Storm Sewers, Page 2

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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 Sewers, Page 1

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4 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 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 4

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 5

6 Region of Waterloo Design Guidelines for Municipal Services Design Flows the quantity of storm water shall be computed using the Rational Method CiA Q p 360 where Q is the peak runoff rate (m 3 /s) C is the runoff coefficient i 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) Storm Sewers, Page 6

7 all storm sewers shall be designed to a 5 year storm event Rainfall Intensity The values of rainfall intensity shall be determined using i avg t c a b c where a, b and c are defined as follows: a b c -year year year year Storm Sewers, Page 7

8 Time of Concentration (t c ) 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: T c T T i p where T c is the time of concentration (min); T i is the inlet time (min); and T p 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 0 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. Storm Sewers, Page 8

9 for the Region of Waterloo, the following inlet times are specified: Runoff Coefficient Inlet Time less than minutes 0.50 R minutes greater than 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 (T p ) is given by: where T p L V L p is the length of the pipe segment (m) V is the mean velocity of the flow (m/min) p where two of more sewer branches meet at a junction, the time of concentration for the combined sewer is taken to be the longest T c. Storm Sewers, Page 9

10 Runoff Coefficient the runoff coefficient accounts for all catchment losses the coefficient is a subjective parameter and is a function of land use C runoff volume rainfall volume Q p Ai in reality, catchment losses should be a function of various parameters such as infiltration rate slope soil compaction Commercial soil porosity, etc. downtown 0.90 neighbourhood 0.50 for multiple land use catchments an area weighted average is used Residential single-family 0.40 typical published runoff detached multi-units 0.45 attached multi-units 0.60 coefficients are applicable for apartments 0.60 a 5 to 10-year frequency design description lower bound upper bound Industrial Downtown Suburban Parks, open space Storm Sewers, Page 10

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, 55, 600, 675, 750, 85, 900, 1050, 100, 1350, 1500, 1650, 1800, 1950, 100, etc. Manning s n for concrete, PVC and HDPE pipes, a Manning s n of 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 11

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 55 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 100 mm the maximum spacing for maintenance holes shall be based on the sewer diameter Sewer Diameter less than 900mm 900mm Dia < 1350mm 1350 mm Maintenance Hole Spacing 90 m 10 m requires the approval of the Chief Municipal Engineer Storm Sewers, Page 1

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 13

14 catch basin spacing the maximum spacing between catchbasins shall be established from the following: Road Type Road Grade < 3% 3% to 5% >5% lane road 90m 75m 60m 4 lane road 75m 60m 60m 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 directions side inlet catchbasin side inlet catchbasins shall be provided on regional and other arterial roadways Storm Sewers, Page 14

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18 runoff coefficient drainage area delineation of the drainage subcatchments is performed on a catchbasin by catchbasin basis Storm Sewers, Page 18

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 19

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 0

21 let s return to our existing development located north of our study area Storm Sewers, Page 1

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 Storm Sewers, Page 3

24 and the pipe network Storm Sewers, Page 4

25 Now, let s define the contributing drainage area associated with catchbasin-manhole no. 1 (CBMH#1) CBMH #1 Storm Sewers, Page 5

26 Storm water arrives at CBMH # in two forms: overland flow into CBMH#1 followed by pipe flow to CBMH# overland flow directly into CBMH# CBMH # Storm Sewers, Page 6

27 repeating the process for all the remaining catchments, we now have our storm water drainage network defined. CBMH # MH #3 DCBMH #1 MH #5 MH #4 CBMH #1 MH # CBMH #3 CBMH #4 CBMH #5 MH #1 Storm Sewers, Page 7

28 we can now state that we have 10 pipe segments to design/size.. MH #3 MH #5 Pipe From To 1 CBMH1 CBMH CBMH MH3 DCBMH #1 MH #4 3 CBMH3 MH1 4 MH1 CBMH4 5 CBMH4 CBMH5 6 CBMH5 MH 7 MH DCBMH1 8 DCBMH1 MH3 MH # 9 MH3 MH4 10 MH4 MH5 CBMH #4 CBMH #5 CBMH #3 MH #1 Storm Sewers, Page 8

29 we begin by assigning a runoff coefficient and drainage area to each catchment Storm Sewers, Page 9

30 in order to illustrate the computation process, we will prepare a preliminary design for pipe connecting CBMH1 and CBMH 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: CBMH #1 CBMH # a i c tc b mm / hr using the Rational Formula to estimate the peak discharge rate: CiA Q m / s Storm Sewers, Page 30

31 we will adopt a preliminary pipe size of 300 mm a preliminary pipe slope of 1.0 % a pipe length of 5 m a Manning s n of using the Manning Formula, we can establish the capacity of the preliminary pipe: CBMH # Q 1 n AR 1 D n 4 S D m / s 3 1 S CBMH #1 our pipe is oversized even though it is at the minimum permitted slope (1%) and diameter (300mm). Q Q Actual Capacity % Storm Sewers, Page 31

32 This image cannot currently be displayed. now, let s turn our attention to CBMH # 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 CBMH computing the full pipe flow velocity and the corresponding travel time through the pipe (Tp): Within the range of permissible velocities V p Q A Full Full QFull D m / s CBMH # CBMH #1 Storm Sewers, Page 3

33 at CBMH #, the corresponding time of concentration was found to be minutes. as before, the corresponding rainfall intensity can be from: a i c t c b mm / hr CBMH # CBMH #1 applying the Rational Formula to estimate the peak discharge rate at CBMH, we get: CiA Q 360 C1 A1 C 360 A i m / s Storm Sewers, Page 33

34 we now need to size the storm pipe connecting CBMH# 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 using the Manning Formula, we can establish the capacity of the preliminary pipe design: MH #3 CBMH # Q 1 AR n 1 D n 4 S D m / s 3 1 S V p QFull AFull m / s 4 Storm Sewers, Page 34

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 = downstream (D/S) invert elevation = CBMH# finished grade elevation = MH#3 finished grade elevation = CBMH # CBMH # MH #3 At a 1% slope, the 5 metres of pipe has a total drop of 0.5m 5m mm 1% The resulting upstream (U/S) pipe invert elevation at CBMH # is = Storm Sewers, Page 35

36 we must account for losses associated with CBMH# 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 = CBMH # CBMH # MH # m mm 1% 48m mm 1.5% At a 1.5% slope, the 48 metres of pipe has a total drop of 0.7m The resulting upstream (U/S) pipe invert elevation at MH #3 is = Storm Sewers, Page 36

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 Storm Sewers, Page 37

38 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 CBMH CBMH MH Pipe Selection Pipe Inverts Pipe Length Pipe Size Pipe Slope Full Flow Capacity Full Flow Velocity Full Flow Travel Time U/S Invert D/S Invert MH Drop (m) (mm) (%) (m3/s) (m/s) (min) Storm Sewers, Page 38

39 MH #3 MH #5 DCBMH #1 CBMH # MH #4 CBMH #1 MH # CBMH #5 CBMH #3 CBMH #4 MH #1 Storm Sewers, Page 39