FLOOD04447

• County: Fremont
• Community: Florence
• Stream Name: Oak Creek
• Basin: Arkansas
• Title: City of Florence LMMP Study
• Date: 2/1/1993
• Prepared For: FEMA
• Prepared By: Love & Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

[(L) (L ) ] 0.3 .
tp-O. 074 [__~"-__._] 0.669
S...
..."~

These equations were developed by regression analysis during
studies of the Purgatoire river at Trinidad, Colorado. The same
30-min unit hydrograph for the en.tire Oak Cree,k WatershE,d from ref.
12 was used in this study.

In the dam breach analysis of the railroad embankment it was
necessary to break the 71.58 sq. mile watershed into ~wo separate
subareas, a 6.7 sq. mile area upstrE~am of the embankmE~nt and the
area of the rest of the watershed. Because the purpose of the dam
breach analysis portion of the st,udy is to determine the change in
flow caused by a dam breach rather tban the mi'igni tude of the flow,
approximate unit hydrographs were us,~d in this part of .the analysis
for the two subareas. The same unit hydrograph used for the entire
Oak Creek watershed area was also used for the larger subarea in
the dam breach analysis. For the smaller subarea Snyder
coefficients Cp and tp, were derived from the regression equations
ab:lVe and used in the model. Although the Clark method did not
converge to the given Snyder coefficients, it is believed that the
co~puted flows will be reasonable enough for the dan breach
an'ilysis.

INE'ILTRATION I,OSSES. The SCS curve number method ivas used to
determine rainfall loss rates. A cur've number of 77 \"as selected,
based on soil and vegetation types within the basin. The same
103s rate was applied to all frequency storms. The curve number
wa3 developed from "Soil Survey of Custer County Area, Colorado,"
AU9ust 1982, Soil Conservation Service, and guidance in "National
En9ineering Handbook, Section 4," revised 1988, Soil Conservation
Service. Actual losses for the 100-'year flood are shmm in Table
1.

DESIGN STORMS.

N-vr 24-hr Rainfall: precipitation data storm was obtained
from NOAA Atlas II (ref. 6). Tot:al rainfall for a 24 hour period
fo= the 100-, 50-, 25-, and la-year storms is 4.13, 3.75, 3.31, and
2.78 inches, respectively. It was broken into lS-minute intervals
and arranged according to the SCS Type II storm pattern as shown in
Table 1. This pattern places thE: mOist intense period of rainfall
in the 49th 1S'-minute period.

6-hr Local Storm PMP: The 6-hr local storm PMP V/ClS obtained
from HMR #SSA (ref. 15) and the incr~mental rainfall was arranged
in a critical distribution pattern in accordance with EM 1110-2-

2