Tidal Inlet Development and Migration in Isles Dernieres Barrier Island Arc, Louisiana

Bruce E. Jaffe, Asbury H. Sallenger, Jeffrey H. List

The Isles Dernieres barrier arc formed about 420 years ago in response to the abandonment of the Bayou Petit Caillou delta of the Mississippi River. Over the past 100 years, the arc has undergone rapid erosion, with much of the Gulf shorelines retreating more than 1 km (10 m/year). Today, the island area is less than 25% of what it was 100 years ago. Most of this areal loss is from a narrowing of the islands as both the bay and Gulf shorelines retreated. A significant portion, however, is due to development of two large tidal inlets that segmented the 32 km long arc into three groups of islands from what was a nearly continuous island in 1890. The presence of these inlets and two recently formed smaller ones alters longshore transport and island erosion patterns. To predi t the future of the Isles Dernieres, we must understand the history and processes of tidal inlet development.

Bathymetric charts show that the inlets formed sometime between 1890 and 1934. We used surface modeling software to investigate their development. Coupe Colin, the westernmost inlet, was 2,500 m wide and 3 m deep in 1934. The eastern inlet, Whiskey Pass, was 800 m wide and 4 m deep. Between 1934 and 1986, both inlets widened and deepened, resulting in an increase in cross-sectional area. As hypothesized by others, this increase could be in response to a greater tidal prism volume caused by relative sea level rise enlarging the bays behind the arc. If this is the case, the recently formed inlets might be maintained or grow due to the continuing increase in tidal prism volume. As the inlets grew, their channels migrated westward (downdrift) about 15 m/year in response to sediment influx from the east, even though there was no alongshore growth of the islands. Preliminary results indicate a balance between the volumes of channel fill and cut. However, if the volume of sand lost in the cut differs from the volume gained in the fill, the inlets could be a sink or source of sand. An additional sink of sand associated with inlet development is the growth of deltas. An analysis of the sand content and volume changes of channel cut and fill and of deltas should indicate the role these inlets played in the erosion of the Isles Dernieres and should be useful in predicting future changes to the islands.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.