Let Them Count An Argument for Inclusion of the Impact of. Household Rainwater Tanks When Designing an Urban Drainage. Network

Transcription

1 Let Them Count An Argument for Inclusion of the Impact of Household Rainwater Tanks When Designing an Urban Drainage Network L.Strauch* 1 *Presenting Author Household rainwater tanks are currently an optional extra' for new homebuilders and existing homeowners. However, the consistent use of rainwater tanks by a critical mass of homeowners has the potential to significantly reduce the total volume of stormwater leaving an urban catchment, lower peak flows in storm events and hence potentially reduce the required capacity of the local drainage networks. This paper presents a small hypothetical residential catchment of approximately 100 lots and demonstrates how the site-specific design and mandatory use of rainwater tanks and overflow storage on individual allotments has the potential to halve peaks in the 20% and 1% AEP events, reduce the required diameter of pipes in the minor drainage network, achieve 75% of the current Best Practice target for the removal of Total Nitrogen (reducing the size of downstream water quality treatment assets), supply homeowners with an alternative water supply in the order of 50kL per year, and reduce the total annual volume of stormwater leaving the catchment by one third. At present, 1 Associate - Water, Spiire, PO Box Melbourne VIC 8007, leah.strauch@spiire.com.au 1

2 the impact of rainwater tanks is not considered in the calculation of storm flows or drainage capacity in Victoria. Understanding these results and their ability to be replicated will allow for a stronger argument for authorities to accept the inclusion of tanks in drainage calculations, and developers and asset owners will be able to derive benefit in the form of reduced asset sizes and are more likely to promote this scenario. I. Introduction Household rainwater tanks are currently an optional extra' for new homebuilders and existing homeowners. Size and end use of the tanks are not controlled or inspected for ongoing performance. However, the consistent use of rainwater tanks by a critical mass of homeowners has the potential to significantly reduce the total volume of stormwater leaving an urban catchment, lower peak flows in storm events and hence potentially reduce the required capacity of the local drainage networks. At present, the impact of rainwater tanks is not considered in the calculation of storm flows or drainage capacity in Victoria. Where their potential performance can be better understood, calculated and mandated, authorities are more likely to accept their inclusion in drainage calculations, and developers and asset owners will be able to derive benefit in the form of reduced asset sizes and are more likely to promote this scenario. Advancing this approach can ultimately produce benefits for: Homeowners - lower water bills Retailers and water authorities - reduced asset sizes (networks and retardation requirements) Developers and Councils - less pressure on drainage networks, reduced asset sizes for maintenance and land use 2

3 Waterways - less urban stormwater entering waterways (lower volume, frequency, pollutants and velocity), when the balance of control, maintenance, cost and confidence can be fully understood. II. Objectives The objective of this theoretical exercise is to allow comment on: Potential reductions in downstream drainage infrastructure when detention storage (and reuse) in household rainwater tanks is included in flow rate and velocity calculations Potential savings in potable water through the use of rainwater tanks for annually consistent demands (for example, toilet flushing rather than garden watering) Potential reductions in catchment-scale stormwater treatment assets due to the removal of pollutants at the allotment level Potential reductions in retardation assets where urban stormwater flows need to be reduced to predevelopment levels. III. Method The impact of this approach on a single catchment is calculated by: Estimating the fraction impervious of the typical lot and road reserve in the catchment Estimating pre-development peak flows and volumes for individual roof areas, and the full catchment Establishing household demand for harvested rainwater and modelling the reliability of supply Determining required storage and outflows to restrict flows from roofs to pre-development levels Calculating the equivalent fraction impervious of the catchment with rainwater tanks in operation Sizing a drainage network for the catchment with and without tanks Sizing a water quality treatment asset to remove target pollutants to Best Practice levels for the catchment with and without tanks 3

4 Sizing a retardation asset to reduce 1% AEP peak flows to pre-development for the catchment with and without tanks Quantifying the differences between the two scenarios in terms of simple costs and annual stormwater volumes. The scenarios are noted as Scenario A, traditional drainage approach, and Scenario B, individual rainwater collection and retardation. For this exercise, a small 7.6ha urban layout containing 104 individual lots was selected. Lots ranged in size from 350m 2 to 816m 2, with an average of 555m 2. This slice of the suburbs is hypothetically located in Melbourne s Western Growth Corridor. In order to apply a realistic impervious fraction to calculations, area measurements were taken on aerial photography of similar lots sizes with established houses, gardens and paved areas. The final typical lot was estimated to have an area of 530m 2, a roof of 250m 2, and 65m 2 of further impervious surfaces. The proposal is to either capture or retard flows off an individual roof to the level generated by a pervious catchment. The adopted fraction impervious is therefore: Scenario A 59% Scenario B 12% The typical 16m road reserve had a fraction impervious of 68%. Rainwater tanks are assumed to supply toilets, laundry and individual gardens. Indoor demand is estimated as a uniform 175L/hh/day based on a household of 3. Garden demand is estimated as 36kL annually, distributed across September to April. MUSIC has been used to model the generation and harvesting of stormwater. The Melbourne Airport station has been used for rainfall data, and the decade from modelled. A range of tank sizes from 2kL to 20kL is modelled at a 6 minute time-step to determine reliability of supply. Tanks are assumed to be a charged system and connected to the entire roof. 4

5 IFD polynomial coefficients have been generated for a location in the Western Growth Corridor. The rational method with Adams estimation for time of concentration has been used to calculate the expected rural/predevelopment peak flows. The rational method has also been used to estimate the size of the local drainage network, using expected velocities to determine time of concentration, for Scenarios A and B. The Swinburne Tech Method has been used to identify the maximum storage required either at an individual house to permit a maximum outflow of pre-development levels for the 20% and 1% AEP events, and also to establish the total end of line storage required to restrict outflows from the catchment to pre-development levels for the 1% AEP event in both Scenario A and B. The alternative system (Scenario B) is expected to function as follows: Rainfall hits an individual roof, and generated runoff is directed to a storage tank. When the nominated capacity of the tank is exceeded, retardation storage is engaged, and the outflow from the tank is restricted to pre-development 20% AEP flows. Where an event larger than the 20% AEP occurs, a second outlet will be engaged, allowing the equivalent of the 1% AEP pre-development flow to discharge. Stored water in the tank is used for toilet, laundry and garden watering. When insufficient water is available, either potable or recycled Class A water will be used to make up the deficit, depending on the location. The major and minor drainage networks are directed to a single point to allow for an individual stormwater quality treatment asset and a retardation basin to achieve Best Practice pollutant removal targets and restrict flows out of the total catchment to pre-development 1% AEP levels. 5

6 IV. Results When comparing possible sizes for rainwater tanks on individual lots, the following results were achieved: Nominal tank size To make use of the consistent demands on the rainwater tanks, the minimum available volume within the nominal tank volume at least 95% of the time over ten years was calculated, and assumed to be available in a rain event. This reduces the volume of extra overflow storage required to maintain pre-development flow levels. The 5kL tank is selected as Scenario B. It is expected that tanks will be located in dead space (ie along the side wall of a house), so the difference in size between the 9.15kL storage and the 6.80kL storage should not be a concern, and supplies an extra 10kL to each household per year. The selected 5kL reuse scenario provides a 50% reliability of supply, or approximately 50kL of rainwater to each household per year. The total household demand is expected to be approximately 179kL per year, which means that the introduction of the tank is able to reduce each household s demand on the reticulated supply by around 30%. Depending on the make-up of the catchment, it was determined that introducing the Scenario B rainwater tanks reduced the 20% and 1% AEP peak flows by anywhere between 30-50%. Once the contributing catchment exceeds approximately 1ha, this reduction in peak flow manifests in a reduction in the required size of the pipe diameters in the minor drainage network. Reliability of supply 95% available volume Extra storage for 20% AEP Extra storage for 1% AEP Total volume 2kL 39% 0.3kL 3.20kL 4.80kL 6.80kL 5kL 50% 0.95kL 2.55kL 4.15kL 9.15kL 10kL 56% 3.20kL 0.30kL 1.90kL 11.90kL 15kL 59% 4.50kL kL 15.60kL 20kL 62% 5.40kL kL Table 1. Tank sizing results. Indicating both the nominal tank size (available for reuse) and total tank volume required to reduce peak flows. tank 6

7 For the catchment in this exercise, around 25% of the drainage network was able to be reduced, and the reductions occurred in the larger diameter pipes. Generally reductions were in the order of one standard size smaller (ie 525mm diameter reducing to 450mm diameter). The reuse of rainwater at the household level removes both water and pollutants from the drainage network. The use of rainwater tanks on every lot in the catchment was found to reduce Total Nitrogen leaving the catchment by 33%, or 75% of the Best Practice pollutant removal requirements. The inclusion of a 200m 2 sedimentation basin at the end of the catchment generated the further required pollutant removal. As a comparison, a 150m 2 rain garden was required in conjunction with a 200m 2 sedimentation basin to achieve Best Practice pollutant removal requirements for Scenario A. In order to restrict stormwater flows from the entire catchment to the pre-development 1% AEP peak, a storage volume of 1888m 3 is required for Scenario A compared to 687m 3 for Scenario B. A simple estimate of relative costs is presented to facilitate comparison of the two scenarios. Maintenance and general operating costs have not been considered at this point. A B Rainwater tanks $ - $572, Pipe drainage1 $ 27, $ - Reticulated water2 $340, $ - Sedimentation basin $ 50, $ 50, Rain garden $ 60, $ - Retardation basin3 $ 37, $ 13, Cost of land (WQ and basin) $ 64, $ 27, Total $580, $662, Table 2. Indicative cost of Scenarios A and B. I 1. Relative additional cost of piped drainage in the traditional network 2. Relative additional cost of reticulated water over 25 year life of tank 3. Estimate of earthworks only. 7

8 I. Conclusions This exercise examined a single residential catchment, and the results cannot be directly translated to all locations. The key outcomes were that with the introduction of overflow storage on household rainwater tanks, peak flow rates for the major and minor drainage networks were able to be significantly reduced, and the diameter of pipes in the minor drainage network was able to be reduced. A further result of the inclusion of rainwater harvesting and reuse was a significant reduction in the requirement for end of line water quality treatment and retardation (where retardation to pre-development levels is required). A simple cost analysis (not including maintenance savings or distribution) suggests that in its basic form, Scenario B is within 15% of the traditional Scenario A. There are also benefits in terms of use of public land, and total volume of stormwater reaching local waterways. Broader investigations into this style of drainage network should be undertaken to understand applicability across different locations, catchment sizes and types, or varied end uses at the household level. It is heavily reliant on the involvement and engagement of local residents and appropriate use of household tanks. Before drainage authorities can accept this style of drainage network, a level on confidence in ability to control and predict use must be achieved to complement the modelling and mathematics that suggests its positive potential. 8