19.4.1 Can land management be an effective runoff management option?
The advent of a 'whole catchment' approach to flood risk management has concentrated attention on the impact of rural land use management on catchment flood response. A comprehensive review by O'Connell et al. (2004, 2007a) looked at the results from over 80 studies based on field monitoring at various scales, numerical modelling and empirical data analysis. One of the key conclusions of the study was
that there is substantial evidence that changes in land use and management practices affect runoff generation at the local scale, but only limited evidence was found that local changes in runoff manifest in the surface water drainage network as changes in stream flow regime.
There are measures that can be taken to mitigate local flooding by delaying runoff. Examples include the maintenance of grass buffers, temporary storage ponds (Fig. 19.9), ditches and 'leaky weirs' that may be built structures (Fig. 19.10) or use willow or other natural vegetation.
An integrated approach is needed in applying these measures to obtain maximum overall benefit for flood mitigation and also in reducing erosion and promoting water quality. Nafferton Farm in Northumberland, UK, is the site of a full-scale integrated runoff management experiment trialling a combination of measures, such as those illustrated above, that can potentially be implemented cost effectively on farm land. Fig. 19.11 shows an overview of the experimental features and monitoring at the site.
These experiments have shown evidence of reducing total phosphorus loads in storm runoff by up to 77 per cent, largely by trapping suspended sediments (Jonczyk et al., 2008). Runoff reduction studies are continuing to determine whether these measures
Fig. I9.II Overview of runoff management structures installed at Nafferton Farm. (Jonczyk et al., 2GG8. Reproduced with permission of the British Hydrological Society.)
could have an appreciable benefit for downstream flood risk. It is clear that monitoring of hydrological processes is needed at multiple scales to provide new information about hydrological responses for different magnitudes of storm event, different environments and different land management practices.
More information is expected as a result of detailed experimental studies such as the Pontbren catchment in mid-Wales, where there has been monitoring of rainfall, soil moisture content, flow in field drains and overland flow. Early results from Pontbren (Jackson et al., 2008) suggest that field-scale interventions such as planting strips of trees might offer some benefits for runoff management visible at the small (c. 12 km2) catchment scale.
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