Impact of changing hydroclimate on bedload yield

Advances in hydroclimate change predictions in rivers include regional estimates of changes in annual flow volumes and storm peakedness. Such nuanced projections are used here to estimate relative changes in annual coarse sediment load and grain size in transport using a novel modification to magnitude–frequency analysis, the method routinely used to estimate bed material yield. The approach employs highly discretised annual flow distributions obtained from gauging station records, with data split by season, that are transformed to simulate projected multipliers on flow volumes and peak flow magnitudes over the existing flow regime. Initial testing is reported for three steepland catchments in south and west Scotland (drainage areas of 2.1–37.5 km2) to reveal the impact of increases in winter flow volumes, compensatory decreases in summer flow volumes and increases in winter and summer peak flow magnitudes. Using Recking's transport function with projected hydroclimate changes suggests annual bedload yield potential increases by 27%–127%, varying inversely with drainage area. Increasing winter rainfall volume by 20%–30% over baseline increases yields by 15%–28%, whereas increasing winter peakedness by 40%–50% only increases yields by 2%–16% because of shorter flow durations. Increasing flow peakedness by 50% also increases the potential maximum grain size transported by 6%–14%, again inversely with catchment area. Monte Carlo-based sensitivity tests on ±10% changes in input variables singly and ±20% in combination illustrate the approach to be robust to parameter variability (or error), with an average signal-to-noise ratio across the test sites exceeding 5:1 for the ±20% sensitivity tests. Hindcasting suggests that sediment yields 60 years ago may have only been 50%–65% of current totals. The model (MFACC, Magnitude–Frequency Analysis for Climate Change) provides a rapid screening tool for estimating ‘first order’ fluvial geomorphology risks attributable to changes in hydroclimate that could be subsequently combined with estimates of sediment supply changes and mediation by channel morphology adjustment.