River roughness - the integration of diverse knowledge

摘要

In 2000, the Environment Agency of England and Wales identified the need to reduce the uncertainty associated with flood level prediction through using recent research advances on river and flood plain conveyance (Samuels et al, 2002). This has led to the development of the Conveyance Estimation System (CES), involving a partnership between researchers and experts in hydraulics, aquatic vegetation and software development and with close consultation with typical users. A key component of this development was taking knowledge on river resistance from a diverse set of sources, covering different types of vegetation in the river and on the flood plain and, different channel dimensions and bed material type. The calculation methods are documented elsewhere (McGahey & Samuels, 2003) and are based on the Darcy friction factor to represent local river bed stresses coupled with other fluid dynamic processes. Most data sets on flow resistance provide average values of Manning's n for whole river sections and include the influence of bed material, vegetation and larger scale topographic variation. A key requirement of the CES project was to produce a roughness advisor to allow the user to access this information in a structured manner and construct information for the CES conveyance calculation. The field data also needed further interpretation to assess the local resistance properties to include in the calculations. This paper describes the integration of this information on flow resistance into the conveyance calculations in the CES software including an approach for the estimation of friction factors when the flow depth is comparable to the effective roughness length scale.