DRAFT
BASIC DATA NARRATIVE
FLOOD INSURANCE STUDY
SACRAMENTO COUTY, CALIFORNIA
1.0
INTRODUCTION
1.1
Purpose of Study
This Flood Insurance Study/Letter of Map Revision for Cosumnes River is to
update and revise the existing Zone A of the Flood Insurance Rate Maps for
Sacramento County Unincorporated Areas Panel Numbers 275D, 375C and 400D
dated July 1998.
Community Name - Sacramento County (Community Number 060262)
State California
1.2
Authority and Acknowledgements
The hydraulic analysis for this study is prepared by Civil Engineering Solutions,
Inc. (CESI), the Study Contractor for the Sacramento County. The study was
completed in March 2010.
1.3
Coordination
In July of 2008, an initial Consultation and Coordination Meeting was held with
representative of the Sacramento County Department of Water Resources
(SCDWR) and the Study Contractor. The stream to be studied and limits of the
study were identified at the meeting. Available mapping and other data identified
at a subsequent meeting between the SCDWR and the Study Contractor.
During the conduct of the study, meetings, telephone calls and e-mail discussions
have been held between the Study Contractor and representative of the SCDWR.
The SCDWR has provided the funding for this study and all relevant data such as
hydrologic data, field survey and topographic mapping.
2.0
AREA STUDIED
2.1
Study Limits
The stream studied by detailed methods in this study is the Cosumnes River
located within the easterly portion of the Sacramento County and flows from
northeast to southwest. See attached vicinity map.
The downstream study limit begins at Dillard Road and ends approximately one
(1) mile upstream of Michigan Bar Road. The total study reach is approximately
9.8 miles.
This reach of the Cosumnes River is studied by Approximate Method and shown
as Zone A on the effective Flood Insurance Rate Map for Sacramento County
Panel Number 275D, 375C and 400D dated July 1998 (reference 1).
2.2
Community Description
The community description is presented in the Flood Insurance Study for the
unincorporated areas of Sacramento County.
2.3
Principal flood problems
There are documented significant flood problems along the Cosumnes River in
the study area.
Two levee systems are maintained within the study reach by the Rancho Murieta
Community Services District (CSD). These levee systems have been granted
provisional accreditation scheduled to expire in January of 2011. For this study
these levees were studied as accredited.
An additional non-accredited man made levee was identified along the south (left)
bank of the river, near the downstream end of the study reach. Alternatives for
with and without levee protection here were provided per the guidelines of
Appendix H.
2.4
Flood protection Measures
The Cosumnes River within the study reach is in natural conditions and no major
channelization or other flood protection measures exists along the study reach.
There is water diversion dam constructed across the Cosumnes River channel
approximately 8300 feet upstream of State Highway 16 Bridge. The dam is a
concrete structure about 10 feet high and 3 feet wide and diverts irrigation water
during low flow periods. There are four (4) bridges crossing Cosumnes River
along the study reach.
3.0
ENGINEERING ANALYSES
3.1
Hydrologic Analyses
The 10-, 50-, 100- and 500-year peak discharges used in this study were
determined by U. S. Geological Survey (USGS) (reference 2). The USGS has
operated a stream flow gauging station (No.11335000) on Cosumnes River from
1907 to 2007. The gauging station is located on the downstream side of the
Michigan Barr Road Bridge. The peak discharges are summarized on Table 1.
TABLE 1
Summary of Peak Discharges
Flooding Source
&
Location
Drainage Area
(Sq. Mi)
10-Year
(cfs)
50-Year
(cfs)
100-Year
(cfs)
500-Year
(cfs)
Cosumnes River at
Michigan Barr
536
34,200
66,800
82,900
125,000
3.2
HYDRAULIC ANALYSES
Water surface elevations were computed utilizing the U. S. Army Corp of
Engineers (COE) HEC-RAS computer program (reference 3). The following
parameters were used:

The geometry of the Cosumnes River channel and overbank floodplain
areas were primarily derived from topographic mapping based on the
Sacramento County latest LIDR data. The areas within the channel banks
were field surveyed to verify the accuracy of the topographic mapping and
to get a better definition of the channel invert (thalweg) elevations.
Bathymetry was obtained where necessary.

As a part of the reconnaissance effort, a field inventory and measurement
process was completed to determine the location, size and characteristics
of all existing bridges and other hydraulic structures.

The Manning’s roughness coefficient ‘n’ values were established based on
field observations and the USGS publication (reference 4). The channel
roughness values between 0.028 and 0.036 was used for earthen natural
channel and overbank roughness values ranges from 0.041 to 0.069. In
some cases, the main channel ‘n’ values were increased by a factor
between 0.02 and 0.03, to stabilize the HEC-RAS computations, and
prevent HEC-RAS from arbitrarily assuming a critical depth
inappropriately. The Manning’s ‘n’ values were calibrated to the USGS
Rating Curve published flow and stage values. The following graph shows
that an exact calibration was not obtained. Conservative estimates for ‘n’
values needed in order to stabilize the HEC-RAS analysis, as well as
computational limitations of the 1-diminsional analysis contribute to this.
Additionally, the HEC-RAS model does not account for stream bed
mobility and scour which is known to occur at the location and vicinity of
the gage. The computed rating curve shown below represents the rating
curve for the situation of the surveyed conditions and assuming the stream
bed does not mobilize or scour.
Cosumnes With Levees Oct 4- 08
RS = 949
198
Plan: rating curves-pals only
12/15/2009
BR Michigan bar bridge
Legend
W.S. Elev
Obs WS
196
194
Obs WS (ft), W.S. Elev (ft)
192
190
188
186
184
182
180
20000
40000
60000
80000
100000
120000
140000
Q Total (cfs)

Contraction and expansion coefficients for open channel sections of
0.1and 0.3 were used in accordance with the COE guidelines. Contraction
coefficients of 0.3 and expansion coefficients of 0.5 were used at bridges.
An additional modification to the contraction and expansion coefficients
was made where water surface elevations were rising in the downstream
direction due to rapid velocity changes. The coefficients were increased to
values such that water surface elevations no longer increased in the
downstream direction. The contraction coefficient was limited to a
maximum value of 0.5 and the expansion coefficient was limited to a
maximum value of 0.9.

The downstream starting water surface elevations for 10-year through
500-year are based on known water surface elevations as determined by
U.S. geological Survey (reference 2) at the Dillard Road Bridge (upstream
side), which was the upper limit of their study reach. The U. S. geological
Survey has performed a detailed hydraulic analysis of Cosumnes River
using WSPRO computer model to determine water surface elevations for
5-year through 500-year and100-year floodplain limits under several
conditions. The U. S. Geological Survey study started at State Highway 99
(at the downstream) and ended at Dillard Road. This study begins at
Dillard Road and continues upstream to the Sacramento County limit line.
The U. S. Geological Survey study is included with this report. A portion
of the water surface profile plot from the report illustrating the estimated
downstream water surface elevations is reproduced below

Supercritical flow conditions can occur in some channel reaches. In
accordance with FEMA’s guidelines, subcritical analyses were conducted
to determined base flood elevations for this study of the Cosumnes River.
‘n’ values were also adjusted to stabilize the subcritical computations,
were supercritical flow was likely.

Calibration of the model is the process used to insure that the model
predicts actual system behavior as closely as possible. The HEC-RAS
model for Cosumnes River was calibrated with stream gauge data (water
surface elevations) for different storm events at Michigan Barr Bridge.
Parameters in the HEC-RAS model such as Manning’s roughness

coefficient ‘n’ values are then adjusted until the model output (water
surface elevations) matched closely the historical gauge data for a specific
storm event.

4.0
Surveys and Datums: The Sacramento County Base LiDAR obtained for
the limits of this Study, and other County areas, was obtained in 1997 and
2003. We considered this LiDAR to be recent and accurate enough to
establish base topography for this study. We verified the LiDAR data, by
obtaining spot shots near the channel bank full locations of the stream.
Supplemental surveys were also performed to obtain THALWEG
elevations at the cross section locations. All data included in these studies
was prepared using the North American Vertical Datum of 1988 (NAVD
88), and the 1983 North American Datum (NAD83) for horizontal
measurements.
FLOODPLAIN MANAGEMENT APPLICATION
The floodplain and floodway boundaries as determined by the hydrologic and
hydraulic analyses have been delineated on Sacramento County one inch equal
300 foot horizontal scale topographic mapping with 1 foot contour interval.
4.1
Flood Boundaries
In accordance with the FEMA guideline, floodplain boundaries have been defined
based on base flood elevations as determined by subcritical flow analyses. In this
study two sets of base flood elevations were studied in the lower reaches, were a
non accredited levee was identified near the left bank. Base flood elevations
adjacent to this non-accredited levee for the channel and right bank are mapped
for the “with levee” condition. Base flood elevations for the left bank are based on
the “without levee”. The profiles show both conditions.
4.2
Floodway
The floodway presented in this study is initially computed on the basis of equal
conveyance reduction from each side of the floodplains, based on the “without
levee” base model for the non-accredited levee in the downstream reach at the left
bank.. Then minor adjustments were made to the initially computed encroachment
stations at each cross section so that no more than 1 foot of water surface rise
would occur anywhere in the model reach. In Table 2 the final floodway analysis
results are shown. On the floodway mapping, some adjustments to the direct
connections from section to section were necessary to maintain the floodway
within the 100-year floodplain.
4.3
Annotated copies of the affected FEMA FIRMs are included with this report.
BIBLIOGRAPHY AND REFERRNCES
Federal Emergency Management Agency, Flood Insurance Study,
Sacramento County, California (unincorporated Areas), Washington, DC.
U. S. Geological Survey, Flood – Inundation Map and Water
Surface Profiles for Floods of Selected Recurrence Intervals, Cosumnes
River and Deer Creek, Sacramento County, California, 1998.
U. S. Department of the Army, Corps of Engineers, Hydrological
Engineering center, HEC-RAS River System Analysis, Version 3.1
November 2002.
U. S. Geological Survey Water – Supply Paper 1849, Roughness
Characteristics of Natural Channels, Washington, DC, 1967.
FEMA FORMS
TABLE 2
FLOODWAY TABLE
HEC-RAS HYDRAULIC SUMMARY TABLE
WATER SURFACE PROFILES
FLOODPLAIN and FLOODWAY
BOUNDARY MAPS
(WORK MAPS)
ANNOTATED FEMA FIRM
HEC-RAS FILES CD
UPPER COSUMNES RIVER FLOOD MAPPING
DRAFT
BASIC DATA NARRATIVE
FLOOD INSURANCE STUDY
SACRAMENTO COUTY, CALIFORNIA
Community No. 060262
November 2008, Revised December 2009.
Prepared By:
CIVIL ENGINEERING
SOLUTIONS, INC.
1325 Howe Avenue Suite 202
Sacramento, CA 95825
(916) 563-7300
U. S. Geological Survey, Flood – Inundation Map and Water
Surface Profiles for Floods of Selected Recurrence Intervals,
Cosumnes River and Deer Creek, Sacramento County,
California, 1998