Drainage Impact Assessment
Drainage Impact Assessments (DIA) are frequently undertaken in parallel with Flood Risk Assessments and as a precursor to drainage designs. As a result of the requirements within Planning Policy Statement 25 (PPS25), Scottish Planning Policy 7 (SPP7) and Technical Advice Note (Wales) TAN15 all new developments must provide effective and robust drainage systems that do not increase the risk of flooding or pollution. A DIA is often required to demonstrate that the impacts of site drainage have been thoroughly considered relative to the pre-development and post-construction situations including appropriate allowances for climate change scenarios.
In general, there is a presumption that new developments or re-development will be carried out with the principles of sustainable drainage (SuDS) techniques. There is also a presumption that the drainage system will deliver some betterment as part of the wider need to reduce flood risks by controlling and reducing surface water runoff.
The DIA is required to consider existing drainage conditions (greenfield or brownfield), drainage routes, ground conditions, site topography, hydrology, hydraulics and future site runoff to demonstrate that surface water runoff can be adequately managed.
The Building Regulation Approved Document H sets out the basic hierarchical approach to surface water disposal as discharge to:
- Soakaway or other infiltration system, subject to soil permeability
- Discharge to watercourse, subject to maximum flow rates and discharge consent
- Discharge to a sewer, subject to a maximum flow rates and discharge consent
Through our drainage, SuDS and flood risk assessment teams, Clear provides a comprehensive service for undertaking DIAs. A key aspect to our approach is our thorough understanding of the hydrological and hydraulic processes that affect any given site. Our reporting and recommendations are always concise, robust and substantiated to ensure that there are minimal delays in obtaining planning approvals.
We always aim to deliver a service that is appropriate to the size and complexity of each development to ensure that we provide maximum cost effectiveness and value for money for our clients.
Where necessary, our specialist hydraulic modelling capability can be used to fully assess existing drainage system performance design and proposed drainage systems to demonstrate that all regulatory requirements are met.
Clear has a proven track record of providing successful and cost effective DIAs to public and private sector clients throughout the UK. Our DIAs are undertaken by experienced staff from our Drainage, SuDS and Flood Risk Assessment teams to provide thorough assessments and reports.
Clear can provide specialist advice on conceptual drainage solutions and SuDS designs to obtain planning approval.