The river Orge : one valley, one river, one Association.
The management of a river is the concern of all the inhabitants of a same valley.
It is for this reason that 37 communes are regrouped together in the Association of Communes of the Downstream Orge Valley (Syndicat de l'Orge).
As a result, the upkeep of the river and its banks, hydraulic management, decontamination and the monitoring of water quality are carried out more efficiently.
37 communes of the Essonne :
Arpajon, Athis-Mons, Avrainville, Ballainvilliers,
Boissy-sous-Saint-Yon, Brétigny-sur-Orge, Breuillet
Bruyères-le-Châtel, Courson-Monteloup, Égly, Épinay-sur-Orge,
Fleury-Mérogis, Fontenay-lès-Briis, Grigny, Guibeville, Janvry, Juvisy-sur-Orge,
La Norville, La-Ville-du-Bois, Leuville-sur-Orge,
Linas, Longpont-sur-Orge, Marcoussis,
Marolles-en-Hurepoix, Montlhéry, Morsang-sur-Orge,
Nozay, Ollainville, Paray-Vieille-Poste,
Saint-Germain-lès-Arpajon, Saint-Michel-sur-Orge, Savigny-sur-Orge,
Villemoisson-sur-Orge, Villiers-sur-Orge, Viry-Châtillon
Elected representatives and technicians working for the environment
64 delegates form committees dealing with ecology / the landscape, construction projects, finance…and decide upon the main decisions concerning the environment of the valley at a general meeting.
94 employees work in the operations, construction, water-quality, landscaping departments…
These employees are trained in the use of modern management tools - flowmeters, remote processing, geographical information systems - but they are also "ground troops" able to intervene at any time in the event of pollution or flooding in the valley.
Actions for the river Orge
Controlling the water levels
12 compensating / balancing reservoirs have been created to store the excess volumes of water which are transported by the river.
Furthermore, the acquisition by the Syndicat of Orge of land located in areas prone to flooding will prevent any further urbanization on the banks of the Orge.
Operating an inter-communal decontamination network
The Syndicat operates a decontamination network which runs from Breuillet to the lift station at Crosne: 142 km of canalisation and 22 lift stations to monitor, upkeep, rehabilitate…
The upkeep of areas of unspoiled nature
Each year, the Syndicat looks after 180 hectares of areas of unspoiled nature : mowing, clearing, pruning and water weed cutting. Much work has also been carried out to restore the banks of the Orge.
Improving water quality
As a slow-flowing urban river, the Orge recuperates water which flows into it and sometimes domestic water coming from the towns.
As a result of the decontamination work carried out by the Syndicat and the communes, the deterioration of the Orge has been halted.
Some key figures
120 km of canalisation
31km of waterways
282 hectares of areas of unspoiled nature
The Syndicat of Orge managed its decontamination networks, to establish a flood prevention system and to survey water quality.
All the water pipes which discharge into the Orge have been listed.
More efficient means of fighting against pollution
All water discharged into the Orge and its tributaries has been subject to a thorough survey.
All the storm water pipes flowing into the Orge have been located.
The drainage basins for each water pipe have been identified.
A survey has been carried out to identify those installations which can be classified as being a risk for pollution (chemical plants, storage of hydrocarbon, etc.).
It is now possible to know through which water pipe accidental or chronic pollution could be discharged into the Orge.
GEOGRAPHICAL INFORMATION SYSTEM (SIG)
A database working for the environment
850 000 objects (trees, sluice gates, main drains, etc.) are listed in the SIG. An information sheet is linked to each object.
As a result, simply by selecting one of the 72 reservoirs listed, one can find out if it is a storage or settling reservoir, a dry or full reservoir, its capacity and the date of the most recent cleaning operation.
The same process can be carried out for decontamination…Statistics for each main drain are also included :
length, diameter, material, most recent cleaning operation…
In addition to storing quantities of useful information, the SIG also allows this information to be shown on plans, maps or aerial views of the Orge valley.
It is also a management assistance tool in as much as it allows the calculation of the intensity of flow into the main drains to gauge the amount of maintenance work required (cleaning, cladding, etc.), or to calculate the volume stored in a reservoir during high water levels.
*The Association has also received subsidies from the Seine-Normandy Water Agency, the department of the Essonne and the Ile-de-France region.
The SIG is also available for use by the communes of the Association.
They are equipped with the material and software necessary for exploiting their own data concerning urbanism (Land use plans (POS), land registry) and the environment (decontamination network, green spaces…).
REMOTE MONITORING AND REMOTE CONTROLLING OF THE ASSOCIATION'S WORK
Efficient management of the river and the decontamination network
There are more than 90 sites which are remote-controlled throughout the valley.
The equipment is connected by a telephone line to the remote-control room which is located in the premises of the Syndicat.
It is now possible to be notified in real time of any malfunctioning of the installations : overflowing of a lifting station, fault in the functioning of a pump, electrical breakdown of a sluice gate…and therefore to intervene very rapidly.
In addition, remote-controlling allows permanent monitoring of the water course : 30 ultrasonic sounding lines measure the height of water in the river. Any changes in water level are notified to Syndicat headquarters in real time. An alarm signal is set off automatically as soon as the river reaches a level which is dangerous for the riverside areas / residents.
This electronic surveillance operates 24 hours per day.
Outside working hours, the alarm signals are immediately directed to the home of an on-call executive who can send out a team to carry out the necessary measures to combat high water levels.
Remote-controlled equipment in the Orge
30 sounding lines to measure water height
20 sluice gates
20 water lifting stations
1 network point (interconnection of the sewage networks)
6 stations to measure water quality
6 flowmeters (to measure the flow rate of the sewage)
11 rain gauges
Establishing scenarios to combat high water levels
Before the remote-control system was installed in 1995, the workings of the sluice gate were mainly carried out on an empirical basis.
Today, numerous data concerning the position of the sluice gates, the amount of rainfall or the height of water is handled by computers.
Using this data, the Syndicat has installed equipment which simulates the flow of the Orge and its tributaries.
This is useful for two main reasons :
- to evaluate the efficiency of sluice gate movements during high water levels,
- to simulate strategies to combat high water levels in similar situations in order to be able to use the best strategy in emergency situations.
The water height curve predicted by the modelling follows that which was actually recorded by the height sounding line.
MEASURING WATER FLOW
Permanent diagnosis of the decontamination network
The Association possess 50 flowmeters to measure the flow of sewage over its territory. At each measuring site two sensors - one to record flow rate and the other to measure water levels - are connected by a cable to an electronic box.
This material stores data every five minutes. This data is verified and reproduced in the form of a curve or a chart.
The benefits of being able to permanently carry out these measurements are not negligible. The network can be constantly diagnosed as variations in sewage flow result partly from leaks, infiltration or poor connections.
Parasite water (infiltrations or rain water) are mixed with sewage and cause the main drains to overflow.
Being aware of the volume of parasite water allows the necessary rehabilitation work to be scheduled and in the medium term, to verify the impact of this work.
DETECTING AND MEASURING RAINFALL
Establishing a connection between rainfall and the rise in the height of water in a river
The Syndicat has acquired the Météotel P.C. This software allows the consultation in real time of the radar images sent by the MétéoSat satellite.
These images show, most importantly, the intensity of the rainfall as well as the direction of the rain which has fallen. This data is only relatively precise. (one pixel corresponds to 2km2).
Bearing this in mind, the Association has backed-up this software by installing a network of rain gauges at 11 sites in the valley.
Using these rain gauges and radar images, a connection can now be established between the rain which falls in the catchment area and the rise in the height of the water in the river. The accuracy of this information is especially important to the on-call teams.
MEASURING WATER QUALITY
The health of the river Orge under the microscope
Six times a year the Syndicat takes samples from 33 points in the valley.
On each site the water temperature and the amount of oxygen is measured before physical, chemical and bacteriological analyses are carried out.
On a less frequent basis samples are taken to detect the presence of certain pollutants such as metals, pesticides and radio-elements.
As the sporadic nature of this sample taking does not allow the detection of intermittent pollution (accidental discharges, periodical effluents, the impact of storm water, etc.), the Syndicat has installed 6 stations to permanently measure the quality of the water.
Every 15 minutes, these "quality stations" record water temperature, the presence of hydrocarbons and ammonium, the pH, the conductivity, the oxygen rate, the turbidness.
INFORMATION FOR THE PUBLIC
Various public relations initiatives
The publishing of the magazine "Au Fil de l'Orge" as well as brochures concerning flood prevention or the upkeep of the valley floor keep the elected representatives, residents and associations well informed. The inhabitants are also offered exhibitions relating to the management of the valley as well as information workshops.
The development of audiovisual and multimedia supports - Video, web site - allow the Syndicat to reach a wider public.