FLOOD07885

• Designation Number: 506
• County: Weld
• Community: Unincorporated Weld County
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Floodplain Information Report - Cache La Poudre River, Weld County
• Date: 4/1/1996
• Designation Date: 9/1/1999
• Floodplain - Doc Type: Floodplain Report/Masterplan

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There are no structures within this portion of the Poudre River which provide a major
reduction in flood flows. Any existing levees along the river are able to contain frequent floods
(i.e. 10 to 25 year floods), but provide little protection against the 100 year flood.

PROPOSED ENCROACHMENT

An encroachment has been proposed and analyzed in order to confine the Cache La Poudre
River to a narrower floodway, within the area of this investigation only. By establishing a
narrower floodway, the amount of usable land increases.
Under current topographic conditions the 100 year storm, which is estimated to be 13 ,200
c.f.s., floods a very substantial amount of the area investigated in this study. So large is the
flooded area that little, to no, usable land exists inside of 1000 feet either side of the existing
banks of the river.
For purposes of investigation, a 1- Yz foot elevated encroachment was added to the existing
ground surface at each designated cross section. The elevation increase was imposed at distances
ranging from 500 to 800 feet either side of the river centerline. After analyzing the flow of the
river under these new conditions, it was found that a significant amount of usable land could be
gained without altering characteristics to a great extent. The water elevations and velocities at
each of the cross sections vary only slightly from existing conditions. Water elevations for the
encroachment analysis range anywhere from Yz foot to 1 foot below the top of the encroachment.

FLOODW A YS

Encroachment on flood plains, such as artificial fill, reduces the flood carrying capacity and
increases flood heights, thus increasing flood hazards in areas beyond the encroachment itself.
One aspect of floodplain management involves balancing the economic gain from floodplain
development against the resulting increase in flood hazard. For purposes of the National Flood
Insurance Program, the concept of a f100dway is used as a tool to assist local communities in this
aspect of floodplain management. Under this concept, the area of the 100 year flood is divided
into a floodway and a floodway fiinge. The floodway is the channel of a stream, plus any adjacent
floodplain areas, that must be kept free of encroachment in order that the 100 year flood be
carried without substantial increases in flood heights. As minimum standards, the Federal
Insurance Administration limits such increases in flood heights to 1.0 foot, provided that
hazardous velocities are not produced..
The floodway in this study was computed on the basis of average conveyance for each side of
the floodplain. The results of these computations are tabulated at selected cross sections for each
stream segment for which a floodway is computed (Tables 4 & 5).
The area between the floodway and the boundary of the 100 year flood is termed the floodway
fringe. The floodway fringe thus encompasses the portion of the floodplain that could be
completely obstructed without increasing the water surface elevation of the 100 year flood more
than 1. 0 foot at any point. Typical relationships between the floodway and the floodway fringe
and their significance to flood plain development are shown in Table 4.

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