Michael D. Miner

Hurricane-induced failure of low salinity wetlands

During the 2005 hurricane season, the storm surge and wave field associated with Hurricanes Katrina and Rita eroded 527 km2 of wetlands within the Louisiana coastal plain. Low salinity wetlands were preferentially eroded, while higher salinity wetlands remained robust and largely unchanged. Here we highlight geotechnical differences between the soil profiles of high and low salinity regimes, which are controlled by vegetation and result in differential erosion. In low salinity wetlands, a weak zone (shear strength 500–1450 Pa) was observed ∼30 cm below the marsh surface, coinciding with the base of rooting. High salinity wetlands had no such zone (shear strengths > 4500 Pa) and contained deeper rooting. Storm waves during Hurricane Katrina produced shear stresses between 425–3600 Pa, sufficient to cause widespread erosion of the low salinity wetlands. Vegetation in low salinity marshes is subject to shallower rooting and is susceptible to erosion during large magnitude storms; these conditions may be exacerbated by low inorganic sediment content and high nutrient inputs. The dramatic difference in resiliency of fresh versus more saline marshes suggests that the introduction of freshwater to marshes as part of restoration efforts may therefore weaken existing wetlands rendering them vulnerable to hurricanes.

Hurricane-associated ebb-tidal delta sediment dynamics

Bathymetric surveys conducted before and after the 2005 hurricane season at Little Pass Timbalier in the Mississippi River delta plain, United States, demonstrate that 9.1 (±2.4) × 10 6 m 3 of sediment was eroded from a 47.9 km 2 area. Between the two surveys, Hurricanes Cindy, Katrina, and Rita passed within 300 km of the tidal inlet. Comparison of before and after bathymetric data sets shows that the distal portion of the ebb-tidal delta (ETD) was the site of 63% of the total erosion, locally resulting in 400 m of shoreface retreat. Shoaling (0.75–1.0 m) in the seawardmost portion of the ebb channel and erosion in the landward portion (~1.5 m) resulted in a 160 m landward shift of the inlet throat. Collectively, these processes forced the landward migration of the entire tidal inlet and ETD system. There has been considerable discussion about large volumes of mineral sediment deposited on the Mississippi River delta plain interior marsh surface as a result of hurricanes; however, the origin of this sediment is unknown. We identify the distal portion of an ETD as one possible sediment source and hypothesize that ETD and shoreface sediment is mobilized by hurricane waves and transported landward by surge-induced currents. Our results emphasize the role of frequent and intense hurricanes in long-term coastal evolution and as a mechanism for regional sediment retention within the transgressive system beyond typical barrier overwash processes.

Impacts of Rising Sea Level to Backbarrier Wetlands, Tidal Inlets, and Barrier Islands: Barataria Coast, Louisiana

The Barataria barrier system within the Mississippi River delta plain, is experiencing some of the highest relative sea-level rise (SLR) rates in the continental USA (0.94 cm/yr). This has led to substantial wetland loss in Barataria Bay (16.9 km 2 /yr, from 1935-2000). This conversion of wetlands to intertidal and subtidal environments results from several linked processes including subsidence, marsh front erosion, and catastrophic scour during large magnitude hurricanes. Increasing open water within Barataria Bay has amplified tidal exchange with the ocean. Between 1880 and 2006, an increase of 400% took place in the combined cross-sectional areas of the major tidal inlets of Barataria Bay, associated with the enlarging tidal prism. This expansion of the inlets has been at the expense of the adjacent barrier islands, evident in the concomitant progradation of the ebb-tidal deltas. Since the 1880s the ebb delta at Barataria Pass built seaward more than 2.0 km, sediment cores show that sand constitutes the upper 1-2 m of the ebb delta. Movement of sand offshore, regional subsidence and increasing bay tidal prism produce segmentation of the barriers, forming new inlets such as Pass Abel. Acceleration in eustatic sea level rise will lead to further wetland loss and thus ultimately barrier disintegration. The Barataria barrier chain will be transformed into an island-only system similar to the Isle Dernieres and Timbaliers.

Runaway Barrier Island Transgression Concept: Global Case Studies

The regime of accelerating sea-level rise forecasted by the IPCC (2013) suggests that many platform marshes and tidal flats may soon cross a threshold and deteriorate/drown as back-barrier basins transform to intertidal and subtidal areas. This chapter explores how marshes may succumb to rising sea level and how the loss of wetlands will increase the extent and the overall depth of open water in the back-barrier, causing greater tidal exchange. Here, we present a conceptual model that depicts how increasing tidal prism enlarges the size of tidal inlets and sequesters an increasingly larger volume of sand in ebb-tidal delta shoals. The conceptual model is based on empirical relationships between tidal prism and inlet parameters, as well as field and theoretical hydraulic studies of tidal inlets showing that long-term basinal deepening intensifies the flood dominance of existing inlet channels and transforms some ebb-dominated channels to flood-dominated channels. This condition leads to sand movement into the back-barrier, which builds and enlarges flood-tidal deltas, filling the newly created accommodation space. The model hypothesizes that sand contributed to the growth of the ebb and flood tidal delta shoals will be at the expense of barrier reservoirs. This will result in diminished sand supplies along the coast, eventually leading to fragmentation of barrier island chains and the transition from stable to transgressive coastal systems. Several historical studies of barrier island systems throughout the world demonstrate barrier response to changing tidal prism and illustrate different stages of this conceptual model.

Sub-decadal submarine landslides are important drivers of deltaic sediment flux: Insights from the Mississippi River Delta Front

Submarine mass failures triggered by energetic forcing events such as hurricanes and earthquakes are relatively well studied due to the potential for infrastructure damage and tsunami generation; such failures are common on heavily sedimented margins where underconsolidated deposits are preconditioned to fail. However, studies of seafloor sediment movement between large events remain scarce. Using repeat bathymetric surveys of the Mississippi River Delta Front (MRDF), we document substantial seafloor movement in absence of major hurricanes. About 1 m/yr of deepening was observed within preexisting failures, with downslope sediment transport on the order of 10 5 m 3 /yr. Outside failure features, seafloor depths remained stable or showed minor (<20 cm/yr) accretion. MRDF volumetric sediment flux during hurricane-driven mass failures is an order of magnitude greater than the annual flux during a quiescent interval. When normalized by time, however, sediment flux during the quiescent interval (5.5 × 10 5 m 3 /yr) was half that of hurricane-driven mass failures (1.1 × 10 6 m 3 /yr). These observations corroborate our wave modeling results, which infer that even waves of 1 yr recurrence interval can generate differential seafloor pressures sufficient to trigger submarine landslides; this does not exclude the possibility of river floods also being agents of failure. These findings indicate that sub-decadal submarine landslides are important to MRDF dynamics, comparable to the role of major hurricanes, and observation during seemingly quiescent periods is necessary to holistically assess sediment flux. The periodicity and prevalence of moderate-scale mass transport documented here corroborates similar recent studies offshore other deltas globally, indicating that highstand mass and time budgets of shelf to deep-sea sediment flux, in addition to organic carbon and bioreactive particles, may need to be revised.

1880 to 2005 Morphologic Evolution of a Transgressive Tidal Inlet, Little Pass Timbalier, Louisiana

Abstract The majority of changes to barrier island shorelines can be attributed to the influence of tidal inlets, and therefore an understanding of inlet processes is important to effectively manage barrier systems. High rates of relative sea-level rise within the Mississippi River delta plain have resulted in a highly transgressive coastal regime and a rapid landward-migration of barrier island and tidal inlet systems. Moreover, ongoing conversion of back barrier and interior wetlands to open water increases tidal exchange. Enlarging bay-tidal prisms together with the landward migration of the barrier systems results in a dynamic environment within which tidal inlets undergo vast changes in position, geometry, and shoreline morphology. Historic bathymetric maps (dating to the 1880s) and newly acquired bathymetric data for Little Pass Timbalier are used to construct a series of digital elevation models and ultimately an evolutionary model for the area. The evolution of Little Pass Timbalier is complex and has encompassed periods of landward and lateral channel migration (43 m/yr and 23 m/yr, respectively) and avulsion to breaches along the adjacent barrier shoreline. The breaching event widened the inlet throat from 1.5 km in 1890 to 8.6 km by 1930. The increasing bay tidal prism resulted in inlet widening and the formation of multiple channels separated by ephemeral shoals. During the same time, the ebb tidal delta grew in size and prograded seaward while the adjacent barriers migrated landward.