RES 17-981 - Laserfiche WebLink

1. Identify 25 Percent Over/Under Slopes. An initial task involved mapping slopes over and under 25 percent. Slope is generally defined as rise (vertical distance up a hill) divided by run (horizontal distance across the landscape), multiplied by 100 to generate a percentage (see Figure 1). Most local communities with hillside protection ordinances recommend using mapping with contour intervals less than 5 feet. The City has very granular topographical mapping of the Southeast Hills that uses 1-foot contours, dating from 2014, which was used as part of the mapping presented in this report. The granularity of the topography was next averaged-out over 3-foot by 3-foot areas, consistent with standard Geographic Information Systems (GIS) slope mapping protocol. This approach is thus protective in that even small variations in slope using 1-foot contour intervals are mapped, but the "noise" of minor variations in topography is reduced by averaging out slopes over 9-square-foot areas. These 9-square-foot areas are the smallest mapping unit under this methodology and should not be combined to undertake further slope averaging. In other words, if a 9-square-foot area has an average slope above 25 percent, it would be subject to Measure PP considerations. Attachment 6 is the resultant map of slopes over and under 25 percent using the methodology described above. Figure 2 illustrates sample slopes on Longview Drive, a road that climbs steeply west from Foothill Road, up Pleasanton Ridge. Figure 1: Calculation of Slope Vertical Change Rise /Rise Slope = = X 100= Slope %Horizontal Change 2. Define Ridgeline. There is no formal definition of "ridge" or "ridgeline" in Measure PP or the General Plan, but Chapter 18.76 of the Municipal Code for the Hillside Planned Development District defines ridge as "a connected series of major and minor hills" and a ridgeline as "a ground line located at the highest elevation of the ridge running parallel to the long axis of the ridge." This latter definition is useful in the Southeast Hills, which is characterized by long ridgelines running in a generally northerly or northwesterly direction. While the long ridgelines in the Southeast Hills seem to obviously meet the definition of the landform, many ridges contain short "spurs" that extend in a perpendicular fashion off the main ridge. These spurs, likely created by two parallel drainages flowing down the side of the ridge from the ridgeline, typically fall steeply to the valley floor. If portions