Guide to the instrumentation of springs and surface waters

A spring is a place where water naturally emerges from the ground. They can be perennial, i.e. present throughout the year with more or less variation in flow. Others are said to be temporary or intermittent, as they are only present for part of the year, often due to heavy rainfall or melting snow.

Groundwater is always underground, but its position in mountainous areas means that it can be collected by gravity using drains. Like springs, they can be perennial or intermittent.

A catchment is the structure used to extract the water resource. It is a man-made infrastructure that must be adapted to its environment (borehole, pump, gravity-fed, etc.).

The presence of a spring or a drained water table will lead to a difference in the way it is captured. There are different types of collection chamber for each type of flow, often determined by their environment and use.

The following are the regulations governing the use of a water catchment for drinking water supply to private individuals or local authorities:

As stated in part 1, a spring or water table may be permanent or temporary. In our case, the catchments used to supply drinking water are, for the most part, perennial, but can experience significant variations in flow over the seasons. What’s more, with increasingly frequent droughts, some of the springs or groundwater used may also dry up.

Monitoring water catchments is part of monitoring the drinking water supply. It is essential to ensure the long-term supply of water to the population. This monitoring is carried out by regularly measuring the flow rate throughout the year, in order to identify seasonal variations and check whether these are normal (linked to the season) or exceptional (outside the seasonal average).

Regular, close monitoring of a water catchment can be complicated by a lack of time, personnel or access to the manhole. Instrumentation of a water catchment is a way of ensuring long-term monitoring of resources, while increasing the accuracy of measurements. There are a wide variety of solutions that can be adapted to each situation. Even when the infrastructure work becomes major, the installation of instrumentation remains a significant asset in monitoring the resource.

In most cases, the flow rate from a spring or gravity-fed water table is measured by circulating the water through a weir or sized channel over which the height of water is measured. A formula is used to convert this height into flow rate. If the water is channelled through pipes, a meter can also be used. We will look at the different systems in detail below.

• V-shaped: These are the most common. The standard angles are 28.4°; 53.8°; 90° and 120° depending on the flow rate to be measured (standardisation is based on easy-cut sizes according to the base and height of the triangle). They can be used to measure low to medium flow rates with wide fluctuations.
• Rectangular: A little less common, this weir is used for high flow rates without major fluctuations.
• Trapezoidal: Rarer, this is a combination of the previous two. They are used to measure high flow rates subject to wide variations.

Although it is not always easy to create a box with a weir, it is possible to adapt a box with an integrated weir directly to the inlet pipe.

The systems described above work in conjunction with a water level sensor to establish the level/flow relationship. All have different measurement ranges and must be adapted to the site. For springs, the lowest ranges (0-1 m or less) will give the best accuracy. There are different types of measurement technology:

The equipment described above is commonly used. There are other systems which, depending on the installation, may be more practical but less accurate. More anecdotal, they are not to be preferred to other more common and therefore more tried and tested systems. Here are a few examples:

PLCs are the most complete devices. Data can be processed directly before being transmitted, and several different types of sensor can be connected. Their large capacity also makes them more energy-intensive. Even if a battery and a solar panel can meet their electrical needs, it is advisable to use them only with 230V domestic current.

Once the data has been acquired, there are two ways of retrieving it. As previously stated, we can retrieve the data manually from the device, but the most interesting solution is of course to automate the sending of remote data. To do this, the remote management system must have a modem and a network connection. There are various networks that can be used:

• GPRS/3G/4G: This is the best known of all, being the network used by mobile phones and smartphones. To use this solution, the datalogger must contain a 3G or 4G modem, a SIM card and an operator subscription. There are multi-operator subscriptions that automatically select the best network (€5/month).
• Satellite: The location of springs in medium and high mountains is not always conducive to the telephone network. In the absence of a mobile network, satellite communication is a good alternative. The subscription is higher (€20 to €30/month) for the same cost of equipment. However, the democratisation of technology is gradually driving down the cost of packages.
• Low-speed networks: The best-known are LoRa Wan and SigFox. Often dependent on traditional telephone networks, they are more effective in urban areas and less present in rural and hilly areas. Their use requires a little more planning, but they enable data to be exchanged where operator networks are too weak. On the other hand, subscriptions are much cheaper (€5 for more than 100 sensors).

Where no network is available for remote transmission, the use of a logger/recorder may be a good idea. This will enable daily data to be acquired, with a visit to the site planned once or twice a year.

Depending on the method or product used, the data will not be transferred in the same way. It will either be sent directly to the customer server, or via the manufacturer’s platform.

• Direct transmission: In this case, the PLC or datalogger is programmed with the client server’s identifiers and sends the data to a folder on the server (usually via FTP or SFTP protocol). The data can then be processed directly from the files sent.
  
• Via platform: With this solution, the datalogger manufacturer retrieves the data and makes it available to the customer via a web platform. Depending on the supplier, a file with the data can also be transferred to the customer. The only advantage of this solution is that the data can be viewed on the manufacturer’s web platform, but this comes at a cost and the customer will have to pay a subscription fee for this service. In addition, it is often sufficient to send the data once, as it is then managed by an internal viewing platform or one offered by another service provider.

Below, we will look at two special cases, but which correspond to situations we encounter regularly.

Table 2: Summary of costs to help with instrumentation costing

A day’s work is rarely carried out by a single person. It is therefore advisable to double the price of labour to include a second person.

The above prices are based on an average suggested by suppliers. They may vary depending on the manufacturer, technology, precision or material chosen. Labour costs may also vary according to the accessibility of the collector and the difficulty of the work.

Table 3: Gant chart for project time management

This methodology does not take into account all the administrative procedures that are useful for the deployment of the project, but which are tedious and rarely reducible.

The length of time taken to prepare the project varies according to the workload of the company responsible for the works. Note that in some cases, the site may not be accessible all year round (snow, mud, etc.). An assignment schedule should always be drawn up in advance with the company responsible for the work.

For the “diagnostic” part, the intervention of a specialist company may be required if the installer is unable to carry out the camera inspection. Video inspection of a drain is more complex than for a borehole (broken drain, filled drain, source in karstic networks, etc.).

Installing the instrumentation is extremely important. It is the key to measurement accuracy. It is therefore important to work with a competent and reliable company. imaGeau has compiled a list of partners with whom it has already worked and which it recommends.

The company below is not an installer, but offers complete satellite communication solutions (intelligent logger + data transmission).

References

Type of sources :

• http://id.eaufrance.fr/nsa/918
• https://www.memoireonline.com/10/13/7528/m_Les-eaux-souterraines-captage-exploitation-et-gestion12.html

Thresholds :

• https://abt.fr/wp-content/uploads/2015/10/Comptage_Caracteristiques-Ouvrages-ABT-%C3%A0-seuil-mince.pdf

Regulations :

• https://www.ecologie.gouv.fr/protection-ressource-en-eau
• https://solidarites-sante.gouv.fr/IMG/pdf/guide_protection_des_captages_d_eau.pdf
• https://solidarites-sante.gouv.fr/IMG/pdf/CAPTAGE_WEB.pdf