Forthbank Wind Energy Development - Partnerships for Renewables

Forthbank Wind Energy Development
13.3.21 The FEH CD-ROM also provided data relating to the Base Flow Index (BFI) and Standard
Percentage Runoff for the site catchments. The BFI is a measure of the proportion of a
catchment's long-term runoff that derives from stored sources, with the BFI ranging from 0.1 in
relatively impermeable clay catchments to 0.99 in highly permeable catchments. The Standard
Percentage Runoff values represent the percentage of rainfall that is likely to contribute to
runoff.
13.3.22 The BFI for the site catchments range from 0.363 to 0.437 indicating that around a third of the
catchment's long-term runoff is derived from stored sources. The Standard Percentage Runoff
for the site catchments range from 38.2 to 42.2, indicating that approximately just over a third
of the rainfall during an event will contribute to direct runoff rather than be stored. The BFI and
SPR values show that the site is located on a slowly permeable catchment and is likely to have
a relatively slow response to rainfall events.
Pre Development Flood Risk
13.3.23 A review of SEPA’s flood map indicates that most areas, within the site, are at risk from the
200 year tidal inundation envelope from the River Forth and from the north, fluvial inundation
envelope from the Black Devon. As part of the Flood Risk Assessment for the site (Appendix
13.1), hydraulic modelling was carried out to confirm the extent of the flooding envelope from a
200 year plus climate change allowance storm with a 15 year return period tidal boundary on
the Black Devon and from a 1000 year costal flooding event. The only infrastructure shown to
be at risk from flooding is the met mast, which is considered to be with flood zone 3b. The rest
of the site was shown to be unaffected and hence is considered to be within flood zone 1 in
November 2010 Chapter 13 Page 11
Copyright Partnerships for Renewables Development Co. Ltd 2010 ©