The use of fibre-optic Distributed Temperature Sensing (DTS) is a promising new technique to measure morphological changes. Fibre-optic distributed temperature sensing uses a single fibre-optical cable as a sensor, and allows measuring temperature with a resolution below 0.1˚C, spatial resolution of less than 1 meter and sub-minute temporal resolution (Hausner et al, 2011). It was originally developed by the oil and gas industries as a borehole logging tool (e.g. Kersey, 2010). Other applications include pipeline monitoring and fire detection and protection. For several years, the technology has been used increasingly to monitor environmental temperatures (Selker et al., 2006; Tyler et al., 2009; Vogt et al., 2010). Here, fibre-optic DTS is applied to monitor morphological changes.
Traditionally, the challenge was to correlate high-resolution hydrodynamic measurements with changes of the bed topography, for data concerning the latter are usually acquired at intervals of several months at least. This measuring interval makes it difficult to pinpoint the hydrodynamic conditions which are driving morphological changes. Using fibre-optic DTS makes it possible to measure morphological changes on an hourly basis, thereby enabling to establish the correlation with hydrodynamic measurements. In addition, real-time monitoring of morphological changes makes it possible to act fast when too much erosion or sedimentation is taking place.
Monitoring morphological changes with fibre-optic DTS works as follows:
Building with Nature interest
Continuous monitoring of dynamic sedimentary systems leads to a better understanding of these systems. This enables adaptive management and improved formulation of adaptive measures. As a result of the continuous character of DTS-monitoring, it is possible to couple instantaneous hydrodynamic conditions to the morphological development of the area. Applying fibre-optic DTS around hydraulic constructions that suffer from scouring, such as bridge pillars, can delay maintenance until an erosional threshold is exceeded. This can make maintenance more cost-effective. The same holds true for continuously monitoring sedimentation in navigational channels.
The tool is applicable in any project in which sedimentation and/or erosion has to be monitored.
To apply fibre-optics in the field, experience with splicing (i.e. precise welding of fibre-optics to connectors) is crucial. Knowledge of computer systems and the DTS system is practical. The data from the DTS-system can be analysed using Matlab.
1. Phased plan process
Identify the environment in which the fibre-optical cables have to be placed. Carefully investigate the depth, water level variations, sediment types, sediment dynamics, and the method to be used for construction. Also take into account activities that can damage the experimental setup, such as fishing activities, recreation and weather conditions.The phased plan process is as follows:
2. Installation of a Fibre-optic DTS System
The fibre-optical cables have to be placed in the field, preferably in an easy-to-demobilize setup. Depending on the required information, one could choose between a horizontal grid to measure lateral displacements, or coiled fibre-optical cables around a vertical pole to measure changes in height, or a combination of the two. Coiling increases the resolution of the measurements because more cable length is wrapped over a short distance around a pole. In the case of a horizontal grid, the cable should be dug in or placed before any sediment body is placed on top of it. In the case of a vertical pole, pole and cable should be dug partly into the ground.
The exact position of the cable should be known in order to properly map the measured erosion and sedimentation. The exact position can be accurately determined in the field with a (Differential Global Positioning System) DGPS and distance indicators (numbers) that are printed on the cable (every meter). The end of a cable has to be spliced (precise welding of fibre-optical cables) to a connector, which connects the cable to the DTS system. This system can be made stand-alone and installed on a platform (containing a battery pack, solar panels and telemetry).
Types of fibre-optic cables
The following software is needed for processing the data:
3. Advice and recommendations
For the Sand engines - IJsselmeer (NL) the cable had to be placed in shallow water (< 1m). A rubber raft was used to hold the cable-winch and roll out the cable. As corner points, Ribbed steel welding nets were used with a flange welded on top of it. These nets were kept in place by steel pegs.
Fibre-optics are applied to monitor the morphological changes of an experimental sand nourishment on the shallow foreshore of the Workumerwaard, the Frisian coast of the IJsselmeer in the Netherlands. The goal of this pilot is to investigate potential strategies for revitalising the ecologically valuable wave-attenuating wetland in front of the primary flood defence, and see to what extent this enables this area to follow a gradual increase of the lake level. The effects of the nourishment on terrestrial and aquatic ecology are monitored, as well as the hydrodynamics and the changes in morphology.
In order to correlate measured hydrodynamic and morphological changes on an intra-day basis, 4 kilometres of fibre-optical cable has been placed on the lake-bed in a 0.5 km2 horizontal grid. The grid is designed to capture the expected morphodynamics of the sand nourishment, which initially covered part of the grid covered. Using laser-based technology, an accurate temperature measurement was taken every two hours in every running meter of the cable. The difference in heat capacity between water and deposited sediment causes an attenuation of the water temperature signal, e.g. due to the daily variation of irradiation, where the cable is covered by sediment. As a result, the position of the nourishment and changes in its morphology can be monitored accurately, at a high space and time resolution. Since the monitoring is on an intra-day basis, individual hydrodynamic events can be matched with changes in morphology. The pilot does show that this innovative fibre optic method is an alternative for traditional echo-sounding and can be applied to high-frequency monitoring of erosion and deposition of sedimentary systems.
The first results of the monitoring program on the Workumerwaard are promising (see figure and videos). Daily changes in temperature and variations of temperature with water-depth are clearly visible. In addition, the exact position of sand bars as observed in the field and by air-photographs can be derived. The position of the nourishment is visible by the deviating temperature. Strikingly, dynamic sand bars form on top of the nourishment (Video 3). Yet, no clear movement of the nourishment itself can be derived from these data. This suggests that the nourishment is less dynamic than expected, at least during the period of time covered by the measurements. The sediment cover, as inferred from temperature measurements, will in a later stadium be validated with bathymetric measurements of the exact lake bed topography.
Movie 1. Interpolated spatial variations in temperature measured by the fibre-optical cable over time. In the left-bottom corner, the cable is covered by the nourishment. Also notice the sand-waves.
Movie 2. DTS measurements along the cable in time. Note the bar-code temperature anomaly pattern caused by sand-waves and the larger anomalies around 3000 and 3500 meters caused by the nourishment.
Movie 3. Blow-up of the the DTS measurements along the fibre-optical cable in time around the first anomaly caused temperature attenuation where the cable is covered with sand. Notice the development and dynamics of sand waves on top of the nourishment.