Calculations for 1996 Slide Deposit at H.J.A. Watershed #3 On February 6th, 1996, around 2100hr heavy rain combined with snowmelt triggered several road and slope failures in Watershed #3. Material from these failures entered the stream channel and raced downstream scouring the channel down to bedrock. This material came to rest in the area of the stream gauging station and sediment basin covering both with a deposition of saturated soil, logs, and rock. Visual observations and the step-like deposition profile suggest multiple deposition occurrences over time rather than a single episode. An effort was made to duplicate the measurement methods used at WS #3 after the 1964 slide event, but several factors required some changes. First, the basic shape of the stream channel did not follow the assumption of a half cylinder form of the valley floor used in 1964. The addition of the access road to the gauging station alters this assumption. Second, monuments of known location and elevation could not be found. Finally, the deposit appeared to go much further upstream than the 1964 deposit. A separate measurement of large organic debris (LOD) was also done due to increased interest in this component. Measurements The measurement of LOD was done using log calipers and a "Sonin" electronic distance meter. Only material greater than 2 meters in length, and 15 centimeters was measured. The length was recorded and diameters were taken at each end. Lengths were estimated for partially buried pieces, and the known diameter was used for both ends. The deposition soil and fine OD component measurement was done using a staff compass, abney level, and 100m nylon/fiberglass tape. First, a mapping baseline was laid-out the length of the deposit using the paved surface of RD.1506 and a bedrock outcropping found near the assumed location of the flume as elevational controls along the baseline. Slope distance, abney reading, and bearings were recorded. At horizontal distances of 15 meters along the baseline a wire pin was set. Next, a traverse at right angles to the baseline was done at each 15m pin to each edge of the deposit. Slope distance, abney readings, and bearings were recorded. Care was taken to make sure elevational measurements were done on the soil component. The baseline and associated right angle traverses were plotted to generate longitudinal and cross-sectional elevation profiles of the deposit and a area map of the stream valley (See fig.1 and fig.2). The right angle traverses also sectioned off the deposit into 15m long sections for volume calculations. Material that flowed down Road 1506 that was then plowed to the side was measured with a 100m tape and a meter stick. Over a distance of 185 meters the width and height of the berm was measured every 5 meters. Large boulders were measured individually for height, width, and length. LOD Calculations LOD measurements were entered into a FOXPRO database. Volume was calculated using the program LOGVOL developed by Hazel Hammond. The following equation was used to calculate the volume of individual pieces: Volume = Length*[BAB + SQRT(BAB*BAT) + BAT] /3 where BAB is the basal area of the base, and BAT is the basal area of the top. Basal area = 3.146*(Diam/2)^2 where Diam is the diameter of the base or top end in the same units as length. Using LOGVOL a total volume of 1801.0 cubic meters of woody debris larger than 15cm in diameter and 2 meters in length is calculated. Slide / Bedload Calculations Since all monuments of known elevation and position were destroyed, and no profiles of the valley floor were done after the 1964 station reconstruction several assumptions had to be made to establish a pre-event elevational and area profile of the valley floor. First, it was assumed that the bedrock outcropping found 82.23m from Rd. 1506 is at the same relative distance up the channel as the flume (See fig1). Second, it was assumed that the post-slide stream position is now occupying the old access road. And is also assumed that the grade between the baseline point P2 (which is on the paved surface of Rd. 1506), and points P3 through P5 (which are in the post- slide streambed) is approximately the same as the access road (See fig.1). By extending this grade line out the distance of the bedrock outcropping an estimate of the flume elevation was made. Finally, it was assumed that a line projected from the estimated flume elevation to the estimated elevation of the post-'64'culvert (The post '64' culvert was found and measured, but only at outflow end) represents the pre-slide depth and grade of the stream channel (See fig.1). The pre-slide stream elevation and grade above the flume was established by plotting from the estimated flume position distance and elevation measurements from a stream elevational profile survey done in 1991 (See fig.1). Using the elevational profile values calculated from the right angle traverses at the 15m points along the baseline a mean profile elevation was determined and plotted above the map baseline profile (See fig. 1). This profile was estimated past baseline point P10 by extending the profile using the grade slope between P9 and P10 as the estimated grade above P10. The ending distance of 275.5 m was estimated. Any deposition beyond this distance was considered insignificant. The width, depth, and length of the pre-slide sediment basin was taken from the sediment basin survey record. The basin position in the valley floor was estimated from memory and was plotted on figure 2. The width of the pre-slide stream channel was also taken from the sediment basin survey record and was plotted (See fig.2). After plotting up the survey information and establishing some elevational references, calculations of volume were done for each 15m section within 3 distinct zones of the deposit. The zones are: 1. The area between the mean baseline elevation profile and the estimated access road grade up to the flume. 2. The area between the estimated access road grade and the estimated stream gradient up to the flume (Area confined to pre-slide channel and sediment basin). 3. The area above the flume between the mean baseline elevation profile and the stream gradient above the flume profile. In zone #1 it was assumed that the deposit in each section was rectangular in shape, with the deposit on the old road surface, and above the pre-slide stream channel. Section widths, lengths, and mean elevations at each end were averaged then multiplied to get volume. The following equation was used: Volume = [(W1 + W2)/2] * [(L1 + L2)/2] * [(E1 + E2)/2] W = Section width at each end L = Section length at each side E = Mean section elevation at each end In zone #2 it was assumed that the pre-slide stream channel and sediment basin are the shape of a half-cylinder. Stream channel/sediment basin widths at each end of the section were averaged with the section end difference in elevation between the estimated road grade and estimated stream gradient. This was used as the cylinder radius. The following equation was used: Volume =[ 3.146 * (R^2 * L) ] /2 R = [(W1 + W2)/2 + (H1 + H2)/2] /2 W = Section width at each end H = Section height at each end L = (L1 + L2)/2 = Mean length of section In zone #3 the same assumptions of a half-cylinder stream channel form was made, so the same equation used in zone #2 was used to calculate volume. The elevation or height of each section end are equal to the difference in elevation between the 1991 survey channel grade above the flume (See fig.1) , and the map baseline elevational profile ( This profile was estimated above station P10.). The volume of the roadside accumulation was calculated by averaging the 5m widths and heights and using the total distance (185m). The equation used was: Volume = W * H * L W = Mean width of 5m measurements H = Mean height of 5m measurements L = Total length