Thursday, 21 December 2017

How much gravel we need to have in a gravel-bed river?

In the past decades we observe gradual degradation of Czech and Polish Carpathian rivers. Whereas afforestation in Tatra Mts (reducing sediment supply), channelisation works and gravel mining are the main problems in Poland for channel incision (e.g., Zawiejska et al. (2010),  Wyzga et al. (2016),  Hajdukiewicz et al. (in press)), Czech rivers are often affected by the presence of water reservoirs stopping sediments from upstream and we noted more intensive channelisation works owing to the high population density in the Czech Carpathian forefields (e.g. Škarpich et al. (2013), Galia et al. (2016), Škarpich et al. (2016)). Unfortunately, this channel degradation (narrowing of channels, incision up to bedrock, loss of gravel bars) endangers also preserved wandering fluvial system in Skalická Morávka Nature Reserve. 

Presently, large volumes of gravels is stopped in the upstream part of the Morávka basin by a weir in Vyšní Lhoty, by Morávka Reservoir and we should also note the presence of check dams in small tributaries together with increase in forested area during 20th century. Therefore, Nature Conservation Agency of the Czech Republic is preparing a new management plan for the following decade, which includes management of gravels in the nature reserve. We have participated in calculations of missing gravels in the nature reserve, calculations of loss of bar habitats between 1950s and the present and we tried to determine dominant discharges, which would be able to sustain gravel bars and migration of channels in the reserve. It seems that huge volume of gravels (ca. 20 000 - 25 000 meters cubic) are missing in the upper part of the reserve. Much of this incision occurred during 20-50y flood in 2010. We expect that two-years daily discharges as well as large floods are important for preservation of the wandering system, reflecting natural hydrologic regime of streams based in flysch lithology. On the other hand, the river need to carry sufficient amount of gravels (up to several thousands meters cubic) during individual high-flow events to prevent from further incision. Other actions as artificial disturbations of gravel-bar surfaces, artificial flooding and close-to-natural bed stabilisation elements were suggested.       

Partially degraded wandering channel in Skalická Morávka Nature Reserve