Influence de l'extraction des granulats en mer sur l'équilibre du littoral

Influence of offshore coarse-grained material extraction on coastal equilibrium

Département Sédimentologie Laboratoire Central d'Hydraulique de France

Abstract

Sand or gravel extraction from the sea requires respecting precise dredging conditions if damaging consequences of these extractions on the coastline are to be avoided. Theoretical studies and field measurements yeld the importance of sedimentary movements under wave action at various distances from shore. For the Bay of Biscay, where waves are particularly severe, 20 m depths under lowest low water level have to be reached to find negligible seasonal sand movements on gently sloping bottoms. Flume and wave tank studies gave indications on the evolution of sand extraction trenches dredged at several depths between shore and - 25 m bottoms. These trenches were 6 meters deep under sea bottom and 200 m wide. Results obtained show that such trenches start filling when the critical wave height (Hc) reaches 0.28 times the depth df at the considered point : Hc = 0.25 to 0.30 df The filling rate of these dredging trenches may be determined from the H1/10 wave amplitude by : V in m3/m/day = (H1/10 - Hc)1.5 Deposits mainly occur by softening of the landward trench slope and generate erosions in shallow bottoms and, consequently, on the beach. If the dredging trench is further than - 21 m under low water level, its action on surrounding bottoms and the shoreline is practically inexistant as long as the maximum wave amplitude doesn't go beyond 9 meters. The results of this study help forecast filling conditions of dredging trenches in sea and to fix limit depths for extracting granulates with respect to local oceanographical conditions so that consequences on the shoreline are negligible.