Open-File Report 2008–1127
Coastal Louisiana is a dynamic and ever changing landscape. From 1956 to 2004, over 297,000 ha of Louisiana’s coastal wetlands were lost because of the effects of natural and human-induced activities. Studies show that, in 2005, Hurricanes Katrina and Rita transformed over 56,200 ha of wetlands to open water in various parts of coastal Louisiana. Besides the catastrophic hurricanes, factors such as subsidence, sea-level rise, freshwater and sediment deprivation, saltwater intrusion, the dredging of oil and gas canals, navigation canals, shoreline erosion, and herbivory are all contributors to wetland loss in Louisiana. Various scientific literatures have well described the direct impacts associated with an immediate physical conversion of habitat in coastal Louisiana; however, the indirect impacts that are subtle and operate over longer time horizons (such as salinity intrusion) have been difficult to discern. In this report, long-term influences on salinity patterns and landscape configuration are evaluated for pre- and postconstruction periods of the Houma Navigation Canal (HNC), which is located in the coastal region of southeastern Louisiana.
As a means of quantifying the distance and degree of influence that the HNC had on marsh degradation, a 3-km interval buffer array and comparable years of vegetation data were used to describe the changes in primary metric values across the three project dates (1958, 1968/69, and 1998). The patterns across landscape metrics varied, and it was difficult to discern direct relationships based on proximity to the canal. Even though the canal may have an influence on marsh degradation, these analyses show that the degree and distance of that influence is not discernible through change in class-level landscape metric values.
The vegetation zone analyses showed that fresher marshes were more susceptible to degradation. This pattern was observed in fresh and intermediate marshes where the primarily solid landscapes in the pre- and postconstruction dates converted to highly fragmented marsh or open water by 1998. Conversely, and as a result of higher connectivity to the Gulf of Mexico and subsequent shoreline and bank erosion, the brackish and saline marshes had higher percentages of water in 1958 and therefore experienced lower levels of degradation than did the fresh and intermediate zones by 1998. The analyses based on vegetation zones by distance buffers showed high variability, which appear to be dominated both by north-south vegetation delineation and by year; however, the influence based on distance from the HNC was not evident.
Even though the patterns of land loss differed spatially, there were four areas of extensively degraded marsh within the project landscape. These areas or “hotspots” were distinguished through significant changes in landscape metric values. All of the four hotspots were located near the intermediate to fresh and brackish marsh boundaries. Three of the hotspots (those closer in proximity to the Houma Navigation Canal) were more directly affected by increased salinity and altered hydrology from the canal and its distributaries. Of those three, one experienced land loss at a slower rate than the others because of its proximity to preexisting natural channels, which were capable of delivering higher salinity waters even prior to the construction of the canal. During the period of 1958-98, the fourth hot spot was observed primarily within a brackish marsh and was therefore less affected by higher salinity waters. Previous studies show that this hotspot is located within two oil and gas fields, and therefore, its minimal configuration change and rapid conversion to water may be linked to fluid withdrawal and faulting coupling.
Posted July 2008
Steyer, G.D., Sasser, C., Evers, E., Swenson, E., Suir, G., and Sapkota, S., 2008, Influence of the Houma Navigation Canal on salinity patterns and landscape configuration in coastal Louisiana: an interagency collaboration: U.S. Geological Survey Open-File Report 2008–1127, 190 p.
Marsh Fragmentation Classification System
Appendix 1: Salinity Level Plots
Appendix 2: FRAGSTATS Classification Criteria and Thresholds
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