501 / 2024-05-14 16:18:15

Numerical study of vegetation effects on intradelta lobe avulsion

Intradelta lobe avulsion,vegetation,modeling,river discharge

Subtheme 3: Eco-hydraulics and Environment > Topic 3: Eco-environmental Effect of Water Engineering

Intradelta lobe avulsion, a channel shift inside the delta lobe, can create new coastal wetlands and benefit wetland restoration. Previous studies suggest that the presence of vegetation can affect the hydrodynamics and sediment deposition patterns and further influence the process of the intradelta lobe avulsion. However, the specific effects of vegetation on the avulsion, especially its location and timescale are still elusive. To address this issue, we coupled a vegetation model to the Delft3D hydro-morphodynamic model and set up a host of simulation scenarios with a wide range of vegetation density to reproduce the process of intradelta lobe avulsion in vegetated and unvegetated deltas. The vegetation model automatically updates vegetation distribution based on the simulated and prescribed critical water depth and velocity. Our results show that the location and timescale of avulsion are dictated by discharge magnitude and sediment deposition patterns (along-channel versus back-filling) in the unvegetated delta. When the discharge is relatively low, the avulsions are dictated by the sediment deposition patterns, and the presence of vegetation promotes along-channel sedimentation in the deltaic channel, which leads to more rapid occurrence of the avulsion at more upstream channel locations. The timescale of avulsion increases with increasing vegetation density. Alternatively, when the avulsions are induced by relatively high river discharge, the presence of the vegetation tends to reduce the bed shear stress in the natural levees, thus requiring an increased sediment deposition volume in the deltaic channel for triggering avulsion and delayed avulsion compared with unvegetated scenarios. Our findings can inform water and sediment management strategies in global deltas under ever growing environmental changes to restore wetlands.