The effect of weirs on nutrient concentrations

The removal of a weir in 1999 from the River Nidd in Yorkshire, UK, was assessed in terms of its impact on in-stream nitrate removal along a 15.8 km long stretch of river. Models of channel hydraulics and denitrification quantified the impact on an annual basis, using, as inputs, river flow, water temperature, water quality data and cross-section geometry collected both before and after the weir was removed. To remove the confounding influences of year-specific conditions, two counterfactual simulations were set up whereby the pre-removal configuration was driven by data from the post-removal period (and vice versa). Results revealed the removal of the weir to have reduced the annual fraction of the upstream nitrate load being retained along the stretch by 2.6% (i.e. 812 kg) and 1.8% (382 kg) for the years 1997 and 2000 respectively. Differences resulting from the presence or absence of the weir were most marked during low flow summer conditions. (C) 2015 Published by Elsevier B.V.