The valley of the river Neretva is located south of the Croatian Adriatic coast, where the river mouth forms a delta. The surface area of the delta is 12,000ha, and the surface of the drainage basin is approximately 10,500 km2. The total length of the river Neretva is around 225 km.
In the past, the Neretva Delta consisted of 12 tributaries. More recently, the number of tributaries has been reduced to only four through numerous hydro-technical reclamation processes. By gradually draining out the swamp, the delta has been transformed into a fertile agricultural area that is abundant with great natural wealth and agricultural diversity.
The Neretva valley has been used to serve a dual function: during high-water levels, it is used as a relief channel, and in the dry season, it is used as a reservoir of fresh water for irrigation. Between 1968 and 1972, two dams with floodgates and ship locks were built on the Neretva: one upstream of the agricultural area in Opuzen and the other at the delta. The dam in Opuzen controls the water inflow, while the other dam prevents the penetration of salty seawater into the Neretva. Salt intrusion represents a significant threat to the whole Neretva ecosystem and can result in reduced crop yields.
State and local authorities, the national water management company Hrvatske vode, and the academic community in Croatia are continuously improving flooding protection in the Neretva valley and its irrigation capacity, as well as preventing the intrusion of saltwater in the agricultural area. The University of Zagreb's Faculty of Civil Engineering is performing continuous hydrological monitoring of water level and flow in irrigation channels in the Neretva River valley. The Faculty of Civil Engineering has procured Geolux instruments for continuous water level and flow monitoring at four locations on the delta. Geolux has provided the devices and related equipment and performed the equipment installation.
A critical decision factor for the Faculty of Civil Engineering when selecting Geolux instruments was that the devices use non-contact radar technology to measure both water level and water flow (discharge). The instruments are installed above the water surface and are not in contact with the water itself. This makes equipment installation more effortless, and the maintenance is minimal. Water flow is calculated using the index velocity method by measuring the water level and surface velocity.
Non-contact radar instruments are commonly installed on an existing overhead structure, such as a bridge, over a water surface. As there are no such structures near locations chosen by the Faculty of Civil Engineering, Geolux had to propose a different solution for mounting the instruments above the water surface. Custom-designed aluminium poles with an extended arm were constructed and installed at the irrigation channel bank so that the arm that holds the radar instruments extends over the water.
All four stations were installed in two days. During the first day, an excavation was made in the ground of the channel bank for a concrete base, and concrete was poured to form a foundation for mounting the aluminium pole. A few days later, after the concrete had been cured, the external formwork was removed, and the aluminium pole was attached to the foundation. Next, the instruments — radar level meter and surface velocity radar — were connected at the end of the extended arm. In contrast, the enclosure with datalogger, solar panel, and backup battery were attached directly to the pole.
Gaining accurate measurements
To provide accurate and reliable measurements, it was essential to ensure that the wind did not cause noticeable vibrations to the mounting pole and extended arm. Therefore, the concrete foundation, the aluminium pole, and the arm were designed to withstand the maximum expected wind at the installation sites with minimal vibrations to ensure measurements are not affected.
In general, the accuracy of the radar level sensor is not significantly affected by the vibrations when the device is configured to use a more extended averaging filter. However, the accuracy of the surface velocity radar can be decreased if the instrument vibrates while it makes the measurement. Therefore, as a general rule, the arm with the surface velocity radar should not oscillate more than a few centimetres in each direction, with a period not shorter than half a second.
Aquatic vegetation at the monitoring sites poses another obstacle to measurement accuracy. The vegetation growing at the channel banks protrudes from the water surface near the banks and grows up to 2m above the water. If the vegetation is within the range of the radar beam, it can affect the radar measurements of both the water level and surface velocity.
Unlike most similar products on the market, the Geolux radar level sensor has a unique operation mode where it can automatically detect the vegetation above the water and measure the actual distance to the water. However, the vegetation affects the surface velocity radar measurements when the wind moves the vegetation back and forth. Such problems can be minimized by configuring the surface velocity radar to use a longer averaging filter length.
To achieve the most accurate measurement results, it is recommended to configure the instruments accordingly and adequately maintain the monitoring site by regularly cutting down the vegetation. To ease site maintenance, Geolux has installed a camera at each location. Geolux HydroCam's camera takes periodic snapshots of the monitoring site and delivers the images and measurements to the central server. The recent site photos from the camera show that it is evident when the vegetation is too high and needs to be cut down.
The four monitoring stations were installed in the Neretva River valley almost two years ago. The analysis of the measured data indicates no decrease in measurement accuracy caused by the pole vibrations. The design of the concrete foundation and the aluminium pole with an extended arm has been proved to keep the instruments above the water surface firmly. The aquatic vegetation on sites requires both proper configurations of the devices and periodic site maintenance. Maintenance planning is made simple by the availability of near-real-time photos from each monitoring site.
Geolux is dedicated to supporting its clients, from suggestions about site selection to instrument installation and configuration of operating parameters. This project from the Neretva River valley is an example of how Geolux has closely cooperated with its customer, from the planning stage and data analysis, to obtain the best possible results.
Rapid Deployment of Hydrological Monitoring Station
Following a 3.2 magnitude earthquake on September 7th, the water level of the karst river Vrljika started dropping very quickly. This rare phenomenon has last occurred in 2004, and, before that, in 1942. Low water levels of Vrljika can be dangerous for several endemic fish species; and the whole Imotski region gets its water supply from Vrljika. Less than 24 hours after the event, Geolux technicians had installed a remote water level monitoring station to monitor water levels of Vrljika in 5 minute time intervals.
Geolux has installed its integrated hydrological monitoring station consisting of a radar-based water level sensor (LX-80-15), a surface velocity radar sensor (RSS-2-300 W), a SmartObserver datalogger with integrated GPRS modem and battery charger, a 10 W solar panel and an 8 Ah backup battery. Using an integrated station which relies solely on non-contact instruments allowed Geolux to setup the station in less than 2 hours, giving an extremely short response time to a critical situation.
Discharge Measurement on Small Channels
To monitor water discharge on a small stream, Geolux has installed HydroStation - highly integrated hydrological monitoring station that consists of radar-based water level sensor, surface velocity radar, Lithium battery pack, solar panel, Geolux HydroCam camera and GPRS datalogger. The station is installed on a bridge above the channel, to measure water level and total discharge. The measurement site is specific because on such a small channel, expected discharge is very small. To improve discharge measurement when the water level is very low, we ave installed additional V-notch weir. This channel is used for irrigation of a nearby field, and the study that uses Geolux instruments aims to determine if more fields can use the same channel for irrigation, or additional construction work will be required.
The main benefit of using Geolux instruments on this site is simple and quick installation. HydroStation comes pre-installed with all instruments and solar panel already attached, and on-site installation requires only attaching the station to the fence that exists on the bridge. The calibration for discharge measurement is also easy, as the channel below the bridge is rectangular and made from concrete. Geolux HydroCam is used to take images of the staff gauge for redundancy, and to provide visual information about water icing or aggregation of driftwood and garbage.
Measured data is transmitted to Geolux HydroView cloud-based software, that stores it in an internal database. HydroView provides a user interface that allows users to monitor hydrological data in real time. HydroView also allows the users to setup and remotely re-configure the operating parameters of a SmartObserver datalogger, and to remotely change the operating parameters of hydrological instruments that are connected to the datalogger. Integrated in HydroView software is a water discharge calculation module. This software module calculates water discharge based on indirect measurements of water level and surface velocity in one or more points on the river profile. If surface velocity radars are not present on-site, discharge can be calculated using a predefined Q-H curve. Or, if a V-notch is used on-site, the V-notch parameters together with the water level are used for discharge calculation.
Monitoring Station at Lazina, Croatia
Geolux has recently installed an automatic hydrological monitoring station at a location in Croatia. The monitoring station is based on a compact Geolux HydroStation system. It consists of a radar level sensor LX-80, a surface velocity radar RSS-2-300W, a HydroCam camera, SmartObserver datalogger, a 20 W solar panel, and a 20 Ah lithium-ion backup battery. All instruments use contactless methods for making measurements, and the equipment is mounted on a bridge over the waterway. The complete installation and setup procedure took less than 30 minutes on-site.
The monitoring station was configured to measure water level and surface velocity every 15 minutes, while the attached camera was configured to take a photo every hour. The measured data is uploaded immediately to Geolux HydroView on-line service using a GPRS modem that is integrated with the SmartObserver datalogger.
Off-Grid Power Supply
Under normal operating conditions, a solar panel provides power to all instruments during the daytime, and the backup battery provides power during the night and under low-light conditions. The solar panel is large enough to provide sufficient power both for the operation of all instruments and for battery recharging. The battery charger circuit integrated into SmartObserver datalogger uses the MPPT technique to maximize power extraction, even when the solar panel is not exposed to direct sunlight.
The backup battery that is used inside HydroStation was carefully selected to meet several requirements. The battery must have a high cycle life and lifespan to provide stable power over prolonged periods and to minimize maintenance costs. The battery should be safe to operate. The battery capacity must be sufficient to provide the power to the instruments and the data logger during longer periods when the solar panel is not producing electricity. After comparing and testing multiple different types of batteries, we have decided to use a lithium-ion battery - more specific, LiFePO4 battery.
The solar panel can stop producing electricity for a multitude of reasons. Iz Zagreb, where Geolux office is located, it is common to have thick clouds obscuring sunlight for a few weeks during the winter. Severe air pollution, fog, and smoke from wildfires also obstruct solar panels from producing electricity. Snow can accumulate on the solar panel for weeks and completely block the sunlight. And finally, the solar panel can fail, the cables between the solar panel and the battery charger can be ripped or torn, or the solar panel can be vandalized or stolen.
The battery capacity was selected to ensure autonomous operation for at least 30 days, even without the solar panel attached. Having 30 days of battery autonomy is enough when the solar panel gets temporarily obstructed, and in case of complete failure of the solar panel, it gives enough time for the servicing team to arrive on location and repair or replace the solar panel. We have calculated that a battery with a capacity of 20 Ah can provide power for at least 30 days of operation of all instruments, with 15-minute readout intervals.
Running the Test
After installation of all equipment, we have decided to run an actual test of battery autonomy. The battery was fully charged before the test, and the solar panel was disconnected from the battery charger. We have continuously monitored the battery voltage over time. The test was run for 30 days.
We have started the test on April 7th, with the battery voltage at 13.82 V, which corresponds to a 100% charge. The test was completed on May 7th, with the battery voltage measured at 12.86 V, which corresponds to 19% charge capacity. A 20 W solar panel was reconnected to the battery charger at 10:00 AM, and the battery was recharged back to the full capacity by the end of the day.
After reviewing the test data, we have concluded that 20 Ah LiFePO4 battery comfortably supports the 30-day autonomy of a Geolux hydrological station. All instruments were operating correctly, even when the battery capacity dropped below 20%. Recharging a battery from 20% capacity to 100% capacity on a sunny day took no more than 10 hours.