Influence of vegetation maintenance on flow and mixing: case study comparing full cut with high-coverage conditions

Abstract

Abstract. In temperate climates, agricultural ditches are generally bounded by seasonal vegetation, which affects the hydrodynamics and mixing processes within the channel and acts as a buffer strip to reduce a load of pollutants coming from the surrounding cultivated fields. However, even if the control of such vegetation represents a key strategy to support sediment and nutrient management, the studies that investigated the effect of different vegetation maintenance scenarios or vegetation coverage on the flow and mixing dynamics at the reach scale are very limited. To overcome these limitations and provide additional insights on the involved processes, tracer tests were conducted in a 500 meters long agricultural ditch close to Warsaw in Poland, focusing on two different vegetation scenarios: highly vegetated and fully cut. Additionally, under the highly vegetated scenario, sub-reaches differing in surficial vegetation coverage are analysed separately to understand better the influence of the vegetation conditions on the flow and mixing parameters. Special attention has been paid to the longitudinal dispersion coefficient in complex natural conditions and its dependency on vegetation coverage (V). The vegetation maintenance decreased the travel and residence times of the solute by 3–5 times, moderately increasing the peak concentrations. We found that the dispersion coefficient decreased approximately linearly with the increase of vegetation coverage at V>68 %. Further research is needed at lower vegetation coverage values and different spatial plant distributions. The obtained longitudinal dispersion coefficient values complement the previously published data, which are barely available for small natural streams. The new process understanding supports the design of future investigations with more environmentally sound vegetation maintenance scenarios.

Longitudinal dispersive coefficient in channels with aquatic vegetation: A review

“…x k gradually increases with the growth of vegetation coverage and tends to stabilize when a stable vegetation landscape is reached [91,94] .…”

“…In such rivers, aquatic vegetation is often arranged in patches or strips against seasons. Currently, validated predictors for estimating the influence of patchy vegetation on the dispersion coefficient are lacking, and we are aware of only two existing recent studies [41,94] focusing on real-scale vegetation patches in compound channels. Experimental or quantitative models describing the dispersion features with the effects of real-scale plants patches are in their infancy.…”