Some Hydrologic Effects of Canal 111 near Homestead, Florida

Barnes, Harry; Meyer, Frederick; Hartwell, James

The feasibility of a salinity-control structure on Canal 111 in the immediate vicinity of U.S. Highway 1 was under consideration. One aspect of the overall evaluation was fresh-water seepage into the canal. An analysis of ground-water records in the basin and of the streamflow record for structure S-18C revealed the following information: 1. For the condition where the earthen plug is removed and Canal 111 below S- 18C is considered a sea-level, saline-water canal, the average annual seepage of fresh water into Canal 111 between s - 18C and U.S. Highway 1 from the limestone aquifer is estimated to be 39,000 acre-feet. This volume of fresh water will be discharged to the ocean each year. The direct loss of this fresh-water volume will reduce the overall capability of the aquifer to resist saltwater encroachment. A salt-water condition in Canal 111 below S-18C and in the proposed aerojet spur canal will accelerate inland encroachment of salt water especially during drought periods. During wet periods overland flow from the area immediately north of Canal 111 between S-18C and U.S. Highway 1 will discharge into the canal through eleven (11) gated culverts. After reaching the canal, this water will be discharged to the sea via the canal and, at times, by southward flow through the gaps in the spoil banks on the south side of the canal. The rate of overland flow into the canal from the north side and its subsequent disposal depend on factors such as tide level, wind speed and direction, the discharge rate through S-l8C, and operating procedures for the culverts. The rate of flow southward through the spoil gaps will also depend on these factors. An evaluation of the overland flow from this area was impractical considering the meager data available. 2. For the condition where Canal 111 is maintained as a ·freshwater canal by means of a salinity-control structure at U.S. Highway 1, the rate of fresh-water seepage into the reach of Canal 111 between S-18C and U.S. Highway 1 will depend on the elevation of water levels in the canal and the adjacent water table. The rate of fresh~water seepage into the canal will decrease as the canal level is increased. For example, the average annual fresh-water seepage into this seven-mile reach with the canal level maintained at 0.5, 1.0, and 1.6 feet msl (mean sea level) is estimated to be 39,000, 22,000, and 3,000 acre-feet, respectively. These reductions in the volume of seepage as the canal level is increased imply an increase in fresh-water storage above the structure. Water in excess of the storage capacity of the area , that is not lost to the atmosphere by evapotranspiration, will help maintain higher water levels to the east and southeast. During wet periods, when the ground-water levels in the area north of S-18C are high, .seepage will accrue to Canals 111 and 111 E. The seepage flow discharged through S-18C will recharge the aquifer along C-111 downstream to U.S. Highway 1. If 'the structure at U.S. Highway 1 is designed to maintain water in the canal at levels of about 1.0 foot or higher, then some of the seepage flow through S-18C will flow southward through the spoil gape.