Infiltration and erosion characteristics of selected tephra deposits from the 1980 eruption of Mount St. Helens, Washington, USA

Abstract

Abstract Infiltration and erosion characteristics of two tephra deposits from the 1980 eruption of Mount St. Helens, Washington, were evaluated using a rainfall simulator and selected process models. The deposits were a 20 cm thick silty-sandy profile in the Shultz Creek drainage and a 35 cm thick profile containing a 15–20 cm pumice-gravel layer in the Clearwater Creek drainage. At Shultz Creek, infiltration was affected by surface crusting and erosion. Steady-state infiltration rates ranged from 0.21 to 0.51 cm h−1 in September 1980, and from 0.41 to 0.71 cm h−1 in August 1981. Rill erosion countered the effects of crusting by exposing the more permeable tephra and pre-emption surface. Erosion rates in 1980 decreased rapidly with successive rainfall simulations. Erosion rates in 1981 were 65–80% less than those in 1980 and were more stable. At Clearwater Creek, surface crusting was less evident and sheet erosion was dominant. The steady-state infiltration rate in 1981 was 2.92 cm h−1. The surface runoff volume was small but subsurface flow through the pumice gravel was substantial.