Projects eligible for this method.
1. Did your lot exist prior to September 10, 2002?
2. Is the use of your lot a single family detached dwelling?
3. Are you proposing 2500 sq. ft. or less of new impervious surface?
4. Will you disturb less than 5000 sq. ft. of earth during construction?
6. The project is not a subdivision?
If you answered yes to each of the above, you may use this method.
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 send to Springfield Township?
Even though a formal drainage 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 sketch plan showing the following information shall be submitted to the Township prior to construction.
• Locations and dimensions of proposed structures, driveways or other paved areas with impervious surface area in square feet. (See “How to Draw A Plot Plan”).
• Location of any existing or proposed onsite septic system and/or potable water wells.
• The direction of flow (slope of the ground)
• Bucks County Conservation District erosion and sediment control “Adequacy” letter (for projects over 1000 square feet of earth disturbance).
Determination of Facility Dimensions
Step 1 – Determine how many controls are needed (see Fig C-1). Applicant can utilize single or multiple controls to fit topography, drainage and impervious requirements. The added stormwater runoff generated from new impervious surface must enter into a control facility, either via surface flow or piping.
Step 2 – Determine Total Impervious Surfaces Served by Each Proposed Facility (Fig C-1)
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 are not included in this calculation. Gravel is impervious.
In Figure C-1, the applicant is using 2 stormwater control facilities because of topography; a dry well for the house and a trench drain for the driveway. The controls must be placed so the water enters it.
House (front roof) | 12 ft. x 48 ft. | = | 576 sq. ft. |
House (rear roof) | 12 ft. x 48 ft. | = | 576 sq. ft. |
Total for house (drywell) | 1152 sq.ft. to Dry Well | ||
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. |
824 sq. ft. to trench |
Step 3 – Determine Required Infiltration Volume (Rv) using the following equation for each facility.
Step 4 – Add required infiltration voids (spaces between stone) by dividing Rv by 40%
Step 5 - Compute linear dimensions (L x W x D) for each facility.
For dry wells D = 3.5 ft (in this example).Then compute the square root of Total Facility Volume/D.
This trench has to be at least 40 ft long based on topography so the water flows into it.
117 ft/40 ft = 3 ft
So the trench will be: 40ft long x 3ft wide x 3ft deep
Construction
The appropriate type of facility 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. Water flows downhill. The increased runoff from new impervious surfaces must enter into the facility via gravity flow.
Dry Well Construction (see Figure C-2)
Dry wells are effective methods to infiltrate runoff using roof leaders. These facilities must be located to reduce potential basement seepage problems but not less than a minimum of twenty (20) feet from the building foundation. They should be kept away from septic drainage areas and wells. A dry well may be either a structural prefabricated chamber or an excavated pit filled with aggregate. Dry wells 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 (3-1/2) ft are prohibited, unless warranted by soil conditions demonstrated to the satisfaction of the Township Engineer. Clean gravel fill should average one and one half to three (1.5-3.0) inches in diameter.
Infiltration Trench Construction (see Figure C-3)
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. 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.
Post Construction Maintenance
Dry wells should be inspected at least four (4) times annually as well as after large storm events. Trenches should be inspected after every large rain event to ensure leaves and debris have not clogged the surface stone and that the backstop berm is intact and functioning.
Figure C-1
Figure C-2
Figure C-3
Note: In both examples shown above, a leaf/debris collection inlet (not shown in the sample drawings) must be installed between a roof downspout/drain and the dry well or infiltration bed.
Field Conditions/Construction
Construction of the stormwater management facility must be observed by the Township Engineer pursuant to Section 501 of this Ordinance. 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.
The Township Engineer will inspect at least twice during construction:
1. After excavation and with geotextile installed, but before stone is added.
2. After facility is complete, but not backfilled.