Surface water runoff calculations

There is a legal requirement to limit surface water runoff from a completed development site to the same level as before the site was constructed.There should be no increase in discharge into nearby watercourses.

Surface water runoff calculations are required to assist developers in demonstrating that they are complying with the latest guidance on managing surface water run-off. you must provide a drawing showing a schematic of the drainage layout, with all pipes, manholes, ponds, etc clearly numbered or referenced.

The calculations are used as part of a sustainable Urban drainage system.

SuDS (sustainable urban drainage systems) try to replicate natural drainage systems in a cost effective way to collect,store and clean any dirty run off water before releasing it into water courses.

This is opposed to conventional systems that cause flooding,polution and harm wildlife.They should be easy to manage,use no energy except that from renewable sources like solar or wind Turbine.They should also be non intrusive and attractive in the enviroment.

This type of development includes the use of reed beds and wetlands that clean water and provide wildlife habitats.

The SuDs system is designed to reduce the amount of surface water entering the sewers,this causes overflowing and contamination of groundwater

Some of The following methods are used in SuDS.

Permeable paving

This can be used for roads,cycle paths and car parks and allows movement of water and air,Porous materials allow built up areas to drain to the soil below.

Permeable paving keeps any polution in the soil and allows for groudwater recharge.It also prevents stream errosion.Heavy metals are captured by void spaces and car oils are digested by micro organisms.

Storm water detention.

 Detention basins are a water management system installed near rivers and lakes they protect against flooding and river errosion.

Also known as dry ponds some are designed to retain some water and are used to control water quantity,They have a limited effect on water quality.

Evidence Required for surface water runoff report(code for sustainable homes)

Taken from Code for Sustainable Homes, Sur1, Page 124-125

Design Stage:

Mandatory Elements require a statement from the appropriately qualified professional that they are qualified in line with the Code and that they provide a report containing the following information:

Demonstration of compliance with peak rate of run-off and volume rate of run-off
Areas of permeable and impermeable surfaces (pre and post development)
A flood risk assessment
Drawings showing pre development drainage
Drawings showing proposed drainage
Confirmation that a drainage system failure would not increase flood risk to site or surrounding area
Design specifications, calculations and drawings to support achievement of credits

Post Construction

Mandatory elements require confirmation from the developer or appropriately qualified professional that the solutions have been implemented which were presented at the design stage. Alternatively if the solutions are different, evidence must be provided to design stage requirements.

Evidence of maintenance responsibilities for SUDS (Sustainable Urban Drainage Systems)
Information provided in the Home User Guide
Confirmation of Flood Risk Assessment

Key Points for Compliance

To establish the risk of post development flooding at the site, a Flood Risk Assessment (FRA) should be undertaken. The level of detail required within the FRA report depends on site size and development density. Small sites with an area less than or equal to 2000m2 will require a less detailed FRA, which may comprise a short report containing information from the local water or sewage utility company, the site investigation, local knowledge and the environment agency, produced by the client's engineer. Sites with an area less than 10000m2 will require a more detailed FRA report, with further detail required for a densely packed site over a sparsely packed site.

Allowances now have to be made for climate change over the lifetime of the development in all post construction calculations.

Credits cannot be awarded if the development of the site has been undertaken against the recommendations of the Environment Agency regarding flood risks at the site.

Detailed requirements for compliance and Surface water runoff calculations can be provided by our team at UK Accreditation LTD.

Contact us.