Assessment of changes in storm and seasonal runoff response of watersheds impacted by Mt. St. Helens Ash Deposition

Datta, Bithin, Lettenmaier, Dennis P., and Burges, Stephen J. (1983) Assessment of changes in storm and seasonal runoff response of watersheds impacted by Mt. St. Helens Ash Deposition. Report. University of Washington, Seattle, WA, USA.

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The most significant effects of the ash deposition following the major Mt. St. Helens eruptions in the state of Washington, U.S,A., during May to July 1980 were twofold. The immediate consequence of the ashfall was either to increase or decrease the runoff from snow-covered areas, depending upon the thickness of the ash cover. The thick­ ness of ash deposits over some of the catchments in the vicinity of Mt. St. Helens were of the order of the most effective thickness for accelerating the ablation of a snowpack. This effect however, was short-lived and the changed rate of snowmelt returned to pre-eruption conditions by the end of water year 1980.

The second consequence of the ashfall was to change the hydrologic re­sponse of ash covered catchments to a given amount of precipitation. To study the changes in the hydrologic response of a catchment, it was necessary to model both the snowmelt process to determine effective precipitation, and the transformation of effective precipitation to runoff. Difficulties in cali­brating existing snowmelt models due to large variations in topography and precipitation patterns led to rejection of classical approaches. As an alternative, it was decided to modify the Constrained Linear System (CLS) model in such a way that the physical characteristics of the catchments could be explicitly incorporated.

The overall conclusion that may be drawn from this investigation is that, for months of high flows the overall response of the catchment to a given gross precipitation remains almost unaffected. However, for the summer months June to September, the post-eruption runoff occurring for a given gross precipitation appears to be lower than that expected from pre-eruption conditions. However, this condition should influence possible increases in flood hazards, especially for the Toutle, where the increases in flood hazards are more related to the changes in travel time and channel carrying capacity due to direct deposits of silt and debris, than to changed runoff response. These changes were analyzed on a monthly basis and on a daily time scale. Therefore, similar inferences can be drawn for seasonal responses, when the effects applicable to daily events are lumped together to predict changes in seasonal response of the catchment

Item ID: 78591
Item Type: Report (Report)
Keywords: Hydrologic Modeling, Volcanic eruption response of catchments, Hydrologic Risk assessment
Date Deposited: 24 May 2023 22:56
FoR Codes: 40 ENGINEERING > 4005 Civil engineering > 400513 Water resources engineering @ 100%
SEO Codes: 18 ENVIRONMENTAL MANAGEMENT > 1803 Fresh, ground and surface water systems and management > 180301 Assessment and management of freshwater ecosystems @ 100%
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