SPR J2 SPRtHOSTt i1

where HOSTi is the proportion of the catchment mapped to HOST class i. For the other components of (13.3)

DPRrain = 0

This dynamic adjustment requires a value of the catchment wetness index (CWI). This changes over time as the catchment wets and dries, although for design purposes the FEH also gives a relationship between a design CWI and the standard annual average rainfall in a catchment (Fig. 13.5).

For particular events, a more complex calculation of the initial CWI is carried out, depending on the antecedent rainfall and soil moisture deficit for that event (see FEH, Vol. 4, Appendix A).

Table 13.3 Recommended standard percentage runoff (SPR) values for different HOST classes

HOST class

SPR

HOST class

SPR

(%)

(%)

1

2.0

16

29.2

2

2.0

17

29.2

3

14.5

18

47.2

4

2.0

19

60.0

5

14.5

20

60.0

6

33.8

21

47.2

7

44.3

22

60.0

8

44.3

23

60.0

9

25.3

24

39.7

10

25.3

25

49.6

11

2.0

26

58.7

12

60.0

27

60.0

13

2.0

28

60.0

14

25.3

29

60.0

15

48.4

Fig. 13.5 Relationship between standardised value of catchment wetness index and standard annual average Rainfall, SAAR (FEH Vol. 4, 3.7, Copyright NERC (CEH)).

A final modification can take account of the effects of urban development in a catchment as:

where URBEXT is the fraction of urban area in the catchment.

The calculations required for the FEH estimation of effective rainfall are greatly facilitated by the availability of the FEH software that allows the calculation of the relevant catchment characteristics for any point on the river networks of the UK. The method has provided a standard for estimating flood runoff in the UK and continues to do so through the evolution of the 'revitalised' flood hydrograph method (see later). What is currently missing from the FEH methodology, however, is any consideration of uncertainty in the estimation methods outlined above.

13.5.4 The FEH unit hydrograph

The UK Flood Estimation Handbook (Institute of Hydrology, 1999) continues the practice of the earlier Flood Studies Report in using a standardised triangular form of the unit hydrograph (Fig. 13.6). It is defined by only two parameters which can be related to catchment characteristics for application to ungauged catchments.

The FEH suggests a number of methods of estimating the peak of the instantaneous unit hydrograph, Tp, including the analysis of several flood events at gauging sites or by scaling from a nearby donor gauging site. At ungauged sites, where no events are

Fig. 13.6 The FEH unit hydrograph.

available for analysis, the regression relationship

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