The Contribution of Bedrock Groundwater Flow to Storm Runoff and High Pore Pressure Development in Hollows
Many analyses of runoff generation, pore pressure development, and slope stability in colluvium-man tled hollows assume that the bedrock-colluvium boundary forms a major hydrologie barrier during periods of intense precipitation. Here we present detailed field observations from one year of intensive monitoring at our 16,000 m2 grassland basin in Mt. Tamalpais State Park, Marin County, California, U.S.A. These data demonstrate that significant large- and small-scale interactions between storm flow in the bedrock and the colluvium occur at our site. The basin-scale, topographically driven, flow system terminates at the channel head where a low-permeabili ty block of bedrock forces groundwater to the surface with an average exfiltration gradient of 0.5 m/m and excess pressure head of 1 m within the basal colluvium. This low- permeability block also forces the water table to remain close to the ground surface along the hollow axes, enabling several storms per year to generate significant saturation overland flow. Surface erosion by saturation overland flow may be the dominant process causing channel incision in our relatively low-gradient hollow. Local bedrock permeability heterogeneities cause fluctuations in the position of the bedrock water table and result in unexpected midslope high pore pressures and a persistent discontinuous zone of saturation overland flow. Presently, exfiltration along the hollow prevents build up of pore pressures in the bedrock and colluvium sufficient to cause landsliding, but on steep hillslopes local upwelling of bedrock storm flow may strongly influence soil instability.