Reconstruction of Morphostratigraphy and Dynamics of Former Tidal Inlets Along a Wave-Dominated Barrier Island and Impacts on Island Evolution: Assateague Island, MD-VA

This dissertation is a study of former tidal inlets along Assateague Island, Maryland-Virginia. A multi-technique approach, including analysis of historical nautical charts and maps, LIDAR datasets, true-color and infrared aerial imagery, ground-penetrating radar (GPR) surveys, and analysis of sediment cores, was pursued to reveal the dynamic nature of former tidal inlets along Assateague Island. Assateague Island has experienced many breaching events throughout its history as a result of extra-tropical and tropical storm impacts. An updated inlet chronology of the mixed-energy, wave-dominated barrier island concluded that 12 former tidal inlets and breach zones were identified from north to south: Northern Assateague Breach zone, North Sinepuxent Inlet, Sandy Point Inlet, North Beach Inlet, Sinepuxent Inlet, Fox Hills and Winter Quarter Breach zone, Slough Inlet, Green Run Bay Inlet, Green Run Inlet, Cherry Tree Inlet, Swan Pool Breach zone, and Tom's Cove Breach zone. Evidence of former tidal inlets and breaches include such geomorphic features as relict recurved-spit ridges, relict flood-tidal deltas (FTD) and flood channels, relict inlet channel scars (topographic low areas of immature vegetation), relict-inlet ponds, and relict inlet-closure ridges. Inlet-closure ridges are more subtle than recurved-spit ridges, with the tallest inlet-closure ridge at about 1.6 m, which is equivalent in height to the shortest recurved-spit ridge (1.6 m). In general, recurved-spit ridges were also documented to decrease in height toward the inlet throat within the former inlets. In total, 34% of Assateague Island is estimated to be comprised of tidal-inlet fill.;A five stage, life-cycle model of Green Run Inlet was constructed utilizing the multi-technique method. GPR data revealed that the former Green Run Inlet migrated 680 m to the south and had a final channel position 100 m wide and 3.75 m deep resulting in a spring tidal prism of 2.79 x 106 m3. While active, Green Run Inlet established a well-developed FTD that was preserved along the backbarrier of Assateague Island. During the waning and shoaling stage of Green Run Inlet, the channel rotated 30° counterclockwise before infilling.;The former Sinepuxent Inlet was subjected to a more complex history and sediment cores yielded a three-stage evolutionary inlet model. Sediment cores and historical aerial photography suggest multiple breaching events occurred at Sinepuxent Inlet. Sediment cores also underscore the importance that energy pulses (i.e., higher flow velocities associated with increased tidal prism during storms, spring high tides, and perigean spring high tides) play in inlet-fill stratigraphy. Unlike the former Green Run Inlet, Sinepuxent Inlet does not exhibit a well-preserved FTD. Three hypotheses are presented to explain why Sinepuxent Inlet lacks a well-developed FTD: 1) the former Sinepuxent Inlet was subjected to multiple ephemeral breaching events, which deposited small FTDs and possibly washover fans that over time coalesced to form a single amalgamated FTD; 2) Holocene-aged, paleo-barrier islands were preserved as backbarrier features along Assateague Island impeding tidal prism flow through Sinepuxent Inlet, thus restricting FTD deposits and causing the former inlet to infill quickly after opening; and 3) a combination of hypotheses 1 and 2---multiple ephemeral breaches with FTDs and possibly washover fans constrained by a backbarrier, Holocene-aged, paleo-barrier-island chain. Sediment core data, surficial geomorphic features, and historical aerial photography suggest multiple breaching events within the former Sinepuxent Inlet area. More work is necessary to confirm the existence of a backbarrier, Holocene-aged, paleo-barrier-island chain. Reconstruction of the tidal prism of the former Sinepuxent Inlet yielded a value of 8.71 X 106 m3 when the inlet was open.;A regional overview of wave-dominated tidal inlets yielded a generalized life-cycle model based on the rotational nature of tidal inlets when they were active. Wave-dominated tidal inlets may form by landward- or seaward-directed breaching and are classified into three categories based on channel rotation direction: clockwise rotation, counterclockwise rotation, or non-rotation. The rotation of wave-dominated tidal inlets appears to be primarily controlled by the lateral shifting of the FTD depocenter in response to available accommodation space. Flood-tidal delta deposits will fill in and thus exhaust accommodation space locally within the estuary (i.e., creating bathymetric highs), and over time, cause the tidal inlet to become less hydraulically efficient. It appears that the natural process of a wave-dominated tidal inlet is to rotate and wane, because of excess sediment deposition within the flood-tidal delta. The critical variables that are responsible for overall tidal inlet rotation are tidal prism, sediment supply, and accommodation space.