Mendocino County Municipal Code Ch. 20.492 Grading, Erosion and Runoff

The approving authority shall review all permit applications for coastal developments to determine the extent of project related impacts due to grading, erosion and runoff. The approving authority shall determine the extent to which the following standards should apply to specific projects, and the extent to which additional studies and/or mitigation are required, specifically development projects within Development Limitations Combining Districts.

(Ord. No. 3785 (part), adopted 1991)

(A) Grading shall not significantly disrupt natural drainage patterns and shall not significantly increase volumes of surface runoff unless adequate measures are taken to provide for the increase in surface runoff.

(B) Development shall be planned to fit the topography, soils, geology, hydrology, and other conditions existing on the site so that grading is kept to an absolute minimum.

(C) Essential grading shall complement the natural land forms. At the intersection of a manufactured cut or fill slope and a natural slope, a gradual transition or rounding of contours shall be provided.

(D) The cut face of earth excavations and fills shall not be steeper than the safe angle of repose for materials encountered. Where consistent with the recommendations of a soils engineer or engineering geologist, a variety of slope ratios shall be applied to any cut or fill slope in excess of two hundred, (200) feet in length or ten (10) feet in height. For individually developed lots, a variety of slope ratios shall be applied to all cut or fill slopes when a building pad area exceeds four thousand five hundred (4,500) square feet, or when the total graded area of the lot exceeds nine thousand (9,000) square feet. The steepest permissible slope ratio shall be two to one (2:1), corresponding to a fifty (50) percent slope.

(E) The permanently exposed faces of earth cuts and fills shall be stabilized and revegetated, or otherwise protected from erosion.

(F) Adjoining property shall be protected from excavation and filling operations and potential soil erosion.

(G) The area of soil to be disturbed at any one time and the duration of its exposure shall be limited. Erosion and sediment control measures shall be installed as soon as possible following the disturbance of the soils. Construction equipment shall be limited to the actual area to be disturbed according to the approved development plans.

(Ord. No. 3785 (part), adopted 1991)

(A) The erosion rate shall not exceed the natural or existing level before development.

(B) Existing vegetation shall be maintained on the construction site to the maximum extent feasible. Trees shall be protected from damage by proper grading techniques.

(C) Areas of disturbed soil shall be reseeded and covered with vegetation as soon as possible after disturbance, but no less than one hundred (100) percent coverage in ninety (90) days after seeding; mulches may be used to cover ground areas temporarily. In environmentally sensitive habitat areas, the revegetation shall be achieved with native vegetation. In buffer areas adjacent to environmentally sensitive habitats, non-native vegetation may be used provided that it is non-invasive and would not adversely affect the environmentally sensitive habitat area.

(D) Mechanical or vegetative techniques to control erosion may be used where possible or necessary providing that they are fully discussed in the approved development plan.

(E) To control erosion, development shall not be allowed on slopes over thirty (30) percent unless adequate evidence from a registered civil engineer or recognized authority is given that no increase in erosion will occur.

(F) Development of sites that will result in road cuts, which pose a hazard and/or which create the potential for uncontrollable problems and adverse impacts from erosion and sedimentation, shall not be allowed. Alternative road routes may be required for projects. Where possible, roads in hilly areas should follow ridgetops to avoid extensive cuts and fills.

(G) Erosion control devices shall be installed in coordination with clearing, grubbing, and grading of downstream construction; the plan shall describe the location and timing for the installation of such devices and shall describe the parties responsible for repair and maintenance of such devices.

(Ord. No. 3785 (part), adopted 1991.)

(A) Sediment basins (e.g., debris basins, desilting basins, or silt traps) shall be installed in conjunction with initial grading operations and maintained through the development/construction process to remove sediment from runoff wastes that may drain from land undergoing development to environmentally sensitive areas.

(B) To prevent sedimentation of off-site areas, vegetation shall be maintained to the maximum extent possible on the development site. Where necessarily removed during construction, native vegetation shall be replanted to help control sedimentation.

(C) Temporary mechanical means of controlling sedimentation, such as hay baling or temporary berms around the site, may be used as part of an overall grading plan, subject to the approval of the Coastal Permit Administrator.

(D) Design of sedimentation control devices shall be coordinated with runoff control structure to provide the most protection.

(E) The grading plan when required shall set forth a schedule for the construction and maintenance of any structure to be developed under this section, and shall include a statement designating who shall be responsible for the long-term management of the devices.

(Ord. No. 3785 (part), adopted 1991.)

(A) Water flows in excess of natural flows resulting from project development shall be mitigated.

(B) If the Coastal Permit Administrator determines that a project site is too small or engineering, aesthetic and economic factors make combined drainage facilities more practical for construction by the County, the County may require a fee and dedication of land, which the County shall use to construct these facilities. The County may allow several developers to jointly construct facilities to approved County specifications.

(C) The acceptability of alternative methods of storm water retention shall be based on appropriate engineering studies. Control methods to regulate the rate of storm water discharge that may be acceptable include retention of water on level surfaces, the use of grass areas, underground storage and oversized storm drains with restricted outlets or energy dissipators.

(D) Retention facilities and drainage structures shall, where possible, use natural topography and natural vegetation. In other situations, planted trees and vegetation such as shrubs and permanent ground cover shall be maintained by the owner.

(E) Provisions shall be made to infiltrate and/or safely conduct surface water to storm drains or suitable watercourses and to prevent surface runoff from damaging faces of cut and fill slopes.

(F) Adequate maintenance of common and public retention basins or ponds shall be assured through the use of performance bonds or other financial mechanisms.

(G) Subsurface drainage devices shall be provided in areas having a high water table and to intercept seepage that would adversely affect slope stability, building foundations, or create undesirable wetness.

(H) A combination of storage and controlled release of storm water runoff shall be required for all development and construction that drains into wetlands.

(I) The release rate of storm water from all developments that drains into wetlands shall not exceed the rate of storm water runoff from the area in its natural or undeveloped state for all intensities and durations of rainfall. The carrying capacity of the channel directly downstream must be considered in determining the amount of the release.

(J) Where coastal development projects within the Gualala Town Plan planning area have the potential to degrade water quality, the approving authority shall require all relevant best management practices to control polluted runoff, as appropriate.

(K) All development that is within, or drains into, environmentally sensitive habitat, is a commercial or residential subdivision, is a service station or automotive repair facility or that includes commercial development or a parking lot, shall capture and infiltrate or treat, using relevant best management practices, including structural best management practices, all runoff from storms of a magnitude such that the runoff from eighty-five percent (85%) of storms is encaptured or treated.

(Ord. No. 3785 (part), adopted 1991; Ord. 4083 (part), adopted 2002.)