The spur dike technique for preventing check dam fishway blockages
Photo 1: A fishway entrance blocked
Photo 2: A spur dike installed at
a fishway entrance
(click to enlarge)
Photo 3: Experimental channel model
(click to enlarge)
Fig. 1: River bed variation after water was
introduced to the experimental
channel model(click to enlarge)
1. Background to related technological development
In response to recent increased public interest in river environments, fishways have been installed at dams, headworks and other structures that affect fish swimming upstream in order to create favorable conditions for their passage.
At least 2,300 fishways have been installed across Hokkaido, and at least 40 percent of these are located at check dams or other erosion/flood control facilities.
To secure habitats for a wide range of fish migrating upstream and downstream (including masu salmon, whose spawning sites are located in upstream areas), it is important for check dams and other upstream structures to have facilities that allow fish to swim upstream.
However, as check dams are intended to stop sediment runoff, extremely large amounts of sediment transported during floods may block fishways (Photo 1).
To address this problem, such sediment accumulating at fishway entrances should be removed when a blockage arises. However, due to recent cuts in public works spending, low-cost measures are required to prevent this type of blockage on an ongoing basis.
Against this background, a technology involving the use of spur dikes to control sediment deposition was developed. Its adoption is expected to reduce construction costs due to the use of boulders accumulating at check dams and thereby reduce expenses relating to maintenance (Photo 2).
Spur dikes project from river banks into the channel to divert flow and reduce its strength.
2. Technology overview
An experimental channel model consisting of a spur dike and a check dam with a fishway entrance was used to examine how dike shape and location affected sediment accumulation at a fishway entrance (Photo 3 and Fig. 1).
A spur dike designed based on the results was installed at an actual check dam whose fishway had previously been blocked by flood-borne sediment (Photo 2).
After the occurrence of several floods in the post-installation period, no further major sediment deposition was observed at the fishway entrance. Accordingly, the spur dike method can be seen as a very cost-effective approach to preventing such blockage.
3. Publication of results
The study results have been published in research papers and manuals. For technical advice on specific related matters, please contact the Watershed Environmental Engineering Research Team at the Civil Engineering Research Institute for Cold Region.
(Contact: Watershed Environmental Engineering Research Team, CERI)