When a river overtops its flood banks, water running down the landward side of the bank can rapidly erode the soil surface and scour the bank, sometimes leading to breaches and collapse. A covering of living vegetation, particularly grass, can reduce this risk of water erosion. As part of a project to assess the effectiveness of different management regimes on bank vegetation cover, direct measurements were required of the erodibility of the soil surface. A portable erosion measurement device (EMD) was developed by LAB Coastal with the support of the Environment Agency. This could direct water flowing at known velocities across areas of the flood banks, and it was used to test directly the erosion resistance of vegetated grass banks at three sites. The EMD gave a direct measure of the erodibility of a small sample of flood bank. While measurements of soil strength and assessments of vegetation cover were useful, they did not always correctly characterize the stability of the bank surface as measured directly by the EMD. The EMD and its use are described and the results obtained are discussed. The results indicate that flood banks need to be mown at least once a year to make them less vulnerable to erosion.