(A) Calculation of project density. The following requirements shall apply to the calculation of project density:
(1) Project density shall be calculated as the total built-upon area divided by the total project area;
(2) A project with existing development shall calculate project density as the difference of total built-upon area minus existing built-upon area divided by the difference of total project area minus existing built-upon area;
(3) Total project area shall exclude areas below the Normal High Water Line (NHWL); and
(4) On a case-by-case basis as determined by the town during application review, projects may be considered to have both high and low density areas based on one or more of the following criteria:
(a) Natural drainage area boundaries;
(b) Variations in land use throughout the project; and
(c) Construction phasing.
(B) Design requirements.
(1) Low density projects. Low density projects shall meet the following minimum design criteria:
(a) Dispersed flow. Projects shall be designed to maximize dispersed flow through vegetated areas and minimize channelization of flow;
(b) Vegetated conveyances. Stormwater that cannot be released as dispersed flow shall be transported by vegetated conveyances. A minimal amount of non-vegetated conveyances for erosion protection or piping for driveways or culverts under a road shall be allowed by the town when it cannot be avoided. Vegetated conveyances shall meet the following requirements:
1. Side slopes shall be no steeper than 5:1 (horizontal to vertical) unless it is demonstrated to the town that the soils and vegetation will remain stable in perpetuity based on engineering calculations and on-site soil investigation; and
2. The conveyance shall be designed so that it does not erode during the peak flow from the ten-year storm as demonstrated by engineering calculations.
(c) Curb outlet systems. Low density projects may use curb and gutter with outlets to convey stormwater to grassed swales or vegetated areas. Requirements for these curb outlet systems shall be as follows:
1. The curb outlets shall be designed such that the swale or vegetated area can carry the peak flow from the ten-year storm at a non-erosive velocity;
2. The longitudinal slope of the swale or vegetated area shall not exceed 5%, except where not practical due to physical constraints. In these cases, devices to slow the rate of runoff and encourage infiltration to reduce pollutant delivery shall be provided;
3. The swale's cross-section shall be trapezoidal with a minimum bottom width of two feet;
4. The side slopes of the swale or vegetated area shall be no steeper than 5:1 (horizontal to vertical);
5. The minimum length of the swale or vegetated area shall be 100 feet.
(2) High density projects. High density projects shall meet the following minimum design criteria:
(a) Treatment requirements. SCMs shall be designed, constructed, and maintained so that the project achieves either "runoff treatment" or "runoff volume match" as those terms are defined in § 158.007.
(b) Off-site stormwater. Stormwater runoff from off-site areas and existing development shall not be required to be treated in the SCM. Runoff from off-site areas or existing development that is not bypassed shall be included in the sizing of on-site SCMs at its full built-out potential.
(c) Off-site SCM.
1. A project that controls runoff through an off-site SCM shall be allowed on a case-by-case basis as determined by the town if the off-site SCM meets the MDC for the specific device.
2. Legal documents are required for clarification of ownership and maintenance responsibilities.
3. Adequate access and maintenance easements are required to be recorded to a public right-of-way.
(d) Expansion or replacement of existing development. When new built-upon area is added to existing development or existing development is replaced with new built-upon area, only the area of net increase shall be subject to this subchapter.
(e) MDC for SCMS. SCMs shall meet the relevant MDC set forth in the Design Manual.
(Ord. 2020-11-01, passed 11-16-20)