Watershed Hydrology & Drainage and Rainwater Harvesting. David Watkins CE 5993

Transcription

1 Watershed Hydrology & Drainage and Rainwater Harvesting David Watkins CE 5993

2

3 Components of Streamflow Direct runoff (over land) Interflow (through near surface soil layers) Baseflow (from groundwater) Precipitation onto channel Small contribution

4 Baseflow

5 Measuring Streamflow Manual approaches Hand-held meters Orange peel? Float gage, bubbler Requires rating curve developed through another method Acoustic (Doppler) methods

6 Handheld Velocity Meter Method Measure velocity and depth at different points along a cross section. W i D i V i Q V A V W D i i i i i i i

7 Water Budget Analysis Surface Storage S = P I ET Q where S = change in surface storage (amount of ponded water) P = precipitation I = infiltration ET = evapotranspiration ti (which h may also include interception of rainfall by plants) Q = runoff

8 Runoff Water Budget Analysis Q P S , S 25.4 P 0.8S CN where P = precipitation (mm) S = potential abstraction (mm) CN is the curve number, which depends on land cover, hydrologic condition, and soil classification

9 Variables in the SCS method of rainfall abstractions; I a = initial abstraction, P e = rainfall excess, F a = continuing abstraction, and P = total rainfall. Water Resource Engineering, 2005 Edition by Larry W. Mays Copyright 2005 by John Wiley & Sons, Inc. All rights reserved.

10 Solution of the SCS runoff equations (from U.S. Department of Agriculture Soil Conservation Service (1972)). Water Resource Engineering, 2005 Edition by Larry W. Mays Copyright 2005 by John Wiley & Sons, Inc. All rights reserved.

11 Water Budget Analysis Evapotranspiration PET * e a Ta 29.8D T where D is the day length (hrs) and e a* (T a ) is the saturation vapor pressure (kpa) at the mean daily temperature, T a ( C) C). If P i PET i, then ET i = PET i If P i < PET i, then i i 1 PET P i fc ET i = P 1 exp i i i 1 a

12 Water Budget Analysis Recharge Ri Ii ETi Dw i where, 1 0 I i = infiltration in period i (mm) ET i = evapotranspiration in period i (mm) D w,i-1 is the soil water depletion at end of period i-1 (mm). The soil water depletion represents the depth of water that is needed to fill the soil to field capacity, θ fc : D where θ i is the soil moisture and d r is the depth of the root zone. d wi, fc i r The water budget equation allows recharge to occur only if the soil moisture in the root zone is above field capacity. At the end of each time step, the soil water depletion is updated as: Dwi, Dwi, 1 Ii ETi Ri 0

13 Modeled runoff and recharge for two watersheds in Bolivia (Fry et al., 2010). Run noff (mm) Mapuruchqui IBTA % 50% Run noff (mm) % 20 25% with ag 15 50% with ag 10 75% with ag % 50% 75% 25% with ag 50% with ag 75% with ag Time from present (years) Time from present (years) Recharge (mm m) % 50% 75% 25% with ag Recharge (mm m) % with ag 10 25% 50% 75% 25% with ag 50% with ag % with ag % with ag Time from present (years) Time from present (years) Runoff and recharge were estimated at 30 years and 75 years from present, using the median, 25th and 75th quartile estimates from the regional averages of precipitation responses from 21 multi-model data sets reported by the IPCC. All are modeled using the median temperature response (Solomon et al. 2007).

14 Peak Discharge Analysis Consider one of the simplest models: Rational Method: Q = CiA C = runoff coefficient (dependent on soils, land use) i = design rainfall intensity for a duration equal to tc tc = time of concentration; time for rainfall at the most remote portion of the basin to travel to the outlet A = area of watershed Assumes uniform rainfall over the watershed. Time-Area methods developed to address non-uniform rainfall (larger areas).

15 Example Problem A 10-acre parking lot in Houston, TX is found to have a time of concentration (tc) of 5 minutes. With a runoff coefficient (C) of 0.98, and the rainfall IDF curves given, use the Rational Method to compute the peak outflow corresponding to the 25-year rainfall event.

16 Example Problem Solution: Q = CiA i = 9.0 in/hr A = 10 acres Q = (0.98)(9.0)(10) = 88.2 ft 3 /s Note: 1 acre-ft/hr = ft 3 /s

17 Rainwater Harvesting Relieves demand and reduces reliance on surface and underground sources. Not subject to many pollutants discharged into surface waters Cost effective: reduces water bills and O&M costs are low. Simple yet flexible technology. People can easily be trained to build, operate and maintain the system. Scalable. Rainwater systems are decentralized and independent of topography and geology. Water is delivered directly to the household, relieving women and children from the burden of carrying it, saving time and energy. Can be used for agricultural purposes.

18 Project Planning Successful rainwater harvesting projects are generally associated with communities that t consider water supply a priority (Gould, 1999). It may not be a perceived as an immediate need, or there may be other priorities iti depending di on the season. Cultural perceptions and religious views regarding the use of water, as well as traditional preferences for its taste, smell or color should also be taken into consideration.

19 300 mm rainy season 120 liters per day for 200 days 80 m 2 roof 24,000 liter storage (Assumes water captured in a distinct rainy season for use in a distinct dry season.)

20 Rainwater Harvesting Analysis Runoff Calculation: Q = CPA C = runoff coefficient (dependent on roof material) P = rainfall depth A = collection area Storage Calculation: S t = S t-1 + Q t D t S t = storage at end of period t (S t < K = storage capacity) Q t = runoff collected in period t D t = water use (demand) in period t

21 Collection Efficiency The recommended values of are often used for the runoff coefficient; however, it may be as high as 0.9 or as low as 0.24 depending on the surface material, and other factors which may occasionally reduce the efficiency (Gould, 1999). A smooth, clean, impervious surface yields better water quality and greater quantity (TWDB, 2005). The coefficient value is also a measure of the performance of the gutters and downspouts, as this is where most of the system losses tend to occur (Gould, 1999).

22 Roof Material Organic: Straw, Grass, Palm Leaves, Attracts rodents and insects Bamboo, Mud, Clay, Slate, Yields contamination Thatch h Adds color to water Wood : Shingles Not for potable uses if chemically treated Concrete/ Masonry: Cement, Concrete, Tiles colored tiles will oxidize and color to the water may require non-toxic coating/liner Other: Asphalt, Asbestos Asphalt contributes grit Asphalt requires pre-filtering of water Fibro-cement Asbestos fibers are dangerous to health, Fiberglass Shingles especially when inhaled during Plastic Liner / Sheet handling/construction Cloth Not a known risk in drinking water Not recommended for use Plastic liner may degrade in high temperatures Paints and Lead-based Paint Lead is toxic to health Coatings: (or paints with lead or zinc) Triggered by acidity Acrylic Paint Will leach dissolved chemicals including Bitumen-based materials (tar) detergents in first few run-offs. May add unpleasant taste to water Not for potable uses Metals: Iron, Tin, Lead Based metals Galvanized roofing is a source of zinc Galvanized Steel Iron may rust and leach into tank, but is not considered a significant health hazard.

23 Roof System with no Gutters

24 Use of a Glide as a Gutter

25 Gutters

26 Roof Washing (First Flush) This second pipe pp should have a volume that corresponds to the 10 gallons per 1000 square feet guideline. Or clean out manually (can use first flush for irrigation) i

27 Tank Components Inlet Access hatch Overflow pipe pp Tap Clean out pipe

28 Ferrocement and Plastic Tanks

29 Tank Materials MATERIAL ADVANTAGES DISADVANTAGES CONCRETE/MASONRY Durable, long lasting Subject to cracks and Above / below ground leaks Monolithic/Poured-in-Place Can decrease corrosiveness Subject to underground (Reinforced Concrete) of rainwater (allows stresses (especially in dissolution of calcium clay soils) carbonate from walls and Not portable / heavy floors) Concrete Block Durable Difficult to maintain Not portable / heavy Portable May require more Ferro-cement (steel mortar composite) Stone PLASTICS Garbage Cans (20-50 gallon) Fiberglass Polyethylene/ Polypropylene Liners, other plastics Durable maintenance Flexible in design Subject to cracks and Easy repairs leaks Durable Difficult to maintain Keeps water cool Not portable / heavy Inexpensive Use only new cans Alterable Degradable Moveable / portable Needs interior coating Sensitive to sunlight Above/below ground Degradable Alterable (can have many Not suitable for outdoor openings) use (requires exterior Moveable / portable coating /enclosure /UV Long life expectancy inhibitor) (25yrs) Must be FDA approved More durable than fiberglass Smooth interior, easy to clean Large storage capacity Used to line concrete tanks Needs support Used to repair leaks May be sensitive to light METALS Steel Drums (55 gallon) / Galvanized Steel Tanks WOOD Durable Lightweight Portable Alterable Subject to corrosion and rust May require liner/coating Verify prior use for toxics Fluctuating ph may release zinc May contain lead

30 Protecting Water Quality Sunlight should not be permitted to enter the tank as this will cause algae to grow, which in turn can feed other micro organisms in the tank. Flood levels higher than the entry point of RWH tank entrances can contaminate the stored water. This is especially a concern when cisterns are used to store the water.

31 Rainwater Harvesting Analysis Runoff Calculation: Q = CPA C = runoff coefficient (dependent on roof material) P = rainfall depth A = collection area Storage Calculation: S t = S t-1 + Q t D t S t = storage at end of period t (S t < K = storage capacity) Q t = runoff collected in period t D t = water use (demand) in period t

32 Example: Falelima, Samoa (Tim Martin, 2008)

33 Example: Falelima, Samoa

34 Example: Falelima, Samoa

35 Traditional Water Harvesting Storage ponds Methods Either to store surface water for direct use, or to enhance ground water recharge Underground tanks/cisterns May be accompanied by landscape modification (e.g., berms, bunds, channels to direct runoff into storage)

36 Underground/Sand Dams (Kankam-Yeboah et al., 2003) (

37 References Gould, J., and E. Nissen-Peterson (1999). Rainwater Catchment Systems. UK: Intermediate t Technology Publications. Martin, T. (2009). An Analysis of Household Rainwater Harvesting Systems in Falelima, Samoa. M.S. Report, Civil & Environmental Engineering, g, Michigan Technological University, Houghton, MI. McCuen, R.H. (2004). Hydrologic Analysis and Design, 3 rd edition, Prentice Hall. Texas Water Development Board (2005). Rainwater Harvesting Manual, 3 rd edition. tx on.pdf. (accessed 1 April 2012.) Kankam-Yeboah, K., S. Dapaah-Siakwan, M. Nishigaki, and M. Komatsu (2003). The Hydrogeological Setting of Ghana and the Potential for Underground Dams,. Okayama University, it 8(1), Journal of fthe Faculty of fenvironmental lscience and Technology