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.
