Abstract
Inspired by the Boxing Day 2015 flood of the River Aire in Leeds, UK, and subsequent attempts to mitigate adverse consequences of flooding, the goals considered are: (i) to revisit the concept of flood-excess volume (FEV) as a complementary diagnostic for classifying flood events; (ii) to establish a new roadmap/protocol for assessing flood-mitigation schemes using FEV; and, (iii) to provide a clear, graphical cost-effectiveness analysis of flood mitigation, exemplified for a hypothetical scheme partially based on actual plans. We revisit the FEV concept and present it as a three-panel graph using thresholds and errors. By re-expressing FEV as a 2 m -deep square lake of equivalent capacity, one can visualise its dimensions in comparison with the river valley considered. Cost-effectiveness of flood-mitigation measures is expressed within the FEV square-lake; different scenarios of our hypothetical flood-mitigation scheme are then presented and assessed graphically, with each scenario involving a combination, near and further upstream of Leeds, of higher (than existing) flood-defence walls, enhanced flood-plain storage sites, giving-room-to-the-river bed-widening and natural flood management. Our cost-effectiveness analysis is intended as a protocol to compare and choose between flood-mitigation scenarios in a quantifiable and visual manner, thereby offering better prospects of being understood by a wide audience, including citizens and city-council planners. Using techniques of data analysis combined with general river hydraulics, common-sense and upper-bound estimation, we offer an accessible check of flood-mitigation plans.
Highlights
Precipitation records show that, whilst annual rainfall has remained roughly constant in the United Kingdom (UK) in recent decades, within that period there has been an increase and decrease in, respectively, winter and summer rainfall [1]
At 22:30 on 12 December 2016, the Armley river level reached a peak of about 3.8 m, and inspection at the nearby river bank revealed that there was still circa 0.5 m to go before it would flow over the walled banks, which roughly matches the findings shown in Figure 5: such local knowledge can be important in setting the desired threshold and it motivates our choice of T f (h T) = 3.9 m
To facilitate both the understanding of and comparison between extreme flooding events, we have revisited the concept of flood-excess volume (FEV), which is the product of the difference between the flood volume of river discharge and the discharge Q T with the flood duration
Summary
Precipitation records show that, whilst annual rainfall has remained roughly constant in the United Kingdom (UK) in recent decades, within that period there has been an increase and decrease in, respectively, winter and summer rainfall [1]. Our second estimate of FEV, denoted by Ve2 , is important in situations where automatic river-level measurements and rating curves are absent, while discharge estimates are required to make flood-mitigation estimates, for example, in local urban or remote rural areas, or in developing countries This second estimate is useful when only the maximum discharge Qmax at the peak level hmax is known while the relationships Q(h) and Q(h T ) are unknown. That case corresponds for example to a flood-alleviation measure in which flood-defence walls are raised, which raises h T under the assumption that this barely alters the in-situ rating curve While these three definitions of FEV (2), (3) and (5) are straightforward given choices of thresholds h T , their accuracy depends on the error bars, in the mean or maximum discharge Qm , Qmax or the rating curve Q = Q(h), which uncertainties are often ignored in practice
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