FLORIDA GEOLOGICAL SURVEY


greatest danger of salt-water encroachment occurs during periods of low
rainfall in areas of heavy pumping.
 Lateral salt-water encroachment occurred along the north and south
shoreline of Gulf Breeze peninsula in 1954. This encroachment was a
result of low ground-water levels caused by the low rainfall. The salt
water moved inland 30 to 40 feet from Pensacola Bay and 20 to 30 feet
from Santa Rosa Sound.
 PERIODS OF HIGH RAINFALL
 Periods of moderately high rainfall are important because the amount
of ground water in storage is increased and streamflow becomes greater.
However, severe problems such as floods or ponded water may result
from excessive rainfall.
 Every stream in the area responds to rain falling in its basin, The
 height to which a stream will rise depends on the amount and distribu-
 tion of rain and the physical characteristics of the river basin, The stages
 of some of the smaller streams in the area vary as much as 15 feet while
 stages of some of the larger streams vary as much as 36 feet. The
 magnitude and frequency of floods are important to engineers in charge
 of designing river appurtenances (bridges, dams) and other structures
 in the flood plain. Data on the magnitude and frequency of floods in the
 area are presented in figure 20.
 Ponded water occurs in areas where the land is flat and drainage fa-
 cilities are inadequate. Examples of this are found near Pensacola. Water
 stands in low spots for varying lengths of time after each intense rain.
 This pounded water leaves some areas only by evaporation and infiltra-
 tion. In these areas the problem can be made worse by developments
 such as paved streets, houses, and lawns, that cause an increase in the
 rate of runoff to the pounded areas. The problem of ponded water can
 be solved by providing adequate drainage.
 Ponded water can also occur where the water table intersects the
 land surface. This happened near the city of Gulf Breeze during the fall
 of 1959 when intense rains caused the water table to rise rapidly (fig.
 3S) and low lands were flooded. The figure shows the slow decline of the
 water table following the abrupt rises. Surface drainage, buried tile
 drains, or pumping are the most effective methods for removing the
 excess water.

 MAN-MADE PROBLEMS
 Water resource problems caused by man are usually associated with
 heavy withdrawal of ground water in an area; the pollution of ground