Tuesday, 10 November 2015

Effect of grade-control structures on bed sediments and local channel parameters

Grade-control structures (GCSs) including check-dams and bed sills represent the typical management of torrential streams, preventing massive bed erosion and bedload transport. On the other hand, they strongly affect connectivity of aquatic biota and sediments. A lot of papers have been published about the effect of large dams on discontinuity of bed sediments and channel incision so far, but the clear relationship between check-dams and channel processes in dynamic mountain channels remains less understood. Our last paper published in Geomorphology deals with issues of downstream grain-size trends and development of scours in the longitudinal profile of the Mohelnice River (A = 40 km^2, S = 1-2 %, W = 15-20 m) with 18 GCSs in various stage of their destruction. These GCSs were (re)constructed in 1970s and since that, they were damaged by high-magnitude floods in 1996, 1997 and 2010. We measured present grain-sizes by Wolman method in sedimentary wedges of these GCSs and also scour geometry related to GCSs. In addition, present geometry of GCSs was measured and compared to their original stage in 1970s to determine their stage of destruction.

Partly destructed check-dam in the Mohelnice River

Besides other things, obtained results demonstrated that downstream coarsening especially of higher grain-size percentiles occurs throughout the sequence of GCSs. Present grain sizes do not correlate with the present channel slope or the present height of GCSs. However, the sequence of 18 GCSs at 2.6 km longitudinal distance is still passable for grain sizes up to 200 mm (at least during floods close to 20 R.I. intervals) as documented development of gravel bar downstream this sequence. The geometry of scour holes (maximum depth, lenght) relatively well corresponded to the present height of GCSs, showing dynamic adjustments of scours to changed geometry of damaged GCSs and thus, the present water drop heights. It is also evident, that these damaged GCSs still stabilise channel bed against erosion and their lowered height increases connectivity for sediments and some aquatic species.

Grain sizes of sedimentary wedges in the Mohelnice River: D16 - green, D50 - red, D84 - blue, D95 - yellow. Grain sizes of downstream channel-bar (black squares) and present height of check-dams (present water drops) are also displayed.  

Our research dealing with GCSs continues on the nearby Bystrý Torrent. There we obtained grain-size characteristics of ca. 50 small gravel bars in the heavy channelized stream with ca. 35 GCSs on 2 km longitudinal distance (A < 10 km^2, S = 2-3 %, W = 5 m). Preliminary results show a good relationship between the equilibrium channel slope upstream individual check-dams and grain-sizes, but some other variables need to be tested. So, the article is still in preparation to be submitted during the first months of next year. However, both studied will provide insight into the effect of GCSs management on downstream calibre of bed sediments via the artificial adjustment of transport capacity of the stream.

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