regions when maximized for moisture and transposed. The 10,000-sq.
mi. 24-hr. moisture maximized value for each of these storms is denoted on figure 2.22 in its place of occurrence by boxed values.
Serial numbers to identify the storms are also included in the boxes.
2.5.5.2 Moisture maximized 10,000-sq. mi. 24-hr. depths from these and other storms were transposed to each of a number of grid points and the resulting largest value at each grid point is shown on the chart. The serial number of the storm (or two storms-see below) producing this largest value is shown beneath the rainfall depth at each grid point.
2.5.5.3 Depth-duration smoothing. At grid points with two identification numbers, the 10,000 sq. mi. value is derived by drawing a smooth depth-duration curve through highest maximized values for all durations from 6 to 72 hours. Figure 2.23 is an example of such a depth-duration curve for the point at latitude 40 oN. and longitude 9SoW. This point, close to the center of figure 2.22, is identified by an asterisk. It is seen here that the 10,000-sq. mi. 24-hr. value is "help up" by two storms:
No. 5 for 12 hours and No. 3 for 36 hours.
2.5.5.4 Depth-area smoothing. Smoothing and envelopment across area sizes should be used in the same manner as the durational smoothing. In such a procedure, a 10,000-sq. mi. value might be
"held up" by 5000- and 25 ,OOO-sq. mi. values. This test TNas applied to figure 2.22 but in this particular instance the cross-area smoothing did not increase the 10,000 sq. mi. values.
2.5.5.5 Drawing of generalized map. Isop1eths of P}W
UPPER FIGURE IN BOX: MOISTURE-ADJUSTED
S;,TORM RAINFALL, INCHES.
OTHER UPPER FIGURES: MOISTURE-ADJUSTED rRANSPOSED STORM RAINFALL, INCHES.
LOWER FIGURE: STORM NUMBER. lil'~
~
I'\)
Figure 2.22 - Generalized 24-hr. PMP for 10,000 square miles (tentative)
\D W
94 ESTI~ffiTION OF MP.XIMUM FLOODS
18
•
2curve
STORM NO. DATE STORM
LOCATION
5 JUNE 1905 SE IOWA
3 MAY 1943 E OKLA.
2 OCT. 1908 C OKLA.
Example of enveloping depth-duration for 10,000 square miles. Applies to 40oN.,980W • (Location is denoted by star near center of
figure 2.22)
are drawn to the transposed, maximized and durationally smoothed storm values of figure 2.22. Guidance for the pattern is given by several climatological rainfall charts. (See par. 2.5.4.6.) These included envelopes of maximum 24-hr. rain, a map of all areal
storm values maximized in place of occurrence, and maximum
weekly rain within climatological subdivisions. The result gives 24-hr. PMP values for any drainage area of 10,000 sq. mi.
2.5;5.6 PMP for other durations and area sizes. As mentioned in paragraph 2.5.4.5 similar procedures can be carried out for mapped values for any other combination of duration and area size. A start on such maps are the smooth depth-duration and depth-area curves for each grid point described in paragraphs 2.5.5.3 and 2.5.5.4. From these values for combinations of areas and durations including those at the extremities of the area1 and durational requirements are selected, plotted on maps, and enveloping isopleths drawn. A resulting pattern may need refinement in light of other rainfall parameters. For example, a 24-hr. rainfall for lOO-sq. mi. PMP map should have more resemblance to mapped and enveloped highest observed I-day rains than the 24-hr. 10,000 sq.
mi. map.
2.5.5.7 From maps for the selected key duration-area combinations, depth-duration-area (DDA) diagrams of PMP can be made for each grid point. If these are expressed as a percent of a basic duratio~ and area, relatively few such DDA diagrams may suffice for the user to obtain Pt~ for any intermediate duration and area.
96
ESTIMATION OF MAXIMUM FLOODS2.5.6 Generalized PMP charts for mountainous regions 2.5.6.1 The role of topography in determining locations and amounts of rain is very evident from annual or seasonal precip-itation charts. These show large differences in amounts over short distances. Individual storm rainfall patterns in mountain regions often have similar distributions, but the degree of similarity varies from storm to storm depending on the extent that non-orographic
storm mechanisms are effective on the orientation of slope relative to inflowing moisture, and other variables.
2.5.6.2 In constructing generalized charts of P~~ in such a topographically influenced region one encounters the following problems:
A. The general level - topography may place stringent limitations on the transposibility of storms.
B. The pattern - how heavy precipitation varies with
topography is known only for a few regions where there are sufficient observational data. The general character of the basic physical processes is clear enough and has been discussed in section 2.4.
But the precipitation variation with slope, elevation, and other topographic factors is not too well defined quantitatively. One reason for this is that throughout the world rain gauges tend to be located in valleys where people live.
C. The labour required - if the generalized charts are constructed for a considerable area of rugged terrain, then depicting all the precipitation variations is indeed a great task.
2.5.6.3 These problems are in part offset by the fact that orographic rainfall is more subject to theoretical synthesis
than non-orographic precipitation, as discussed in sections 2.2 and 2.4 The three problems enumerated are handled as follows:
A. An indirect method of transposition is applied, or reliance is placed on computation of precipitation by the orographic model or a combination of these, to yield enough values to control the general level.
B. Where inflow wind direction in storms is relatively constant and mountain forms relatively simple and of approximately normal orientation to this wind, a sufficient number of orographic
computations with the orographic model (par. 2.4.7.4) can be carried out to define the pattern as well as the general level.
This procedure was followed in one recent study of the D.S. Weather Bureau (32). This requires sufficient wind data to establish
maximum winds and sufficient rainfall data at different elevations in at least a few places to verify the model. Othen~ise either of two courses is followed. One course is to develop relations of intense storm precipitation to elevation and other topographic factors from available storms, then apply these relations in connection with the topographic map. Another course is to assume proportionality between PMP and some rainfall parameter that has already been mapped such as mean seasonal precipitation 2-yr. 24-hr.
precipitation (for caution see par. 2.5.4.8).
c.
The matter of avoiding excessive detail is handled by relating the Pl~ estimates on the generalized chart to atopographic map of adequate but not too much detail or by the direct comparability assumption mentioned above.
2.5.6.4 By all the above methods professional judgment