Dr Isabelle NIANG Dpt Géologie/Université de Dakar [email protected] The background
Around 70% of sandy coasts in the world are eroding Global causes : sea level rise, population’s concentration along the coasts Local causes: changes in the sedimentary budget; human interactions with natural processes Yenne sur mer
Rufisque UEMOA Project / Ndour 2010 Variable evolution rates (Sénégal)
Saint -Louis - 3.3 to + 0.6 m / year
Yoff - 3.7 to + 0.9 m / year
Dakar cliffs - 0.4 to – 1.4 m / year
Rufisque - 3.8 to + 1.3 m / year
Joal - 9.1 to + 8.5 m / year
Sangomar - 137 m / year Coastal erosion in West Africa
Mean rates of 1 to 2 m per year Sometimes exceptional events (some tenths of m per year) (the breach in the Sangomar sand spit) Variable causes: natural and human Sedimentary deficits (protection works, beach sand mining) Sea level rise The Nouakchott harbour (Mauritania)
Accumulation
Erosion
Photo: Ould Elmoustapha, 2000
Between 1980 and 1998, retreat rate of 25 m per year (Ould Elmoustapha, 2000) Djiffere and the Lagoba breach
An exceptional and natural event A rate of retreat of 137 m per year (1991-1995)
Map of the Sangomar sand spit evolution (1991-1995) Djiffere, May 2010
Djiffere, October 2010 Palmarin Ngallou, May 2010
Palmarin Ngallou, October 2010 Saint Louis, 2 March 2010 Global sea level rise observed byTopex/Poseidon (1993-1999)
Global sea level rise since 1982 (in red ,tide gauges and Topex in blue)
Sea level rise as observed by satellite
Acceération of sea level rise (1-2 mm per year → 3 mm/yr) Climate change in coastal zones
The first consequence of global warming will be a sea level rise
20252025 20502050 21002100
33 –– 1414 cmcm 55 –– 3232 cmcm 99 –– 8888 cmcm Sea level rise : 3rd IPCCC Assessment Report Source: UNEP Report « Snow and ice » 2008 Direct impacts of CC in coastal zones
Acceleration of coastal erosion Inundation of low lying areas Salinization of soils and waters, especially groundwaters Ecosystem degradation Modification of fish populations Increase in extreme events(storms, cyclones) Sea level Wave energy
Sedimentary budget
Beach profile ou
Any change in one of these components of the coastal equilibrium induces a desequiloibrium either positive (accreion), or negative (eosion) The Bruun rule (1962) R= (L/B+h) s
50 to 100 Indirect consequences
Linked to changes in rainfall and winds (but high uncertainties) Change in sediment supply through possible Changes in streams and their flows Modifications in soil erosion (links with desertification) Changes in coastal energy Changes in swells (winds) Changes in winds Cap Vert peninsula Inundated areas (hatched) with a 2 m inundation level (3% of the total area) Saloum estuary Inundable zones (hatched) with a 2 m inundation level
52% of the study zone - Mangroves - Salt exploitation zones Adaptation options
Will depend on: the causes of coastal erosion Socio-economic and environmenta characteristics of the zone Available expertise Available ressources (human and financial) Defined strategies for coastal zones Exisiting constraints (physical, institutional, etc.) Available options Structural solutions Non structural solutions Planning, management Information, communication, education
The institutional, legislative and political contexte Accepted principles (precaution, participation, etc.) Sustainable development options Adaptation options Non structural options Structural options Land use strategies Soft (beach Containment(zoning, nourishment, dune retreat lines) building, etc.) Planning technics Hard (seawalls, groins, (planified retreat) breakwaters, etc.) Planning, management options
Structural options
Eurosion project Options in West Africa Mainly protection options And generally hard structural options (seawalls: Rufisque) Limited experience in soft structural options (The Gambia: beach nourishment) The Diokoul concrete wall (Rufisque)
Conception
10 years after! Rufisque concrete wall in august 2004 Rufisque, February 2010 Options in West Africa Options regarding planning, legislation and management Planning tools (director schemes, planning maps, etc) are most of the time existing but not applied Existing laws and regulations The Abidjan convention (combat coastal erosion) Laws regarding beach sand mining Laws on the public maritime domain But not enforced/considered/understood Options in West Africa
Options regarding planning, legislation and management Limited experience in Integradted Coastal Zone Management and mainly related to biodiversity Coastal planning in Guinée Bissau Marine protected areas Institutional frameworks
Generally inadequate framework Competence conflits between Ministries Lack of technical expertise and of funds Absence of a structure encharged of monitoring, enforcement of politics for coastal zone management Possible solutions at the sub regional scale
Combat against beach sand mining Existing regulations Coordination of coastal zone management politics Exchange of experiences regarding coastal protection options