The distribution and management of channelized streams in Denmark

TLDR

The Danish Watercourse Law of 1982 mandates that major river works and maintenance be planned with regard to water quality and channel form, and fluvial geomorphology is essential for nature‑based river management. The study proposes giving special consideration to Denmark’s few naturally sinuous channels in stream management practices. The authors surveyed Danish stream channels using maps, field surveys, and engineering documents, and measured approximately 300 sites to identify five major adjustment processes in straightened reaches, including bed degradation, bank slumping, bed armouring, sinuous thalweg development, and meander restoration. The survey shows that 97.8 % of Danish watercourses are artificially straightened, with channel‑work density 300 × higher than in the U.S.

Abstract

Abstract The Danish Watercourse Law of 1982 states that major river works and maintenance procedures must be planned and undertaken with regard to water quality and the physical form of the river channel. Fluvial geomorphology is an essential component of alternative procedures of river management which work with nature rather than against it. A survey of Danish stream channels has been produced by collating data obtained from maps, field surveys and engineering documents. This indicates that 97.8 per cent of Danish watercourses have been artificially straightened and that only 2.2 per cent (880 km) have natural morphological characteristics. The density of channel works is 300 times greater than in the U.S.A. and 15 times greater than in England and Wales. It is suggested that since there are only a few naturally sinuous channels in Denmark these should be given special consideration with regard to stream management practices. Measurements undertaken at approximately 300 sites in Denmark indicate five major processes of adjustment within straightened reaches. These adjustments include degradation of the channel bed with associated bank slumping, armouring of the bed, the development of a sinuous thalweg by the deposition of sediment at low gradients, and restoration of meanders. Channel stability is related to specific stream power and the results may be useful in predicting potential adjustments following channel straightening. Deposition in natural reaches below straightened channels often obliterates pool and riffle sequences but several channel adjustments may provide a greater variability of physical habitats than the initial uniform straightened channel; their significance for the biology of the watercourse is best considered before traditional management practices are carried out to control the adjustment.