipe size, and• at the termination point of all sewers
invert drops across maintenance holes where pipes enter and leave in-line, the drop from invert to invert across the
maintenance hole shall be the slope of the pipe where pipes enter and exit at angles between 0 and 45º, the minimum drop from
invert to invert across the maintenance hole shall be 30 mm where pipes enter and exit at angles between 45º and 90º, the minimum drop from
invert to invert across the maintenance hole shall be 60 mm
changes in flow direction changes in the direction of flow through a maintenance hole greater than 90º will
not be permitted in pipe sizes of 675 mm or greater, the change in direction through a maintenance
hole shall be no greater the 45º
Storm Sewers, Page 14
catch basin spacing the maximum spacing between catchbasins shall be established from the
following:
catchbasin location catchbasins shall be located on the upstream side of all intersections where the
road grade falls towards the intersection
double catchbasin double catchbasins shall be provided at all low points where water is collected
from 2 directions
side inlet catchbasin side inlet catchbasins shall be provided on regional and other arterial roadways
Road TypeRoad Grade
< 3% 3% to 5% >5%
2 lane road 90m 75m 60m
4 lane road 75m 60m 60m
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delineation of the drainage subcatchments is performed on a catchbasin by catchbasin basis
drainage area
runoff coefficient
Storm Sewers, Page 19
Preliminary Design Procedure the drainage systems are typically sized by first designing the minor flow systems
and then checking the hydraulic performance of the major storm system. the design of storm sewers is typically completed using a spreadsheet approach.
• most major cities will provide a standard design sheet which presents and summarizes the design information required under the approval process.
• in general, establishing a storm sewer design is done by starting at the upstream end of the system and progressing downstream, one pipe at a time.
• at the upstream end of the first pipe reach, a discharge is computed using the Rational Method based on the specified inlet time to the catch basin.
• based on the discharge, a tentative pipe size and grade are selected which can negate the friction losses through the pipe.
• at each manhole, care should be taken to match the proposed road grade, the required depth of cover and the required pipe slope.
• at each manhole, the required upstream and downstream inverts are identified
Storm Sewers, Page 20
a mean velocity is computed along the pipe segment and a pipe travel time is estimated.
the pipe travel time plus the upstream inlet time provides the new time of concentration for the next downstream pipe segment.
based on the new time of concentration, a new peak flow is computed for the next pipe segment.
the design should continue downstream until you have sized all pipes and have reached the storm water management facility.
Storm Sewers, Page 21
let’s return to our existing development located north of our study area
Storm Sewers, Page 22
based on the existing topography, let’s consider the following catchbasin locations
Storm Sewers, Page 23
now, let’s include maintenance holes at the required locations…
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and the pipe network…
Storm Sewers, Page 25
CBMH #1
Now, let’s define the contributing drainage area associated with catchbasin-manhole no. 1 (CBMH#1)
Storm Sewers, Page 26
CBMH #2
Storm water arrives at CBMH #2 in two forms:
•overland flow into CBMH#1 followed by pipe flow to CBMH#2
•overland flow directly into CBMH#2
Storm Sewers, Page 27
CBMH #4
repeating the process for all the remaining catchments, we now have our storm water drainage network defined.
CBMH #3
CBMH #5
DCBMH #1
MH #1
MH #2
MH #3
MH #4
MH #5
CBMH #1
CBMH #2
Storm Sewers, Page 28
CBMH #4
we can now state that we have 10 pipe segments to design/size..
CBMH #3
CBMH #5
DCBMH #1
MH #1
MH #2
MH #3
MH #4
MH #5
Pipe From To
1 CBMH1 CBMH2
2 CBMH2 MH3
3 CBMH3 MH1
4 MH1 CBMH4
5 CBMH4 CBMH5
6 CBMH5 MH2
7 MH2 DCBMH1
8 DCBMH1 MH3
9 MH3 MH4
10 MH4 MH5
Storm Sewers, Page 29
we begin by assigning a runoff coefficient and drainage area to each catchment
0.4
0.4 0.4
0.4
0.4
0.4
0.51 0.71
0.65
0.470.55
1.15
Storm Sewers, Page 30
in order to illustrate the computation process, we will prepare a preliminary design for pipe connecting CBMH1 and CBMH2
the contributing drainage area to CBMH1 is 0.47 ha
the runoff coefficient is 0.40 the inlet time is specified as 15
minutes the corresponding rainfall intensity
can be from:
using the Rational Formula to estimate the peak discharge rate:
CBMH #1
CBMH #2
0.4
0.47
hrmm
btai c
c
/0.867.1215
1395839.0
sm
CiAQ
/045.0360
47.00.864.0360
3
Storm Sewers, Page 31
we will adopt • a preliminary pipe size of 300 mm• a preliminary pipe slope of 1.0 %• a pipe length of 52 m• a Manning’s n of 0.013
using the Manning Formula, we can establish the capacity of the preliminary pipe:
our pipe is oversized even though it is at the minimum permitted slope (1%) and diameter (300mm).
CBMH #1
CBMH #2
0.4
0.47
sm
SDDn
SARn
Q
/097.0
01.04300.0
4300.0
013.01
441
1
3
213
22
213
22
21
32
%4.46097.0045.0
Capacity
Actual
Storm Sewers, Page 32
0.4
0.47
CBMH #2
CBMH #1
now, let’s turn our attention to CBMH #2 • runoff can reach the outlet of
the second CBMH by either :• overland flow to the
catchment (inlet time = 15 min), or..
• overland flow to CBMH#1 (inlet time of 15 minutes) plus the travel time associated with the flow through the storm sewer connecting CBMH1 with CBMH2
computing the full pipe flow velocity and the corresponding travel time through the pipe (Tp):
0.4
0.47
0.4
0.55
This image cannot currently be displayed.
sm
DQ
AQ
V
Full
Full
Fullp
/37.1
4300.0097.0
4
2
2
Within the range of permissible velocities
Storm Sewers, Page 33
0.4
0.47
CBMH #2
CBMH #1
at CBMH #2, the corresponding time of concentration was found to be 15.63 minutes.
as before, the corresponding rainfall intensity can be from:
applying the Rational Formula to estimate the peak discharge rate at CBMH2, we get:
0.4
0.47
0.4
0.55
hrmm
btai c
c
/4.847.1263.15
1395839.0
sm
iACAC
CiAQ
/096.0360
4.8455.04.047.04.0360
360
3
2211
Storm Sewers, Page 34
we now need to size the storm pipe connecting CBMH#2 and MH#3
we can adopt the following preliminary numbers• a preliminary pipe size of 300 mm• a preliminary pipe slope of 1.5 %• a pipe length of 48 m• a Manning’s n of 0.013
using the Manning Formula, we can establish the capacity of the preliminary pipe design:
CBMH #2
MH #3
sm
SDDn
SARn
Q
/118.0
015.04300.0
4300.0
013.01
441
1
3
213
22
213
22
21
32
sm
AQ
VFull
Fullp
/68.1
4300.0118.0
2
Storm Sewers, Page 35
let’s look at our design in profile and clean up a few loose ends. for this tutorial, let’s adopt the following:
• CBMH#1• finished grade elevation = 345.60• downstream (D/S) invert elevation = 343.000
• CBMH#2• finished grade elevation = 344.00
• MH#3• finished grade elevation = 343.60
CBMH #1MH #3345.60
343.000
52m. - 300mm pipe @ 1%
CBMH #2
344.00
At a 1% slope, the 52 metres of pipe has a total drop of 0.52m
The resulting upstream (U/S) pipe invert elevation at CBMH #2 is 343.000 – 0.52 = 342.480
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we must account for losses associated with CBMH#2• where pipes enter and exit at angles between 0 and 45º, the minimum drop
from invert to invert across the maintenance hole shall be 30 mm the resulting downstream invert is then established at 342.480 – 0.030 = 342.450
CBMH #1MH #3345.60
343.000
52m. - 300mm pipe @ 1%
CBMH #2
344.00
342.480342.450
48m. - 300mm pipe @ 1.5%
At a 1.5% slope, the 48 metres of pipe has a total drop of 0.72m
The resulting upstream (U/S) pipe invert elevation at MH #3 is 342.450 – 0.72 = 341.730
341.730
343.60
Storm Sewers, Page 37
we have completed one leg of our drainage network
the entire process will then be repeated beginning at the upstream end of the other drainage leg
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While numerous software programs are now available to complete the computations, it is still useful to work through a standard design spreadsheet typical of major towns and cities for presenting and summarizing the design information required under the approval process. Let’s examine the previous computations in a spreadsheet format:
Location Drainage Area Runoff
From To Area(A) Runoff AC Total Inlet Rainfall Discharge
Manhole Manhole Coeff (C) AC Time Accum Intensity
(ha) (min) (min) (mm/hr) (m3/s)
CBMH1 CBMH2 0.47 0.4 0.188 0.188 15 15.00 86.0 0.045
CBMH2 MH3 0.55 0.4 0.220 0.408 15 15.63 84.4 0.096
Pipe Selection Pipe Inverts
PipeLength
PipeSize
PipeSlope
Full FlowCapacity
Full FlowVelocity
Full FlowTravel Time
U/SInvert
D/S Invert
MHDrop
(m) (mm) (%) (m3/s) (m/s) (min)
52.0 300 1.0 0.097 1.37 0.63 343.000 342.480 0.030
48.0 300 1.5 0.118 1.68 0.48 342.450 341.730
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CBMH #4CBMH #3
CBMH #5
DCBMH #1
MH #1
MH #2
MH #3
MH #4
MH #5
CBMH #1
CBMH #2