ML20037A604

Revised Pages 4,5 & 6 of Review & Evaluation of Dames & Moore Verification Study of Hurricane Storm Surge Model

ML20037A604
Person / Time
Site:	Crystal River
Issue date:	10/15/1973
From:	ARMY, DEPT. OF, CORPS OF ENGINEERS
To:
Shared Package
ML20037A603	List: ML20037A601 ML20037A604
References
NUDOCS 8003160179
Download: ML20037A604 (3)

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Cc=nents on the Statistical Treatment and Accuracy of the DSM Results.

1.

The accuracy of calculated hydrograph relativ'e to the observed -

hydro;raph was checked by.DSM by the following nethods: a) A point-by-point. comparison of percent difference between the two hydrographs at each-tiac ste-b) computing the sun of-the squares of the difference between the two.ky,drographs' for the duration of the hydrograph, and c) conparing the average percent differences of sections of the hydrographs.

2.

All three methods are very similar.

Mathed (a) presents the probica in that if the recorded and cal.culated hydrographs are out.of phase with the same general shape, something.which is'not uncormon, point-(

by-point comparison of pprcent differences is very misleading.

Furthermore there may be positive as well as negative differences when the hydrographs cross.

All of the hydrographs analy:ed by DSM are out of phase, and cross.

Method (b) that of using the sum of the squares of differences between the two hydrographs presents the problen in that the hydrographs again are.

out of phase and, in addition, by squaring the sums even the negative differences become positive and the "best fit" is not a true "best fit."-

Method (c), that of looking at the av_erage percent differences of sections of the hydrographs and comparing.each one-third portion with. emphasis on the middle-third portion of the hydrograph,.is ecually ineffective as a statistical method of cocparison for the same reasons as cited.for method (a) and (b).

3.

To check on the validity of the DO! wind stress-equation using their coefficients and their pressure-dependent factor and in order to ccapare it to CERC's wind stress equation, using the same.botton friction factor, the following test was applied. Using the parameters of the' probable maximum hurricane, P:!H, (Po = 26.7 in. of Hg., Pn = 31.25 in. Hg.,

R = 24 n. ci., Vt : 20 knots, and U 3x = 149.9 cph) wind speeds and pressures were calculated along the hurricane's price vector for increasing integer distance intervals of the ratio, r/R.

Utilizing these data, both the DSM and CERC wind stress equations were solved and respective values of wind stress coefficients were tabulated in Table 1.

To illustrate graphically the differences of both the CERC with DSM wind stress coefficients with-increasing wind speeds, a plot was prepsred (Figure 1).

Tha' percent difference between the CERC and the DS'i wind stress coefficients were similarly calculated for each wind speed interval and plotted on the same graph.

Exc=ination of these graphs indicates that both the CERC and DSM wind stress coefficients have approxinately the same initial values up to wind speeds of 20 mph.

Beyond wind speeds of 20 mph, the CERC calculated wind stress coefficbnt increases at a greater rate than the DSM coeffi-cient; at 30 mph, the difference increases to 94, at 50 mph to 13.5%,,.and at 100 mph to 25.25.

Also apparent from this graph.is that the pressure factor introduced in the DSM wind stress calculation,. cnnnot possibly WlH RB 4

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8.0 03160 I-7f

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compensate for increased str-ass due to' stronger uinds as'the region'off

-r:axinum winds. is approached. As shown, the speeds between--80-90 uph the.

U;' wind stress coefficient approaches a~ plateau value and retains th'e

same or an even lower value in the vicinity of maxinum winds ~ (149.9,cph).

This does not appear 1to be consistent with provious investigations ~since:

wind speed is a nore important factor than atrospheric pressure int

~.

j determining the wind stress, a

TABLE 1 Comparison of CERC and Dames and Roore-i Wind Stress Coefficients

1. /R W'

P ~

CERC - k

~D4M - K Difference J4-

-6

-6 1

149.9 28.4 3.40 X 10 2.37 X 10 30.39 2

137.9, 29.5 3.36

- 2.44 -

= "

27.'47' 3

100.0 30.0 3.15 2.36

~25.11 4

86.6 30.2 3.04 2.31 j

.23.9 5

76.4.

2.26' 30.4 2.931 22.7 6

68.2 30.6 2.82 2.22

'21.46 7

60.6 30.6-2.70 2.15 20.5 S

53.7 30.7 2.57 2.07-19.2 9

48.0 30.8 2.43 2.00 17.7 10 43.0 30.8 2.29 1.92 16.3.

12 34.8 30.9-

2.01 1,76 12.8 l

14-28.9 30.9 17 6 1.60l 9.0 l

16 24.3 31.0/

1.53 1.46.

4.6-

{

18 20.5-31.0 1.34 1.33 0.62 20 16.9 31.0 1.22 1.23

- 1.29

.22 13.8 31.0 1.21 1.21 0

Wind Stress Equations'.

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Figure 1.

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