FLOOD07101

• County: Statewide
• Basin: Statewide
• Title: A Numerical Modeling Study to Investigate the Assumptions Used in the Calculation of Probable Maximum Precipitation
• Date: 3/1/1999
• Prepared By: Water Resources Research
• Floodplain - Doc Type: Educational/Technical/Reference Information

ABBS: INVESTIGATION OF PROBABLE MAXIMUM PRECIPITATION ASSUMPTIONS

Depth-area curves are calculated for the standard durations of 400
2, 12, 24, 36, and 48 hours.
Depth-area curves, for durations of 2 hours and 24 hours, 3S0
are shown in Figure 8 for the control simulation, EC86_3, and
EC86 _5. AlSD shown is the "observed" curve for this storm for 300
a duratiDn of 24 hours, A comparison of the observed and
control curves shows that the model is producing high rainfall 250
amounts for small areas. The reason for this discrepancy is ~
E
unknown, but it may be associated with the inability of the E zoo
model to adequately resolve features even at the relatively high ~
ci.
resolution used in this work. For areas >1000 km2, there is u
good agreement between the two CllIVCS. G 150
.
"-
The depth-area curves for EC86_3 and EC86_5 show
grealer rainfall depths for all areas and for durations of 2 and 100
24 hours. The maximization factors associated with these sim~
ulations are 1.15 and 1.3, respectively. These values are much 50
0
10
(a)
..,..,....Loo.,~..,.. .........j.... 650
600
50 SSO
500
4!lO
~ 400
E 3S0
.5-
ci. 300
U 250
L_ooo.o G
.
"-
zoo
150
...-................ u..........u........ .r................. 100
50
0
10

iP
LI604.3

.. ......... ~. ......... .......

L-25.0

H..!.
no. ..~..

L_...,

Figure 7. The (a) convergence and (b) topographic compo-
nenrs of the storm. Contours are as for Figure 3b.

793

(a)

- - - M.F. = 1.30
- ...5 deg
- - - M.F. = 1.15
- t-3 d.g
- Control

100

tOOOOO 1oo00b0

1000

10000

Area (km')

(b)

w

H

. - . Observ-d
-- - J.l.r. = 1.30
- 1"5 d.g
--- ).t.r. = 1,1'
- t-3 deg
- Control

100

10000

100000 1000000

1000

Area (km')

Figure 8. Depth-area curves for (a) 2 hours and (b) 24 hours
for the control simulation (thick, solid black curve), EC86_3
(solid, shaded curve), and EC86_5 (thin, solid, black curve).
Also shown are curves corresponding to maximization factors
of 1.15 (dashed, shaded curve) and 1.3 (dashed, black curve).
The observed depth-area curve (dashed, thick black curve) for
a duration of 24 hours is shown in Figure 8b. The letters 0, C,
W, and H indicate the 2- and 24-hour probable maximum
precipitation (PMP) values for the catchments of the Oberon,
Chiftey, and Warragamba Dams and the Hawkesbury-Nepeun
catchment, respectively.

lower than the maximization factor of 1.8 used by the Austra-
lian Bureau of Meteorology to maximize this storm (L Minty,
private communication, 1997). For both cases the maximized
curves (derived by applying (5)) lie below the corresponding,
simulated depth-area curve. This indicates that for durations of
2 and 24 hours the current maximization method is unueres.
timating the precipitation for all areas. For this case study the
model produces between 15% and 25% more precipitation
than the maximization relationship of the current PMP tech-