Ioannis Y. Georgiou

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.

The value of wetlands in protecting southeast louisiana from hurricane storm surges.

The Indian Ocean tsunami in 2004 and Hurricanes Katrina and Rita in 2005 have spurred global interest in the role of coastal wetlands and vegetation in reducing storm surge and flood damages. Evidence that coastal wetlands reduce storm surge and attenuate waves is often cited in support of restoring Gulf Coast wetlands to protect coastal communities and property from hurricane damage. Yet interdisciplinary studies combining hydrodynamic and economic analysis to explore this relationship for temperate marshes in the Gulf are lacking. By combining hydrodynamic analysis of simulated hurricane storm surges and economic valuation of expected property damages, we show that the presence of coastal marshes and their vegetation has a demonstrable effect on reducing storm surge levels, thus generating significant values in terms of protecting property in southeast Louisiana. Simulations for four storms along a sea to land transect show that surge levels decline with wetland continuity and vegetation roughness. Regressions confirm that wetland continuity and vegetation along the transect are effective in reducing storm surge levels. A 0.1 increase in wetland continuity per meter reduces property damages for the average affected area analyzed in southeast Louisiana, which includes New Orleans, by

Hurricane-associated ebb-tidal delta sediment dynamics

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

133, and a 0.001 increase in vegetation roughness decreases damages by

Salinity, Nutrient, and Sediment Dynamics in the Pontchartrain Estuary

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Runaway Barrier Island Transgression Concept: Global Case Studies

43. These reduced damages are equivalent to saving 3 to 5 and 1 to 2 properties per storm for the average area, respectively.

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

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.

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

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.

Multidimensional Modeling of the Lower Mississippi River

Abstract Algal blooms have been recorded in the Pontchartrain Estuary when elevated nutrient loads combine with other ambient conditions, e.g., low salinity and low turbidity. Nutrient and sediment loads were quantified and incorporated into a dynamic mass balance model for the upper Pontchartrain Estuary to determine the timing of conditions that can lead to algal blooms. General nutrient and sediment loading relationships were developed for the tributaries of the Pontchartrain Estuary where data were available; these relationships were extended to estimate the loading for ungauged areas. The annual water yield for the drainage basin was found to be 500 mm. The annual nutrient loadings from all sources were found to be 21,000 t (13 g m−2 y−1) of total nitrogen and 2700 t (1.6 g m−2 y−1) of phosphorus for Lakes Maurepas, Pontchartrain, and Borgne. The basin sediment yield was calculated to be 42 t km−2. Nutrient and sediment concentrations in the Mississippi River were combined with the estimated leakage through the Bonnet Carré Spillway to obtain nutrient loads related to the river stage. The mass-balance model was applied to assess the occurrence of algal blooms for the period 1990–2008; the model predicted all five observed algal blooms but also indicated a potential for two more that were not documented. The strongest potential for algal blooms was in the northwest quadrant of Lake Pontchartrain although the highest dissolved inorganic nitrogen concentrations were predicted for the southwest quadrant in connection with spillage from the Bonnet Carré Spillway. The lower than expected response of the southwest quadrant may be associated with the higher turbidity due to the Spillway plume. Abstract La afloración de algas ha sido observada en el Estuario Pontchartrain cuando las cargas elevadas de nutrientes se combinan con otras condiciones ambientales (e.g., baja salinidad y baja turbiedad). Las cargas de nutrientes y de sedimentos fueron cuantificadas e incorporadas en un modelo dinámico de balance de masa en la region superior del Estuario Pontchartrain para determinar el momento en que las condiciones son propicias para la afloración de algas. Para los casos donde habían datos disponibles, se establecieron relaciones entre los nutrientes generales y las cargas de sedimentos de los tributarios del Estuario Pontchartrain; en las áreas donde no hubo datos disponibles, estas relaciones fueron utilizadas para estimar las cargas. El rendimiento de agua anual para la cuenca hidrográfica fue de 500 mm. Las cargas anuales de nutrientes de todas las fuentes fue de 21,000 toneladas de nitrógeno total y 2700 toneladas de fósforo. El rendimiento de sedimentos fue calculado en 42 toneladas/km2. Las concentraciones de nutrientes y sedimentos en el Río Mississippi fueron combinadas con las de las estimadas por la fuga de agua por Vertedero Bonnet Carré para obtener cargas de nutrientes relacionadas con el nivel del Río. El modelo de balance de masa fue aplicado para evaluar la ocurrencia de la afloración de algas para el periodo entre 1990 y 2008. Este modelo predijo las 5 afloraciones de alga observadas y también indicó el potencial para 2 afloraciones más que no fueron documentadas. El área con el mayor potencial al desarrollo de afloración de algas fue el cuadrante noroeste del Lago Pontchartrain, a pesar de que las mayores concentraciones de nitrógeno inorgánico disuelto fueron predichas para el cuadrante suroeste en relación con la descarga del Vertedero Bonnet Carré. La respuesta, más baja de la esperada, en el cuadrante sureoeste puede estar asociada con mayores niveles de turbiedad debido a la mancha de la descarga del vertedero.

Stratification and Circulation in Lake Pontchartrain

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.