FLOOD07436

• County: Statewide
• Basin: Statewide
• Title: Arkansas River from John Martin Dam to the Colorado-Kansas State Line: Channel Capacity and Riparian Planning Study
• Date: 7/1/1999
• Prepared For: CWCB
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

hydraulic analysis. This program is applicable to steady, gradually varied flow conditions. It
was assumed that the controlling hydraulic conditions were represented within the boundaries
of the geometry provided, i.e., that conditions downstream of the areas would not substantially
influence the profiles within the computer models. The downstream boundary condition was
calculated using the Slope-Area method. Cross-section geometry was provided in ASCII
format by Kucera International, Inc., under contract to CWCB. Reach lengths between cross-
sections were scaled from the contact prints. Bridges and diversion structures were not
modeled since the corresponding geometry was not provided. This should not significantly
effect the results for the relatively low discharges modeled. Manning's "n" rouglmess values
were assigned based on a combination of previous studies, field conditions, sediment transport
theory, and engineering judgement. Channel "n" values were adjusted using Brownlie's bed
roughness equation to account for bed forms. The specific "n" values for the individual
problem areas are shown in Table 3-2. Contraction and expansion coefficients used were O.l
and 0.3, respectively.

Table 3-2. Manning's roug1mess values.

3
4
5
6
7

0.065-0.100
0.050-0.065
0.050-0.070
0.070-0.100
0.090-0.100

Channel

0.D35
0.032
0.032
0.033
0.033

Right Overbank

0.050-0.l00
0.045-0.100
0.070-0.100
0.070-0.1 00
0.080-0.100

Problem Area

Left Overbank

The 3,000 cfs operational peak discharge was modeled to indicate potential problem
areas under current conditions. The flow area extents for this discharge (i. e., the points where
the water surface "daylights" with ground) were connected to generate "pseudo-floodplains"
and plotted over the photographic images. The figures generated are for illustrative purposes,
and are not true floodplains, since they were produced without sufficient topographic
information. The following assumptions were used to create these:

l) Flow was initially confined to the generally bermed flow corridor.
2) Flows that escaped the corridor were allowed to spread out at subsequent
upstream cross-sections, unless one of these upstream cross-sections showed a
strong likelihood of returning flows to the bermed flow area.
3) Correspondingly, potential ponding outside of berms at cross-sections
downstream from a "break-out" location were ignored.

These simplifying assumptions were necessary due to the lack of topographic detail between
cross-sections. In actuality intervening features, or the lack thereof, would have a bearing on

l7