Rural Land Management Impacts on Catchment Scale Flood Risk

Abstract

This thesis examines the relationship between rural land management and downstream flood risk. The recent increase in flood frequency and magnitude has been hypothesised to have been caused by either climate change or land management. The theoretical basis for why these factors might increase flood risk is well known, but showing their impact on downstream flood risk remains a challenge. Field scale studies have found that changing land management practices does affect local runoff and streamflow. Upscaling these effects to the catchment scale continues to be problematic,both conceptually and, more importantly, methodologically. Conceptually, upscaling is critical. As land management may impact upon the relative timing as well as the magnitude of runoff, any changes in land management practice may lead to changes in the synchronisation of tributaries flows, either reducing or increasing downstream flood risk. Methodologically, understanding this effect requires capturing the spatial resolution associated with field-scale hydrological processes simultaneously with the upscaling of these processes to the downstream locations where flood risk is of concern. Most approaches to this problem aim to upscale from individual grid cells to whole catchments, something that restricts the complexity of possible process representation,produces models that may not be parsimonious with the data needed to calibrate them and, faced with data uncertainties, provides computational limitations on the extent to which model uncertainty can be fully explored. Rather than upscaling to problems of concern, this thesis seeks to downscale from locations of known flood risk, as a means of identifying where land use management changes might be beneficial and then uses numerical modelling to identify the kinds of management changes required in those downscaled locations. Thus, the aim of this thesis is to test an approach to understanding the impacts of rural land management upon flood risk based upon catchment-to-source downscaling. This thesis uses the case study of the River Eden catchment (2400 km2) as a test case. Firstly the downstream flood risk problem was assessed using both gauged data and documentary evidence to investigate the historical flood record. This found the last decade does not differ significantly from previous flood rich periods, which were defined as 1) 1873-1904; 2) 1923-1933; and 3) 1994-present. Second, the potential causes of floods within the catchment were investigated; firstly climate variability was assessed using Lamb weather types, which found that five weather types were responsible for causing 90% of the floods in the last 30 years. Third, spatial downscaling of catchment-scale flood risk was undertaken using two methods; databased statistical analysis; and hydraulic modelling. Both approaches consider the magnitudes and the timing of the flows from each major sub-catchment. The statistical approach involved a principal components analysis to simplify the complex subcatchment interactions and a stepwise regression to predict downstream flood risk. The hydraulic modelling approach used iSIS-Flow to undertake a series of numerical experiments, where the input hydrographs from each tributary were shifted individually and the effect on downstream peak stage assessed. Both these approaches found that the Upper Eden and Eamont sub-catchments were the most important in explaining downstream flood risk. The Eamont sub-catchment was chosen for future analysis as:(1) it was shown to have a significant impact on downstream flood risk; and (2) it had range of data and information needed for modelling land use changes. The second part of this thesis explored the land management scenarios that could be used to reduce flood risk at the catchment scale. The scenarios to be tested were determined through a stakeholder participation approach, whereby workshops were held to brainstorm and prioritise land management options, and then to identify specific locations within the Eamont sub-catchment where they could tested. There were two main types of land management scenarios chosen: (1) landscape-scale changes,including afforestation and compaction; and (2) channel modification and floodplain storage scenarios, including flood bank removal and wet woodland creation. The hydrological model CRUM3 was used to test the catchment scale land use changes,while the hydraulic model iSIS-Flow was used to test the channel and floodplain scenarios. It was found that through changing the whole of a small sub-catchment(Dacre Beck), the scenarios of reducing compaction and arabilisation could reduce catchment scale (2400 km2) flood risk by up to 3.5% for a 1 in 175 year flood event(January 2005). Changing localised floodplain roughness reduced sub-catchment (Lowther) peak stage by up to 0.134 m. This impact diminished to hardly any effect on peak flow magnitudes at the sub-catchment scale (Eamont). However, these scenarios caused a delay of the flood peak by up to 5 hours at the sub-catchment scale, which has been found to reduce peak stage at Carlisle by between 0.167 m to 0.232 m, corresponding to a 5.8% decrease in peak discharge. A key conclusion is that land management practices have been shown to have an effect on catchment scale flooding, even for extreme flood events. However, the effect of land management scenarios are both spatially and temporally dependent i.e. the same land management practice has different effects depending on where it is implemented, and when implemented in the same location has different effects on different flood events