Flood frequency analysis for the River Avon at Bath

The flood hydrology of the River Avon at Bath was investigated by combining data on past flood events from a number of different sources using a maximum likelihood function derived for use with censored events. The different data sources include: annual maximum series of peak flow from the HiFlows-UK databse (1969-2009), a literature survey of past studies (1725 - present), and physical flood marks found beneath the Widcombe footbridge located behind the Bath Spa railway station (1825-1947). Peak flow values for the historical events
were derived by combining a survey of the water level marks with historical information on the cross-section geometry. The resulting data series contains 89 individual events covering the period 1725 – 2009. Defining a censoring threshold value of 250m3/s, the resulting at-site flood frequency curve was derived using a Generalised Logistic (GLO) distribution, and the recurrence interval of the maximum flow of 260 m3/s observed during the winter 2013/14
estimated to be about 30 years. A sensitivity analysis of assumptions made to derive the historical events (Manning‟s number, hydraulic gradient, and level of historical floods not marked on the Widcombe Bridge) show that the resulting design flood estimates are most sensitive to assumptions made about the hydraulic grade when converting levels to flow using Manning‟s formula.