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

Overview

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 (SEPA, 2015). 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 (see Figure 1). 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 (Pattison et al., 2014). 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?

Methodology

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.

To assess travel and response times of water, hydrometric (discharge) data and natural tracer data (electrical conductivity) will be collected in the headwaters of the Wear catchment, where several different natural flood management features (see Figure 1) have recently been installed by the Environmental Agency. Additionally, Injections of artificial tracers will be carried out to constrain travel time estimates. These data will allow quantifying response and travel times of water during low and high flow periods, and thus asses the slowing of flow induced by flood interventions at those sites. Time series of electrical conductivity measurements can serve as a conservative tracer for the short travel times of interest here (Vogt et al., 2010). Through the use of convolution techniques (e.g, Cirpka et al., 2007), upstream-downstream response and travel times at various flow conditions can be assessed from these times series of electrical conductivity.

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 (Pattison et al., 2014). 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 (Pattison et al., 2014) simulating the interactions of different tributary contribution to flooding will be expanded to incorporate estimates of response times and tributary contributions.

Project Timeline

Year 1

Literature review, review of existing data sets, planning of field work and collection of field data

Year 2

Collection and analysis of field data, model setup

Year 3

Evaluation of model outputs, paper & thesis writing

Year 3.5

Completion of thesis and paper writing

Training
& Skills

This project would suit a student with a degree in Earth or Environmental Sciences or Physical Geography (or a related field) and a strong interest in hydrology.

Excellent time management skills coupled with strong numerical, verbal and written communication are important. Training will cover field methods and data analysis techniques. The student will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment.

References & further reading

Cirpka, O.A., Fienen, M.N., Hofer, M., Hoehn, E., Tessarini, A., Kipfer, R. and Kitanidis, P.K., 2007. Analyzing bank filtration by deconvoluting time series of electric conductivity. Groundwater, 45(3), pp.318-328. https://doi.org/10.1111/j.1745-6584.2006.00293.x
Pattison I, Lane SN, Hardy RJ, & Reaney S, (2014). The role of tributary relative timing and sequencing in controlling large floods, Water Resources Research, 50, 5444-5458. https://doi.org/10.1002/2013WR014067
Scottish Environmental Protection Agency (SEPA), 2015. Natural Flood Management Handbook. https://www.sepa.org.uk/media/163560/sepa-natural-flood-management-handbook1.pdf

Vogt, T., Hoehn, E., Schneider, P., Freund, A., Schirmer, M. and Cirpka, O.A., 2010. Fluctuations of electrical conductivity as a natural tracer for bank filtration in a losing stream. Advances in Water Resources, 33(11), pp.1296-1308. https://doi.org/10.1016/j.advwatres.2010.02.007

Further Information

For further information on the project, please contact Julia Knapp (Julia.L.Knapp@durham.ac.uk)

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