Advice, methods and tools for estimating channel roughness

Roughness plays an essential role in water level estimation for rivers, watercourses and drains. It reduces the discharge capacity through energy expenditure from boundary-generated turbulence as well as physical blockages caused by vegetation. Today's practitioners are constantly challenged with identifying, quantifying and simulating these processes to provide vital information for a range of flood risk management tasks. This paper introduces the roughness advisor, one of a series of tools embedded in the recently developed Environment Agency's conveyance and afflux estimation system. The roughness advisor provides an extensive database of roughness information from a diverse set of sources (>700) and presents it in a structured manner to enable users readily to select from a range of in-channel and floodplain vegetation and bed material types. Traditionally, flow resistance datasets are based on average values of Manning's n for whole river sections. Here, the notion of a local unit roughness n_l is introduced and its performance is demonstrated through flow prediction for a range of channel types. Further applications are used to compare the unit roughness with the traditional Manning n, to trial a boulder roughness approach and to describe the sensitivity and relative importance of roughness in the calculation of channel flow.