1. Dept. of Civil Engineering, Univ. of Bristol, Queens Building, University Walk, Bristol BS8 1TR, UK (corresponding author).
2. Environment Agency, King’s Meadow House, King’s Meadow Rd., Reading RG1 8DU, UK.
3. HR Wallingford, Howbery Park, Wallingford OX10 8BA, UK.
4. Environment Agency, Rio House, Waterside Dr., Aztec West, Almondsbury, Bristol BS32 4UD, UK.