OCR Text |
Show as the lake level increased, deepening the water in the breach, and as the head differential has been reduced. Causeway Permeability GSL brine can flow not only through the breach and culverts in the causeway but also through the porous crushed- rock fill of the causeway itself. The permeability of the SPRR causeway fill material is a major factor in determining the rate at which water moves either south- to- north or north- to- south through the causeway. This rate is also influenced by the head differential across the causeway, and to a lesser degree by the viscosities of the different saline waters. Under a given head differential, the greater the causeway permeability, the greater the rate of flow. The causeway's permeability is controlled by the size and number of the interconnected openings within the fill material, and the length of the fill path the water has to travel. For a given interconnected opening size and number, the overall permeability near the narrow top of the causeway is greater than at the wide bottom. Within a narrow lake- elevation range, and similar causeway- permeability conditions, the head differential across the causeway should be similar as the net inflow into the south arm is balanced by flow through the causeway. Somewhat higher head differential can be expected during a rising lake scenario than during stable- or dropping- lake conditions. If the permeability decreases under the given conditions, the head differential will increase to compensate for the change. The increase in head differential provides a greater south- to- north driving force. It is believed that the permeability of the SPRR causeway was decreased during the 1980s high- water years as shown by a comparison of the two pre- breach head- differential groups and the post- breach group shown in figure A. The time periods of the three groups, plotted on a south- arm hydrograph are shown in figure B, and the head differentials between the south and north arms across the causeway, over time, are shown in figure C. Lake- elevation data used in figures A and B are USGS provisional lake- level data; south- arm data are corrected using Wold/ Waddell corrections developed for the lake- salinity model. Pre- Causeway Salinity Conditions It is assumed that the entire lake was relatively well mixed vertically as well as laterally prior to the construction of the SPRR causeway as there is little or no evidence to the contrary based on north/ south or top/ bottom brine densities observed at the same time. Figure D shows the relationship between lake level and wt% salt in the brine from about 1850 to about 1965, or just after the construction of the SPRR causeway. Data for the pre- causeway plot came from a number of early publications. For comparisons, the lake level- wt% salt in brine plots for the post- causeway south and north arms of the lake are also given for the 1966 to 1987 period of time. Data for the south and north- arm data are from UGS GSL brine chemistry database. 374 |