DISCONNECTED IMPERVIOUS AREA (DIA)
FOR CARBONATE GEOLOGY AREAS (ONLY)
ROOFTOP DISCONNECTION
When rooftop downspouts are directed to a pervious area that allows for infiltration, filtration, and increased time of concentration, the rooftop may qualify as completely or partially DIA, and a portion of the impervious rooftop area may be excluded from the calculation of total impervious area.
A rooftop is considered to be completely or partially disconnected if it meets the requirements listed below:
- The contributing area of a rooftop discharge is 500 square feet or less, and
- The soil, in proximity of the roof water discharge area, is not designated as hydrologic soil group “D” or equivalent, and
- The overland flow path from roof water discharge area has a positive slope 2% to of 5%.
For designs that meet these requirements, the portion of the roof that may be considered disconnected depends on the length of the overland path as designated in Table F.1.
Table F.1: Partial Rooftop Disconnection
Length of Pervious Flow Path* | Roof Area Treated as Disconnected |
(ft.) | (% of contributing area) |
0 – 14 | 0 |
15 – 29 | 20 |
30 – 44 | 40 |
45 – 59 | 60 |
60 – 74 | 80 |
75 or more | 100 |
*Flow path cannot include impervious surfaces and must be at least 15 feet from any impervious surfaces.
If the discharge is concentrated at one or more discrete points, a gravel strip or other spreading device is required to develop sheet flow.
PAVED SURFACE DISCONNECTION
When runoff from paved surfaces is directed to a pervious area that allows for infiltration, filtration, and increased time of concentration, the contributing pavement area may qualify as disconnected. This applies to only small or narrow pavement surfaces such as driveways and walkways. Paved surfaces may be considered disconnected if they, or the adjacent areas, meet the following requirements:
- The contributing area of paved surface discharge is 500 square feet or less, and
- the contributing flow path over the impervious area is not more than 75 feet, and
- the length of overland flow is greater than or equal to the maximum flow length over the impervious surface, and
- the slope of the contributing impervious surface area and overland area is 2% to 5%, and
- if the discharge is concentrated at one or more discrete points, a gravel strip or other spreading device is required to develop sheet flow. For non-concentrated discharges along the edge of the pavement, this requirement is waived; however, there must be a provision for the establishment of vegetation along the pavement edge and temporary stabilization of the area until vegetation becomes stabilized.
- the slope of the contributing impervious surface area and overland area is 2% to 5%, and
- if the discharge is concentrated at one or more discrete points, a gravel strip or other spreading device is required to develop sheet flow. For non-concentrated discharges along the edge of the pavement, this requirement is waived; however, there must be a provision for the establishment of vegetation along the pavement edge and temporary stabilization of the area until vegetation becomes stabilized.