Assessing the Performance of Grass and Soil in Resisting Erosion

Tender description

Grass cover on flood embankments and embankment dams acts to protect the surface of the structure from initiation of erosion during overflow or overtopping. The performance of the grass cover is therefore an important part of the overall structure performance during extreme conditions for both embankment dams and flood embankments. Current guidance on grass performance is limited, with recent research highlighting a number of issues of concern in both scope and use of the current guidance. In particular, the current guidance is inappropriate for use in reliability assessments and does not address conditions during initial overflow or at higher flow velocities. Soil erodibility depends upon both the soil type and the soil state. New models and methods for predicting internal erosion, breach growth and surface cover performance depend upon a measure of soil erodibility, but there is little information available to support expert judgement on the choice of erodibility values and any associated uncertainty other than via laboratory or field testing of soil samples. The effect of varying soil erodibility can be very large, changing both the magnitude and characteristics of a breach flood hydrograph. Hence uncertainty in identifying the soil erodibility contributes greatly to uncertainty in the associated flood risk analyses and flood mapping. With climate change effects resulting in more extreme loading conditions it is likely that overflow / overtopping will occur more frequently and that defining and designing acceptable overflow / overtopping conditions will be sought as part of overall structure performance design.