Hurricane-induced failure of low salinity wetlands

Abstract

During the 2005 hurricane season, the storm surge and wave field associated with Hurricanes Katrina and Rita eroded 527 km 2 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.

Freshwater river diversions for marsh restoration in Louisiana: Twenty-six years of changing vegetative cover and marsh area

“…Fertilization of brackish marshes would promote lower rhizome and root biomass [Valiela et al, 1976], accelerated decomposition rates [Godshalk and Wetzel, 1978;Sundareshwar et al, 2003] and stems vulnerable to collapse (lodging) from high winds [Resio and Westerink, 2008]. In fact, research on Louisiana wetland vegetation [Darby and Turner, 2008;Turner, 2011] documented shallow and limited rooting in Spartina alterniflora, resulting in weak substrate structure and shear strength [Howes et al, 2010;Turner, 2011]. Moreover, nitrogen additions to freshwater wetlands enhance carbon losses [Bragazza et al, 2006;Mack et al, 2004].…”

“…Further, the available data indicate that vertical accretion is correlated with in situ organic accumulation (principally root and rhizome material), not mineral sediment inputs [Nyman et al, 1993;Turner et al, 2000]. Excessive nutrient influx into the Caernarvon diversion marshes (the largest freshwater diversion created) is linked to the widespread occurrence of low soil strength (by field shear vane tests), indicating that these marsh sediments are potentially highly erodible; in fact, land losses from Hurricane Katrina were especially high [Howes et al, 2010]. The strong support given diversions by some local wetland scientists [Boesch, 2006;Day et al, 2007] [5] The effectiveness of diversions to mitigate marsh loss has not been quantitatively evaluated.…”

Wetland Accretion Rates Along Coastal Louisiana: Spatial and Temporal Variability in Light of Hurricane Isaac’s Impacts

“…Despite their role in sediment deposition and accretion, recent events including Ike, Gustav, Katrina, and Rita were responsible for significant levels of ecological disturbance, marsh flooding, and denudation throughout Louisiana. This includes, but is not limited to, increasing the size of open water bodies [43], marsh erosion [44], increasing wetland salinity, landscape change from wetland to mudflat, removal of floating marsh, and creation of small ponds [45]. Balancing the results from this study with a similar large-scale assessment of wetland erosion and loss due to recent hurricanes would be an ideal "two-prong" approach to understanding regional coastal dynamics.…”

A Hydrograph-Based Sediment Availability Assessment: Implications for Mississippi River Sediment Diversion

“…The lack of positive response to the freshwater diversions during this period has been attributed to inappropriate timing of discharge, insufficient discharge magnitude (need > 100 m 3 s −1 to induce sheet flow needed to carry sediment across the marsh at Caernarvon Diversion, while DPFD is too small and channelized to induce sheet flow [15], and/or overloading of nutrients affecting sediment stability [14,16]. After Hurricanes Katrina and Rita, in 2005, these areas sustained even larger losses in vegetation and overall marsh area, when compared to similar marshes of the adjacent reference sites [14,17]. The West Bay Sediment Diversion was designed for sediment capture.…”