JEFFERSON COUNTY, ARKANSAS AND INCORPORATED AREAS

Transcription

1 JEFFERSON COUNTY, ARKANSAS Community Name Community Number ALTHEIMER, CITY OF HUMPHREY, CITY OF JEFFERSON COUNTY (UNINCORPORATED AREAS) PINE BLUFF, CITY OF REDFIELD, CITY OF SHERRILL, TOWN OF WABBASEKA, CITY OF WHITE HALL, CITY OF REVISED DATE: Federal Emergency Management Agency FLOOD INSURANCE STUDY NUMBER 05069CV000B

2 NOTICE TO FLOOD INSURANCE STUDY USERS Communities participating in the National Flood Insurance Program have established repositories of flood hazard data for floodplain management and flood insurance purposes. This Flood Insurance Study (FIS) may not contain all data available within the repository. It is advisable to contact the community repository for any additional data. Part or all of this FIS may be revised and republished at any time. In addition, part of this FIS may be revised by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS. It is, therefore, the responsibility of the user to consult with community officials and to check the community repository to obtain the most current FIS components. Initial Countywide FIS Effective Date: March 16, Revised Countywide FIS Revision Date:

3 TABLE OF CONTENTS 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgments Coordination AREA STUDIED Scope of Study Community Description Principal Flood Problems Flood Protection Measures ENGINEERING METHODS Hydrologic Analyses Detailed Study Streams Incorporated from a LOMR Redelineated Detailed Study Streams Revised Countywide Analyses Hydraulic Analyses Detailed Study Streams Incorporated from a LOMR Redelineated Detailed Study Streams Revised Countywide Analyses Benchmarks Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATIONS FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES 38 i

4 TABLE OF CONTENTS - continued FIGURES FIGURE 1 - FLOODWAY SCHEMATIC 35 TABLES TABLE 1 - SCOPE OF STUDY 3 TABLE 2 - SUMMARY OF DISCHARGES TABLE 3 - SUMMARY OF ROUGHNESS COEFFICIENTS: REDELINEATION 18 TABLE 4 - SUMMARY OF ROUGHNESS COEFFICIENTS: PMR ANALYSIS 20 TABLE 5 - FLOODWAY DATA TABLE 6 - COMMUNITY MAP HISTORY 37 EXHIBITS Exhibit 1 Flood Profiles Arkansas River Arnold Creek Arnold Creek Tributary Bayou Bartholomew Bayou Bartholomew Tributary No. 1 Bayou Bartholomew Tributary No. 2 Boggy Bayou Boggy Bayou Tributary No. 1 Boggy Bayou Tributary No. 2 Brumps Bayou Caney Bayou Caney Bayou Fork Caney Bayou Tributary A Caney Bayou Tributary A1 Caney Tributary No. 1 Caney Tributary No. 2 Carter Creek Dancing Rabbit Creek Drainage Ditch East of School Eden Park Drain Flat Bayou Gamble Creek Panels 01P 05P Panels 06P 07P Panels 08P 09P Panels 10P 14P Panel 15P Panels 16P 17P Panels 18P 19P Panel 20P Panel 21P Panels 22P 24P Panels 25P 27P Panels 28P 30P Panels 31P 32P Panels 33P 34P Panels 35P 36P Panels 37P 38P Panels 39P 40P Panel 41P Panel 42P Panel 43P Panel 44P Panels 45P 47P ii

5 TABLE OF CONTENTS - continued EXHIBITS - continued Exhibit 1 Flood Profiles - continued Harding Drain Henslee Creek Industrial Creek Industrial Creek Tributary Interceptor Canal Nevins Creek Nevins Tributary Oakland Heights Creek Oakland Heights Tributary Outlet Canal Outlet Canal Tributary A Park Creek Piney Creek Pitts Drain School Drain Spring Hill Creek Sulphur Springs Creek Sulphur Springs Tributary Wabbaseka Bayou Panels 48P 49P Panels 50P 51P Panels 52P 53P Panel 54P Panels 55P 56P Panels 57P 59P Panels 60P 61P Panels 62P 63P Panel 64P Panels 65P 66P Panel 67P Panels 68P 69P Panels 70P 72P Panel 73P Panels 74P 75P Panels 76P 77P Panels 78P 79P Panels 80P 82P Panel 83P Exhibit 2 Flood Insurance Rate Map Index Flood Insurance Rate Maps iii

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7 FLOOD INSURANCE STUDY KANSAS 1.0 INTRODUCTION 1.1 Purpose of Study This Flood Insurance Study (FIS) revises and updates information on the existence and severity of flood hazards in the geographic area of Jefferson County, Arkansas, including the Cities of Altheimer, Humphrey, Pine Bluff, Redfield, Wabbaseka, and White Hall; the Town of Sherrill; and unincorporated areas of Jefferson County (hereinafter referred to collectively as Jefferson County), and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 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, Please note that the City of Humphrey is geographically located in both Jefferson and Arkansas Counties. The FIS and Flood Insurance Rate Map (FIRM) for Jefferson County, Arkansas, and Incorporated Areas will show only those portions located within Jefferson County. See the separately published FIS report and FIRM for Arkansas County, Arkansas and Incorporated Areas (Reference 1) for flood hazard information outside of Jefferson County. 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 be able to explain them. 1.2 Authority and Acknowledgments The sources of authority for this FIS are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of The hydrologic and hydraulic analyses for the original FIS for the City of Altheimer (Reference 2) were performed by Miller-Newell Engineers, Ltd., for the Federal Insurance Administration, under Contract No. H This study was completed in February The hydrologic and hydraulic analyses for the original FIS for the City of Pine Bluff (Reference 3) were performed by the Vicksburg District of the U.S. Army Corps of Engineers (USACE-Vicks), for the Federal Emergency Management Agency (FEMA), under Interagency Agreement (IAA) No. H-18-78, Project Order No. 32. This study was completed in June The hydrologic and hydraulic analyses for the original FIS for the unincorporated areas of Jefferson County (Reference 4) were performed by the Little Rock District of the U.S. Army Corps of Engineers (USACE-LR) for 1

8 FEMA, under IAA No. EMW-84-E-1506, Project Order No. 1, Amendment No. 32. This work was completed in April The hydrologic and hydraulic analyses for the original FIS for the City of Wabbaseka (Reference 5) were performed by Garver and Garver, Inc. for FEMA, under Contract No. H This study was completed in June The hydrologic and hydraulic analyses for the original FIS for the City of White Hall (Reference 6) were performed by USACE-LR for FEMA, under IAA No. EMW-84-E-1506, Project Order No. 1, Amendment No. 32. This study was completed in August The hydrologic and hydraulic analyses for a portion of Gamble Creek in the first revision were performed during the preparation of the original FIS for the unincorporated areas of Jefferson County (Reference 4). The hydrologic and hydraulic analyses for the March 16, 2009 countywide FIS for Jefferson County, Arkansas and Incorporated Areas (Reference 7) incorporates the results of a Letter of Map Revision (LOMR) performed by FTN Associates Ltd., issued for reaches of Bayou Bartholomew, Industrial Creek, Industrial Creek Tributary, Nevins Creek, Spring Hill Creek, Dancing Rabbit Creek, and Outlet Canal for the City of Pine Bluff (Reference 8), with minor modifications using available additional updated topographic data. The hydraulic analyses for the countywide study were performed by FTN/Taylor JV for FEMA, under Contract No. EMT-2002-CO-0050, Task Order No This study was completed in May The hydraulic analyses for the levee analysis revision were performed by Risk Assessment, Mapping, and Planning Partners (RAMPP) for FEMA, under Contract No. HSFEHQ-09-D-0369, Task Order HSFE06-09-J This study was completed in January Base map information shown on this revision s FIRMs was provided in digital format by the Arkansas Geographic Information Office and FEMA s effective digital FIRM (DFIRM) database (Reference 9). 1.3 Coordination An initial Consultation Coordination Officer s (CCO s) meeting is typically held with representatives of FEMA, the community, and the study contractor to explain the nature and purpose of an FIS, and to identify the streams to be studied by detailed methods. A final CCO meeting is typically held with the same representatives to review the results of the study. The dates of the initial and final CCO meetings for the countywide FIS were March 22, 2005, and May 30, 2007, respectively. These meetings were attended by representatives of FEMA, the communities, and the study contractor. For the levee analysis revision, an initial CCO meeting was held on October 28, 2009, and attended by representatives of FEMA, the community, and the study contractor. 2

9 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Jefferson County, Arkansas, including the incorporated communities listed in Section 1.1. The March 16, 2009, FIS combined the individual community studies into a countywide FIS and converted all FIRMs to DFIRM format. In the March 16, 2009, countywide study, reaches of Bayou Bartholomew, Dancing Rabbit Creek, Nevins Creek, and Spring Hill Creek, studied as part of a recent LOMR with Case No P (Reference 8), were incorporated with minor modifications using available additional updated topographic data. All stream reaches with prior effective detailed mapping not studied for the countywide study were redelineated. All stream reaches with approximate mapping not studied for the countywide study were adjusted to reflect updated topographic data where available (Reference 7). In the levee analysis revision, reaches of the Arkansas River and Caney Bayou were restudied as a result of the de-accreditation of Tucker Lake Levee and the western segment of the Plum Bayou Levee. All other effective information was retained. For this revision, limits of detailed studies for the revised streams are shown in Table 1. Stream Name TABLE 1 - SCOPE OF STUDY Limits of Detailed Study Arkansas River Caney Bayou From approximately 0.80 mile upstream of Union Pacific Railroad to approximately 0.85 mile downstream of the County Boundary From approximately 0.95 mile downstream of Regional Park Drive to approximately 0.87 mile downstream of Jefferson Parkway No LOMRs have been incorporated into this revision. 2.2 Community Description Jefferson County is located in the south-central portion of Arkansas and is bordered by Pulaski and Lonoke Counties to the north, Arkansas County to the east, Cleveland and Lincoln Counties to the south, and Grant County to the west. The county seat is the City of Pine Bluff. The City of Altheimer is located in the northeast portion of Jefferson County. The City of Humphrey is located in northeast Jefferson County along the Arkansas County line. The City of Pine 3

10 Bluff lies on the south bank of the Arkansas River in central Jefferson County. The City of Redfield is located in the northwest portion of Jefferson County. The Town of Sherrill is located in the north-central portion of Jefferson County, on the north bank of Plum Bayou. The City of Wabbaseka is located on U.S. Highway 79 in the northeast portion of Jefferson County about 10 miles northeast of the Arkansas River. The City of White Hall is located in the west-central portion of Jefferson County, adjacent to the City of Pine Bluff and approximately 40 miles southeast of Little Rock, the state capital of Arkansas. According to the U.S. Census Bureau, the population of Jefferson County has decreased from 85,487 in 1990 to 81,700 in 2005 (Reference 10). The population of the City of Pine Bluff has decreased from 57,140 in 1990 to 53,419 in 2004 (Reference 10). For the City of Altheimer, the majority of the study area is urban with lawns and a variety of trees and plants. The lower elevation areas along Flat Bayou support heavy growths of water-tolerant trees and plants. The floodplains include public utilities, residential developments, and several city streets. The Arkansas River, which flows from northwest to southeast immediately north of the City of Pine Bluff, is a major river in the south-central United States, and affects floodwater drainage in the northern part of the City. An old, cut-off, river bendway (Lake Langhofer) acts as part of the northern boundary of the City. The Arkansas River drains parts of Texas, New Mexico, Colorado, Kansas, Missouri, Oklahoma, and Arkansas within its 160,645 square miles of drainage area above its confluence with the Mississippi River. Caney Bayou, which flows into Lake Langhofer from the west, drains approximately 38.4 square miles along its 21 miles of length. Tributaries A and A1 drain the northwest corner of the City of Pine Bluff. Brumps Bayou, with a length of 3.7 miles, drains 2.3 square miles within the City through Lake Pine Bluff, an Arkansas Game and Fish Commission project, into Lake Langhofer. The 2.1 square miles of area drained by Harding Drain Town Branch lie in the north-central part of the City of Pine Bluff and include a large portion of the downtown area. When stages are low on the Arkansas River, flows pass through a floodgate beneath East Second Avenue into Lake Langhofer; however, during high water on the Arkansas River, excess flows pass into Bayou Bartholomew through the Outlet Canal. The Outlet Canal drains the northeastern third of the City of Pine Bluff by acting as a flow outlet for Harding Drain Town Branch, the Admiral Benbow area, Pitts Drain, Tributary A, and other minor areas, which comprise its 10.2 square miles of drainage that empties into Bayou Bartholomew. Pitts Drain (1.5 square miles) and Outlet Canal Tributary A (1 square mile) drain the east-central portion of the City. Bayou Bartholomew is a major east-bank tributary of the Ouachita River and drains 1,718 square miles. The Bayou runs along the southern corporate limits of 4

11 the City of Pine Bluff and drains the southwestern third of the City via the Interceptor Canal, Eden Park Drain, and other small tributaries. The Interceptor Canal drains 2.5 square miles of the southwestern part of the City of Pine Bluff, while Eden Park Drain drains 2.2 square miles of the south-central part of the City. As for the City of Wabbaseka, the Wabbaseka Bayou borders the south side of the City. The drainage from the City enters the Bayou. Wabbaseka Bayou travels southeast through the corner of Arkansas County, where it enters Long Pond. Long Pond drains into Little Bayou Meto, which travels south into Jefferson County to empty into the Arkansas River. The City of White Hall is mostly on gently rolling hills, with some areas within the floodplain of Caney Bayou and its tributaries. The unincorporated areas of Jefferson County are composed primarily of upland and floodplain areas. The upland areas are gently rolling hills covered by woodlands with some residential areas. The Arkansas River bisects the county diagonally from the northwest to the southeast, creating large floodplain areas that are primarily under cultivation. Commercial and residential development is generally along each of the streams in this study. The watersheds in the unincorporated areas of Jefferson County lie to the northwest, west, southwest, and southeast of the City of Pine Bluff. The topography of the area is gentle, and ranges in elevation from 198 feet at the lower end of Bayou Bartholomew to approximately 420 feet at the headwaters of Caney Bayou. Nearly all streams lie on east- or north-facing slopes. The overbanks of all streams within the County are heavily vegetated, although the stream channels are generally clear. The Pine Bluff Arsenal is located in central Jefferson County, approximately 8 miles northwest of the City of Pine Bluff. According to the Center for Land Use Interpretation, the Pine Bluff Arsenal is an active military arsenal that stores 12.5 percent of the Nation s obsolete chemical weapons. Biological weapons operations were conducted here from the 1950s to the 1970s. The 14,943-acre installation has 267 munitions storage igloos, and employs around 1,200 people (Reference 11). The climate in Jefferson County is humid, with variable temperature. The average annual temperature is 61 degrees Fahrenheit ( F). Summers are moderately long and hot, with daily maximum temperatures occasionally exceeding 100 F. Winters are moderately short and cold. The average annual precipitation for the area is approximately 50 inches. Precipitation is fairly uniformly distributed throughout the year, with heavier amounts occurring in the spring and lesser amounts occurring in the summer. Intense thunderstorms occur in the summer and may release large amounts of rain over a small area in a short period of time. Snowfall is light, with the area receiving approximately 4 inches annually. 5

12 2.3 Principal Flood Problems The Arkansas River has produced many devastating floods in the past, causing property damage in the millions of dollars. The upstream reaches of the streams in this study are continuing to develop. This will require floodplain management to avoid flood damages since increased runoff will occur. The 10 highest recorded annual stages for the Arkansas River at Pine Bluff (Reference 4) are shown in the following tabulation: Stage Elevation (in feet, North American Vertical Datum of 1988 Date of Crest [NAVD 88]) May 28, June 24, April 21, April 22, February 23, May 31, November 8, May 22, February 3, May 31, Major flooding along the Arkansas River caused extensive damage in 1927 due to multiple levee failures and blocked drainage from tributaries that emptied into the river. Extensive flooding occurred in the low-lying portions of downtown Pine Bluff. Though higher stages occurred on the Arkansas River at Pine Bluff in both 1935 and 1943, the extent of flooding in Pine Bluff was much less because the levees along the river were not overtopped or breached during those floods. Major agricultural development has occurred in the floodplain, resulting in agricultural flood damages that can be quite extensive. While residential and commercial development has occurred mostly on the fringes of this large floodplain, significant flood damage has resulted from this encroachment. Continuing developmental pressures within the floodplain could result in extensive urban flood damages. The flood of record on Bayou Bartholomew occurred on May 2, 1958, when approximately 5 inches of rain fell during a 15-hour period at the Pine Bluff National Weather Service (NWS) station. This flood was estimated to have a recurrence interval of approximately 40 years based on stage records for a gage on the State Route 15 bridge. During the period from December 25 to 28, 1982, 9.25 inches of rain were recorded at the Pine Bluff NWS station, causing a flood on Bayou Bartholomew with a recurrence interval of approximately 30 years based on records at the State Route 15 gage. The next largest recorded flood on Bayou Bartholomew was 6

13 caused by a storm that produced 4.18 inches of rain at the Pine Bluff NWS station from March 16 to 21, Its recurrence interval was approximately 25 years. Continuous records of river stages on the Arkansas River at the gage in Little Rock are available from 1927 to the present. Since flows in Little Rock do not vary appreciably from the flows in Pine Bluff, they provide a good indication of Arkansas River discharges in the study area. Stage readings from the headwater and tailwater gages at Locks and Dams Nos. 3, 4, and 5 are also available on the Arkansas River for the period since the McClellan-Kerr Navigation System was completed. Records of maximum water elevations are available for Bayou Bartholomew from 1952 to the present from the USACE gage at the State Route 15 bridge in Pine Bluff. USACE also maintains three other crest stage gages on Bayou Bartholomew in Jefferson County that record high-water elevations. The locations and periods of record are as follows: Old Warren Road, 1952 to present; U.S. Route 79, 1964 to present; and Outlet Canal, 1957 to present. A crest stage gage was also installed on a small tributary of Piney Creek by the U.S. Geological Survey (USGS) in 1961 and is currently published as Nevins Creek Tributary near Pine Bluff. Flooding along Caney Bayou and its tributaries within the corporate limits of the City of Pine Bluff is usually of the flash flood type produced by short-term, highintensity storms. While extensive flooding occurred in July 1951 and August 1970 from summer thunderstorms, no record of the extent of flooding is available, as flooding in that area of the City of Pine Bluff caused little concern. However, continued development in this area has increased concern for flood problems. No high-water marks or flood records are available for Caney Bayou within the City of White Hall corporate limits. The upstream portions of the study streams within the corporate limits of the City of White Hall have been developed in recent years, and this requires floodplain management to avoid flood damages since increased runoff will occur. Although Brumps Bayou is also subject to flash flooding from short duration, intense thunderstorms, no determination of the extent of flooding from the previously mentioned storms is available. Along Harding Drain, significant flooding occurred during the storms of July 1951, May 1958, August 1970, and June High-water marks were established along Harding Drain for all of these storms, as the entire stream is highly developed both residentially and commercially. The estimated recurrence intervals of the maximums in various reaches of the stream were approximately 10 years. The extent of flooding along Outlet Canal from some of the previously mentioned storms was significant, but only minimal damage occurred due to the lack of development along the canal at that time. However, continued development in this area has increased concern for flood problems. 7

14 Along Pitts Drain, extensive flooding occurs from the above-mentioned short duration storms. While little damage occurs in the lower reach, extensive damage occurs in the middle and upper reaches, where moderate to heavy residential development in the floodplain has occurred. Outlet Canal Tributary A is also subject to widespread flooding from short duration storms. Significant damage occurs in the middle and upper reaches, where moderate to heavy residential development exists in the floodplain. Eden Park Drain suffers widespread flooding from these local intense thunderstorms as well as longer duration storms affecting Bayou Bartholomew. While little damage occurs in the lower reach, extensive damage occurs in the upper reach, where moderate to heavy residential development exists in the floodplain. Flooding along Interceptor Canal is also the result of local intense storms. While flood flows are passed within banks on the lower two-thirds of the canal, extensive damage occurs in the upper reach of the stream, where moderate to very heavy residential development has occurred in the floodplain. The only rain gage in Jefferson County maintained by the NWS is located in the City of Pine Bluff. 2.4 Flood Protection Measures The levees along the Arkansas River provide protection from flooding of the Arkansas River. Completion of the mainline levees along the south bank of the Arkansas River, which protect the downtown area of the City of Pine Bluff from the Arkansas River, was accomplished by the Federal Government following the disastrous 1927 flood. The subsequent construction of 28 major reservoirs that provide flood control storage in the Arkansas River Basin upstream of the City of Pine Bluff resulted in a significant reduction in floodwater levels along the river. The Harding Drain drainage structure was constructed in conjunction with the mainline levee to provide an outlet for interior stormwater whenever the Arkansas River is low. The USACE ensures proper maintenance by annual inspections of all accredited levees along the Arkansas River. FEMA specifies that all levees must have a minimum freeboard of 3 feet above the 1-percent-annual-chance flood event. The USACE levees exceed the FEMA freeboard requirement for 1-percent-annualchance flood protection and provide greater than 0.2-percent-annual-chance flood protection. Plum Bayou Levee (Plum Bayou Segment) is an accredited levee along the eastern bank of Plum Bayou that extends to the confluence of the Arkansas River. New Gascony Levee is an accredited levee along the eastern bank of the Arkansas River that extends from the confluence of Plum Bayou to the confluence of Kings Bayou. Jefferson County Levee No. 3 is an accredited levee along the eastern bank of the Arkansas River that extends from the New Gascony Levee to the Farelly Lake Levee. Farelly Lake Levee is an accredited levee along the eastern bank of the Arkansas River that extends from the Jefferson County Levee No. 3 to the southern county boundary. The French Town Auburn 8

15 Levee is an accredited levee along the western bank of the Arkansas River that extends from the confluence of Caney Bayou to the southern county boundary. Tucker Lake Levee is a de-accredited levee that runs along the western bank of the Arkansas River, the western bank of Lake Langhofer, and the northern bank of Caney Bayou, upstream of the confluence of Caney Bayou and Lake Langhofer. Plum Bayou Levee (Arkansas River Segment) is a de-accredited levee along the eastern bank of the Arkansas River that extends from the northern county boundary to the confluence of Arkansas River and Plum Bayou. Locks and Dams Nos. 3, 4, and 5 on the Arkansas River are located in the southeast, central, and northwest portions of Jefferson County. They have no flood control storage and are part of the McClellan-Kerr Navigation System. In 1954, USACE constructed the Interceptor Canal, enlarged the Outlet Canal, and enlarged and cleaned out the channel of Harding Drain and a short segment of Bayou Bartholomew. This work was an effort to reduce flood problems in the highly developed residential and commercial area drained by Harding Drain. The City of Pine Bluff has accomplished drainage improvement projects along portions of Caney Bayou Tributary A and Pitts Drain. The Caney Bayou Tributary A project had a significant impact in reducing damages on part of the stream for all but major floods. The work on Pitts Drain provided some flood relief from more frequent storms on a portion of the stream, but had little effect in reducing overall flood damages. The City has an active program for cleanout and maintenance of existing ditches. The City of Pine Bluff has implemented an ordinance designed to control land use and development in designated floodprone areas subject to its jurisdiction. 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods 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 are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 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-percent, 2-percent, 1-percent, and 0.2-percent-annual-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 that equals or exceeds the 1-percent-annual-chance (100-year) flood in any 50-year period is approximately 40 percent (4 in 10); for any 90- year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 9

16 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for each Jefferson County flooding source studied in detail Detailed Study Streams Incorporated from a LOMR Reaches of Bayou Bartholomew, Dancing Rabbit Creek, Nevins Creek, and Spring Hill Creek were studied as part of a recent LOMR, Case No P (Reference 8). The discharges for these flooding sources were based on hydrologic determinations for the previous studies Redelineated Detailed Study Streams In the original FIS for the unincorporated areas of Jefferson County, dated April 16, 1991 (Reference 4), hydrologic analyses for the Arkansas River were obtained from the detailed and extensive engineering studies that have been previously performed in connection with the design, construction, and maintenance of the McClellan-Kerr Arkansas River Navigation System. Continuous records of river stages on the Arkansas River at the Little Rock gage are available from 1927 to date. These flood data were investigated to determine the discharge frequency used in this study. These data were adjusted as required for the effects of the appropriate upstream storage reservoirs. In the original study for the unincorporated areas of Jefferson County (Reference 4) and the original City of Wabbaseka study, dated June 1980 (Reference 5), it was determined that the flat terrain along most of Wabbaseka Bayou made the drainage areas indeterminate in the vicinity of this study. USACE-Vicks provided stage data and gage information for the U.S. Route 79 stream gage on Wabbaseka Bayou. The stage-frequency curve was used to obtain elevations for the 10-, 2-, and 1-percent-annualchance floods. Gage data were used to construct a rating curve for Wabbaseka Bayou at U.S. Route 79, and used with the stage data to determine discharges for the 10-, 2-, and 1-percent-annual-chance floods. The 0.2-percent-annual-chance flood discharge was determined from a log-probability relationship of the lower frequency flows. The 0.2-percentannual-chance flood stage elevation was obtained from the rating curve. For the remaining streams studied by detailed methods in the original study for the unincorporated areas of Jefferson County (Reference 4), the HEC-1 flood hydrograph package (Reference 12) was used to develop flood-frequency discharge values. Design storms for floods of the selected recurrence intervals were generated using rainfall data from NWS Technical Paper No. 40 (TP-40) and Technical Paper No. 49 (TP-49) and NWS Technical Memorandum Hydro-35 (HYDRO-35) (References 12, 13, and 14, respectively). Time distribution of these annual series rainfalls and adjustment of rainfall by area was done using options in the HEC-1 computer program (Reference 12). Snyder s synthetic unit hydrographs were developed for each of the sub-basins. The unit hydrograph 10

17 coefficients were developed using the modified Fort Worth method consisting of unit hydrograph relations and unit graph adjustments for urban areas. Routing through the reaches was accomplished using the modified Puls option of the HEC-1 computer program (Reference 12). HEC-2 storage runs were prepared and executed to provide storageelevation-discharge data for routing each reach (Reference 16). A summary of the drainage area-peak discharge relationships for the streams studied by detailed methods is shown in Table 2, Summary of Discharges. TABLE 2 - SUMMARY OF DISCHARGES DRAINAGE AREA (sq. miles) 10% Annual Chance PEAK DISCHARGES (cfs) 2% 1% Annual Annual Chance Chance 0.2% Annual Chance FLOODING SOURCE AND LOCATION Arkansas River At Lock and Dam No , , , , ,000 Arnold Creek At confluence with Caney Bayou ,050 2,850 3, 4,050 Arnold Creek Tributary At confluence with Arnold Creek ,300 1,450 1,900 Bayou Bartholomew At confluence of Outlet Canal ,600 8,200 9,400 12,000 At City of Pine Bluff corporate limits ,600 4,900 5,600 7,200 Bayou Bartholomew Tributary No. 1 At confluence with Bayou Bartholomew , 1,950 2, 2,850 Bayou Bartholomew Tributary No. 2 At confluence with Bayou Bartholomew ,450 2,100 2,400 3,000 Boggy Bayou At confluence with Sandy Bayou ,650 7,100 8,200 10,900 Boggy Bayou Tributary No. 1 At confluence with Boggy Bayou ,300 1,950 2, 2,900 Boggy Bayou Tributary No. 2 At confluence with Boggy Bayou ,050 1,800 2,100 2,750 Brumps Bayou At station 2,000 feet 2.3 1,279 1,680 1,883 3,316 Caney Bayou At confluence with Lake 11

18 Langhofer ,000 4,600 5,400 7,200 At City of White Hall corporate limits ,700 5,700 6,700 8,800 Caney Bayou Fork At confluence with Caney Bayou ,050 6,300 7,300 9,600 TABLE 2 - SUMMARY OF DISCHARGES (CONTINUED) DRAINAGE AREA (sq. miles) 10% Annual Chance PEAK DISCHARGES (cfs) 2% 1% Annual Annual Chance Chance 0.2% Annual Chance FLOODING SOURCE AND LOCATION Caney Bayou Tributary A At Station 3,900 feet ,004 1,150 2,085 Caney Bayou Tributary A1 At confluence with Caney Bayou Tributary A ,884 Caney Tributary No. 1 At confluence with Caney Bayou ,150 Caney Tributary No. 2 At confluence with Caney Bayou Carter Creek At confluence with School Drain Dancing Rabbit Creek At confluence with Bayou Bartholomew Drainage Ditch East of School Section A Section B Eden Park Drain At confluence with Bayou , , , ,675 Bartholomew ,270 2,300 Flat Bayou Section A Section B Gamble Creek At confluence with Caney Bayou ,400 1,550 1,950 Harding Drain At 8 th Avenue 1.2 1,720 2,200 2,430 3,100 Henslee Creek At confluence with Caney Bayou Industrial Creek At State Route ,000 2,850 3,200 4,100 Industrial Tributary At confluence with ,

19 TABLE 2 - SUMMARY OF DISCHARGES (CONTINUED) DRAINAGE AREA (sq. miles) 10% Annual Chance PEAK DISCHARGES (cfs) 2% 1% Annual Annual Chance Chance 0.2% Annual Chance FLOODING SOURCE AND LOCATION Industrial Creek Interceptor Canal At confluence with Bayou Bartholomew 2.5 1,052 1,330 1,541 2,988 Nevins Creek At confluence with Bayou Bartholomew ,800 8,900 10,500 14,500 Nevins Tributary At confluence with Nevins Creek ,600 2,350 2,600 3,050 Oakland Heights Creek At confluence with Caney Bayou Tributary ,300 1,550 2,150 Oakland Heights Tributary At confluence with Oakland Heights Creek Outlet Canal At confluence with Bayou Bartholomew ,800 2,400 2,750 5,500 Outlet Canal Tributary A At Ohio Street ,147 Park Creek At confluence with Oakland Heights Creek Piney Creek At confluence with Nevins Creek ,800 8,800 10,600 14,800 Pitts Drain At confluence with Outlet Canal ,749 School Drain At confluence with Caney Tributary No ,150 Spring Hill Creek At confluence with Bayou Bartholomew ,100 1,650 2,100 2,200 Sulphur Springs Creek At confluence with Nevins Creek ,975 3,050 3,875 5,100 Sulphur Springs Tributary At confluence with Sulphur Springs Creek ,500 1,700 2,150 Wabbaseka Bayou At U.S. Route 79 Indeterminate 1,870 2,350 2,500 3,000 13

20 In the original City of Altheimer study dated February 1980 (Reference 2), peak discharges for the 10-, 2-, and 1-percent-annual-chance flood events were obtained from Water Resources Circular Number 11 published by the USGS (Reference 17). Discharges for the 0.2-percent-annual-chance flood were determined by straight-line extrapolation of a semi-log graph of flood discharges computed for frequencies up to once in 100 years. In the original City of Pine Bluff study dated June 1, 1983 (Reference 3), rainfall frequency data were obtained from TP-40 and TP-49 (References 12 and 13). Rainfall data for the Standard Project Flood were derived from USACE Engineer Manual (EM) , Standard Project Flood Determinations (Reference 18). Rainfall excess values were applied to unit hydrographs by Snyder s Method (Reference 19) to develop peak flow values for each frequency storm. In the original City of Pine Bluff study (Reference 3), it was determined that peak flows along Bayou Bartholomew could not be developed by Snyder s Method due to the extensive overbank storage along the stream. A storage routing model was developed to reproduce available flow data for two previous storms (May 1958 and May 1968). The unit hydrographs developed by the routing model, which used the Working Value R Routing Method, described in EM (Reference 20), were then applied to various synthetic storms for which infiltration rates developed by the model were also used. In the original City of Wabbaseka study dated June 1980 (Reference 5), only a small portion of Wabbaseka Bayou was studied, so constant discharges were used throughout the study reach. In the original study and the FIS for the City of White Hall revised August 15, 1990 (Reference 6), discharges for the streams studied by detailed methods, including the portion of Gamble Creek studied in the first revision, were developed using the HEC-1 Flood Hydrograph Package (Reference 21). Design storms for floods with return periods of 10-, 2-, 1-, and 0.2-percent-annual-chance were generated using rainfall data from TP-40 and HYDRO-35 (References 12 and 14). Time distribution of these annual series rainfalls and adjustment of rainfall by area was done using the options in the HEC-1 computer program (Reference 21). Snyder s synthetic unit hydrographs were developed for each of the sub-basins. The unit hydrograph coefficients were developed using the modified Fort Worth method consisting of unit hydrograph relations and unit graph adjustments for urban areas. Routing through the reaches was accomplished using the modified Puls option in the HEC-1 program (Reference 21). 14

21 3.1.3 Revised Countywide Analyses 3.2 Hydraulic Analyses No new hydrologic analysis was performed as part of this levee PMR. The effective flows were applied in the analysis. 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. Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data tables in the FIS report. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are cautioned to use the flood elevation data presented in this FIS in conjunction with the data shown on the FIRM. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the FIRM (Exhibit 2). Along certain portions of the Arkansas River, a profile base line is shown on the maps to represent channel distances as indicated on the flood profiles and floodway data tables. The hydraulic analyses for this study and the previous studies were based on unobstructed flow. The flood elevations shown on the profiles (Exhibit 1) are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail Detailed Study Streams Incorporated from a LOMR Reaches of Bayou Bartholomew, Dancing Rabbit Creek, Nevins Creek, and Spring Hill Creek were studied as part of a recent LOMR, Case No P (Reference 8). These streams have been incorporated with minor modifications using additional updated topographic data. Water surface elevations for the 10-, 2-, 1-, and 0.2-percent-annual-chance floods for these flooding sources were computed using the USACE HEC-RAS version step-backwater computer program (Reference 22). The channel and near overbank ( feet from channel) elevation and structure data for the hydraulic models were generally obtained from detailed field survey information. For each survey cross section, the field elevations were blended with overbank topographic data obtained from the detailed topographic data provided by the Arkansas Highway and Transportation Department, as part of a recent LOMR, Case No P (Reference 8) Redelineated Detailed Study Streams In the original FIS for the unincorporated areas of Jefferson County, dated April 16, 1991 (Reference 4), cross sections for the streams studied by 15

22 detailed methods were developed by field survey. Cross sections were extended beyond survey data using USGS topographic maps (Reference 23). All cross sections are computer-plotted and show field survey data as well as data obtained from the topographic maps. Any additional cross sections that were not surveyed were estimated using the topographic maps or were synthetic sections made from actual cross sections. Dimensions of hydraulic structures were obtained by field survey. In the original study for the unincorporated areas of Jefferson County (Reference 4), water surface elevations of floods for the selected recurrence intervals for the Arkansas River were computed using USACE s Program LRD-1 (Reference 24). Water surface elevations for Wabbaseka Bayou were picked from the stage-frequency curves at U.S. Route 79. The 0.2-percent-annual-chance flood elevations were picked from a rating curve developed from gage data. For the remaining streams studied by detailed methods, water surface elevations were computed using the USACE HEC-2 step-backwater computer program (Reference 16). Starting water surface elevations were determined using the slopearea method except when the drainage areas of two confluent streams were within 60 percent of one another and coincident peaks were assumed to occur. When coincident peaks were assumed, the mainstem water surface elevation at the confluence was used as the starting elevation for the tributary. Starting water surface elevations for the following streams were determined using the slope-area method: Gamble Creek, Bayou Bartholomew, Outlet Canal, Dancing Rabbit Creek, Spring Hill Creek, Nevins Tributary, and Sulphur Springs Creek. Starting water surface elevations for the remaining streams studied by detailed methods were determined using the coincident peak method previously described. In the original study for the unincorporated areas of Jefferson County (Reference 4), it was determined that Boggy Bayou was affected by backwater effects from Bayou Bartholomew that were determined using a discharge-drainage area analysis. In the original City of Altheimer study dated February 1980 (Reference 2), water surface elevations of floods of the selected recurrence intervals were computed through use of the USACE HEC-2 step-backwater computer program (Reference 25). Starting water surface elevations were calculated using the slope-area method. In the original City of Altheimer study (Reference 2), channel roughness factors (Manning s n ) used in the hydraulic computations were chosen by engineering judgment and based on field observations of the streams and floodplain areas. Roughness values for the channels ranged from to 0.090, with floodplain roughness values ranging from to for all floods. In the original City of Pine Bluff study dated June 1, 1983 (Reference 3), channel roughness factors (Manning s n ) used in the hydraulic 16

23 computations were chosen by engineering judgment and based on field observations of the streams and floodplain areas. Roughness values ranged from 0.04 to 0.08 for the main channels of streams and 0.07 to 0.25 for the overbank areas. The acceptability of all assumed hydraulic factors, cross sections, and hydraulic structure data was checked by computations that duplicated historic floodwater profiles. In November 1973, USACE-Vicks published a Floodplain Information Report (FPI) in two parts for the City of Pine Bluff (Reference 26). In the original City of Pine Bluff study (Reference 3), this FPI report was used and corrected in subsequent reports published by USACE-Vicks as part of the Pine Bluff Metropolitan Area, Arkansas, Urban Water Management Study. These subsequent studies were Analysis of Urban Drainage Related Flooding, Sheet Flow or Ponding Area (Reference 27); Phase I Report Flood Control Alternative Study (Reference 28); and Comprehensive Flood Control Plan (Reference 29), which is the source of flood profiles and the additional flooding added to Brumps Bayou. Also, the area not included near river mile 351 on Bayou Bartholomew was incorporated, and detailed flooding in these areas was added. In the original City of Wabbaseka study dated June 1980 (Reference 5), channel roughness factors (Manning s n ) used in the hydraulic computations were estimated by field inspection. Roughness coefficients of in the channel and 0.10 in the floodplains were used for all floods. In the original City of Wabbaseka study (Reference 5), flood elevations for the 10-, 2-, and 1-percent-annual-chance floods at U.S. Highway 79 on Wabbaseka Bayou were picked from stage frequency curves. The 0.2- percent-annual-chance flood elevations were picked from a rating curve developed from gage data. A rating curve was developed from surveyed cross section data at the lower limits of the study. Flood elevations of the desired frequencies were picked from this curve using the flood discharges determined at U.S. Highway 79. In the original study and the FIS for the City of White Hall revised August 15, 1990 (Reference 6), water surface elevations of floods of the selected recurrence intervals for the streams studied by detailed methods, including the portion of Gamble Creek studied in the first revision, were computed using the USACE HEC-2 step-backwater computer program (Reference 16). Flood profiles were drawn showing computed water surface elevations for floods of the selected recurrence intervals. In the original study and the FIS for the City of White Hall revised August 15, 1990 (Reference 6), starting water surface elevations for Caney Bayou, Caney Tributary No. 1, Caney Tributary No. 2, Caney Bayou Fork, Park Creek, and Oakland Heights Tributary were determined from the slopearea method. For School Drain, Carter Creek, Industrial Creek, Oakland 17

24 Heights Creek, Henslee Creek, and Gamble Creek, the starting water surface elevations were determined from coincident peaks. Channel roughness factors (Manning s n ) used in the hydraulic computations were assigned on the basis of engineering judgment and field inspection of the stream and floodplain areas. Channel and overbank n values for the streams studied by detailed methods in the current study and the previous study of the unincorporated areas of Jefferson County are shown in Table 3. TABLE 3 - SUMMARY OF ROUGHNESS COEFFICIENTS: REDELINEATION Stream Channel n Overbank n Arkansas River Arnold Creek Arnold Creek Tributary Bayou Bartholomew Bayou Bartholomew Tributary No Bayou Bartholomew Tributary No Boggy Bayou Boggy Bayou Tributary No Boggy Bayou Tributary No Caney Bayou Caney Bayou Fork Caney Tributary No Caney Tributary No Dancing Rabbit Creek Gamble Creek Henslee Creek Industrial Creek Industrial Creek Tributary Nevins Creek Nevins Tributary Outlet Canal Piney Creek Spring Hill Creek Sulphur Spring Creek Sulphur Springs Tributary Wabbaseka Bayou Revised Countywide Analyses The analyses consisted of determining water surface elevations (WSELs) for the 1-percent-annual-chance flood events for with-levee, withoutleft-levee, and without-right-levee scenarios, and the floodway for the without-both-levees scenario on Arkansas River and Caney Bayou within this revision. The analyses also consisted of determining WSELs for the 2-percent-annual-chance flood events for with-levee and 18

25 without-left-levee scenarios of Caney Bayou and with-levee and without-right-levee scenarios of Arkansas River within this revision. The with-levee conditions represent the corrected effective hydraulic analyses and were used to prepare the base model for the without-levee scenarios. The hydraulic methods used for the without-levee analysis were HEC-RAS version 4.0 (Reference 30) for riverine modeling. RAMPP prepared hydraulic analyses using geometry data from the effective model cross sections; an electronic file of a modified version (dated March 3, 2008) of this effective model from the USACE and National Elevation Dataset data was obtained from the USGS Seamless Server (Reference 31). The Arkansas River without-right-levee analysis consisted of removing the levee on the right overbank (Tucker Lake Levee) and leaving the left levee in place. The Arkansas River without-left-levee analysis consisted of removing the levee on the left overbank (Plum Bayou Levee, western segments) and leaving the right levee in place. The Arkansas River without-both-levees analysis consisted of removing the levee on the left overbank (Plum Bayou Levee, western segments) and right overbank (Tucker Lake Levee). Due to the de-accredited status of the adjoining levee segment in Pulaski County, the Arkansas River without-left-levee and without-both-levees analyses included removing that levee on the left. Breakout flow was found to occur within the behind-levee floodplain for the Arkansas River without-left-levee and without-both-levees analyses. The breakout flow was modeled in HEC-RAS version 4.0 (Reference 30) using flow optimization over lateral weirs between the Arkansas River and two other flooding sources: Main Canal No. 9 and Plum Bayou. Breakout flow was analyzed by linking the cross sections of two flooding sources by equivalent location along the lateral weirs. The analysis of Arkansas River incorporated breakout flow into Plum Bayou. The corresponding Plum Bayou analysis included levee crests from USACE for an accredited levee, the Plum Bayou segment of the Plum Bayou Levee, which follows the left overbank of Plum Bayou and is landward of the western segments of the Plum Bayou Levee. The without-left-levee and without-both-levees analyses include this accredited levee, which was certified by USACE in July The Caney Bayou without-levee analysis consisted of removing the levee on the left overbank (Tucker Lake Levee). Channel roughness factors (Manning s n ) used in the hydraulic computations were assigned on the basis of engineering judgment and field inspection of the stream and floodplain areas. Channel and overbank n values for the streams studied by without-levee analyses in the current study are shown in Table 4. 19