Zoe J. Hughes

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.

Tidal Channels on Tidal Flats and Marshes

In shallow coastal settings channels provide a pathway for the tide to propagate and are, thus, a primary control on the sedimentation and ecology of these environments. Being shaped by bidirectional flows, tidal channels exhibit morphologies, which, despite apparent similarities, bear significant and fundamental differences to fluvial channels, specifically their scaling with size. This chapter considers the classification of tidal channels and the networks they form. We examine the hydrodynamics of shallow tidal channels, including asymmetry in period or velocity between the ebb and flood tides, and the hysteresis seen in stage-velocity curves in regions with large intertidal areas. Channel initiation may occur either through incision or by variations in rates of deposition. Tidal channels evolve over time and a number of relationships are presented that have been derived to describe the geometry of tidal channels. Mutually-evasive pathways of flood and ebb flows may produce cuspate meanders; a morphology unique to tidal channels. Of particular importance, in terms of preservation potential, is the development of meanders in channels and the resulting pointbars. Pointbars in tidal environments are often fully or partially detached from the bank by a channel formed by the subordinate tidal current, however their exact morphology varies being dependent on channel sinuosity and tidal asymmetry. Channels are preserved through infilling (as tidal prism is reduced) and through lateral accretion, particularly at meanders. Pointbars in tidal regions are generally heavily bioturbated in the upper tidal range, and mid-tidal zones will exhibit inclined stratigraphy, often with intercalated beds of muddier and sandier deposits.

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.

Impact of Multiple Freshwater Diversions on the Salinity Distribution in the Pontchartrain Estuary under Tidal Forcing

Abstract Numerical experiments of multiple freshwater diversions into the Pontchartrain Estuary under tidal forcing were conducted to evaluate the impact on salinity and tidal flow distribution. A validated numerical hydrodynamic and transport model was used to assess the impacts on tidal flows, circulation, and salinity as a function of additional freshwater input in the estuary from hypothetical diversions combined with channel modifications in the Mississippi River Gulf Outlet. The cumulative and specific impacts were compared with existing conditions. It was concluded that upper and middle estuarine salinity regimes are coupled, and diversion flows need to be managed in accordance with historic inputs. This study also showed that if total freshwater input is not of an order similar to the existing natural tributary flow, the average salinity in the upper estuary could be reduced by 1.5 ppt (±0.5 ppt), which is approximately 40% of the existing long-term salinity of the upper estuary. The additional flow into the upper estuary will produce changes in the flow through the tidal passes on the order of 5%–6%, will decrease hydraulic detention times in the estuary, and will cause an additional increase in the ebb-dominance of the estuary.