Overland Flow in Wetlands: Vegetation Resistance

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Overland Flow in Wetlands: Vegetation Resistance

Introduction to Vegetation Resistance in Wetlands

Wetlands are unique ecosystems where vegetation plays a crucial role in influencing the flow of surface water. The resistance provided by vegetation significantly impacts the hydrodynamics of overland flow, affecting water movement, sediment transport, and soil erosion. Understanding the mechanisms of vegetation resistance is essential for effective wetland management and restoration.

Emergent Vegetation and Flow Resistance

Emergent wetland vegetation, such as reeds and grasses, often provides the primary resistance to overland flow. The stems of these plants are typically spaced apart, necessitating the computation of fluid friction based on drag on individual objects rather than using channel or packed bed equations. This resistance is further complicated by the vertical variation in vegetation density and the non-uniform spatial variation of soil elevations. Additionally, the flow in wetlands often transitions between laminar and turbulent states, making traditional equations like Manning's equation inappropriate for accurate predictions.

Influence of Vegetation Lodging

Vegetation lodging, where plants bend downstream under the force of flowing water, alters the flow regime and resistance characteristics. Experiments have shown that as the slope or degree of vegetation lodging increases, the flow patterns evolve from slow to rapid, and the resistance of overland flow decreases with an increase in the lodging angle. For instance, vegetation with lodging angles of 20°, 40°, 60°, and 80° showed a decrease in resistance by 10.28%, 32.53%, 52.45%, and 61.37%, respectively, compared to vertical vegetation . This phenomenon highlights the importance of considering vegetation morphology in hydraulic models to avoid large deviations in overland flow forecasts.

Vegetation Density and Flow Resistance

The density of vegetation cover is a critical factor in controlling soil erosion and influencing hydraulic resistance. Studies on wheatgrass patches with varying densities have demonstrated that increased vegetation density leads to a transition from transient to turbulent flow, significantly affecting the Froude number and overall flow resistance. The total resistance of overland flow under vegetation cover can be divided into form resistance and grain resistance, with form resistance increasing from 95% to 99% as vegetation density increases. This underscores the importance of vegetation density in managing soil erosion and water conservation.

Synthetic Vegetation and Resistance Coefficients

Experiments using synthetic grass and stems have provided insights into the variation of resistance coefficients under different vegetation covers. The relationship between the resistance coefficient (f) and the Reynolds number (Re) is not straightforward but depends on the specific vegetation coverage. A critical coverage threshold was identified, beyond which the resistance coefficient's behavior changes significantly. This threshold decreases with increasing slope gradient, indicating that slope and vegetation coverage must be considered together to predict flow resistance accurately.

Theoretical Models and Experimental Validation

Theoretical models, such as those based on power-velocity profiles, have been tested against experimental data to predict flow resistance under various vegetation types and rainfall intensities. These models have shown that flow resistance increases with rainfall intensity for laminar overland flow and that a single flow resistance equation can be applied across different vegetation types, influenced by the flow regime represented by the Reynolds number. Such models are essential for improving the accuracy of hydrodynamic simulations in wetland environments.

Flexible vs. Rigid Vegetation

The type of vegetation, whether flexible or rigid, also plays a significant role in determining flow resistance. Flexible vegetation tends to oscillate with the flow, reducing flow momentum and adding extra resistance, while rigid vegetation creates turbulent vortices and wakes, dissipating flow energy and increasing resistance. Flume experiments have shown that rigid vegetation generates significant form drag, which dominates flow resistance and increases with flow depth, whereas flexible vegetation causes much smaller flow resistance due to its tilting behavior.

Conclusion

Understanding the resistance provided by vegetation in wetlands is crucial for effective water management and ecological restoration. Factors such as vegetation density, lodging, type (flexible or rigid), and coverage significantly influence the hydraulic characteristics of overland flow. Accurate models and experimental validations are essential for predicting flow resistance and managing soil erosion in wetland ecosystems. By considering these factors, we can better design and implement strategies for wetland conservation and restoration.