Riverbank protection removal to enhance bar formation for habitat diversity

Abstract

Over the past centuries natural riverbanks have been transformed into banks with artificial revetments or sheet piles to protect them from erosion, which led to disappearance of important river features for flora and fauna. River restoration projects show that the removal of man-made bank protections may lead to the formation of bars (e.g. Van den Berghe et al., 2012; Schirmer et al., 2014). Habitat diversity in rivers may be enhanced through the formation of river bars with preferably diversity in bar height, size and location. This research aims at gaining knowledge into the effects of removing riverbank protections on bar formation to enhance habitat diversity. Mobile-bed flume experiments were performed in the Fluid Mechanics Laboratory of Delft University of Technology. The experiments were focussed on bar formation related to three variables of a bank protection removal; length, location and flow asymmetry. Geometrical and morphodynamic characteristics were selected for the experiment having a bar mode of one to obtain a system with alternate bars (Crosato and Mosselman, 2009) and showed gravel-bed river similarity based on bankfull river characteristics of Parker (2004). The experimental flume consisted of a 6.2 metre long and 0.2 metre wide straight channel with 0.5 metre wide floodplains on the sides. On both sides of the channel, bank protection could be removed over a limited length with optionally a groyne upstream. After seven hours at the end of each test, photos of the bed topography were taken and the longitudinal bed profile was measured. Bar wavelengths and bar heights in each experimental test were determined from detrended bed profiles. The bar height was divided in two classes: low and high. The areas of low bars, high bars, floodplains and the deep channel were determined from photos of the final bed topographies. Bar types were indicated with terminology from Duró et al. (2015) as forced, free or hybrid. Removal of bank protections resulted in lateral erosion of channel banks over a limited length. The eroded bank line moved in downstream direction, which agrees with the downstream meander migration observed in field and laboratory investigations by many authors (e.g. Odgaard, 1987). Scour holes developed downstream of the widened section, due to turbulent eddies forced by the channel geometry. In the widened reach, flow decelerated which resulted in a lower sediment transport. Consequently, sediment was deposited in the widened reach and caused rising of the mean bed level along with formation of bars. In areas with higher flow velocities an increased sediment transport deepened the channel. When the riverbanks were fixed, relatively low, small, side bars were formed in the channel. In general, the total bar area and bar height increased for an increase in channel widening, due to the removal of bank protection. An increased bank protection removal length up to nine times the channel width or an upstream asymmetrical flow forcing, i.e. groyne, increased the total bar area, whereas a bank protection removal at three different locations with a total length of nine times the channel width did not significantly increased the total bar area. It is recommended to remove riverbank protections over a length of at least nine times the river width on both sides of the river with optionally a groyne upstream as a measure to enhance habitat diversity. It is also valuable to remove the bank protection on one side of the river only. It is advised to construct a groyne upstream of the bank protection removal to increase habitat diversity. This research led to results that can be used in further research. It is advised to conduct larger scale experiments and eventually move to the field.