Examples of Section 50 Applications

The following examples are provided as guidance in respect of the completion of consent applications:

This example uses the Unit Hydrograph Method from the Flood Studies Report (FSR), (NERC, 1975), to estimate the design flood and HEC_RAS software package (US Army Corps of Engineers) to evaluate the hydraulics of the bridge.

This example uses the FSR Catchment Characteristics method and compares it with gauged flows for the catchment. The comparison highlights the requirement for the use of standard errors to ensure that flows are not significantly underestimated.

The Institute of Hydrology method, as described in the publication IH124: Flood Estimation for Small Catchments, is used to estimate the flow for this catchment. The value obtained is compared to the OPW design flow for the Broadmeadow & Ward Drainage Scheme, which is somewhat higher and is adopted as the design flow. A quick method, based on the average velocity, is used to estimate the head loss caused by the culvert. The culvert size is confirmed using a proprietary software package.

In this example, a number of estimates are made for the flood in the river. An estimate of the ebb flow is added to this for analysis of the capacity of the bridge. The tide level is based on an extreme flood coinciding with a high tide recorded in 1999. A sensitivity analysis is carried out with a much higher flood to confirm the viability of the design.

The design flow was estimated using the catchment characteristics equation from the FSR. An estimate of the tailwater level was obtained using Manning's equation and this was used in a hydraulic analysis using a proprietary software package.