1. Can floodplain lowering help to mitigate river bed erosion?
Ralph M.J. Schielen a,b, Hermjan Barneveld c, Aukje Spruyt d, Michiel van den Berg e, Kees Sloff d and Alouette van Hove c
a University of Twente, Faculty of Engineering Technology, Water Engineering & Management, The Netherlands b Ministry of Infrastructure and Water Management-Rijkswaterstaat, The Netherlands
c HKV Lijn in Water, Lelystad, The Netherlands
d Deltares, Delft, The Netherlands
e World Wide Fund for Nature, Zeist, The Netherlands
a P.O. Box 2232, 3500 GE Utrecht, The Netherlands, Telephone: ,
Problem:
The Dutch Rhine branches erode at a rate of 2-3 cm/a. This causes problems for navigation, threatens stability of sluices and bridges and causes desiccation of floodplains.
A possible solution: use the floodplains as extra flow capacity
The slope of a river is a function of (a.o.) the width. Increasing the width means that the river slope increases. Hence, the bed aggrades. Floodplain lowering is one of the possibilities to increase the river width at intermediate and high flow.
Source: Rijkswaterstaat, Blom (2016)
Principles of river widening: left – restricted flow with summerdikes, middle – opening of side channel, right – lowering or removal of summerdikes. Source: Bureau Stroming and WWF
Method: 1D-Sobek
modelling
We use a 1D model (based on calculated 2D flow patterns) of the Waal River to simulate the ongoing bed erosion. River widening is modelled by a general lowering of the floodplain of 2 meters and more. We apply a step wise yearly returning hydrograph.
2D-Waqua profile
Sediment inflow (m3/s) at upstream boundary, resulting in average erosion in upper part Waal River of 2 cm/a
Hydrograph
Results:
The model is capable of simulating the eroding trends and the morphological impact of floodplain lowering and sediment nourishments. River widening indeed slows down bed erosion.
State after 100 years for the reference
Difference reference and 2 m. lowering of floodplain
Upstream sediment load needed for trend of 2 cm/a and 0 cm/a
Conclusions: Based on a simple 1D model, widening the river is a promising measure to counteract bed erosion. However, implementation asks for a tailor-made approach. Most probably, the system still needs smart sediment nourishments to mitigate bed erosion completely.