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 Hydrocoast Maps.
Subscribe: To receive Hydrocoast products by email please email Hydrocoast@saveourlake.org.
Click on link below map for larger image.
The Hydrocoast maps for the week of May 6 -12 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.). The trend for the current Hydrocoast is freshening of the basin when compared to the last hydrocoast period(April 8-14), due to high rainfall in the basin during this hydrocoast period and the week prior. Inland waters were fresh with Lake Pontchartrain below 3.0 ppt and Lake Borgne below 5 ppt. Fresh water was also in Breton Sound and along the Bird's Foot Delta. During this Hydrocoast period there was up to 7 inches of rain over the Bird's Foot Delta and 2 to 5 inches over the rest of the basin. The resultant winds for this period oscillated were variable coming from all directions at some point in the week except from the north. Winds ranged from 2 to 11 m/s (0 to 25 miles/hr). Average daily discharge for the week increased from the last Hydrocoast period all local rivers and diversions. Average daily discharge difference between last Hydrocoast and the current was:
- Pearl River = 31,002 to 33,757 cfs
- Tangipahoa = 611 to 1,602 cfs
- Tickfaw = 223 to 543 cfs
- Amite = 1,840 to 4,464 cfs
- Caernarvon Diversion = 1,221 to 1,220 cfs
- Mardi Gras Pass = 1,339 to 3,310 cfs
- Violet Siphon = 0 to 280 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 and dashed lines represent 0.5 increments. The salinity it highest out past the Chandeleur Islands (red lines, 36 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 and canals (with associated spoil banks). This time period is characterized the movement of fresh water into the basin. 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 Borgne.
The second map shows the salinity contours for the week of April 8-14 laid on top of a habitat/land -use map, showing where fresh to salt marsh and swamps are found in the basin. The third map shows the salinity contours in conjunction with weekly rainfall and wind roses in the Pontchartrain basin. There was 2 to 5 inches of rain in the Pontchartrain Basin during this time period with 7 inches over the Bird's Foot Delta. Winds ranged from 2 to 11 m/s (0 to 25 miles/hr) and were variable, coming from all directions except the north.
You can find more information on the Hydrocoast Maps in our May 2012 edition of LPBF's Lake & Coast (page 20).