On using flood-excess volume to assess natural flood management, exemplified for extreme 2007 and 2015 floods in Yorkshire

Abstract

This paper offers a protocol for conducting a quantified assessment of the relative merits of both existing and proposed methods of Natural Flood Management (NFM). Assessment is based on the rarely used concept of flood-excess volume (FEV), which approximately quantifies the volume of water one wishes to eliminate via flood-mitigation schemes, and is exemplified using publicly available river-gauge data for recent well-known extreme-flood events in Yorkshire, UK. The following question motivates the study: what fraction of the FEV is reduced, and at what cost, by a particular (suite of) flood-mitigation measure(s)? The approach presented admits juxtaposed cost assessments, of disparate and popular NFM measures, that are neither available nor considered in existing flood-mitigation policy. With promulgation of a societally useful protocol in mind, quantification and interpretation of alternative cost scenarios are facilitated using the authors' novel visualisation of flood-alleviation basins as partially filled, realistically constructible, two-metre-deep square lakes of side-length approximately one-to-two kilometres. In the first case study, a hypothetical flood-alleviation scheme for the River Calder at Mytholmroyd, comprising flow-attenuation features, tree planting and peat restoration, and reservoir storage, is critically assessed alongside reasonable cost estimates. The clear quantification and visual representation of the analysis indicate that the fractions of FEV reduced by the NFM measures, while not insignificant, are dwarfed by the careful draw-down of reservoirs. The second case study, of the River Don at Sheffield, extends the analysis via a range of scenarios that attempt to sample realistic seasonal rainfall distributions across a catchment, and in doing so elucidates both the potential and uncertainty of numerous mitigation schemes under different conditions. We corroborate the growing consensus that, while NFM measures can reduce low-level flooding locally, the spatial scale at which they are effective is limited and may not upscale to the catchment level. Our FEV analysis and protocol thus not only offers a concise quantification of the effectiveness of disparate and in-tandem flood mitigation measures but also further highlights the issue of NFM scalability.

Communicating (nature‐based) flood‐mitigation schemes using flood‐excess volume

“…Conceptually, FEV offers a straightforward and comprehensible means of quantitatively underpinning flood‐mitigation strategies. It is presently assumed that the stage–discharge relationship of the river is known in terms of a recorded and accurate time‐series (see also Bokhove et al, ). Given an in situ rating curve Q = Q ( t ) explicitly as function of time t over a flood duration T f , or implicitly as a function of the in situ river level , the approximation of FEV used (which comprises the shaded and hatched areas in Figure a) is in which Q T = Q ( h T ).…”

“…In general, rating curves are not exact due to errors in the relation and measurement errors in . Generally, river levels are measured and, using theoretical or phenomenological rating curves (Bokhove et al, ; Environment Agency, ), they are converted into discharge rates .…”

“…For both NFM and civil‐engineering measures, first assessments of the effectiveness of flood‐mitigation strategies can be quantified straightforwardly using flood‐excess volume (FEV; see Hui & Lund, ; Schneider, ; Bokhove et al,). FEV concerns the fraction of the total volume of river discharge, over a certain period, which caused flooding at a certain river location during an extreme event.…”

“…Our analysis is intended to provide an insightful and accessible protocol for assessing flood mitigation, aimed at increasing public understanding and assisting policy makers in decision making. We build upon the analysis of a (hypothetical) flood‐alleviation scheme in Bokhove et al (), in which more available physical and economic data is incorporated. The paper outline is as follows.…”

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

“…Since in the calculation of the excess volume the rating curve is used twice, via a subtraction, the cumulative error in the excess volume will remain circa 5.5%, following error propagating techniques, yielding the quoted 0.51Mm 3 error in (3.2). In a companion article (Bokhove et al 2018), we also determine the FEV for the Boxing following flood-excess volumes V e = (9.34 ± 0.51, 1.65 ± 0.22, 3.00 ± 0.24)m 3 were found for chosen threshold levels of h T = (3.9, 4.5, 2.9)m for the River Aire, River Calder and River Don, respectively.…”