Surface-Water Hydrology of the Gulf Intracoastal Waterway in South-Central Louisiana, 1996-99
U.S. Geological Survey Professional Paper 1672
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The flow of freshwater and suspended sediment from the Lower Atchafalaya River (LAR) and Wax Lake Outlet (WLO) into and along the Gulf Intracoastal Waterway (GIWW) and selected adjacent surface-water bodies between Cypremort and Larose in south-central Louisiana, from October 1996 to December 1999, was characterized using instantaneous and computed continuous discharge measurements and measurements of suspended- sediment concentrations. The GIWW parallels the entire Louisiana coast near the wetland/ upland interface. Following natural hydraulic gradients, the GIWW captures water and sediment from the southward flowing LAR and the WLO where it crosses those waterways, and distributes this freshwater and sediment to points east and west.
East of Morgan City, La., an average of 12,200 ft3/s (cubic feet per second) of water flowed from the LAR into the Avoca Island Cutoff Channel. The LAR was the primary source of water to the GIWW east of Morgan City. Drainage from the Verret Subbasin through Bayou Boeuf contributed an average of 1,000 ft3/s to the eastward flow in the GIWW. Eastward flow in the GIWW near Bay Wallace east of Morgan City and to the west of the Houma Navigation Canal (HNC) at Houma, La., averaged about 5,700 ft3/s. Average flow in the GIWW east of the HNC at Houma was 2,610 ft3/s to the east, and 2,200 ft3/s east of Bayou Lafourche at Larose, also to the east.
Measured discharge in the GIWW was always to the west between the LAR and WLO. Water entered this stretch of the GIWW from the LAR. The WLO was the primary source of water to the GIWW west of WLO. Discharge in the GIWW averaged 9,460 ft3/s west of WLO south of Calumet and 8,230 ft3/s east of Jaws Bay west of Franklin. Average discharge in the GIWW west of Jaws Bay near Cypremort was 3,310 ft3/s and at Cypremort was 1,350 ft3/s. Average discharge was to the west at all four locations, but discharge as high as 2,830 ft3/s was measured flowing eastward toward Jaws Bay in the GIWW at Cypremort.
In bayous and canals in most of coastal Louisiana, including the GIWW, stage narrowly fluctuates around the Gulf of Mexico level. Where the GIWW crosses the LAR and WLO, stage can reach 3 ft (feet) or more above the North American Vertical Datum of 1988 (NAVD88). Flow in the GIWW results from these differences in stage. When the LAR at Morgan City reached 3 to 4 ft above NAVD88, flow in the GIWW became more predictable. Discharge at most sites between the HNC and Jaws Bay increased in varying amounts as stage of the LAR at Morgan City increased beyond 3 ft above NAVD88. At sites in the GIWW east of HNC, discharge did not increase predictably. For all measurements made when the LAR at Morgan City was 3 ft or more above NAVD88, average discharge was about 3,100 ft3/s in the GIWW east of HNC at Houma and 2,880 ft3/s east of Bayou Lafourche at Larose. The LAR at Morgan City is 3 ft or more above NAVD88 for about 7 months in a normal year.
When the LAR at Morgan City was less than 3 ft above NAVD88, water in the GIWW flowed along the prevailing water-level gradients, to the east between Bay Wallace and the HNC and to the west between WLO and Jaws Bay. However, local runoff and drainage from areas adjacent to the GIWW became more significant in maintaining flow at low LAR stage. Discharge was consistently higher in the GIWW west of the HNC at Houma than farther west near Bay Wallace east of Morgan City, when the LAR at Morgan City was less than 3 ft above NAVD88. Westward flow in the GIWW between Houma and Morgan City was observed near Bay Wallace east of Morgan City but was never observed west of the HNC at Houma. Discharge in the GIWW east of Jaws Bay west of Franklin, La., frequently was higher than in the GIWW west of WLO south of Calumet, La., at low LAR stage.
East of the LAR, suspended-sediment concentrations averaged about 162 mg/L (milligrams per liter) at the two sites closest to the LAR, Avoca Island Cutoff Channel and Bayou Penchant south of Morgan City. Suspended-sediment concentrations averaged 91 mg/L in the water entering the GIWW through Bayou Boeuf at railroad bridge at Amelia. The water in the GIWW near Bay Wallace east of Morgan City to the HNC was intermediate in suspended-sediment concentrations (108- 114 mg/L), reflecting the mixture of the two water types. Suspended-sediment concentrations in the GIWW near Bay Wallace east of Morgan City and east and west of the HNC were comparable, but in the GIWW east of Larose, they increased by about 45 percent. To the west of WLO, suspended-sediment concentrations averaged between 167 and 191 mg/L.
Freshwater flow directly affected salinity patterns in the GIWW and adjacent surface-water bodies. Monthly-average salinity at many sites, both in and adjacent to the GIWW, was related to monthly-average stage of the LAR at Morgan City.
When the LAR at Morgan City is 3 or more ft above NAVD88, the GIWW effectively distributes freshwater and sediment from the LAR and WLO to points in coastal Louisiana 30 to 50 miles east and west of Morgan City. The freshwater and sediment, some of which originate indirectly from the Mississippi River, are valuable resources for building wetlands and could be incorporated into ongoing efforts to restore coastal Louisiana.
Purpose and Scope
Description of Study Area
The Gulf Intracoastal Waterway
Physiography of Coastal Louisiana
Approach and Methods
Discharge and Suspended Sediment
Stage and Water-Level Gradients
Hydrologic Conditions During Study
Discharge in Relation to Stage of the Lower Atchafalaya River and Precipitation
Between Lower Atchafalaya River and Houma Navigation Canal
East of Houma Navigation Canal
Between Lower Atchafalaya River and Wax Lake Outlet
West of the Wax Lake Outlet
Discharge in the Gulf Intracoastal Waterway from 1997-99
Net Gain or Loss of Water Along Major Segments
Magnitude, Duration, and Timing of Flow
Stage and Water-Level Gradients
Selected Water-Quality Constituents
Summary and Conclusions
Appendix: Regression Equations
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