FLOOD08833

• County: Las Animas
• Community: Near Trinidad
• Stream Name: Purgatoire
• Basin: Arkansas
• Title: Flood Control Problem Purgatoire near Trinidad
• Date: 1/1/1944
• Floodplain - Doc Type: Project

tary water supply at the Mammoth, I. S., or Purgatoire No.2 sites are
negligible and that developments at these sites would have value only
in the Arkansas River Basin. They may be of value in the futlll'e as
possible replacement for storage capacity in.John Martin Reservoir
that may be lost by silt accumulation. '
101., In a flood plain such as that of Purgatoire River" in which
the overflow limits are narrow, the area benefited by channel rectifi,
cation, levees, or bank stabilization is small. The costs per unit
length of channel ,exceed the benefits which would accrue from such
flood'eontrol works. ' ,
102. The erratic stream flow of Purgatoire River and demand for
all available water for other purposes precludes the development of
bydroclectl'ic power in this area. There' is no need for further munic,
ipal water supply in, the basin as existing reservoirs meet the present
needs. There is no stream pollution problem in Purgatoire River
under present conditions.
103. The dual,purpose reservoir, plan 1, at the Sopris site, which
WitS designed to control a flood of probable occurrence of once in 100
years, would control all experienced floods and provide additional
water for irrigation which is urgently needed by agricultlll'al areas in
the valley. The reservoir would provide complete protection from
small volume floods, but those of suflicient magnitude to produce
a sustainod outlet discharge would damage irrigation structures and
cause, bank eaving. Even though a dual,purpose lltructure would
more adequately serve local interests in the valley, it is not economi,
cally justified at this time as the ratio of annual benefits ,to annual
charges is 0.46. '
104, The dual-purpose reservoir, plan 2, with spillway crest at
elevation 6,195 was investigated in the light of data on irrigation
benefit.q furnished by the Bureau, of Reclamation and the :results of
the study we}'e discussed with representatives of that agency. It was
found that a large portion of probable irrigation benefits obtainable,
and some flood-control benefits through detention of small volume
floods, would be provided. The fleod-control benefits would be small.
The ratio of total benefits to costs being only 0,51 shows that a develop-
ment of this type is not economically justified,
105. ,A reservoir at the Sopris site, for flood control only, which would
control the maximtlln flood of record would require a capacity not
much smallor than the dual'purpose reservoir controlling a flood of
the same magnitude. 'l'herefore, it is evident from the sms,ll economic
ratio (0.46) for the dual.purpose reservoir that a reservoir for flood
control only is not economically justified.. '
10(\.. It was considered that a channel improvement through the
urban area of Trinidad should provide flood protection for the maxi-
mum flood of record, if eponomically justified, so as not to induce a
false senso of security among inhabitants in the area.. The most
eeonomical degree of improvement is indicated on chart No.1,'
appendix A: This degree is provided by the recommended plan of
improvement. The economic comparison of other plans considered
is also shown on chart No. 1.1
" 107. The recommended plan of improvement will safely pass a flood
discharge of 45,000 eubic feet per second which is, equal in magnitude

I 'l'hls chart Is not printed.