preliminary flo assessment r preliminary flood risk assessment report ...

Preliminary Flood Risk Assessment
5. FUTURE FLOOD RISK
5.1 Locally Agreed Surface Water Information
The Environment Agency has produced two national datasets
showing predicted surface water flooding:
• Areas Susceptible to Surface Water Flooding (AStSWF)
• Flood Map for Surface Water (FMfSW)
The data shown on both the AStSWF and FMfSW maps for the
Knowsley area was reviewed by representatives of the
Environment Agency, United Utilities and Knowsley Council on
24 th January 2011. It was compared to known, locally agreed
surface water data including the effects of an estimated 1%
rainfall event (an event that has a 1 in 100 chance of happening
in any one year) that occurred on 20 July 2011 and it was
agreed that the FMfSW was more representative of the flood
risk. As such, the FMfSW was adopted for use as the map
identifying “locally agreed surface water information”.
The Environment Agency has used the information within the
National Receptor dataset to count the number of properties at
risk of flooding identified on the FMfSW. The data is based on
the assumption that a rainfall event with a 1 in 200 chance of
happening in any one year is equivalent to the chance of
flooding on the ground in the order of a 1 in 100 chance in any
given year.
Table 5-1 – Properties in Knowsley identified to be at risk of surface water
flooding. Source: Environment Agency, 2011
1 in 200 Rainfall - Flooding Greater than 0.3m depth
All properties Residential properties Non-residential properties
3000 2400 600
5.2 Local Drainage Capacity
Data provided by United Utilities covering recorded incidents
from 2008 to date was reviewed and Table 4-1 shows the
location of surface water and combined sewers that have been
identified to be hydraulically inadequate. These locations tie in
to the areas of predicted surface flood risk identified on the
FMfSW.
KMBC – Preliminary Assessment Report 13