FLOOD06536

• Designation Number: 43
• County: Las Animas
• Community: Trinidad
• Stream Name: Purgatory River and Tributaries
• Basin: Rio Grande
• Title: Floodplain Information Report - Purgatory River and Tributaries, Vicinity of Trinidad, Las Nimas County, Colorado
• Date: 4/1/1974
• Designation Date: 2/1/1975
• Floodplain - Doc Type: Floodplain Report/Masterplan

heights of rise, rates of rise, and durations of flooding measured

above bankfull stage.
rloodflow velocities depend on the general waterway oon-
fi~~ration, and the si~e, shape, and slope of strea~ed and overbank
flow areas. Velocities may range from the averaged rnaxi~~s appear-
ing in ~able 5 to less than 50 percent of these values. On 29-30

September 1904, the flooding duration was approximately 8.5 hours as
co~red to 4.5 and 6.0 hours estimated for the Inte~ediate Regional
and Standard Project Floods, respectively. The shorter flooding
duration results from storms emanating OVer the steep Raton Creek
watershod, the pri~ry contributor to the Purgatoire River.
Bigh rate of rise shown on Table 5 indicates that little
flood warning may be expected, particularly along the tributaries.

Obstructions

Of major concern is maintenance of channel and overbank
capacity of a stream to convey flood waters without causing serious
flood damage. Reducing waterway openings by approach embankments at
bridges and otherwise ch~nging channel and ov<,rbank capacities will
impede floodflow~ and cause greater backwater depths with conse-
quent increase in flooded areas. Obstructive new developm~nts and
future land filling in or nea~ the stream floodways can be a ~jor
factor in multiplying the adverse effp.ct~ Jp.~crihen.
Estimated future flood crests and pertinent elevations
at bridge crossings within the study area are listed in Tabl~ 6. Of
the various bridges crossing thc strcum~ in the study area, most ure
obstructive to the Intermediate Regional Flood and even more are

obstructive to th~ Standard Project Flood.

In some ca~es, bridges

may be high enough so as not to be inundated by flood flews; however,
tne approaChes are often at lower elevations and subj~ct to flooding.
For study purposes herein, it has been ass~~d that limited clogging
C3n be expected and all bridge structures ~~uld stand intact. Sig-
nificant changes in this premise, imposed by differing eondltions of
a futur~ flood, could alter the e.timat~d flood crests and flood

limlts shown in the tables and related plate~.

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