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DISCOVER THE POWER
Enhanced Subbase Attenuation
ENHANCED SUBBASE ATTENUATION FOR SOURCE CONTROL SuDS
Our mission is to provide engineered systems for the next generation of source control subbase attenuation. With well thought out accessible inlet & outlet products along with specially selected geofabrics to compliment the Formavoid attenuation units, we have developed the ultimate enhanced subbase attenuation system.
This page covers how the FORMA enhanced subbase attenuation system works, how it can be implemented with multiple surface types and how we can effectively deal with sloping sites using checkdams.
1. System Characteristics
Increase Water Storage Capacity
The space beneath the Formavoid arches remains open void for rainwater storage and therefore increases the void ratio of open graded subbase from 30% to a combined void ratio of up to 70%.
This broadens the opportunity for implementing source control subbase attenuation where capacity would normally be a limiting factor.
Reduce Construction Depth
The increased void capacity of Formavoid enhanced subbase attenuation means more stormwater can be attenuated in shallower profile than open graded subbase alone.
Particularly beneficial for reducing cut & fill costs and avoiding risks associated with contaminated ground & high water table.
Reduce/Remove Traditional Attenuation Tanks
Given sufficient area, the increased void capacity of Formavoid enhanced subbase attenuation makes it possible to provide all of the required attenuation volume within the subbase and above formation without the need for a geocellular Attenuation tank.
This can save on huge costs and risks associated with installing deep tanks.
Lower Carbon Footprint
Formavoid is made from 100% recycled & carbon neutral polymer and nest stacks to reduce transport carbon emissions.
Coupled with the reduced carbon emissions from reduced excavation depths and reduced stone import, the Formavoid enhanced subbase attenuation system is an extremely carbon conscious solution.
Improve Aggregate Stability
When the Formavoid columns are filled with open graded aggregate, they act as anchors/plugs, providing excellent stability and anti shear characteristics to the subbase.
Reduced Underground Pipe Network
As the Formavoid enhanced subbase attenuation system manages stormwater at source, generally we are dealing with very low flows and therefore require a small and shallow underground drainage network not exceeding 160mmØ, with chambers not exceeding 600mmØ.
2. Extend Subbase Attenuation to All Surfaces
To take full advantage of the system characteristics, it is important to understand that subbase attenuation can be implemented far beyond permeable paving. Formavoid enhanced subbase attenuation can be extended beneath permeable areas (parking bays for example), impermeable asphalt/concrete (roadways & service yards) or a combination of all surface types.
Permeable Surface
Permeable surfaces such as permeable block paving & permeable asphalt provide the ideal opportunity for enhanced subbase attenuation as the permeable surface allows stormwater to drain directly into the open graded subbase whilst providing sufficient proprietary treatment against hydrocarbons.
The Formavoid technology allows us to provide stability to the permeable construction whilst maintaining a shallow construction depth.
Project Example: Manchester Airport, Clough Bank A
Project Example: Cornford Road ph1
Impermeable Surface
Often not considered an option...it is perfectly possible and advantageous to implement enhanced subbase attenuation beneath and entirely impermeable surface.
Traditional surfacing can be used in conjunction with traditional channel drains & gullys. However, rather than draining into a pipe network, the channel drains and gullys simply connect directly into the Formavoid enhanced subbase which allows stormwater to surcharge back into the subbase and leave the system via a Formaflo flow controlled outlet.
Project Example: North Manchester Hospital CP26
Mixed Surface
The most elegant designs take advantage of both permeable & impermeable surfaces with Formavoid enhanced subbase attenuation extended beneath all areas.
Impermeable surface are generally used in high wear areas such as roadways & service yards, and permeable surfaces implemented beneath low wear areas such as parking bays. Stormatwer runs off impermeable areas, draining through permeable surfaces allowing the stormwater to surcharge the subbase beneath all areas. This approach takes advantage of the innate strength of impermeable surfaces and the source control hydrocarbon treatment provided by permeable surfaces.
Generally this can be achieved with a uniform formation and minimal underground drainage pipe.
3. SUBBASE ATTENUATION ; ON A SLOPE
Most sites are not perfectly flat and have slopes/gradients due to the natural topology or engineered to falls to drain the surface. It is still possible to implement and optimise enhanced subbase attenuation on sloping site by introducing subsurface checkdams to terrace the water storage, as shown in the diagram below.
Each checkdam area is dealt with as a micro-catchment with a dedicated Formaflo orifice flow controlled outlet. Each micro-catchment discharges into a small and shallow drainage spine not exceeding 160mmØ, where the flows from each micro-catchment accumulate, not exceeding the combined designed flow rate for the area.
We have developed a clear methodology and product range to implement checkdams very simply and effectively.
IN SECTION
Precast concrete edgings are a simple and effective way to create checkdams within the subbase (terminating just below surface level). Formatextile Geotextile & Formaliner Geomembrane simply lap against the face of the checkdam to create a sealed area.
Formaflo Flow control chambers are recommended 1 per 1,000m², providing a non invasive flow controlled outlet from each micro-catchment into the central drainage spine no deeper than 300mm below formation level.
The Formaflo flow control chamber has a removable upstream protected orifice and remains accessible via manhole cover for future maintenance.
ON PLAN
Checkdams are aligned with proposed surface contours and generally positioned every 250mm - 300mm change in level.
Formaflo flow control units connect into the main drainage spine via Formaflo catchpit chambers, providing access for rodding/CCTV/air testing etc.
As stormwater is attenuated at source with very low flows, the main drainage spines is incredibly discrete and can deal with significant areas. The below is an example project whereby just 150 linear meters of 160mmØ drainage pipe was needed to drain a 8,500m² car park area.
Design Assistance
We have a team of experts on hand to provide support and guidance to develop the most suitable solution for your project. We provide support at high level concept design in addition to detailed pre-planning design including CAD details, attenuation calculations & orifice diameter calculations. It is our mission to provide designers with the tools to implement innovative source control subbase attenuation as part of a SuDS strategy whilst remaining on hand to provide support where required.
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