Abstract
Gully networks developed on coastal hillslopes in San Mateo County, California, extended nearly 300% from 1941 to 1978, primarily by subsurface erosion. In one drainage, subsurface erosion expanded the gully network by 80 m, nearly doubling the gully length from 1982 to 1987. Intense gullying is restricted to areas underlain by members of the Pliocene Purisima Formation, a unit rich in unstable volcanic lithic fragments, which weather to produce expansive smectite clays. Gully development proceeds through a sequence of lateral eluviation of clays near the base of the A horizon (piping), development of an open conduit in which erosion proceeds by turbulent, concentrated flow (tunneling), and finally, collapse of the tunnel roof, exposing an open gully. Measurements made in one gully system during a 25-yr storm showed that the subsurface drainage network carried four times as much flow as the surface drainage net and had average suspended sediment concentrations an order of magnitude higher.
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