Projects eligible for this procedure
The following two types of projects may utilize the simplified procedure contained in this appendix to meet volume requirements of this Part and are not required to submit detailed stormwater management plans as required by Article IV to the township:
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Any proposed individual home construction projects, accessory structures projects, and/or other projects involving the construction or installation of impervious surfaces, taking place on residential lots of one acre or less, which, solely and/or cumulatively, propose from 500 square feet up to 1,000 square feet of additional impervious surface; and
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Individual-home construction projects on single family lots which result in less than 2,500 square feet of new impervious area (including the building footprint, driveway, sidewalks and parking areas) and less than 5,000 square feet of earth disturbance.
This procedure may not be utilized for proposed subdivisions or land developments.
Are professional services necessary to meet these requirements?
This appendix has been developed to assist the individual homeowner in meeting the water quality and groundwater recharge goals of the Stormwater Management Ordinance. If the guidelines are followed, the homeowner will not require professional services to comply with these water quality and groundwater recharge goals.
What do I need to submit to Bedminster Township?
Even though a formal stormwater management plan is not required for individual lot owners, a brief description of the proposed infiltration facilities, including types of material to be used, total impervious areas and volume calculations, and a simple sketch plan showing the following information shall be submitted to the township prior to construction:
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Location of proposed structures, driveways or other paved areas with approximate surface area in square feet.
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Location of any existing or proposed onsite septic system and/or potable water wells showing proximity to infiltration facilities.
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Bucks County Conservation District erosion and sediment control “Adequacy” letter.
Determination of Recharge Volume
The amount of recharge volume that must be provided is determined by following the simple steps below. Impervious area calculations must include all areas on the lot proposed to be covered by roof area or pavement which would prevent rain from naturally percolating into the ground, including sidewalks, driveways or parking areas. Sidewalks, driveways or patios that are constructed with turf pavers and are not included in this calculation.
Example Recharge Volume:
STEP 1 - Determine Total Impervious Surfaces.
Driveway | 12 ft. x 50 ft. | = | 600 sq. ft. |
House roof (front) | 12 ft. x 48 ft. | = | 576 sq. ft. |
House roof (rear) | 12 ft. x 48 ft. | = | 576 sq. ft. |
Parking pad | 12 ft. x 12 ft. | = | 144 sq. ft. |
Walkway | 4 ft. x 20 ft. | = | 80 sq. ft. |
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1,976 sq. ft. | |||
STEP 2 - Determine Required Infiltration Volume (Rv) Using the Following Equation.
Rv = 3.26 inches x (total impervious area in square feet) = _________ cubic feet of recharge
12
12
Rv = 3.26 x 1,976 sq. ft. = 537 cu. ft.
12
12
Note: This example is located within the Neshaminy Creek Watershed. Use 2.0 inches in other watersheds.
STEP 3 - Sizing of Selected Infiltration Method.
The following pages identify several methods to infiltrate stormwater runoff. Their appropriateness depends on the amount of required infiltration volume and amount of available land. More than one method may be implemented on a site, depending on site constraints. Dry wells may be used only for receiving runoff from roof drains. Infiltration trenches are appropriate for receiving runoff from driveways, sidewalk or parking areas. Other methods may be appropriate, but these must be reviewed with the Township Engineer prior to installation.
Dry Wells
Dry wells are effective methods to infiltrate runoff from roof leaders. These facilities must be located based upon a determination by the design professional to reduce potential basement seepage problems but not less than a minimum of 20 feet from the building foundation. A dry well maybe either a structural prefabricated chamber or an excavated pit filled with aggregate. Dry well shall not be constructed until all other areas of the site are stabilized, to avoid clogging. During construction, compaction of the subgrade soil shall be avoided, and construction may be performed with only light machinery. Depth of dry wells in excess of three and one-half feet should be avoided unless warranted by soil conditions. “Clean” gravel fill should average one and one-half to three inches in diameter. Dry wells should be inspected at least four times annually as well as after large storm events.
Example Sizing For Drywells:
STEP 1 - Determine Total Impervious Surfaces
House Roof Area: 12 ft. x 48 ft. = 576 sq. ft.
STEP 2 - Determine Required Infiltration Volume using Equation
3.26 in. x 576 sq. ft. = 156 cu. ft.
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156 cu. ft. = 390 cu. ft. (*assumes 40% void ratio in gravel)
0.4*
0.4*
STEP 3 - Sizing of Selected Infiltration Method
Volume of facility = Depth x Width x Length
Set D = 3.5 ft; Set W = L for a square chamber
390 cu. ft. = 3.5 x L X L ; L = 10.5 ft.
Final Facility Dimensions: 3.5 ft. (D) x 10.5 ft. (W) x 10.5 ft. (L)
Infiltration Trenches
An infiltration trench is a long, narrow, rock-filled trench with no outlet that receives stormwater runoff. Runoff is stored in the void space between the stones and infiltrates through the bottom and into the soil matrix. Infiltration trenches perform well for removal of fine sediment and associated pollutants. Pretreatment using buffer strips, swales, or detention basins is important for limiting amounts of coarse sediment entering the trench which can clog and render the trench ineffective.
Example Sizing Infiltration Trenches:
STEP 1 - Determine Total Impervious Surfaces
Driveway | 12 ft. x 50 ft. | = | 600 sq. ft |
Parking pad | 12 ft. x 12 ft. | = | 144 sq. ft. |
Walkway | 4 ft. x 20 ft. | = | 80 sq. ft. |
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824 sq. ft. |
STEP 2 - Determine REquired Infilitration Volume using Equasion
3.26 in. x 824 sq. ft. = 224 cu. ft
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224 cu. ft. = 560 cu. ft. (*assumes 40% void ratio in gravel bed)
0.4*
0.4*
Note: This example is located within the Neshaminy Creek Watershed. Use 2.0 inches in other watersheds.
STEP 3 - Sizing of Selected Infiltration Method
Volume of facility = Depth x Width x Length
Set D = 3 ft; determine required surface area of trench
560 cu. ft. / 3 ft. = 187 sq. ft.
The width of the trench should be greater than 2 times it depth (2 x D); therefore, in this example a trench width of 6 feet is selected;
Determine trench length: L = 187 sq. ft. / 6 ft. = 31 ft.
Final trench dimensions: 3 ft. (D) x 6 ft. (W) x 31 ft. (L)
Rain Gardens
A Rain Garden is a planted shallow depression designed to catch and filter rainfall runoff. The garden captures rain from a downspout or a paved surface. The water sinks into the ground, aided by deep rooted plants that like both wet and dry conditions. The ideal location for a rain garden is between the source of runoff (roofs and driveways) and the runoff destination (drains, stream, low spots and the like).
Design Considerations:
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A maximum of 3:1 side slope is recommended.
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The depth of a rain garden can range from six to eight inches. Ponded water should not exceed six inches.
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The rain garden should drain within 72 hours.
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The garden should be at least ten to 20 feet from a building’s foundation and 25 feet from septic system drain fields and wellheads.
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If the site has clay soils, soil should be amended with compost or organic material.
∙ Choose native plants. See http://pa.audubon.org/habitat/PDFs/RGBrochure_complete.pdf for a native plant list. To find native plant sources go to www.pawildflower.org.
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At the rain garden location, the water table should be at least two feet below the soil level. If water stands in an area for more than one day after a heavy rain you can assume it has a higher water table and is not a good choice for a rain garden.
Maintenance:
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Water plants regularly until they become established.
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Inspect twice a year for sediment buildup, erosion and vegetative conditions.
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Mulch with hardwood when erosion is evident and replenish annually.
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Prune and remove dead vegetation in the spring season.
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Weed as you would any garden.
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Move plants around if some plants would grow better in the drier or wetter parts of the garden.
Sizing Example for Rain Garden
1. Pick a site for the rain garden between the source of runoff and between a low lying area, a.k.a., a drainage area.
2. Perform an infiltration test to determine the depth of the rain garden:
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Dig a hole eight inches by eight inches
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Fill with water and put a popsicle stick at the top of the water level.
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Measure how far it drains down after a few hours (ideally four).
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Calculate the depth of water that will drain out over 24 hours.
3. Determine total impervious surface area to drain to rain garden:
House roof (front) | 14 ft. x 48 ft. | = | 672 sq. ft. |
4. Sizing the rain garden:
For this example, the infiltration test determined six inches of water drained out of a hole in 24 hours. The depth of the rain garden should be set to the results of the infiltration test so six inches is the depth of the rain garden. The sizing calculation below is based on controlling one inch of runoff. First divide the impervious surface by the depth of the rain garden.
(672 sq. ft. / 6 ft.) = 112 sq. ft.
In order to control two inch of runoff volume, the rain garden area needs to be multiplied by two.
112 sq. ft. * 2 = 224 sq. ft.
The rain garden should be about 225 sq. ft. in size and six inches deep.
Field Conditions/Construction
Construction of the stormwater management facility must be observed by the Township Engineer pursuant to § 151.075. If soil conditions indicate a limiting zone (e.g. high water table, depth to bedrock) will encroach within the infiltration/dry well, the stormwater management facility construction details (width/depth/length) must be revised as determined by the Township Engineer and field documented.
(Ord. 239, passed 10-14-2020; Ord. 248, passed 7-14-2021)