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 2 through May 8, 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 decreased in Lake Pontchartrain as salinity across the lake was less than 2 ppt. Salinity decreased in Lake Borgne, Chandeleur Sound and Breton Sound as well. Discharge increased at all Northshore rivers except the Pear River and decreased at all river outlets except Mardi Gras Pass. Average daily discharge difference between last Hydrocoast and the current was:
- Pearl River = 35,471 to 8,144 cfs
- Tangipahoa = 1,021 to 1,627 cfs
- Tickfaw = 525 to 791 cfs
- Amite = 3,333 to 4,653 cfs
- Caernarvon Diversion = 211 to 164
- Mardi Gras Pass = 7,600 to 9,800 cfs
- Violet Siphon = Closed
- Bohemia Spillway = 378 to 0 cfs
- Bonnet Carré Spillway = 0 to 0 cfs
- Bayou Lamoque = 2,305 to 2,002 cfs
- Fort St. Philip = 94,745 to 89,132 cfs
- Baptiste Collete = 74,613 to 69,924 cfs
- Grand and Tiger Pass = 61,266 to 57,147 cfs
- Main Pass = 58,184 to 54,669 cfs
- West Bay = 35,885 to 33,630 cfs
- Pass A Loutre = 49,278 to 46,514 cfs
- Southwest Pass = 234,085 to 220,959 cfs
- South Pass = 68,639 to 64,790 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 less than an inch of rainfall over Breton Sound and no rain across the rest of the basin. Resultant winds were variable but mostly from the northwest. Wind speeds ranged from 2 to greater than 11.2 m/s (2 to greater than 25 miles/hr).
The fourth "Water Quality" shows the results of LPBF's water quality sampling around Lake Pontchartrain, reporting the fecal coliform counts. High counts fecal coliform counts were found at Laketown, Bonnabel Boat Launch, Old Beach, Pontchartrain Beach, Bogue Falaya, and Bayou Castine. 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.
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.