Surface Water Drainage Strategy For Planning
A Surface Water Drainage Strategy is required for most planning applications in England in accordance with the National Planning Policy Framework, unless there is clear evidence that this would be inappropriate. There are only few exceptions to not provide a Surface Water Drainage Strategy.
Without a Surface Water Drainage Strategy your planning application could be rejected.
What are SuDS?
In natural environments, rain falls onto permeable surfaces (grass, etc.) and soaks into the ground. Through urbanisation, this process will be impacted with many surfaces become sealed by buildings, paving, and other impermeable surfaces. As a result the natural process of mitigating runoff is limited.
Sustainable Drainage Systems, or more commonly known SuDS, are designed to manage the flood and pollution risk from urban runoff and to contribute wherever possible to environmental enhancement and place making. SuDS are designed to manage storm water locally (as close to its source as possible) to mimic natural drainage and encourage infiltration where possible and then attenuate and passively treat it.
When you implement SuDS it’s possible to achieve lowered flow rates, increased water storage capacity, and reduced pollution transportation to the surrounding water environment.
It’s highly unlikely that a development would require just one type of SuDS device. It will frequently consist of a management train to achieve, where possible, all four pillars of SuDS design.
SuDS can use an array of techniques and components of drainage systems that when combined together form a management train.
As surface water flows through a site, specific components are selected to support the SuDS system and mitigate the flow velocity and pollutants within the water system. The management train may include the following stages:
Source control methods to decrease the volume of water entering the drainage/river network by intercepting run-off water on roofs for subsequent re-use (e.g. for irrigation) or for storage and subsequent evapotranspiration (e.g. green roofs). Where space is limited these techniques can be valuable.
Pre-treatment steps, such as vegetated swales or filter trenches, that remove pollutants from surface water prior to discharge to watercourses or aquifers. These can be used where there is more space and the ability to incorporate such systems. When these aren’t possible there are man-made systems that replicate the removal of pollutants but will likely require greater maintenance.
Retention systems that delay the discharge of surface water to watercourses by providing storage within ponds, retention basins, or wetlands. These can be advantageous for major applications and become areas which can increase biodiversity and be an area of amenity for communities.
Infiltration systems, such as infiltration trenches and soakaways, mimic natural recharge. This allows water to soak into the ground and is a preferred technique. These should be consider first on all applications where possible. There is a requirement to appoint an additional specialist to conduct the tests.
Unlocking Development through Drainage Strategies
A Drainage Strategy is required to be tailored to your development and specific to your needs. Our team can support your planning application and help you better understand the risk of flooding and drainage to your site. Whether you are in a critical drainage area, need a surface water, or foul drainage assessment we can help. From outline planning to detailed design, Aegaea can support you throughout.
Frequently Asked Questions
SWDSs review multiple datasets to fully assess the existing and proposed site drainage conditions, including; surface (permeable, like grass or greenspaces, or impermeable, like concrete), topography, existing drainage (if any), invert levels/cover levels/capacity checks, soil type, existing surfaces, existing infrastructure, rainfall data, and other environmental conditions (river proximity, nitrates/phosphates, groundwater, contamination, ecology, etc).
After assessing each dataset, scientific and engineering calculations can be conducted to get a complete picture of the impact of your proposed development. This then informs the levels of water, or runoff rate, needed to mitigate across the site, either through traditional drainage or Sustainable Drainage Strategies, determining if your development is feasible.
The SWDS is then assessed by the Lead Local Flood Authority, Water Company, and Local Planning Authority to determine whether it’s sufficient to control the runoff from your site without overloading the existing systems or surrounding environment. It also needs to mitigate any uncontrolled discharges that could lead to pollution.
The overall outcome of a sustainable drainage strategy is to control a water source on your development by mimicking natural processes. The end goal is to provide a ‘betterment’ to the area and reduce the impact of your development on the surrounding site.
To satisfy planning, there are many Sustainable urban Drainage Systems (SuDS) that can be used to support achieving this goal. Each solution can be assessed on the 4 key SuDs benefits, but one solution does not need to meet all 4. You can use a variety of solutions together to meet these benefits. The 4 parts to assess the suitability and benefit of a SuDS solution are;
- Water Quantity
- Water Quality
The National Planning Policy Framework of 2019 introduced the use of SuDS as a requirement unless it is deemed to be inappropriate for the development.
When determining any planning applications, local planning authorities should
ensure that flood risk is not increased elsewhere. Where appropriate, applications should be supported by a site-specific flood-risk assessments. Development should only be allowed in areas at risk of flooding where, in the light of this assessment (and the sequential and exception tests, as applicable) it can be demonstrated that:
a) within the site, the most vulnerable development is located in areas of lowest flood risk, unless there are overriding reasons to prefer a different location;
b) the development is appropriately flood resistant and resilient such that, in the event of a flood, it could be quickly brought back into use without significant refurbishment;
c) it incorporates sustainable drainage systems, unless there is clear evidence that this would be inappropriate;
d) any residual risk can be safely managed; and
e) safe access and escape routes are included where appropriate, as part of an agreed emergency plan.
This new policy means it’s highly encouraged to use sustainable drainage systems, however, there are limited exemptions.
A common exemption would be an extension atop the existing built footprint. If it doesn’t exceed the existing building footprint and will connect into former/existing roof drainage, it can be exempt from needing a new SuDS strategy.
Each application is unique and although this is a common exemption, we would strongly recommend liaison with your Local Planning Authority and Lead Local Flood Authority before proceeding with similar projects.
The main difference between outline planning and full planning is the level of detail that’s required. You need significantly more data and information in a full planning application than an outline plan. Outline planning could also be viewed as a well thought out conceptual plan.
Outline Planning is focused to the concept and principles of drainage. How will the runoff be mitigated post development? Where could the sustainable urban drainage systems be located on the site? How will water be discharged from the site? Planning Authorities will expect these questions to be answered, with surface water drainage calculations and a plan demonstrating the potential future arrangement.
Outline planning may not be as detailed as full planning or pre-construction, but it still has to be implementable and detailed enough to justify the selection and specific sizing of the chosen SuDS (such as tank size, etc).
Due to it being more conceptual, it can lead to more information being requested as planning consent is being issued before permission to start the works can go forward.
Full Planning has much greater detail allows for a works to go forward (provided no conditions have been imposed).
Full planning, when focused to a surface water drainage strategy, evolves the conceptual elements to a level that is near to pre-construction detail. This includes things like pipe sizes, diameters, lengths, rain water pipes, manhole covers, invert levels, discharge points, flow rates, and agreements with LLFAs/water companies. It further details the composition and maintenance of each drainage device or SuDs and creates a fully detailed plan that can be evolved by a contractor once the project progresses to construction.
The National Planning Policy Framework (NPPF) defines a planning condition as, ‘A condition imposed on a grant of planning permission (in accordance with the Town and Country Planning Act 1990) or a condition included in a Local Development Order or Neighbourhood Development Order.’
Conditions are imposed on some applications as an alternative to issuing an objection or refusal of a planning application. These conditions might require additional information or approvals for specific aspects of the development. On occasions there are opportunities to engage with the Local Planning Authority before the application is submitted to broadly agree the conditions that could be imposed and recommended. This saves you and them time and cost as you won’t have to resubmit anything.
Our team are able to progress a planning applications that has been imposed with planning conditions. If a condition has been issued for a drainage strategy it is likely that more information is needed. More information will likely require more surveys to be conducted in order to source the data needed to inform the strategy and discharge the condition.
Our team are able to review your project and the condition imposed to advise whether further surveys could be required whilst working with you on how we can overcome discharging of the conditions.
Each Surface Water Drainage Strategy is unique, however, typically our expert team can produce a surface water drainage strategy within 10 to 20 working days from receiving all necessary data.
Much of this time-frame depends greatly on the size of the project, complexity of the site (brownfield, existing connections), environmental constraints (geology, topography, ground water etc.) and the amount of data that is already available. Review of all of these elements is required to allow our experts to cross review and interrogate each element and piece of data in order to provide you with a national planning policy compliant report for planning.
It is the responsibility of the client/applicant to source the majority of the datasets and necessary pieces of information. However, our experts are able to support in sourcing information and supporting you through the project. We have a network of specialists who can provide additional services as and when required.
If you’re looking for a much shorter time frame, our team can provide reports in an accelerated time-frame at an additional cost to meet your requirements. We’re able to provide this rapid service due to our teams experience of supporting our clients and the tried and tested methodologies and approaches.
Surface Water Drainage Experts from Aegaea
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