The links between water profile, net deposition and erosion in the design and performance of stiff grass hedges

Strips of dense and stiff grass are commonly used to reduce sediment delivery to aquatic systems. Grass strips cause changes in resistance to the flow, which can lead to a reduction in transport capacity and an increase in sediment deposition. Experiments were carried out in controlled conditions and the profile of water surface was recorded upstream and within narrow and stiff hedges for different slope length and steepness, and for different flow rates. The results showed that water surface profiles in stiff hedges no longer follow the classic M2 profile and the friction slope can be quite high within the narrow hedges. A model is developed to simulate the water profile upstream and within grass strips. Simulated water surface profiles compare well with observations, and this study re-confirms the non-linear relationship between the Manning’s n and Froude number in densely vegetated areas. As water approaches the downstream end of stiff hedges, the reduction in water depth leads to flow acceleration, and an increase in friction slope and stream power, and as a consequence net deposition of sediment is unlikely to occur.