The role of tributary relative timing and sequencing in controlling large floods

Hydrograph convolution is a product of tributary inputs from across the watershed. The time-space distribution of precipitation, the biophysical processes that control the conversion of precipitation to runoff and channel flow conveyance processes, are heterogeneous and different areas respond to rainfall in different ways. We take a sub-watershed approach to this and account for tributary flow magnitude, relative timing and sequencing. We hypothesise that as the scale of the watershed increases so we may start to see systematic differences in sub-watershed hydrological response. We test this hypothesis for a large flood (T > 100 years) in a large watershed in northern England. We undertake a sensitivity analysis of the effects of changing sub-watershed hydrological response using a hydraulic model. Delaying upstream tributary peak flow timing to make them asynchronous from downstream sub-watersheds reduced flood magnitude. However, significant hydrograph adjustment in any one sub-watershed was needed for meaningful reductions in stage downstream, although smaller adjustments in multiple tributaries resulted in comparable impacts. For larger hydrograph adjustments, the effect of changing the timing of two tributaries together was lower than the effect of changing each one separately. For smaller adjustments synergy between two sub-watersheds meant the effect of changing them together could be greater than the sum of the parts. Thus, this work shows that whilst the effects of modifying biophysical catchment properties diminishes with scale due to dilution effects, their impact on relative timing of tributaries may, if applied in the right locations, be an important element of flood management.