Environmental Engineering Reference
In-Depth Information
line condition; and full aforestation causes up to
60% decrease in flood peaks from the baseline
condition.
Within FRMRC1, therefore, a new modelling
procedure has been defined, which allows explicit
representation of detailed management changes
at the scale of individual fields and hillslopes,
and provides, through metamodels, the upscaling
necessary to simulate catchment-scale effects.
The results of Figure 3.11 show some relatively
large effects for relatively frequent events. Clearly
extreme events are of particular interest for flood
risk assessment, and Wheater et al. (2008) report a
speculative simulation for Pontbren using the
extreme rainfall that generated the Carlisle flood
of January 2005. The rainfall total over 2 days
was 140mm, with an estimated return period of
180 years.
They also generally form sensible individual
units, due to the tendency of farmers to set ditches
and drainage outlets at field boundaries.
We illustrate the impacts at the catchment-
scale in Figure 3.11, for a 4-km
2
Pontbren sub-
catchment. The baseline is the current-day land
use at Pontbren, the first scenario removes the
effect of the recent Pontbren tree plantings (and
hence takes the catchment back to something
approximating the intensive use of the early
1990s), the second adds shelter belts to all grazed
grassland sites, and the third assumes the entire
catchment is woodland. The changes in flood
peaks observed for the three scenarios are: remov-
ing all the trees causes up to 20% increase in flood
peaks from the baseline condition; adding tree
shelter belts to all grazed grassland sites causes
up to 20% decrease in flood peaks from the base-
Fig. 3.11
Catchment-scale response for a 4-km
2
Pontbren subcatchment for current land use and a set of scenarios:
1990s intensification, further addition of tree shelter belts, and full aforestation. (see the colour version of this figure in
Colour Plate section.).
