How can we measure the coastal currents of Negombo?

1. Where is Negombo?

Located on the west coast of Sri Lanka lies Negombo. It is a vibrant coastal destination steeped in history and culture. The city borders the Indian Ocean and boasts long stretches of sandy beaches and one large lagoon.

The Negombo Lagoon is one of the key features that define the area. It is a haven for a wide variety of bird species and serves as an important fishing ground. The lagoon is connected with the ocean, allowing a very particular exchange of water and nutrients. The coastline is dotted with fishing villages where local fishermen depend on the sea for their livelihood. The city also connects to the Portuguese and Dutch colonial periods, as reflected in its architecture and old forts.

The Negombo climate is tropical, with marked wet and dry seasons. The wet season provides heavy rainfall that can influence the water level and flow in both the lagoon and the coastal area. Vegetation in the area is highly mixed with coconut palms, mangroves, and other tropical plants which have preferred this coastal environment.

2. What is the situation of the coastal currents near Negombo?

Various factors influence the coastal currents near Negombo. One of the most important among them is the monsoon winds. During the southwest monsoon from May to September, the strong winds push the surface waters in a northeasterly direction. The wind reverses the direction of the current during the northeast monsoon, from December to February, in a southwesterly direction.

The topography of the bottom off the coast further influences the pattern of currents. Underwater ridges, sandbars, and channels may accelerate or decelerate the currents. The shape and depth of the entrance to a lagoon and its interaction with the ocean currents further complicate the coastal current system. Tidal currents are another major influence. It was found that this area consists of semi - diurnal tide variation, which can be caused from considerably developing the current velocities and changing current directions, particularly in a lagoon area.

3. How to observe the coastal water flow of Negombo?

Surface Drifter Buoy Method

The surface drifter buoy is a simple, workable method for observing the flow of surface water. These buoys are set on the surface of the water and drift along with the currents. With the use of satellite-based tracking systems or other positioning techniques, the buoy movements over time can be followed. Such methods provide information on the directions and speeds of surface currents. But this method will provide the data about only the surface layer and cannot provide the entire structure of the current of the total water column.

Moored Ship Method

The ship is moored in a particular position in an anchorage somewhat offshore from the coast. Instruments aboard the ship are used to measure the water flow at different levels. Thus continuous measurement at one point may be done. But it has its own limitations. The ship's own movement due to waves and wind can introduce errors into the measurements. Also, it only provides data for the area around the moored ship and may not represent the overall coastal current situation.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP profiler method is a more advanced and comprehensive way to measure coastal currents. ADCPs send out acoustic pulses through the water and measure the Doppler-shifted echoes that come back. In that way, they can simultaneously measure the water velocity at several depths. This provides a detailed profile of the current from the surface down to a certain depth, depending on the instrument's capabilities. It provides a better overview of the present status of the water column and is thus very well suited for studies of coastal currents off Negombo.

4. How do ADCPs using the principle of Doppler work?

The ADCPs operate on the principle of Doppler. They emit high-frequency acoustic signals into the water. Interaction of these signals with the moving particles in the water-suspended sediments or small organisms-produce a change in frequency of the reflected signals due to the Doppler effect.

If the particles are moving towards the ADCP, the frequency of the reflected signal is higher than the emitted frequency, and if they are moving away, it is lower. By measuring these frequency shifts at different angles and depths, the ADCP can calculate the velocity of the water at various points in the water column. Thus, ADCPs can give a quite detailed current profile with a pattern of speed and direction at depth. The data from the ADCPs are able to be used to analyze both the spatial and temporal variability in coastal currents.

5. What's needed for high-quality measurement of Negombo coastal currents?

The equipment should be of reliable material for high-quality measurement of the coastal currents near Negombo. It needs to be able to withstand seawater, which is corrosive in nature and the harsh marine environment. A compact size, light weight, and low power consumption are also wanted.

It would be more easy to deploy and recover if the device is compact and lightweight from a boat or coastal installation. The use of low power can permit longer-term monitoring without replacing the batteries so frequently. Low cost is an essential factor in cost-effectiveness, especially when a measurement project has to be performed on a large scale.

The housing of the ADCP meter shall be of titanium alloy. It is a very corrosion-resistant metal alloy, and therefore it is suitable for use in the marine environment. Additionally, it has a great strength-to-weight ratio, thus giving the structural integrity the device needs without making it overly heavy.

6. How to Choose the right equipment for current measurement?

Based on Usage

Ship-borne ADCP: In case the measurements are to be made from a moving vessel, the ship - borne ADCP flow meter would be a suitable option. It will give continuous data as the ship travels along the coast to allow for better understanding of the spatial variability of the currents.

Bottom-mounted ADCP: A bottom-mounted ADCP current profiler is appropriate for long-term and fixed-point measurements near the seabed. It will be able to give current profiles in detail at one location, which is useful for studying the local current behavior.

Buoy-mounted ADCP: When flexibility in deployment and measurement of surface and upper-water-column currents is required, a buoy-mounted ADCP is the way to go.

Based on Frequency

Frequency will of course depend on water depth, which should be considered in selection: for relatively shallow waters usually a 600kHz ADCP would be appropriate; from 70 m up to 110 m, the use of a 300 kHz is a better fit. Finally, when the water is very deep-for instance, as might occur around Negombo in areas, an ADCP operating at 75kHz should allow measuring up to about 1000 m depth.

There are well-known ADCP current meter brands like Teledyne RDI, Nortek, and Sontek. However, for a cost-effective option with excellent quality, the China Sonar PandaADCP is a good alternative. It is made of titanium alloy and offers a great price-performance ratio. You can find more information at https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and moels.