WATER RESOURCES OF ORANGE COUNTY 43
in the use of flow-duration curves for predicting future flows are applicable to the prediction of future stages by use of stageduration curves based on short records can be adjusted to longer periods by correlation with records for a long-term station. This has been done for the stage stations established for this investigation.
Knowledge of the magnitude and probable frequency of floods is essential to the proper design and location of water-related structures such as dams, bridges, culverts, levees, etc., and any other structures that may be located in areas subject to periodic flooding. Such knowledge is also useful in solving problems associated with flood insurance and flood zoning.
Because flood-frequency information is often needed for locations where no flow data are available, methods have been devised that permit determination of the probable magnitude and frequency of floods at any point along a stream.
Methods of determining the probable magnitude and frequency of floods of recurrence intervals from 1.1 to 50 years on streams in Orange County are given in U. S. Geological Survey Water-Supply Paper 1674 (Barnes and Golden, 1966).
Because of the importance of stage in floods on the main stem of the St. Johns River, stage-frequency data are presented in the form of water-surface profiles for floods of recurrence intervals of 2.33, 5, 10 and 30 years (figure 17). These profiles are for the reach between the southern Orange County line and State Highway 46.
A low-flow frequency curve shows the average interval between the recurrence of annual low flows less than the indicated values. Curves for durations of 7, 30, 60, 120, and 183 days, 9 months, and 1 year are given. These curves are useful in determining whether the natural flow of a stream is adequate for a particular development and, if not, how much the natural flow must be augmented from storage or some other source. In Orange County only the St. Johns River, the Econlockhatchee River, and the Wekiva River has sufficient low flow to warrant analysis.
SURFACE DRAINAGE
Surface water from the southwestern 341 square miles of Orange County drains southward into the Kissimmee River. Surface water from the eastern and northern 662 square miles of the county drains northward into the St. Johns River. Figure 18