(a) Purpose. The purpose of this section is to protect areas with steep slopes from erosion caused by surface runoff due to development. These provisions apply to all properties in Single-Family R-1 and R-1A, and Large Lot Single-Family Residential RS-1 Zoning Districts with 12% or greater slope shown in a final plat or otherwise approved by the City Council after May 1, 1993.
(b) Lots with an average slope of 12% or greater. For lots with an average slope of 12% or greater impervious surface coverage shall be limited according to the following table:
Average Slope | Maximum Coverage | Average Slope | Maximum Coverage |
Average Slope | Maximum Coverage | Average Slope | Maximum Coverage |
12% | 34% | 22% | 24% |
13% | 33% | 23% | 23% |
14% | 32% | 24% | 22% |
15% | 31% | 25% | 21% |
16% | 30% | 26% | 20% |
17% | 29% | 27% | 19% |
18% | 28% | 28% | 18% |
19% | 27% | 29% | 17% |
20% | 26% | 30% | 16% |
21% | 25% | ||
Impervious surface coverage for properties with average slopes greater than 30% shall be decreased by 1% for each percent of slope greater than 30%.
(c) Average percentage slope. Average percentage slope shall be calculated according to one of the following methods, based on existing conditions prior to the proposed development:
(1) In those instances where the lot is rectangular or regularly shaped, and the contour lines are generally parallel throughout the property, the average percentage slope may be calculated by determining the slope of a line drawn from the highest point on the property to the lowest point on the property, according to the following formula:
S = (VH - VL)/H S = Average percentage slope VH = Elevation of the highest point of the line above mean sea level VL = Elevation of the lowest point of the line above mean sea level. In no event shall the lowest point used in this calculation have an elevation lower than 722 feet above mean sea level. H = The horizontal distance separating the highest point from the lowest point |
(2) In those instances where the lot is rectangular or regularly shaped, and the contour lines are not generally parallel through the property, the average percentage slope may be calculated by determining the average of the slopes of several lines, evenly spaced across the property, drawn from the highest points on the property to the lowest points on the property. The slope of each line shall be calculated in the manner specified in subsection (c)(1) above;
(3) In those instances where the lot is irregularly shaped, the following formula may be used:
S = (0.00229 x L x F)/A S = Average percentage slope L = Length of all contours (above 722 feet above mean sea level) in feet F = Contour interval in feet A = Area of the lot (above 722 feet above mean sea level) in acres |
(4) Any other methodology commonly recognized in the field of civil engineering and which is approved by the issuing authority. In no instance shall the calculation incorporate elevations below the 722 foot contour elevation above mean sea level.
(d) Special provisions.
(1) Surface runoff redirected away from slopes. In those instances where surface water runoff from impervious surface is directed away from steep slopes and into a public storm sewer or public or approved private sedimentation pond, such areas shall not be considered as impervious surface. However, in no instance shall the total impervious surface, including those areas affected by this provision, exceed the maximum impervious allowed in the zoning district.
(2) Development occurring on areas with less than 12% slopes. In those instances where all impervious surfaces on lots with steep slopes are located in an area of the lot with slopes of less than 12%, and surface water runoff from impervious surface is directed away from steep slopes and into a public storm sewer or public or approved private sedimentation pond, the restrictions of this section shall not apply.
(3) Mitigation. When the City Council grants relief from the requirements of this section by granting of a variance, approval of a Planned Development (PD) Overlay District, approval of a conditional use permit for a Neighborhood Unit Development or other action, the City Council may require mitigation of the effects of surface runoff on steep slopes by any reasonable method, including but not limited to:
(i) Trees. The City Council may require applicants to plant additional trees to intercept rain water on open portions of the lot. Any overstory tree of two and one-half caliper inches or greater or any evergreen tree of six feet or more feet in height at the time of planting, shall qualify as meeting this requirement. This provision shall not be required for any lot which has more than one tree of qualifying size (but not species) for each 3,000 square feet of lot area;
(ii) Redirection of surface water. The City Council may require applicants to redirect surface water runoff away from steep slopes by the provision of gutters and downspouts, reorientation of roof slopes, regrading and/or installation of an approved private storm sewer system; and
(iii) Removal of existing impervious surfaces. The City Council may require the removal of certain existing impervious surfaces which will reduce surface water runoff. This requirement may include replacement of impervious plastic sheeting under landscaping materials with permeable landscaping membrane, replacement of parking areas (other than in driveways) with permeable surfaces, or removal of any impermeable surface which is in violation of any of the provisions of this code.
(Ord. 82-54, passed 12-27-1982; Ord. 93-14, passed 3-29-1993; Ord. 2004-44, passed 11-15-2004)