§ 18.108.060. Slope regulations—Prohibited uses.

A.

Uses Prohibited Without an Exemption or an Exception. To minimize the risks associated with project development in areas characterized by steep slopes, high erosion potential, unstable soils, combustible vegetation and other sensitive environmental resource areas as defined by Section 18.08.270 or designated in the conservation element, the recreation and open space element and the safety element of the general plan, no construction, improvement, grading, earthmoving activity or vegetation removal associated with the development or use of land shall take place on those parcels or portions thereof generally having a slope of thirty percent or greater as defined in Section 18.108.060(C) unless exempt under Sections 18.108.050 or 18.108.055 or unless an exception through the use permit process is granted pursuant to Section 18.108.040.

B.

Uses Prohibited Without an Exemption. No structure, improvement, grading, earthmoving activity, vegetation removal or development shall be permitted on a slope greater than fifty percent unless exempt under Section 18.108.050 or 18.108.055 of this chapter.

C.

Slope Determination Methodology. The percent slope for a proposed project is described as the ratio of the vertical distance to the horizontal distance, or the elevation change in feet divided by distance in feet measured perpendicular to the contours. The percent slope of the development area described herein is the natural slope of the existing terrain, not the finished or proposed percent slope resulting from the project.

1.

Structural Development.

a.

The percent slope of the structural development area is measured perpendicular to the contours across the building pad and driveway when the driveway is less than 50 feet in length. The slope determination shall be made by evaluating a plot plan identifying contour intervals of two (2) to five (5) feet, with a scale of 1" = 20' or better.

b.

When a driveway exceeds 50 feet in length, the slope of the structural development area is measured perpendicular to the contours across the foundation and area of ground disturbance around the foundation of the proposed structure. The driveway slope shall be measured separately as identified in subsection (C)(2) of this section.

2.

Road/Access Development. The analysis for slope determination for grading involving a roadway longer than 50 feet shall be determined using the following criteria and shall be based on mapping with a maximum scale of 1" = 50' with contour intervals of five (5) feet or less.

a.

The approximate centerline of the proposed roadway shall be stationed with 0+00 being assigned to the point where grading commences.

b.

Cross sections shall be taken at each station 100 feet apart, i.e., 1+00, 2+00 etc., extending to the outer limits of grading. When the road is less than 200 feet long, then three equally spaced cross sections shall be taken. The axis of each cross section shall be perpendicular to the contours pertinent to the section. These sections shall be drawn to a scale of 1" = 10' horizontal and vertical. An average cross section slope shall be calculated by dividing the difference in elevation of the cut and/or fill catch points by the intervening distance.

c.

The average slope of the project is to be determined by averaging all of the cross sections excluding those measured at less than five percent slope. The average slope shall not exceed thirty percent and no more than three (3) cross sections shall exceed fifty percent.

3.

General Land Clearing. When earth-disturbing activities, land clearing or grading involves 30 acres or less, the slope of contiguous lands (i.e., not separated by streams, roads, or noncleared areas) is measured from a map with a scale of 1" = 100' (maximum) with contour intervals of 20 feet, or at a contour interval acceptable to the director. When earth disturbing activities, grading or removal of vegetation involves greater than 30 acres, a contour interval of two (2) feet in the Napa River Watershed and five (5) feet in other areas shall be applied. If any portion within the contiguous area to be cleared is greater than thirty percent slope, then the following standards apply:

a.

If the total area of any contiguous earthmoving, grading or land clearing is larger than one (1) acre, subareas up to one acre in size in the thirty to fifty percent slope range may be cleared subject to administrative approval by the director. No earthmoving activities, grading or land clearing exceeding one (1) acre shall occur on slopes greater than thirty percent.

b.

If the total contiguous area to of proposed earthmoving, land clearing and/or grading is less than one (1) acre, no more than ⅓ (one third) of the project area to be cleared or graded may exceed thirty percent slope, subject to administrative approval by the director.

(Ord. 1269 § 3, 2005: Ord. 1082 § 11, 1995; Ord. 1062 § 5, 1994; Ord. 991 (part), 1991: prior code § 12453)

(Ord. No. 1438, § 9, 4-9-2019)