Response of Bed Sediments on the Grade‐Control Structure Management of a Small Piedmont Stream

Abstract

AbstractThe grain sizes of 53 small lateral and mid‐channel deposits were evaluated on the 2.7 km long channel‐reach of a headwater piedmont stream that has been affected by check‐dams and bed sills constructions (ca. 14 constructions per km), riprap bank stabilisations and the removal of instream wood. Such interventions have led to the loss of natural vertically oscillating bedforms, and pool‐riffles have been replaced by plane beds. The evaluated sediment deposits were divided based on their positions relative to grade‐control structures on downstream, intermediate and upstream types. The individual types of deposits produced different downstream trends in the grain sizes that reflected their positions with respect to the grade‐control structures (GCSs). The downstream deposits generally produced slow downstream coarsening, which may point to the increased efficiency of the energy dissipation of the GCSs under a gradual downstream decrease in the channel slopes. However, they were generally not coarser than the other two types. The upstream deposits did not indicate any notable downstream grain‐size trend. Despite the significant downstream decrease in the channel slopes and related bankfull shear stresses at the positions of the intermediate deposits and the lack of a lateral sediment supply in the studied longitudinal profile, the sediment calibre of the deposits did not produce any systematic downstream fining. Thus, the observed grain sizes most likely pointed to the selective scouring of the fine sediments, which was caused by the presence of the GCSs and their effects on the decreasing sediment connectivity. Copyright © 2016 John Wiley & Sons, Ltd.