“Carry out topographic survey of the water course and adjacent floodplain from the weir upstream of the culvert to the railway culvert downstream of the culvert; “Assess the catchment hydrology to give culvert inflow hydrographs for return periods of 5, 25, 50 and 100 years; “Build a HEC-RAS model of the watercourse with the culvert in place and with the culvert replaced by an open channel with a bank top width of 10m; “Run the two models for the various return periods and report on the results, including identifying the route that any overland flows would take and the impact of these on existing and proposed buildings. “The report will enable a comparison to be made of the potential for flooding with the culvert in place and following its removal.”
“The culvert is approximately 72 metres in length and has a total fall of 3.25 metres giving an overall hydraulic gradient of about 1 in 22. The un-surcharged capacity of the culvert in estimating using Colebrook-White formula with friction ks of 0.6mm. The theoretical capacity is 6.6 m 3/sec. However this figure is reduced significantly by the hydraulic characteristics of the entry to the pipe and a coefficient between 0.3 and 0.4 might be applicable. This being so, the capacity would be reduced to between 2 and 3.3 m 3/second.”
“The watercourse from its point of issue south of Roman Road down as far as the railway embankment was first surveyed and then a mathematic model was prepared using a steady state HEC-RAS [v2.2] approach. A second model was also prepared omitting the culvert under the concrete hardstanding to investigate the possible sizing of an open channel if the culvert were to be replaced.”
“If the existing culvert is removed then a channel having a 10 metre width across top of banks will accept peak flows. The bank slopes would be a maximum of 1 vertical to 1.5 horizontal. Alternatively one sloping bank could be replaced by a vertical wall to limit the loss of concrete area.”
“I now understand that you require confirmation that if the culvert were to be removed it could be replaced by an open watercourse contained within a 10m strip adjacent to the fence along the northern boundary of your site. To be certain of this we will need to produce proposals which would be acceptable to the Environment Agency (without necessarily consulting them). Preparation of these proposals and submission to you will involve some additional work. This is likely to take 6 hours. We would undertake this in accordance with our proposal letter to you dated9 November 2001 at the rate of£70 /hr.”