CONTROL STRUCTURE LITTLE SIOUX RIVER, IOWA Hydraulic Model Investigation By

TECHNICAL REPORT NO. 2-762 CONTROL STRUCTURE LITTLE SIOUX RIVER, IOWA Hydraulic Model Investigation by

T. E. Murphy

February 1967

Sponsored by U. S. Army Engineer District Omaha

Conducted by U. S. Army Engineer Waterways Experiment Station CORPS OF ENGINEERS TECHNICAL REPORT NO. 2-762 CONTROL STRUCTURE LITTLE SIOUX RIVER, IOWA Hydraulic Model Investigation

T. 2. Murphy

February 1967

Sponsored by U. S. Army Engineer District Omaha

Conducted by U. S. Army Engineer Waterways Experiment Station CORPS OF ENGINEERS Destroy this report when no longer needed. Do not return it to the originator.

The findings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. FOREWORD

Model investigation of the control structure for Little Sioux River was authorized by the Office, Chief of Engineers on 25 May 1962, at the request of the U. S. Army Engineer District, Omaha. The study was conducted in the Hydraulics Division of the Waterways Experiment Station during the period July to December 1962. The investigation was conducted under the general supervision of Mr. E. P. Fortson, Jr., Chief of the Hydraulics Division, and Mr. F. R. Brown, Chief of the Hydrodynamics Branch, and under the direct supervision of Mr. T. E. Murphy, Chief of the Structures Section. The engineer in immediate charge of the model was Mr. E. S. Melsheimer, who was assisted by Mr. B. P. Fletcher. This report was prepared by Mr. Murphy. During the course of the investigation Messrs. E. R. Bloomquist, W. M. Linder, H. E. Hormann, and V. S. Horihan of the Omaha District and Mr. D. C. Bondurant of the Missouri River Division visited the Waterways Experiment Station to discuss model results and to correlate these results with design studies. Director of the Waterways Experiment Station during the testing program was Colonel Alex G. Sutton, Jr., CE. Director during preparation of this report was Colonel John R. Oswalt, Jr., CE. Technical Director was Mr. J. B. Tiffany.

iii CONTNTS

FOREWORD 0 0.. ... , ii i CONVERSION FACTORS, BRITISH TO METRIC UNITS OF MEASUREMVENT ...... vii

PART I: INTRODUCTION ...... 1 Pertinent Features of the Project ...... 1 The Control Structure...... 1 Purpose of Model Study...... 2 PART II: THE MODYEL...... 3 Description...... 3 Model Apparatus...... Scale Relations...... 4

PART III: TESTS AND RESUJLTS...... 5 Preliminary Tests...... 5

Riprap Requirements...... 5 Approach Channel Roughness...... 7 Recommended Structure ...... 7 PART IV: DISCUSSION ...... 9 PHOTOGRAPHS l-4~ PLATES 1-5

V CONVERSION FACTORS, BRI TI SH TO METRIC UNITS OF MEASUREMENYT

British units of measurement used in this report can be converted to metric units as follows:

Multiply y To Obtain inches 25.)+ millimeters feet 30.148 centimeters miles 1.609344 kilometers pounds 0.45359237 kilograms pounds per cubic foot 16 .0185 kilograms per cubic centimeter cubic feet per second 0.028317 cubic meters per second

vii SUMMARY

The control structure in the Little Sioux River in Monona County, Iowa, was designed to provide nonscouring velocities upstream in the channel and on the channel berms between the levees. The structure consists of a rectangular concrete drop in the central channel flanked by rock sills on the berms. Tests on a 1:30-scale hydraulic model were concerned with capacity of the structure, effectiveness of the concrete drop, and adequacy of the rock protection for the sills, channel, and berms. Model tests verified the adequacy of the concrete drop and the overall capacity of the structure. Modification of the riprap plan was required in certain areas to prevent failure and was recommended in other areas in the interest of economy.

ix -N-I VICINITY MAP

MONONA COUNTY HARRISON COUNTY

SCALE IN MILES 2 0 2 4

Fig. 1. Location map CONTROL STRUCTURE, LITTLE SIOUX RIVER, IOWA

Hydraulic Model Investigation

PART I: INTRODUCTION

Pertinent Features of the Project

1. The Little Sioux Project is located in Woodbury, Monona, and Harrison Counties, Iowa, along both banks of the Little Sioux River from Smithland, Iowa, to the mouth (fig. 1). The plan of improvement consists of remedial work on the channel and three existing sills at the mouth of the river, and the construction of a channel control structure about 5.75 miles* above the mouth. 2. The provision of a control structure on the Little Sioux is an essential hydraulic design feature of the project. Prior to the construction of the three control sills (1959), channel degradation had progressed upstream approximately 3.5 miles from the mouth. Between 1959 and the present, degradation advanced another 2.5 miles or a total of 6 miles. This erosion and degradation have now advanced so far upstream that it is no longer practical to attempt to control its advance by increasing the stage at the mouth through the use of additional sills at that location. The proposed method of stopping the degradation is the construction of a control structure just downstream of the upper limits of serious erosion.

The Control Structure

3. The control structure consists of a rectangular drop in the central channel with a width of 50 ft flanked by rock sills on 158-ft-wide berms extending to the levees. The channel has bottom widths of 40 and 60 ft upstream and downstream from the structure. Channel side slopes are 1 On 2-1/2 and the bed is approximately at elevation 1013.** The weir of

* A table of factors for converting British units of measurement to metric units is presented on page vii. ** Elevations are in feet above mean sea level.

1 the rectangular drop (see plate 1) is at elevation 1020, and the berms at elevation 1032 are surmounted by rock sills at elevation 1034. Rock protection is provided upstream and downstream from the structure. The design discharge is 35,000 cfs.

Purpose of Model Study

4. The purpose of the model study was to check the overall feasi- bility of this type of structure. Specifically, tests were conducted to: a. Provide data for the adjustment of the concrete crest width, the transverse length of the rock sills on the berms, or the height of the rock sills to produce headwater curves for both present and future tailwater conditions that fall within a specified zone. b. Verify the adequacy of the stilling basin for the concrete drop structure. c. Assure adequate riprap placement to protect the structure and obtain the most economical use of the rock.

2 PART II: THE MODEL

Description

5. The model (fig. 2) was constructed to an undistorted scale of 1:30 and reproduced about 700 ft of the channel and berms upstream of the structure, the drop structure, and about 1300 ft of the channel and berms downstream of the structure. Portions of the levees containing the structure were reproduced adjacent to the berms. The approach channel and berms were initially molded in sand; later these areas were molded of cement mortar to sheet-metal templates. Prototype roughness was simulated by expanded metal mesh (7/8 in.) placed on the upstream berms. The concrete portion of the drop structure was fabricated of plastic-coated plywood.

Fig. 2. The model looking downstream Model Apparatus

6. Discharge in the model was measured with venturi meters. Water- surface profiles were obtained with a point gage and velocities were measured with a pitot tube. Scour measurements below the structure were made with a portable sounding rod. Tailwater elevations in the lower end of the model were regulated by an adjustable flapgate.

Scale Relations

7. The accepted equations of hydraulic similitude, based on the Froudian relation, which assumes gravity to be the predominant factor of flow, were used to express mathematical relations between dimensions and hydraulic quantities of model and prototype. General relations for transference of model data to prototype equivalents are as follows:

Dimension Ratio Scale Relation

Length Lr =L 1:30 r Area A = L2 1:900 r r Weight L = L3 1:27,000 r r Velocity V = VI / 2 r r 1:5.477 Discharge Qr = L5,r1:4929

Roughness n = Ll/ 6 1:1.762 r r

8. Quantitative transfer of measurements of discharge, water-surface elevation, and velocity from model to prototype dimensions by these scale relations is considered reliable. Experimental data also indicate that the prototype-to-model scale ratio is valid for scaling riprap in the sizes used in this study. However, scour tendencies in the sand bed of the model are valid only for purposes of comparison and to indicate critical scour areas. PART III: TESTS AND RESULTS

Preliminary Tests

9. Design of the rectangular drop structure (plate 1) was based on results of model tests on similar structures for the Gering Valley Project.* In the present tests, observations revealed the desired types of flow conditions in the rectangular drop and this was considered verification of the adequacy of this element of the control structure. Also, preliminary measurements indicated the capacity of the structures to be about as antic- ipated. Thus, calibration was deferred pending development of a recommended riprap plan.

Riprap Requirements

10. Tested at various locations in the model were three sizes of riprap with individual rock sizes as follows:

a. Derrick stone. Maximum size 2000 lb, minimum size 800 lb. b. 250-lb riprap. A graded mixture with 250-1b rocks as the median and 650-lb rocks as the maximum. c. 80-1b riprap. A graded mixture with 80-1b rocks as the median and 250-1b rocks as the maximum.

The rock used in the model to simulate the riprap described above was a limestone with a specific gravity of 2.63 (164 pcf). In the model tests a fiber glass cloth was placed over the sand bed to serve as a filter and prevent migration of the sand through the voids in the rock. Plan 1 11. Details of the plan 1 riprap are shown in plate 2. Results of a scour test of 90-min duration with the discharge varied from 2750 to 35,000 cfs and the tailwater at elevations corresponding to the future tailwater curve (plate 5) are shown in photograph 1. Observations of flow

* U. S. Army Engineer Waterways Experiment Station, CE, Drop Structure for Gering Valley Project, Scottsbluff County, Nebraska; Hydraulic Model In- vestigation, by T. E. Murphy, Technical Report No. 2-760 (Vicksburg, Miss., February 1967). conditions and of the scoured model resulted in opinions that modifications of the riprap, discussed below, would be feasible and desirable. a. The extent of the rock protection on the channel side slopes upstream from the structure should be reduced. b. The protected channel side slopes downstream from the structure should be changed from 1 on 4 to 1 on 3. c. The cap rock on the berms at the tops of the downstream side slopes should be reduced in width but should be extended downstream to the end of the side slope riprap. d. The top width of the sills across the berms should be reduced, but additional rock is needed at the downstream toe of these sills.

Plans 2 and 3 12. Riprap plans 2 and 3 included the modifications discussed above. However, in order to define the areas where larger rock is required, only the smallest (80-lb median) of the three rock sizes was used in the plan 2 riprap. Based on results of tests of the plan 2 riprap, the intermediate size rock (250-lb median) was placed in certain areas to form the plan 3 riprap. Results of tests of plan 3 riprap indicated that the largest size rock (800- to 1000-lb derrick stone) was needed in certain areas. Plan 4 13. Details of the plan 4 riprap are shown in plate 3. However, prior to testing this riprap, roughness of the model approach was adjusted as discussed in paragraph 14 in order to approximate expected prototype roughness in the approach channel. Scour of the model bed resulting from a test of 90-min duration with discharge varied from 2750 to 35,000 cfs and tailwater at elevations corresponding to the future tailwater curve (plate 5) is shown in photograph 2. The only significant move- ment of rock occurred immediately downstream from the headwalls. It is recommended that the largest size rock be used in this location. There was considerable scour of the sand berms in the model, but the integrity of the rock sills across the berms was maintained. Also, there was no significant erosion of the berms at the top of the side slopes of the central channel.

6 Approach Channel Roughness

14. Originally the model approach, berms, and central channel were molded in sand. However, in the prototype, vegetation is expected to grow on the berms and it is estimated that the Manning's roughness coefficient n will be about 0.033 for the berms and only 0.020 for the central channel. In order to approximate the prototype roughness values, and thus the distribution of flow, the model approach was molded in cement mortar and the berms were covered with 7/8-in. expanded metal mesh (see photograph 2) with the long dimension parallel to flow. Previous model tests had indicated that this should result in n values of about 0.011 for the channel and 0.018 for the berms, simulating prototype values of about 0.020 and 0.032, respectively.

Recommended Structure

Description 15. The recommended structure consisted of the original concrete drop (plate 1) and the plan 4 riprap (plate 3); however, the largest size rock should replace the middle size rock in the riprap immediately downstream from the concrete headwalls. Flow conditions 16. Flow conditions are shown in photographs 3 and 4i. At discharges of less than about 8000 cfs (photograph 3a) flow was confined to the concrete drop structure and dissipation occurred in the stilling basin and in the scour hole, which developed as expected downstream from the stilling basin. At flows slightly greater than 8000 cfs, the headwalls of the concrete structure and the lateral rock sills were overtopped. The most severe attack on the riprap occurred at discharges of 10,000 to 20,000 cfs (photographs 3b, 3c, and 4 a). At discharges greater than about 20,000 cfs (photographs 4b and 4c), there was sufficient tailwater over the berms to cause flow to remain on the surface as it passed downstream from the structure and the attack on the riprap was not as severe as at the

7 intermediate discharges. Transverse and longitudinal water-surface profiles are plotted in plate 4. 17. Observations were made with obstructions placed in the approach channel to produce unsymmetrical flow. However, these unsymmetrical flow conditions did not compromise the performance of the structure. Calibration 18. The structure was designed to provide headwater curves within specified limits under either existing or future (degraded) tailwater conditions. Tailwater and headwater curves are plotted in plate 5. The model indicated headwater elevations slightly above the allowable zone at discharges of 6000 to 12,000 cfs. At the design discharge of 35,000 cfs and future tailwater conditions, the model headwater was in close agree- ment with the computed headwater. Also, for these conditions the headwater was at the lower limit of the allowable zone. No alterations were made to the structure in an effort to modify the rating curves. 19. About 10 in. of riprap was added on top of the berms upstream from the structure to determine the effect on the rating curve of the accretion of material on the berms. No change in headwater levels could be detected. PART IV: DISCUSSION

20. The primary purpose of the model tests was verification of the riprap plan. Since the concrete drop structure was designed with the benefit of extensive data obtained in the model tests of the Gering Valley Project, there was little reason to doubt that its performance would be adequate. 21. Of particular concern were flows of 10,000 to 20,000 cfs when the depth of flow over the rock sills was 1 to 4 ft (headwater elevations 1035 to 1038) and tailwater varied from 3 ft below to 3 ft above the channel berms (elevation 1029 to 1035). Under these conditions all or a large part of the flow passing over the rock sills returned to the central channel downstream from the structure. It was uncertain whether this would cause failure of the side slopes of the central channel. Although runnels formed on the channel berms, tests indicated that the plan 4 riprap would provide adequate protection to the central channel side slopes. However, in the prototype the lodging of debris or successive flows could result in the development of runnels of such size as to cause severe flow concentrations and failure of the central channel side slopes. A reasonable schedule of inspection is recommended, particularly until it has been established that the channel berms immediately below the structure will not erode excessively.

9 2505.18

Photograph 1. Plan 1 riprap after test of 90-min duration; discharge varied from 2750 to 35,000 cfs 2505-36

Photograph 2. Plan 14riprap after test of 90-min duration; discharge varied from 2750 to 35,000 cfs Photograph 3. Flow conditions at low flows 2505-32

Photograph 4. Flow conditions at high flows - HEADWALL

T1 t

FLOW f4

5' *--, SO'

HALF PLAN

CENTER-LINE SECTION

NOTE: ALL ELEVATIONS ARE IN FEET ABOVE MEAN SEA LEVEL. DETAILS OF DROP STRUCTURE SCALE IN FEET so to 20o

T I PL ATE I ::

EL 1034, N LIIELIEL

SECTION E-E

SECTION F-F

NOTE: GRADING E ANDF RIAMT

DERRICK STONE,8OO TO 2000 LB SECTIONS O-D AND G-G S250-LB RIPRAP, GRADED 250-LB MEDIAN, 650-LB MAXIMUM S80-LB RIPRAP, GRADED SO-LB MEDIAN, 250-LB MAXIMUM

SECTION C-C SCALE IN FEET PLAN SCALE IN FEET GRADING AND PLAN I RIPRAP 1 s0 0 II 00I1 I I IIIILIIIIV SECTION A-A HHL /032

ON0I !S' A8' /0' /O

SECTION B8-B SECTION E--E

SECTION F -F

l EL /032 1 ONu

NOT LEGEND

ALL ELEVATIONS ARE IN SECTION D-D MEAN SEA LEVEL. SDERRICK STONE, aoo To OO00LB S250-LB RIPRAP, GRADED250-LB MEDIAN, 050-LB MAXIMUM 8 0-LB RIPRAP, GRADED 80-LB MEDIAN, 250-LB MAXIMUM

SECTION C-""C

SCALE IN FEET 2p 0 20 40 PLAN '-4 SCALE IN FEET GRADING AND PLAN 4 RIPRAP 50 0 50 100 w i t ~ ItL /CILV ii I r I DISCHARGE OF 25,000 CFS

1037PRFL30USTRAFRMHAWL

1032-

270240 210 180 I50 20 0 0 30 0 30 80 0 20 150 180 210 240 270 DISTANCE.FT DISCHARGE OF 15,000 CFS TRANSVERSE PROFILES

1040 h " r

1030

1020

1010F

s0 40 20 20 10 0 10 20 30 40 20 60 70 60 0 00 I10 120 130 1052 rCENTER LINE OF CHANNEL ~1052

0 40 20 20 t0 0 10 20 30 40 s0 90 70 80 0 00 110 120 130 CENTER LINE OF HEADWALL 1042F2,0 r , 2500 S 000 CFS.OCS

50 40 30 20 10 0 to 20 30 40 50 90 70 80 0 00 I10 120 130 OISTANrCL,Ft CENTER LINE OF BERM LONGITUDINAL PROF ILES

WATER-SURFACE PROF ILES

PLATE 4 1030

CHANNL BEMS IEXISTING TA/LWATEP

'U ~FUTURiE TA IL WA TER i 'U

10281

10286__

1024

___ CREST OF DROP STRUCTURE WEIR4

0 2 4 8 8 10 12 14 I6 18 20 22 24 28 28 30 32 4 36 3 DISCHARGE, THOUSANDS OF CVFS

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Announcement of Availability by Tech Liaison Branch: CIVIL ENGINEERING THE MILITARY ENGINEER ENGINEERING NBWS -RECORD

5 Unclassified Security Classification DOCUMENT CONTROL DATA " R&D (Security classificationof title, body of abstract and indexing annotation must be entered when the overall report is classified) SORIGINAPTIVE NOESITY(Corporae uhort)PnluRTeLdIte

Ucasfe U. S., Army Engineer Waterways Experiment Station

2 RU Vicksburg, Mississippi

3 REPORT TITLE

oe netgto CONTROL STRUCTURE, LITTLE SIOUX RIVER, IOWA; Hydraui

4. DESCRIPTIVE NOTES (Type of reort and inclusive dates) Final report S AUTHOR(S) (Last name, firstname, initial)

Murphy, Thomas E.

6 REPORT DATE 7s TOTAL NO OF PAGES 7b NO OF REFS

February 1967 24 I $a, CONTRACT OR GRANT NO, 9. ORIGINATORS REPORT NUMBER(S)

b PROJECT NO. Technical Report No. 2-762

c. 9b. OTHER REPORT NO(S) (Any othernumbers thatmay be assigred this report)

d. 10 A VA IL AILITY/LIMITATION NOTICES

Distribution of this document is unlimited.

11. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY U. S. Army Engineer District, Omaha, Nebraska

13 ABSTRACT The control structure in the Little Sioux River in Monona County, Iowa, was designed to provide nonscouring velocities upstream in the channel and on the channel berms between the levees. The structure consists of a rectangular drop in the central channel flanked by rock sills on the berms. Tests on a 1:30- scale hydraulic model were concerned with capacity of the structure, effectiveness of the concrete drop, and adequacy of the rock protection for the sills, channel, and berms. Model tests verified the adequacy of the concrete drop and the overall capacity of the structure. Modification of the riprap plan was required in certain areas to prevent failure and was recommended in other areas in the interest of economy.

DD'FC1JAN64~'C~ 1473II~llt OLU~~L1VL UnclassifiedI-Z: DD 1 AAN4 147 Security Classification Unclassified Security Classification.

I A .. . WOD 14, LINK Bwr LINK A LINK B LINK C KEY WORDS ROLE ROLE Wr ROLE WT ROLE ST

Control structures (Hydraulics) Little Sioux River Hydraulic models Riprap Scour

INSTRUCTIONS

1. ORIGINATING ACTIVITY: Enter the name and address 10. AVAILABILITY/LIMITATION NOTICES: Enter any lim- of the contractor, subcontractor, grantee, Department of De- itations on further dissemination of the report. other than those fense activity or other organization (corporate author) issuing the report, imposed by security classification, using standard statements such as: 2a. REPORT SECUITY CLASSIFICATION: Enter the overall security classification of the report. Indicate whether (1) "Qualified requesters may obtain copies of this report from DDC." "Restricted Data" is included. Marking is to be in accord- ance with appropriate security regulations. (2) "Foreign announcement and dissemination of this authorized." 2b. GROUP: Automatic downgrading is specified in DoD Di- report by DDC is not rective 5200.10 and Armed Forces Industrial Manual. Enter (3) "U. S. Government agencies may obtain copies of the group number. Also, when applicable, show that optional this report directly from DDC. Other qualified DDC markings have been used for Group 3 and Group 4 as author- users shall request through ized. 3. REPORT TITLE: Enter the complete report title in all capital letters. Titles in all cases should be unclassified. (4) "U. S. military agencies may obtain copies of this If a meaningful title cannot be selected without classifica- report directly from DDC. Other qualified users shall request through tion, show title classification in all capitals in parenthesis immediately following the title. 4. DESCRIPTIVE NOTES: If appropriate, enter the type of (5) "All distribution of this report is controlled. Qual- repo.t, e.g., interim, progress, summary, annual, or final. ified DDC users shall request through Give the inclusive dates when a specific reporting period is covered. If the report has been furnished to the Office of Technical 5. AUTHOR(S): Enter the name(s) of author(s) as shown on Services, Department of Commerce, for sale to the public, indi- or in the report, Enter last name, first name, middle initial. cate this fact and enter the price, if known. If military, show rank and branch of service, The name of the principal author is an absolute minimum requirement. 11. SUPPLEMENTARY NOTES: Use for additional explana- tory notes. 6. REPORT DATE: Enter the date of the report as day, month, year; or month, year. If more than one date appears 12. SPONSORING MILITARY ACTIVITY: Enter the name of on the report, use date of publication,. the departmental project office or laboratory sponsoring (pay in* for) the research and development. Include address. 7a. TOTAL NUMBER OF PAGES: The total page count should follow normal pagination procedures, i.e., enter the 13. ABSTRACT: Enter an abstract giving a brief and factual number of pages containing information. summary of the document indicative of the report, even though it may also appear elsewhere in the body of the technical re- 7b. NUMBER OF REFERENCES: Enter the total number of port. If additional space is required, a continuation sheet references cited in the report. shall be attached. 8o. CONTRACT OR GRANT NUMBER: If appropriate, enter It is highly desirable that the abstract of classified re- the applicable number of the contract or grant under which ports be unclassified. Each paragraph of the abstract shall the report was written. end with an indication of the military security classification Sb, 8c, & Sd. PROJECT NUMBER: Enter the appropriate of the information in the paragraph, represented as (TS), (5), military department identification, such as project number, (C). or (U) subproject number, system numbers, task number, etc. There is no limitation on the length of the abstract. How- 9.. ORIGINATOR'S REPORT NUMBER(S): Enter the offi- ever, the suggested length is from 150 to 225 words. cial report number by which the document will be identified 14. KEY WORDS: Key words are technically meaningful terms and controlled by the originating activity. This number must or short phrases that characterize a report and may be used as be unique to this report. index entries for cataloging the report. Key words must be 9b. OTHER REPORT NUMBER(S): If the report has been selected so that no security classification is required. Iden- assigned any other report numbers (either by the originator fiers, such as equipment model designation, trade name. mnili- or by the aponsor), also enter this number(s). tary project code name, geographic location, may be used as key words but will be followed by an indication of technical context. The assignment of links, rules, and weights is optional.

Unclas sified Security Classification