The geomorphological effectiveness of floods—a contribution stimulated by two recent events in mid‐wales

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Abstract
The occurrence of two ‘rare’ floods (August 1973, August 1977) in the Plynlimon experimental catchments has confirmed the susceptibility of small upland catchments to summer flooding and provided clues to complications in the geomorphological interpretation of floods in terms of their magnitude and frequency. Magnitude may be treated both in terms of work and effectiveness; the emphasis here is on effectiveness, as revealed by simple surveys. The first Plynlimon flood was more effective on slopes and the second in channels, despite peak discharges of similar return periods and almost identical rates of work revealed by bedload trapping. Effectiveness/frequency studies are likely to require a much more detailed approach, subdividing both the characteristics of the flood and the spatial elements of the affected catchments; a simple slope/channel subdivision is found to be suitable for accounts of effectiveness found in the literature on British floods this century. Effectiveness studies also require regular surveys throughout the recovery period following major flooding; in upland catchments these surveys should concentrate on identifying threshold phenomena and illustrating the relationship between effectiveness and work assessments of magnitude.

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