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FLOOD INSURANCE STUDY

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					 FLOOD INSURANCE STUDY


BUTLER COUNTY, NEBRASKA


        July 2003
                                 TABLE OF CONTENTS

                                                     Page

1.0 INTRODUCTION                                       1

    1.1 Purpose of Study                               1
    1.2 Authority and Acknowledgments                  1
    1.3 Coordination                                   1


2.0 AREA STUDIED                                       2

     2.1   Scope of Study                              2
     2.2   Community Description                       2
     2.3   Principal Flood Problems                    3
     2.4   Flood Protection Measures                   3


3.0 ENGINEERING METHODS                                3

     3.1 Hydrologic Analyses                           3
     3.2 Hydraulic Analyses                            5

4.0 FLOODPLAIN MANAGEMENT APPLICATIONS                 7

     4.1 Flood Boundaries                              7
     4.2 Floodways                                     8

5.0 INSURANCE APPLICATIONS                             8

6.0 FLOOD INSURANCE RATE MAP                           9

7.0 OTHER STUDIES                                      9

8.0 LOCATION OF DATA                                   9

9.0 REFERENCES AND BIBLIOGRAPHY                        10
                                     TABLE OF CONTENTS (continued)

                                                           Page
FIGURES

Figure 1 - Vicinity Map                                      2

Figure 2 - Floodway Schematic                                8

TABLES

Table   1 - Platte River Peak Flows, Butler County Reach     4


EXHIBITS

Flood Profiles; Platte River - Exhibit 1

Flood Insurance Rate Map - Exhibit 2
                                FLOOD INSURANCE STUDY
                               BUTLER COUNTY, NEBRASKA

1.0 INTRODUCTION

1.1 Purpose of Study

This Flood Insurance Study investigates the existence and severity of flood hazards in Butler County,
Nebraska, and aids in the administration of the National Flood Insurance Act of 1968 and the Flood
Disaster Protection Act of 1973. This study has developed flood risk data for various areas of the
community that will be used to establish actuarial flood insurance rates and assist the community in
its efforts to promote sound floodplain management. Minimum floodplain management
requirements for participation in the National Flood Insurance Program (NFIP) are set forth in the
Code of Federal Regulations at 44 CFR, 60.3.

In some states or communities, floodplain management criteria or regulations may exist that are more
restrictive or comprehensive than the minimum Federal requirements. In such cases, the more
restrictive criteria take precedence and the State (or other jurisdictional agency) will explain them.


1.2 Authority and Acknowledgments

The sources of authority for this Flood Insurance Study are the National Flood Insurance Act of 1968
and the Flood Disaster Protection Act of 1973.

The hydraulic analyses for this study were performed by the U.S. Army Corps of Engineers, Omaha
District, (the Study Contractor) under Inter-Agency Agreement EMW-97-IA-0140 Project Order
Number 3. The Nebraska Department of Natural Resources (DNR) provided surveys used in this
study. Some data used in this study was developed for the Corps’ Lower Platte River and Tributaries
Feasibility Study. The study was completed in July 2003.

1.3 Coordination

The study at Butler County was part of a study of the lower Platte River in Nebraska from the
Missouri River to the City of Columbus. The scope of study was determined through discussions
between the Study Contractor, FEMA and the Nebraska Department of Natural Resources.

One of several regional initial meeting for the Lower Platte River, Nebraska, FIS was held on
April 29, 1997 at North Bend, Nebraska. The meeting included representatives of FEMA, the
study contractor, the Lower Platte North Natural Resources District and communities in the study
area.



                                                  1
2.0 AREA STUDIED

2.1 Scope of Study

This Flood Insurance Study covers the area of Butler County along the Platte River. The area of
study is shown on the Vicinity Map (Figure 1). Only flooding from the Platte River was studied in
detail.

2.2 Community Description

Butler County is located in east central, Nebraska. The Platte River is the northern county boundary.
The population of Butler County in the year 2000 census was 8,767.




                                            Figure 1
                                   Butler County and Vicinity


                                                 2
2.3 Principal Flood Problems

The Platte River was the only flooding source included in this study. The Platte River is the northern
border of Butler County.

2.4 Flood Protection Measures

There are no government sponsored flood protection measures along the Platte River in Butler
County. Some private developments along the river may have constructed localized dikes for flood
protection.

3.0 ENGINEERING METHODS

For the flooding sources studied in detail in the community, standard hydrologic and hydraulic study
methods were used to determine the flood hazard data required for this study. Flood events of a
magnitude that is expected to be equaled or exceeded once on the average during any 10-, 50-, 100-,
500-year period (recurrence interval) have been selected as having special significance for floodplain
management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and
500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded
during any year. Although the recurrence interval represents the long term average period between
floods of a specific magnitude, rare floods could occur at short intervals or even within the same
year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered.
 For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent
chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and, for
any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported
here in reflect flooding potentials based on conditions existing in the community at the time of
completion of this study. Maps and flood elevations may be amended periodically to reflect future
changes.

3.1 Hydrologic Analyses

        Peak flood flows for the Platte River at Butler County, including the 10-, 50-, 100-, and
500-year floods were developed through analysis of records from the U.S. Geological Survey
stream gage station at North Bend and Duncan, Nebraska using the Corps of Engineers HEC-
FFA computer program (Reference 2). The HEC-FFA program follows the procedures outlined
in Water Resources Council Bulletin 17B to produce the desired seasonal computed flow
frequency. Peak flows developed for the North Bend gage were used for the Platte River from
the Elkhorn River through North Bend to the Loup River confluence. Peak flows developed for
the Duncan gage were used for the reach upstream from the Loup River. Data from the U.S.G.S.
gage stations on the Platte River at Grand Island, Ashland and Louisville was used with the
North Bend and Duncan gage data to develop regional skews. Periods of record for the gage
stations associated with this study included only those years that provided complete and pertinent
data. The respective record lengths used for analyses for each station are as follows:

                                                  3
               Platte River at Louisville              1954-1994
               Platte River near Ashland               1929-1960, 1989-1994
               Platte River at North Bend              1950-1994
               Platte River at Duncan                  1942-1994
               Platte River at Grand Island            1942-1994


Information from the gage stations allowed for the determination of preliminary computed
discharge-frequency relationships and station skews for both the snowmelt season (December 15-
March 31) and rainfall season (April 1-December 14). The preliminary seasonal station skews
from the five gage station locations were then used to develop a regional skew for the snowmelt
season as well as a regional skew for the rainfall season. The preliminary snowmelt and rainfall
season computed flow-frequency relationships for the gage stations at North Bend were adjusted
to reflect the seasonal regional skews. Adjustments for seasonal regional skews produced a final
computed flow-frequency relationship for each season. A combined season flow-frequency
relationship for the stations was computed using the combined probability equation from the
final snowmelt season and rainfall season computed flow-frequency relationships:

               Pcombined=Prainfall+Psnowmelt-(Prainfall*Psnowmelt)                (1)

Pcombined is the combined probability of a given flow being equaled or exceeded.
Prainfall is the probability of a given flow being equaled or exceeded in the rainfall season.
Psnowmelt is the probability of a given flow being equaled or exceeded in the snowmelt season.
The peak flows for the Platte River at Butler County for the 10-, 50-, 100-, and 500-year flood
frequencies are given in Table 1.

                                     Table 1
               Platte River Peak Flood Flows, Butler County Reach

    Location       Drainage Area       10-year       50-year   100-year         500-year
                     (sq. mile)         (cfs)         (cfs)      (cfs)            (cfs)

  North Bend
    Gage                82,900         62,000        106,000    132,000         220,000
                        D
 Duncan Gage            54,630         17,500        29,000     35,000       53,000


A detailed description of the hydrologic analyses is provided in Reference 3.




                                                 4
3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to
provide estimates of the elevations of floods of the selected recurrence intervals.

Water-surface profiles for all streams studied in detail were developed using the U.S. Army
Corps of Engineers HEC-2 backwater computer program (Reference 4).

Cross sections for the Platte River were provided by the Nebraska DNR. The DNR cross section
data was developed using photogrammetric methods from aerial photographs taken on November
8, 1999. All elevation data used in this study was to the National Geodetic Vertical Datum of
1929 (NGVD29).

Field surveys of the Platte River bed were obtained at the bridges. At cross sections where river
bed elevations were not available, additional cross section points were added to provide a flat-
bottomed bed about 1 to 2 feet below the water surface. The selected bed elevations were
adjusted by computing water surface profiles for the mean daily flow on the date that the aerial
photographs were taken. The bed points were adjusted so that the computed water surface
profile was close to the measured water surface elevations from the aerial photogrammetry.
Selected cross sections used in the hydraulic analyses are shown on the profile plates. Cross
section locations are also shown on the Flood Boundary and Floodway Map.

Bridge hydraulics were computed using the HEC-2 normal bridge methods. Bridge data for the
Nebraska Highway 15 and the U.S. Highway 81 bridges were provided by the Nebraska
Department of Roads. The Nebraska DNR provided field surveys for the Burlington Northern
Santa Fe Railroad Bridge. The hydraulic analyses for this study are based only on the effects of
unobstructed flow through bridges and other hydraulic structures.

Roughness coefficients (Manning's "n") for the Platte River were determined according to
calibration to available high water marks and to the U.S. Geological Survey stream gages at
North Bend and Leshara, Nebraska. For the Platte River channel, the roughness coefficients
varied from 0.025 to 0.030. The roughness coefficients used in for the overbanks varied from
0.050 to 0.095. Starting Platte River water surface elevations for the hydraulic models through
the county were taken from computed water surface profiles of downstream river reaches.

Because of the history of ice effects in this reach, ice-affected water surface profiles were
developed for the Platte River through Butler County. Ice-affected water surface profiles were
developed using the snowmelt season peak flows and the HEC-2 ice option. While a flood of a
given frequency can occur during the snowmelt season, the flood stages may not always be ice-
affected. In order to determine base flood elevations that include the possibility of ice-affected
stages, the composite probability method described in FEMA Publication 37 (Reference 5) was
used.


                                                 5
The composite stage-frequency plot for establishing the elevations of the various return interval
floods were obtained at selected cross sections by combining the open-water (free flow) stage-
frequency and the ice-affected stage-frequency using the combined probability equation:

                       P(s) = P(si) + P(sq) - P(si) * P(sq)                  (2)
Where:
P(s) is the probability of a given stage “s”being equaled or exceeded in any year, by either an ice-
affected or open water event.
P(si) is the probability of the annual maximum stage exceeding a given stage “s” in the ice-
affected (snowmelt) season.
P(sq) is the probability of the annual maximum stage exceeding a given stage “s” in the open
water (rainfall) season.
Because the maximum annual stages in the snowmelt season were not always ice-affected, the
term P(si) needed to be expanded as:

P(si) = P(sw)*P(si=ice-jam event) + P(so)*P(si = free-flow event)                    (3)

Where:
 P(sw) is the probability of a given stage “s” in the snowmelt season that is ice-affected.
P(si=ice jam events) is the fraction of years during the snowmelt season that peak stages are ice-
affected .
P(so) is the probability of exceeded a given stage “s’ in the snowmelt season from free flow.
P(si=free flow event) is the fraction of years during the snowmelt season that peak stages are
from free flow.
Combining equations (2) and (3) gives the equation for computing the composite stage-
probability:

P(s) =[P(sw)*P(si=ice jam event) + P(so)*P(si = free flow event)] +P(sq)

- [(P(sw)*P(si=ice jam event) + P(so)*P(si=free flow event)) *P(sq)] (4)

Equation (4) was to replace the composite probability equation given in the current edition of
FEMA 37 (Reference 5), which is in the process of revision. The computation required the
fraction of snowmelt season floods that are ice-affected be known. Based on records of ice-
affected stages at the Ashland and North Bend gages, a fraction of 0.48 was used.

When the results of Equation 4 for various flood stages were plotted for the selected cross sections,
composite stage-probability curves were obtained. The base (one percent chance) flood elevations
at the cross sections were obtained from the stage-probability plots. A 100-year flood water surface
profile using composite probability is shown on the water surface profile plots



                                                 6
4.0 FLOODPLAIN MANAGEMENT APPLICATIONS

The National Flood Insurance Program encourages State and local governments to adopt sound
floodplain management programs. Therefore, each Flood Insurance Study provides 100-year flood
elevations and delineations of the 100- and 500-year floodplain boundaries to assist communities in
developing floodplain management measures.

4.1 Flood Boundaries

To provide a national standard without regional discrimination, the 1-percent annual chance
(100-year) flood has been adopted by FEMA as the base flood for floodplain management purposes.
The 0.2-percent annual chance (500-year) flood is employed to indicate additional areas of flood risk
in the community. For each stream studied in detail, the 100- and 500-year floodplain boundaries
have been delineated using the flood elevations determined at each cross section. Between cross
sections, the flood boundaries were interpolated using the U.S. Geological Survey 7.5 minute
quadrangle maps (Reference 5).

The 100- and 500-year floodplain boundaries are shown on the Flood Insurance Rate Map (Exhibit
2). On this map, the 100-year floodplain boundary corresponds to the boundary of the areas of
special flood hazards (Zones A, AE, AH, AO) and the 500-year floodplain boundary corresponds to
the boundary of areas of moderate flood hazards. In cases where the 100- and 500-year flood plain
boundaries are close together, only the 100-year flood plain boundary has been shown. Small areas
within the flood plain boundaries may lie above the flood elevations, but cannot be shown due to
limitations of the map scale and/or lack of detailed topographic data.


4.2 Floodways

Encroachment on floodplains, such as structures and fill, reduces the flood-carrying capacity,
increases flood heights and velocities, and increases 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, a floodway is used as a tool to assist local communities in this aspect of
floodplain management. Under this concept, the area of the 100-year floodplain is divided into a
floodway and a floodway fringe. The floodway is the channel of a stream plus any adjacent
floodplain areas that must be kept free of encroachment so that the 100-year flood can be carried
without substantial increases in flood heights. Minimum Federal Standards limit such increases to
1.0 foot, provided that hazardous velocities are not produced. The floodways in this study are
presented to local agencies as minimum standards that can be adopted directly or that can be used as
a basis for additional floodway studies.




                                                 7
The area between the floodway and 100-year floodplain boundaries is termed the floodway fringe.
The floodway fringe encompasses the portion of the floodplain that could be completely obstructed
without increasing the water surface elevation of the 100-year flood by more than 1.0 foot at any
point. Typical relationships between the floodway and the floodway fringe and their significance to
floodplain development are shown in Figure 2.




                      Figure 2 - Floodway Schematic

The floodway along the Platte through the Butler County was delineated based on equal conveyance
reduction on both sides of the channel. The floodway boundaries are shown on the Flood Insurance
Rate Map. Where the 100-year flood boundary and floodway boundary are collinear, only the
floodway boundary is shown.


5.0 INSURANCE APPLICATIONS

For insurance rating purposes, flood insurance zone designations are assigned to a community based
on the results of the engineering analyses. These zones are as follows.

Zone A

Zone A is a flood insurance rate zone that corresponds to the 100-year floodplains that are
determined in the Flood Insurance Study by approximate methods. Because detailed hydraulic
analyses are not performed for such areas, no base flood elevations or depths are shown within this
zone.


                                                8
Zone AE

Zone AE is the flood insurance rate zone that corresponds to the 100-year floodplains that are
determined in the Flood Insurance Study by detailed methods. In most instances, whole-foot base
flood elevations derived from the detailed hydraulic analyses are shown at selected intervals within
this zone.

Zone X
Zone X is the flood insurance rate zone that corresponds to areas outside the 100-year floodplain,
areas of 100-year flooding where average depths are less than 1 foot, areas of 100-year flooding
where the contributing drainage area is less than 1 square mile, and areas protected from the 100-year
flood by levees. No base flood elevations or depths are shown within this zone.


6.0 FLOOD INSURANCE RATE MAP

The Flood Insurance Rate Map is designed for flood insurance and floodplain management
applications. For flood insurance applications, the map designates flood insurance rate zones as
described in Section 5.0 and, in the 100-year floodplains that were studied by detailed methods,
shows selected whole-foot base flood elevations or average depths. Insurance agents use the zones
and base flood elevations in conjunction with information on structures and their contents to assign
premium rates for flood insurance policies.

For floodplain management applications, the map shows by tints, screens, and symbols the 100- and
500-year floodplains, the floodways, and the locations of selected cross sections used in the hydraulic
analyses and floodway computation.


7.0 OTHER STUDIES

There was no previously published detailed flood hazard information for the Platte River through
Butler County.

8.0 LOCATION OF DATA

Information concerning the pertinent data used in the preparation of this study can be obtained by
contacting the Natural and Technology Hazards Division.




                                                  9
9.0 REFERENCES AND BIBLIOGRAPHY

1. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-FFA, Flood Frequency
Analysis Program, Version 3.0, July 1992, Davis, California.

2. U.S. Army Corps of Engineers, Omaha District, Hydrologic Analysis, Lower Platte River,
Nebraska, Flood Insurance Study, March 1998.

3. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-2 Water-Surface Profiles,
Version 4.6.2, Davis, California, May 1991.

4. U. S. Geological Survey, 7.5-Minute Series Topographic Maps, Scale: 1:24,000, Contour
Interval 10 feet, Columbus, Nebraska, 1958 (photo revised 1976); Richland, Nebraska, 1968;
Rogers, Nebraska, 1968.




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