Risk Assessment Methods for Structure Blockage During Flood Flows

The UK Environment Agency (EA) has a duty to prepare statutory 1 in 100 year flood-maps known as Section 105 Surveys.  Validation of these maps by the South Western Region appeared to show that standard maps were under-predicting floodplain extents.

This discrepancy was attributed to the occurrence of blockage at structures causing elevated flood levels.  The EA commissioned a major R & D study to investigate the causes, prediction and modelling of blockage, with the intention of preparing standard consistent procedures for incorporation into future S105 studies.

The project was carried out jointly by Kennedy and Donkin Consultants and Brian Faulkner.

Theoretical 'standard' culverts were modelled in HEC-RAS™ for a wide range of blockage scenarios. Practical procedures such as an Excel™ based Blockage Risk Probability Assessment pro-forma were developed for consistent application.

A key area of innovation was to apply state of the art risk assessment techniques to blockage prediction, using @RISK™.  Based on a stochastic analysis of various blockage causing factors, probability distributions of blockage risk can be determined for any structure.

The adopted methods have been found useful for other important aspects of flood management, especially -

• maintenance planning for existing structures
• optimised design of new culverts

Client - UK Environment Agency Date - January 1997 Funding - Client Scheme value - n/a Support tools - HEC-RAS, Excel™, @RISK™

www.environment-agency.gov.uk www.kdltd.com www.palisade.com