FLOOD11628 ‎(2)‎

• County: El Paso
• Community: Manitou Springs, Colorado Springs
• Basin: Arkansas
• Title: Paleohydrologic Flood Investigations for Streams in Manitou Springs and Colorado Springs
• Date: 12/3/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

INTRODUCTION
As diecuased with Mr. Uon Soards on October 25, 1987, the U. S.
Army Corpe of Engineere, Albuquerque Dietrict, is developing a
realtime flood warning system for Colorado Springs and Manitou
Springa, Colorado. Drainage baeins whoae etreame flow through these
communitieB heve been identified. A general area map is shown in
Figure 1. Rainfall gages have been inetalled in these b~sins that
relay rainfall amounta and in con~unction with a rainfall-runoff model
will provide warning ae to expected flood magnitude and timing. Mr.
5oarde, indicated that results from the report "Flood hydrology of
foathill and mountain streams in Colarado" (Jarrett, 1987), would be
used as an aid to design and develop the flood warning system. (This
report is preeently in review ae a U.S. Geological Surve~y professional
paper). Specifically the repart demonatratea that there is an upper
elevation limit for significant rainfall-runoff flooding of about
7,'S(~0 feet in Colorado. The~ etudy also indicated that below about
7,500 feet in Colorado streams are eub~ect to infrequent cataetrophic
flash tloode; whereas, above 7,SQ0 feet snowmelt flows predominate and
are generally within stream bank:s. Many reported "floode" above 7,500
feet were actually debris flowe. The source of these flaeh floods is
intense cloudburst stormB that have been shown to be limited to emall
areaa. Some of the etreame that drain into Colorado Springs and
Manitou Springs originate up to elevations of 14,000 feet on Pikea
Peak. The Pike'a Peak Maasi.ve rises abruptly from the plains and
2looding may differ from other mountain areaB. Also, aite specific
paleohydrologic investigations r,ave not previoualy been done in theses
streams. Site specific inve,stiqations need to be done to determine if
elevation limits of eignificar~t reinfall differ from those developed
for the Arkanese River basin (.Iarrett, 1987>. These reaults can then
be used to determine the contri.buting drainage area in the new flood-
warning system.
Because of the importance of the flood-warning system, Robert
Jarrett conducted paleohydrolc~gic investigatione in the streams that
drain into the two communitiea on Hovember 7-8, 1987 to provide eite
epecific information for input into the deeign of the flood-warning
system. Since the flood-warninc~ syetem is being developed during this
winter season for use next year, onsite investigations were made prior
to snowfall t~at would cover stream channele for the winter. The
purpoae of these investigations were to (1> determine if the elevation
limit of 7,500 feet applies to ihese baains that drain off tt~e eastern
flank of Pikes Peak and (2~ roughly estimate the magnitude and
frequency of past floodiny ueing paleohydrologic techniques.
Subaequent to the paleoflood investigations a review of flooding
history and review of available streamflow-gaging etation records was
made. Two pointe need to be made (1) these identified paleofloods are
eatimateB at thiB time with an error eatimate o.f about 50 p~rcent and
the estimates of time o~E floods is approximate (detailed
paleohydrologic methods can improve the estimates of the number of
past floods, their magnitude, and age of occurrence) and (2) the
magnitude of floods variefa dramatically over ahort diatances