BARFIELD BEND POTOMOLOGY STUDY UPDATE MISSISSIPPI RIVER Hydraulics and Hydrology Branch. July 1996

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1 BARFIELD BEND POTOMOLOGY STUDY UPDATE MISSISSIPPI RIVER Hydraulics and Hydrology Branch July 1996 I. Purpose. The purpose of this report is to update investigations pertaining to overbank scour at Barfield Bend made as presented in the February 1986 report "Overbank Scour Problem at Barfield Bend Arkansas, Mississippi River Miles 89-87". This report update will assess the following: I) potential for additional top bank scour at Barfield Bend, 2) potential for adverse hydraulic impacts of overbank flow on main-line levee, 3) potential formation of a cutoff across Barfield Bend, 4) impacts of the Nebraska Point dike field on overbank flow lines, 5) impacts of stone closure degradation during overbank events, and 6) benefits of recent tree screen improvements (Figure 1-1 ). II. General. Conclusions from the 1986 report summarized that there was no threat to the main line Mississippi River levee (at that time), that the top bank control should be maintained throughout the Barfield revetment by maintaining the scour closures, and that a monitoring program be utilized to evaluate the situation with subsequent overbank flow periods. Federal actions recommended in the 1986 report were to monitor the location and to maintain the top bank controls. Local interests actions recommended in the 1986 report included degrading the abandoned private levee and establishment of a tree screen along top bank and around each ofthe scour holes. The Memphis District Corps of Engineers has maintained the scour closures and continued a monitoring program as recommended. Local interests have completed a tree screen around the scour holes as the previous report recommended. None of the private levee has been degraded to date. Iii.. Analysis. Analysis ofpotential for additional top bank scour at Barfield Bend consisted of analyzing overbank flow measurements made during several flood events and historical growth rates ofthe scout holes (Figure 111-1) as plano metered from aerial photography. No significant trends could be identified from the data to indicate excessive overbank scour should be expected in the future.. Local scour will likely continue where the private levee breaches had originally occurred and where the scour holes were closed by placement 'of riprap. The closures were constructed to approximately 3 feet above natural ground elevations in the surrounding overbank and about 4 feet below the private levee grade. The closures, therefore, function as weirs during higher

2 stages and focus flow energy in a relatively small area. With little tail water on the closures when first overtopped, the scour holes have continued to grow during past flood events. Since historic growth of the scour holes has been limited, however, formation of a cutoff across Barfield Bend appears unlikely in the near future. Degradation of the riprap closures during overbank flow conditions tends to increase the flow energy through and over the closures. The increased energy results in increased scour rates until repair can be completed. Sour holes 1 and 3, where natural depressions occur in the floodplain topography, convey a greater percentage of flow during overbank events, and top bank scour at these locations and along the resulting flow paths has been the greatest. These lower areas exhibited the highest observed velocities for the overbank flow. Negative effects on the main line levee were determined to be minimal when evaluating velocities measured in the overbank near the levee. Water surface elevations for pre and post Nebraska Dike field were computed for frequency discharges on the Mississippi River using HEC-2. As expected, the dike field in concert with the entire channel improvement program, has improved channel capacity over time and a reduction in Water surface elevations has resulted. Increased channel capacity results in less discharge being conveyed in the overbank areas. Table III-1 indicates computed reductions in water surface elevations with the dike field in place since A decrease in Water surface elevations is also supported by downward shifts in specific gage records and discharge rating curves downstream of Caruthersville, Missouri in recent years. Evaluation of buffer strips created by tree screen plantings were qualitative since several years growth will be required to fully establish the stand. However, vegetative cover between the trees in the interim will decrease the potential for soil movement until the trees begin to grow and mature. A mature stand of trees will significantly increase the resistance to flow resulting in decreased soil movement. IV. Summary. Following are a list of conclusions from observations and analyses completed. A. The Mississippi River main line levee is not presently threatened by overbank scour or overbank flows at Barfield Bend. B. The potential for additional top bank scour at Barfield Bend is primarily a localized problem exaggerated by concentration of f.low through the rip rap closures constructed at scour holes 1 U through 5. Additional scour holes will form should the private levee fail at other locations. Private levee failure at other locations is probable since several locations were observed where overtopping has eroded part of the levee section. C. Riprap closures constructed approximately 3 feet above the natural ground elevations increase the amount of flow energy to be dissipated and the duration of the higher flow energy (tailwater takes longer to develop) as flow overtops the closures. The higher energy increases the potential for downstream scour and for failure in the riprap cfosure. Failures in the riprap closures focus even greater flow energy into the overbank areas accelerating the

3 ) rate and magnitude of the scour holes. D. The lower overbank topography downstream of scour holes 1 and J-(and to a lesser extent 4 and 5) collects overbank flow and acts as primary drainage paths for the majority of overbank discharges. Flow along these paths is affected by the roadway perpendicular to the flow. E. The risk for formation of a cutoff across Barfield Bend is low given that the top bank control will be maintained by Federal actions. F. Dike field construction at Nebraska Point, and for the channel improvement project in general, has lowered water surface elevations and reduced overbank discharges. G. Tree screen improvements will increase resistance to overbank scour in the future, but will have a lesser, although still positive, immediate effect. V. Recommendations. Following are recommendations for reducing maintenance ofthe closures and for reducing top bank scour at Barfield Bend. Figure V -1 provides locations and further description of recommended work. A. Continue overbank monitoring program to ensure main line levee integrity. B. Degrade abandoned private levee from scour hole 1U downstream to scour hole 5. Degradation of the levee will permit flow to enter the overbank unifom1ly and substantially reduce scour potential. As stated in the 1986 report, no major scour problems were identified at Barfield Bend prior to construction of the private levee and its subsequent failure in C. Degrade riprap closures to ground elevations consistent with overbank elevations around each scour hole. Flow will then enter the overbank in a more uniform fashion thereby reducing the amount ofenergy to be dissipated. D. Raise the county roadway that runs near top bank at locations (where the highest velocities have been observed) to create a tevel road profile and uniform flow across the roadway. A 5 foot wide tree screen and grassed area should also be established on the downstream side ofthe road to further dissipate flow energy as it crosses the road. E. Protect the downstream perimeter of the existing scour holes with riprap to minimize their elongation during future overbank flow events. F. Place rip rap at active scour areas around the perimeter of the scour holes.

4 TABLE Water Surface Elevation Comparisons from HEC-2 for Pre-Nebraska Point Dike Construction and Post Nebraska Point Dike Construction Conditions Mississippi River Barfield Bend Potomology Study Update RM July Year Exceedance Frequency 5 Year Exceedance Frequency 1 Year Exceedance Frequency Water Surface Elevations (ft.) Water Surface Elevations (ft.) Water Surface Elevations (ft.) (1) (2) (2)- (1) (3) (4) (4)-(3) (5) (6) (6) - (5) Approximate River MileAHP Description Pre-Nebraska Dikes Post- Nebraska Dikes Delta (-is lower) (+ is higher) Pre-Nebraska Dikes Post- Nebraska Dikes Delta (-is lower) (+is higher) Pre-Nebraska Dikes Post- Nebraska Dikes Delta (-is lower) (+ is higher) Surveyed section Crossing Below Tamm Bend 1 Dike Below Tamm Bend 2 Dike Surveyed section Below Tamm Bend 3 Dike Crossing Surveyed section Crossing Island 25 1 Dike Surveyed section Island 25 2 Dike Crossing Forked Deer 1 Dike Surveyed section i. -- I 71189G 1 33 AM

5 ... )....: QA!L! - 1 ) DISCHARGE RANGE TOMATO Q!SARI"! ELO I"'t NT I"OKfO DEER I S.LAHO o"' t s it,f CEST GAGE L,)Cl.TI..,5 t r ' '""'1 to'"' MISSISSIPPI RIVER AND TRIBUTARIES MONITORING PLAN

6 . Qf.L -Ill NEMASKA NT ORIGINALLY CONSTRUCTED PRIVATE LEVEE TOMATO BAIH'I ELO POl NT FKED Df.f. I SLAHO MISSISSIPPI RIVER AND TRIBUTARIES PRIVATE LEVEE ORI<?INAL CONSTRUCTION )

7 ) AMDtfL l Ill NEMAS KA 1"1 NT ) RESTORED PRIVATE LEVEE WITH SETBACKS AROUND SCOUR HOLES TOMATO BARI"I ELD f"'l NT MISSISSIPPI RIVER AND TRIBUTARIES PRIVATE LEVEE RECONSTRUCTED LEVEE AFTER 1976 REPAIRS

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10 { '"" '" L COTTCNW """' '\ COTTONWOOD...w2 S!H(2 SCOUR HOLE # 14 SCOUR HOLE # 1,:;-;; '_ J. \ \ 'Oo'O :... -N NO <:}Joc.\ c;l)\ % '; v oo'v >A() '1. \)\S,,'._ c' 1_,", os\ o' () c,\ (:, ' -'..- ooo"l\ ":> --a\ () oz+'" \:.. '-t: o"'\ ()' (). 'v\ o<,.''" COITONWOOD TREESCREEN TREES PLANTED ON 1 FT. X 1 FT. CENT RS S''f'r'"J 1'\'\\, c,' <1) I...; i D I v $ <o-;:vo ;;,;:v "') ; o "')' (J ' -"' " ',_,... PU"'""'ENT # TltE NWY M MPHlS DISTlOCT, CORPS OF IHEERS f ,..., nlllfolua& NVEI'I (2W'UIIELII.fi\''Dii.Hl BARFIELD BEND, ARKANSAS POTOMOLOGY STUDY UPDATE FIGURE J

11 u Q\1<-E NU r; :'(r.-\ ' % 17 r::\j'j \ () 1 \)\+-s o'l " X ) o-,'l SCOUR HOLE # 1 \. o\jx\j "..- \Of oo_,.ocl \ pat\\ +\) f\"' af'l.---- pt\11' oz..-<;_ \s, 2 <::>'?:Jo'D \-\. ax(), :...- C) '-' \)x... a: l (!)?J rn <:: rn -1 rn z -1 \'- c.' X v Lowest Road Elevations )44+ ary Aow -f>ath for Scour Hole # "- ::) V) i- "J _,;;, rt I oec:x.to DeMtNENT Of THE NWr MEMPHIS DISTRICT, CORPS Of EHGIN EIIS (J IMIS8ISSIPt'1 JWH BARAELD...nHUSS&: POTOMOLOGY STUDY FIGURE MILE aw... IMMOVW HT BEND,ARKANSAS UPDATE 1 OltAIING 11. I 1 i,-, SHUT 1 OF 1

12 LJ \ o"e " I \ ()-'() () 'J. 7' "7 5 Foot Tree Screen and Grassed Ar.. SCOUR HOLE # 1 Along Perimeter of Scour Hole -N _ ()11\ '(.. oo" 'o \'+-<(,_ " \) '1..,""'" -) -o X()... c :J '1 --' \ s' J 2 "or. <oo<o egrde Clo.ure Elevation to Nat. Ground _K Along Perimeter of Seour Hole '%. "<(. ""9...,..,. o--o"' \ "=> oz."'<;_c,\ ""o (),+ \..\ oo'o \..\ \)c,_,x 5 Foot Tree Screen and Grassed Area-?J rn <::: rn --< :z. -L " 3111\"'oo \ HOLE # 3 Degrade Closure Elevation to Net. Ground Along Perlm;Jter-of Scour Hole ( v,' o, I>." "'>"' "" Degr-ade Closure Elevation to Net. Ground " () ' A..<\; "') A..C) $ "') (J c) A..().,_,;:; "') () sctffi M...,NVBI DEI"Ain'M NT Of TliE NtN( MEMPHIS DISllUCT, CORPS OF ENGINEERS WEMPHIS, nhnu611! BARAELD BEND,ARKANSAS POTOMOLOGY STUDY UPDATE AGURE V-1 [ ,_ SHEE'T1-1