Coral Protection - Puerto Caucedo Dredging, DO
Location: Puerto Caucedo, Dominican Republic
Date: 2002 – 2003
Involved parties: Ballast Nedam (Van Oord), CSX World Terminals, Caucedo Investments Incorporated
Technical Readiness Level: 7 (system prototype demonstration in operational environment)
Environment: Tropical shelf seas and shores, Ports and Cities, Estuaries
Keywords: port, dredging, coral, monitoring
|Building with Nature design||Traditional design|
Historically, the environmental protection measures at dredging projects near sensitive coral ecosystems was passive. Dredging project management was traditionally focused solely on carrying out appointed mitigation or compensation measures covering project impacts. Loss of coral habitat was taken as necessary to enable economic development or coastal safety. No realistic nature based solutions were recognized.
The coral environment had to be protected while dredging works were conducted for the port terminal construction of Puerto Caucedo. Therefore, an Environmental Impact Assessment study was carried out. Water quality was monitored and a special construction plan was implemented.
During the period between August 2002 and April 2004 a new US$250 million container terminal, the Puerto Caucedo Multimodal Terminal, was constructed on a green field site at Andres in the Dominican Republic. Between December 2002 and August 2003, approximately 2 million m3 of material had to be dredged from the harbour basin and the entrance channel. The project is located near a live coral reef and a popular holiday resort at Boca Chica. To meet the requirements of the Dominican Government, the financial lenders and the project sponsors, Caucedo Investments Incorporated, an extensive Environmental Impact Assessment Study had been carried out as part of the project planning. This identified various coral colonies that may be affected by the construction works. Adverse impacts on the coral communities were not desirable from an environmental as well as social and economic point of view. Mitigation measures were recommended for the execution methods to be employed and the incorporation of these measures into the Construction Contract Documents were one of the key conditions for allowing the project to proceed. Also, the water quality and coral was monitored such that no adverse impacts would occur.
The new container terminal construction was ordered by CSXWT and constructed by Ballast Nedam Dredging (presently owned by Van Oord). Attention for ecological issues was high from the beginning of the project; the entire board of stakeholders (Dominican Government, financial lenders and project sponsors) ordered an Environmental Impact Assessment Study (conducted by Mouchel Parkman Consulting Services Ltd. Limited). In that way the entire board of stakeholders demonstrated their determination to carry out the works to a high standard of care. Based on the findings of this report and on stakeholders' requests, the contractors implemented mitigation measures to ensure the ecological safety of the local marine environment.
Pre-feasibility & Feasibility
An Environmental Impact Assessment study has been carried out as part of the project preparation because of the vicinity of a live coral reef and a popular holiday resort at Boca Chica. The EIA study identified various coral colonies that may be affected by the construction works. Adverse impacts on the coral communities were not desirable from an environmental as well as social and economic point of view. Mitigation measures were recommended for the execution methods to be employed and the incorporation of these measures into the Construction Contract Documents were one of the key conditions for allowing the project to proceed. Also, the water quality and coral was monitored to guide the dredging operations such that no adverse impacts would occur.
The Environmental Study identified certain mitigation measures that should be implemented to avoid possible adverse impacts of the construction works on the marine environment. An Environmental Permit was issued by the Dominican Government following the acceptance of the study and the proposed mitigation measures by SEMARN (The Dominican Environmental Protection Department). This Permit allowed the construction and operation of the project and required the implementation of the mitigation measures and control measures identified by the EISA. Main areas of ecological concern were nearby located coral reefs and general water quality within the bay of Andres. The bay was a highly important ecological area until the hurricane George in September 1998 caused large destruction of the various coral communities. The coral reefs are still recuperating from this natural disaster. In addition pollution from sugar processing and waste-water from tourist establishments is affecting water quality which further threatens the health of the coral communities. Despite these problems the coral reefs are still considered of high ecological value and are frequented by tourists from the Dominican Republic and overseas. In addition the nearby beaches of Boca Chica are a major recreation area for people from Santo Domingo and tourists.
The location of the project is approximately 1 km south of the existing port of Puerto Andrés and 5 km from the port of Boca Chica. The Caucedo peninsula has been zoned for industrial development and a number of other industrial sites are currently under development. The site at Cabo Caucedo was chosen as the preferred location for the port. Its proximity to Las Americas International Airport offers the opportunity to develop an intermodal facility and the site is only a 30 minute drive from Santo Domingo. The seabed is also steeply sloping allowing access for large vessels without excessive dredging being required. There is also no requirement for ongoing maintenance dredging. This project comprised of the construction of a new state of the art container terminal that will function as a container hub for the Caribbean market. Vast marine works were necessary besides the extensive civil works (container stack yard, administrative buildings, container cranes etc.).
The main source of threat to the environment was determined to be the dredging works. These works consisted of:
- Dredging of 2.0 million m3 of coral rock (to form the harbour basin and the entrance channel)
- Reclaiming 1.5 million m3 of dredged material on existing land
- Reclaiming 0.5 million m3 of dredged material behind a breakwater under construction
Planning and design phase
To minimize the environmental impact of the dredging works, the EIA study advised the following mitigating measures:
- Relocation of valuable coral communities
- Water quality and current monitoring
- Erection of two settlement basins and a silt screen
- Discharge of reclamation water below the 40m depth contour
In more detail: a balance of cut and fill quantities so that no dredged material had to be disposed offsite, relocation of live coral species likely to be affected by the dredging works, settlement basins, deep discharge of reclamation water to prevent sediment returning into the sea in the area of live coral and the use of split screens during the breakwater construction. A thorough monitoring system of the water quality was installed (including turbidity, coral health and sedimentation rate) to ensure that these mitigation measures were effective.
In this section the 'construction' of the mitigation measures is described in more detail.
1. Relocation of coral communities
The environmental study identified a significant colony of healthy Elkhorn coral that could be affected by the dredging works. Prior to those works a specialist marine diving company was employed to examine the condition of the Elkhorn and advise on possible methods for relocating the colony on a suitable site. It was considered feasible to move the best specimens to a location outside of the harbour basin.
- 300 healthy Elkhorn corals identified for relocation
- Relocation site found to the north of the harbour basin
- transport under water using air bags
- Set in new location using grout.
- Completed in 1 month
2. Water quality and current monitoring
Extensive numerical modelling was carried out to assess the impact of the proposed harbour on the currents and the water quality of the broader area. The modelling showed that the currents would not adversely be affected and that water quality in the lagoon would not deteriorate as a consequence of the works. However the sediment modelling showed that to avoid any return of sediment from the pipe onto the coral reefs, the pipe outlet would have to be located at the 40 m depth contour to the south of the project site.
Since the local economy is highly attached to the existence of coral reef, project and authorities raised great public concern on the possible adverse impact of the works and thus, to alienate these concerns an extensive water quality monitoring was established including continuous turbidity monitoring stations and current monitoring stations, plus vessel based water quality monitoring (turbidity and suspended solids).
An environmental study had been performed well in advance of any construction works to establish the baseline water conditions in the area surrounding the project location. Based on this study and after discussions between governmental representatives, financiers, the owner and the contractor a monitoring protocol was developed that addressed the concerns for the protection of surrounding coral reefs. Up to a distance of 300 m from any construction point the following restrictions apply:
- Early warning level: the 60-minute depth average turbidity level at the monitoring station should
not exceed the 95% of the baseline values plus 20 NTU.
- Action level: the depth average suspended solid level should not exceed 95% of the baseline values
plus 17 mg/l.
- Target level: the depth average suspended solid level should not exceed 95% of the baseline values plus 20 mg/l.
3. Erection of two settlement basins and a silt screen
The dredged material was to be discharged into an onshore and offshore reclamation area. Approximately 85% of the dredged material was to be pumped onshore. The remaining volume was to be pumped along the inside of the breakwater. The excess water was to be treated such that suspended solid levels released back into the sea were kept to a minimum.
At the onshore reclamation this could be achieved by creating two settlement basins. The excess water was to be returned to the sea leaving the second settlement basin via a pipeline.
At the reclamation along the inside of the breakwater such settlement basins were not available. To minimise the spread of suspended solids outside the harbour basin the following measures were put in place:
- Commencement of this reclamation was only permitted to start if a sufficient lead of breakwater core was available.
- A silt screen was to be put in place 100 m downstream of the discharge point. The silt screen was to force settlement of suspended solids close to the reclamation area.
The area of settlement of suspended solids would always be inside the harbour basin. The extremely low currents in the harbour basin would not create sufficient shear strength to allow re-suspension transporting the settled suspended solids outside the harbour basin. The area would be cleaned up by the cutter suction dredger (during the final clean-up phase) or would be covered via dry filling.
4. Discharge of reclamation water below the 40 m depth contour
Due to the volume of excess water being pumped onshore, despite passing through two settlement basins discharge of reclamation water could have an adverse impact on the water quality. Coral surveys had demonstrated that coral communities below the 25 m depth contour were virtually non-existent. As an extra safe guard it was decided to discharge the excess water below the 40 m depth contour via a pipeline.
The above mentioned mitigation measures and water quality monitoring were part of the total contract package. Compliance with these measures, installation, operation and maintenance of the water quality monitoring as well as compliance with the water quality limits were a contractual obligation to the contractor.
Operation and Maintenance phase
The project does not seem to have an adverse effect on the surrounding area and ecology.
After completion of the works, surveys have now been carried out at 3 month, 6 month and 12 month intervals and the evidence shows a high percentage of the transplanted specimens are in good condition, have grown larger and have attracted a thriving fish population. A programme for longer term monitoring of the coral is currently being discussed between the port operator and the local diving community.
Settlement basins and sinker line
The settlement basins proved to work extremely well for a complete different reason. During the first 7 months of dredging no excess water was discharged back to the sea. All excess water seeped into the many voids and crevices of the ground.
During the remaining 2 months some excess water was discharged into the sea via the pipeline. No adverse impact on the sea water quality was noted during this period near the exit of the discharge pipeline. No visible decolourisation was noted at surface level as well. The dredger was mainly involved in clean-up operations during the last two months. The amount of suspended solids that have been returned to the sea will have been minimal.
The settlement basin and pipeline discharge at 40m depth therefore ensured that the impact of the excess water on the sea water quality was kept to an absolute minimum.
The silt screen was located 100m in front of the discharge point. No measurements were executed to research the difference in concentration levels inside and outside the silt screen. The decolourisation of the water at either side of the silt screen demonstrated the little effect it had on containing the suspended solids of the excess water from the reclamation along the inside of the breakwater.
Nonetheless no adverse impact on the water quality was recorded at any station whilst reclaiming along the inside of the breakwater. The main reasons will be:
- Reclaiming only when a sufficient lead of breakwater was available
- Very low currents inside the harbour basin.
The mitigation measures were proven successful since the agreed Environmental Managements Plan was strictly followed and the requested environmental standards were fully met.
The water quality measurements never indicated any deterioration of the water quality and the health of the coral reef. This was confirmed by the monthly dive inspections carried out by an independent auditor. As a consequence there were no stoppages of the works by environmental authorities, both local and international.
The numerical modelling indicated that the chance of adverse impact on the environment was minimal. Only for some relatively short periods the water quality monitoring demonstrated an elevation of suspended solid levels and the sedimentation traps recorded any elevation of the sedimentation rate. In view of these results and the mitigation measures taken, the amount of monitoring would appear to be slightly excessive. Dredge operations are normally a 24 hours continuous operation. Also, the dredging method and material do not normally change during a day's work in areas of constant material type, as encountered at Caucedo. Therefore, the requirement for 24 hour continuous monitoring is conservative.
However these requirements need to be put into perspective. Local communities were afraid that the coral communities would be destroyed thus annihilating the local tourist industry. Their opposition to the works were very strong and could only be alleviated by the monitoring protocol. The financers of the project had similar concern.
- P. Bogers & J. Gardner (2004), Dredging near live coral, published in WODCON XVII 2004, Hamburg, Germany
- S. Wilson & F. Kennedy (2001), Marine Environmental Baseline Report, published in association with Titan Envirnmental Surveys
- N.J.Gimsing & C.Iversen (2001), Oserund Link Project, the Oserund technical publication, dredging and reclamation.
- Mouchel Consulting Limited (2001), Environmental Impact Statement Summary
- Facts About Dredging Around Coral Reefs An Information Update from the IADC – Number 1 – 2011.
- Koningsveld, M. van, Hofstede, R. ter, Elzinga, J., Smolders, T., Schutter, M., Osinga, R. (2017): Reefguard: a scientific approach to active reef rehabilitation; Terra et Aqua, Number 147, June 2017.
- Kramer, A; Kenny, I (2012). Coral relocation: a mitigation tool for dredging and reclamation works at the cruise ship terminal in Jamaica; Terra et Aqua, Number 128, September 2012.
- PIANC (2010); Dredging and port construction around coral reefs; Pianc Report N° 108 Environmental Commission
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