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.