Graduation Year

2011

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Ping Wang

Keywords

beach erosion, berm nourishment, coastal morphodynamics, nearshore bar, nearshore sediment transport

Abstract

Dredging is often conducted to maintain authorized depths in coastal navigation channels. Placement of dredged sediment in the form of nearshore berms is becoming an increasingly popular option for disposal. Compared to direct beach placement, nearshore berms have fewer environmental impacts such as shore birds and turtle nesting, and have more lenient sediment compatibility restrictions. Understanding the potential morphological and sedimentological evolution is crucial to the design of a nearshore berm. Furthermore, the artificial perturbation generated by the berm installation provides a unique opportunity to understand the equilibrium process of coastal morphodynamics.

Matanzas Pass and Bowditch Point, located on the northern tip of Estero Island in west-central Florida were dredged in October 2009. The dredged material was placed approximately 600 ft offshore of Fort Myers Beach and 1.5 miles southeast of Matanzas Pass, in the form of an artificial berm. Time-series surveys and sediment sampling were conducted semi-annually in order to quantify sedimentological characteristics and morphological changes within the first year after construction of the berm.

The artificial berm at Fort Myers Beach is composed mainly of fine sand. Patches of mud were found throughout the study area, with the highest concentrations being in the trough landward of the berm, and offshore southeast of the berm area. The highest concentration of carbonates was found in the swash zone, as well as at the landward toe of the berm, which coincides with the coarsest sediment. The overall mud content of the berm is lower than that of the dredged sediment, thus indicating a coarsening of the berm over time. The reduction in fines as compared to the original dredged sedimet could also indicate a selective transport mechanism that moves finer material offshore, and coarser material landward, a desirable trend for artificial berm nourishment.

During the course of the first year, the berm migrated landward and increased in elevation. Onshore migration occurred mostly within the first 6 months. Along with onshore migration, the shape of the berm changed from a symmetrical bell curve to an asymmetrical shape with a steep landward slope. There is no clear spatial trend of volume change alongshore within the berm area, indicating that sediment transport is mostly cross-shore dominated. A salient was formed landward of the northern portion of the berm. Several gaps were created during berm construction due to dredging and placement techniques. These dynamic gaps are likely maintained by rip currents through them. This study showed that the Fort Myers Beach berm is active, due to its landward migration during the first year after construction.

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