Two large, high-velocity lahars (volcanic debris flows) were triggered by a pyroclastic surge during the first few minutes of the May 18, 1980, eruption of Mount St. Helens. The initial surge cloud evolved progressively by gravity segregation from a gas-mobilized, highly inflated density flow to a dense, water-mobilized, basal debris flow (lahar) and accompanying ash cloud as it flowed down the east flank of the volcano. The main source of the water for the lahars was probably from eroded snow and ice incorporated into the flow by turbulent mixing, but ground water, expelled together with the rock debris by the initiating volcanic explosions, also may have contributed.

Peak lahar discharge from the Pine-Muddy fan, upper Smith Creek, and Ape Canyon probably exceeded 250,000 m3/s initially but decreased exponentially in the downstream direction. Total volume of the lahars was in excess of 1.4 × 107 m3. Initial peak-flow velocities in excess of 30 m/s also decreased markedly downstream. Where flow was not impeded, velocity was strongly related to the depth-slope term (R2/3S1/2) from the Manning uniform-flow equation as a power-law function. During much of the route traveled, lahar flow appears to have been supercritical. Deposits left in channels were generally thin relative to flow depth (0 to 2.5 m). Particles up to small boulder size were randomly distributed in the poorly sorted, nonstratified matrix, indicating complete suspension in a fully developed debris-flow slurry; however, much larger clasts were transported as “bed-load.” Computed sediment concentrations of matrix slurry samples ranged from 84% to 91% solids by weight and were similar for the two lahars.

Two indirect methods for computing peak-flow velocity, previously only tentatively applied to debris flows, were tested for accuracy by comparing computed lahar arrival times with recorded arrival times at Swift Reservoir. The computed velocities appear to be ∼15% slower than the recorded velocities, which is consistent with the restriction that the velocity formulas produce only minimum values.

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