Hydrological Disturbances Caused By Explosive Volcanic Eruptions
Friday, 19 December 2014: 3:05 PM
Explosive eruptions can drastically alter hydrogeomorphic regimes of drainage basins. The extent and degree of eruption-induced alteration scale with eruption magnitude, volcanic process, and basin proximity to a volcano. The most important effects of explosive eruptions on basin hydrology are ones that alter production and routing of runoff: (a) vegetation damage, which decreases (or eliminates) interception and evapotranspiration (ET); (b) reduction of surface infiltration owing to tephra deposition, which increases overland flow; (c) alteration of stream-channel hydraulics, which enables efficient transport of water and sediment; and (d) alterations to drainage networks, which accelerate or delay geomorphic response. In combination, these effects alter flood magnitude and frequency and rates of sediment transport. Vegetation loss allows more water to fall directly to the ground surface and reduces ET, which affects soil moisture, water storage and runoff pathways. Tephra fall, which typically paves the landscape with nearly impervious sediment, can reduce infiltration by as much as 2 orders of magnitude compared to pre-eruption rates and can increase direct runoff from near zero to as much as 90%. Even very thin layers (2–5 mm) of extremely fine tephra can increase runoff and decrease lag times between peak rainfall and peak runoff. Volcanic sedimentation in river valleys can increase channel gradient, reduce planform resistance, and smooth channel hydraulics, allowing for more efficient flow routing and producing larger, flashier flows. Hydrological effects of eruptive disturbance can linger for decades, but the most extreme effects typically last but a few years. However, lake formation through tributary blockage by thick deposits can delay response and extend the hydrologic legacy of eruptive disturbances. Failures of lake-impounding dams can produce large floods that renew downstream channel instability and rejuvenate headwater erosion.