Is the whole greater than the sum of its parts? Assessing the combined effect of multiple natural flood management features on downstream flooding.

Flooding from rivers is an increasing problem in the UK which threatens infrastructure and livelihoods every year. To reduce the effect of river flooding, a combination of management interventions in downstream and upstream regions of the river systems is typically used. These consist of flood barriers and other types of engineered defences to prevent overtopping of rivers or reduce their damaging effect at the critical points in urban areas. In addition, smaller interventions in the headwater regions can help to slow down the flow in the upstream regions of the river course, and thus reduce the pressure on the downstream flood barriers. Natural flood management features are often chosen for these headwater interventions, which are land management techniques that work with natural hydrological and morphological processes to manage flooding. These interventions provide a possibility to reduce runoff peaks and manage their timing.
The functioning and effect of individual natural flood management features have been studied extensively in recent years. However, less is known about how different features work in concert, how far down stream they have an impact and about their cumulative effect on downstream flooding. By managing the timing and contributions from different tributaries, the cumulative effect of several features together may be optimised for the greatest downstream benefit.
However, tributary contributions are often difficult to constrain with common hydrological models used in flood forecasting. Since antecedent conditions and precipitation intensities vary from event to event, different sub-catchments /tributaries may be activated to different degrees during different events. Consequently, a better constraint of sub-catchment contributions to downstream flooding has the potential to improve model predictions.
The project aims to quantify how the contributions from various sub-catchments and their timing can be regulated through natural flood management interventions, by addressing the following research questions:
1) How do natural flood management features affect travel and response times through the catchment?
2) Can we improve the prediction of downstream flooding through the estimation of relative contributions from tributaries and their timing?
The project combines field work, numerical analysis and catchment modelling to assess travel and response times of water, and estimate the cumulative effect of numerous natural flood management features on downstream flooding.
The relative contributions from tributaries and their timing plays a substantial role in controlling large floods, but these are often difficult to constrain with common hydrological models used in flood forecasting. One way of doing this is through response time estimations and end-member mixing analyses based on natural tracer signals, which can provide upper and lower bounds of sub-catchment contributions on downstream flow processes. This PhD project will use the signals of natural solutes to assess tributary contributions to downstream flooding and how they vary as a function of antecedent conditions (how dry or wet is the catchment before the rain event?), event characteristics (how much does it rain and for how long?), and seasonal effects (how much vegetation is there?). For this purpose, an existing model simulating the interactions of different tributary contribution to flooding will be expanded to incorporate estimates of response times and tributary contributions.