storm sewers, page 2 · 2018. 8. 28. · storm sewers, page 5 storm sewer software traditionally,...

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


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 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


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


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


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


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


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


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.


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


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


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


delineation of the drainage subcatchments is performed on a catchbasin by catchbasin basis

drainage area

runoff coefficient


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


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.


let’s return to our existing development located north of our study area


based on the existing topography, let’s consider the following catchbasin locations


now, let’s include maintenance holes at the required locations…


and the pipe network…


CBMH #1

Now, let’s define the contributing drainage area associated with catchbasin-manhole no. 1 (CBMH#1)


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


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


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


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


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


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

QQ


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


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


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


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


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


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


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


CBMH #4CBMH #3

CBMH #5

DCBMH #1

MH #1

MH #2

MH #3

MH #4

MH #5

CBMH #1

CBMH #2

storm sewers, page 2 · 2018. 8. 28. · storm sewers, page 5 storm sewer software traditionally,...

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