(A) Minimum requirements.
(1) These minimum requirements apply to all new development and redevelopment projects that disturb one acre or more of land, including projects less than one acre that are part of a larger common plan of development or sale that will result in disturbance of one acre or more. Projects whose final level of impervious cover is less than 20% are exempt, since the low level of development inherently results in little or no impact to water quality. Impervious cover includes but is not limited to:
(a) Pavement including streets, sidewalks, driveways, parking lots, etc.;
(b) Rooftops if not part of a rainwater harvesting system;
(c) Compacted road base, such as that used for parking areas; and
(d) Other surfaces that prevent the infiltration of water into the soil.
(2) Bicycle and pedestrian paths separated from other impervious surfaces by a distance of at least ten feet, except at intersections, are considered sustainable and do not require any special runoff management.
(3) When the development project includes residential tracts that will be developed subsequently, and whose future impervious level is unknown, the assumptions presented in the following table should be used. The values in this table do not include the area of the streets in the development.
Table: Impervious Cover Assumptions for Residential Tracts
Lot Size | Assume Impervious Cover (sq. ft.) |
> 3 acres | 10,00 |
Between 1 and 3 acres | 7,000 |
Between 15,000 sq. ft. and 1 acre | 5,000 |
Between 10,000 and 15,000 sq. ft. | 4,000 |
Between 5,000 and 10,000 sq. ft. | 3,500 |
< 5,000 sq. ft. | 70% of lot size |
(4) There are two options available to meet the sustainable drainage requirements: payment in lieu or use of at least one of the structural controls described below to manage stormwater runoff from the site.
(a) Payment in lieu. The payment in lieu option requires the developer to make a payment of $50,000 per impervious acre created (not total project size) into an account whose funds may only be used for drainage improvements that reduce the discharge of pollutants from the drainage system.
(b) Structural controls. Stormwater runoff generated on the site must be managed through the use of one or more of these structural practices (guidelines available from City Hall):
1. Vegetated swale;
2. Vegetated filter strip;
3. Porous pavement;
4. Enhanced detention;
5. Bioretention/rain gardens; and
6. Infiltration basins.
(5) For the structural practices that are sized based on runoff volume (bioretention, enhanced detention, infiltration basins), the capture volume must be sized to accommodate the runoff from a 1.0" rainfall event at a minimum. The runoff coefficient is a function of the impervious cover and is calculated as:
Rv = 0.05 + 0.90IC
Where:
Rv = Runoff Coefficient
IC = Fraction of impervious cover in the catchment of the structural practice
The minimum capture volume is then calculated as:
V = P x A x Rv/12
Where:
V = Minimum required capture volume P = Rainfall depth (1.0 inches)
A = Catchment area of the practice (ft2) Rv = Runoff Coefficient
(6) Additionally, watercourse edge buffers should be provided between any development and all natural bodies of water. The buffer width needed to perform properly will depend on the size of the stream and the surrounding conditions, but a minimum 25-foot undisturbed vegetative buffer is recommended for all water courses, even the smallest perennial streams. Where feasible, watercourse edge buffers should be sized to include the 100-year floodplain.
(7) This first 25-foot section should be a zero-development zone. Buffer zones should generally remain free of construction, development, or other alterations. The number of roadways crossing through the buffer zones should be minimized and constructed only when necessary, such as when a significant portion of the site can only be reached by crossing a buffer zone. Other alterations within buffer zones beyond the 25-foot minimum could include utility crossings, but only when necessary, fences, low impact parks, and open space. Roadways and utilities crossings should be approximately perpendicular to the buffer zone. Low impact park development within the buffer zone should be limited to trails, picnic facilities, and similar construction that do not significantly alter the existing vegetation. Parking lots and roads significantly alter existing vegetation and are not considered low impact.
(Ord. 593-B, passed 7-12-2022)