42-257. Design and Performance Standards

1. Except as provided in §42-257 (Design and Performance Standards) B(2) of this subpart, erosion and sedimentation control measures, structures, and devices shall be planned, designed, and constructed to provide protection from the calculated maximum peak rate of runoff from the ten (10) year storm. Runoff rates shall be calculated using the procedures in the United States Department of Agricultural (USDA), Natural Resource Conservation Service’s “National Engineering Field Handbook", or other acceptable calculation procedures.
2. HQW Zones. In High Quality Water (HQW) zones the following design standards shall apply:
   1. Limit on Uncovered Area. Uncovered areas in HQW zones shall be limited at any time to a maximum total area of 20 acres within the boundaries of the tract. Only the portion of the land- disturbing activity within a HQW zone shall be governed by this section. Larger areas may be uncovered within the boundaries of the tract with the written approval of the Director upon providing engineering justification with a construction sequence that considers phasing, limiting exposure, weekly submitted self-inspection reports, and a more conservation design than the Twenty-Five Year Storm.
   2. Maximum Peak Rate of Runoff Protection. Erosion and sedimentation control measures, structures, and devices within HQW zones shall be planned, designed and constructed to provide protection from the runoff of the 25-year storm which produces the maximum peak rate of runoff as calculated according to procedures in the latest edition of the United States Department of Agriculture Natural Resources Conservation Service’s “National Engineering Field Handbook" or according to procedures adopted by any other agency of this state or the United States or any generally recognized organization or association.
   3. Sediment Basin Design. Sediment basin within HQW zones shall be designed and constructed according to the following or association.
      1. use a surface withdrawal mechanism, except when the basin drainage area is less than 1.0 acre:
      2. have a minimum of 1800 cubic feet of storage area per acre of disturbed area;
      3. have a minimum surface area of 325 square feet per cfs of the Twenty-Five Year Storm (Q25) peak flow;
      4. have a minimum dewatering time of 48 hours;
      5. incorporate 3 baffles, unless the basin is less than 20 feet in length, in which case 2 baffles shall be sufficient.

 Upon a written request of the applicant, the Director may allow alternative design and control measures in lieu of meeting the conditions required in subparagraphs (3)(ii) through (3)(v) of this sub-section if the applicant demonstrates that meeting all of those conditions will result in design or operational hardships and that the alternative measures will provide an equal or more effective level of erosion and sediment control on the site. Alternative measures may include quicker application of ground cover, use of sediment flocculants, and use of enhanced ground cover practices.

4.  Grade. Newly constructed open channels in HQW zones shall be designed and constructed with side slopes no steeper than two (2) horizontal to one (1) vertical if a vegetative cover is used for stabilization unless soil conditions permit a steeper slope or where the slopes are stabilized by using mechanical devices, structural devices or other forms of ditch liners proven as being effective in restraining accelerated erosion. In any event, the angle for side slopes shall be sufficient to restrain accelerated erosion.