Step 2. Physical Feasibility Factors
This matrix allows the designer to evaluate possible options based on physical conditions at the site. More detailed testing protocols are often needed to confirm physical conditions at the site. Five primary factors are:
Soils. The key evaluation factors are based on an initial investigation of the NRCS hydrologic soils groups at the site. Note that more detailed geotechnical tests are usually required for infiltration feasibility and during design to confirm permeability and other factors.
Water Table. This column indicates the minimum depth to the seasonally high water table from the bottom elevation, or floor, of an STP.
Drainage Area. This column indicates the minimum or maximum drainage area that is considered optimal for a practice. If the drainage area present at a site is slightly greater than the maximum allowable drainage area for a practice, some leeway is warranted where a practice meets other management objectives. Likewise, the minimum drainage areas indicated for ponds and wetlands should not be considered inflexible limits, and may be increased or decreased depending on water availability (baseflow or groundwater), mechanisms employed to prevent clogging, or the ability to assume an increased maintenance burden.
Slope. This column evaluates the effect of slope on the practice. Specifically, the slope guidance refers to how flat the area where the practice is installed must be and/or how steep the contributing drainage area or flow length can be.
Head. This column provides an estimate of the elevation difference needed for a practice (from the inflow to the outflow) to allow for gravity operation.The Physical Feasibility Matrix provides specific feasibility criteria. The criteria in this matrix are adapted from the Maryland Stormwater Design Manual. Each community needs to select specific criteria that are appropriate for their region, and consistent with local design experience.
STP Selection Matrix 2. Physical Feasibility
A soils may
require pond liner.
|2 foot separation if hotspot or aquifer||10 min*||No more than 15%||6 to 8 ft|
|Wet ED Pond|
|Pocket Pond||OK||below WT||5 max**||4 ft|
A soils may
|3 to 5 ft|
|Pocket Marsh||OK||below WT||5 max||2 to 3 ft|
|4 feet||5 max||No more than 6%||1 ft|
|Shallow I-Basin||10 max||3 ft|
|Filters||Surface Sand Filter||OK||2 feet||10 max **||no more than 6%||5 ft|
|Underground SF||2 max **||5 to 7ft|
|Perimeter SF||2 max **||2 to 3 ft|
|Organic SF||5 max**||2 to 4 ft|
|Pocket Sand Filter||5 max **||2 to 5 ft|
|Bioretention||Made Soil||5 ft|
|Open Channels||Dry Swale||Made Soil||2 feet||5 max||No more than 4%||3 to 5 ft|
|Wet Swale||OK||below WT||5 max||1 ft|
|Grass Channel||OK||2 feet||5 max||1 ft|
OK= not restricted, WT= water table, PT = pretreatment, fc =soil
* unless adequate water balance and anti-clogging device installed ** drainage area can be larger in some instances.