Sediment accounting and management has become a critical consideration for those who own and maintain Blue-Green Infrastructure of any size – from small SuDS treatment ponds and detention basins to large reservoirs and catchment-scale river management.
Blue-Green Infrastructure can be threatened by sediment (where artificial basins or reservoirs become filled in and ineffective), or it can be an integral component in sediment management (such as vegetated buffer strips to trap sediment before it reaches watercourses). A key consideration is the upslope sediment yield at the point that Blue-Green Infrastructure is being proposed. Rough mapping of sediment yield and its spatial variation across the UK was carried out by McHugh et al (2002), but more detailed and accurate sediment accounting is challenging in many cases because of uncertainties about sediment inputs and dynamics (and limitations in our modelling abilities – eg Barry et al, 2004). Some estimates have been produced from empirical studies – Heal et al (2006) for example, forecast sediment removal frequency from a specific set of SuDS in Scotland.
There are strong arguments for designing Blue-Green Infrastructure (and traditional grey infrastructure, too), in order to maintain relatively ‘natural’ sediment levels. Excessive sediment can be harmful by smothering fish nests and gills; but equally, some regular sediment inputs have been shown to confer a ‘flood pulse advantage’ by providing nutrients to the river ecosystem.
For managers of reservoirs and detention basins, integrating sediment accounting into long-term management decisions allows better decisions to be made. For example, Palmeiri et al (2001) extended the work of Basson (1997) to conclude that certain reservoirs could be operated indefinitely, and profitably, if certain maintenance practices such as silt flushing, routing or dredging are followed. For other basins and reservoirs, the usual strategy is to accept a finite life to the reservoir due to sedimentation, and plan for substantial end-of-life costs when the dam is retired. Some unusual exceptions exist, such as reservoirs on the Loess Plateau in China that are predicted to have a significant end-of-life salvage value due to sedimentation producing valuable agricultural land (Voegele, 1997).
Heal, K.V., Hepburn, D.A. and Lunn, R.J. (2006). Sediment management in sustainable urban drainage (SUD) ponds. Wat. Sci. Tech., 53(10), 219-227.