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Open-File Report 2008–1127

In Cooperation with Minerals Management Service, Gulf of Mexico Outer Continental Shelf Region

Influence of the Houma Navigation Canal on Salinity Patterns and Landscape Configuration in Coastal Louisiana

By Gregory D. Steyer, Charles Sasser, Elaine Evers, Erick Swenson, Glenn Suir, and Sijan Sapkota

Abstract

Thumbnail of and link to report PDF (8.9 MB)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.
     Analysis of daily and hourly salinity data from long-term data collection stations within the areas surrounding the HNC indicated that there were no obvious patterns in increasing salinity levels following the completion of the canal, except for the immediate increase in salinity spikes that occurred toward the completion of its construction in 1961. Increases in salinity spikes were also observed during a severe drought in 1999–2000. Data from Bayou Grand Caillou at Dulac, however, show a longer term trend of increasing salinity levels, which is similar to the pattern observed at the Houma Water Treatment Plant. A potential explanation for these patterns is based on the dredging history of the HNC, where dates of maintenance dredging correspond fairly closely to the salinity peaks in Bayou Grand Caillou and the canal. It appears that the dredging events opened up a deeper route from the canal to Crozier and into Grand Bayou Caillou, but it also may be a result of the general breakup of the marsh in the adjacent area, which resulted in greater exchange of bay water and subsequently higher salinity levels. Although the available salinity data were insufficient to conduct statistical correlations, there was close agreement between salinity changes and specific dredging events of the HNC.
     A procedure for analyzing marsh landscapes, which utilizes the FRAGSTATS landscape statistical application and a two-part marsh classification system, was developed as a means of determining the connectivity and configuration of marsh and water patches within the study area. Individual landscape metrics were used to determine the percentage and rate of land change and the shifts in density, shape, and cohesiveness of water within the marsh. Wetland loss rates for coastal Louisiana and Terrebonne basin were compared to the long- and short-term loss rates of the Houma Navigation Canal study area that were quantified by using the FRAGSTATS landscape analysis method. These results suggest that the canal study area was losing land at a significantly faster rate than both the marshes of coastal Louisiana (over all periods) and the other highly degraded neighboring marshes within Terrebonne basin. Overall, 37 percent (17,625 ha) of the project area marsh was lost between 1958 and 1998.

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.

Version 1.0

Posted July 2008


Suggested citation:

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.


Contents

Introduction

Salinity Assessment

Landscape Metrics

Marsh Fragmentation Classification System

Acknowledgments

References Cited

Appendix 1: Salinity Level Plots

Appendix 2: FRAGSTATS Classification Criteria and Thresholds

Glossary


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