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 July 13, 2015 through July 19, 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 in Lake Pontchartrain, Biloxi Marsh and Chandeleur Sound remained similar to the last Hydrocoast period. Lake Borgne freshened slightly. Breton Sound was also slightly fresher. 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 = 3,417 to 2,812 cfs
- Tangipahoa = 874 to 526 cfs
- Tickfaw = 671 to 360 cfs
- Amite = 3,347 to 1,200 cfs
- Caernarvon Diversion = 157 to 228 cfs
- Mardi Gras Pass = 4,288 to 4,882 cfs
- Violet Siphon = Closed
- Bohemia Spillway = 5,244 to 15,976 cfs
- Bonnet Carré Spillway = 510 to 3,500 cfs
- Bayou Lamoque = 2,423 to 2,887 cfs
- Fort St. Philip = 45,024 to 52,566 cfs
- Baptiste Collete = 94,154 to 106,346 cfs
- Grand and Tiger Pass = 70,483 to 84,885 cfs
- Main Pass = 65,950 to 77,839 cfs
- West Bay = 54,490 to 63,226 cfs
- Pass A Loutre = 47,210 to 57,327 cfs
- Southwest Pass = 255,000 to 289,153 cfs
- South Pass = 113,356 to 129,470 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 light rainfall across most of the basin. Rainfall ranged from 1 to 3 inches. Resultant winds were mostly from the southwest but also the northwest. Wind speeds ranged from 2 to 8.8 m/s (2 to 20 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 counts at Bogue Falaya and Amite at Idle Hour Road. High counts were found at Abita River, Yellow River/LA22, Natalbany River/LA22, and Bayou Manchac at Amite. 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 results of an aerial oyster boat survey conducted at 7 am on July 13th are shown. There were 20 oyster boats counted, mostly in the Biloxi Marshes. 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.