Chapter 1. Introduction
1.5. Document structure
The thesis is comprised of this introduction, a literature review of modelling of catchment processes and natural flood mitigation strategies (Chapter 2), and four multi-author papers, on three of which the thesis author is first author. A summary of the author’s contributions to each of the included papers can be found in the supporting statement preceding each paper. Three appendices detail the mathematics behind the models developed.
The conclusion summarises the findings and details how the aim and objectives of the project have been met. It suggests wider-scale and more diverse applications of the framework that was developed.
Dynamic TOPMODEL: A new implementation in R and its sensitivity to time and space steps
This describes a new, open source, implementation of a semi-distributed model developed to simulate runoff and soil moisture deficits. The paper describes its underlying methodology, modular structure and an application to a simulated landscape and a small (3.65km²) catchment in Wales, UK. Various spatial and temporal discretisations are applied to a single calibration period. The implementation demonstrates robustness to the various schemes and convergence to a limiting output. Appendix 1 details the flux calculations and new surface routing method introduced in this implementation.
A modelling framework for evaluation of the hydrological impacts of nature-based approaches to flood risk management, with application to in- channel interventions across a 29 km² scale catchment in the United Kingdom
This study develops a depth-averaged 1D hydraulic routing model that can be applied to route in and out of bank channel flows in river networks of arbitrary scale and structure. It can take into account the effect of insertion of any configuration of structures designed to mitigate storm flows. It applies parameterised channel geometries and a simplified flood plain representation. The model is applied to an agricultural catchment where a NFM -type approach is suggested to reduce flood risk but where land use and regulatory constraints preclude the use of hillslope measures. The Dynamic TOPMODEL implementation developed in Chapter 3 is used to simulate the hillslope runoff for a storm period. The hydraulic model applied to investigate the effects of various configuration and geometries of in-channel barriers on the response.
Appendix 2 details the hydraulic routing scheme for channel routing. and Appendix 3 suggests formulations for the hydraulic characteristics of selected in-channel flood mitigation measures.
Strategies for testing the impact of natural flood risk management measures.
This chapter describes a four month long project undertaken for the Rivers Trust in liaison with Lancaster Environment Centre and JBA Consulting. The author acted as research associate and developed the computer model with which the detailed modelling was undertaken. The project aimed to investigate the potential for applying a nature-based approach for flood mitigation across the headwaters of three Cumbrian catchments badly affected by the extreme storm events of the winter of 2015-2016. The runoff model described in Chapter 3 was further developed to allow simulation of the effect of the addition of various types of widely-distributed hillslope interventions for flood mitigation. These include peat restoration, soil structural improvements, tree- planting on hillslopes and riparian areas, and deepening of existing accumulation areas to provide detention storage area for surface runoff.
A catchment partner workshop was undertaken and opportunity mapping based on runs of the fully-distributed JFLOW surface runoff model applied to identify priority areas. The chapter describes the tiered methodology developed to incorporate the opportunity mapping with JFLOW, catchment partner input, and detailed modelling of runoff with and without intervention measures using an enhanced version of Dynamic TOPMODEL. It outlines the evidence and literature base for the parameter and input data changes applied to simulate these larger-scale interventions.
Simplified representation of runoff attenuation features within analysis of the hydrological performance of a natural flood management scheme
A measure simulated in the project described in Chapter 5 was the insertion of runoff attenuation features (RAFs) intended to intercept and store overland storm runoff. The new surface routing module developed in Section A.1.4 was modified to allow modelling of these features. This chapter describes the modelling using this approach, through the storm period from the previous study, of approximately 4500 of such features across the 228km² headwaters of the Eden, Cumbria, UK.
The study applies an uncertainty estimation framework to the simulated runoff for unmodified and intervention cases. Features with a number of different “leakiness” characteristics are applied. Some conclusions are drawn on the effects of different drain-down times across extreme events such as those studied. The chapter discusses whether the simplified representation is valid and suggests some experimental approaches to determining the behaviour of such features and their effects on the catchment response.
The issue of synchronisation or desynchronisation of subcatchment flood waves, and its impact on the effectiveness of NFM interventions, is considered. Experimental approaches, both computerised and field-based are suggested as a means of investigating, in further studies, this issue and the robustness of the modelling results