|
<br />A regional relationship between basin characteristics and stream flow characteristics, as
<br />developed by the USGS (Reference 21), was used to define the discharge-frequency data for
<br />Monument Creek Tributary. The peak discharges for Monument Creek Tributary were
<br />reduced to account for temporary storage upstream from small culverts situated under two
<br />railroad embankments, Peak discharges downstream from the railroad embankments were
<br />computed by combining the reduced peak discharges through the culverts and the peak
<br />discharges from the intervening drainage area, The 500-year peak discharge on Monument
<br />Creek Tributary near the mouth also includes the 2,410 cfs that spill out of Monument Creek
<br />and flow around the north side of the island near the confluence of the two streams, Based
<br />on hydraulic analysis, this overflow channel, called Monument Creek Bypass, would convey
<br />floodwater only during the 500-year flood, For the 500-year flood, the water-surface
<br />elevation would be high enough on Monument Creek to produce a peak discharge of 2,410
<br />cfs in Monument Creek Bypass, This overflow enters Monument Creek Tributary at Cross
<br />Section 8 and then flows back into Monument Creek approximately 1,100 feet downstream
<br />of Cross Section A (Reference 6),
<br />
<br />For Fountain Creek, through the City of Manitou Springs, flood discharges were computed
<br />by the USACE in the 1974 FPI report (Reference 12), A log-Pearson Type III analysis of
<br />streamflow records at the Pueblo gage from 1935, and 1941 to ]965, was used to determine
<br />flood frequency (Reference 26). Other gages on Fountain Creek were not used due to short
<br />length of record (Colorado Springs and Security) and inaccurate representation of the flood
<br />potential of the stream (Fountain),
<br />
<br />The SPFs for Fountain Creek at Manitou Springs, Colorado Springs, the EI Paso-Pueblo
<br />County boundary, and Pueblo were developed by the USACE (Reference 12) using the June
<br />1921 storm, The SPF is defined as the flood that may be expected from the most severe
<br />combination of meteorological and hydrological conditions that is considered reasonably
<br />characteristic of the geographical area in which the drainage basin is located, excluding
<br />extremely rare combinations. Drainage areas above an elevation of 9,000 feet were not
<br />included in the SPF computations for Fountain Creek because they do not make a significant
<br />contribution to storms of the SPF magnitude, A discharge-drainage area curve was developed
<br />and used to compute SPF discharges for Fountain Creek at locations in Manitou Springs,
<br />
<br />The SPF discharge of 219,000 cfs at the Pueblo gage is approximately equivalent to the
<br />magnitude of the 350-year flood, Based on this, frequency curves for other locations were
<br />developed by assigning a recurrence interval of 350 years to each SPF and constructing
<br />curves parallel to the frequency curve at the Pueblo gage, These curves were extended to
<br />obtain the 500-year discharges for Fountain Creek, From these frequency curves, the 10-,
<br />50-, 100-, and 500-year flood discharges were determined for use in this study,
<br />
<br />A frequency analysis of the Pueblo gage based on a log-Pearson Type III distribution
<br />(Reference 26) was made for this study using flow records of 1935, 1941 to 1965, and 1971
<br />to 1977, In lieu of the computed skew, a regional skew of 0.4 was used in computations of
<br />peak discharges and confidence limits, The USACE frequency curve falls within the 50-
<br />percent confidence interval of this more recent study for recurrence intervals of greater than
<br />30 years, while the IO-year flood is only slightly larger than the 0,25 limit. It was concluded
<br />that the USACE discharge for Fountain Creek at Manitou Springs would be adopted for this
<br />study,
<br />
<br />21
<br />
|