“ Enhancing resilience of Vulnerable Coastal Areas and Communities to Climate Change in the Republic of Gambia” Jonathan McCue and Jeremy Hills
PPG Work Programme WORK “PHASES” EXPECTED DATES Project PPG Approval February 2012 Procurement of Consultants and Approval March to July 2012 PPG Inception Phase August 2012 Consultation Phase (Vulnerability Assessment and August to September 2012 Feasibility) Result Compilation October 2012 Project Document (Draft) compilation November 2012 Project Document Draft Workshop Presentation December 2012 (11-18 Dec?) Final Acceptance of Project Document and January 2013 submission to GEF Anticipated start July 2012
Issues linked to Coastal Protection decision making in Gambia • Is there mechanical equipment available for construction purposes?. • If a project requires earth moving equipment (trucks or loaders) they would have to be shipped in at considerable expense. • What are the locally available materials for a coastal structure ?. • Are large rocks available for armour stones?
Issue (cont) • Construction projects may rely only on manual labour; hence, the size and quantity of rock which can be moved is limited. • Supervision of construction projects in remote areas can be an issue, • Mechanical equipment, brought in from outside, is very expensive • Need to use local material and local labour - benefit of the project can be economically felt in the local area.
Issue (cont) • Rubble mound walls are the least reflective but require specific rock sizes and precise construction supervision. • Artificial nourishment of a beach is a soft solution with no negative effects on the beach. • Artificial nourishment must be repeated periodically (often not understood by local land owners). • Artificial nourishment requires a adequate supply of sand and equipment to move it onto the beach.
Living with the Sea - Possible intervention techniques 1. Shore Management Plan 2. Use of dredged material to construct embankments or for intertidal recharge 3. Construction of realigned earth embanked defences within lagoons (managed realignment) 4. Sand bag offshore breakwaters 5. Sediment bypassing at lagoon entrances (beach recycling) 6. Adaptive Mangrove/tree planting Techniques
The need for a Shoreline Planning Framework for Gambia • A Shoreline Planning Framework is needed to help strategically plan for the suitable implementation of sustainable sea defence management (soft and hard) and other maritime works. • It can be used by GoG when considering applications for reclamation or for new private development proposals on islands. • Help to enforce environmental permits and EIA regulations
Shoreline Planning for Gambia
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Adaptive Options for a “Road” scenario
Sediment Dynamics – Cape Point
Sediment Dynamics – Cape Point
Setback Policies? 150m?
Intervention Measure 1 - Beach Replenishment • Sand pumping and beach replenishment, commonly takes place • Consider wider geographic impacts of the practice and how this might alter the prevailing sediment budget. • Localised remedies often lead to neighbouring island problems. • High monitoring costs and maintenance commitment • Alteration to sediment budgets • “Relic” footprints that are left behind by sand pumping (depression hollows) which can increase the risk of unstable footing.
Beach Recycling and Reprofiling All locations, including those with limited blown Trinidad sand for natural recovery. Appropriate locations Low to moderate, but requires ongoing maintenance (US$2000- US$40,000 /100m length plus fencing, transplanting, etc, with similar repeat Costs costs). Short term defence against erosion, and enhancement of natural recovery. Moderate Effectiveness resistance to single storms. Enhanced by fencing and vegetation transplanting, and can be successfully used to bury hard defences. Accelerates natural recovery of beaches and Benefits provides short term defence against single severe storms. Removes material from other sites, possibly transferring erosion or environmental damage to a Problems different frontage. May introduce beach debris, non-indigenous sediment and/or vegetation, potentially damaging local ecology. Useful low cost solution to localised problems in Applicability to specific areas. Needs to be combined with a robust Gambia? monitoring framework to be able to deduce and predict volumes and timing of recycling.
Associated measure : Sediment bypassing at river outlets (beach recycling)
Intervention Measure B : Sand bag revetment or related structures
Structural Measure : Sand bag revetment
Intervention Measure C : offshore breakwaters A Multi-Purpose Reef is a type of coastal structure that can reduce erosion, enhance marine habitat and provide a valuable recreational resource. The key to the effectiveness of a Multi-Purpose Reef is the formation of a salient – a wider, more stable section of a beach caused by the reef. Multi-Purpose Reefs actually consider the value that beaches provide to local communities by serving to protect the beach itself. Seawalls and other forms of coastal armouring do not actually protect the beach; they only protect the land behind it.
Coastal Engineering Design - Recommendations
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Artificial Offshore Reefs Trinidad Appropriate locations Exposed beach of high ecological and landscape value. Moderate to high, and may need some shoreline maintenance (USS40,000 to US$120,000/100m of Costs structure, plus minor works for unprotected areas). Causes lee side accretion, but least effective during Effectiveness storm surge conditions. Unlimited structure life. Natural processes are only partly disrupted, allowing Benefits beach to stabilise. Rocks create new intertidal habitat. May cause navigation hazard for local fisherfolk. Visually intrusive at low tide. Disrupt amenity use of Problems beach. Likely to disrupt natural beach replenishment of offshore sand bar stores. Also would be visually Applicability to Gambia intrusive and impact on beach and surfing recreation activities
Associated measures : Sand bag, timber or rock groynes
Timber Revetments Trinidad High value sites suffering modest and periodic Appropriate locations erosion. Moderate (US$4000 to US$100,000/100m frontage Costs length) – See Appendix C. Provide good protection if only occasionally Effectiveness exposed to waves. 5-30 year life. Normally acceptable to the public. Less expensive Benefits than seawalls or rock revetments Limited life, particularly where exposed to wave Problems action. Visually intrusive. Alters beach-beach processes as sand interchange is disrupted. Not appropriate as shore normal groynes on the foreshore, or as foreshore breastwork (see Figure 21), possible consideration as a backshore sand Applicability to Gambia? trap structure using local hardwood and assist with other timber designs as part of the Master Plan.
Groynes and Jetties • ensure appropriate consideration is given to the spacing, composition and height of coastal structures. • It is apparent that most groynes, jetties and breakwaters constructed are not based on quantitative information that relates to the hydrodynamic conditions of the immediate area. • Consider the aesthetic appeal of the local environment!!!
Intervention Measure D : Construction of realigned earth embanked defences within the estuary (managed realignment)
Integrated aquaculture with sea defence measures Road Level Land level Ag. land level Sea level Road Level Sea Defense System Land level Ag. land level Sea level Fish Ponds
Intervention Measure E : Adaptive Mangrove/tree planting techniques
Beach Grass Planting Trinidad Above normal limit of wave run-up at any location with Appropriate locations available blown sand. Unlikely to succeed where erosion is severe. Low, but labour intensive with on-going management (up to Beach Grass Planting Costs $4000/100m length for each visit). Enhancement to natural beach recovery. Reservoir of sand Effectiveness held in planted beach will provide a buffer to resist storm erosion. Compliments natural system. Can be used to improve other Benefits management options. Potentially self sustaining. Normally requires beach fencing or thatching to achieve Problems success. May be completely lost to storm erosion. Native beach grasses, are needed. Hardy species and can be used to help assist beach sediment binding as part of the Applicability to the Nile Delta? structure particularly on the seaward face of structure.
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