Advice, methods and tools for estimating channel roughness

• Published in: Proceedings of the Institution of Civil Engineers. Water management 2009-12, Vol.162 (6), p.353-362
• Main Authors: MCGAHEY, C, KNIGHT, D. W, SAMUELS, P. G
• Format: Article
• Language: English
• Subjects: Applied sciences; Buildings. Public works; Channels; Computation methods. Tables. Charts; Estimating; Exact sciences and technology; Floods; Freshwater; Hydraulic constructions; Risk management; Rivers; Roughness; Structural analysis. Stresses; Vegetation
• ISSN: 1741-7589; 1751-7729
• DOI: 10.1680/wama.2009.162.6.353

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.