Locating and Planning Cistern Installations for Stormwater Management Downspout Screened inlet Overflow Drain to garden hose For more information, see the Cistern factsheet and other resources at www.seattle.gov/util/rainwise
Seattle Public Utilities8 Is this a good site for a cistern? A cistern installation needs: A level location near a downspout, outside (not inside building) A solid base (6” of packed 3/8 crushed rock) To allow ingress and egress for people Don’t block doors, windows, vents, or utilities accesses A safe place to discharge the overflow, a minimum of: –5 feet from your home (with no basement) –10 feet from your home, if you have a basement –5 feet from a property line, and 10 feet from neighboring buildings –or connect overflow to sewer (requires side sewer permit – call DPD at # on RainWise factsheet)
Example Cistern Layout Follow manufacturer’s instructions! Gutter (ideally with leaf screens) Downspout Self-cleaning leaf filter Screened inlet Finer screens keep more dirt out. Overflow – 3-4 inch pipe, with ”P” trap Cistern – typically heavy plastic or fiberglass Disperse overflow into landscape A minimum of: - 5 ft. from house, or 10 ft. with basement - 5 ft. from property line, - 10 ft. from neighboring buildings Level foundation – 6 inches of packed 3/8” crushed rock, or concrete block, over packed subsoil Drain – 3/4 inch pipe with faucet Connect garden hose to drain to yard in winter. Close faucet in May to store water for summer. screen Cleanout plug (3 to 4 inch diameter) Clean tank annually by squirting hose into inlet and cleanout, and scrubbing with long- handled brush.
Seattle Public Utilities11 What makes a cistern work for storm water detention? The “low-flow orifice” In this case, a faucet that’s left open October-May, to allow tank to drain out between storm events, so there’s space to store and slowly release the next rainfall. Cisterns must have a ¼-inch orifice plumbed to the landscape or sewer. For non-rebate homeowner projects, a faucet & garden hose works fine. If the cistern is full when it rains hard, it provides no stormwater benefit – it must be able to drain between storms.
Low-flow orifice plumbing for Code-permitted cisterns (example from Lakewood project) Seattle Public Utilities12
Modeling % Reduction for Rebate Value Seattle Public Utilities14
15 Step 1: Build a level foundation to support a cistern full of water Excavate topsoil at least 3 inches: create hard a level surface – don’t place on un-compacted fill Pack subsoil with hand tamper or mechanical compactor Place a level foundation: –Ground-contact-treated lumber box, filled with 6 inches of 3/8 crushed rock, well compacted. May top with 1 inch of sand or fine gravel to get smooth level surface under cistern. –Concrete blocks or heavy (min. 3-inch thick) pavers – perfectly level on top –Existing concrete pad, if level. (Installing a new concrete pad would require a building permit.)
Seattle Public Utilities18 Step 2: Place cistern tank Don’t block opening of doors & windows, emergency egress, vents, utilities access, etc. Plan for downspout flow to cistern, and overflow routing Consider appearance and consult neighbors Follow manufacturer’s instructions If the tank is taller than it is wide, secure for earthquake safety (usually impractical, so best not to use tall, narrow tanks) Downspout Screened inlet Overflow Drain to garden hose
Seattle Public Utilities19 Connect additional tanks, if used
Seattle Public Utilities20 Step 3: Install screened inlet, to keep debris and mosquitoes out At minimum, wrap and secure aluminum screen over inlet opening Additional protection against clogging: –Gutter screens and wire cages in gutter outlets to exclude leaves –Self-cleaning leaf excluder in downspout run to cistern – commercially available or home- assembled – Google “Downspout filters, screens” –Divert dirtier initial flow after dry period: Google “First flush diverters” “Roof washers” or “Cistern Installation” for ideas
Seattle Public Utilities24 Step 4: Make gutter connections 3-4” Aluminum, ABS, or PVC NDS pipe Secure all connections with stainless steel screws & silicone seal, or glue Strap & support as needed May put a self-cleaning leaf excluder in line: typically a sloped screen so leaves are pushed aside Arrange so water falls into screened inlet, with access to clean screen
Seattle Public Utilities25 Step 5: Install overflow pipe that’s as big as the inlet pipe Watertight bulkhead fitting at top, or internal overflow riser, or both (if less that 3 inch) Must be big enough to carry full gutter flow once cistern fills – Install “P” trap somewhere, to prevent mosquito and rat entry
Seattle Public Utilities26 Step 6: Extend overflow pipe to a safe discharge point 5’ from house or 10’ if basement 5’ from property line, 10’ from neighboring buildings Rubber “hubless” unions protect against breakage, allow maintenance Use rocks or gravel to prevent erosion and disperse overflow into landscape If reconnected to sewer, requires a permit – call DPD
Seattle Public Utilities29 Consider appearance Can screen cistern with fencing, latticework, cedar or bamboo wrap, etc. Plastic paint works on ABS & PVC pipe Blend with existing architectural finishes Consult with neighbors South Seattle Community College cistern overflowing to rain garden in a downpour, Nov. 2009
Seattle Public Utilities38 Step 7: Install drain valve for use as “low-flow orifice” Use oversize (1-3”) bulkhead fitting, so entire fitting can be removed for cleaning tank. Or install separate cleanout plug. “Hose bib” garden hose faucet Connect garden hose, and run it to safe discharge point to drain cistern slowly between storms Close faucet in May to store water for summer. Open again in October to detain and slowly drain winter storm water
Questions? Guide, factsheets, and design tools: www.seattle.gov/util/RainWise www.seattle.gov/util/RainWise RainWise Tools www.rainwise.seattle.gov Bob Spencer David McDonald Seattle Public Utilities