Quantifying the benefits of natural flood management methods in groundwater-dominated river systems

Abstract

This thesis aims to address the gap in the Natural Flood Management (NFM) evidence base concerning its potential to reduce flooding in groundwater-dominated chalk catchments. Due to the difference in hydrological processes between groundwater-dominated chalk catchments and other typical UK river systems with a relative dominance of surface runoff, it is proposed that the flood reduction benefits of NFM in chalk catchments is currently unknown. A statistical approach is used to ascertain the likelihood of successful NFM implementation in 198 chalk catchments in England, by matching physical catchment properties hydrological characteristics with the associated NFM measures. Three catchment typologies were identified using redundancy analysis and hierarchical clustering: 1) large catchments, 2) headwater catchments with permeable soils, and 3) catchments with impermeable soils and surfaces. Based on the properties identified for Groups 1 and 2, it can be assumed that NFM will have limited flood reduction potential in these chalk catchments. Group 3 chalk catchments were identified as being most suitable to NFM based on catchment properties and hydrological processes. The findings of the statistical analysis were tested by taking an example Type 2 chalk stream, the River Test, and modelling in-channel NFM implementation and increasing landcover of deciduous woodland using the numerical model, SHETRAN. Model parameterisation of in-channel NFM scenarios are based on field-measured Manning’s roughness values and the literature. Modelling results show that increasing tree cover, revegetating river channels, and installing large woody debris can generate a flood peak reduction averaging 5%. However, this 5% flood peak reduction does not translate to a reduction in flood duration, meaning that flooding in groundwater-dominated catchments will likely remain highly disruptive. The effectiveness of NFM under projected future climate change scenarios is tested by comparing NFM flood mitigation scenarios to a non-NFM baseline. Under future projected climate changes, NFM maintains its effect when compared to non-NFM baseline scenarios. This equates to an average 5% flood peak reduction and insignificant reductions in flood duration. All together these findings suggest that NFM has a reduced flood reduction impact in chalk groundwater-dominated catchments compared to non-groundwater dominated catchments. It is recommended that future research in this area focus on targeted application of NFM in sources of groundwater emergence, improved mapping of hydrogeological features, and manipulating soil properties via land management practices.

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Identifiers

ORCID for David Sear: orcid.org/0000-0003-0191-6179

ORCID for Justin Sheffield: orcid.org/0000-0003-2400-0630

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