12/00218/FUL - Armscroft Park PDF 393 KB - Democracy ...

Flood Risk Statement
The Flood Risk Statement utilises the Manning’s Equation to compare estimates of the
pre- and post-mitigation bridge and channel capacities. The equation appears to have
been applied correctly, but this approach is considered too simplistic for supporting a
significant change to channel hydraulics in a sensitive flood risk area, as it omits the
impact of surcharging and hydraulic interactions with the upstream and downstream
system (e.g.: backwater effect). However, the exercise does suggest that the proposals
will increase the efficiency of the fluvial system through the improvement reach. It also
demonstrates that the upstream ‘Gallows Bridge culvert’ is the hydraulic control on flows
reaching the improvement reach. The report compares the proposed 30l/s discharge rate
from the railway triangle development against the flows through the culvert, which
represents a 0.4% increase to the 1 in 100 year flows.
The report estimates the maximum runoff from the development site over a 24hr period
(2592m 3 ), and offers to provide 5000m 3 of floodplain storage within the park, thereby
over compensating for the runoff volume from the development.
However, it is considered that the potential upstream flood risk benefits or downstream
impacts of the proposals have not been quantitatively or qualitatively assessed
sufficiently within this report alone.
Modelling Technical Note
The modelling technical note utilises the Environment Agency’s ISIS-TUFLOW hydraulic
model of the Wotton Brook to compare the proposed mitigation (with additional inflow
from the development) against the existing conditions. It should be noted that the
proposed flood detention basins are not referenced within the technical note; therefore it
is assumed that these have been omitted from the modelling exercise.
A hydraulic model offers a much more comprehensive and robust assessment of the
complex in-channel hydraulics and interactions, but it relies on the model providing a
stable and accurate representation of the fluvial system – this review assumes that the
model is fit for purpose.
As part of the modelling exercise it is reported that the existing and post-mitigation
models were tested against the following scenarios: 1 in 100 year; 1 in 100 year plus
climate change; and a 25% and 50% blockage of Blinkhorn’s Bridge at the 100 year plus
climate change event. Flood levels for the scenarios were compared at 5 locations along
the improvement reach and 2 additional locations within Armscroft Park.
The report shows that water levels within the improvement reach are lowered as a result
of the flood mitigation works (by between 440mm to 20mm). It is reported that the most
significant betterment is observed upstream of Blinkhorn’s Bridge (reductions of up to
440mm), where the replacement bridge resolves the significant surcharging as observed
on the existing structure – i.e.: instead of flood flows passing over the old bridge they
pass under the new bridge. It is considered that this would offer betterment to the local
flooding issues; it is also likely that the elevated soffit and larger bore area of the
redesigned bridge would reduce the risk of debris accumulation and blockages. The
report also demonstrates that the replacement bridge performs better than the existing
during the hypothetical blockage scenarios.
The impacts of the proposed works downstream of Blinkhorn’s Bridge are shown to be
less pronounced, a reduction in flood levels of between 20mm and 60mm; it is reported
that this is a result of the widening of the channel. Peak flood levels within Armscroft
Park, downstream of the mitigation works, are shown to be essentially unaffected.