The contribution of gully erosion to the sediment budget of the River Leira

Abstract

This paper reports the results of a research project aimed at quantifying the sediment budget of the River Leira in southern Norway in order to reduce its sediment load and improve water quality. The sediment load arises from the erosion of agricultural land and from extensive gullying in the drainage basin. The sediment budget was determined through a monitoring programme that included several stations along the main stem of the river and in the gullies. Long term variability in sediment transport was determined by analysis of lake deposits, overbank sediment from flood plains and long term records of floods and landslides. An examination of the stability of the gullies revealed the presence of two types. In the unstable type, the dominant process was landslides, whereas lateral erosion in streams and soil creep characterized the stable ones. The measured annual sediment load was 5431 and 5071 respectively in the two gullies, corresponding to sediment yields of 1131 t km 2 year1 and 141 t km 2 year1. The mean annual sediment load was measured to be 73 0001 in the lower part of the main stem. Using the USLE equation, the erosion of agricultural land was estimated to contribute about 45% of the total load. Gully erosion and river bank erosion thus contribute 55% of the total load. The long term record of major floods and landslides shows a corresponding periodic behaviour. Subsequent to large floods, the baselevel of the gullies is lowered and the frequency of landslides increases. The sedimentation rate of lake and overbank sediments increases with the combined effect of frequent floods and landslides and the changes in agricultural practice that took place in the last part of the 1960s.