Rainwater harvesting for stormwater management Proof that it works

Transcription

1 Rainwater harvesting for stormwater management Proof that it works Richard Kellagher, Juan Gutierrez Andres, Aurelie Gerolin 1 (1) Centre d Etudes Techniques de l Equipement HR FWR Wallingford 3/11/

2 Rainwater harvesting - context SuDS Standards Volume control Regulator position Ambivalent on RWH for stormwater control Added value if stormwater control potential can be demonstrated Page 2

3 Speaker Notes pages 2 to 8 Page 4 Speaker Notes Some countries only look at peak reduction with attenuation. Runoff volume reduction is important RWH is important. Specially in areas where infiltration is not an option (UK ¾ of the soils is not suitable for infiltration!) Type of soils, high ground water table /pollution or risk of polluted soils. Nobody internationally has accepted RWH can achieve storm water management. More work is needed. Design by house hold occupancy. Uncertainties: Main is population. You don t know how many occupancy and how will it change in the future. Number of bedroom = population (statistical analysis). De Page 5 Yield: Annual approach in places where annual rainfall is more or less averaged. Other areas you should work more in wet season basis Page 6 Inverse of what they are seeing. Explain the graph Page 7 UK little difference in monthly rainfall Uncertainty of property occupancy Storage AD allowance for stochastic uncertainty f(y/d) Storage CS Coefficients f(y/d) Tank storage = Coef x Tank volume + Ad Ad to address uncertainty of depth f(y/d) Page 3

4 RWH stormwater control potential - Why is rainfall harvesting potentially so useful? - Save water (protect the environment) - Reduction of runoff volume (SuDS Standards) - Only alternative to infiltration which is commonly not possible - Ambivalent position in the UK - BS8515 (RWH Code of Practice) & Code for Sustainable Homes - EA current position: RWH can not reliably provide stormwater benefits - Objectives of the Study - Demonstrate RWH s storm water control potential - Test the proposed sizing methodology (pilot study) - Develop a practical approach to various uncertainties Page 4

5 Sizing methodology key parameter (Y/D) Regular Daily Demand (D) Toilets Washing machines Yield (Y) Annual Rainfall Event losses are important (150+events /yr) Roof Area Page 5

6 Tank Storage availability Storage behaviour f(y/d) must be <0.95 Page 6

7 Allowance depth (mm) Mean absolute error (mm) Storage coefficients Y/D V/N=1 V/N=1.5 V/N=2 V/N= Y/D Greenwich Ringway Elmdon Page 7

8 Mean monthly rainfall (mm) Rainfall exceedence ratio (nth year / mean year) rainfall depth (mm) Uncertainties Cherwell District 2.5 Number of bedrooms in the property Overall Properties sample size Number of occupants Mean occupancy Standard deviation Roof area Drop bars are + /- 1 standard deviation 1 Toilets / washing 0.5 machine use Elmdon, TSRrain, 24 hour storms Oxford Oxford Proportion of record Y/D = 0.7 Y/D = 0.95 DJF JJA 1 TSR rainfall MAM SON T(X) = 1 / P(X) Annual annual spring summer autumn winter 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Page 8

9 Pilot study - Banbury Detailed IWCS model Page 9

10 Speaker Notes pages 9 to 21 Page 9 How many ha? No worries Attenuation pond; no volume control just attenuation control Page 11 Modelling individually everything Page 12 Speaker Notes Stat. Popul. Number of bedroom and occupancy distribution for this type of house; give examples; (31 surveyed Y/D <0.95 communal Y/D was also Population uncertainty desapeir when considering all the people. Page 15 For bands bigger than 60mm Y/D closer to 0.9 are helping to give a good performance; For lower bands are the Y/D < 0.7 which give a good performance; complementing each other. Imperial College-Rainfall; accurately representative based on 15 years of observed data. Dry periods and wet periods. TSR sim was used. Page 16 For bands bigger than 60mm Y/D closer to 0.9 are helping to give a good performance; For lower bands are the Y/D < 0.7 which give a good performance; complementing each other. Imperial College-Rainfall; accurately representative based on 15 years of observed data. Dry periods and wet periods. TSR sim was used. Page 17 7 properties which did not complai Page 10

11 Pilot study -Design Rainfall Depth = 60mm -100 year stochastic rainfall series (17099 events!) - Regular Daily Demand (D) Toilets (21l/c/d) Washing machines (19/l/c/d) - Yield (Y) Annual Rainfall (730mm) Roof Areas x Modelling tool: InfoWorksCS Page 11

12 Pilot study Y/D characteristics Model 1 Model 2 (Communal tank) - Model 3 (Y/D = 0.76) Page 12

13 Model 1- Example of result for a tank Y/D = 0.65 Page 13

14 Model 1- Example of result for a tank Y/D = 0.91 Page 14

15 Statistical population spill performance Page 15

16 Model 2 (Actual population) spill performance Page 16

17 Model 2 (Actual population) with Y/D > of 34 Properties Page 17

18 Model 2 (Actual population) with Y/D < 0.95 Page 18

19 Model 3 (stat. population) Communal tank Page 19

20 Passive system Active or Passive storage management? A proportion of tanks will have insufficient storage for some events The uncertainty allowance is an additional 25 to 40mm Active system (pumping station wet well ON & OFF switches, 3 day timer) Storage for water supply saving + 60mm for event Y/D can be >0.95 Easier to design downstream system Page 20

21 Conclusions 1. Rainwater harvesting can be designed for controlling stormwater to a specific storage rainfall depth 2. Sizing methodology works but an allowance is needed to account for real occupancy. This has been developed. 3. Communal performance removes occupancy uncertainty. Ref: SR736-Developing Stormwater Management using Rainwater Harvesting (HR Wallingford, 2011) Page 21