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Southwest Environmental Limited
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Environmental Consultancy Services to Industry, Business and Individuals

Drainage Strategy

If you need to Drainage Strategy for your project then please contact us us using the details above and we will be glad to produce a report for you containing details of your Foul and Surface Water option on site. 

Sustainable Drainage has recently become very much more of a focal point within planning applications. As recent as 2014 these report were very light on detail, but now they have become fairly considerable documents in there own right. Work that could usually be conditioned until recently now has to be done during the planning period, which can often lead to work such as soak-away tests being required at short notice. Another time consuming request can be a application to sewage undertakers to check capacity for the proposed discharge.

However primarily there are two main aspects considered within a drainage strategy. Foul Water . . . . and Surface Water. However the other items such as Soak-Away tests and Sewer Connections or Sewer Capacity Enquiries are often requested.

Foul Water 

There are numerous standards which can be followed in terms of calculating foul sewer flow rates, for inclusion is foul water drainage strategies. It is a simple procedures for domestic properties, however it become more complex for commercial properties such as hotels and depends somewhat on visitor numbers etc.

Foul water ordinarily would discharge is foul sewers, but in some instances such as rural settings a small sewage treatment plant may be a requirement. If flow rates are over thresholds then, a permit will be required for this sewer treatment plant.

If connecting to sewer then it is sometimes required that either a Application for Sewer Connection is undertaken. Within this application it will be required to details such as:

The above information is normally included within our report dependent on the scope of the Drainage Strategy agreed at the time of commissioning.

Surface Water

A surface water management plan is sometime required as part of a flood risk assessment. A surface water drainage strategy considers factors such as run-off and attenuation volumes, but also where the water will go, there is a massive amount of guidance to be followed in the form the the CIRIA documentation. It was porobaly written with larger sites in mind, buit is frequently applied to smaller single dwelling sites.

At first this seemed rather excessive, but the the latest CIRIA guidance see a move away from attenuation tanks, in favor of over specified permeable paving. A large sub base can be provided to give adequate volumes and other criteria that are included (stemming from CIRIA guidance) such as water quality can be counted all under this same method.

Of course is some instances this approach is not possible, and a tank, or oversize sewer pipe may be recommended as an alternative. There are so many consideration in producing the surface water report that they are impossible to list here but they do include:

The report is written by practical and experienced professionals, we would do are very best to avoid presenting unworkable solutions. 

An important note is that if your ground is contaminated, then it is very unlikely that you will able to use soak-away drainage.

Soak-Away Tests

It is becoming more common that soak-away tests are used to inform a Drainage Strategy. This is consider (by us) to be very annoying, as soak-away tests are time consuming to arrange, relative expensive in cost and mostly importantly are not nessassry until after planning is granted, they could easily be conditioned but often are not.

Ordinarily we would try to avoid this requirement for our clients by including two of more option for surface water strategy. For example discard to ground if post planning soak-away tests are good, and discharge to surface water body (river etc.) if they are bad.

However, as mentioned above this is not always accepted as a way forward, so BRE 365 soak-away tests are carried out. These can be done with hand dug pits if access is tight, but expect only to achieve 1 or 2 tests in a day, due to labor time. A mini digger is the normal method, where 4 tests (if required) can be completed in a day. If the water level in the hole has not dropped significantly over the course of a working day, then it is likely that calculation will prove sok-away is unviable.

Application for Sewer Connection

If soak-away tests prove poor ground and  that you will need to discharge to surface water sewer, then normally is it required at planning that you seek permission from the undertaker, and put the results in to your drainage strategy report. This can be in the form of a capacity enquiry, which is similar to an outline planning application in that you only have to give a low level of details, they will then agree "in principal" to a connection.

If you have more detail, then you can apply for a connection, which is bit more complicated but ultimately more useful.