Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Geology

Major Professor

Ping Wang

Keywords

beach-nearshore nourishment, coastal morphodynamics, equilibrium beach profile, nearshore bar, nearshore sediment transport

Abstract

Inlets and channels are dredged often to maintain navigation safety. It is beneficial to reintroduce the dredged material back into the littoral system, in the form of beach or nearshore nourishments. Nourishment in the nearshore is becoming an increasingly utilized method, particularly for dredged material that contains more fine sediment than the native beach. This research examines the morphologic evolution of two different nearshore nourishments. A nearshore berm was constructed at Fort Myers Beach, Florida using mixed-sized sediment dredged from a nearby channel. The nearshore berm was placed in water depths between 1.2 and 2.4 m with the berm crest just below MLLW in the shape of a bar. The nearshore berm migrated onshore while the system was approaching a dynamic equilibrium. Near the end of the fourth year, the beach profiles had returned to the equilibrium shape characteristic of the study area. Gaps in the berm allowed water circulation and should be considered as a design parameter. The fine sediment fractions in the original placed material was selectively transported and deposited offshore, while the coarser component moved onshore. The dry beach maintained the same sediment properties throughout the study period and was not influenced by the fine sediment in the initial construction of the berm. Another nearshore nourishment was placed along eastern Perdido Key, Florida in 2011-2012 using maintenance dredged material from nearby Pensacola Pass. Different from the Fort Myers Beach berm, the material was placed within the swash-zone, with a maximum elevation of +0.91 m NAVD88 (or 0.62 m above MHHW). The low constructed berm elevation allowed natural overwash processes to occur frequently, which resulted in net onshore sediment transport and growth of the active beach berm. Sediment volume gain west of the project area due to longshore spreading of the nourishment occurred mostly in the trough between the shoreline and the bar, rather than on the dry beach. The swash-zone berm evolved back to the natural equilibrium profile shape maintained in the study area within 8 months. The performance of the swash-zone nourishment was compared to two previous beach nourishments at the same location in 1985 and 1989-1991, with higher berm elevations, at +3 m and +1.2 m NAVD88, respectively. The 1.2-km 1985 nourishment performed the poorest with a shoreline retreat rate of 40 m/year. The 7.3-km 1989-1991 nourishment performed the best with a retreat rate of 11 m/year. This suggests that high berm elevations do not necessarily lead to better nourishment performance. Longshore extent of a nourishment may play an essential role. The distant passage of two tropical storms (Tropical Storm Debby and Hurricane Isaac) generated high waves for the study areas. The two berm nourishments responded differently to the storm. Response was also compared to a beach nourishment in Sand Key. The bar-shaped Fort Myers Beach berm was split into two smaller bars, while a storm berm developed for the swash-zone nourishment at Perdido Key. In both cases, the energetic storm conditions accelerated the evolution of the berm profiles toward equilibrium. As compared to the measured nearshore waves by this study, CMS-Wave accurately propagated the WIS Hindcast waves. SBEACH accurately captured the maximum water elevation, consistent with measured upper limit of morphology change. The model correctly predicted beach and nearshore erosion during the storms. The growth of the storm berm at the Perdido Key swash-zone nourishment was predicted reasonably well by the SBEACH model. However, the magnitudes of the storm-induced erosion and the locations of the offshore bar were not accurately predicted consistently.

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