SEDIMENTOLOGICAL STUDY ON DREDGED MATERIAL DISPOSAL SITES IN THE ST-LAWRENCE-RIVER .2. QUANTITATIVE-ANALYSIS OF THE STABILITY OF DEPOSITED SEDIMENTS

This paper presents the second part of a sedimentological study on four disposal sites of dredged material along the St. Lawrence River between Sorel and Saint-Pierre-les-Becquets. A quantitative analysis of the stability of noncohesive sediments disposed at those sites comprises, on one hand, a study of the erosion process by currents at those sites and, on the other hand, of the erosion by currents and waves at Yamachiche in Lake St-Pierre. The results show that the daily eroded quantity at the site of Cap-de-la-Madeleine varies from 0.1 to 6 times the capacity of a barge of 500 m(3) when the river discharge varies from low-water to flood conditions with a 20-year recurrence. When wave conditions corresponding to a significant height of 0.5 m and a peak period of 3.5 s (event with a probability of the order of 10%) are superposed on currents at the Yamachiche site, the calculated erosion corresponds to 0.7 to 2.5 barges. Under high-wave conditions of 0.7-m height and 4 s period (event with a probability of the order of 0.1%), the erosion rises to about 2.5 to 7 barges. These results are in good agreement with measures taken at Cap-de-la-Madeleine and Yamachiche.