Sustainable Water Management

Flood Risk and Drainage Impact Assessment

The Environment Agency estimates that year on year flooding in the UK poses a huge risk to people and property.

UK government policies for minimising flood risk to property and the environment are set out in The National Planning Policy Framework (NPPF), Scottish Planning Policy [Sections 196 – 211] / Flood Risk Management (Scotland) Act 2009, Northern Ireland Strategy Planning Policy Statement (SPPS), and the guidance for Wales Technical Advice Note 15 (TAN 15), which outline the significant issues that need addressing.

Local Authorities, in accordance with the guidance, ensure that flood risk is a consideration in the planning process for all new developments to lessen the risk of the damage caused by flooding.

To comply with the NPPF, a proposed development will require a Flood Risk Assessment (FRA) if situated:
Within Flood Zone 2 or Flood Zone 3; or
Within a Critical Drainage Area (CDA); or
Within a site curtilage above 1ha in size.

The FRA will accompany the planning application, as the document outlines the threat of flooding and the measures by which the risk can be mitigated or avoided.

A Flood Risk Assessment is determined by the scale and location of a development; therefore, we will ask the right questions to ensure we provide the correct advice in relation to the scope and detail of a project.

In conjunction with the above, we will identify the potential source(s) of flooding, review flooding history, obtain critical water levels and determine the influences on river hydraulics. The FRA will consider flooding from tidal, fluvial, groundwater, sewer, highway and on-site drainage sources. The assessment can cover a number of other factors such as loss of floodplain storage, washland modelling, flood plans, access and egress, sequential testing, exception testing and surface water drainage issues.

Through a series of alleviation measures and engineering options, we will determine the most cost effective and practical solutions to completely avoid or mitigate, within acceptable limits, the risk of flooding on-site or elsewhere.

A Drainage Impact Assessment (DIA), which often compliments an FRA, provides a preliminary review of the potential options for drainage design that may be developed in avoidance or mitigation of flood risks. It also considers the physical changes that the proposed drainage measures may cause to the development and the off-site area drainage efficiency beyond the proposed development. The DIA is supplemented with Microdrainage or XP Drainage calculations to prove the existing runoff rates from site (Brownfield or Greenfield) and to determine the betterment of the proposed drainage system.

The DIA invariably offers an outline design for an appropriate sustainable drainage system that will consider benefits in comparison with the existing flood protection measures.

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