In association with the Environment Agency Medway Council and
Swale Borough Councils commissioned Mott MacDonald to undertake a
strategic flood risk assessment of the River Medway area, including
proposals for extensive development and regeneration associated
with the Thames Gateway.
From its head-waters near East Grinstead in Sussex to its outfall
into the Thames Estuary the Medway catchment covers an area of some
1,800 km
2. This catchment is an area of complex
topography and geology which provides considerable challenges in
predicting and managing fluvial flooding. In particular, the
confluence of two sub-catchments with markedly different responses
to rainfall can result in sudden rises in river water levels.
Together with tidal influences from the wide Thames Estuary,
flooding has become a perennial hazard for many settlements in the
flood plain. The most significant flood event of recent times was
the East Coast Flooding in 1953. This event caused the loss of 300
human lives along England’s eastern seaboard.
Strategic flood risk assessment (SFRA) study
We carried out an SFRA to provide an accurate predictive system and
map coverage for different flooding scenarios in the area. The
study comprised a report, a computer model capable of modelling
varying flood scenarios, and maps relating to 1 in 200 and 1 in
1000 flood events (generated by our specialist computer modelling).
- In order to assess the flooding potential of an area a SFRA
model takes into account the differing mechanisms of flooding. In
principle these are:
- Fluvial flooding – where river flows exceed the capacity of the
river channel
- Coastal and tidal flooding – where high tides, storm surges and
high fluvial flows combine
- Estuarine flooding and watercourses affected by tide lock –
often involving high tidal levels and high fluvial flows
- Overland overflow due to torrential rainfall
- Flooding from artificial drainage systems due to system failure
eg. blocked sewers
- Flooding from infrastructure failure eg. structural, hydrologic
or geotechnical failure of infrastructure that retains, transmits
or controls flooding
The mechanisms are combined with the likelihood of a flood hazard
occurring together with the magnitude of the flood event. The SFRA
of an area requires a highly technical modelling exercise, we used
results from detailed hydrological and 2-D hydraulic modelling to
obtain accurate flood levels, combined with highly accurate ground
level information predominantly sourced from a LiDAR system, an
airborne laser ground height measurement, accurate to +/- 15cm.
This gave us the extent and magnitude of tidal flood events.
Purpose of flood risk assessment in the development
process
Flood risk, or rather its potential, is a material consideration in
planning applications and appeals. In the Medway area, the
development plan currently comprises the Kent and Medway Structure
Plan, the Medway Local Plan plus the Thames Gateway Planning
Framework – which must have regard to planning for flood risk,
including sites and opportunities already identified in the
development plan.
Our findings in Medway
Our model indicates flood risk for the principle regeneration sites
in Medway, based on existing flood defences. The study confirms
that many of Medway’s riverside regeneration areas have a degree of
risk to flooding events. The risks apply to both current climatic
conditions and those predicted for 2100, simulated in the model.
Mott MacDonald has had a long association with the River Medway.
Two previous projects we’ve undertaken included:
- Strategic flood risk mapping (Section 105 study)
- Flood forecasting and warning
