1. Pennine Water Group, Dept. of Civil and Structural Engineering, Univ. of Sheffield, Sheffield S1 3JD, UK (corresponding author).
2. Dept. of Architecture, Univ. of Cambridge, Cambridge CB2 1PX, UK.
3. Pennine Water Group, Dept. of Civil and Structural Engineering, Univ. of Sheffield, Sheffield S1 3JD, UK.