Coastal dune consideration for erosion and marine submersion characterization along the Aquitaine coast (SW France)

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1 Coastal dune consideration for erosion and marine submersion characterization along the Aquitaine coast (SW France) Integrated Coastal Dune Management in Europe Case studies EUCC Merlimont, June 2014 C. Mallet, D. Rosebery, F. Desmazes

2 Aims of the study > How geomorphological beach and dunes parameters can contribute in coastal hazards characterization (erosion, submersion)? > Erosion: coastline cartography in 2020, 2040, )Coastline evolution rates 2) Maximum length (Lmax) of erosion which can occur during an event, using the measures of eroded dunes after particular event such as storms, strong waves, etc. > Submersion: sensitivity of coastal zones at a regional scale, in complement of numerical modeling approaches which are used at a local scale within coastal risk prevention plans 1) Definition of a dune typology classification using : geomorphological parameters, erosion rates, future coastline projections, etc. 2) Sensibility analysis to marine submersion (direct, indirect) using reference marine levels 3) Cartography of the sensibility concerning marine submersion > Ongoing study! > 2

3 General context > Aquitaine coast is 270km long, with 230km of sandy beach-dune system > More than 50% is concerned by erosion, average : 1-3m/y, reaching sometimes and locally more than 20m/y > Increasing anthropic pressure (tourism, population) > More than 10% of the sandy coast is urbanized > Since 1996, the Aquitaine Coast Observatory is a regional partnership (Government, authorities, operators) for ICZM > His missions : coastline surveys (beach & dune), coastal hazards analysis (erosion, submersion), expertise > 3

4 Inheritance of geological history, geomorphology and anthropic actions Old dunes Coastal dune Modern dunes > 4

5 Dunes as protection Case 1: Natural erosion grooves a cliff which does not affect the structure s stability Case 2: Very strong marine erosion that removes the capacities of the dune to behave «like a dyke» Case 3: Generalised marine erosion Adaptation of dune interventions in order to use dune as protection against erosion and submersion Natural sedimentary transport between beach and dune must be preserved > 5

6 Erosion characterization: evolution rate Virtual baseline Coastline evolution Example Coastline dataset (orthophotos, satellites, DGPS, etc.) Transects (regularly espaced, ex. 100m) Extrapolated coastline within XX years GIS: Arcmap v9 ( ESRI) + DSAS > 6

7 Example of results > This study has been used to define erosion hazards within the Aquitaine strategy of coastline management realised by «GIP Littoral Aquitain» > 7

8 Erosion: Maximum length (Lmax) of erosion > Since 2002: survey of dune erosion (notch) by ONF all along the sandy coast after each storm event (wave, storms, etc.) or after winter > Lmax corresponds to average of maximum notch > Mean Lmax = 20m > Note: winter not taken into account yet (Lmax>30m?) C5.1 C5.2 C5.3 C6 Profondeur maximale Moyenne/Lmax Maximum erosion notch along the dunes by sedimentary cells along the Landes Coast for > 8

9 Submersion characterization The regional typology is based on 3 steps > 1/ Identification of sectors relatively high, relatively low or intermediates Relatively high sectors: not very concerned by submersion Relatively low sectors: concerned by submersion Intermediates sectors: potentially concerned by submersion At a first step, altitude levels are taken as hypothesis (5m, 8m), depending on sea water level, etc. + 8m NGF Zones relativement hautes Zones intermédiaires + 5m NGF Zones relativement basses Valeurs provisoires - Hypothèses de travail > 9

10 Submersion characterization > 1 st step : identification of sectors relatively high (>8m), relatively low (<5m) or intermediates (5-8m), example (altitudes are hypothesis) > 10

11 Submersion characterization > 2/ Difference between direct and indirect character of submersion Direct: no natural obstacle (dune) relatively high between ocean and land Indirect: presence of a relatively high obstacle (dune) between ocean and low land Erosion can affect dune and must be taken into account Sometimes direct or indirect type is not simple, ex.: sandspit along an estuary > 11

12 Submersion characterization > 3/ Geomorphological classification of the coast Aim: define a classification to take into account geomorphology, sedimentary and submersion Coastal type Relatively high dune Relatively low dune Estuarine mouth, mud-sand coast Description Dune and inland relatively high Dune relatively low near the coastline Relatively low coast, mud or sand, close to an estuary or lagoon Dune and inland relatively low, near an estuary Intermediate dune Dune and inland intermediate Dune and relatively low inland Dune and relatively low and wet inland Relatively high dune, inland with possible connection to an estuary Dune of intermediate altitude Relatively high dune and intermediate inland Relatively high dune and low inland Relatively high dune and low and wet inland Submersion types No sensitive to submersion Submersion by overflow Corresponding color class Submersion by waves Submersion by overflow within estuary or lagoon Submersion by waves after erosion of beach and dune > 12

13 Synthesis of the method Levels relatively high, low and intermediates Submersion direct and indirect Geomorphological criterions Geomorphological cartography Geomorphological types Submersion possibilities Regional cartography of sectors sensitive to submersion > 13

14 Submersion characterization: results > Example of relatively high dune and low inland Submersion possibilities : wave surge in case of dune erosion and overflow > 14

15 Submersion characterization: results > Example of relatively low dune and inland : Hossegor > 15

16 Example: Lacanau > Possibility to use this analysis to take into account the submersion sensitivity within the local strategies of coastline management which are undertaken TEMPORARY TEMPORARY > 16

17 Regional cartography of coastal types Légende 1 - Dune relativement haute 2 - Littoral rocheux 3 - Littoral sablo-rocheux 4 - Dune relativement basse 5 - Littoral sablo-rocheux relativement bas 6 - Littoral sablo-vaseux et embouchure 7 - Dune et a-p rlt bas, proche d'une embouchure 8 - Dune d'altitude intermédiaire 9 - Littoral sablo-rocheux d'altitude intermédiaire 10 - Dune et arrière-pays d'altitude intermédiaire 11 - Dune et arrière-pays relativement bas 12 - Dune et arrière-pays humide relativement bas TEMPORARY > 17

18 Conclusions > Ongoing study > Next steps for submersion evaluation: Hotspots identification (relative to submersion) Water level evaluation (setup, runup) for hotspots Regional cartography analysis of submersion sensitivity (GIS and statistical analysis) > Interests Geomorphological analysis at a regional scale in complement to model studies (combining water level, wave propagation, etc.) which are more complete (and more expensive at the geographical scale) Both erosion and submersion characterized at a regional scale Used for regional and local management plans (risk prevention) and strategies Role improvement of dune as natural protection environment > 18

19 Thank you for your attention More about the Aquitaine Coast Observatory : > 19