§ 10-4.308. Low impact development.

(a)

Generally. Low impact development (LID) is a comprehensive land planning and engineering design approach with a goal of maintaining and enhancing the predevelopment hydrologic conditions of developing watersheds. LID is a manner of land development that seeks to mimic predevelopment hydrology to protect watercourses, habitat, baseflow and groundwater recharge. It also protects water quality by minimizing the pollutant loading from developed areas to our surface waters. The basic premise of LID is to decentralize stormwater treatment by reducing runoff and designing infiltration techniques before the runoff reaches the stormwater facility.

(b)

Minimization of directly connected impervious area. It is the intent of this subsection to minimize directly connected impervious area in an effort to improve water quality treatment by decentralizing stormwater runoff and percolating this runoff wherever possible. Environmental permit applications that include stormwater treatment, with the exception of projects located in the Multi-Modal Transportation District, must demonstrate that directly connected impervious area is minimized to the greatest extent practicable, i.e., swales versus piped system.

(c)

LID options. The following are examples of LID options that will be evaluated on a case by case basis by using acceptable engineering practices with verification that maintenance entities are provided to ensure that the LID will provide continuing benefits after construction:

(1)

Bioretention;

(2)

Retaining natural vegetation;

(3)

Minimize directly connected impervious areas;

(4)

Pervious pavement;

(5)

Green roofs;

(6)

Rain gardens;

(7)

Rain barrels/cisterns;

(8)

Resource efficient landscapes;

(9)

Stormwater reuse; and

(10)

Vegetated swales.

(d)

Minimum standards. The following are determined to be acceptable LID techniques for which credit shall be available (other techniques may be approved at the discretion of the county administrator or designee):

(1)

Shallow retention areas. This category includes bioretention areas, inverted landscape areas and planted retention areas, if these areas are less than two feet in depth and integrated within the landscaped areas of a site. Landscape credit for these shallow retention areas shall be provided at a ratio of 2:1 toward the landscape area requirement in section 10-4.344 after all other landscaping requirements, including interior landscaping requirements, have been met. These retention areas may provide stormwater treatment credit if recovery requirements have been met.

(2)

Natural area on residential lots. Stormwater treatment will not be required for areas on residential lots specifically designated as "conservation area," intended to remain in a natural predevelopment condition. If possible, conservation areas should be located on the downstream side of the lot to provide infiltration of runoff from impervious areas. "Conservation areas" shall be so indicated on the site plan, plat, or environmental permit for the development. Covenants and restrictions for residential subdivisions shall include specific language to ensure preservation of dedicated areas. The treatment volume for the subdivision must meet the minimum state water quality treatment standards.

(3)

Pervious pavement. Pervious pavement techniques utilize a variety of materials and construction designs that allow movement of stormwater flow through sustainable materials that contain pores or separation joints with eventual seepage into a base material. Types of pervious pavement include, but are not limited to, porous asphalt and concrete, plastic or concrete grid systems with gravel-filled voids, and grass pavers/turf blocks (gravel, asphalt millings, crushed limerock or equivalent are not considered pervious pavement).

Pervious pavement is required for parking as required in section 10-7.545. Credit toward landscaping and stormwater treatment requirements are as follows:

a.

Grass pavers/turf blocks. Landscape credit for grass pavers/turf blocks shall be provided at a ratio of 1:1 toward the landscape area requirement in section 10-4.344 after all other landscaping requirements, including interior landscaping requirements, have been met.

b.

For calculating the treatment volume required for pervious pavement, initially such surfaces shall be assumed to be 100 percent impervious, then deductions in the required treatment volume for such areas can be taken that is equivalent to:

(i)

The porosity of the pavement material multiplied by the thickness of the paving material multiplied by a safety factor of five-tenths.

(ii)

If, and only if, the soils immediately underlying the pavement for a depth of 18 inches have a permeability of three inches per hour or greater, as demonstrated by onsite percolation tests, then a further deduction can be taken equivalent to the porosity of the soil strata multiplied by four inches multiplied by a safety factor of five-tenths.

(iii)

The above deductions will be allowed provided that the applicant specifically commits, in the stormwater operating permit, to regularly sweep/vacuum the area covered with pervious pavement and to verify the percolation capacity of the pavement upon each renewal of the operating permit.

(4)

Green roofs. A green roof is a roof of a building that is partially or completely covered with non-invasive vegetation and a growing medium, planted over a waterproofing membrane. The purpose of a green roof is to absorb and treat rainwater, provide insulation, create habitat for wildlife and mitigate the heat island effect. Landscape credit for the green roof area shall be provided at a ratio of at least 1:1 toward the landscape area requirement in section 10-4.344 after all other landscaping requirements, including interior landscaping requirements, have been met. Appropriate credit for water quality treatment provided shall be applied towards the applicable treatment requirements for the development. Appropriate credit shall be calculated based upon the actual water quality benefit provided by the green roof as calculated by a licensed engineer applying professionally acceptable standards.

(5)

Resource efficient landscapes. Resource efficient landscapes must use in-situ native soils, avoid soil compaction, provide wildlife habitat, reduce cover of turfgrass species, use native plant species best adapted to the conditions present before planting and will not need supplemental water, fertilizer and pesticides once established. Landscape credit for resource efficient landscapes shall be provided at a ratio of 1.5:1 toward the landscape area requirement in section 10-4.344 after all other landscaping requirements, including interior landscaping requirements, have been met.

(6)

Stormwater reuse. Reuse of stormwater for irrigation of landscaping and natural areas can assist in retaining stormwater runoff onsite. In all areas where retention is not required and recovery cannot be met solely by percolation, reductions to the treatment volume will be given for irrigation of stormwater on a case by case basis. The treatment volume for the areas draining to the stormwater pond must still meet the minimum state water quality treatment standards.

(e)

LID credit. All LID techniques must include provisions for maintenance and enforcement to ensure that the original design remains intact and continues to provide the benefits originally designed. Except as otherwise provided herein, credit towards water quality requirements shall be given on a 1:1 basis and shall be based upon the actual water quality benefit provided by the LID technique as calculated by a licensed engineer based upon professionally acceptable methodology. Credits must be reviewed and approved by the county administrator or designee based on the site conditions and proposed LID techniques. The county administrator or designee will provide clarification and/or interpretation of requirements in this section if multiple interpretations become the source of conflict.

(Ord. No. 13-25, § 1, 12-10-13)