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Show levels increased the culverts were both submerged and wave action plugged them with .5 to 1.5 inch gravel. Since submergence and culvert plugging seemed to occur together, this was simulated by code in the model which set culvert flows to zero and bypassed culvert subroutines whenever submergence was predicted. Another change in GSL from 1986 to 1987 was the pumping of brines from the north arm of GSL to the Newfoundland Evaporation Basin for flood control. It was estimated that this pumping left a total of 600 million tons of salt in the Newfoundland Evaporation Basin, decreasing the total salt in GSL from 4.9 to 4.3 billion tons. In the most recent study of GSL water and salt balance by USGS for the period from 1987 to 1999, it was discovered through model calibration that the causeway dike has tightened further to twenty percent of the 1973 study permeability. The more recently constructed portion of the dike above elevation 4200 feet is approximately 80 percent as permeable as the 1973 dike hydraulic conductivity values. As a result of the plugging of the causeway culverts and tightening of the causeway fill, higher salinity north arm brines have not been able to adequately replenish the dissolved solids load of the south arm. Brine shrimp populations, which require a salinity range somewhere between 13- 19 percent salinity to thrive, have plummeted and harvests of eggs have been disappointing. South arm salt industries are now seeing the lowest south arm salinity since the 1986- 87 period when lake elevation reached record highs. The importance of the causeway culverts has become apparent since they were designed to return north arm brines to the south and now were plugged and inoperative. DWRe and USGS began to see the need of modeling free- flowing culverts to see if the answer to the lake's salinity problems could be found in keeping the culverts cleaned. Studying the submerged culverts was difficult with the model modified by Holley, et. al. ( 1976) since it could only model flow regimes with a free upper surface. Dr. Edward Holly was again commissioned by USGS in 1998, this time to derive the equations governing submerged culvert flow. These newly derived equations were incorporated into water- salt balance model subroutines by Craig Miller of DWRe in 1999 and used to study the effect that the free- flowing submerged culverts might have on the salt balance of the lake. Dr. Holly determined that the equation for critical flow under submerged conditions remained the same as equation ( 1) above. From the general equation for momentum and the boundary conditions of a submerged culvert, Dr. Holly derived the equations for the depth of the culvert versus distance along the barrel to be as follows: 365 |
