CASE STUDIES

Flood Risk Assessment and Drainage Strategy

99 High Street, Staines.

Jomas undertook a Drainage Strategy and Flood Risk Assessment for the construction of a new multi-unit residential development at High Street, Staines. The site is currently developed land containing a commercial unit and associated hard standing. The proposals are for a conversion of the existing roof space into two residential units and the construction of 7 residential units at the rear of the property. The development area of the site is mainly located in the Environment Agency’s (EA) Flood Zone 2 (medium risk) with a small part of the site at the rear within flood zone 3 (high risk).

Added Value

A review of the existing site levels and the EA flood map revealed that the flood risk to the site is actually significantly less than shown by the EA flood map.

 

Due to this, the required flood mitigation works can be minimised providing a significant cost saving in the construction of the development. 

 

In order to further minimise construction costs while also complying with the local authority requirements, the proposed drainage strategy aims to reduce the surface water discharge from the site by at least 50% in accordance with best practice and the expected requirements of Thames Water. Part of the existing surface water drainage system and all the new development area will drain through an attenuation system. Surface water discharge from the newbuild section will be limited to 5 litres/second in accordance with best practice. 

 

Attenuation and reduced discharge will be provided for all storm events up to and including the 1 in 100-year storm plus 40% allowance for climate change. Sustainable Drainage Systems (SuDS) shall be used, including an underground tank and silt traps. 

 

Overall, the proposed development has an acceptable flood risk within the terms and requirements of the NPPF. The proposals provide a high level of water treatment, runoff reduction and flooding protection for the proposed development and are in accordance with all requirements of the Lead Local Flood Authority (LLFA).

Flood Risk Assessment and Drainage Strategy

Sycamores, Kenley Lane, Kenley.

Jomas undertook a Drainage Strategy and Flood Risk Assessment for the construction of a new multi-unit residential development at Sycamores, Kenley Lane, Kenley.

The Environment Agency flood map shows the site is at high risk of surface water flooding. However, Jomas investigations revealed that the site levels are raised above the railway line to the rear and so any surface water would actually flow down the railway line and not through the site. 

Added Value:

The discovery of the revised flow path is an important stage in the study of the flood risk as if the investigation did not reveal the level difference, the Local Authority would be likely to require a building redesign to ensure the overland flow path is maintained.

We stated that due to the discovery of the large level difference, the risk of surface water flooding is actually very low. In order to ensure this low risk is maintained, the design of the levels and drainage system design will ensure the risk of flooding is minimised as far as possible. 

 

Option 1 considers the use of infiltration. This is a relatively simple design, with the front driveway area of the site proposed to be permeable paved and all surface water infiltrated via this area. The benefits of this option are both the sustainability (preferable to the Lead Local Flood Authority) and also having the paving at the front of the site, in the case of drainage system failure, overflow will be to the sewers in the street. 

 

Option 2 considers the case where the surface water discharges to the sewer. This option has been kept as simple as possible by converting the permeable paving to a tanked system where an impermeable liner is used to surround the paving and act as a tank.

By designing the drainage with 2 options and keeping these similar it enables the cost of the design to be accurately estimated with the use of either option. This strategy ensures that the quality of surface water discharge is of an acceptable level to the LLFA through the use of permeable paving and also satisfies the requirement for a maximum discharge of no greater than the greenfield rate.

Overall, the proposed development has an acceptable flood risk within the terms and requirements of the NPPF. The proposals provide a high level of water treatment, runoff reduction and flooding protection for the proposed development and are in accordance with all requirements of the Lead Local Flood Authority (LLFA).

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