John T. Wells

Restoration of the Mississippi Delta: Lessons from Hurricanes Katrina and Rita

Hurricanes Katrina and Rita showed the vulnerability of coastal communities and how human activities that caused deterioration of the Mississippi Deltaic Plain (MDP) exacerbated this vulnerability. The MDP formed by dynamic interactions between river and coast at various temporal and spatial scales, and human activity has reduced these interactions at all scales. Restoration efforts aim to re-establish this dynamic interaction, with emphasis on reconnecting the river to the deltaic plain. Science must guide MDP restoration, which will provide insights into delta restoration elsewhere and generally into coasts facing climate change in times of resource scarcity.

Longshore Transport at Cape Lookout, North Carolina: Shoal Evolution and the Regional Sediment Budget

Abstract The Cape Lookout cuspate foreland has undergone significant offshore accretion since the first rudimentary field studies were conducted in the 1800s. Despite the wave-dominated setting, however, little is known about littoral processes under the wide range of wave conditions that impact the complicated coastal geometry at the cape. In this study we examined littoral processes, driven by longshore currents, using a numerical wave refraction/diffraction model (Ref/Dif 1) and through use of aerial photographs and nautical charts. Results show that longshore current direction and speed, as expected, are highly variable and depend primarily on incoming wave direction. Southerly longshore currents on Core Banks predominate under northeast and east wave approaches, whereas weaker northerly currents are generated under southeast and south wave approaches. The result of these patterns provides a source of sediments to Cape Lookout Shoal. Results show the sediment input from Core Banks to the shoal is of the order 512,000 to 581,000 m3/yr. The western limb of the system receives a portion of these sediments, which are released from the shoal and transported north by waves that approach from the southeast, south, and southwest. Predicted longshore currents on the shoal indicate that repeated extension and retreat of Cape Lookout Point would result from the imbalance between southerly longshore currents on the east side of the shoal and northerly longshore currents on the west side. Cape Lookout Shoal may play a role in protecting sections of adjacent barrier islands by interfering with shoreward wave propagation and by allowing waves to focus energy onto their offshore subaqueous areas. It is still unclear, however, how sediments on the northern proximal regions of the shoal are transported to the distal regions farther south.

Aeolian Dynamics along Scraped Shorelines, Bogue Banks, North Carolina

Abstract Aeolian transport rates were measured for six months (March–September 1999) at paired scraped and natural shorelines on Bogue Banks, North Carolina. Although wave action was principally responsible for dune volume losses at scraped shorelines, substantial aeolian losses were observed. Beach scraping induced rapid onshore aeolian sediment movement by increasing sediment availability, altering dune morphology, denuding dune foreslopes and modifying dune sediment characteristics. In response to prevailing onshore winds, net transport of sediments across the foredune crest was 1.54 × 10−3 m3/m/day, resulting in dune growth at scraped locations. Dune planting with American beach-grass (Ammophila breviligulata) did not substantially reduce aeolian transport rates at a scraped beachfront. Minimization of aeolian losses at scraped dunes by installation of sand fencing is desirable from a management perspective. Accelerated dune migration and depletion of the longshore sediment budget are potential long-term effects of scraping-induced onshore sediment flux on Bogue Banks.

Monitoring Beach Renourishment along the Sediment-Starved Shoreline of Grand Strand, South Carolina

Abstract In this paper, we examine temporal and spatial beach profile volume changes, sediment budget changes, and side-scan sonar images at nourished beaches of northeastern South Carolina. Results of bulk volume change indicate that most sands eroded from the subaerial beach section remain and circulate within the coastal system. The results also indicate more active sediment exchange between the nearshore and offshore zones than expected, indicating that the offshore can be both a sink and a source for the nearshore morphologic change. Our profile volume change results do not show a unidirectional net southerly transport pattern in the study area during the initial postnourishment period. Instead, results show that the net downdrift direction alternates along the shore and that longshore volume drift patterns are often disrupted by the seaward cross-shore transport events that occur at erosional hotspots. Prenourishment erosional patterns, particularly local areas of elevated erosion rates, were re-established after nourishment. Furthermore, those erosional locations often correspond to the offshore locations of paleoriver channel fill sands with low relief, indicating existence of the specific seaward transport pathways along the shore in the study area.

Spit Growth and Downdrift Erosion: Results of Longshore Transport Modeling and Morphologic Analysis at the Cape Lookout Cuspate Foreland

Abstract This study examined geomorphologic changes, littoral processes and sediment budget along the west side of the Cape Lookout, North Carolina, cuspate foreland, where a prograding spit at right angles to an eroding barrier island forms prominent features. Progradation of the spit is of interest geologically because it occurs within a transgressive, sediment-starved coastal setting in which adjacent barrier island limbs are eroding and in which immediate sediment sources and transport mechanisms are not obvious. Results indicate that as the spit extends to the north, it also widens to the west through swash bar attachment, leaving a well-defined record of dune ridges that mark former shoreline positions. Although northerly longshore transport occurs only under SW wave conditions, the average rate, ∼0.18 × 106 m3/y, is considered high given the sheltered position of the spit within the cuspate foreland. Results of modeling and development patterns of a series of ridge and runnel systems along the spit indicate that sediments are derived from the eastern limb of the cape or, under certain conditions, from sediments stored in Cape Lookout Shoal by longshore currents. Because the spit forms the distal end of a terminal littoral cell, it has an important effect on the regional sediment budget. Shackleford Banks, oriented at right angles and located immediately downdrift of Power Squadron Spit, is sheltered by the spit at its eastern end but suffers high erosion rates in its central section from lack of sediment input from the updrift direction. This result suggests that change in the input sediment budget can be a major factor in controlling coastal geomorphologic change under uniform physical conditions.

Sediment Diversions on the Lower Mississippi River: Insight from Simple Analytical Models

ABSTRACT Dean, R.G.; Wells, J.T.; Fernando, H.J., and Goodwin, P., 2014. Sediment diversions on the lower Mississippi River: insight from simple analytical models. River diversions offer a mechanism by which sediment-laden waters can be introduced into interdistributary basins and bays to build new land that provides a substrate for wetland growth. Two geometric models were developed to allow calculation of future performance of river diversions with emphasis on the Mississippi River. These geometries, a truncated cone and a uniform width geometry, assume a constant discharge of sediment into the receiving basin and thus avoid many of the complexities of the evolutionary processes. Model results from both geometries show a clear life cycle of growth and deterioration in a diversion that experiences relative sea level rise and, under certain combinations of relative sea level rise, depth of receiving waters and sediment discharge rate, situations in which a subaerial platform will never form. A comparison of subaerial deposits in larger versus smaller diversions, assuming the same total sediment discharge in both cases, reveals that the total subaerial land area for the larger diversions is substantially greater than the sum of the two volumes of the smaller diversions. Model results have been used to illustrate, through examples, the effect of bottom slope and sea level rise on diversion performance. Recommendations have been made for the selection (quantification) of diversion parameters to be used in the models. These include annual sediment input, proportion of sediment retained in the diversion deposit, sediment bulking factor, foreset slope, and subsidence rate. Other required input parameters are considered to be known, namely, average diversion water discharge, initial depth of receiving waters, bottom slope, deposit angle or width, and future sea level rise. We also include general recommendations for selection and utilization of diversion sites.