River channel adjustment and sediment budget in response to a catastrophic flood event (Lainbach catchment, Southern Bavaria)

Abstract

In summer 1990 the Lainbach catchment experienced a thunderstorm-induced flood with a recurrence interval far in excess of 100 years of a magnitude never registered before. The Lainbach has and the effects of the flood could be observed directly in the field. For the 120 m long measuring reach a new topographic map was drawn after the flood on the same level of resolution (0.1 metre contours) as the basemap produced in 1988. A comparison gave detailed information on river bed changes. Using pre-flood and post-flood longitudinal and cross profile surveys, sediment budgets were calculated for the mountain reaches of the Lainbach and Kotlaine. The main sources of eroded channel sediments lay upstream of the broken check dams. River segments with low slopes were generally most affected by high accumulation rates. In the geodetically surveyed reaches considerably more material was accumulated than eroded, resulting in an overall mass balance surplus of about 35000 m³. A large part of this material was mobilized in the tributary channels and slope rills. Slope failures were only of minor importance, characterizing this catastrophic event as a channel flood. The flood caused a fundamental transformation of the channel system.