FLOOD10114

• County: Saguache
• Stream Name: Saguache Creek
• Basin: Rio Grande
• Title: Radar Rainfall Reconstruction of Saguache Creek Flash Flood
• Date: 7/25/1999
• Prepared For: CWCB
• Prepared By: HDR Engineering, Inc.
• Floodplain - Doc Type: Flood Documentation Report

~~' -

0.72"
1.20"
1.92"
2.40"
3.36"
3.36"
Table 2
Relationship Between Peak 60-Minute and Peak 30-Minute
Rainfall Rates and Radar Reflectivity Levels.

.''''':" 55 ,,,.t'.t.J.1'-,,'''o;"
~. .'f....,

0.50
0.85
1.34
1.68
2.38"
2.38"

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If you insert the values for PWI and the depth of the warm layer into
equations (2) and (3), the peak 60-minute rainfall rate is 3.42" and the peak 30-
minute rainfall rate is 2.38 inches. These rainfall rates are assigned to the grid
squares covered by radar reflectivity values of 50 dBZ or greater. Lower rainfall
rates are assigned to lower reflectivity values as shown in Table 2.

,'~ :';-'" t \ Radar.':Ii:-\:j:h~ 1'~!
_ . .P11ft.......'" . ,1"1:.'
'.' - dBZ - Level.
". K, "'25W'If."I;.;;"'F

30'- -.

35 . "
.", ~ _r 40 "''4''T'lT'.

- 45 'hJ.,,'" .
"50, .

:. 'fi" :

Peak 60-mln

HMS utilized the Doppler radar from the National Weather Service (NWS)
WSR-88D located at Pueblo, Colorado. This radar is located about "'115 miles
from the basin and provides fairly accurate radar reflectivity observations, HMS
notes that the Pueblo is slightly affected by the passage of its beam over higher
terrain to the west. HMS estimates that up to 30% of the lower half of the beam
may be attenuated as it shoots towards the Saguache location along a 255-
degree radial. However, this factor matters little as HMS does not use the
absolute strength of the radar reflectivity to calculate the rainfall rates but rather
uses the strength of the maximum reflectivity to identify the location of the
heaviest rainfall. The rate is calculated by the HMS 2-D cloud model,

As the radar reflectivity data field is received and mapped; each grid
square is assigned a reflectivity value of 0 through 7. Table 2 above shows the
reflectivity values and their associated dBZ values.

3.4 Analysis of Radar Reflectivity and Rainfall

This section will encompass an examination of the radar reflectivity
pattern and the rainfall pattern that accompanies it. The resolution of the radar
reflectivity data allows it to define the radar reflectivity for 0,65 by 0.65 square
mile area, Figure 6 shows a summation of the reflectivity value for each grid
square for the duration of the storm. Note that a rainfall rate (See Table 3) is
assigned to each radar reflectivity factor for each observation and the storm total
rainfall is a summation of these individual rainfall observations. Note that the
grid is segmented into an outer and inner grid and that the inner grid is split
along the stream location of the valley of the basin,

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