On the coast, rainfall mixes with seawater from the Gulf, resulting in a coastal system called an estuary. Many of the external influences on an estuary are the same influences that affect the weather, like rainfall or winds; but the estuary is also impacted by an additional set of factors, including tides or river diversions. This daily interaction of freshwater and seawater is as complex as our local weather, and it is almost as important.
Lake Pontchartrain Basin Foundation (LPBF) sees a parallel between water monitoring and weather forecasting. Just as weather networks and websites use maps to forecast the weather, LPBF will use a map to show water movement and the most recent distribution of salinity across the basin. LPBF has developed a map to display hydrology for the Pontchartrain Basin. We call our map the “Hydrocoast Map.” Please see below to view LPBF's latest Hydrocoast Maps. You can also view our archived maps and methodology.
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Click on link below map for larger image.
The Hydrocoast maps for the week of May 16 through May 22, 2016 were produced using field data, MODIS, satellite imagery, precipitation data, wind data and permanent monitoring stations in the basin (USGS buoys, Coastwide Reference Monitoring System (CRMS), etc.). During this Hydrocoast period the salinity remained similar in Lake Pontchartrain as salinity across the lake was still less than 2 ppt. Salinity decreased in the Cat Island Channel, Chandeleur Sound and Breton Sound. Discharge decreased at all Northshore rivers and increased at all river outlets. Average daily discharge difference between last Hydrocoast and the current was:
- Pearl River = 8,144 to 4,300 cfs
- Tangipahoa = 1,627 to 1,214 cfs
- Tickfaw = 791 to 670 cfs
- Amite = 4,653 to 4,109 cfs
- Caernarvon Diversion = 164 to 131 cfs
- Mardi Gras Pass = 9,800 to 10,100 cfs
- Violet Siphon = Closed
- Bohemia Spillway = 0 to 3,333 cfs
- Bonnet Carré Spillway = 0 to 20 cfs
- Bayou Lamoque = 2,002 to 2,294 cfs
- Fort St. Philip = 89,132 to 109,194 cfs
- Baptiste Collete = 69,924 to 81,399 cfs
- Grand and Tiger Pass = 57,147 to 67,282 cfs
- Main Pass = 54,669 to 63,271 cfs
- West Bay = 33,630 to 39,149 cfs
- Pass A Loutre = 46,514 to 53,198 cfs
- Southwest Pass = 220,959 to 252,705 cfs
- South Pass = 64,790 to 74,099 cfs
The first map (Hydrocoast map without precipitation) shows the salinity contours and freshwater discharge across the Pontchartrain Basin. The solid line salinity contours are at 1 ppt salinity increments. The salinity is highest out past the Chandeleur Islands (red lines, 32 ppt) and decreases to fresh conditions (dark blue lines) in the basin. Sea water generally has a salinity of 32 ppt. Green asterisks represent salinity leak points, usually in the form of gates in storm surge protection features but also in passes, portions of roads that are raised, canals, and bayous that are holes in the salinity barriers (pink lines) in the form of levees, roads, natural ridges and canals (with associated spoil banks). Salinity contours that are close together represent an area where salinity changes quickly over a short distance, which is seen slightly offshore throughout the basin during this Hydrocoast period. Contours that are farther apart represent a more gradual change over longer distances which can be seen in throughout the interior of the basin, in Lake Pontchartrain and Lake Maurepas.
The second map shows the salinity contours laid on top of a habitat/land -use map, showing where fresh to salt marsh and swamps are found in the basin. This map also shows soil water salinity contours across the land masses. The third map shows the salinity contours in conjunction with weekly rainfall and wind roses in the Pontchartrain basin. During this Hydrocoast period there was rain across the basin ranging from 0.25 to 3 inches. Resultant winds were from the southwest, south and southeast. Wind speeds ranged from 2 to 11.2 m/s (2 to 25 miles/hr).
The fourth "Water Quality" shows the results of LPBF's water quality sampling around Lake Pontchartrain, reporting the fecal coliform counts. Elevated fecal coliform counts were found at Bayou Castine, Abita River and Yellow River/LA22 and high counts were found at Bogue Falaya, and Fontainebleau Beach. The water quality map also shows the impaired water bodies for Primary Contact (swimming, immersion likely) and Secondary Contact (boating wading, immersion unlikely) in the basin as prepared by the Louisiana Department of Environmental Quality under the EPA 305(b)/303(d) guidelines. Hypoxia was detected in Chandeleur Sound on May 18th and in Breton Sound on May 19th. In addition there were areas with stratification but no hypoxia.
The fifth “Biological Map” shows oyster harvest area closure, as determined by the Louisiana Department of Health and Hospitals and the location of the public oyster seed grounds. The biological map also shows the impaired water bodies for fishing and oyster propagation in the basin as prepared by the Louisiana Department of Environmental Quality under the EPA 305(b)/303(d) guidelines. Impairments due to metal contamination and fecal coliform are shown.