River Engineering & Restoration
Purbeck Keystone Project River Frome Water Level Management Plan Pre-Feasbility Study
Keywords: Water Level Management, Eco-Hydrology, Flood Risk Assessment.
Client, & Other Organisations
Purbeck Keystone, Environment Agency, Footprint Ecology, Cain Bio-Engineering
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1. Sample LiDAR coverage of the Lower Frome |
2. Lower River Frome & floodplain |
Description
Edenvale Young Associates Ltd. was approached to act as part of a multi-disciplinary consortium to identify opportunities to manage water levels to restore floodplain grazing marsh on the River Frome in Dorset. The project primarily aimed to re-introduce favourable wetland conditions for breeding waders but also examined opportunities to enhance, maintain and manage the existing drainage network to the benefit and conservation of important ditch flora and fauna.
Edenvale Young were contracted to provide the essential hydrological and hydraulic analysis central to achievement of the above objectives. The Company has made novel use of its strong knowledge of 1D-2D hydraulic modelling to construct detailed models of the main rivers, drainage network and floodplains of the two catchments in order to simulate the existing hydrological regime and to examine proposed modifications to the watercourses within the catchments.
3. 100-year CC baseline flood map
In order to achieve the above objectives, an entirely new broad-scale ISIS-TUFLOW model of the Lower Frome catchment was constructed. Topographical survey, LiDAR data and information about the position of pre-existing water level management structures were combined to produce a model framework to assess the existing behaviour of the local catchments. Post calibration, this baseline information was used to assess the implications of the addition of further structures and changes to the local catchment – principally to the benefit of the local ecology but, crucially, in terms of any change to the flood risk posed to local properties and land holdings.
In analysing both low and high flow scenarios, amongst other benefits, the use of the powerful ISIS-TUFLOW modelling approach allowed:
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Production of detailed and robust flood map outputs in which local landowners can invest a high degree of confidence.
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Eco-hydrological analysis of probable water levels within the proposed system in order to ensure that existing species assemblages are not adversely affected.
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Ready manipulation of the floodplain to represent the introduction of scrapes designed to attract breeding waders.
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Easy communication of results to stakeholders.
Public consultation post model construction saw a largely favourable response to the model outputs which are consistent with the patterns of flooding observed by occupants of the valley.