FLOODING & DRAINAGE Understanding river flooding
Photogrammetry has been used to compile topographic maps for decades. Now modern and automated close-range digital photogrammetry has provided hydraulic engineers with an insight into why rivers flood. In a research project funded by the EPSRC, the Department of Civil and Building Engineering at Loughborough University has been conducting work on two physical flumes that simulate flow within a flooding river channel and flood plain.
o prevent flood events such as those of November 2000, the flow processes leading to flooding of rivers need to be better understood. The research at Hydraulics Research, (Wallingford) and Loughborough Univ- ersity has been able to assist in this. Numerical flow models are important to hydraulics research but are insufficiently accurate to simulate the complex interaction of fluid flow, sedi- ment entrainment and bedform develop- ment. Experiments conducted in physical models (two flumes of different sizes) allow the real world to be simpli- fied so that critical parameters can be controlled and measured.
To measure the detailed bed-forms created by the flowing water in the flume rapidly, digital photogrammetry was used, generating accurate and dense digital elevation models (DEMs) to represent the bed-surface.
When a river floods, the flow structure becomes complex in the main channel owing to floodplain flow plunging into the main
Digital image of bed-surface of flume
channel – see figure. This become more prominent as water depth increases. The interaction between main channel flow, flood plain flow and bed-form increases overall
flow resistance, which raises the water level and causes increased incidence and extent of flooding. Designing methods to prevent the development of such complex flow could therefore allow a reduction in flooding incidence in the future.
The data gathered during the flume experiments combined with other fluvial parameters has been used to increase our understanding of the processes involved in a flooding river and the critical controls. The photo- grammetric methods have also been applied successfully in monitoring bed changes in a real braided river in the Canadian Rockies using oblique, ground-based digital photographs.
For further information please contact Dr Jim Chandler, Department of Civil and Building Engineering, Loughborough University, LE11 3TU (01509 222624, fax: (01509 223981; E-mail: J.H.Ch
Catchment flood management planning HR Wallingford is leading a team developing a Modelling and Decision Support Framework (MDSF) to support
the implementation of Catchment Flood Management Plans in the UK. The work has been funded by DEFRA and the Environment Agency and the partners include Halcrow, CEH Wallingford and the Flood Hazard Research Centre.
looding is often the result of rainfall and drainage conditions some distance away from the affected area. DEFRA and the Environment Agency are now developing ‘Catchment Flood Management Plans’ (CFMPs), the aim being to identify preferred flood management policies under present and future conditions, taking account of whole catchments rather than just individual rivers. These must be based on a sound understanding of the hydrological, hydraulic and hydrogeological processes at work in each catchment. The purpose of the MDSF is to provide a customised geographical information system to help with data management, along with computational tools that undertake the complex calculations needed within a CFMP. The MDSF obtains flood water levels from hydrological and hydraulic models, and advice on appropriate catchment models is provided within the system.
The MDSF allows the rapid appraisal of different flood management policies to help the user select the preferred policy. Policies
HRW. ‘Core data has been collected on each river catchment, including OS background maps, indicative flood plain maps and a digital elevation model needed to predict flood depths.’ Data sets for the economic damage and social impact calculations were assembled for use in the MDSF.
are assessed under present-day and predicted future conditions of land use and climate change, and provides advice on climate change and land use scenarios. ‘Pilot CFMPs are being prepared by con- sultants on five different UK catchments. Two of these – the Medway and the Irwell – are being used to trial the MDSF,’ explains David Ramsbottom who leads the project at
HRW coordinated the EUROTAS project (European River Flood Occurrence and Total Risk Assessment System) from 1997 to 2000 and their involvement with this study has informed development of the MDSF, particularly the modelling approaches. The first release of the MDSF was completed in January 2002. The system will be further enhanced as more and better data and procedures become available in the future.
For further information about this work, or a CD presentation/demonstration, please contact David Ramsbottom at HR Wallingford (01491 822218; fax: 01491 825916; E-mail: email@example.com
Research Focus NO. 48 FEBRUARY 2002
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