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Stormdrain System Design

Stormdrain System Design. CE154 Hydraulic Design Lectures 10-11. Stormdrain System. Definition - A system that collects, conveys and discharges stormwater runoff from the drainage basin to designated outflow collection points - Typically used in urbanized areas

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Stormdrain System Design

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  1. Stormdrain System Design CE154 Hydraulic Design Lectures 10-11

  2. Stormdrain System • Definition- A system that collects, conveys and discharges stormwater runoff from the drainage basin to designated outflow collection points- Typically used in urbanized areas • Elements of design- hydrology: design flow and volume- hydraulics: inlet, conveyance in open channel and closed conduit, temporary storage in detention basin, & outfall

  3. Applications • Land development – municipal ordinances require runoff not to exceed pre-project level • Industrial plants (power, chemical, oil refinery, etc.) require that facilities be protected from X-year floods • Municipal storm sewer design typically to transport 5-25 year flood runoff

  4. Useful References • California Stormwater Best Management Practice Handbook, Calif. Stormwater Quality Association, 2004 (a broad description of systems and elements) • US EPA Stormwater Best Management Practice Design Guide, EPA/600/R-04/121, September 2004 • Local county or city public works design standards

  5. Study Objectives • Be cognizant of storm drain system elements and design criteria • Be able to conduct preliminary design

  6. Definitions • Detention basin: a natural or artificial basin that receives and temporarily holds storm runoff to reduce downstream peak flows for flood control purposes • Drainage pipe or channel: part of a stormwater conveyance system that transport stormwater from one place to another • Manhole: a junction where two or more drainage pipes confluence and where maintenance access is provided to the drainage system

  7. Schematic GIS drainage map

  8. Typical Manhole

  9. Definitions (cont’d) • Catch Basin: A basin, typically with a grated cover, to which surface runoff drains. The basin may be along a curb side or in the middle of a field. The bottom of the basin is typically connected to a drainage pipe, and the basin serves as an inlet to the storm drain system.

  10. Catch Basin

  11. Storm Drain System Design • Layout drainage channels and pipes to provide transport of runoff • Delineate the drainage area from which runoff drains toward a pipe or channel • Determine drainage pipe or channel size • Design catch basins, manholes, detention basins, and other pertinent structures • Conduct system-wide drainage analysis to ensure connectivity and system capacity

  12. Design Considerations • Free surface flow exists for the design discharge. Practical design limit for free surface (open channel) flow is 80% full. • Use commercially available pipe sizes >8” in diameter. Sizes include 8, 10, 12, 15, 18, 21, 24, 27, 30, 36, 42, 48 inches, etc. • A minimum flow velocity of 2 ft/sec is desirable to reduce deposition

  13. Design considerations (cont’d) • Reasonable velocity may be 10 ft/sec • At any junction or manhole, the downstream pipe should not be smaller than any of the upstream pipes • Typically, the rational method is used to determine design discharge because of its simplicity and suitability to small urban drainage areas

  14. Rational Method • Q = iCAQ: discharge in cfsC: dimensionless runoff coefficient depending on surface condition and area slopei: rainfall intensity in inches per hourA: drainage area in acres • when there is more than one basin that drains into a junction, useQ = i(CA)

  15. Rational Method Runoff Coeff. C

  16. Rainfall Intensity “i” • Typically prepared by local water agency as part of rainfall intensity-duration-frequency curve such as Figure I-1 of DSD • “i” is a function of design return period and rainfall duration (which is equal to time of concentration)

  17. Rainfall Intensity “i” (cont’d) • Where Tr = design return period in yearsTc = rainfall period in hours which is assumed to be the same as the time of concentration • Sonoma County proposed this relationship for the local area (note: this Tc is in minutes): • For either case, need to determine Tc

  18. Time of Concentration Tc • Usually a function of watershed slope, length, surface roughness and rainfall intensity • May be computed by runoff calculation or from flood hydrograph • Simplified time of concentration estimate by Yen and Chow [FHWA-RD-82-063, 064 & 065, 1983]

  19. Time of Concentration Tc • Tc = time of concentration in hours • N = overland texture factor (see next slide) • L = length of longest flow path in feet • So = average slope • K = constant defined below

  20. Time of Concentration Tc • N – overland texture factors

  21. Example of Tc calculation • Matadero Creek in Palo Alto:L = 7.2 miles = 38000 ftS = 2% = 0.02N = between suburban and dense residential = 0.05 from tableK = heavy rain > 1.2 in/hr = 0.012 • Tc = 0.012 (0.05*38000/(0.02)^0.5)^0.6 = 3.6 hours

  22. Example of “i” calculation • Use the Sonoma County relationship and the Matadero watershed time of concentration to compute the 10-year and 100-year design rainfall intensities: • Tc = 216 min., for 10-year rain intensity, i =0.42 in/hr • For the 100-year event, i = 0.59 in/hr • Note that the ratio between a 10-year and 100-year rainfall intensity is only 1.4

  23. Rational Method • For each drainage area, knowing A (in acres), estimating C, and computing Tc to get i, the design discharge (Q) can be computed. • The minimum pipe diameter (for nearly full flow) that is required to convey the design discharge may be computed using one of the 2 formulae below:

  24. Pipe Sizing • If using Manning’s formula (in English units): • If using Darch-Weisbach formula (any consistent unit):

  25. Pipe sizing • 2 useful relationships to relate Manning’s n and Darcy f • Where es = equivalent sand grain roughness in ft D = pipe diameter in ft

  26. Example – pipe sizing • Size a storm drain pipe to convey a design runoff of 280 cfs from a junction at El. 545 ft to a junction at El. 523 ft. The linear distance between the 2 junctions is 1200 ft. Assume reinforced concrete pipe. • Answer: Using the Manning’s formula Q = 280 ft n = 0.015 (estimated average condition) So = (545-523)/1200 = 0.0183 D = 4.84 ft

  27. Example – pipe sizing • Now use the Darcy-Weisbach formula • es = 0.0128 ft Using D = 4.84 ft, es/D = 0.00265 f = 0.025 And computing for pipe diameter, we have D = 4.85 ft Say use D = 5 ft = 60 in.

  28. Circular pipe flow geometry

  29. Junctions • Design considerations:- located at every change of pipe size, horizontal direction or vertical alignment- spaced at no more than 400 ft- Minimum diameter of 36 - 48” to allow access and maintenance activities, at least large enough to accommodate all pipes connected with a minimum of 3 inches of wall thickness on both sides of all pipes

  30. Loss Coefficient for Junctions • At junctions, the losses may be classified as pipe exit loss and entrance loss. • There are 2-way, 3-way, and 4-way junctions most commonly seen. • Extensive experimental data to develop loss coefficients. See Chap 14, Hydraulic Design of Urban Drainage Systems, of Hydraulic Design Handbook by L. Mays

  31. 2-way Junction • Same size pipes upstream and downstream of junction • No change in direction of flow • Noticeable high head loss and vortex and instability when ratio of junction depth (Y) to pipe diameter (D) is between 1 and 2. • Head Loss = K V2/2g

  32. 2-way Junction – same pipe size

  33. 2-way Junction – different pipe sizes

  34. 2-way Junction – pipe location effect

  35. 3-way Junction – same pipe sizes

  36. 3-way Junction – same pipe sizes

  37. 3-way Junction – different pipe sizes

  38. 3-way Junction – different pipe sizes

  39. System Analysis • Taking energy balance between upstream and downstream junctions of a pipe for surcharged (full) flow condition • Applying culvert flow considerations for open channel flow condition • Starting from the downstream end and moving upstream to determine water levels in junctions • Maintain sufficient freeboard at junctions

  40. Detention Basins • Also called dry pond, since only retains water during wet weather (A wet pond is a retention basin) • Main flood control objective is to reduce peak flood flow in the downstream • May improve water quality of the downstream flow as well • Need inflow hydrograph, elevation-storage curve, and outflow rating curve for design

  41. Detention Basin • Regulatory requirements now dictate that the peak storm flow rate do not exceed the pre-project condition for all events (from 2-year to 100-year). • Also there are requirements for runoff not to exceed certain water quality criteria • These requirements result in installation of detention basins that delay and reduce storm runoffs.

  42. Detention Basin

  43. Detention Basin • Routing follows the same procedure provided in Table 9-1 (p. 343) of Design of Small Dams • Outflow may be provided by a conduit (pipe or box culvert). Under full flow condition, the discharge is governed by an orifice-flow condition

  44. Detention Basin • C = discharge coefficientA = conduit areaH = total energy headQ = discharge • Loss coefficient ko is related to C by:

  45. Orifice discharge characteristics • C ko

  46. Example Design Problem I II III 11 IV 21 12 31

  47. Example (K=0.7 assumed)

  48. Example

  49. Example

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