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

of moss and lichen are a function.of climate (and microclimate) and
geochemical weathering of rocks; hence, detailed studies are needed to
determine reliable datea since a flood. The use for this
inveatigetion ie to provide a r~lative time frame of floodin~ between
the digferent etreams in the s~tudy area. As a general rule mosB can
take 10 or more yeara to ree~tablish following a flood; lj.chen can
take 10 ta 20 years ~o recoloni~e and then hundreds of years for the
coloniea to merge together end completely cover a surface. A near
completely covered lichen surface is indicative of a very ol~ surface
(and undisturbed by flooding).
Depoaitianal featuree, whirh occur where streamflow velocity can
not transport the sediment in motion, include large boulder bars in
the channel and alluvial fane et tributary ~unctione, at.rapi~ channel
expansiona, and at ~ecreasea in channel alope. Along many etreame
terracea on the channel margins are formed during the flood. Flood
bd'ulders can easily be diatinquished from boulders that have rolled
oYf valley walle as they are more rounded because of their movement
and abrasion by flowing water` and sediment. Generally the more
frequent the floods (and particle movement) the more rounded the
particle. Magnitude of floo~s can be determined based on th~ size of
boulders moved. In these basins there are boulders of essentially all
sizes available for traneport (boulders as large as houses occur in
these baeins). The larger a boulder that was transported by a flood,
the larger the flood velocity and depth. Interpretation of these
sediment deposits can provide both the number and the magnitude of
paat floods.
It hae been ehown (Jarrett, 1987) that debris flowe generally
occur above 7,500 feet in Colorado because of steep slopee and that
there has been a problem misidentifying debris flows ae watex floods.
Becauee debria-flow deposits were identified in one basin
investigated, a brief overview is provided. Debris flows are defined
as maes-movement intermediate between landsliding and water flooding
that contain very large quantities of sediment. Debris flows are a
heterogeneous mixture of sediment sizes and water; bec~use the
pracesses differ, mitigation of their hazards differ. De~ris-flow
deposits can be differentiated from water flood deposits. Debris-flow
deposits have characteristic levees on the sides of the ~~hannels.
Theae depoeits are poorly sorted and the largest boulders ~re on the
top surface, arid there is no evidence of "flooding" downstream from a
characteristic terminsl lobe marking the end of the debris flow.
Water-flood depoeite are sorted by grain size, cross-bedded,
atratified, and with gradetional boundaries; sediments ~ften are
deposited as bare or alluvial fane. The largest boulders are
concentrated near the bottom center of the depoeite (where velocity
and ~epth are largeBt).
When gravel bare are depoeited, vegetation regenerates on top of
the bars (or on other flood deposits). Recent flood deposits support
little vegetation; older depo~its have increasingly more vegetation.
Trees growing on the surface can be cored, annual tree rings counted,
and approximate date of the fl.ood determined. (A tree corer was not
available for this survey, but the diameter and types of these trees
3.