1 / 49

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

gail-zamora
Download Presentation

Stormdrain System Design

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related