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 August 10, 2015 through August 16, 2015 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 increased on the eastern side of Lake Pontchartrain, from 11 ppt to 13 ppt. The western half of the lake remained fresh. Salinity also increased in Lake Borgne and Biloxi Marsh, Chandeleur Sound but decreased in Breton Sound. Discharge decreased at all rivers and outlets except the Pearl River and Mardi Gras Pass. Average daily discharge difference between last Hydrocoast and the current was:
- Pearl River = 1,798 to 1,834 cfs
- Tangipahoa = 445 to 372 cfs
- Tickfaw = 275 to 119 cfs
- Amite = 973 to 553 cfs
- Caernarvon Diversion = 227 to 199 cfs
- Mardi Gras Pass = 5,091 to 5,300 cfs
- Violet Siphon = Closed
- Bohemia Spillway = 18,037 to 3,545 cfs
- Bonnet Carré Spillway = 4,620 to 40 cfs
- Bayou Lamoque = 3,028 to 2,165 cfs
- Fort St. Philip = 55,310 to 41,605 cfs
- Baptiste Collete = 110,627 to 88,317 cfs
- Grand and Tiger Pass = 87,170 to 62,900 cfs
- Main Pass = 79,697 to 59,544 cfs
- West Bay = 64,550 to 49,561 cfs
- Pass A Loutre = 58,951 to 41,981 cfs
- Southwest Pass = 297,067 to 234,377 cfs
- South Pass = 131,734 to 103,341 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 rainfall across the entire basin. Rainfall ranged from 1 to 4 inches. Resultant winds were mostly from southeast but also from the east. Wind speeds ranged from 2 to 11.1 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. During this Hydrocoast period there were elevated fecal coliform counts at Abita River and Alligator Bayou and high counts were found at Bogue Falaya, Fontainebleau Beach, Yellow River/LA22, Muddy C. Road and Harrell Lane Public Launch. 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. Two hypoxic areas were observed, one in Chandeleur sound at 88 square miles observed on June 18th and one in Breton Sound at 205 square miles observed on June 24th. We have seen Hypoxia develop on a yearly basis in this area (http://saveourlake.org/PDF-documents/our-coast/Chandeleur-2013-Hypoxia-July2013.pdf ).
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. During this Hydrocoast period, oyster harvest area 8 (spanning the Bird's Foot Delta) was closed while all other areas east of the Mississippi River remained open. 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, fecal coliform and remnants from the BP Oil Spill are shown.