Hydrologic Analysis of a Watershed-Scale Rainwater Harvesting Program. Thomas Walsh, MS, PhD Candidate University of Utah

Transcription

1 Hydrologic Analysis of a -Scale Rainwater Harvesting Program Thomas Walsh, MS, PhD Candidate University of Utah

2 1. Hydrologic analysis of watershed-scale RWH networks targeting stormwater runoff volumes, rates, and recurrence of events Long-term Annual 2. Quantification of watershed-scale impacts 3. Normalization of benefits to the net present costs (NPC)

3 Hypotheses 1. As RWH capacity increases, runoff reductions increase (e.g. volume, peak, and average flow rates) 2. are dependent on watershed conditions and LID application (i.e. extent and configuration), with maximum benefits from full allocation 3. Stormwater management networks comprised of RWH alone can not provide significant reductions in stormwater runoff (i.e. volume or rate)

4 Precipitation Depth (in) Evaporation (in), San Diego, CA Avg Precipitation Avg Evaporation Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec th Percentile: 1.65 cm Average Annual Precipitation: 25.9 cm Drainage Area: 30.7-km 2 Hydrologic Soils Group: C and D Total Impervious: 50% Direct Connect Impervious: 38%-46% Housing Density: 5,400 ppsk Flow Regime: Intermittent Schools 5% Freeway and Highway 5% Commercial/Ind ustrial facilities 7% Open Space 7% Cemeteries 2% Road 22% Miscellaneous 6% Residential 48%

5 GIS data (City of San Diego, U.S. Census Bureau, San Diego County) Drainage network Parameters Land Use Climate data (NCDC, NOAA) Precipitation Hourly, , San Diego Lindbergh Field SCS Type I, 24-hour duration (1-yr, 2-yr, 5-yr, 10-yr, 25-yr) Evaporation Monthly Averages Infiltration Green-Ampt Limited Flow All streamflow attributed to urban runoff

6 SWMM Calibration Iterative approach with 7 events Annual water quality and urban runoff monitoring reports (Weston Solutions, Inc. 2007; 2010; 2010a) Average of 3.1% difference for peak flow rates Validation Iterative approach with 7 events Monitoring and Modeling of Chollas, Palleta, and Switzer Creeks, Technical Report 513 (Schiff and Carter 2007) 5% - 20% difference in volume and outflow rates for all events from yr, 24-hr design storm simulated within 5.8% difference Discrepancies attributed to watershed characteristics and temporal variations of rainfall patterns Model was deemed acceptable for application of RWH scenarios and in meeting goals and objectives of watershedscale study

7 RWH Nominal sizes: 227 liters 7,571 liters Allocation Households ,817 7,571 % # Liters Liters Liters Liters Liters 25 5,197 1E6 5E6 19E6 76E6 39E ,394 2E6 9E6 38E6 151E6 79E ,590 4E6 14E6 57E6 227E6 118E ,787 5E6 19E6 75E6 302E6 157E6

8 Rain Barrel Drain Equation q = outflow (in/hr) h = height of stored water (in) H d = drain height (in) C = underdrain coefficient n = underdrain exponent (0.5 typical of orifice) Estimating the Discharge Coefficient D = depth of stored water (in) T = time to drain (hr) Drain Delay: 24- and 48-hours of dry conditions q h - H d South Coast Irrigation: Average of 22,220 63,141 liters/month (i.e. Dec July) or liters/hour Discharge Coefficient: Varied to represent 24- and 48-hour discharge rates

9 Average Outflow Rate Reduction Peak Outflow Rate Reduction WY % 14% 12% 10% 8% 6% 4% 2% y = x R² = y = x R² = % 0% 2% 4% 6% 8% 10% 12% 14% Outflow Volume Reduction change in Avg Flow change in Max Flow Linear (change in Avg Flow) Linear (change in Max Flow) 8% 7% 6% 5% 4% 3% 2% 1% Capacity = Volumetric Reductions = Flow Rate Dampening

10 year Moving Average Annual Outflow Volume Reduction WY % 12.5% 12.0% 11.5% 11.0% 10.5% 10.0% 50 per. Mov. Avg. (7,571) 50 per. Mov. Avg. (1,817) 50 per. Mov. Avg. (908) 50 per. Mov. Avg. (454) 50 per. Mov. Avg. (227) Water Year

11 Cunnane s Method T N 1 2a M a Exceedences per Year E 1 T Where: N: record (years); M: event magnitude rank; a: constant (0.4); T: event frequency (years); E: event exceedence (1/years)

12 Peak Discharge (CFS) Base 227 Liters 454 Liters 908 Liters 1,817 Liters 7,571 Liters Exceedence Probability (%)

13 Percent Reduction in Peak Flow Rate Recurrence Events 18% 16% 14% 12% 10% 8% 6% 4% 2% 0% T (yr) 224 Liters 454 Liters 908 Liters 1,817 Liters 7,571 Liters Scenario Q 0.5 Reduction Q 1 Reduction Q 2 Reduction Q 5 Reduction Q 10 Reduction Liters CFS % CFS % CFS % CFS % CFS % N/A 789 N/A 1,026 N/A 1,508 N/A 1,723 N/A % % % 1, % 1, % % % % 1, % 1, % % % % 1, % 1, % 1, % % % 1, % 1, % 7, % % % 1, % 1, %

14 Stormwater runoff reductions increase linearly with capacity Target Variable Volume Q peak Q avg 2.5% % Outflow Volume 0.4% - 1.9% Peak Outflow Rate 2.6% % Average Outflow Rate Total, Long Term Annual Average for 227 liters 454 liters 908 liters 1,817 liters 7,571 liters Total 11.0% 11.2% 11.6% 12.1% 12.8% Wet 10.1% 10.3% 10.7% 11.2% 12.2% Dry 11.6% 11.8% 12.2% 12.7% 13.2% Total 10.9% 11.2% 11.8% 12.5% 13.9% Wet 8.6% 8.8% 9.3% 10.1% 12.3% Dry 12.5% 12.8% 13.4% 14.2% 15.0% Total 11.0% 11.2% 11.6% 12.1% 12.8% Wet 10.1% 10.3% 10.7% 11.2% 12.2% Dry 11.6% 11.8% 12.2% 12.7% 13.2%

15 Drain Delay and Duration Increasing drain delay Lengthens detention times; however Negligible increases in the ratio of overflow to underdrain flow Statistically insignificant increases in Subcatchment runoff coefficients Total watershed outflow Extending drain duration Increased RWH overflow, subcatchment runoff coefficients Ultimately leading to greater watershed outflow, flood potential, and limitations for future available capacity

16 Reduction per Dollar (L/$) Full replacement at 50 years accounted for in NPC 8,000 Discount rate of 3.0% and inflation rate of 1.5% (April 2013) Costs increase linearly with volumetric capacity 227-liter is conservatively $100 per barrel 7,571-liter is roughly $1,000 per cistern 227-liter 7,000 6,000 5,000 4,000 3,000 2,000 1, liter 908-liter 1,817-liter 7,571-liter 0 25% 50% 75% 100% Percent of Implementation throughout

17 With a watershed-scale RWH network in Reductions in runoff volumes and rates increase with available capacity Maximum average annual reductions range between % for runoff volume % for runoff peak flow rate Wet year reductions average lower than Dry years Drain delay does not significantly impact the overall reduction benefits for watershed-scale RWH Extending drain duration, via the governing outflow parameters, increases watershed outflow and reduces the long term available capacity of the RWH unit Transfer of O&M responsibilities may not be as problematic as traditionally believed given outflow durations are managed up front with recommended orifice sizes

18 Greatest overall reduction is the 7,571-liter RWH cistern at 100% watershed extent NPC: $30.8 million 763 liters/$ (overall) Greatest normalized reduction (per invested dollar) is the single 227-liter RWH unit at 50% watershed extent NPC: $1.5 million 6,700 liters/$ (overall) 110 liters/$/yr (average) 5:1 (overflow:underdrain) 13.7% Reduction in Runoff Volume 5.5% Reduction in Runoff Volume

19 RWH alone is not capable of providing volumetric reductions on the scale of traditional stormwater management practices; however, RWH, and other LIDs, may be able to increase the effectiveness of existing stormwater management within the watershed Future Expanding this study to include other LID networks that are representative of the underlying land cover and use to design sustainable stormwater reductions for urban watersheds, employing

20 Contact Information and Questions Thomas Walsh:

21 SCS Design Storm Pervious Area Impervious Area Overflow YES % of Impervious to LID Underdrain Drain to Pervious? Y/N NO