Category Archives: Surface Water Management

Surface Water Drainage Reporting – Essex

Surface Water Drainage Reporting – Essex

Acting on instructions from a repeat client we have prepared a number of drainage reports and plans for a project in Thurrock , Essex.

The work included a BRE 365 Report, A Surface Water Drainage Strategy and a Detailed Drainage Plan.

Surface water drainage requirements have become more and more onerous over the last few years, and you can expect a good deal of detail to be required at planning.

We used falling head test results from borehole soak-away tests to derive a required soak-away size in accordance with BRE 365.

We were then able to work up a detailed drainage scheme, include attenuation volumes, and drainage layout, drainage details and invert / cover levels.

Drainage Strategy – Amersham, Buckinghamshire

Southwest Environmental were contact my a firm of architects who required a Surface Water Management Plan (Drainage Strategy) for their site is amersham. The project involves the extension of an existing warehouse.

The additional roof area has the potential to create run-off. A design was put forward to counter this problem.

Run-Off is when water falls on a hard surface, and does not soak in to the ground. It then will “run-off” . . . but where will it go. Perhaps it is caused flooding elsewhere. This is the purpose of a Surface Water Drainage Strategy. It sets out a method for dealing with Run-Off

The report was prepared on a quick turnaround (5 working days) and submitted for review. Pending some minor amendments at request of the client the report was submitted to Chiltern District Council for Scrutiny.

Are current design avoids the use of soak-away drainage although is does recommend for some attenuation.

Farm Environmental Audit – Dorset

Southwest Environmental Limited were commissioned by a national charity to make an Farm Environmental Audit or Review of there operations as a site in Dorset.

SWEL do undertake Agricultural work, this aspect suited or remit well in that the main focus of Environmental Audit was Surface Water Management and Waste Management both are topics we have considerable experience in.

A review was made of the premises, and a 21 page report was issued within 3 days of the visit, detailing findings and recommendations. We focused on low cost solutions to the problem in hand avoiding large capital expenditure requirements focusing on improvements to operating techniques and site management. 

Drainage Report – Cullompton

Drainage Report – Cullompton

Some areas of Cullompton are designated as a critical drainage area. You may have a planning condition which states that:

“As the application site falls within the Cullompton Critical Drainage Area, please submit a Flood risk Assessment”

A flood risk assessment typical is focused on flood risk to the site, when what is required in a Critical Drainage is a Surface Water Drainage Strategy.

The way to deal with most domestic / residential drainage requirement is permeable paves, with added features. Mots consider this an expensive option, but this is due to lack of understanding.

In order to capture run off you must provide a volume of “holding” (an attenuation volume) this usually provided my the spaces between gravels in a soak-away trench.

But why not use these same gravel as a sub-base for permeable paving? You will have to import fill for driveways anyhow. You end up using the aggregate twice for soak-away and for sub-base.

Water from roofs can be fed in to the sub base, and drive water just soaks straight in, avoid the requirement for gullies etc. There are no blockages from leaves.

Surface Water Drainage Strategy & Flood Warning and Evacuation Plan Tilbury

Surface Water Drainage Strategy & Flood Warning and Evacuation Plan Tilbury

Southwest Environmental were instructed to undertake a Surface Water Drainage Strategy for a commercial development in the Port of Tilbury.

A few weeks after this initial instruction we were also advised to proceed with a Flood Warning and Evacuation Plan.

In both cases the key was  building up sufficient evidence in order to ascertain risks accurately, and then implement migratory measures and evacuation procedures based on those risks.

Flood Warning and Evacuation Plan

There has been historic flooding in Tilbury, and flood water can take many days to recede after an event. The flood defenses in Tilbury do provide a good level of protection but, it is important to consider over topping events.

Although the site should remain high during a flood it is thought, that workers may be isolated for a number of days. To that end a list of long life provisions was included in the Flood Warning and Evacuation Plan. Image from here.

Surface Water Drainage Strategy

The site is to be entirely covered in concrete surfaces, there is also wash water from the intended use that must be disposed of.

Surface Water and Foul Water systems were kept separate in the conceptual design, additional measures were taken to ensure that this was so, although changes to design were kept to a minimum.

The port has a complex private drainage system, and this was studied in deatil to suggest applicable options moving forward.


Surface water was disposed of to the Thames River (which runs through London) via the Open Botany Drainage Channel and Botany Sluice. Water Qaulity to the Thames was subject to new CIRIA Mitigation Indices (above).

Surface Water Drainage Design – Newton St. Cyres, Devon, UK

Surface Water Drainage Design – Newton St. Cyres, Devon, UK

Southwest Environmental have produced drainage reports on a very rapid turnaround for a private client in Devon. 
Planning officers at Devon County Council concerned with the changes to site run-off characteristics pending an extension. 
Planning was due to be determined just a few days ahead of when Southwest Environmental were commissioned. We produced a report on a 24 hour turn around, this was then put before the concerned planning officer and the application as a whole was approved. 
Impermeable Surfaces were Removed
We later provided further details of the surface water scheme to be implemented. Including size and design of the soak away in accordance with BRE 365 and various options which might be considered under Part H of the building regulations . 

Application for Connection to Thames Water – Barking

Application for Connection to Thames Water – Barking

Southwest Environmental Limited have recent applied for a connect to thames water sewers, on behalf of a repeat client. The application procedure is not overly onerous, but it does help to have background knowledge of the site.

Back in 2014 SWEL produce a Surface Water Drainage Strategy for this site and so we are familiar with the asset arrangement in the area, along with the flow rates from surface and foul water outputs.

It helps if the assets in the area are understood. . . 

. . . location plans were sourced. . . 

. . . invert levels should also be considered. 

This is the more recent of numerous applications we have undertaken on behalf of developers in London. The process does take a while Fees to Thames Water are around £500, and then we would charge time on top of this.

If you need any help with applications to Thames Water or Anglian Water or others, then please get in touch.

Environmental Consultants London 

Surface Water Drainage Strategy – East Devon

Surface Water Drainage Strategy – East Devon

In proving a Surface Water Drainage Strategy to the planning office when submitting a planning application, it makes clear how surface water drainage will be handled within the new development.

Surface water drainage will have to be carefully considered when the project involves large areas of impermeable surfaces.

In this instance an artificial sports pitch surface was to be renewed in and the opportunity was taken to check the adequacy of the surface water drainage on site.

SWEL undertook attenuation calculation in line with the relevant guidance, but were careful to estimate an appropriate attenuation value for the pitch surface.

The current pitch surface would seem to be impermeable but this would not have been the case when the pitch was first installed. By research the subject SWEL were able to determin that the pitch sand (which would be free draining) had becomh contaimanted with fine grained soils (clays etc.) that had reduced its permeability and thus increases run-off potentially.

These characteristics had no doubt developed over the life spane of the exist surface and so to specify draining based on the aged, low permeability surface would have been to over specify.

Instead an average value was used between that of a new surface and that of the surface as was observed.

Legacy Drainage Plan, were used to derive current state of drainage.

Pitch condition reports were used to derive condition of surfaces.

High Quality Legacy Plans helped to facilitate the writing of the report.

Surface Water Management Plan

Flood Risk Assessment – Bristol

Southwest Environmental Limited were commissioned to write a flood risk assessment to accompany a planning application.

SWEl were informed that that the site was within flood zone 1, and was at low risk of flooding from tides and rivers. However, there was a question with regards to surface water flooding.

Unlike most developments that see the introduction of built elements, with a subsequent decrease in attenuation, the proposal saw the introduction of large areas of woodland and meadow land which resulted in a 10% increase in attenuation.

Flood Risk Bristol

Attenuation Tank Design – Surrey

Following on from planning level reporting resulting in the conditioning of various surface water management requirements. Southwest Environmental are finalising design of surface water management system for a site in Surrey.

Design of attenuation tank in line with SUDs manual – Outflow requirements are negotiated with local sewer provider Thames Water, and tank capacity is adjusted to minimise capital outlay for developer.

Application is made to Thames Water for Sewer Connection. 

Note: The size of a surface water attenuation tank is based on the peak inflow rates from the site  (the run-off coefficient is taken it to account when calculating this), and the maximum outflow rate. For example a 5 litre per second outflow rate would require an attenuation tank size of 20 cubic meters, whilst a 10 litre per second out flow would require an 8 cubic meter attenuation tank.

As well as the attenuation tank design, a maintenance plan was submitted for it continued upkeep.