Estimating the Thickness of Colluvial Fill in Unchanneled Valleys from Surface Topography

Abstract

Two procedures for estimating the thickness of colluvium in unchanneled valleys using easily measured surficial characteristics are proposed. Depth-to-bedrock predictions are tested at 12 sites in northern California and Washington where surface topography is known and the depth of colluvial infilling has been independently determined. In the contour displacement technique, the bedrock geometry is approximated as a tipped triangular trough. The contour displacement is defined as the map distance between the intersection of tangents to side-slope contour segments on opposite sides of an unchanneled valley and the actual position of the contour in the valley axis. Colluvium thickness is estimated from a trigonometric relationship between the contour displacement and hollow gradient. This technique accurately predicts the depth to bedrock for sites with planar side slopes but underpredicts this depth for sites with convex side slopes. The curve fitting technique fits second-order polynomial equations to side-slope profiles on either side of the hollow and solves for their intersection beneath the depositional zone. The thickness of the colluvial deposit is estimated as the distance between this point and the true ground surface. This procedure gives reasonable estimates for sites with distinct side-slope convexity but overestimates the thickness at sites with nearly planar side slopes. Accurate depth-to-bedrock predictions are obtained by using the contour displacement technique at sites with planar side slopes and the curve fitting technique for sites with convex side slopes.