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<br />return period and a regional skew (2). values of normalized stage(NS) can
<br />
<br />be read from Fig. 5. The normalized stage is then multiplied by the "stage
<br />
<br />index slopel" (Eq. 6) to obtain the height of the desired flood stage above
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<br />that for the 10-year flood. The stage for a flood of frequency f is thus
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<br />Sf SIS (NSf) + S10""," ,.......""",,(7)
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<br />Relationshio of Sta2e Index Slooe to Basin Characteristics
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<br />One would generally expect the stage index slope to increase with
<br />
<br />tributary drainage area (because of larger flows) and the steepness of
<br />
<br />the relief near and the degree of flow constriction at the flood plain
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<br />Accuracv of Results Usinll: the "Sta2e Index Slooe" Method
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<br />location. Fig. 6 plots drainage areas versus "stage index slope" for the
<br />
<br />51 stations used in this analysis. The "stage index slope" is seen gen.
<br />
<br />erally to increase with drainage area; however, there is a wide range of
<br />
<br />"stage index slopes" for any particular drainage area size. The topo-
<br />
<br />graphy associated with extreme values of the IIstage index slope" is
<br />
<br />shown on Figs. 7 through 10. High vslues are generally associated with
<br />
<br />steep topography, particularly where the stream has a relatively flat
<br />
<br />gradient, Whereas, low values are generally associated with flat topo.
<br />
<br />graphy or steep stream slope. High values are also found in flat topo.
<br />
<br />graphy in coastal areas where flooding is affected by hurricane tides.
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<br />In order to evaluate the accuracy of the "stage index slope" method,
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<br />37 stage frequency curves were obtained from U. S. Geological Survey's
<br />
<br />Hydrologic Atlases, normalized, and tabulated with regional skews (2) on
<br />
<br />Table 3. Stages for the rarer floods were estimated by the "stage index
<br />
<br />slopett method and compared with the stages published in the .Atlases.
<br />
<br />17
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