FLOOD09312

• Designation Number: 370
• County: Crowley
• Community: Ordway
• Stream Name: Local Drainage
• Basin: Arkansas
• Title: Floodplain Study and Master Drainage Plan - Ordway, Colorado
• Date: 6/1/1987
• Designation Date: 7/1/1994
• Floodplain - Doc Type: Floodplain Report/Masterplan

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silt
SiMONS, Li & AssociATES, INC.

1660 SOUTH ALBION STREET
SUITE 1010
DENVER, COLORADO 80222

TELEPHONE [phone]

November 4, 1986

Mr. Bill Stanton
Colorado Water Conservation Board
State Services Building
1313 Sherman Street
Denver, CO 80203

RE: Town of Ordway I.Iaster Drainage Pl anni ng Study (SLA Project
No. PCO.ORD.01l

Dear Bi 11 :

We have completed our analysis of the off-site drainage areas, adja-
cent to the Town of Ordway. Enclosed is a map depicting the location
of the design points, as well as a summary table of the peak flows for
the various methodologies used.

Three separate stonn patterns were applied in the Soil Conservation
Service (SCSl method, namely to 2-, 6- and 24-hour duration storms.
As can be seen from the table, peak flow variances of less than ten
percent are effected when applying the different storm patterns. With
respect to the basi ns under analy si s, I wou1 d reconmend that ei ther
the 2- or 6-hour pattern be used for the Ordway study.

The 10-year peak fl ow rates are extremely compatible between the
various methodologies. However, a wide variation of flow rates is
seen for the 100-year frequency, particularly for the northwest basin.
There is no clear explanation for the variances, other than that the
methods by their very nature are quite different in their application
and data base.

Within the context of this master drainage planning study, there is
some doubt as to whether there will be an economic basis for
justi fyi ng a 100-year design level, particul arly for the northwest
channel. It is becoming clear that if an 8,200 cubic feet per second
(cfsl design flow is used for planning, that substantial areas will be
shown in the floodplain. Even at 4,100 efs (as per the 2-hour SCS
calculations) the lack of channel capacity will result in a wide
100-year flood zone, probably not substantially deeper than the B,200
efs case. However, in the case of a 4,100 efs flow rate this type of

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