Mitigation actions that reduce the threat to coral reefs to low levels


This page summarizes what we now know about mitigation of sand harvesting impacts to the adjacent coral reef. On 10th April observation onboard the dredger Willem van Oranje was done, to see the operations in person, and identify what practices, if any, could be effective to reduce the threat to coral reefs. See the tab below for information on these three key questions:

1) What are the primary threats from sand harvesting?

2) How do the sediment plumes impact reefs?

3) How can the threats and impacts be reduced?

Click here to see answers to these questions – on the sand harvest risks to the coral reef, and how to mitigate them
1) What is the primary threat?2) How do the sediment plumes impact reefs?3) How can the threats be reduced?

Dredging and sand harvesting essentially remove sediment from one location and relocate it to another location. There are direct impacts where the sand is taken from, and where the material is deposited.

For the sand harvesting activity, of course anything in the path of the ‘draghead’ is picked up, and all organisms within the collected sediment are likely killed on pick-up or when deposited for land-fill at the terminal. Most relevant to the concerns of stakeholders are what happens with sediment stirred up but that is not retained in the vessel and taken to the port.

There are four parts to this:

  • A plume of sediment is stirred up at the draghead on the bottom but not sucked into the vessel. This forms a ‘bed plume’ that settles on the bottom.
  • Sediment and water mixture is sucked into the vessel. The body of the dredger is a massive tank and as it fills, coarser sediment settles to the bottom of the tanks, while water with fine sediment is overflowed to make space for coarse sediment to build up to the capacity of the vessel. The overflowed sediment and water forms a dense plume in the sea. The heavier elements sink to the bottom relatively quickly, the ‘discharge plume’, while the finest elements form a ‘surface plume’ that may remain in suspension over a long time.
  • Back on the seabed, the falling sediments from the surface may join the ‘bed plume’.
  • All of these may be stirred up further by the propeller action and turning motions of the ship – the discharge plume, the surface plume and the bed plume. The effect is particularly strong on the discharge plume and bed plumes in shallow water, whereas in deeper water they may settle below the depths that are affected by the ship and propeller.


What we see from the surface comprises a few elements:

  • The main discharge plume is very dense and close to the ship, can be seen very strongly, and a certain distance behind the vessel, once heavy elements have sunk, it transitions into the lighter surface plume.
  • The surface plume may be visible for a long period, made of fine particles that remain suspended, particularly if fine air bubbles are mixed with the silt and keep it afloat.
  • In general, we cannot see the bed plume, particularly off the reef at 30-60 m depth.
  • When the ship is turning actively, stirring of the surface plume can be seen, and of the main plume when turning is very dynamic, as in this aerial photo.

These videos illustrate aspects of this process:

1) this video shows how water and sediment is pumped into the main tank, and the view over the side when there is no overflow discharge. The green water shows fine silt and air bubbles from operations, but without the dense discharge plume from overflow.

2) this video shows the dense discharge plume when the hopper is full, and overflow water and fine sediments are discharged. In this video, the vessel is not moving, and the sinking/billowing discharge plume is clearly visible. This dense plume is now being kept away from the reef (see next section).


Over time, the following happens to the plumes:

  • They all dissipate over time, which happens by sediment settling to the bottom.
  • At the surface, this is seen as:
  • the colour of the plume changes from dense, white/sandy colour to a lighter green
  • the remnant surface plumes (in the shape of blobs where discharge is done, and trails showing the track of the vessel) slowly merge with one another and into the background water colour.

Rough conditions keep sediment in suspension for longer. Preventing settlement in this way has two opposing effects:

  • it helps prevent stress to bottom habitats, but
  • it allows spread of sediments over a larger area (but at lower concentrations).

The principal stress is settlement of suspended sediments on reef surfaces and biota. A lesser stress is reduced light due to turbidity; the reef system in the project area extends only to 15-18 m deep, and low light over extended periods of time is not likely to happen.

The principal effects of settled sediments include (and see ‘Effects of sediment disturbance on corals’)

  • Smothering of corals and other benthic organisms – causing a stress response in them, reduced health and potential mortality. Hard and soft corals are the dominant invertebrates, and sensitive to settled sediment so provide the most useful indicators of stress.
  • Sediment settled on algae may induce stress but it is not so clearly visible. However it does affected the palatability of algae, so this may affect herbivory and the behaviour of herbivores (fish and mobile invertebrates).

The effect of sediments in algae is likely proportionate to the impact of sediment on benthic invertebrates, so since the former is more direct to measure, it is used as the primary biotic indicator of stress/impact from sediments.

This video shows the appearance of sediment settled onto the bottom:

From the perspective of the sediment plumes, the following order of importance is clear:

  • the discharge plume is the most damaging, as it results in massive settlement of sediment;
  • the surface plume cuts down light and results in some sedimentation, but at much lower levels than the discharge plume;
  • the draghead plume on the bottom is a relative unknown. Technical publications indicate it is much less significant in amount of sediment than the main plume, and does not travel far, but further information is needed on this.

A main purpose of an EIA, and then of the Environmental Monitoring and Management Plan produced from the EIA, is to identify what mitigation actions are needed, their priority and importance, and under what conditions they should be activated.

Based on my observations on the reef and on the vessel, and discussions with the vessel engineers (who understand the technical performance of the vessel and its operations), we can identify the following key mitigation options: 

MAJOR The discharge plume is very heavily laden with sediment that sinks relatively rapidly and causes heavy sedimentation on the bottom The main discharge must happen at a distance from the reef that is sufficient to ensure that none of the discharge plume reaches the reef, and that both surface and bed plumes are also as minimal as possible.
UNKNOWN The bed plume caused by the draghead This contribution is for the moment unquantified and more information is needed, to determine if mitigation is needed and/or possible
MINOR Leakages – there are leakages of sediment and water from the seal between the suction pipe and vessel side, and during the start/stop of harvesting. Leakages of the main sediment/water slurry to be reduced as much as possible by assuring prompt and precise controls on starting and stopping suction during harvesting, and the fit of the dredge arm to the side of the vessel. While these pulses are relatively small they will be very visible (especially with the main plume being absent from the sand harvest zone).
MINOR The surface plume. This is intensified and buoyed by fine/micro bubbles present in the water/ sediment that is discharged from the vessel. A device can be used that minimizes entrainment of air when the main overflow is discharged from the vessel (much like when a funnel of air is sucked into the draining water from a bathtub). Operating this ‘green valve’ significantly reduces the buoyancy of the fine silt, so increases the among of sediment sinking in the discharge plume, and reducing the amount of sediment remaining in the surface plume.


Based on the above, 3 main mitigation actions can be undertaken:

1) Overflow shuttling

The most important action is to discharge the overflow water and sediment from the vessel far enough away from the reef that it does not affect the reef or other inshore systems.

To do this, the vessel must:


  • Prevent any primary discharge/overflow within the sand harvesting area that is close to the reef;
  • Turn away from the reef and start overflow at a set distance out from the coral reef.
  • Return to the harvest area and harvest sand until the hopper is full, then repeat from step 1.

The effect of this mitigation is shown in these three satellite images, that show, from left to right: 1) background conditions, on 19 January before any sand harvesting activities; 2) 23 March, harvesting proceeding without any mitigation measures. The sediment plume is being pushed strongly onto the reef by the wind, and multiple layers can be seen corresponding to successive tracks when harvesting and discharging overflow simultaneously.; 3) 15 April, when the overflow shuttling is occurring. The discharge plume is farther offshore, and because it is not continuous, there is less merging of plumes over time.

19 January – control/no harvesting

23 March 2019 – sand harvesting without mitigation

15 April 2019 – sand harvesting with “overflow shuttling” producing loop tracks.

This mitigation approach is called “overflow shuttling” and results in ‘loop tracks’. These have been implemented since the re-start of sand harvesting on 4 April 2019, and the difference in sediment delivery to the reef is clearly seen in satellite images before and after this date.How far off the reef is the right distance? The monitoring component of the EMMP will help identify the optimal distance, as it will vary with the prevailing wind, offshore current, waves, tidal cycles and other factors. But operationally, a distance of 1.5 km away from the landward border of the sand harvest area has been set as a conservative limit (so overflow is done close to 2 km out from the reef).

2) Operate the ‘green valve’

During discharge of the overflow water/sediment operating the green valve minimizes the inclusion of air in the discharge, such that the discharge plume has fewer fine bubbles, so is more dense and the silt sinks more quickly. Operating this ‘green valve’ makes the proportion of sediment in the dynamic plume larger, and the surface plume smaller.

The green valve has also been operated since the re-start of sand harvesting on 4 April, during overflow discharge at all times.

3) Minimizing overflow leakages and surface plume resuspension

The minor overflow leakages should be reduced to as close to zero as operationally possible.

Turning the vessel and propeller action stir and buoy the discharge and surface plumes. This effect should be reduced to minimum during discharge. Operationally, once the vessel is at the mitigation line, it could turn 90 degrees to travel parallel to the reef, straighten its course and reduce propeller thrust to zero, then perform the discharge. Once the discharge is stopped (after a a number of seconds, e.g. 30), the vessel could be turned using the rudder, and then propeller thrust restarted.

Given the mitigation actions applied since 4 April, the risk of damage to Kwale coral reefs is reasonably low.

SCUBA surveys to date have shown LOW impact to the reefs in terms of coral mortality, and observations summarized above indicate the risk to coral reefs can be reduced even further.

The dredging vessel has been implementing the shuttle overflow and green valve since it re-started operations on 4 April. Some modifications to the procedure (e.g. the 1.5 km mitigation line for discharge in part 3 above) and additional measures such as identified under mitigation action 3 may reduce even further the risks from sand harvesting.

Many of the fears expressed by stakeholders, in particular the claim “sand harvesting is killing Diani coral reefs” are not true. Stakeholders are afraid of this, as we were when there was no clear information. But having observed the reef we know that it is not true. The risk of future damage, which is still a concern, is now fully addressed by the mitigation actions above, and the new monitoring plan which is ready to implement.

Active monitoring will ensure that critical thresholds are not exceeded for:

  • a) the amount of sediment in the water reaching the reef (monitored daily);
  • b) the amount of sediment settling on the reef (monitored weekly); and
  • c) the health of hard and soft corals (monitored weekly).


IN SUMMARY- The way the sand harvesting was being done before 27 March was demonstrably showing build-up of impact, and it was stopped BEFORE impacts became serious. The current mitigation and monitoring actions SHOULD have been developed from the original EIA and in the EMMP. But the main mitigation actions are already being implemented, and the monitoring actions are implementation-ready. Together we are confident they WILL be able to prevent significant damage as wanted by all stakeholders.