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:
With laser technology, a temperature measurement in the cable can be obtained with an accuracy of 0.1˚C and a spatial resolution of 1 m. If a cable is placed in or under sediment (or installed under a nourishment), the difference in heat capacity between water and sediment causes an attenuation of the water temperature signal where the cable is covered by sediment. From these temperature data, the position and ultimately the thickness of the sand cover can be derived. Since measurements are taken on an hourly-day basis, individual events, like storms or floods, can be matched with changes in erosion and sedimentation.

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.

In Building with Nature projects, thorough systems knowledge is essential. Detailed coupling of hydrodynamic conditions and sedimentation/ erosion behaviour provides insight into the system’s functioning and its response to the driving factors. This knowledge can be used to validate models, be it conceptual or computational.

The tool is applicable in any project in which sedimentation and/or erosion has to be monitored.

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:

1. 1. Determine the data-storage type: if the location is easy to reach, one can work with data storage on the DTS. Otherwise, one may opt for a modem connection.
      
      

   2. Make a design of the fibre-optical cable installation. The resolution of the measurements determines the type: horizontal grid and / or vertical coiled poles. In the design it is important to realize that for several locations of the grid an exact cable number must be linked to an exact GPS coordinate. Furthermore, one has to keep in mind that the cable has to be attached to the DTS-system. The DTS can be placed on a floating structure, a platform or on land. In each case the cable length should be sufficient to cover the distance to the DTS-system. The cable is usually rolled off from a large (2 m high) winch weighing several hundred kilos. This should be kept in mind when designing the grid and its installation procedure.

      
      

   3. Place the corner points and cables; note the cable locations and GPS coordinates. Keep in mind that the quality of the placement and the time it takes will benefit from good weather circumstances.
      
      
   4. Finish the setup in the field by splicing connectors to the cable ends and reinforce this connection with a synthetic resin.
      
      
   5. Create a safe place for the DTS-system
      
      
   6. Connect the cables to the DTS and check if the cable is in one peace.
      
      
   7. Start measuring

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

Many types of fibre-optical cables are commercially available, and innovation in this field develops quickly. In the case Sand engines - IJsselmeer (NL) a black reinforced cable of 1 cm thick was chosen, but thinner cables more suitable for coiling are also available. Lately, cables have been introduced that can be heated from the inside by applying an electrical current. These cables enable the monitoring of erosion and sedimentation in areas with minor day-to-day temperature variations, such as deeper seas. In this case the dissipation of the heat generated by the cable is a function of the sediment thickness.

The following software is needed for processing the data:

• Programming software (e.g. Matlab or Java, C++) to write scripts that derive the position and thickness of the sediment layer above the cable from the temperature data. These scripts have to be customized for every situation, for instance depending on experimental setup, sediment properties and temperature variability.

• Visualisation software (Matlab, ArcGiS, OpenEarth) to visualise morphological changes

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.

• The cable was held down by steel pegs which held the cables from moving.
• Make sure the position of the grid is well indicated so no boats can damage the fibre-optical cables.

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 km^₂ 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. 

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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.

• Hausner, M.B., Suarez, F., Glander, K., Van der Giesen, N., Selker, J.S. and Tyler, S.W., 2011. Calibrating Single-Ended Fiber-Optic Raman Spectra Distributed Temperature Sensing Data. Sensors 11, 10859-10879, doi:10.3390/s111110859.
• Kersey, A.D., 2010. Optical fiber sensors for permanent downwell monitoring applications in the oil and gas industry. IEICE Trans. Electron. E83-C, 400-404.
• Selker, J.S., Thévanaz, L., Huwald, H., Mallet, A. Luxemburg, W., Van der Giesen, N., Stejskal, M., Zeman, J., Westhoff, M.C., Parlange, M.B., 2006. Distributed fiber-optic temperature sensing for hydrologic systems. Water Resources Research, 42, W12202, doi:10.1029/2006WR005326.
• Tyler, S.W., Selker, J.S., Hausner, M.B., Hatch, C.E., Torgersen, T., Thodal, C.E., Schladow, S.G., 2009. Environmental temperature sensing using Raman spectra DTS biber-optic methods. Water Resources Research 45, doi:10.1029/2008WR007052.
• Vogt, T, Schneider, Ph., Hahn-Woernle, L., Cirpka, O.A., 2010. Estimation of seepage rates in a losing stream by means of fiber-optic high-resolutionvertical temperature profiling. Journal of Hydrology 380, 154-164.