Effect of vegetation on the deposition of fine-grained sediment on the Wax Lake delta, southern Louisiana, USA

Abstract:

In many coastal environments, such as salt marshes and deltas, it is increasingly apparent that vegetation exerts a strong control on where and when sediment is deposited. This suggests that changes in vegetation should have a measurable effect on the deposition of sediment, especially fine-grained or organic sediment. However, it is difficult to directly measure the effect changes in vegetation have on sedimentation in natural systems. Consequently, most of the estimates come from flume experiments using highly simplified stand-ins for vegetation or from relatively few locations and vegetation types. At Wax Lake delta, a young, prograding delta in Louisiana, we have the unique opportunity to measure the effect of more structurally complex vegetation types on sedimentation. Here we present a series of experiments conducted in two field flumes constructed at Wax Lake delta. In the experiments, fluorescent-dyed silt-sized organic and siliciclastic sediment was injected into the flume at known rates and measuring the concentration at the outlet. The field flumes were constructed over winter to allow vegetation to grow naturally through the growing season. One field flume was dominated by Colocasia esculenta and represented a typical intertidal environment and the other was sparsely vegetated and represented a typical subtidal environment. Experiments were conducted early and late in the growing season and at high and low discharges. We used data- and process-driven models to estimate the capture efficiency and the effective settling rate in all the experiments. We find that while discharge still exerts a first order control on the amount of sediment deposited, vegetation density has a significant influence on the amount of sediment that is ultimately removed from the flow, primarily by changing the effective settling rate for siliciclastic sediment or by changing the capture efficiency of direct interception for organic sediment. Our results imply that whether sediment is captured or bypasses the deltaic environment may be influenced by what vegetation grows within the environment and how the growth season aligns with the timing of the sediment supply and river discharge.