The problem in management of rainwater on real estate projects has required new techniques known as “alternatives” for some time now. The principle of these techniques is to reproduce existing phenomena to the natural state from both a quantitative viewpoint with control of flow, and a qualitative viewpoint with treatment of waste-polluted water.

Using the forces of nature rather than artificial means, living with water rather than having it disappear at any price, is surely a very environmental attitude that leads to efficiency, safety and sustainable development. Stanislas Grassien, Head of R&D at GSE, gives us the key to a principle that tends to develop itself.

What do we understand by “Rain water management” and what is its purpose?

Stanislas Grassien: A real estate project regardless of type involves large waterproofed surfaces such as roofs, roads and car parks.
This has two major consequences:

• on the hydraulic plan, the reaction to pluviometry of the constructed parcel is different from that of the initial natural land  Waterproofing of the surfaces has the effect of accelerating the runoff speeds of the rain water and concentrating them more quickly in the project’s low points. In the case of heavy rain, this increases considerably the instantaneous waste flow downstream of the parcel called the “peak flow”. 
• as regards the quality of rainwater, it is affected by pollution: repeated vehicle traffic on the roadway causes a chronic deposit of hydrocarbons that mix with dust and foul up the roads and the car parks. With precipitation come particles called “suspended matter” that are carried by the rain. The water thus polluted cannot be discharged into nature as such.

The principle of rainwater management thus consists of protecting what is downstream of the constructed parcel, and under the parcel (the water table), quantitatively (control of peak flow) and qualitatively (pollution control). To do this, the constructer must carry out collection and regulation of rainwater, using networks and buffer tanks, for example and de-pollution using hydrocarbon cleaner/separators.

How does this work in practice?

S.G.: In concrete terms, using traditional techniques,

• the water collected on building roofs is drained via rain water downspouts to networks buried under the roadway, 
• and to drain and treat the water collected on the ground, trash racks distributed on the roadway and the car parks are connected to buried networks that end at a hydrocarbon cleaner/separator. 
• According to the requirements of the authorised leak flow or seepage from the parcel, a downstream buffer tank, normally sealed, is connected to the system.

The principle is to differentiate the water coming from the roofs and that coming from the roadways and to dry, as quickly as possible, all coated surfaces, regardless of the type of rainfall. In meteorology, the statistics tend to demonstrate that the heavier the rainfall, the rarer it is: for certain projects every ten years, or even every century ,is mentioned. It is thus that very large systems using traditional techniques are rarely used at their nominal capacity. Moreover, the sometimes complex networks and the hydrocarbon separators present access problems for maintenance. Lastly, the buffer tanks often take up a lot of space and are difficult to integrate into the landscape.

Being aware of all these drawbacks, we tended towards the “alternative” techniques, since they fulfil the same functions, are easier to implement and are better adapted to construction projects.

So what are these new techniques and what is the principle?

S.G.: These techniques known as “alternatives” require management and treatment of rain water that exists in the natural milieu. They are substitutes for the traditional artificial hydraulic work.

From the hydraulic viewpoint, the topography of the land is developed: the roadway gradients are exploited and the open spaces ensure control of surface runoff and may cause stopping up distributed over the whole parcel.

This prevents concentration of rain water at a single point, slows down its flow speed, and thus reduces the size of the works.
In concrete terms, the runoff from coated surfaces is authorised by direct channelling to works alongside them of model and planted landscape types. For example, the landscape valleys alongside the roadways and car parks and, in green spaces, hollows in the form of tanks called “normally dry”, that are filled during rainfall. The slight slopes of the contours of these works facilitate their integration into the landscape project and allow easy maintenance, similar to that of classic green spaces.

As regards qualitative pollution treatment, natural works involving decanting of pollutants in suspension are created (efficient for about 90% of pollution); they are associated with semi-aquatic plantations that absorb the dissolved pollutants (the remaining 10%). A first compartment called a “pre-treatment tank”, large and quite shallow, allows a slowing down of rain water drainage and favours decanting of particles in suspension carrying pollution; rhizome plants, such as reeds, planted at the periphery, complete treatment of the dissolved pollution. A second compartment downstream from the first, called the spreading tank, allows reduction of the peak flow to an authorised flow on the parcel and ensures additional treatment in case of an exceptional rainfall.
 
What can we do today about the implementation of such techniques?

S.G.: These techniques have been known and used for more than 10 years in the U.S.A., and are being used in England and Germany.

In France, a theoretical approach, stemming from conferences and numerous theses on the subject from 2000/2001, has been mastered for some years by engineering companies. Up to then it was generally reserved for small-scale projects not subject to the law on water, or to very urbanised projects.

Since 2003, significant projects have been taking shape for public developments subject to the Environmental Code (article 214 and after – e.g. law on Water), particularly ZAC projects, thanks to the aid from DIREN in certain “pilot” regions. GSE was a pioneer on the subject with a first example in 2004 applied to a logistical park.

These projects became reference points of efficiency and were the basis for specifications developed by DIREN, whose aim is to promote these alternative techniques by professionalising them. Little by little these solutions, following reports on their efficiency, have been used in other areas with good results.

The core question of sustainable development has speeded up development of projects to alternative techniques. Our experience of the subject enables us, today, to design our construction projects by taking account of these different methods of rain water management.

Finally, this new rain water management seems to offer numerous advantages that are good for everybody

S.G.: In effect, this design of rain water management by alternative techniques, when well controlled, offers advantages for all those involved in the project.

• For the investor, there are generally savings linked to the optimisation of the land space thanks to a better integration of the works on the parcel (particularly holding tanks) and increased safety linked to control of water distribution on the whole of the land involved.
• For the constructor, there are doubtless gains regarding reduction of completion time and the causes of heavy losses, thanks to a reduction in the number and size of buried structures.
• For the user, there is a simplification of operation and follow up of these rain water management works that become accessible, easier to maintain, and better performing from an environmental viewpoint due to better control of chronic pollution, the whole integrated in an agreeable and qualitative landscape project.

In summary, these alternative techniques are a very efficient way to reconcile environmental performance, the optimum use of space and safety in all weathers!