Assessing multiple benefits of nature-based solutions for flood protection on water quantity and quality

Overview

Flooding of rivers is a substantial problem in the UK. It results in loss of infrastructure, farmland, and livelihoods, and damage by flooding costs the UK economy around £1.3 billion each year. In addition to the damage of infrastructure, flooding also affects water quality and ecosystem services both directly and indirectly (Hrdinka et al., 2012). Particularly in agricultural areas, the flushing of pesticides, manure, and fertilizers into the river systems occurring during flood events impacts water quality. Additionally, flooding may have indirect effects on water quality and ecosystem services, resulting from the erosion of sediment from riverbanks and the streambed. As sediments are mobilized, they act as transport vehicles for sorbed pollutants. In 2019, only 15% of England’s rivers were considered to be at Good Ecological Status (DEFRA, 2020), and this is expected to get worse given that flooding probabilities have increased in recent years due to climate changed induced weather extremes becoming more frequent (Otto et al., 2018).

Although still costly, flood prevention is often cheaper than the repair of flooding damage. For this purpose, nature-based solutions for flood mitigation by have been proposed in recent years. These include landscape-scale interventions such as agricultural and soil management, and channel-based interventions such as woody-debris dams. However, nature-based solutions are often considered to be not financially viable for flood mitigation alone. It has, however, been suggested that the effects of nature-based solutions may not be limited to flood prevention, but that they may have multiple benefits, including an improvement in water quality, increased biodiversity, and reduced soil erosion (Dadson et al., 2017). In consequence, nature-based solutions for flood management may also be able to mitigate effects of diffuse pollution on water systems, thus decreasing the direct cost of their implementation.

In this project, we want to use natural tracer data to assess larger-scale impacts of localized flood modification systems. Passive tracers like stable water isotopes or chloride, which are applied with precipitation, can be used to quantify transit times of water through the catchment (Hrachowitz et al., 2016). Through the estimation of these catchment transit times we can assess how natural flood management impacts internal catchment functioning like the transport, storage, and release of water, nutrients, and contaminants.

Natural tracer techniques furthermore provide a possibility of assessing subcatchment contributions to downstream flooding. Varying and (a-)synchronous contributions from different tributaries and subcatchments can substantially affect the size of a flooding event, but they are often difficult to constrain with common hydrological models used in flood forecasting (Pattison et al., 2014). Because antecedent conditions and precipitation intensities vary from event to event, different subcatchments /tributaries may be activated to different degrees during different events. Consequently, a better constraint of subcatchment contributions to downstream flooding has the potential to improve model predictions. One way of doing this is through transit time estimations and end-member mixing analyses based on natural tracer signals, which can provide upper and lower bounds of subcatchment contributions on downstream flow processes. Through a better understanding of when certain subcatchment contributions are dominant, this can also help to mitigate point-source pollution effects from sources within individual subcatchments.

The purpose of this project is to identify and quantify various benefits of nature-based solutions for flood prevention. This includes, but is not limited to the improvement of water quality and ecosystem services of river systems across the UK. The project furthermore aims to quantify how the contributions from various subcatchments and their timing can be regulated to mitigate flooding effects.

Methodology

The aim of the project is to assess and quantify the multiple benefits of nature-based solutions for flood mitigation. These will be achieved through a combination of different methods. These methods include statistical data analysis techniques, model development, as well as field sampling of hydrological event data.

During the project, existing time series of hydrometric and water quality data will be analyzed using various statistical techniques to identify differences across systems and over time. The purpose of this initial analysis is to answer how nature-based flooding solutions have impacted water quality compared to control sites without comparable modifications.

To assess the larger-scale impact of localized flood modifications, natural tracer data will be used to estimate transit times and sub-catchment contributions. These can help to identify how and under which conditions different subcatchments are contributing to downstream flooding, and how natural flood management impacts internal catchment functioning like the transport, storage, and release of water, nutrients, and contaminants.

To gain a more detailed understanding of the processes occurring at the hydrological event time scale, river water samples will be collected during individual events. This high-frequency data set will provide helpful insights into fast-flow processes and their timing.

In a final step, we aim to test the hypotheses developed from the results of the data analysis through numerical modelling. A range of conceptual and physically-based hydrological models would be used to simulate the effect of nature-based solutions on the travel time and interaction of different tributaries.

Project Timeline

Year 1

Literature review, analysis of existing data sets, planning of field work

Year 2

Analysis of existing data sets and field data, model development

Year 3

Analysis of numerical model results, paper writing

Year 3.5

Thesis writing, paper writing

Training
& Skills

Data analysis, statistical methods, field techniques, laboratory analysis with IC, ICP-MS, modeling (details on software)
Field methods: water sample collection and handling
Laboratory skills: water sample handling and preparation, training in chemical analysis of water samples: IC (ion chromatography), ICP-MS (inductively coupled plasma mass spectrometry), and stable water isotope analysis
Data analysis: statistical methods; handling of big data
Modeling: set-up and calibration of different hydrological models
Training in writing skills through detailed feedback on manuscripts and thesis drafts
Presentation skills: presentation of research at national and international conferences
Project management skills: how to plan, initiate and execute a project efficiently and successfully

References & further reading

Dadson, S. J., Hall, J. W., Murgatroyd, A., Acreman, M., Bates, P., Beven, K., … & O’Connell, E. (2017). A restatement of the natural science evidence concerning catchment-based “natural” flood management in the UK. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 473(2199), 20160706. https://doi.org/10.1098/rspa.2016.0706

Department for Environment, Food and Rural Affairs, UK (2020). UK Biodiversity Indicators 2019. https://jncc.gov.uk/our-work/ukbi-b7-surface-water-status/
Hrachowitz, M., Benettin, P., Van Breukelen, B. M., Fovet, O., Howden, N. J., Ruiz, L., … & Wade, A. J. (2016). Transit times: The link between hydrology and water quality at the catchment scale. Wiley Interdisciplinary Reviews: Water, 3(5), 629-657. https://doi.org/10.1002/wat2.1155

Hrdinka, T., Novicky, O., Hansli­k, E., & Rieder, M. (2012). Possible impacts of floods and droughts on water quality. Journal of Hydro-environment Research, 6(2), 145-150. https://doi.org/10.1016/j.jher.2012.01.008

Otto, F. E., van der Wiel, K., van Oldenborgh, G. J., Philip, S., Kew, S. F., Uhe, P., & Cullen, H. (2018). Climate change increases the probability of heavy rains in Northern England/Southern Scotland like those of storm Desmond – a real-time event attribution revisited. Environmental Research Letters, 13(2), 024006. https://doi.org/10.1088/1748-9326/aa9663

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

Further Information

For further information please contact Julia Knapp at julia.knapp@usys.ethz.ch

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