Open-File Report 96-517
Vertical elevation changes associated with an earthquake cause the topographic slope to be altered and this in turn can result in shifts in flood plain boundaries. The elevation changes created by the Northridge earthquake caused minor changes in flood plain levels. The horizontal shift in flood plain boundaries, dx , is determined by subtracting the postseismic position of the flood plain boundary from its preseismic position. This is calculated using where x is the preseismic and x ' the postseismic position of the flood boundary, a is the preseismic topographic slope, B [beta] is the change in slope caused by the Northridge earthquake and y is the original height of the flood surface above a certain point (Figure 14). In the Northridge area the topography generally became steeper as a result of the thrust earthquake. The horizontal shift in flood plain level is greatest where the topographic slope is most gentle and where the earthquake-associated ground tilt is largest.
In the Canoga Park, Reseda and Sepulveda Dam areas, where the topography is very gentle, flood plain changes were small since the elevation changes were less than 10 cm. Where elevation changes were large, the topography was steeper and the effect of the coseismic uplift on the topography was small. Elevation changes were greatest in the Granada Hills area and the Santa Susana Mountains. The 300 m topographic contour level, which runs through Chatsworth and Granada Hills and borders the Hansen Dam and Chatsworth Reservoir, received the largest shifts. The topographic gradient ranged from 1/41 (1.4deg.) to 1/115 (0.5deg.) and the corresponding northward shift in the 300 m contour level ranged from one meter to 28 meters. The change in the flood plain boundary around the Chatsworth Reservoir was at most 12 meters while at the Hansen Dam the shift in contour level was less than one meter and insignificant. The change in tilt (metres per kilometer ) across the San Fernando Valley and the Santa Susana Mountain Range are contoured in Figure 15. The tilt is positive towards the north, i.e., uplifted towards the north in the valley, and negative on the north side of the Santa Susana Mountain range.
Flood plain areas are affected by the rate of delivery of floodwater to the plain. Since the Northridge earthquake altered the gradient of stream channels the change in gradients for streams flowing into the San Fernando Valley were also calculated. The Browns Canyon, Limekiln and Aliso Canyon are three main stream channels which drain into the San Fernando Valley from the Santa Susana Mountain Range. The change in gradient for the Brown Canyon is about 4.2 to 5 cm/km (0.0025deg.) in the foothills and as far as Chatsworth. Gradient changes along the Limekiln Canyon were slightly larger at ~6.5 cm/km (0.0037deg.). Between the 500 m and 300 m contour intervals, the change in gradient along the Aliso Canyon and Aliso Canyon Wash was 5 to 7 cm/km (0.0028deg. to 0.004deg.). The gradient change in stream channels in the Santa Susana Mountains was therefore very small.
U.S. Department of the Interior |
U.S. Geological Survey
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