Kronvang, B. & Larsen, S.E. 2023. Virkemiddel for brinkerosion og fosfortab ved restaurering af vådområder og vandløb. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 33 s. - Teknisk rapport nr. 263, http://dce2.au.dk/pub/TR263.pdf
Statistical analyses of a comprehensive set of previously collected data on bank erosion in Danish stream types from River Odense and River Skjern have been used in this report to develop proposals for a management tool allowing estimation of the effect of different types of stream restoration on bank erosion and the resulting phosphorus loss to surface water. The tool will be used in the planning of wetland and stream restoration projects to obtain an estimation of the significance of bank erosion in the pre-and post-situation on the basis of collected standard information on the stream.
The statistical analyses of data from River Odense showed that there is a difference between bank erosion in different sizes of streams (Strahler order), categorised into channelised and meandering streams, respectively. In small streams (Strahler Order 1 & 2), the statistical analyses identified a major difference between the annual average bank erosion in the meandering streams in the younger moraine landscape (18 mm m-1 year-1) and the meandering streams in the old heath plain and moraine hill landscape (89 mm m-1 year-1). In the analyses, an almost similar difference was found for the annual average bank erosion in the channelised streams in the two landscape types (21 and 45 mm year-1). Measurements of the bank erosion 10 years after the remeandering of the lower part of River Skjern also demonstrated a relatively large annual average bank erosion rate (41-64 mm m-1 year-1) just as the erosion rates recorded for heath plain and moraine hill streams. In general, there is no significant difference between bank erosion in channelised and meandering streams in the moraine landscape, whereas the difference is relatively significant in streams in the heath plain and older moraine hill landscape. However, relatively few measurements are available on streams in the heath plain and the older moraine hill landscape; thus, the data basis needs to be improved.
The slope of the bank also has an impact on the extent of bank erosion. A model was developed showing a significant increase of bank erosion with enhanced bank slope based on statistical analyses of all data from River Odense. The model developed for the impact of the bank slope will therefore be included in the efficiency estimations of the management tool based on collected data on the slope of the banks in the pre-and post-situation in connection with the establishment of wetlands and actual stream restoration projects. Finally, a significant impact was found of the occurrence of high vegetation (trees) along streams in relation to e.g. grass in the buffer for the extent of the bank erosion in the different stream types. It is thus possible to estimate the effect of establishing trees along streams in connection with a wetland or stream restoration project. Trees in the buffers along streams must be naturally occurring species such as willow or alder that either immigrate or are planted in connection with the restoration project.
Conversion of the tool’s bank erosion rates to an expected phosphorus loss is carried out by using the many analyses of the phosphorus content of bank material in nine georegions of the country. The developed management tool for phosphorus loss in connection with bank erosion therefore contains the following parameters: 1) the location of the stream in the two main landscape types; 2) the size of the stream (Strahler order); 3) the location of stream in the georegions; 4) stream type (channelised/meandering); 5) stationing (sections) of the stream; 6) the slope of the bank in the pre-and post-situation; 7) information about trees in the buffer along the stream before and after the restoration.
In the report, the management tool was applied to three older stream restoration/wetland projects as well as three recent wetland projects. Only in one of the three older restoration projects did the bank erosion decrease as an effect of the remeandering of the stream. In the estimations for the three recent wetland projects, bank erosion and phosphorus loss decreased in one of the projects and increased in two of the projects. None of these six restoration projects included planting of trees in the buffer along the streams, which would effectively have contributed to reducing the bank erosion. The results clearly show that it is important to include bank erosion when estimating the efficiency of management tools. This can contribute to optimising the projects so that the extent of the bank erosion after the restoration is minimised, which will also minimise the phosphorus loss to streams.
