Contact
Deltares | R&D Highlights 2015
The link between river
hydraulics and vegetation
patchiness
Overview of experimental setup at the River Experiment Centre in Andong (South Korea)
Vegetation plays a fundamental role in shaping streams and rivers due to its effects on morphodynamic processes, particularly in small streams, where vegetation can lead to significant changes in the bed morphology of the main stream by affecting erosion and deposition processes. Vegetation therefore acts as a system engineer: it adapts the environmental conditions by retaining and fixing the sediment, and by changing soil moisture characteristics, therefore altering hydro-morphodynamic processes. In turn, hydro-morphodynamic processes affect the development of floodplain vegetation.
Models are regularly used to predict the effect of floodplain vegetation roughness on water conveyance. The same models are also used to explore different management options for floodplain vegetation. The assumptions on which they depend are, however, not always accurate. For instance, the roughness factor of vegetation is often considered to be constant in time and place but, in reality, vegetation roughness and therefore water conveyance are affected by variations over time in vegetation density and patchiness. Improving the accuracy of these models by collecting relevant data in the field or in experimental studies will therefore be tremendously valuable. In this project, Deltares and the Korean Institute of Civil Engineering and Building Technology (KICT) created a validation dataset for flows around and through patches of vegetation at the River Experiment Centre (REC) in Andong (South Korea). Two doctorate candidates from the RiverCare programme are also working on this project. The KICT-REC facility is designed for full-scale tests with three prototype channels (600 m long and 11 m wide) and a large capacity pump facility (maximum flow rate of 10 m3/s). The
natural bed allows for real vegetation to root and establish itself in a natural way, providing a unique setting for obtaining data about vegetated flows that are unscaled but completely controlled. Seven willow patches were planted in 2014 in
alternative strips 4 m long and 1.5 m wide. The density was 38 stems/m2 in four of those patches and 10 stems/m2 in
three patches. Moreover, because the willows had already been planted in 2014, understory vegetation had time to develop in the patches, reproducing natural conditions more exactly than the sticks usually used in the laboratory.
Flow velocities in front, above, in and behind the patches were measured in 2015 with an Acoustic Doppler Velocity meter and an Acoustic Doppler Current Profiler. Measurements were performed in three distinct conditions: relatively low water tables (emerged vegetation), medium water tables (partially submerged vegetation) and high water tables (fully submerged vegetation). Results showed that the effect of vegetation on flow velocities depends on the flow velocities themselves but that the density of the vegetation patches and the level of submergence also had considerable effects on flow velocity. In the coming months and years, models will be calibrated and validated using the detailed dataset. Moreover, the effect of understory vegetation and the level of the drag force on roots (in other words, the critical flow velocity for uprooting) will be explored in more detail.
ellis.penning@deltares.nl T +31(0)6 2387 8545
< Vertical flow profile near a vegetation patch
Ecosystems and Environmental Quality
The full team working on the joint project
