Greenfield run off and flood estimation on small catchments

Abstract

Using evidence from 46 gauged small catchments in the United Kingdom, this paper demonstrates that the methods most commonly used for estimating design flows and greenfield run off rates on small catchments do not perform as well as alternative methods. Their results show larger error and a bias towards underestimation of the median annual flood. In contrast, newer methods from the Flood Estimation Handbook (FEH), when applied to small catchments, tend to have lower error and less bias. The paper investigates the theoretical and empirical support for four methods: Institute of Hydrology Report 124, Agricultural Development and Advisory Service (ADAS) Report 345, the FEH Statistical method and the Revitalised Flood Hydrograph method. It compares the results from all four methods with those from direct analysis of flood peak data and discusses the implications of scaling down flows to the development plot scale.The paper recommends that the many guidance documents on run off estimation and urban drainage are amended to recommend use of Flood Estimation Handbook methods.

Exploring the Linkage between Urban Flood Risk and Spatial Patterns in Small Urbanized Catchments of Beijing, China

“…Geophysical factors that affect catchment runoff, such as soil condition, climate and land cover, can be easily configured by altering the input parameters in hydrological models [21,23]. Furthermore, hydrological models provide a reductionist way of characterizing various spatial scenarios in detail without considering the complexity of the real world [24,25,26]. As such, model-based analysis is necessary to generalize the specific hydrological process, and thus provides guidance for future urban storm water management efforts [27].…”

An improved method for estimating the median annual flood for small ungauged catchments in the United Kingdom

“…(), more recent studies by Faulkner et al . (2009, ) found that it gave poorer flood estimates than either the FEH QMED equation or the Revitalised Flood Hydrograph method (ReFH) discussed in the following sections. The Environment Agency (2009) also concluded that QMED estimates based on IH 124 are, on average, 39% too small.…”

“…Urbanised catchments (URBEXT1990 > 0.125) where floods occur predominantly in the summer half of the year, and highly permeable (BFIHOST >0.65) catchments, were also poorly represented in the datasets used to calibrate the ReFH model. Faulkner et al (2011), who used the evidence from 46 gauged catchments in the UK to investigate the theoretical and empirical support for various methods of design flood estimation found that: • Although the ReFH method had a fairly low bias when considering the entire dataset, this overall average masked the fact that the method tended to underestimate QMED for permeable catchments and overestimate QMED for less permeable and wet catchments (i.e. SAAR >800 mm); and • The combination of high permeability and part urbanisation appeared to result in particularly poor performance from ReFH.…”

“…only 23% in total); and • The sensitivity of the equation to the inclusion of three additional permeable catchments; excluded from the sample utilised, because their urban fractions (of 0.03, 0.05, and 0.06) were above the arbitrary limit of 0.025 for the urbanisation factor URBAN50k (see Cawley and Cunnane, 2003). Although the approach given in IH 124 is recommended for use by Kellagher (2004) and by Woods-Ballard et al (2007), more recent studies by Faulkner et al ( , 2011 found that it gave poorer flood estimates than either the FEH QMED equation or the Revitalised Flood Hydrograph method (ReFH) discussed in the following sections. The Environment Agency (2009) also concluded that QMED estimates based on IH 124 are, on average, 39% too small.…”

“…Guidance available on which design flood estimation method to use for small catchments and greenfield runoff (Cawley and Cunnane, ; Kellagher, ; Woods‐Ballard et al ., ; Environment Agency, 2009; Faulkner et al ., , ) does not provide a uniform viewpoint. These guides highlight that current methods are often applied to areas with characteristics outside the range for which the method was calibrated, and the results obtained typically have very wide error bounds.…”