FLOOD02086

• County: Gilpin
• Community: Black Hawk/ Central City
• Title: Black Hawk/Central City Colorado Radar Rainfall Reconstruction
• Date: 7/30/1998
• Prepared For: Gilpin County
• Prepared By: HDR Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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I!

(3) Peak 30-minute rainfall = 0.70 X (Peak 60-min rain)

(4) Peak 15-minute rainfall = 0.60 X (Peak 30-min rain)

where the Precipitable Water Index (PWI) is a measure of the amount of water in the atmosphere
from the surface to about 20,000 feet above the ground. A matrix of rainfall rates, which are
derived from surface temperature and dew point fields are used to initialize the 2-D model output.
For each set of surface temperature-dew point combinations, a unique radar-rainfall relationship is
created for precipitation mapping, In effect the peak 60, 30, and 15-minute rainfall rates are
related to the 50 dBZ or greater radar reflectivity values within the precipitating cloud. Lower
rainfall rates are down,stepped to correspond with lower radar reflectivity values.

2.1 Event specific radar-rainfall estimate methodology

A nearby ALERT weather station's temperature/dew point values were used to initialize the HDR
2-D cloud model for rainfall estimation, For the event of July 30th, 1998 the Blue Mountain ALERT
weather station, owned and operated by the Urban Drainage and Flood Control District, was used
in the model initialization process.

An example of this relationship and the calculations for the afternoon of July 30, 1998 will now be
discussed to illustrate the points just made. The temperature and dew point information were
plotted on a Skew-T, Log P diagram, containing information derived from a radisonde, launched at
Denver, Colorado around 500AM MDT, to calculate the PWL The calculated PWI was 1.28"
adjusted for an elevation of 8,500 feet above sea level, while the depth of the warm updraft layer
was 2.3 KM. The next step entailed solving equation (2) to calculate peak 60-minute rainfall rate.

Values for PWI and the depth of the warm layer were inserted into equation (2), and the result
was inserted into equation (3), resulting in a peak 30-minute rainfall rate of 2.74 inches, This
rainfall rate was divided by 5, which corresponded to a 6-minute peak rainfall of 0.55". The peak
6-minute rainfall values were assigned to the grid squares covered by radar reflectivity values of
50 dBz or greater. Lower rainfall rates were assigned to grid squares associated with lower
reflectivity values and are shown in Table 1, The peak 30-min rainfall rate was used, based on
the fact that the duration of the heaviest rainfall was around 30 minutes,

0.09"
0.13"
0.19"
0.27"
0.38"
,,50.0.. 0.55"
Table 1 Relationship between peak 6-minute rainfall rates and radar reflectivity values,

The radar reflectivity data field was navigated to their corresponding grid square and assigned a
reflectivity value of 0 through 11. Table 2 shows the reflectivity values and their associated dBz
values, The resolution of the radar reflectivity data allows it to define the radar reflectivity for 0,60
by 0.60 square mile area, Figures 1 and 2 show the HDR

3