On using flood-excess volume in flood mitigation, exemplified for the River Aire Boxing Day Flood of 2015

The goals of this paper are threefold, namely to: (i) define the rarely used concept of flood-excess volume (FEV) as the flood volume above a chosen river-level threshold of flooding; (ii) show how to estimate FEV for the Boxing Day Flood of 2015 of the River Aire in the UK; and, (iii) analyse the use of FEV in evaluating a hypothetical flood-alleviation scheme (FASII+) for the River Aire, largely based on the actual Leeds Flood-Alleviation Scheme II (FASII). Techniques employed are data analysis combined with general river hydraulics and estimation using bounds. By expressing FEV equivalently in terms of a square lake with a certain side-length and depth (of one to a few metres), with the same capacity, it becomes easy to visualise its dimensions and compare it with those of the river valley considered. FEV analysis provides cost-effective estimates of new flood-mitigation measures, either prior to or in tandem with more detailed hydrodynamic numerical and laboratory modelling of river flows. It is used to illuminate five different scenarios of flood mitigation for our new FASII+, with each scenario involving a combination of higher (than existing) flood-defence walls and enhanced flood-plain storage sites both closer to and further upstream of Leeds. An integral part of this approach is a cost-benefit analysis. For policy makers, a further advantage of FEV is that it can be used to analyse and choose between flood-mitigation measures in a direct and visual manner, thereby offering better prospects of being understood by a wide, particularly non-technical, audience and city-council planning departments.