Millburn, NJ, Residential Area Stormwater Analyses

Transcription

1 Base Conditions Parameter Files: Millburn, NJ, Residential Area Stormwater Analyses R. Pitt March 12,

2 Millburn Typical Medium Density Home: Source Area Area (acres) Description Roofs Pitch roofs, directly connected (to dry well) [3,500ft 2, large homes!) Roofs Flat roof, draining to pervious area (silty soil) Paved parking Directly connected (to dry well) Driveways Directly connected (to dry well) Sidewalks Directly connected Sidewalks Draining to pervious area (silty soil) Streets ft wide, intermediate texture, use model accumulation rates and initial street dirt loading value Small 0.36 Silty soil landscaped areas 1 Other partially connected impervious areas Draining to pervious area (silty soil) Total Area acres Model results for base condition (no dry well): 2

3 Dry Well for Directly Connected Roofs, Paved Parking, and Driveways: Split the residential area to move the areas for the dry well into residential 2 (zero their areas out in residential 1 so the total area still equals acres): 3

4 Dry Well as a Biofilter (but with no vegetation and only native soil infiltration and no engineered soil and only broad crested weir overflow) 4

5 Dry well removed about 80% of the influent water from the directly connected roofs, parking, and driveway stormwater directed to it. The overall site runoff volume was reduced by about 46%. Easy to evaluate if only roof runoff to dry well (move the paved parking and driveway areas back into Residential 1). 5

6 Irrigation Beneficial Uses of Stormwater Maximum Irrigation of Kentucky Bluegrass for Essex County, New Jersey Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average monthly rain (in/mo) Lawn moisture needs (in/mo)* Deficit irrigation need (in/mo) Deficit irrigation needed (gallons/day/house) 0.36 acres to irrigate * this is greater than the minimum ET requirements in an attempt to use as much roof runoff for irrigation without damaging the turf (0.5 inch/week during the dormant season and up to 2 in/week during the hot months) Cistern to be used at Roof 1 (directly connected pitched roofs) Design Property of Cistern Value Top surface area (ft 2 ) 100 Bottom surface area (ft 2 ) 100 Height to overflow (ft) 10 Rock filled depth (ft) 0 Rock fill porosity (0 1) 0 (n/a) Inflow hydrograph peak to average 3.8 (this may be already selected as a default parameter, but flow ratio being redundant won t matter. If this unit was not at a source area, the actual hydrograph shape would be routed and combined from the upland areas). Number of devices in source area or 1 (if more than one, the flow per unit is automatically land use recalculated by the program) Runoff fraction entering device (0 1) 1 (all of the flow enters the cistern, there is no upgradient flow splitter) Therefore 1,000 ft 3 of storage per 3,500 ft 2 of source area; for a storage/source area ratio of

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8 The 0.28 ft 3 /ft 3 storage/area ratio corresponds to a total annual outfall runoff reduction of about 20%, or a roof runoff reduction of about 63%. 8

9 Rain Garden Alternative for Roof Runoff in Area of Shallow Groundwater Roof runoff rain garden characteristics Top area (ft2) 120 Bottom area (ft2) 80 Total depth (ft) 3.0 Total width (ft) (cost ext. 10 only) Native soil infiltration rate 1.0 (in/hr) Infilt. Rate fraction bottom 1.00 (0 1) Infilt. Rate fraction sides 1.00 (0 1) Rock fill depth (ft) 0.00 Rock fill porosity (0 1) 0.00 (n/a) Engineered media type Sandy loam (1 in/hr) Engineered media depth 2.00 Engineered media porosity 0.4 Inflow hydrograph peak to average flow ratio 3.8 (again, the default, but will actually use the routed and combined hydrograph from upgradient areas Number of devices in source area or in upstream drainage system Broad crested weir (surface overflow) Weir crest length (ft) 8.00 Weir crest width (ft) 0.5 Height from datum to bottom of weir opening (ft) (the roof area is very large at 3,500ft 2, resulting in a total rain garden area of 720ft 2, or about 21% of the roof area). No ET, under drains, or other losses considered (but could be added if desired). 9

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11 Therefore, this rain garden that is 21% of the roof contributing area and has an infiltration rate of 1 in/hr, will almost completely infiltrate all of the roof runoff from all events during this 5 year period (99.4%). This corresponds to an outfall runoff reduction of about 33% (the fraction of the annual total flow originating from these connected roofs). 11