FLOOD06964

• County: Adams
• Community: Unincorporated Adams County
• Stream Name: South Platte
• Basin: South Platte
• Title: Unincorporated Adams County Community File
• Date: 1/1/2001
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Community File

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Six Streams Group I Hydrology, Second Creek & Third Creek
April 1, 1975
Page 4

In an effort to verify the reasonableness of the 100-year
flood flows calculated for Second and Third Creeks, the flow
magnitudes were compared to other available information. The
other available sources used were the Corps of Engineers HEC-l
Computer Program, a flow per area curve developed for the
Federal Insurance Administration Studies and flows calculated
for the First Creek Master Plan Phase A Report.

The flow comparisons are shown in Table 5. The most signifi-
cant difference is seen in the flows developed from the FIA
Q vs. Area Curve. The FIA curve was produced by developing
an average curve for the eastern plains area of Colorado.
When using this method, the flows were approximately 50% to
80% greater than those calculated in this analysis. This
difference can be attributed to the fact that statistically
the Second and Third Creek Basins are in an area of minimum
rainfall as shown by the isohetol curves in the Urban Storm
Drainage Criteria Manual.

Another method used was the Corps of Engineers HEC-l Flood
Hydrograph computer program. This program was used to compute
basin flows for Second and Third Creek for the study being
carried out for the Federal Insurance Administration. In this
study, the flows developed for Second Creek were 6% higher than
the flows developed by the FIA study. This is due primarily to
the assumption of future developed conditions. For Third Creek,
the calculated values were 10% lower than the appropriate FIA
Study. Again, this can be attributed tothe different methods
of calculation. With the small error between the two methods,
it can be concluded that the values obtained by the CUHP and
the HEC-l Program are within acceptable limits and are there-
fore reasonable.

The flows calculated in this study were compared to those flows
developed for the First Creek Flood Plain Master Plan. Due to
the difference in basins and areas, a rough comparison was made
of the discharge per unit area index. The flows generated in
this analysis are approximately 30% higher than those generated
for First Creek. A large portion of this difference can be
attributed to the fact that the First Creek Study used a per-
vious storage value of .4" and an impervious storage value of
.1" compared to .35"-.30" pervious storage and .05"-.1" for
impervious storage in this study. In completing this analysis
it became apparent that the values used for pervious and im-
pervious storage have a very significant effect on the total
runoff from each basin. This is shown in Table 6, Effects of
Pervious and Impervious Storage. The table gives an indication
of why the values developed for our study are 30% higher than
those developed for the First Creek Study.