74
Source: P. Quinn, G. O’Donnell, A.Nicholson, M. Wilkinson, G. Owen, J. Jonczyk, N. Barber, M. Hardwick and G. Davies (2013). Potential Use of
Runoff Attenuation Features in Small Rural Catchments for Flood Mitigation. Newcastle University and Royal HaskoningDHV in partnership
with the Environment Agency
M
ain
impacts
&
benefits
Monitoring of benefits regarding flood risk reduction is ongoing. Photographic and video evidence from farmers showed
that the NRWM were clearly holding water upstream of the village.
Modelling
results from the project have demonstrated
a network of runoff attenuation features, similar to those implemented in Belford catchment, to have a positive impact on
flood hazard in small catchments. Modelling and direct comparison work (using monitoring data) indicates that the individual
impact of a single feature on peak flows is relatively small because individual features provide only a small amount of storage
each. The benefit to peak flow rate reduction is achieved through implementing a larger number of measures, distributed
throughout the catchment, which provides a
cumulative positive effect.
Further assessment has been undertaken on the
combined impacts of a hypothetical pond network providing 19 250m
3
of storage, which modelling showed to provide 15
to 30% peak flow reduction. Investigations to assess effectiveness of features for reducing losses of sediment and nutrients
began in 2009. Monitoring showed that after a large runoff event a single retention bund captured an estimated
1 tonneof
sediment
. The overall, cumulative impact of all the NWRM has been found to be difficult to prove, and to require extensive
monitoring. However, it was identified that different features operate to retain pollutants under contrasting flow conditions.
Online features appeared to be functioning to reduce chronic losses of suspended solids, but were less effective in storm events
(being ineffective at retaining pollutants during the rising limb and peak of flood events). In contrast, a multi stage NWRM
(constructed following findings of initial NWRM) that included a sediment trap and willow barriers worked effectively to reduce
sediment and nutrient losses from the catchment during storms: it showed average reduction in pollutant concentrations of
40% SS, 26% TP, 25% soluble RP and 15% NO3 over a 24hour storm event.While clear quantitative evidence is not yet
available at the catchment-scale, improvements in the water quality within the catchment should improve ecological status
and biodiversity. Habitat benefits are also likely to be associated with the creation of pond features within the catchment.
Model results showing the influence of increasing storage volume on reducing peak flows
