Urban Stormwater Drainage: Effective Design Strategies and Techniques

1. URBAN STORMWATER DRAINAGEA typical gully pit

2. URBAN STORMWATER DRAINAGEWhen the design fails!

3. URBAN STORMWATER DRAINAGEDesign of urban stormwater drainage involves • Hydrologic calculations of catchment flow rates • Hydraulic calculations of pit energy and friction losses, and pipe sizes

4. URBAN STORMWATER DRAINAGEHydraulic Design

5. URBAN STORMWATER DRAINAGEHydraulic Design (continue) • Friction slope  Pipe slope • Allow 150 mm freeboard for USWL & DSWL • USWL - DSWL  Losses • Losses = Friction + Pit energy losses • Calculate pipe size to satisfy above condition

7. If we completely fill in the floodplain and develop every bit of space, this is what we get - Lincoln Creek in Milwaukee

8. Channel enlarged because there is no flood plain

13. A solution - picks up 96% of solids. Cost effective

14. after before

15. Total annual precipitation In WI = approx. 30 in no connection no reduction b a a/b

16. Example of Roof Runoff Into Trench (CT)

17. Parking Lot Infiltration Systems (CT)

18. Grass Swales (Waukesha County)

19. Rain Island (MD)

21. Red Cross Headquarters In Madison

23. Natural Wetland Detention Design

28. Hydrology • Meteorology • Study of the atmosphere including weather and climate • Surface water hydrology • Flow and occurrence of water on the surface of the earth • Hydrogeology • Flow and occurrence of ground water

29. Engineering Uses of Surface Water Hydrology • Average events (average annual rainfall, evaporation, infiltration...) • Expected average performance of a system • Potential water supply using reservoirs • Frequent extreme events (10 year flood, 10 year low flow) • Levees • Wastewater dilution • Rare extreme events (100 to PMF) • Dam failure • Power plant flooding Probable maximum flood

30. Flood Design Techniques • Use stream flow records • Limited data • Can be used for high probability events • Use precipitation records • Use rain gauges rather than stream gauges • Determine flood magnitude based on precipitation, runoff, streamflow • Create a synthetic storm • Based on record of storms

31. Sources of Data • Stream flows • tamab • Precipitation • tamab • National Weather Service • Global extreme events

32. Forecasting Stream Flows • Natural processes - not easily predicted in a deterministic way • We cannot predict the monthly stream flow • We will use probability distributions instead of predictions 10 year daily average Seasonal trend with large variation

33. Choice of Return Periods: RISK!!! • How do you choose an acceptable risk? • Crops • Parking lot • Water treatment plant • Nuclear power plant • Large dam • What about long term changes? • Global climate change • Development in the watershed • Construction of Levees Potential harm Acceptable risk

34. Design Flood Exceedance • Example: what is the probability that a 100 year design flood is exceeded at least once in a 50-year project life (small dam design) • =______________________ Not (safe for 50 years) (p = probability of exceedance in one year) probability of safe performance for one year probability of safe performance for two years probability of safe performance for n years probability of exceedance in n years probability that 100 year flood exceeded at least once in 50 years

35. 10 year flood Empirical Estimation of 10 Year Flood Annual Peak Flow Record • Sort annual max discharge in decreasing order • Plot vs. Where N is the number of years in the record How often was data collected? 2 year flood

36. Extreme Events • Suppose we can only accept a 1% chance of failure due to flooding in a 50 year project life. What is the return period for the design flood? • Given 50 year project life, 1% chance of failure requires the probability of exceedance to be _____ in one year • Extreme event! Return period of _____ years! 0.02% 5000

37. Extreme Events • Low probability of failure requires the probability of failure in one year to be very very low • The design event has most likely not occurred in the historic record • Nuclear power plant on bank of river • Designed for flood with 100,000 year return period, but have observations for 100 years

38. Quantifying Extreme Events • Use stream flow records to describe distribution including skewness and then extrapolate • Adjust gage station flows to project site based on watershed area • Use similar adjacent watersheds if stream flow data is unavailable for the project stream • Use rainfall data and apply a model to estimate stream flow • Use local rain gage data • Use global maximum precipitation • Estimate probable maximum precipitation for the site

39. Flood Design Process • Create a synthetic storm • Estimate the infiltration, depression storage, and runoff • Estimate the stream flow We need models!

40. Methods to Predict Runoff • Scientific (dynamic) hydrology • Based on physical principles • Mechanistic description • Difficult given all the local details • Engineering (empirical) hydrology • “Rational formula” • Soil-cover complex method • Many others

41. Engineering (Empirical) Hydrology • Based on observations and experience • Overall description without attempt to describe details • Mostly concerned with various methods of estimating or predicting precipitation and streamflow

42. p. 359 in Chin “Rational Formula” • Qp = CiA • QP = peak runoff • C is a dimensionless coefficient • C=f(land use, slope) • http://ceeserver.Cee.Cornell.Edu/mw24/cee332/scs_cn/runoff_coefficients.Htm • i = rainfall intensity [L/T] • A = drainage area [L2]

43. “Rational Formula” - Method to Choose Rainfall Intensity • Intensity = f(storm duration) • Expectation of stream flow vs. Time during storm of constant intensity Q Qp Outflow point t Watershed divide tc

44. “Rational Formula” - Time of Concentration (Tc) • Time required (after start of rainfall event) for most distant point in basin to begin contributing runoff to basin outlet • Tc affects the shape of the outflow hydrograph (flow record as a function of time)

45. Time of Concentration (Tc): Kirpich • Tc = time of concentration [min] • L = “stream” or “flow path” length [ft] • h = elevation difference between basin ends [ft] Watch those units!

46. Time of Concentration (Tc): Hatheway • Tc = time of concentration [min] • L = “stream” or “flow path” length [ft] • S = mean slope of the basin • N = Manning’s roughness coefficient (0.02 smooth to 0.8 grass overland)

47. “Rational Formula” - Review • Estimate tc • Pick duration of storm = tc • Estimate point rainfall intensity based on synthetic storm • Convert point rainfall intensity to average area intensity • Estimate runoff coefficient based on land use Why is this the max flow?

48. “Rational Formula” - Fall Creek 10 Year Storm • C ­ 0.25 (moderately steep, grass-covered clayey soils, some development) • Qp = CiA • QP = 7300 ft3/s (200 m3/s) • Empirical 10 year flood is approximately 150 m3/s Runoff Coefficients

49. “Rational Method” Limitations • Reasonable for small watersheds • The runoff coefficient is not constant during a storm • No ability to predict flow as a function of time (only peak flow) • Only applicable for storms with duration longer than the time of concentration < 80 ha

50. Flood Design Process (Review) • Create a synthetic storm • Estimate infiltration and runoff • Soil-cover complex • Estimate the streamflow • “Rational method” • Hydrographs