Three approaches for adapting Urban Areas to flooding
Most properties, businesses and related infrastructure at risk of flooding are in urban areas, where more than 80% of the UK population live.
The risk of flooding depends on the vulnerability and exposure in the catchment. The catchment, in this case, is in urban areas. Flooding is also altered by the likelihood of the rainfall event and changes to the catchment permeability.
The likelihood of flooding is influenced by seasonal rainfall patterns, the type of river catchment and the relationship between droughts and flooding. The Pitt Rivers review stated that heavy rainfall that occurs after long periods of drought is more likely to run off the dry compacted soil and cause river levels to quickly rise, increasing the risking of flash flooding. Managing the water cycle as a whole could help to reduce the risks of both floods and drought.
The flood vulnerability and exposure is historically increasing in urban areas as the urbanisation expansion encroaches upon areas prone to flooding. Moreover, in 2015 the Joseph Rowntree Foundation noted that factors including old age, poor health, low income and social isolation contribute towards vulnerability to the impacts of flooding.
The increase in impermeable areas has lead to an increase of runoff entering the sewers which are usually combined drainage systems. During heavy rainfall events, the combined sewer flow by-passes the treatment plants and discharges untreated into the rivers causing pollution. In other cases, the sewers are overwhelmed before reaching the treatment plants and flooding is caused within the catchment areas. The Sewer Undertakers have come together to deliver the Water UK 2st Century initiative which aims to improve the drainage systems; however wide implementation of retrofitting sustainable drainage is still in its infancy.
Creating flood defences may lead to higher likelihood of downstream flooding. Hard defences are susceptible to failure or overtopping can cause a substantial impact on homes, industry and key infrastructure. Flood impact is particularly severe when key infrastructure is affected directly by flooding.
Adaptation 1: Flood risk management approaches
As always there is no single solution that can manage all flood risks. Therefore an integrated flood management approach can have a greater impact than single engineered solutions.
Approaches that consider the whole catchment can reduce flood damage in urban areas without passing on problems to downstream communities. Solutions such as:
Woodland planting and soil conservation can slow down runoff at low-intensity rainfall events and therefore can increase the resilience of urban flood defences.
Reservoirs, embankments and active management of upstream floodplains could control excess water during high-intensity rainfall events. This infrastructure requires land availability and involves a proactive approach with landowners to allow compensation or purchase of land.
The use of these interventions is dependable on the size of the catchment; so far there is limited evidence that they can reduce flood peaks at a larger catchment scale.
Adaptation 2: Use of Green Infrastructure
It includes using parks and sustainable drainage systems that manage water at ground level. These interventions can provide storage and control of runoff rates. They also improve the biodiversity and amenity of urban areas. Retrofitting of SuDS and Green Infrastructure in existing neighbourhoods is required in order to have a greater impact on managing flooding in urban areas.
Sustainable drainage systems should also be designed to manage exceedance flows. This involves using development infrastructure to move excessive overland flows to storage areas and away from properties and buildings. Stakeholder engagement is particularly important to the success of such schemes because residents can perceive that developments create artificial rivers within catchments when non-critical roads are used to channel runoff water.
Adaptation 3: Increase resistance and resilience levels
The property Level Resistance and Resilience can have a particular influence on the impact of the flood. There is a general lack of uptake on resistance and resilience steps. Studies after the flooding in 2007 found that 46% of respondents chose not to include resistance and resilient measures in repairs because they wanted their home exactly as it had been before, and others believed it was not their responsibility. In the UK there is no obligation on insurers to repair or replace flood-damaged properties with resistance and resilience measures as the regulatory framework is not in place. Ideally, Building Regulations should incorporate the levels of resistance and resilience expected in properties at risk of flooding.