Overland Flow in Wetlands: Vegetation Resistance

Introduction

Overland flow in wetlands is significantly influenced by vegetation, which acts as a resistance factor affecting the flow dynamics. Understanding how different types of vegetation and their characteristics impact flow resistance is crucial for effective wetland management and restoration efforts.

Key Insights

Impact of Vegetation Type and Density on Flow Resistance:
- Vegetation increases flow resistance, with different types of vegetation affecting the resistance differently. A single flow resistance equation can be applied across various vegetation types, but the flow regime (represented by the Reynolds number) plays a crucial role .
- Sparse stem vegetation coverage shows that vegetation coverage (Cr) is the dominant factor affecting overland flow resistance under zero slope conditions, but not under non-zero slope conditions.
Effect of Vegetation Morphology and Lodging:
- Vegetation lodging (the angle at which vegetation bends) significantly influences flow resistance. Higher lodging angles result in lower resistance and increased flow velocity, Reynolds number, and Froude number, leading to enhanced turbulence .
- The resistance coefficient (f) decreases with an increase in the vegetation lodging angle, and the morphology of vegetation lodging is a primary factor in controlling hydraulic erosion .
Hydraulic Characteristics and Flow Regimes:
- Vegetation stems increase the Reynolds number and decrease the Froude number and mean flow velocity, indicating a reduction in the runoff's ability to carry sediment. The Darcy-Weisbach friction coefficient increases with vegetation stem diameter, showing higher energy consumption due to hydraulic resistance.
- The flow regime at low depths is similar to porous media flow, while at higher depths, it becomes more turbulent. This indicates the complexity of applying a single power law-type expression for vegetation resistance over varying depths and energy slopes.
Combined Effects of Vegetation and Surface Roughness:
- Surface roughness has a greater influence on overland flow resistance at lower discharges, while vegetation becomes more influential at higher discharges. The combined effects of vegetation and surface roughness are not simply additive, indicating the limitations of using linear superposition principles.
Numerical and Analytical Modeling:
- Numerical models and analytical solutions of partial differential equations (PDEs) are effective in quantifying vegetation resistance and understanding flow dynamics in wetlands. These models help in developing empirical functions to express discharge in terms of water depth and energy slope .

Conclusion

Vegetation significantly impacts overland flow resistance in wetlands, with factors such as vegetation type, density, morphology, and lodging angle playing crucial roles. The interaction between vegetation and surface roughness, as well as the complexity of flow regimes, necessitates advanced modeling techniques to accurately predict and manage flow dynamics. Understanding these factors is essential for effective wetland restoration and management.

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Overland Flow in Wetlands: Vegetation Resistance

Summary

Introduction

Key Insights

Conclusion

Sources and full results

Testing a theoretical resistance law for overland flow under simulated rainfall with different types of vegetation

The theoretical flow resistance equation accurately estimates Darcy-Weisbach friction factor for overland flow under simulated rainfall, with resistance increasing with rainfall intensity for laminar overland flow.

The use of discharge perturbations to understand in situ vegetation resistance in wetlands

The Manning's equation is not applicable for wetlands with thick vegetation, and applying a single power law-type expression for vegetation resistance over a wide range of depths and energy slopes is difficult due to multiple flow regimes and different power exponents.

Effects of simulated submerged and rigid vegetation and grain roughness on hydraulic resistance to simulated overland flow

Simulated vegetation and surface roughness significantly influence overland flow resistance, with surface roughness being more significant at lower discharges and vegetation more significant at higher discharges.

Effects of Vegetation Stems on Hydraulics of Overland Flow Under Varying Water Discharges

Vegetation stems significantly increase the hydraulic resistance of overland flow, with vegetation resistance accounting for 80% of total resistance on vegetated slopes.

A numerical model for assessing the additional resistance to flow introduced by flexible vegetation

Flexible vegetation in wetlands increases flow resistance, affecting flood control and hydroinformatics.

Effects of vegetation lodging on overland runoff flow regime and resistance

Higher vegetation lodging angles lead to enhanced turbulence and weaker flow resistance in overland runoff, affecting flow regime and resistance.

Influence of vegetation lodging on the flow regime and resistance characteristic of overland flow

Vegetation lodging changes overland flow characteristics and resistance, with higher lodging angles reducing resistance and slower flow patterns.

Overland Flow Resistance Law under Sparse Stem Vegetation Coverage

Sparse stem vegetation coverage significantly affects overland flow resistance under zero slope conditions, but its effect varies with different hydraulic parameters.

Effect of flexible vegetation lodging on overland run‐off resistance

Flexible vegetation lodging angle has a quantitative relationship with overland runoff resistance, with unsubmerged vegetation increasing resistance as water depth increases, while completely submerged vegetation decreases resistance with increasing water depth.

Experimental study on the effect of simulated grass and stem coverage on resistance coefficient of overland flow

Synthetic grass and stem coverage significantly affects overland flow resistance, with a critical coverage threshold at 2.72% and a positive correlation between resistance coefficient and Reynolds number at higher coverage levels.

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