Of Maximum Scour at Selected Bridges in Iowa

Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa

U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 95-4051

Prepared in cooperation with the IOWA HIGHWAY RESEARCH BOARD and the HIGHWAY DIVISION of the IOWA DEPARTMENT OF TRANSPORTATION (IOWA DOT Research Project HR-344) Cover: Map of Iowa showing bridges for which potential-scourassessments were made. Solid symbols show bridges for which maximum scour calculations also were made. Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa

By EDWARD E. FISCHER

U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 95-4051

Prepared in cooperation with the IOWA HIGHWAY RESEARCH BOARD and the HIGHWAY DIVISION of the IOWA DEPARTMENT OF TRANSPORTATION (IOWA DOT Research Project HR-344)

Iowa City, Iowa 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY GORDON P. EATON, Director

For additional information write to: Copies of this report can be purchased from:

U.S. Geological Survey District Chief Earth Science Information Center U.S. Geological Survey, WRD Open-File Reports Section Rm. 269 Federal Building Box 25286, MS 517 400 South Clinton Street Denver Federal Center Iowa City, Iowa 52244 Denver, Colorado 80225 CONTENTS

Abstract 1 Introduction 2 Purpose and Scope 2 Acknowledgments 3 Potential-Scour Assessments 3 Potential-Scour Index 3 Data Collection for Potential-Scour Assessment 7 Landform Regions in Iowa 8 Results of Potential-Scour Assessments 7 Evaluation of Potential-Scour Assessment Technique 10 Estimates of Maximum Scour 12 Bridge-Scour Processes and Estimating Maximum Scour 12 Long-Term Aggradation/Degradation of the Streambed 12 Contraction Scour 15 Pier Scour 16 Abutment Scour 16 Channel Stability 16 Total Scour 16 Data Collection and Method of Analysis for Estimating Maximum Scour 16 Rood Discharges 17 Hydraulic Properties 17 Water-Surface Profiles 17 Maximum Scour Equations ~ 17 Results of Estimates of Maximum Scour 17 Middle Raccoon River near Bayard (05483450) 18 Raccoon River at Van Meter (05484500) 20 White Breast Creek near Dallas (05487980) 22 Cedar Creek near Bussey (05489000) 24 Maple River at Mapleton (06607200) 26 West Nishnabotna River at Randolph (06808500) '. 28 East Nishnabotna River near Atlantic (06809210) 30 Nodaway River at Clarinda (06817000) 32 Platte River near Diagonal (06818750) 34 Iowa River at Wapello (05465500) 36 Summary and Conclusions 38 References 41 Appendix—Example of Form Used to Collect Potential-Scour Data 71

FIGURES 1. Maps showing location of bridges assessed for potential scour and landform regions in Iowa 4 2. Histogram of potential-scour indices for 130 bridges in Iowa 10 3. Maps showing location of bridges grouped by selected ranges of potential-scour index, assessment of bed-material component, assessment of bank-erosion component, assessment ofchannel-evolution component, and assessment of bed-protection component 11 4. Map showing location of bridge sites where maximum scour was estimated 13

Contents II! FIGURES—Continued 5.-14. Graphs of streambedaggradation/degradation trend line and elevation view (lookingdownstream) of channel cross section showing calculated maximum scour depths: 5. For the 100-year flood at State Highway 25 bridge in Guthrie County, streamflow-gaging station Middle Raccoon River near Bayard (05483450) 19 6. For the 100-year flood at County Road R16 bridge in Dallas County, streamflow-gaging station Raccoon River at Van Meter (05484500) 21 7. For the point-of-road-overflow flood at county road bridge in Marion County, streamflow-gaging station White Breast Creek near Dallas (05487980) 23 8. For the discharge with maximum measured velocities at State Highway 156 bridge in Marion County, streamflow-gaging station Cedar Creek near Bussey (05489000) 25 9. For the 100-year flood at State Highway 175 bridge in Monona County, streamflow-gaging station Maple River at Mapleton (06607200) 27 10. For the 100-year flood at State Highway 184 bridge in Fremont County, streamflow-gaging station West Nishnabotna River at Randolph (06808500) 29 11. For the point-of-road-overflow flood at county road bridge in Cass County, streamflow-gaging station East Nishnabotna River near Atlantic (06809210) 31 12. For the 100-year flood at State Highway 2 (business route) bridge in Page County, streamflow-gaging station Nodaway River at Clarinda (06817000) 33 13. For the 100-year flood at county road bridge in Ringgold County, streamflow-gaging station Platte River near Diagonal (06818750) 35 14. For the maximum measured discharge at State Highway 99 bridge in Louisa County, streamflow-gaging station Iowa River at Wapello (05465500) 37 15. Graphs showing streambed elevations in main channel at upstream edge of State Highway 99 bridge over Iowa River at Wapello, July-November 1993 39 16. Maps showing streambed elevations in Iowa River at State Highway 99 bridge, Wapello, Iowa, November 17, 1993, and July 15, 1994 40

TABLES 1. Stages of channel evolution 6 2. Summary of assessed values of the potential-scour index components and potential-scour indices at 130 highway bridges in Iowa 9 3. Location,drainage area, median bed-material particle size, and flood-frequency data for bridge sites analyzed for maximumscour .". 14 4. Potential-scour index at selected highway bridges in Iowa 43

CONVERSION FACTORS AND ABBREVIATIONS

Multiply By To obtain

foot (ft) 30.48 millimeter 0.3048 meter foot per second (ft/s) 0.3048 meter per second foot per year (ft/yr) 0.3048 meter per year mile 1.609 kilometer square foot (fr) 0.09290 square meter square mile (mi2) 2.590 square kilometer cubic foot per second (fr'/s) 0.02832 cubic meter per second millimeter (mm) 0.032808 foot meter (m) 3.2808 foot

IV Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa

By Edward E. Fischer

Abstract the long-term aggradation or degradation that occurred during the periodof streamflow data The results of potential-scour assessments collection at each site was evaluated. The stream- at 130 bridgesandestimates of maximum scourat bed appeared to be stable at 6 of the 10sites, was 10 bridges in Iowa are presented. All of the degrading at 3 sites, andwas aggrading at 1 site. bridges evaluated in the study are constructed The estimates of maximum scour were made at bridges (not culverts) that are sites of active or most ofthe bridges using 100-year and 500-year discontinued streamflow-gaging stations and flood discharges. Other discharges also were peak-stage measurement sites. The period of the evaluated at four of the bridges. With respect to study was from October 1991 to September 1994. contraction scour, channel cross sections The potential-scourassessments were made measured during floods show parts of the stream- usinga potential-scour index developed by the bed to be scoured lower than the computed maxi U.S. Geological Survey for a study in western mum contraction-scour depths at 4 ofthe 10 sites. Tennessee. Higher values of the index suggest a The measured discharges at three ofthe sites were greater likelihood of scour-related problems less than the respective 100-year floods used to occurring at a bridge. For the Iowa assessments, compute scour. the maximum value of the index was 24.5, the No pier-scourmeasurements were obtained minimum value was 3, and the median value was in the study except for about 4 feet of local pier 11.5. The two components of the potential-scour scour that was measured at the bridge over the index that affected the indices the most in this Iowa River at Wapello, Iowa. However, the study were the bed-material component, which streambed was below the base of the pier footing, accounted for 27.1 percent of the overall total of which is supported by piling, at the time the the indices, and bank erosion at the bridge, which measurement was made. Discharge-measurement accounted for 18.3 percent of the overall total. cross sections collected at two other bridges, Because the potential-scour index represents which are not supported by piling, show the conditions at a single moment in time, the streambed between the piers to be lower than the usefulness of potential-scour assessments is bases of the piers. Additional investigation may dependent upon regular assessments if the index be warranted at these sites to determine whether is used to monitor potential-scour conditions; the streambed has been scoured below the bases however, few of the components of the index at the upstream edges of the piers. considered in this study are likely to change Although the abutment-scour equation between assessments. predicted deep scour holes at many of the sites, The estimates of maximum scour were the only significant abutment scour that was made using scour equations recommended by the measured was erosion of the embankment at the Federal Highway Administration. In this study, left abutment at one bridge after a flood.

Abstract 1 INTRODUCTION of 1993 are described by Wahl and others (1993), and the flood peaks are described by Parrett and others Bridge scour is the erosion of soil particles by (1993). flowing water from around the piers and abutments The Iowa Highway Research Board (IHRB) that support a bridge. Because of the inherent problem initially addressed bridge scour during the mid-1950s this process poses to bridge stability, bridge scour has by sponsoring laboratory research at the Iowa Institute been the focus of much international scientific of Hydraulic Research at the University of Iowa. Co- research. Yet, "the most common cause of bridge sponsors of the research were the Iowa State Highway failures is floods with the scouring of bridge founda Commission and the Bureau of Public Roads [cur tions being the most common cause of flood damage rently called the Iowa Department ofTransportation to bridges" (Richardson and others, 1993, p. 1). For (IDOT) and the Federal Highway Administration example, a major bridge in Iowa that failed because of (FHWA), respectively]. The results of this work were scour was the 1-29 bridge over the Big Sioux River in reported in IHRB Bulletin No. 4, "Scour Around Woodbury County in 1962. Elsewhere in the United Bridge Piers and Abutments" (Laursen and Toch, States, a scour-related failure that resulted in the loss 1956), and IHRB Bulletin No. 8, "Scour at Bridge of life was the collapse of the New York State Crossings" (Laursen, 1958). According to Vanoni Thruway bridge over Schoharie Creek in 1987. (1975, p. 48), Laursen's studies were influential in the Because of these and other bridge failures around the scientific community because his work on the nature Nation, the Federal Highway Administration of scour (Laursen, 1952) formalized many of the recommended that "every bridge over a scourable scattered theories of scour at the time into some stream, whether existing or under design, should be general principles. The contraction scour equations evaluated as to its vulnerability to floods in order to used in the FHWA manual HEC-18, "Evaluating determine the prudent measures to be taken for its Scour at Bridges" (Richardson and others, 1993), are protection" (U.S. Department of Transportation, 1988, based on Laursen's work. p. 2). The scour assessments described in this report Major flooding in south-central Iowa in developed from IDOT's response to FHWA's September 1992 and throughout most of Iowa during recommendation concerning bridge scour (U.S. the summer of 1993 damaged many bridges in the Department ofTransportation, 1988). IDOT began a State. For example, in 1992 the State Highway 2 bridge-scour review program that evaluated more than bridge over the Weldon River in Decatur County was 2,000 bridges in the State's primary highway system. closed because 10 ft of piling at the left abutment were As part of their review, IDOT and the U.S. Geological exposed by floodwaters. The peak discharge was Survey (USGS) developed a cooperative study that about four times the design flood for the bridge, which assessed scour at selected bridges in Iowa. The study was built in 1985. The flood and resulting scour was comprised of three components: (1) assess poten damage at this bridge aredescribed by Fischer (1993). tial scour at 130 bridges using a potential-scour index Statewide flooding during the summer of 1993 developedby the USGS for a similar study in western causedmany highways andbridges to beclosed. New Tennessee and evaluate the technique, (2) estimate peak discharges of record occurred at 34 streamflow- maximum scour at 10 bridges using 100-year and gaging stations operated bytheU.S. Geological 500-year(or other) design floodsand FHWA scour Survey (Southardand others, 1994, p. 7). Even equations, and (3) obtain scourmeasurements if though floodwaters destroyed only twobridges inthe possible for comparison with the maximum scour State's primary highway system, manybridges were estimates. The study was for the period October 1991 subjected to floodflows thatexceeded their 100-year through September 1994. design floods. Of 83 streamflow-gaging stations on unregulated streams in Iowa with 11 or more years of systematic, continuous-record data, 11 stations Purpose and Scope recorded peakdischarges thatexceeded thetheoretical 100-year flood discharge computed for therespective This report presentsthe results of potential- sites (D.A. Eash, U.S. Geological Survey, written scour assessments at 130 bridges in Iowa using a commun.,September 1994). The meteorological potential-scour index developed by the USGS for a conditions that caused the flooding during the summer similarstudy in western Tennessee and the results of

2 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa maximum-scour estimates at 10 bridges in Iowa using bridges are structures supported by abutments and scour equations recommended by the Federal High possibly one or more piers (that is, none of the bridges way Administration. The potential-scour assessment in this study are culverts). The ages of the bridges technique is evaluated, and estimated scour depths are range from less than 5 to more than 70 years. The compared to measured scour depths. This information study sites are assumed to be a random selection of will assist IDOT in making decisions as to whether the bridges in Iowa because the original selection of the potential-scour assessment technique would be of bridges at streamflow-gaging stations or peak-flow value to the State and whether present bridge-design measurement sites was independent of existing scour criteria with respect to scour are adequate. conditions at each bridge. The potential-scour index, the potential-scour data-collection form used for this study, the results of Acknowledgments the potential-scour assessments, and an evaluation of the potential-scour assessment technique are described IDOT provided bridge plans showing soil in the following sections. A section on the landform boring information and subsurface structural informa regions of Iowa also is included because the assess tion for the bridges in the primary highway system that ment of some of the factors that comprise the were assessed for scour in this study. County engi potential-scour index were clearly related to some neers throughout the State provided bridge plans for of the regions. the county bridges that were assessed. The streamflow data used in this study were collected in cooperation with IDOT, the Iowa Depart Potential-Scour Index ment of Natural Resources, and the U.S. Army Corps of Engineers. The Federal Highway Administration The potential-scour index used in this study was has supported the USGS National Bridge Scour developed by Simon and Outlaw (1989) for a bridge- Project, for which some work was done in this study. scour study by the USGS in western Tennessee. The index is comprised of 11 principal components. A value is assigned to each component according to the POTENTIAL-SCOUR ASSESSMENTS results of an onsite evaluation, and the potential-scour index is the sum of the component values. Larger A potential-scour assessment is used to help values of the index suggest a greater likelihood for determine whether a bridge may be vulnerable to scour-related problems to occur. Evaluation of several scour. Although a potential-scour assessment cannot of the index components is somewhat subjective and predict actual scour during a flood, it provides a assigned values may vary depending on the in?pec- measure of the likelihood ofscour-related problems tor's judgment and experience. The effects of occurring, both during a flood and over time as the variability in the potential-scour index because of channel-evolution processes work on the stream. The differences among persons making scour assessments assessment is accomplished by an onsite evaluation were not tested in this study. However, no single using a scour-inspection form. The scour suscepti component dominates the potential-scour index, and bility of the bridge is expressed as a number called the variations in the assigned values probably tend to potential-scour index. As used in this study, higher cancel each other out when the components are values of the index suggest a greater likelihood of summed to produce the index. The 11 principal index scour-related problems occurring at a bridge. components are described in the following paragraphs. Potential-scour assessments generally are made for approximate bankfull or 1- to 2-year flood event Bed material.—The type of bed material determines conditions. the relative erodibility of the streambed. Listed Potential-scour assessments were performed at in order of increasing erodibility, the values that 130highway bridges throughout Iowa from can be assigned are 0 for bedrock, 1 for November 1991 through May 1992 (fig. 1). All of the boulders/cobbles, 2 for gravel, 3 for sand, and bridges are located at sites of active or discontinued 4 for silt/clay. A value of 3.5 (for alluvium) is USGS streamflow-gaging stations and peak-stage assigned if the bed material cannot be deter measurement sites. The drainage areas upstream from mined during the onsite evaluation. No the bridges range from 23 to 7,785 mi2. All ofthe consideration is given to the cohesive properties

Potential-Scour Assessments Paleozoic Plateau

Southern Iowa Drift Missouri Plain Alluvial Plain Mississippi Alluvial Plain

Landform regions in Iowa, adapted from Prior (1991)

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139 737nZ.U.__'iPaS* .Taylor [Ringgold" J---A--Decatur ""[Waynei72^3|Appanoosei.ivn^.j^"""""Davis jVanBurenF Fremont \ Al ^ JA |29 31; * 2^ A I— iQe !^38. 88 j 1A5 1102 ;A3a0a| 1221 3^| 28 116 _L_A.. Basefrom US Geological Survey digital data.1:100.000.1S83 Universal Traverse Mercator projection. 20 40 60 MILES Zone 15 20 40 60 KILOMETERS

EXPLANATION 1A Bridge and site identification number Boundary of landform region—Adapted from Prior (1991)

Figure 1. Location of bridges assessed for potential scour and landform regions in Iowa.

4 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa of bed materials such as clay. Rather, the basis assigned are 0 for no piers in the main channel, for evaluating bed material is particle size. 1for one or two piers in the main channel, and 2 Bedprotection.—Riprap may be placed at a bridge site for more than two piers in the main channel. to protect the bed and banks from erosion. A Piers not in the main channel are not considered. value of 0 is assigned to this component if the Percentage ofblockage by debris.—This component bed is protected, and 1 if the bed is not has three subcomponents: percentage of protected. A value of 2 is assigned if the bed is vertical blockage, percentage of horizontal not protected but one bank is protected, and a blockage, and percentage of total blockage of value of 3 is assigned if the bed is not protected bridge opening. The values that can be assigned but both banks are protected. The increase in for each subcomponent are 0 for 0- to 5-percent the value because of bank protection is justified blockage, 0.33 for 6- to 25-percent blockage, on the basis that excess stream energy that 0.67 for 26- to 50-percent blockage, 1 for 51- to cannot be dissipated through lateral erosion will 75-percent blockage, and 1.33 for greater than tend to erode the streambed (Simon and Outlaw, 75-percent blockage. A fractional value for the 1989, p. 117). subcomponents is used so that the effect of Stage ofchannelevolution.—This component is based debris blockage on the potential-scour index is on the channel-evolution model developed by not overemphasized (Simon and Outlaw, 1989, Simon (1989). Each of the stages comprising p. 118). the model is described in table 1, which is taken Bankerosion.—The values that can be assigned for directly from Simon and Outlaw (1989, p. 120). bank erosion are 0 for no significant erosion, Listed in the order presented in table I, the 1 for fluvial erosion (erosion at the base of the values and corresponding stages that can be banks), and 2 for mass wasting (large sections of assigned to this component are 0 for Premodi- the riverbank have fallen into the water). A fied, 1 for Constructed, 2 for Degradation, 4 for value is assigned for each bank on the basis of Threshold, 3 for Aggradation, and 0 for the most severe erosion observed in the vicinity Restabilization. Evaluation of this component of the bridge. is perhaps the most subjective of any of the index components because it relies on the Proximity ofriver meander impact point to bridge.— interpretative skills of the inspector. This component is a measure of the likelihood that the outside bend of the river eventually will Percentage ofchannel constriction.—This compo migrate to the bridge, possibly undermining an nent measures the relative constriction of the abutment. The values that can be assigned are 0 main channel by the bridge. The percentage of if the impact point is greater than 100 ft from the constriction is calculated by dividing the bridge, 1 if the impact point is between 51 and difference between the widths of the channel 100 ft away, 2 if the impact point is between upstream of the bridge and at the bridge by the 26 and 50 ft away, and 3 if the impact point is width of the channel upstream and multiplying 25 ft or less away. by 100. Channel width is measured at the top of the banks, and the upstream width is measured Pier skew.—Piers that are not aligned with the princi sufficiently far upstream to be representative of pal direction of flow through the bridge opening the natural channel width; for most bridges this increase the scour potential at a site. The values is approximately one bridge length from the that can be assigned for this component are 0 if structure. The values that can be assigned to the pier is aligned with the flow and 1 if the pier this component are 0 for 0- to 5-percent is not aligned with the flow. A value is constriction. 1 for 6- to 25-percent constriction, determined for each pier in the main channel. 2 for 26- to 50-percent constriction, 3 for 51- to Mass wasting at pier.—A large value is assigned to 75-percent constriction, and 4 for greater than this component for bridge piers that are at the 75-percent constriction. edge of the bank and mass-wasting processes Numberofbridge piers in channel.—This component are occurring in the vicinity of the bridge. The is included because piers represent sites of values that can be assigned are 0 for no mass potential local scour. The values that can be wasting and 3 for mass wasting.

Potential-Scour Assessments Table 1. Stages of channel evolution (from Simon and Outlaw, 1989, p. 120)

Stage Dominant processes

No. Name Fluvial Hillslope Characteristic forms Geobotanical evidence

I Premodified Sediment transport— Stable, alternate channel Vegetated banks to flow mild aggradation: bars; convex top- line. basal erosion on bank shape; flow line outside bends; high relative to top deposition on inside bank; channel bends. straight or meandering. II Constructed Trapezoidal cross Removal of vegeta section; linear bank tion (?). surfaces; flow line lower relative to top. III Degradation Degradation; basal Pop-out failures. Heightening and Riparian vegetation high erosion on banks. steepening of banks: relative to flow line alternate bars eroded; and may lean towards flow line lower channel. relative to top bank. IV Threshold Degradation; basal Slab, rotational and Large scallops and bank Tilted and fallen riparian erosion on banks. pop-out failures. retreat; vertical face vegetation. and upper-bank sur faces; failure blocks on upper banks; some reduction in bank angles; flow line very low relative to top bank. V Aggradation Aggradation; develop Slab, rotational and Large scallops and bank Tilted and fallen ment of meandering pop-out failures; retreat; vertical face, riparian vegetation; thalweg: initial low-angle slides upper bank,and re-establishing deposition of alternate of previously slough line; flattening vegetation on slough bars; reworking of failed material. of bank angles: flow line; deposition of failed material on line low relative to material above root lower banks. top bank; develop collars of slough-line ment of new flood vegetation. plain (?). VI Restabilization Aggradation; further Low-angle slides; Stable, alternate channel Re-establishing vegeta development of some pop-out bars; convex-short tion extends up meandering thalweg; failures near flow vertical face, on top slough line and upper further deposition of line. bank; flattening of bank; deposition of alternate bars; bank angles; develop material above root reworking of failed ment of new flood collars of slough material; some basal plain (?); flow line line and upper- erosion on outside high relative to top bank vegetation; bends: deposition on bank. some vegetation flood plain and bank establishing on bars. surfaces.

Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa Angle ofapproach ofhighflows.—This component Loess Hills, the Southern Iowa Drift Plain, accounts for the effect of bridge crossings that the Iowan Surface, the Northwest Iowa are skewed (that is, not perpendicular) to the Plains, the Paleozoic Plateau, and the Allu main direction of floods. The values that can vial Plains. These regions are distinguished be assigned are 0 for 0 to 10 degrees skew, 1 for on the basis of physical appearance, and their 11 to 25 degrees skew, 2 for 26 to 40 degrees observable differences result from variations skew, 2.5 for 41 to 60 degrees skew, and 3 for in geologic history * * *. Each region con greater than 60 degrees skew. tains distinct landscape patterns and features that resulted from erosional activity at differ ent times, in varying intensity, into variable Data Collection for Potential-Scour deposits of loess, drift, alluvium, or bed Assessment rock. Some regions contrast sharply, with an obvious topographic boundary separating The fundamental data-collection mechanism for the potential-scour assessments was completion of them. Other boundaries are less clear, and the change from one landscape pattern to a form adapted from Simon and Outlaw (1989, another may occur gradually over several p. 115-116). The layout of the form was modified several times during the course of the assessments to miles. facilitate the collection of data; however, no data The principal material comprising the North elements were changed. The latest form is in the west Iowa Plains, Des Moines Lobe, Iowan Surface, Appendix. Additional data were collected at many and Southern Iowa Drift Plain landform regions is sites to characterize a site for future investigations, glacial drift overlying sedimentary bedrock. Drift is including bank heights and angles, bank vegetative the term for deposits of clay, silt, sand, gravel, and cover, bank material, channel-profile description, and boulders left by glaciers or their meltwater streams typeof debris. Some of the elements listed on the (Prior, 1991, p. 132). The thickness of the glacial drift form, such as bridge number and sufficiency rating is variable throughout the regions, ranging from zero (Appendix), were not determined. These elements to hundreds of feet. A layer of loess, which is a wind- were included in the original form for use by the depositedsilt composed predominantly of closely cooperating agencies. packed grains of quartz (Prior, 1991, p. 49), overlies Data were entered into a computer data base, the glacial drift in the Northwest Iowa Plains, the and a computer program was used to calculate the Southern Iowa Drift Plain, and parts of the Iowan potential-scour index on the basis of the factors Surface. The thickness of the loess throughout the described above. The data for each bridge and the regions also is variable, but the loess generally is calculated potential-scour index are presented in thicker in the western part of the State and in the table 4 at the end of this report. The entries in the northern part of the Southern Iowa Drift Plain that is table are sorted by county and within counties by the east of the Des Moines Lobe (fig. 1) (Oschwald and USGS station number. The site identification number others, 1965, p. 6). in the first column of the table is the key to the bridge The Loess Hills landform region is composed location in figure 1. of loess that is generally more than 60 ft thick. Com paredto glacial drift, which is somewhat resistive to erosive processes, loess is highly erodible and Landform Regions in Iowa unstable when wet. "Gully erosion is especially The major landformregions in the State are pronounced, and these deep, narrow, steep-sided described here because some components of the features are characteristic of the region's smaller potential-scour index were assessed larger values in drainages. Gullies lengthen headward, deepen,and some regions more frequently than in others. The widenquickly after rainstorms, cutting into cropland, following introductory description is from Landforms clogging stream channels and drainage ditches, and ofIowa (Prior, 1991, p. 30); the regions shown in forcing costly relocations of bridges and pipelines" figure 1are adapted from the same publication (p. 31). (Prior, 1991, p. 57). [The State is comprised of] seven topo The Paleozoic Plateau landform region is graphic regions: the Des Moines Lobe, the characterizedby shallow sedimentary bedrock and a

Potential-Scour Assessments near absence of glacial deposits. Many deep, narrow of index values is shown in figure 3A. The darker valleys have been eroded into the bedrock by the symbols denote larger values of the index. The sites streamsof the region (Prior, 1991, p. 84). A layer of with a potential-scour index greater than or equal to 15 loess covers most of the region (Oschwald and others, are located predominantly in the western part of the 1965, p. 6). State. Five of the eight sites with the index greater The Alluvial Plains constitute the remaining than or equal to 20 are in or adjacent to the Loess Hills landform region in the State. Although two major landform region in the southwest part of the State. plainsare shown in figure 1,alluvial plains occur With respect to the components comprising the throughout Iowaalong the State's majorstreamsand potential-scour index, bed material had the greatest rivers. The plains are formed by sedimentary effect on the index and accounted for 27.1 percent of processes, which are the erosion, entrainment, the overall total of the potential-scour indices (table transportation, deposition, and compactionof 2). The bed material component was evaluated as sediments (Vanoni, 1975, p. 1). The material sand, silt/clay, or when it could not be determined as comprisingthe alluvial plains, called alluvium, is either sand or silt/clay, as alluvium at 123 of the 130 made up of sediment that has been transported by bridge sites. The distribution of the bridges with water. Bridges over water in Iowa are in alluvial respect to the values assigned to this component is plains and are subject to the effects of the sedimentary shown in figure 3B. The fairly even distribution about processes that created the plains. the State attests to the alluvial nature ofrivers in Iowa. The rivers have carved the State's valleys and partially filled them with layered deposits ofgravel, sand, silt, Results of Potential-Scour Assessments and clay (Prior, 1991, p. 30, 98). Because of the ubiquitous occurrence of sand, silt, and clay in the A summary of the potential-scour indices and State's streambeds, the usefulness of the bed-material components is provided in table 2. Listed for each component in the potential-scour index is diminished component are the minimum, maximum, and median in Iowa. As noted previously, no consideration is values that were assessed, the sum of the values by given to the cohesive properties of bed material, which component for all of the bridge sites, and the percent affects the erodibility of the stream channel. age that each component comprises of the overall total The second most effectual component of the of the potential-scour indices (overall total—sum of potential-scour index was bank erosion at the bridge the 130 potential-scour indices determined in this sites, which accounted for 18.3 percent (sum of left study). The same summary of values for the potential- bank erosion and right bank erosion, table 2) of the scour indices also is listed in the table. overall total of the potential-scour indices. The The numerical distribution of the potential- distribution of bridges with respect to this component scour indices is summarized graphically by a histo is shown in figure 3C. The symbols in the figure gram in figure 2. The median of the 130 indices is reflect the largest value assigned to either bank at each 11.5. The interval estimate of the population median site. About one-fourth (34) of the bridges had mass at the 95-percent confidence level is 10.5 to 12.5 wasting occurring at one or both banks. Almost all of (Iman and Conover, 1983, p. 202), where population is the sites in or near the Loess Hills landform region the set ofall bridges over water in Iowa. The histo were in this category. gram shows that the indices are evenly distributed The third most effectual component of the about the median. Five bridges were assessed with potential-scour index was channel evolution, which indices less than 5, and eight bridges were assessed accounted for 17.9 percent of the overall total of the with indices greater than or equal to 20. The smallest potential-scour indices (table 2). The distribution of index value of 3 was determined for the State High the bridges with respect to this component is shown in way 9 bridge over the Rock River at Rock Rapids in figure 3D. The symbols used for each bridge are Lyon County (table 4, site 74), and the largest value of shown in order ofdecreasing values of the channel- 24.5 was determined for the State Highway 191 bridge evolution component. More than one-half (79) of the over Mosquito Creek near Earling in Shelby County bridges were assigned a value of 3 (Threshold) or 4 (site 105). (Aggradation, see table 1). Most of these bridges are The spatial distribution throughout the State of in the Southern Iowa Drift Plain and Loess Hills the potential-scour indices grouped by selected ranges landform regions.

8 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa Table 2. Summary of assessed values of the potential-scour index components and potential-scour indices at 130 highway bridges in Iowa

Index components

Percentage of Proxim Percent blockage by debris Bank erosion ity of age of Number river- Angle of Poten Stage of channel of piers Hori meander Mass approach tial- Bed Bed channel constric in zon- Vert- Left Right impact Pier wasting of high scour Assigned value material protection evolution tion channel tal cal Total bank bank point skew at piers flows indices

Minimum value 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 assessed Maximum value 4 3 4 3 2 1.33 0.67 0.33 2 2 3 5 6 2.5 24.5 assessed Median ofassessed 3.5 1 3 0 1 0 0 0 1 1 0 0 0 0 11.5 values Sum of values by 431.5 192 286 37 83 6.33 3.66 0.66 143 149 113 34 60 56 1,595.2 component for all sites

Percentage contribu 27.1 12.0 17.9 2.3 5.2 0.4 0.2 0.0 9.0 9.3 7.1 2.1 3.8 3.5 - tion to overall potential-scour index1 (rank) (1) (4) (3) (9) (6) (11) (2) (5) (10) (7) (8) 'Percentages do not sum to 100 because ofrounding.

c/> o o c

> (0 (0 (6 CO to 3 e a ST Mass wasting at piers accounted for 3.8 percent of the overall potential-scour index. Five bridgeswere assigned a value of 6 because of mass-wasting processes near a pieron both banks, and 10 bridges were assigned a value of 3 because of mass-wasting processes near a pier on one bank. The remaining bridges were assigned a value of 0. Angle of approach of high flows accounted for 3.5 percent of the overall potential-scour index. The highest value of this component was 2.5, which was assigned at eight bridges that were judged to have an angle of approach of high flows of about 45 degrees. Eight other bridges were assigned a value of 2, and POTENTIAL-SCOUR INDEX 20 bridges were assigned a value of 1 (table 4). Percentage of channel constriction accounted Figure 2. Histogram of potential-scour indices for130 for 2.3 percent of the overall potential-scour index. bridges in Iowa. The highest value assigned was 3 at one bridge, West Fork Ditch at Hornick in Woodbury County (table 4, The fourth most effectual component of the site 126). The measured constriction at this bridge potential-scour index was bed protection, which was 61 percent. The channel constriction is caused by accounted for 12 percent of the overall total of the vertical abutment walls of an old bridge that were left potential-scour indices (table 2). The distribution of standing just upstream of the current bridge. Six the bridges according to the values assigned to this bridges were assigned a value of 2 for this component, component is relatively uniform (fig. 3£). The bed- protection component isa good indication of bridges and 22 bridges were assigned a value of 1 (table 4). that have had their banks protected either because of Pier skew accounted for 2.1 percent of the changed conditions after a bridge was built, such as a overall potential-scour index. The bridge with the change in the angle of approaching flows, or because highest assigned value for this component (5) crosses the Cedar River at Cedar Rapids in Linn County of unusual conditions, such as highly skewed (table 4, site 70). This bridge has five piers in the crossings. channel that were assessed as being skewed about The seven remaining index components account 10degrees from the approach flow. for 24.6 percentof the overall total of the potential- Very little blockage of the bridge opening by scour indices (table 2). They are discussed in debris was noted during the onsite assessments. As a decreasing order of effect on the overall total. result, the percentage of blockage-by-debris compo Proximity of river meander impact point nent affected the overall potential-scour index the least accounted for 7.1 percent of the overall potential- of all the components and accounted for less than scour index. With respect to the values assigned to 1percent of the overall total of the indices. this component, 31 bridges were assigned a value of 3 because impact points are within 25 ft of the bridge, 6 bridges were assigned a value of 2 because impact Evaluation of Potential-Scour Assessment points are within 50 ft, and 8 bridges were assigned a Technique value of 1 because impact points are within 100 ft. The potential-scour index does not predict Number of piers in channel accounted for scour. Rather, it represents an assessment of the 5.2 percent of the overall potential-scourindex. Ten conditions at a bridge that may cause excessive scour. of the 130 bridges were assigned a value of 2 because Also, it represents an assessment of conditions at a of more than two piers in the main channel, and single moment in time. A single potential-scour 63 bridges were assigned a value of 1 because they assessment may help identify conditions that suggest have one or two piers in the main channel. The additional investigation at the site. The usefulness of remaining bridges do not have any piers or do not have the index in monitoring potential-scour conditions, piers in the channel during normal flows. however, is dependent on regular assessments and is

10 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa B _±W £*^—( s A. A AA i^ ^J^-si k > j k^4»*_«S V—ii4•» i^ j—•—1\k n«/ U*Lw> .2 1^ ^**ilr JI^a _i k A*A Ai A^J NuL j*AJk M^5^**J1P

RANGESIGES OF POTENTIAL-SCOURPOTI INDEX VALUES ASSESSMENT OF BED-MATERIAL COMPONENT -mr 20 to 24.J.9 A10 to 14.9 A Sand, silt/clay, or alluvium A 15to19.J.9 A 0 to 9.9 A Bedrock or boulders/cobbles

ASSESSMENT OF BANK-EROSION COMPONENT ASSESSMENT OF CHANNEL-EVOLUTION COMPONENT A Mass wasting at one or both banks IV(Threshold) A Fluvial erosion at one or both banks V (Aggradation) A No bank erosion (Degradation) II (Constructed) A I(Premodified) or VI (Restabilization)

EXPLANATION Boundary of landform region—Adapted from Prior (1991)

ASSESSMENT OF BED-PROTECTION COMPONENT ~m~ Bed not protected, both banks protected A Bed not protected, one bank protected A Bed not protected, banks not protected A Bed is protected

Figure 3. Location of bridges grouped by (A)selected ranges of potential-scour index, (8) assessment of bed-material component, (C)assessment of bank-erosion component, (D)assessment of channel-evolution component, and (£) assessment of bed-protection component.

Potential-Scour Assessments 11 limited to those components of the index that may chosen because it had the second-largest potential- change between assessments. For example, a river scour index (site 27, table 4). The drainage area of the meander impact point may move closer to a bridge, site with the largest index is 32 mi2 (site 105, table 4). suggesting that some protective countermeasures be The bridge over the Iowa River at Wapello (station installed at the bridge before scour problems occur. 05465500, fig. 4) was chosen because of unusual Also, as will be discussed in the next section, contraction scour that was measured there during the contraction and pier scour may be exacerbated at flood of 1993. The flood and resulting scour at this bridges that trap debris. Several of the components, site originally were described by Fischer (1994); however, very likely will not change between additional information is provided in this report. assessments, such as bed material, bed protection, percentage of channel constriction, and number of Bridge-Scour Processes and Estimating piers in channel. The repeated evaluation of these components would not provide new information. Maximum Scour The values of some of the components of the Bridge-scour processes are classified into three potential-scour index are closely related to the components—long-term aggradation or degradation of landform region in which the sites are located. For the stream channel, contraction scour, and local scour example, the higher valued assessments of channel at piers and abutments. The total scour that can occur evolution occurred predominantly in the Southern at a bridge is the sum of these components. Also Iowa Drift Plain and Loess Hills landform regions affecting scour is channel stability. Stream channels (fig. 3D). The greater likelihood of occurrence of a can migrate laterally, creating flow conditions at a particular value in a landform region will diminish the bridge that are significantly different from the flow value of periodically re-assessing the component conditions that existed when the bridge was built. The because no new information would be gained. maximum scour equations used for this study are those An aspect of potential-scour assessments that presented in the FHWA report "Evaluating Scour at may be beneficial to IDOT is that the assessments Bridges," second edition, Hydraulic Engineering evaluate some of the geomorphologic processes that Circular No. 18 (HEC-18) (Richardson and others, affect scour at a bridge. Currently, evaluation of these 1993). The report is referred to as HEC-18, and the processes is not part of a typical bridge inspection. scour equations are referred to as the HEC-18 equations in the following pages.

ESTIMATES OF MAXIMUM SCOUR Long-Term Aggradation/Degradation of the Streambed An estimate of the maximum scour that may occur at a site during an extreme high flow is made by In geologic time, all streams degrade. The determining the hydraulic properties of the channel process, however, is not evenly distributed; some and bridge opening for a design flood and using scour streams will degrade more quickly or deeper than equations. Two principal types of scour occur at other streams. Still other streams may aggrade as bridges—contraction scour and local scour at piers sediments are deposited. Excessive degradation and abutments. Included in the estimate of maximum creates stability problems at a bridge, and excessive scour is a determination whether long-term aggrada aggradation reduces conveyance through a bridge tion or degradation may be occurring at the bridge. opening that can cause frequent flooding and highway Estimates of maximum scour were made at closure. 10 highway bridges in this study (fig. 4). The location, Human activities can affect degradation or drainage area, median bed-material particle size, and aggradation. Such activities include agricultural flood-frequency data for each of the sites are listed in practices, urban development, mining operations, and table 3. The principal criterion for selecting the river-control works. For example, construction of a bridges was that most of the sites have drainage areas flood-control reservoir on a stream contributes to greater than about 300 mi". In addition, the sites were channel degradation downstream of the dam by selected to represent a variety of bridge and channel trapping much of the sediment and altering the conditions. The bridge over the Raccoon River at Van streamflow characteristics. The natural sediment load Meter in Dallas County (station 05484500, fig. 4) was and flow of the stream were responsible for establish-

-,StouJc rd'BrtetHri^ hi.iTAiSr' rHalôciHcerTo"I iGordo i"0^L-_ L,Chlckasaw|—.. -J |• AV \ ••..!!!' .Fayette iClayton /Plymouth '[Cherokee.T.4_._4._._L._._L._.j_._.j_._.4_.-.Jtokee iRuenaJBuena *"JPocaip/wa |Humboldt [WrightIWrlaht [Franklin jButkr |Bremer I | 191° I |Vista ihontas _ I L—. I I I Webster j '. [Black^ti ûchanaTTJbelawarcMBuchanan 'Delawi IDubuque ôodSTry—^[ida [sic "—[adhoun-1 rHamlitorrTHarfin ""feundy" lHawk { { I '!_{ JI '! J'! J'! J'! j_ JTalSaiJBenton L"pn LT^** i. Uackson .Monona a .Crawford" [CarroU *[Greene ' Tioone ' ]Sto'ry ""[Marshall" ', ' 06607200 II 42" |'• l—[Ctoton

Harrison i [Shelby [Audu-i[Guthrie i|Dallas i _J[Polk IJasperi L[Poweshiek L|Iowa I[Johnson |p*"; | v/.-~~-. i | 05483450 J | I ' I I JJ j -|054845(W j_ _ J J ! _ \mHm (Pottawattamie ,'Cass [Adair [Madison 1WairaP^fMarion " [Mahaska | Keokuk [Wash- \_J I • | [, I 1 llngton Ml .06809210,yuou« I ' 05487980i [I I !ta3ton i ! ij| ! 05487980jA|A ^[05489000J05489000[ I ^_j! ^ •Louisa1 A„A05465500/ \ Mill's ""-"-—TMont- [Adams "TOnYon [clarke 'TLucas [Monroe [Wapello |Jefferson [Henry] ,'gomery \ ' > ' ' [ \ ) iDes I .i j j i _i_ l I • olnc\ A |Page Tfaylor TRmgook! Decatur "[Wayne [Appa- [Datis JVanBurenl I / 068085006808500 I L I! [noose I flee **-. ) ^emont!_i 068~7000>' - 06818750«*0«0-»«r* j^ iii:: ^/ Basefrom U.S. Geological Survey digital data. 20 40 60 MILES 1:100.000,1S83 Universal Traverse Mercatorprojection. 0 20 40 60 KILOMETERS Zone 15

EXPLANATION 05465500 a Bridge site and U.S. Geological Survey streamflow-gaging station number

Figure 4. Location ofbridge siteswhere maximum scour wasestimated. ingthecharacteristics of the channelprior to construc with respectto time shows graphicallywhat has tion of the dam. Clear water (water that is not occurred at the site. Generally, changes in fhe stage transporting sediment) released from thereservoir corresponding to the index discharge imply a similar entrains sediment as it moves downstream, eroding change in theelevation of the streambed. Changes in the streambed and channel banks until equilibrium the width of the flow area of the index discharge that with the new flow characteristics is achieved are due to changes in the streambed elevation are (Vanoni, 1975, p. 2-9). assumed to be minimal. In this study, the long-term aggradation or Historically, an early variation of the rating- degradation of the streambed that has occurred during curve method of measuring changes in streambed the period of streamflow data collection at the site is elevation was the "Specific discharge Gauge" usedby presented. The method of measuring aggradation or Inglis(l949,p. 3, 178-179, 189). According toInglis, degradation is based on changes in the stage the "Specific discharge Gauge" is the "* * * Gauge corresponding to an index discharge. The index reading corresponding toa particular discharge *** discharge used for this study is theavejrage discharge [which] is arrived at by drawing a smoothed—most for the period of streamflow record ateach site. The probable—curve through the Gauge readings observed stage of the index discharge isdetermined from each with discharges approximating to the specific dis rating curve that was developed and is assigned the charges during (R) the rising flood season and (F) the date each curve was developed. A plot of the stage falling flood season" (p. 3). Inglis used several

Estimates of Maximum Scour 13 Table 3. Location, drainage area, median bed-material particle size, and flood-frequency data for bridge sites analyzed for maximum scour [mi2, square miles; mm, millimeters; ft3/s, cubic feet per secondl

U.S. Flood-frequency data Geological Median Survey bed- Number of years streamflow- Stream name and material of record2 gaging vicinity, particle Discharge Discharge station county, Drainage size, Period of of 100-year of 500-year number highway, area D50 peak-flow Syste- His- flood (Q1M) flood (Q500) (fig- 4) date surveyed (mi2) (mm) record1 matic toric (ft3/s) (f^/s)

05465500 Iowa River at Wapello, 12,499 30.60 1903-92 90 0 103,000 121,000 Louisa County, State Highway 99, November 15-18, 1993

05483450 Middle Raccoon River 375 .34 1973-92 14 20 18,800 26,800 near Bayard, Guthrie County, State Highway 25, October 25, 1993

05484500 Raccoon River at Van 3,441 91 1915-92 78 0 49,100 62,600 Meter, Dallas County, County Road R16, November 4-8, 1993

05487980 White Breast Creek near 342 .45 1962-92 31 48 25,800 35,900 Dallas, Marion County, County gravel road, October 19, 1993

05489000 Cedar Creek near 374 .27 1946-92 46 141 45,900 73,500 Bussey, Marion County. State Highwy 156, June 15-16, 1993

06607200 Maple River at 669 .39 1942-92 51 26,200 33,300 Mapleton, Monona County, State Highway 175, October 26, 1993

06808500 West Nishnabotna River 1,326 .41 1949-92 44 45 49,500 59,100 at Randolph, Fremont County, State Highway 184, October 27, 1993

14 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa Table 3. Location, drainage area, median bed-material particle size, and flood-frequency data for bridge sites analyzed for maximum scour—Continued

U.S. Flood-frequency data Geological Median Survey bed- Number of years streamflow- Stream name and material of record2 gaging vicinity, particle - Discharge Discharge station county, Drainage size, Period of of 100-year of 500-year number highway, area peak-flow Syste His flood (Q100) flood (Qsooj (fig. 4) date surveyed (mi2) (mm) record1 matic toric (ftVs) (tf/s)

06809210 East Nishnabotna River 436 0.34 1948-92 32 45 35,600 45,200 near Atlantic, Cass County, County paved road, May 24-25, October 28, 1993

06817000 Nodaway River near 762 .34 1918-25, 66 90 42,700 51,800 Clarinda, 1936-92 Page County, State Highway 2 (business route), June 22-23, 1993

06818750 Platte River near 217 .47 1966-91 24 26 10,000 11,200 Diagonal, Ringgold County, County gravel road, May 25, 1993

Inclusiveyears of systematic peak-flow data collection; gaps may exist in the interval during which the streamflow-gaging station was discontinued. "Systematicrecord—periodduring which streamflow data werecollected. Historic record—theperiod outside the systematic record during which certain peak-discharge information has been determined that enables extension of the peak-flow record. 3Average offive sediment-size analyses made during 1992 at Iowa River atWapello. reference (index) discharges to show changes in the stress that can scour the streambed at the bridge streambed elevation. More recently, Williams and opening. Contraction scour typically is cyclic; the Wolman (1984, p. 4) used the rating-curve method as streambed scours during the rising stage and backfills one way to determine changes in mean bed elevation during the falling stage. Other factors that result in downstream ofdams on alluvial rivers. They used the contraction scour include ice, debris, and the growth discharge that was exceeded 95 percent ofthe time as ofvegetation in the channel or flood plain (Richardson the index discharge (p. 5). and others, 1993, p. 9). Contraction scour is affected by the sediment Contraction Scour transport characteristics of a river. Therefore, two sets A highway embankment built across a flood ofequations in HEC-18 are used to compute maxi plain reduces the flow area of a flooding river. The mum contraction scour, one for live-bed sediment embankment contracts the flow, forcing the water from transport conditions and the other for clear-water sedi the flood plain through the bridge opening. From the ment transport conditions. Live-bed sediment trans principles of conservation of mass and energy, the port conditions occur when the flow is transporting flow velocity at the bridge is greater than the flow sediment along the bottom of the channel. The velocity without the embankment present. The contraction scour depth increases at the bridge increased flow velocity results in increased bed-shear opening, decreasing the bed-shear stress until the

Estimates of Maximum Scour 15 sediment transport rate out of the opening is equal to embankment length as one of the variables, which the sediment transport rate into the opening. Clear results in excessively conservative (very deep) water scour occurs when no upstream bed material is estimates of scour. Richardson and others (1993, transported into the opening. The contractionscour p. 50) also present an alternative abutment scour depth increases until the shear velocities in the equation that maybe usedwhereconditions at a bridge enlargedbridgeopeningare less than the threshold of are similar to the field conditions from which the sediment motion. An equation is presented in HEC-18 equation was developed (scour at the end of a spur to help determine whether to use the live-bed equa dike extending into a river). In this study, however, tions or the clear-water equations to estimate contrac calculations of abutment scour using the alternative tion scour. The equation is based on the critical flow equation generally estimated deeper scour. velocity that will transport the D50 bed material. D50 is the median diameter of the streambed material such Channel Stability that 50 percentby weightof the streambed particles have diameters less than D50. Live-bed sediment The tendency of river channels to migrate or transportconditions are common in most Iowa rivers, shift laterally as the banks erode on the outside edges and clear-water conditions occur on most flood plains. of bends and fill in on the inside edges affects scour at bridges. A migratingstream will change the hydraulic conditions at a bridge. A bridge designed for one type Pier Scour of hydraulic condition may not be appropriate for a Erosion of the streambed around bridge piers is new condition. For example, piers that were aligned caused by redirection of the flow as water is deflected with the flow when the bridge was built but are no downward and accelerated around the pier. The longer aligned because of a change in the angle of the redirected flow increases the shear stress that can approaching flow are subject to greater scour because transport bed material away. Like live-bedcontraction of the increase in the obstructive area the pier presents scour, the maximum live-bed local scour occurs when to the flow. Also, a migrating stream eventually may the rate of sediment transported out of the scour hole cause streamflows to be directed towards an abutment, exceeds the rate of sediment transported into the undermining it. hole. For clear-water conditions, the scour hole will deepen until the shear velocity in the scour hole cannot Total Scour transport additional material. The HEC-18 pier-scour The total scour that can occur is the sum of the equation is recommended to be used for both live-bed components described above. If the streambed is and clear-water sediment transport conditions likely to degrade during the life of the bridge, the (Richardson and others, 1993, p. 39). maximum contraction scour, pier scour, and abutment Many factors affect local pier scour. They scour depths are measured from the expected elevation include pier width, pier shape, flow velocity, flow of the degraded bed. If a pier or abutment is located in depth, and alignment of the pier with respect to the an area where contraction scour also may occur, the approaching flow. Debris piles can increase the maximum pier scour and abutment scour are measured effective width of piers, resulting in deep scour holes from the computed elevation of maximum contraction (Laursen and Toch, 1956, p. 28; Richardson and scour. others, 1993, p. 46).

Abutment Scour Data Collection and Method of Analysis for Estimating Maximum Scour Erosion of the streambed at abutments is caused by the rapid change in flow direction as water enters The scour equations in HEC-18 require quantifi the bridge opening from the flood plain. Abutment cation of variables that can be obtained from a hydrau scour is affected by the type ofabutment (vertical-wall lic analysis of the bridge site. Therefore, the estimates abutments, spill-through abutments), the type of wing of maximum scour in this study were made using the walls, and guide banks. According to Richardson and following methodology: (1) determine the 100-year others (1993, p. 47), all of the abutment-scour equa flood (Qion) arjd 500-year flood (Q500) discharges for tions in the literature include the approach highway a site, (2) determine the corresponding hydraulic

16 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa properties of thechannel and bridge, (3) compute the bridge section for the Q|0o flood discharge with the water-surface profiles for the flood discharges, and stage-discharge rating curve in effect at each site. (4) calculate the maximum scour. Rating curves that did not include the Qiqo fl°°d discharge were extended. Flood Discharges The Qiooar>d Q500 flood discharges used to Maximum Scour Equations compute the water-surface profiles were determined from flood-frequency analyses of the streamflow The HEC-18 scour equations were used to records at each bridge site. The flood frequencies estimate scour. Input variables to the equations, such were determined according to procedures outlined in as channel widths, discharges, flow depths, and flow velocities, were obtained or derived from WSPRO. Bulletin 17B of the U.S. Interagency Advisory The median diameter of the streambed material, D50, Committee on Water Data (1982). The analyses were computed using streamflow recordscollected through was obtained from unpublished data collected by Eash wateryear 1992, exceptone site that was discontinued (1993). The values used for each site are listed in at the end of water year 1991. The 1993 flood peaks table 3. were not used to compute flood frequencies because they were not available at the time of the scour analyses. The peak-flow record, the numberof years Results of Estimates of Maximum Scour ofsystematic andhistoric record usedin the frequency analyses, and the Q100 and Q500 flood discharges are The results of the estimates of maximum scour listed for each site in table 3. In the subsequent are presented foreach site in the following format: hydraulic analyses, flood discharges otherthan Qiqo (1) the channel and bridge at a site are described, and Q5oo were used at four sites for reasons that are (2) the water-surface profiles are discussed, (3) the explainedin the respective analyses. calculated scour depths are tabulated, and (4) the results are discussed. The long-term aggradation or degradation that has occurred is shown in a graph of Hydraulic Properties the river stage corresponding to the average stream- Channel cross-section and bridge-geometry data flow plotted as a function of time. The channel cross werecollected using an electronic surveying instru section at the downstream side of the bridge is shown ment and entered into a step-backwater computer in an elevation view. The scour depths calculated for model so that the hydraulic properties at a bridge the Qiqo flood discharge (orother discharge as noted) could be determined. Cross-section properties were are superimposed on the cross section. The ccntrac- computed for the exitsection, the full-valley section, tion-scour depth is referenced to the streambed at the the bridge-opening section, and the approach section. time the bridge site was surveyed. The local scour If a cross section could not be surveyed, that cross depths for the piers andabutments arereferenced to section was estimated from another cross section using the elevation of the calculated contraction scour the template option of the step-backwater model (Richardson and others, 1993, p. 69), and the (Shearman, 1990,p. 123). All elevations were abutment-scour depths are shown at the toe of the referenced to gage datum. abutment embankment. The cross-section data (dashed line in the figures) were obtained from Water-Surface Profiles discharge measurements made at thebridge. The vertical scale of the elevation view is exaggerated to Water-surface profiles were calculated using the facilitate rendition of the calculated scour depths. The WSPRO step-backwater model (Shearman. 1990; dimensions of the pier footings and pilings were Shearman and others, 1986). WSPRO is a water- determined from bridge plans provided by IDOT. surface profile computation model for one-dimen sional, gradually varied, steady flow in open channels. The bridge sites are presented in downstream The model can estimate hydraulic properties through orderby USGS streamflow-gaging station number bridges and in flood plains. The model was calibrated except Iowa River at Wapello, which is presented last at eachsite by adjusting channel roughness values to because of the unusual contraction scour that occurred match the estimated water-surface elevation at the there.

Estimates of Maximum Scour 17 Middle Raccoon River near Bayard (05483450) at the bridge than upstream (W2 greater than Wj in equation 16, Richardson and others, 1993,p. 33). The This bridge is located on State Highway 25 in scour depths calculated for the bridge at Middle Guthrie County. It crosses the main channel of the river at a 20-degree angle; upstream of the bridge, the Raccoon River near Bayard are summarized in the main channel bends to about a 45-degree angle to the table below. bridge and highway. The river valley is relatively Figure 5A shows that the stage corresponding to narrow and extends about 500 ft from side to side in the average streamflow at the site is 1.4 ft higher in the vicinity of the bridge. Upstream of the bridge, the 1993 than in 1978, which indicates that the streambed channel is near the right edge of the valley,and the left is aggrading. Whether the streambed will continue flood plain is a pasture. Downstream, the channel is aggrading cannot be estimated from the data because near the left side of the valley, and the right flood plain is a cultivated field. Trees cover the narrow flood of the short (15 years) period of record. plainon each side of the bridge, and thin bandsof Figure 5B shows the cross section surveyed trees line the opposite side of the channel. The bridge at the downstream side of the bridge on is a 245-ft by 36-ft, concrete-beam structure resting on October 25, 1993, with the pier- and abutment-scour abutments and two concrete piers, which are skewed depths calculated for the Q10o fl°°d superimposed. 15 degrees from perpendicular to the axis of the Also shown in figure 5B is the cross section obtained bridge. The abutment and pier footings are supported from a discharge measurement made on July 9, 1993. by steel piling. The bridge was built in 1980(Iowa Department of Transportation, 1979). The measured discharge was 23,200 ft3/s, which is The water-surface profile computations show greater than the Qioo flood. The discharge-measure pressure-flow conditions at the bridge for the Q50o ment cross section shows clear evidence of scour in flood discharge. Contraction-scour depths were not the middle of the channel and no evidence of scour at determined because negative values were computed. either abutment. The measured scour was about to the The negative values are due to the channel being wider elevation of the base of the piers.

100-year flood 500-year flood (Qioo) (Qsoo) Remarks

Discharge (fr/s) 18,800 26,800 No road overflow; pressure flow for Q500. River stage at bridge 26.75 28.84 (ft above gage datum)

Contraction-scour depth (ft) -- — Live-bed conditions; negative values computed.

Pier-scour depth (ft) 10.5 11.5 ~ Abutment-scour depth (ft)

Left abutment 11.9 13.9 ~

Right abutment 14.1 18.5 -

18 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa o uj 11 Average discharge = 232 cubic feet per second

UlOLU

8 '' I I I I I L J L rr uj QJ> 1978 1983 1988 1993 > < rr

0 50 100 150 200 250 260 DISTANCE, IN FEET FROM LEFT ABUTMENT (VERTICAL EXAGGERATION X 2)

EXPLANATION

mm Calculated pier scour, 100-year flood KSNS^l Calculated abutment scour, 100-year flood ^^— Surveyed cross section, October 25,1993 —_ Discharge measurement cross section, July 9,1993, discharge = 23,200 cubic feet per second, river stage = 28.49 feet

Figure 5. (A) Streambed aggradation/degradation trend lineand (8) elevationview(looking downstream) of channel cross section showingcalculated maximum scour depths forthe 100-yearflood at State Highway 25 bridge in GuthrieCounty, streamflow-gaging station Middle Raccoon River near Bayard (05483450).

Estimates of Maximum Scour 19 Raccoon River at Van Meter (05484500) Contraction scour is shown only for the overbank (flood-plain) portion of the cross section because the This bridge site is locatedon County Road R16 contraction-scour equations produced a negative value in Dallas County. The bridge is nearthe right edge of for the main channel. The surveyed cross section the river valley and crosses the river at a wide bend in shows the streambed between the first and second the river. The alignment of the piers is perpendicular piers from the right abutment to be about at the to the axis of the bridge; however, the angle of elevation of the base of the piers. approach of floodflows isabout 15 degrees. Theflood Two discharge-measurement cross sections also plain is about 2,000 ft wide at the bridge. Upstream, are shown in figure 65. The discharge measured on the left flood plain is cultivated, and the right flood July 1, 1986, was 38,300 ft3/s with a corresponding plain is covered with trees and marshland, theareaof river stage of 22.25 ft. The other cross section is from which, according to the bridge plans, was formerly a the first discharge measurement made at the site after gravel pit. Downstream, the left flood plain is culti the flood peak, which occurred July 10, 1993(date of vated between the edge of the plain to about 300 ft cross section =July 19, 1993, discharge = 13,600 ft3/s, from the edge of the river; between this point and the river stage =14.01 ft). Unsafe conditions prevented river the floodplain is coveredwith trees. The right measurement of the flood peak at the bridge because flood plain is cultivated. The bridge is a 445-ftby water was flowing against the side of the bridge 24-ft, continuous I-beam structure resting on beams. Discharge measurements were made at abutments and four piers. The abutments and pier another bridge about 5 mi downstream during the footings are supported by steel piling. The two right extreme high flows. The peak discharge at the study piersare in the mainchannel,and the rightabutment is bridge was determined to be 70,100 ft /s; the protected with riprap. The bridge was builtin 1957 corresponding river stage was 26.34 ft (Southardand (Iowa Department of Transportation, 1956). others, 1994, p. 164). This peak discharge was greater The water-surface profile computations indi than the theoretical Q50o flood (table 3). cated submerged pressure-flowconditions for the Because the streambed was nearly at the same Q500 ^00^ discharge. Contraction-scour depths in the elevation in November when the site was surveyed as main channel were not determined because the scour it was when measured on July 19, it was assumed that equations producednegativevalues. The scourdepths the channel did not fill in between the flood peak and calculated for the bridge over the Raccoon River at the discharge measurement 9 days later. The similar Van Meter are summarized in the table below. bed elevations of these two cross sections and of the Figure 6A shows that the streambed has been cross section measured in 1986 suggest that contrac stable at this site since the gaging station was installed. tion and abutment scour at the bridge is much less than Figure 6B shows the cross section surveyed at the the scour predicted by the scour equations. That downstream side of the bridge on November 14, 1993, contraction scour in the main channel is minimal and with the contraction-, pier-, and abutment-scour is likely a consequence of the large size (D50 = 91 mm) depthscalculated for the Qjqoflood superimposed. of the bed material (table 3).

100-year flood 500-year flood (Q100) (Q500) Remarks

Discharge (ft3/s) 49,100 62,600 Road overflow. Discharge through bridge opening 46,600 49,500 Pressure flow for Qsoo- (ft3/s)

River stage at bridge 23.95 25.64 -- (ft above gage datum) Contraction-scour depth (ft) Main channel Clear-water conditions; negative values computed. Overbank 8.7 10.2 Clear-water conditions.

Pier-scour depth (ft) 19.4 20.1 - Abutment-scour depth (ft) Left abutment 26.4 24.5

Right abutment 22.1 18.6 ~

20 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa o its a I I I I I I I I I I I I 1 1 I I I I I I I I I I I I I I I l I l I l l I I I I I I I I l l I I I I I I l I I I I I I I I I I l I I I I I I I I I I I I I I I I I I I I a Average discharge = 1,455 cubic feet per second 2*1 5 _••••••••' ••«•• «•• £*2 4 giug 3

WC< 2 •' 111 1111111111111111111111 111111 1111 1111111111 DC UJ o o o o o o o o CO UJ> cm CO TT IO h- 00 o> o> >< O) O) Oi o> O) s o> O) o> o>

-i—p—i—i—i—i—i—i—i—i—i—|—i—i—i—i—i—i—r—i—i—i—i—i—i i i—i—i—i—i—j—i i i i i i—i—i—i—p—i—i—i—i—i—r d 100-year flood discharge = 49,100 cubic feet per second River stage = 23.95 feet j

Right - abutment

-20 0 100 200 300 400 465 DISTANCE, IN FEET FROM LEFTABUTMENT (VERTICAL EXAGGERATION X 4)

EXPLANATION

Calculated contraction scour, 100-year flood Calculated pier scour, 100-yearflood E^vSSNl Calculated abutment scour, 100-year flood ^^—i Surveyed cross section, November 4,1993 __ Discharge measurement cross section, July 19,1993 (9 days after peak of record) Discharge measurement cross section, July 1,1986, discharge = 38,300 cubic feet per second, river stage = 22.25 feet

Figure 6. (A) Streambed aggradation/degradation trend line and (S)elevation view (looking downstream) of channel cross section showing calculated maximum scour depths forthe 100-year flood at County Road R16bridge in Dallas County, streamflow-gaging station Raccoon Riverat Van Meter (05484500).

Estimates of Maximum Scour 21 White Breast Creek near Dallas (05487980) opening occurat the stage of the stream when flow begins to go over the road on the left flood plain This bridge site is locatedon a gravel road in rather than at the stage of the Q)0o flood western Marion County. The bridge crosses the stream at about a 30-degree angle and is downstream (Qioo =25,800 ft3/s). Therefore, scour calculations about 350 ft from a bend in the stream. The road in were made for a discharge of 17,700 ft3/s, denoted the left flood plain curves 90 degrees to the bridge in Qpro (point-of-road overflow), rather than for Ql00. the upstream direction. Upstream of the bridge, the The maximum scour depths calculated for the bridge right flood plain is a pasture, and the left flood plain is over White Breast Creek near Dallas are summarized cultivated. There is a large clump of trees on the left in the table below. bank near the bridge. Downstream, the flood plain is Figure 1A shows that the streambed has been cultivated on both sides of the stream, and trees line stable since 1962. Figure IB shows the cross section the banks. The bridge is a 250-ft by 20-ft, continuous surveyed at the downstream side of the bridge on I-beam structure supported by abutments and two October 19, 1993, with scour depths calculated for the concrete piersthatareskewed 30degrees to theaxisof point-of-road-overflow flood (Qpro) superimposed. the bridge to be parallel to the flow. The abutments and piers are supported by wood piling. The bridge The extent of the erosion at the right abutment is was built in 1955 (Iowa Department of Transportation, shown by the outline of a discharge-measurement 1954a). The site is marked by active erosion at the cross section made July 5, 1981. The area of the rightbankand abutment. Streamflow occasionally is bridge opening (computed parallel to the axis of the affectedby backwaterfrom a reservoirabout 15 mi bridge)hasenlargedapproximately400 fv since 1981. downstream. The primary cause of the erosion is a river-meander The water-surface profile computations indi impact point occurring at the bridge during normal cated that the higher velocities through the bridge flows (see table 4, site 77).

Discharge, point- of-road overflow 500-year flood (Qpro) (Q500) Remarks

Discharge (ft3/s) 17,700 35,900 Road overflow for Qsoo-

Discharge through bridge opening 17,700 25,100 — (ft3/s) Stream stage at bridge 27.50 33.00 (ft above gage datum) Contraction-scour depth (ft) 1.4 1.7 Live-bed conditions.

Pier-scour depth (ft) 8.5 8.7 ~ Abutment-scour depth (ft) Left abutment 11.9 17.4

Right abutment 10.3 18.0 —

22 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa e

lis -1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1 1 r 1 1 1 1 1 1 1 r §?i a Average discharge = 206 cubic feet per second

7 - sis §£> 5 r 1 1 1 1 1 1 1 1 1 1 1 1 1 \ 1 1 1—1—1—1—1—1—1—1—1—1—l 1960 1965 1970 1975 1980 1985 1990 1993 oc < h- C/D

0 50 100 150 200 DISTANCE, IN FEET FROM LEFTABUTMENT (VERTICAL EXAGGERATION X

EXPLANATION Calculated contraction scour, discharge at point-of-road overflow Calculated pierscour, discharge at point-of-road overflow fc^s^l Calculated abutment scour, discharge at point-of-road overflow — Surveyed cross section, October 19,1993 — — Discharge measurement cross section, July 5,1981, discharge = 11,300 cubic feet per second, river stage = 25.86 feet

Figure7. (A) Streambed aggradation/degradation trend lineand (8) elevationview (looking downstream) of channel cross sectionshowing calculatedmaximum scour depths for the point-of-road-overflow flood at countyroad bridgein Marion County, streamflow-gagingstation White Breast Creek near Dallas (05487980).

Estimates of Maximum Scour 23 Cedar Creek near Bussey (05489000) 26,600 ft3/s in 1981, andno fall for thepeak discharge of 96,000 ft3/s in 1982. The bridge at this site is located on State High Because maximum scour conditions are not way 156 in eastern Marion County. The highway likely to occur when the bridge is not acting as a crosses the river at an angle of about 15 degrees near contracted opening, it was decided to calculate scour the right edge of the river valley and continuesacross using the dischargewith the maximum measured the flood plain for about0.5 mi before leavingthe velocities at the current bridge and compare the results valley. Upstream of the bridge, the streamis approxi with the measurement. The discharge measurement mately parallelto the highway for about0.5 mi. The was made July 6, 1993, and was 16,100 ft3/s; the left flood plainon both sidesof the highway is culti average velocity was 3.82 ft/s. There was no road vated; narrow bands of trees line the riverbank. The overflow. The scour depths calculated for the bridge right flood plain on both sides of the bridge is covered over Cedar Creek near Bussey using this discharge are by trees. The bridge is a 401-ft by 36-ft, pretensioned, summarized in the table below. prestressed, concrete-beam bridgesupported on Figure 8/\ shows that the streambed has been abutments and four concrete piers that are skewed stable for the period of record (1947-93). Figure 85 15degreesfrom perpendicular to theaxis of the bridge shows the cross section surveyed at the downstream to be parallel to theflow. The abutments are supported side of the bridge June 15, 1993, with the calculated by steel piling, and the piersare supported by spread scour depths superimposed on the cross section. The footings on shale and limestone. An earthen guide calculated contraction scour for the main channel is bank extends upstream from the left abutment. The 0.1 ft and is not discernible in figure SB. Abutment bridge was built in 1989 (Iowa Departmentof scour was not calculated for the left abutment because Transportation, 1989). of the presence of the guide bank. The cross section Road overflow begins at discharges greater than from the discharge measurement, measured at the approximately 16,000 ft3/s, which is about one-third upstreamside of the bridge, also is shown in figure SB. The actual scour is much less than the calculated scour the theoretical Qioo flood of45,900 ft3/s. The point except for about 1 ft of contraction scour in the main of road overflow is not in the same hydraulic section channel. Although pier scour was not measured as the bridge but is about 2,500 ft upstream. There during the flood, a post-flood inspection showed fore, it was necessary to divide and route streamflows minor scour at the piers. An inspection after the flood over the road and through the bridge. The water- of September 15-16, 1992 (maximum discharge = surface profile computations indicated that the bridge 20,900 ft3/s, discharge through bridge opening = section is not a contracted opening for discharges 15,900 ft3/s, river stage = 28.28 ft), also showed that greater than about 20,000 ft3/s. This indication is minor scour occurred at the site. The inspection in supported by flood profiles made in the Cedar Creek 1992 revealed that the toe of the upstream end of the drainage basin in 1981 and 1982 (Heinitz, 1986, guide bank had eroded an estimated 5 ft and that the fig. 22, p. 32). The flood profilesshow a fall of 0.3 ft erosion was lateral into the guide bank rather than into at the site (old bridge) for the peak discharge of the ground.

Discharge measured July 6,1993 Remarks

Discharge (ft/s) 16,100 Discharge with maximum measured flow velocities, no road overflow.

River stage at bridge 24.96 ~ (ft above gage datum) Contraction-scour depth (ft) Main channel 0.1 Live-bed conditions. Left overbank 5.3 Clear-water conditions. Right overbank 4.1 Clear-water conditions.

Pier-scour depth (ft) 7.3 ~ Abutment-scour depth (ft)

Left abutment -- Not calculated because of guide bank.

Right abutment 8.9 -

24 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa O uj

i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i a Average discharge = 218 cubic feet per second

ffig£ 7 -

LjjQLU 6 - OUjg

5 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i rr uj LU> 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1993 >< rr

50 T—| 1 1 1 1 1 I I I I 1—I 1 1—I 1—I—I 1 1 1 1111 1 I I 1 I | I I I I I o Maximum-measured-velocity discharge = 16,100 cubic feet per second 45 River stage = 24.96 feet § 40 Right Lett abutment uu Road surface abutment G 35 < Low concrete O 30 UJ Water surface 25 S maximum-measured- CO velocity discharge < 20 I- Ul LU 15 U_ z 10

5 o I- 0 Ik-'*

UJ -5 G —I UJ -10 t_i L _• • • • • ••'•••'• • • • i__j t_ -20 0 100 200 300 400 420 DISTANCE, IN FEET FROM LEFT ABUTMENT (VERTICAL EXAGGERATION X 4)

EXPLANATION

Calculated contraction scour, maximum-measured-velocity discharge Q] Calculated pier scour, maximum-measured-velocity discharge tSSSSSl Calculated abutment scour, maximum-measured-velocity discharge —— Surveyed cross section, June 15,1993 — — Discharge measurement cross section, July 6,1993 (upstream side of bridge), discharge = 16,100 cubic feet per second, river stage = 24.96 feet

Figure8. (A) Streambed aggradation/degradation trend line and (S) elevationview(looking downstream) of channel cross section showing calculated scour forthe discharge with maximum measured velocities at State Highway 156 bridge in Marion County, streamflow-gaging station Cedar Creek near Bussey (05489000).

Estimates of Maximum Scour 25 Maple River at Mapleton (06607200) Figure 9A shows thatthestagecorresponding to theaverage discharge decreased 6.7 ft between 1941 This bridge is on State Highway 175 over the and 1987, which indicates that the streambed degraded Maple River about 1mi southwest of Mapleton in approximately the same amount. The rate of degrada MononaCounty. The highway crosses the river at an angle ofabout 30degrees near the left side ofthe river tion decreased about 1971; between 1971 and 1987 the valley. Thehighway is parallel to theaxisof the streambed degraded about0.5 ft. The rate of degrada valley away from the bridge, and theflood plain is tion fortheperiod of rating-curve changes is0.146 ft/yr about 3,500 ft wide. Small trees and brush cover the (6.7ft in 46 years); the rate of degradationsince 1971 leftflood plain on both sides of the highway in the is 0.031 ft/yr(0.5ft in 16years). The mostlikely immediate vicinity of thebridge. A low leveeextends explanation for thestreambed degradation at this site downstream from the highway about 250 ft from the is the response of the river to channel straightening in riverbank on the right flood plain. The right flood the 1930's. The site is located in the Loess Hills plain iscultivated on the upstream side of the high landform region (site 82, fig. 1). way, and it is cultivated beyond the levee on the downstream side. The bridge is a 240-ft by 26-ft, Figure9B shows the cross section surveyedat continuous I-beam structure supported by concrete the downstream side of the bridge October 26, 1993, abutments and two concrete piers, which are skewed with the calculated scour depths superimposed on the 30degrees from perpendicular totheaxis of the bridge cross section. The cross section of the largest recently to be parallel to the flow. Theabutment and pier measured discharge also is shown in figure 9B. The footings are supported by wood piling. The bridge measurement, made at the downstream side of the was built in 1955, replacing a bridge that was washed bridge June 15, 1991, shows that the streambed out in 1954(Iowa Department of Transportation, scoured in the middle ofthe channel and that the depth 1954b). of scour is below the elevation of the bases of the The bridge site is characterized by degradation piers. This scour, however, is not the resultof floodof the streambedby more than 6 ft since systematic collection of streamflow records began in 1942. The plain flow returning to the main channel because the water-surface profile analyses indicated that the Qiqo streamflowwas approximately bankfull. Rather, the and Q500 flood discharges will pass through the bridge streambed lowered during the flood as the result of a opening. Themaximum scourdepths calculated for general entrainment of bed material causedby the thebridge at Maple River at Mapleton aresummarized rapidly flowing water. The scoured streambed in the table below. backfilled as the flow returned to the base discharge.

100-year flood 500-year flood (Qioo) (Q500) Remarks

Discharge (fr/s) >,200 33,300 No road overflow.

River stage at bridge 18.72 20.34 - (ft above gage datum) Contraction-scour depth (ft) 5.4 7.4 Live-bed conditions.

Pier-scour depth (ft) 8.8 9.4 ~ Abutment-scour depth (ft) Left abutment 14.0 11.8 Qioodepth greater because of large difference in highway embankment length for Q500- Right abutment 15.8 20.3

26 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa O

10 I I I I 1 I i i i i i i i i i i i i i i i i i i i-j-Tri i i i i i i i i i i i i i i i i i i i i i r-rr- Average discharge = 267 cubic feet per second •••••♦.. 2ur§ 8 6 - A rr i<5 "•••••... «. oo< O «2 O 4 v*—...... ouj^ 2 » ♦ •'"*••••

^licog< • i i i i i " i ' ' i i i i i i i i i i i i i i i i i i i i i i i i i rr uj UJ> 1940 1950 1960 1970 1980 1990 1993 >< rr

—I—I—I—I—I—I—I—I—I—I—I—I—.—I—I—I—r 100-year flood discharge = 26,200 cubic feet per second 3 I- River stage = 18.72 feet < Q Right abutment UJ Road surface CD < O UJ > o CD <

UJ U.

5 UJ —I UJ

0 50 100 150 200 250 260 DISTANCE, IN FEET FROM LEFTABUTMENT (VERTICAL EXAGGERATION X 3)

EXPLANATION

Calculated contraction scour, 100-year flood Calculated pier scour, 100-year flood r\^^SN Calculated abutment scour, 100-year flood — Surveyed cross section, October 26,1993 Discharge measurement cross section, June 15,1991, discharge = 15,200 cubic feet per second, river stage = 14.42 feet

Figure 9. (A) Streambed aggradation/degradation trend line and (S)elevation view (looking downstream) ofchannel cross section showing calculated scourforthe 100-year flood at State Highway 175 bridge in Monona County, streamflow-gaging station Maple River at Mapleton (06607200).

Estimates of Maximum Scour 27 West Nishnabotna River at Randolph (06808500) overflow computations. The final calibration was accomplished by increasing the bridgedischarge This bridge is located on State Highway 184in coefficient to 0.98 (from the automatically calculated FremontCounty. The highwaycrossesthe rivervalley value of 0.85). This fact suggests that the contribution and river at nearly right angles. The floodplain is of flow to the bridge opening from the left flood plain about 3,500 ft wide at the bridge. Tree-covered levees is minimal, even though the flood plain is connected line the banks on both sides of the highway, and the hydraulically. The scour depths calculatedfor the left and right flood plainsare cultivated. The bridge is bridge at West NishnabotnaRiver at Randolph are a 384.5-ft by 32-ft, pretensioned, prestressed, summarized in the table below. concrete-beam structure supported by abutments and Figure 10A shows that the streambed at this site three concrete piers, which are supported by steel has been relatively stable for the period of record piling. The bridge was built in 1974 (Iowa Depart (1948-93). Figure 10i3 shows the cross section ment ofTransportation, 1973). surveyed at the downstream side of the bridge The water-surface profile calculations show that October 27, 1993, with the calculated pier-scour road overflow occurs on the left flood plain for the depthssuperimposed on the cross section. Also Qioo andQ5oo flood discharges. Water in the right shown in figure lOflis the cross section of a flood flood plain is ponded at flood stagesand therefore measured May 26, 1987. The peak discharge of the does not contribute to the conveyance of flow; the flood was 35,800 ft3/s, of which 3,100 ft3/s was road Qioo flood stage is about level with the top of the overflow on the left flood plain. The cross section levee,and the Q50o flood stage is less than 1ft higher. shows that the streambed near the downstream side of The levee on the left flood plain is about 2 ft lower the bridge scoured about 6 ft between the left and than the levee on the right flood plain. During the center piers and that it scoured about 5 ft between the calibration of the WSPRO model, it became apparent center and right piers. The scour is attributable to a that adjusting the channel roughness values alone general loweringof the streambedduringthe flood and would not be sufficient to accomplish the calibration, to possible debris caught on the piers. A debris pile nor would it be possible to set the end of the approach was noted on the center pier at the time of the cross section in the left flood plain at the top of the potential-scourassessment in March 1992(see table4, levee because that would have affected the road site 37).

100-year flood 500-year flood (Qioo) (Q500) Remarks

Discharge (ft/s) 49,500 59,100 Road overflow.

Discharge through bridge opening 32,600 37,400 -- (ft3/s)

River stage at bridge 24.97 25.72 — (ft above gage datum)

Contraction-scour depth (ft) - - Live-bed condi values computed. Pier-scour depth (ft) 8.5 8.4 Abutment-scour depth (ft) Not calculated because the levees affect the approach flow like guide banks.

28 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa O

O uj %" 10 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i Average discharge = 594 cubic feet per second

/• •••

7 r i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i rr uj uj> 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1993

LU > 25 o m < 20

UJ 15 LL

o 5 I UJ 0 -J UJ l__l I I •'•'••'•! • ••• I • • i • ; • -5 -20 0 50 100 150 200 250 300 350 400 DISTANCE, IN FEET FROM LEFT ABUTMENT (VERTICAL EXAGGERATION X 5)

EXPLANATION Calculated pier scour, 100-year flood —^— Surveyed cross section, October 27,1993 — _ Discharge measurement cross section, May 26,1987, discharge = 35,800 cubic feet per second, road overflow = 3,100 cubic feet per second, river stage = 24.00 feet

Figure 10. (A) Streambed aggradation/degradation trend line and(S) elevation view (looking downstream) ofchannel cross section showing calculated scour for the 100-year flood at theStateHighway 184 bridge in Fremont County, streamflow- gagingstationWest Nishnabotna River at Randolph (06808500).

Estimates of Maximum Scour 29 East Nishnabotna River near Atlantic (06809210) it was decided to compute scour using the discharge occurring at the point-of-road overflow, Qpro, which The bridge is on a paved county road in Cass was determined to be about 22,000 ft3/s. Thescour County. The highway is perpendicular to the axis of the river valley; it crosses the river at an angle of about depths calculated for the bridge over the East 12 degrees. The river is near the right edge of the Nishnabotna River near Atlantic are summarized in valley;during flooding, road overflow occurs in the the table below. left flood plain. The right flood plain is cultivated on Figure 1\A shows that the streambed at this site the upstreamside of the highway, and the left flood plain is a pasture. Theflood plain iscultivated on both has degraded about 1 ft between 1970 and 1989. The sides ofthe river downstream ofthe highway. Narrow rate of degradation for the period is 0.053 ft/yr (1 ft in bands of trees line the banks along the cultivated 19 years). The points in figure 11A are for the period portions of the flood plain. The bridge is a 240-ft by of record at the current site; before 1970, the gaging 20-ft, continuous I-beam structure supported by station was located 2.2 mi upstream. abutments and two concrete piers that are skewed 15degrees from perpendicular to the axis of the bridge Figure 1lfl shows the cross section surveyed at to be parallel to the flow. The abutments are supported the downstream side of the bridge May 24,1993, with by wood piling, and the piers are supported by spread the calculated scour depths for the Qj0o flood footings on shale and limestone. The bridge was built superimposed on the cross section. Also shown in in 1951 (Iowa Department of Transportation, 1950). figure 1IB is the cross section of a flood measured Considerable erosion of the bank has occurred June 14, 1991. The peak discharge of the flood was at the left abutment. During the potential-scour 21,000 ft3/s, and the river stage was 18.29 ft. There assessments, the site had mass wasting on the left was no road overflow. The discharge-measurement bank, which is caused by a river meander impact point cross section shows that the streambed was scoured at the bridge (table 4, site 16). Sheetpiling has been below the elevation of the base of the footing of the left pier. The measured scour was about 9 ft, which is driven into the channel at the base of the abutment, more than twice the calculated Qioo contraction scour and riprap has been installed on the embankment. depth (3.3 ft). A possible cause for the scour in The water-surface profile analyses indicated that addition to the contraction caused by the highway the bridge section is not a contracted opening at the embankment is debris on the piers. A debris pile was Qioo and Q50O flood discharges. It was necessary to noted on the left pier at the time of the potential-scour composite the bridge and road sections to create a assessment in March 1992 (table 4, site 16). Given the regular (non-bridge) channel cross section to compute facts that the measured scour depth is below the the water-surface profiles (Shearman, 1990, p. 90-91; elevation of the base of one pier, that the pier is not Shearman and others, 1986, p. 40). The analyses also supported by piling, and that debris piles can cause indicated that the conveyance-tube flow velocities at deeper scour holes (see, for example, Laursen and the bridge were less for the Q5qo flood than for the Toch, 1956, p. 30), additional investigation of local Qioo flood. Therefore, rather than use the Q50o flood, scour at the pier may be warranted.

Discharge, point- of-road overflow 100-year flood (Qpro) (Q100) Remarks

Discharge (ft3/s) 22,000 35,600 Road overflow for Qioo-

Discharge through bridge opening 22,000 31,600 ~ (ft3/s)

Stream stage at bridge 18.94 22.64 ~ (ft above gage datum) Contraction-scour depth (ft) 2.4 3.3 Live-bed conditions.

Pier-scour depth (ft) 11.9 12.8 « Abutment-scour depth (ft) Left abutment 17.4 21.7

Right abutment 13.0 16.2 ~

30 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa CD uj t 1 1 1 1 1 1 1 1 1 1 1 r——r 1 1 1—-i 1 i i i r a Average discharge =226 cubic feet per second

gujg

co2< ii i i i i J I I I I L rr uj UJ> 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1993 >< rr

35 -" r -> r 2 100-yearflood discharge = 35,600cubicfeet per second n B 30 h river stage = 22.64 feet Water surface * Right < Q Road surface abutment UJ 25 CD < 20 CD UJ > 15 O CO < 10 I- LU UJ 5

0 z o -5 I-

UJ -10 -J UJ -15 -20 0 50 100 150 200 250 DISTANCE, IN FEET FROM LEFT ABUTMENT (VERTICAL EXAGGERATION X 3)

EXPLANATION

Calculated contraction scour, 100-year flood Calculated pier scour, 100-yearflood t^^l Calculated abutment scour, 100-year flood — Surveyed cross section, May 24,1993 — — Discharge measurement cross section, June 14,1991, discharge = 21,000 cubic feet per second, river stage = 18.29 feet

Figure 11. {A) Streambed aggradation/degradation trend line and (8) elevation view (looking downstream) of channel cross section showing calculated scour depths for the point-of-road-overflow flood at county road bridge in Cass County, streamflow-gaging stationEast Nishnabotna River at Atlantic (06809210).

Estimates of Maximum Scour 31 Nodaway River at Clarinda (08817000) 1.2 ft between 1918 and 1953, an increase of about 1.5 ft between 1953 and 1960, and a decrease of about Thebridge at this site is located on State High 2.5 ft between 1960 and 1987. (The gaging station way 2 (business route) in Page County. The highway was discontinued from 1925 to 1936; hence, there are crosses the river at an angle of about 17degrees near nodataduring that interval.) The rateof degradation the center of the river valley. The flood plain is culti between I960 and 1987 is 0.093 ft/yr (2.5 ft in vated on both sides of the river. Very few trees are 27years). The rating curve has notbeen changed standing in thevicinity of thebridge. The bridge isa since 1987, which suggeststhat the rate of degradation 314-ft by26-ft, continuous I-beam structure supported has decreased. by concrete abutments and three concrete piers, which Figure IIB showsthe cross section surveyed at are perpendicular to the axis of the bridge. According the downstream side of the bridge June 22, 1993, with to thebridge plans, the piers and rightabutment the calculatedscour depths for the Q100 flood previously supported the old bridge, which was built superimposed on the cross section. Also shown in in 1917. The current left pier was formerly the left figure MB is the crosssection of a discharge abutment. The presentleft abutment is supported by measurement made September 15, 1992. The wood pilings, and the right abutment and three piers measured discharge was 24,500 ft3/s, and the river are apparently spread footings on "hardpan." The stage was 18.43 ft. The cross section shows that the right abutment and the piers are sharp-nosed and angle streambed was scoured below the base of the footings outward on the upstream side of the bridge. The between the center and right piers. Because the present bridge was built in 1949 after one of the spans maximum stage was only 1 to 2 ft above bankfull of the previous structure collapsed because of an stage, thescoured streambed is likely the result of overloadedtruck (Iowa Department of Transportation, general entrainment of bed material caused by the 1949). rapidly flowing water andcontraction of flow area The water-surface profile analyses indicate that causedby debris on the bridge piers. Debris was road overflow will not occur for the Qjqo and Q500 noted at this site at the time of the potential-scour flood discharges. The scour depthscalculated for the assessment (table 4, site 88). Given the facts that the bridge over the Nodaway Riverat Clarinda are measured scour depth is below the elevation of the summarized in the table below. baseof the piers, that the piers are not supportedby Figure 12A showsthat the streambed at this site pilings, and that debris pilescan causedeeperscour hasdegraded, aggraded, and againdegraded during holes (Laursen and Toch, 1956, p. 30), additional theperiod of record (1918-93). The data points show investigation of local scour at the piers may be a decrease in the elevation of the streambed of about warranted.

100-year flood 500-year flood (Q100) (Q500) Remarks

Discharge (ft3/s) 42.700 51,800 No road overflow. River stage at bridge 26.22 27.58 (ft above gage datum) Contraction-scour depth (ft) 4.1 6.7 Live-bed conditions. Pier-scour depth (ft) 22.6 23.8 Abutment-scour depth (ft) Left abutment 12.5 17.6 Right abutment 15.5 19.7

32 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa o

0UJ

10 •'' I I II I I I I I I I I I I I I I I I I I I I I I I I l I I I I I I l I I I I I I I | | | | [ | | | M | I I I I I I I I I I II I I I I I I I I I I IJ Average discharge =357 cubic feet per second 9

8 *. ..••• oo< OCOCD 7 LUQUJ ••• gujg 6

cog< ' 11111' 1111111 11111111 I I I Ill 1111- rr uj ui> 1915 1925 1935 T945 1955 1965 1975 1985 1993 >< rr

1 1 1 1 1 1 1 1 1 1 1 r- 100-year flood discharge =42,700 cubic feet per second river stage = 26.22 feet Right Road surface abutment ~

0 50 100 150 200 250 300 330 DISTANCE, IN FEET FROM LEFT ABUTMENT (VERTICAL EXAGGERATION X 2)

EXPLANATION Calculated contraction scour, 100-year flood Calculated pier scour, 100-year flood r^sSNSNl Calculated abutment scour, 100-year flood Surveyed cross section, June 22,1993 — — Dischargemeasurement cross section, September 15,1992, discharge = 24,500 cubic feet per second, river stage = 18.43 feet

Figure 12. (A) Streambed aggradation/degradation trend line and (6) elevation view (looking downstream) of channel cross section showing calculated scour for the 100-year flood at the State Highway 2 (business route) bridge in Page County, streamflow-gaging station Nodaway River at Clarinda (06817000).

Estimates of Maximum Scour 33 Platte River near Diagonal (06818750) scour depths calculated for the bridge over the Platte River near Diagonal are summarized in the table The bridge is located on a gravel county roadin below. Ringgold County. The river is a straightchannel upstreamanddownstreamofthe bridge,andthe banks Figure 13A shows that the streambed at this site are lined with narrow bands oftrees. The flood plain has been relatively stable. The data points show is flat, about 3,500 ft wide, and cultivated on both degradation ofabout 0.7 ft between 1968 and 1980 sides ofthe river upstream and downstream ofthe and aggradationof about 0.5 ft between 1980 and road. The road crosses the river valley and river at a 1987. nearly 90-degree angle. The road embankment is Figure 13B shows the cross section surveyed at raised about 4 ft above the surrounding fields except at the bridge where it is about 7 ft higher. Two culverts the downstream side of the bridge May 25,1993, with cross under the road on the right flood plain, but they the calculated scour depths for the Qioo flood were not considered in the hydraulic analysis. The superimposed on the cross section. Also shown in bridge is a 180-ft by 20-ft, prestressed concrete-beam figure \3B is the cross section ofa discharge structure supported on concrete abutments and two measurement made July 5, 1993. The measured pile bents. Low concrete on the bridge is approxi discharge was 9,650 ft3/s, which iswithin 3.5 percent mately 3 ft above the lowest crown elevation surveyed of the Qioodischarge. The river stage was 23.60 ft. on the road. The bridge was built in 1962 (Iowa Because the gaging station at this site was not active in Department ofTransportation, 1961). 1993, it is not known when the peak occurred. The This site was chosen for analysis because there measurement notes indicate that there was no road were no apparent factors to affect application ofthe overflow at the time. The cross section shows that part scour equations. It is also the only bridge with pile ofthe embankment near the left abutment eroded and bents that was analyzed for scour in this study. The that the erosion is lateral into the embankment.

100-year flood 500-year flood (Qioo) (Qsoo) Remarks

Discharge (ft3/s) 10,000 11,200 Road overflow.

Discharge through bridge opening 9,800 10,400 ~ (frVs)

River stage at bridge 24.20 24.50 — (ft above gage datum) Contraction-scour depth (ft) 6.5 7.8 Live-bed conditions.

Pier-scour depth (ft) 4.2 4.3 ~ Abutment-scour depth (ft)

Left abutment 8.6 8.8 —

Right abutment 15.0 16.4 --

34 Potential-Scour AMessmente and Estimate* of Maximum Scour at Selected Bridges in Iowa o

O UJ

% u- 7 t—i—i—i—r t—i—i—i—i—i—i—i—i—]T—t—i—i—i—r—t—r t—r Average discharge = 129 cubic feet per second g|£6 CCrr ^xOm

ujQ^ 5

4 r i i i i i ) i i i i—i—i—i—i—i—i—i—i—i—i—i—l rr uj 1965 1970 1975 1980 1985 1990 1993 m^ >< rr

0 20 40 60 80 100 120 140 160 180 190 DISTANCE, IN FEET FROM LEFTABUTMENT (VERTICAL EXAGGERATION X 2)

EXPLANATION Calculated contraction scour, 100-year flood Calculated pier scour, 100-year flood ISS^vSl Calculated abutment scour, 100-year flood —— Surveyed cross section, May 25,1993 — — Discharge measurement cross section, July 5,1993, discharge = 9,650 cubic feet per second, river stage = 23.60 feet

Figure 13. (A) Streambed aggradation/degradation trend line and (6)elevation view (looking downstream) ofchannel cross section showing calculated scourfor the 100-year flood at county road bridge in Ringgold County, streamflow-gaging station Platte River near Diagonal (06818750).

Estimates of Maximum Scour 35 Iowa River at Wapello (05465500) exposed below the second pier from the right bank. Scour depths were computed using the maximum This bridge is locatedon State Highway99 at discharge measured at the site during the flooding. the easternedge of the City of Wapello in Louisa Because the hydraulic contraction is upstream of the County. The bridge crossesthe river at an angle of about5 degrees. The river valleyat the bridge is about bridge,the channelsectionat the bridge was notcoded 1.3 mi wide with the main channel at the right edge of as a bridge section for the purposes of computing the the valley. The effective left edge of the flood plain, water-surface profile. The values for the upstream however, is defined by a levee at the end of the bridge. variables used in the contraction scour equations were The levee was built parallel to the river at the bridge, derived from the channel section upstream of the but about 600 ft upstream it was built to the left edge levee. Abutment scour was not calculated because the of the valley at a nearly 90-degree angle to the axis of abutments do not extend significantly into the flow the valley. The effectof thisconfiguration of the levee path and because the right abutment is protected with is a largehydraulic contraction of floodflows. Up riprap. The scourdepthscalculated for the bridgeover stream of the bridge, the right bank is protected by the Iowa River at Wapello are summarized in the table concrete-filled fabric erosion-protection mats. Down below. stream the right bank is generally unprotected, although some areas are protected by broken concrete Figure 14A shows that the streambed at this site pieces. The left flood plain between the main channel has been relatively stable for the period of record and levee on both sides of the bridge is covered with (1914-93). Figure 14flshows the cross section trees. The bridge is a 1,217-ftby 30-ft, multiple-span surveyed at the downstream side of the bridge structure consisting of a 639-ft five-span, continuous- November 15, 1993, with the calculated scour depths deck girder sectionover the main channel and a 576-ft for the maximum measured discharge superimposed eleven-span, continuous I-beamsectionover the flood on the cross section. Also shown in figure \4B is the plain. It is supported by concrete abutments and cross section ofthe maximum discharge measurement, 15concrete piers. The abutments and piers are which was made July 8, 1993. The measured dis supported by wood piling. The right abutment and charge was 106,500 ft3/s, which is greater than the rightmost pier were protectedwith riprap in 1988 theoretical Qioo discharge. The river stage was (Brad Barrett, IDOT, oral commun., February 1994). The bridge was built in 1946(Iowa Departmentof 28.07 ft. At the time of the measurement, the Transportation, 1945). Planimetric views of the river, streambed between the second and third piers from the flood plain, and bridgeare given in Fischer (1994). right abutment was higher than when the cross section Computed scour depths were compared to scour was surveyed in November 1993. depths measuredduring the flood of 1993. There was The discharge measurement cross section, evidence of extensive contraction scour in the main which was made at the downstream side of the bridge, channel at the bridge, and about 10 ft of piling were shows that the streambed was scoured to the base of

Discharge measured July 8,1993 Remarks

Discharge (ft3/s) 106,500 Maximum measured discharge; discharge greater than Qioo- River stage at bridge 28.07 (ft above gage datum) Contraction-scour depth (ft) Main channel: 19.2 Live-bed conditions. Left overbank: Clear-waterconditions; negative value computed. Pier-scour depth (ft) 23.3 Calculated for piers in main channel only. Abutment-scour depth (ft) Not calculated.

36 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa O

o m 16 i i 11 i 11 11 i 11 i i i i 11 111 11 i i i 11 11 i 11 11 i i i 11 i i i i 11 i i m i a Averagedischarge = 8,637cubicfeet persecond

ujQi^ 14 CDUJfi

I IIIIIIIII III !, ||,||!! IIIIIIIIIIIIIIII1IIIIIIIIIIIIIIIIIII IIIII IIIIM IIIII coJf < 13 o o O CO rr ui o O o o o o m 00 o> o> CM CO o> o> 5> o> o> o> 8 O) o> rr

60 r 1 1 • 1 • i • i 5 Maximum measured discharge = 106,500 cubic feet per second B I- 50 E- riverstage = 28.07 feet Right abutment < Road surface Q 40 UJ O < 30 CD UJ 20 § CQ 10 I 5 < ! I 0 UJ U- -10

-20

5 -30 > UJ -40 Ui -50

DISTANCE, IN FEET FROM LEFT ABUTMENT (VERTICAL EXAGGERAl ION X6)

EXPLANATION I | Calculated contraction scour, maximum measured discharge IH11111| Calculated pier scour, maximum measured discharge Surveyed cross section, November 15,1993 __ Discharge measurementcross section, July8,1993, maximum measured discharge (cross section is at downstream edge of bridge)

Figure 14. {A) Streambed aggradation/degradation trend line and (8) elevation view (looking downstream) of channel cross section showing calculated scour for the maximum measured discharge atState Highway 99 bridge in Louisa County, streamflow-gaging station Iowa River at Wapello (05465500).

Estimates of Maximum Scour 37 the second pier. Depth measurements made along the at a bridge. The maximum value of the potential- upstream side of the bridgein the main channel, scour index that was assigned during the Iowa assess however, show that streambed was scoured below the ments was 24.5, and the minimum value was 3. The baseof the pier (fig. 15). The cross sections shown in median of the indices was 11.5; the interval estimate figure 15 were measured betweenJuly 9 and of the median index for all bridges in Iowa at the November 17, 1993. The soil layers shown in the 95-percent confidence level was 10.5 to 12.5. Most of figure are from soil-boring information shown on the the bridges assigned an index value of 15or more are bridge plans (Iowa Department of Transportation, in the western part of the State where loess soil 1945). An unusual characteristic of the flood of 1993 deposits generally are thicker. was the long duration of high water. The river was The component of the potential-scour index that above flood stage from June 8 to September 22, 1993, contributed most to the overall total of the 130 indices a periodof 106days (R.E.Southard, U.S. Geological was bed material, which accounted for 27.1 percent of Survey, written commun., February 1994),and the the overall total. This component was identified as cross sections show a steady decrease in the elevation sand, silt/clay, or alluvium at 123 sites. The cohesive of the streambed in the main channel. Figure 15/1 properties of the bed material were not considered in shows that the bed was already scoured below the base the assessment of this component. The component of the second pier. Figure 15A also shows about4 ft of with the second greatest contribution to the overall local pier scour using the ambient bed as a reference; total of the indices was bank erosion at the bridge, the total scour measured below the base on July 9 was which accounted for 18.3 percent of the overall total. 8 ft. Figures \5B and 15Cshow additional scour of Most of the sites that have mass wasting at one or both the streambed; however, local pier scour at the second banks are located in the parts of the State where the pier is no longer apparent. The maximum measured loess deposits are thicker. Listed in order of scour below the base was 11 ft on August 18, 1993 decreasing contribution to the overall total of the (fig. 15C). No scour was observed at the rightmost potential-scour indices, the remaining components are pier because it is protected with riprap. stage of channel evolution (17.9 percent), bed protec Figure 15D shows the elevation of the bed at the tion (12.0 percent), proximity of river meander impact upstream edge of the bridge on November 17, 1993, point (7.1 percent), number of piers in channel 2 months after the river receded below flood stage. (5.2 percent), mass wasting at piers (3.8 percent), Because the channel did not appear to be backfilling, angle of approach of high flows (3.5 percent), the channel was sounded upstream and downstream to percentage of channel constriction (2.3 percent), pier determine the extent of the scoured bed. Soundings skew (2.1 percent), and percentage of blockage by made November 17 showed that the streambed had debris (0.6 percent). scoured about 1,600 ft upstream of the bridge The potential-scour index represents conditions (fig. 16A). Soundings made July 15, 1994, show that at a bridge at a single moment in time. A single the streambed is filling again (fig. 16/5). The lines of potential-scour assessment may help identify equal streambed elevation in figure 16/5 show a conditions that suggest the need for additional depression in the streambed downstream of the bridge investigation at a site. The usefulness of potential- that is not present in figure 16i4. The depression may scour assessments is dependent upon regular assess have been present in November 1993 but was not ments if the index is used to monitor potential-scour detected because the cross-section spacing was farther susceptibility, although few of the components of the apart in 1993 than in 1994. index considered in this study are likely to change between assessments. Because bridges already are inspected at regular intervals by IDOT, it would be SUMMARY AND CONCLUSIONS possible to include a potential-scour assessment for

Potential-scour assessments were made at one or more of the components described in this study 130 bridges in Iowa. A potential-scour value was in the bridge-inspection report. assigned to each bridge using an index developed for a Maximum scour was estimated at 10 bridges. potential-scour assessment study in western The aggradation or degradation of the streambed that Tennessee. Higher values of the index suggest a has occurred during the period of streamflow data greater likelihood ofscour-related problems occurring collection at each site was determined using a method

38 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa - .. I i ... 1 . . ,, 1 ,,,, , , ,, , , r- Right /\ July9, 1993,discharge= 100,000 cubic feet per second, river stage = 26.95 feet abutment

Water surface 8 (eot total scour, 4 feet local pier scour

1 iL_i L_Lui

50 40 Q August 2, 1993,discharge= 61,800cubic feet per second, river stage = 23.60 feet 30 20 3 10 H 9.5 feet total < Q 0 scour LU -10 O < -20 CD 111 -30 > -40 O CD <

UJ 50 40 Q August 18, 1993, discharge = 78,900 cubic feet persecond, river stage = 25.21 feet 30 20 10 0 -10 •20 -30 -40

50 40 f" D November 17, 1993, discharge = 10,200 cubic feet per second, river stage = 13.81 feet 30 20 10

0 ci -10 -20 -30 -40 600 700 800 900 1,000 1,100 1,200 1,250 DISTANCE, IN FEET FROM LEFT ABUTMENT (VERTICAL EXAGGERATION X 1.8)

Figure 15. Streambed elevations in main channel at upstream edge of the State Highway 99 bridge over Iowa River at Wapello (streamflow-gaging station 05465500), July-November 1993 (modified from Fischer, 1994).

Summary and Conclusions 3S A. November 17, 1993 9r.1v Iowa River Cedar River

City of Wapello

Bridge & piers

41°11'-

EXPLANATION

Main channel at time of survey

. Streambed contour—Shows elevation of streambed in relation to gage datum. Contour B. July 15, 1994 interval 2 meters (6.6 feet) 9i°ir

Edge of water November 17, 1993

500 1,000 1.500FEET 4rir— J , 1 , I 500 1.000 METERS

Figure 16. Streambed elevations inthe Iowa River at State Highway 99 bridge, Wapello, Iowa, (A) November 17,1993 (from Fischer, 1994), and (8) July 15,1994.

40 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa that considers changes in the river stage corresponding abutment scour that was measured was erosion of the to an index discharge. The streambed appears to be embankment at the left abutment at the bridge over the stable at six sites; has degraded at three sites, and has Platte River near Diagonal. Erosion at the right aggraded at one site. The greatest degradation abutment at White Breast Creek near Dallas is the observed in this study was 6.7 ft at the bridge over the result of a river meander impact point occurring at the Maple River at Mapleton. The rate of degradation was bridge during normal flows that has undermined the 0.146 ft/yr for the period 1941-87, although the rate of embankment. degradation since 1971 was 0.031 ft/yr. Maximum scour was estimated using Federal REFERENCES Highway Administration scour equations. The principle discharges used to estimate scour were the Eash, D.A., 1993, Estimating design-flood discharges for 100-year(Qioo)and 500-year (Q50o) floods. Other streams in Iowa using drainage-basin and channel- discharges also were used at four bridges, generally geometry characteristics: U.S. Geological Survey because it was determined that the Qiqo and (or) Water-Resources Investigations Report 93-4062, 96 p. Q50o floods did not represent the conditions that Fischer, E.E., 1993, Scour at a bridge over the Weldon would cause maximum scour. River, Iowa, in Hydraulic Engineering—Proceedings of the 1993 National Conference on Hydraulic Channel cross sections obtained from discharge Engineering: New York, American Society of Civil measurements at four of the study bridges show Engineers, p. 1854-1859. greater scour than the contraction scour predicted 1994, Contraction scour at a bridge over the Iowa using the scour equations. In three of the cases, the River, in Hydraulic Engineering—Proceedings of the measured discharge was less than the respective 1994 National Conference on Hydraulic Engineering: Q100 flood used to estimate maximumscour (West New York, American Society ofCivil Engineers, Nishnabotna River at Randolph, East Nishnabotna p. 31-35. River near Atlantic, and Nodaway River at Clarinda). Heinitz, A.J., 1986, Floods in south-central Iowa: U.S. In the fourth case, the measured discharge was greater Geological Survey Open-File Report 85-100,20 p. than the Q|00 flood, but a negative value was com Iman, R.L., and Conover, W.J., 1983, A modem approach to putedfor contraction scour (Middle Raccoon River statistics: New York, John Wiley and Sons, 497 p. near Bayard). The measured scour at two of the sites Inglis, C.C., 1949,The behaviour and control of rivers and was at or below the base of the piers, although not in canals (with the aid of models): Poona, India, Central the vicinity of the piers (East Nishnabotna River near Waterpower Irrigation & Navigation Research Station, Atlantic and Nodaway River at Clarinda). Research Publication No. 13, Part I, 279 p. Iowa Department of Transportation, Highway Division, No pier-scour measurements were obtained in 1945, Design for 1217' multiple span bridge: Ames, this study except at the bridge over the Iowa River at Iowa, File No. 11861, Design No. 145, 15 sheets. Wapello. The total scour measuredbelow the base of 1949, Design for 314' x 26' continuous I-beam the second pier at this bridge during the floodof 1993 bridge: Clarinda, Iowa, Page County Project No. was 11 ft. Most of the scour at this pier was caused by F 433, File No. 14153, Design No. 149, 6 sheets. contraction scour. About 4 ft of local pier scour was 1950,Design for 15-degree skew 240' x 20' continu measuredduring the early part of the flood, although ous I-beam bridge: Atlantic, Iowa, Cass County the ambient (reference) bed was already below the Project No. SN 1853, File No. 15052, Design 850, base of the pier. Because discharge-measurement 3 sheets. cross sections at two other sites (East Nishnabotna _ 1954a, Design for 250' x 20' continuous I-beam River near Atlantic and Nodaway River at Clarinda) bridge: Knoxville, Iowa, MarionCounty Project show the streambeds to be at or below the elevation of No. SN 1877, File No. 16902, Design No. 2554, the base of the piers, additional investigation may be 5 sheets. _1954b, Design for 30-degree skew 240' x 26' warranted at these sites to determine whether the continuous I-beam bridge: Ames, Iowa, File streambed has been scoured below the upstream edge No. 15846, Design No. 1654A, 5 sheets. of the bases of the piers. 1956, Design for 425' x 24' continuous I-beam The abutments of the 10 bridges analyzed in this bridge: Adel, Iowa, Dallas County Project study were designed as spill-through abutments with No. S 2690-1, File No. 18403, Design No. 256, sloped-earth embankments. The only significant 4 sheets.

References 41 _1961, Design for 180' x 20' prestressed concrete computations: U.S. Department of Transportation, beam bridge: Mount Ayr, Iowa, Ringgold County Federal Highway Administration Report ProjectNo. SN-1990, File No. 20132, DesignNo. 162, No. FHWA-IP-89-027, 177 p. 3 sheets. Shearman, J.O., Kirby, W.H., Schneider, V.R., and Flippo, 1973, Design for 384'-6" x 32' pretensioned pre H.N., 1986, Bridge waterways analysis model— stressed concrete beam bridge: Ames, Iowa, File Research report: U.S. Department of Transportation, No. 24575, Design No. 173, 13 sheets. Federal Highway Administration Report _1979, Design for 15-degree skew 243'-0 x 36' No. FHWA/RD-86/108, 112 p. pretensioned prestressed concrete beam bridge: Ames, Simon, Andrew, 1989, A model of channel response in Iowa, File No. 26155, Design No. 278. 12 sheets. disturbed alluvial channels: Earth Surface Processes _1989, Design for 15-degree skew 40T-0 x 36' and Landforms, v. 14, p. 11-26. pretensioned prestressed concrete beam bridge: Ames, Simon, Andrew, and Outlaw, G.S., 1989, Evaluation, Iowa, File No. 27395, Design No. 286, 19 sheets. modeling, and mapping of potential bridge-scour, west Laursen, E.M., 1952, Observations on the nature ofscour, in Tennessee, in Proceedings of the bridge scour Bulletin 34, Proceedings of Fifth Hydraulic Confer symposium: U.S. Department of Transportation, ence. June 9-11, 1952: Iowa City. University of Iowa, Federal Highway Administration Report p. 179-197. No. FHWA-RD-90-035. p. 112-129. 1958, Scour at bridge crossings: Iowa Highway Southard, R.E., Sneck-Fahrer, Debra, Anderson, C.J., Research Board Bulletin No. 8, 53 p. Goodrich, R.D., and Gorman, J.G. 1994, Water Laursen, E.M., and Toch, Arthur. 1956, Scour around bridge resources data Iowa, water year 1993: U.S. Geological piers and abutments: Iowa Highway Research Board Survey Water-Data Report IA-93-1, 388 p. Bulletin No. 4, 60 p. U.S. Department of Transportation, 1988, Scour at bridges: Oschwald, W.R., Riecken, F.F., Dideriksen, R.I., Scholtes, Washington, D.C., Federal Highway Administration W.H., and Schaller, F.W., 1965, Principal soils of Iowa: Technical Advisory 5140.20, 2 p. Ames, Iowa State University Department of U.S. Interagency Advisory Committee on Water Data Agronomy Special Report No. 42, 76 p. (IACWD), 1982, Guidelines for determining flood Parrett, Charles, Melcher, N.B., and James. R.W., 1993, flow frequency: Hydrology Subcommittee Bulletin Flood discharges in the upper Mississippi River Basin: 17B, 28 p. U.S. Geological Survey Circular 1120-A, 14 p. Vanoni, V.A., ed., 1975, Sedimentation engineering: New Prior, J.C., 1991, Landforms of Iowa: Iowa City, University York, American Society of Civil Engineers, 745 p. of Iowa Press, 153 p. Wahl. K.L., Vining, K.C., and Wiche, G.J., 1993, Precipita Richardson, E.V., Harrison, L.J., Richardson, J.R., and tion in the upper Mississippi River Basin, January 1 Davis, S.R., 1993, Evaluating scour at bridges: through July 31, 1993: U.S. Geological Survey Washington, D.C., Federal Highway Administration Circular 1120-B, 10 p. Hydraulic Engineering Circular No. 18, 132 p. Williams, G.P., and Wolman, M.G., 1984, Downstream Shearman, J.O., 1990, User's manual for WSPRO— effects of dams on alluvial rivers: U.S. Geological A computer model for water surface profile Survey Professional Paper 1286, 83 p.

42 Potential-Scour Assessments and Estimates of Maximum Scour at Selected Bridges in Iowa Table 4. Potential-scour index at selected highway bridges in Iowa [The index is the sum ofthe component values and isapplicable at the time each site was evaluated. Listed below each component value is the assessment description (in parentheses) made during the onsite visit (see pages 3-7 in this report). Sites are listed by county. USGS, U.S. Geological Survey; mi2, square miles; ft, feet; >, greater than; est., estimated]

Index components

Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of Site vicinity (USGS station Percent • river Pier Mass Angle of identifi* number), age of meander skew wasting approach Poten cation drainage area, Bed Bed Stage of channel Number of Hori- impact (num at pier of high tial numbei highway, mate pro channel constric piers in zon- Verti Left Right point ber of (number flows scour (fig-1) date visited rial tection evolution tion channel tal cal Total bank bank (ft) piers) of piers) (degrees) index

Adams County

1 Platte River near 4 1 3 0 1 0 0 0 1 1 1 1 0 0 13.0 Stringtown (06818598), (silt/ (bed not (V (0) (1) (0) (0) (0) (flu (flu (85) (1) (0) (0) 51.7mi2, clay) protected) Aggra vial) vial) U.S. Highway 34, dation) March 23, 1992

Appanoose County

-a o 2 Chariton River near 4 1 4 0 0 0 0 0 1 1 0 0 0 0 11.0 5f Rathbun (06903900) (silt/ (bed not (IV (0) (0) (0) (0) (0) (flu (flu (MOO) (0) (0) (0) (site from 1960 to 1969, clay) protected) Thresh vial) vial) which is downstream old) en o o from current site), c 549 mi2, County road, a. CD Februarys 1992 X CO 3 Cooper Creek at 3.5 1 3 0 0 0 0 0 1 1 0 0 0 0 9.5 CO Centerville (06903990) (allu (bed not (V (0) (0) (0) (0) (0) (flu (flu (>100) (0) (0) (0) © o 47.8mi2, vium) protected) Aggra vial) vial) c? dation) a State Highway 5, z Februarys 1992 IQ 3* 4 Chariton River near 3.5 1 4 2 0 0 0 0 1 1 0 0 0 12.5 i 0 CO (bed not (38) (flu (flu •< Centerville (06904000), (allu (IV (0) (0) (0) (0) (>100) (0) (0) (0) 00 708mi2, vium) protected) Thresh vial) vial)

a State Highway 2, old) Februarys 1992 CD 4k Table 4 . Potential-scour index at selected highway bridges in Iowa—Continued

TJ Index components a o 3 Percentage of Bank erosion Proxim m Stream name and blockage by debris (type) ity of CO river Pier Mass Angle of o Site vicinity (USGS station Percent o meander skew wasting approach Poten c identifi number), age of channel Number of Hori- impact (num at pier of high tial > cation drainage area, Bed Bed Stage of CO Left Right point ber of (number flows scour CO numbei highway, mate pro channel constric piers in zon- Verti © of piers) (degrees) index CO date visited rial tection evolution tion channel tal cal Total bank bank (ft) piers) to

to 3 Audubon County 5) a 0 19.5 3 5 Davids Creek near 3.5 1 4 1 0 0 0 0 2 2 0 0 6 a (mass, (mass, (>100) (0) (2) (0) m Hamlin (06809000), (allu (bed not (IV (8) (0) (0) (0) (0) CO 26.0mi2. vium) protected) Thresh wast wast 3 State Highway 44, old) ing) ing) 5 April 6, 1992 CO o Benton County 3

6 Prairie Creek at 3.5 2 3 0 1 0 0 0 2 2 0 0 3 0 16.5 i e Blairstown (05464560), (allu (bed not (V (0) (1) (0) (0) (0) (mass, (mass, (>100) (0) (1) (0) i 87.0 mi2, vium) protected, Aggra wast wast dation) ing) ing) o State Highway 82, right bank c January 14, 1992 protected)

Black Hawk County

CO o 7 West Fork Cedar River at 3.5 2 3 1 2 0 0 0 1 1 2 0 0 0 15.5 i Finchford (05458900), (allu (bed not (V (9) (3) (0) (0) (0) (flu (flu (50) (0) (0) (0) 09 "1 846 mi2 vium) protected, Aggra vial) vial) a

9 Cedar River at Waterloo 3.5 3 0 0 2 0 0 0 0 0 0 0 0 0 8.5 (05464000), (allu (bed not (1 (0) (6) (5) (5) (0) (none) (none) (>100) (0) (0) (0) 5,146 mi2, vium) protected, Premodi- 6th Street, both banks fied) April 13, 1992 protected) Table 4. Potential-scour index at selected highway bridges in Iowa—Continued

Index components

Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of Site vicinity (USGS station Percent river Pier Mass Angle of identifi number), age of meander skew wasting approach Poten cation drainage area, Bed Bed Stage of channel Number of Hori- impact (num at pier of high tial number highway, mate pro channel constric piers in zon- Verti Left Right point ber of (number flows scour (fig-1) date visited rial tection evolution tion channel tal cal Total bank bank (ft) piers) of piers) (degrees) index

Buchanan County

10 Wapsipinicon River at 3.5 2 3 0 2 0 0 0 1 1 1 0 0 1 14.5 Independence (allu (bed not (V (0) (3) (0) (0) (0) (flu (flu (0*) (0) (0) (20) (05421000), vium) protected, Aggra vial) vial) 1,048 mi2, right bank dation) Buchanan County, protected) State Highway 150, March 20, 1992

11 Pine Creek near 3 1 3 0 1 0 0 0 1 1 0 1 0 1 12.0 Winthrop (05421200), (sand) (bed not (V (0) (1) (0) (0) (0) (flu (flu (MOO) (1) (0) (15) 28.3 mi2, protected) Aggra vial) vial) •v State Highway 939, dation) o CD March 20, 1992 3 £0 Buena Vista County CO o o Little Sioux River at 3.5 1 0 0 1 0 0 0 1 1 0 0 0 0 7.5 c 12 Linn Grove (06605850), (allu (bed not (VI (0) (2) (0) (0) (0) (flu (flu (>100) (0) (0) (0) 3 a 1,548 mi2. vium) protected) Restabili- vial) vial) o X State Highway 264, zation) April 28, 1992

Butler County o & Shell Rock River at 3.5 0 0 0 1 0 0 0 0 0 0 0 0 0 4.5 X 13 CQ Shell Rock (05462000), (allu (protected (VI (3) (2) (0) (0) (0) (none) (none) (>100) (0) (0) (0) 1,746 mi2, vium) bed) Restabili- to «< County Road C45, zation) CD 3. April 14, 1992 a. ca 3.5 1 0 0 2 0 0 0 1 1 0 0 0 0 8.5 o 14 Beaver Creek at New CO Hartford (05463000), (allu (bed not (I (0) (3) (0) (0) (0) (flu (flu (>100) (0) (0) (0) 5" 347mi2, vium) protected) Premodi- vial) vial) o County Road T55, fied) i April 13, 1992

en Table 4 . Potential-scour index al: selecteij highway b•ridges in Iowa—Continued

•o Index components o

3 Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of CO Percent river Pier Mass Angle of O Site vicinity (USGS station o meander skew wasting approach Poten c identifi number), age of cation drainage area, Bed Bed Stage of channel Number of Hori- impact (num at pier of high tial > (0 mate constric piers in zon- Verti Left Right point ber of (number flows scour CO number highway, pro channel CD cal Total bank bank piers) of piers) (degrees) index CO («g. D date visited rial tection evolution tion channel tal (ft) CO 3 CD 3 Calhoun County S" O 0 0 10.0 3 15 Hardin Creek near 4 3 0 0 0 0 0 0 1 1 1 0 a Farnhamville (silt/ (bed not (VI (0) (0) (0) (0) (0) (flu (flu (75) (0) (0) (0) m CO (05482600), clay) protected, Restabili- vial) vial) 3 43.7mi2, both banks zation) to ? State Highway 175, protected) CO April 9, 1992 o 3 to Cass County X 3 c 16 East Nishnabotna River 4 2 4 0 1 0.33 0.67 0 2 1 3 0 0 0 18.0 3 near Atlantic (silt/ (bed not (IV (0) (1) (10) (45) (5) (mass (flu (0) (0) (0) (0) CO o (06809210), clay) protected, Thresh wast vial) o c 436 mi2, left bank old) ing) to County road, protected) CO March 25, 1992 CD

o Cerro Gordo County

CD 17 Winnebago River at 1 3 0 1 0 0 0 0 0 0 0 0 0 1 6.0 a (none) (>100) (15) (O Mason City (cob (bed not (I (8) (0) (0) (0) (0) (none) (0) (0) CD CO (05459500), bles) protected, Premodi- 5* 526mi2, both banks fied) o Thirteenth Street, protected) CD May 14, 1992

18 Willow Creek near 3 1 0 0 1 0 0 0 0 0 0 0 0 0 5.0 Mason City (sand) (bed not (I (0) (1) (0) (0) (0) (none) (none) (>100) (0) (0) (0) (05460100), protected) Premodi- 78.6 mi2, fied) U.S. Highway 18, May 14, 1992 Table 4. Potential-scour index at selected highway bridges in Iowa—Continued

Index components

Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of Site vicinity (USGS station Percent river Pier Mass Angle of identifi number), age of meander skew wasting approach Poten cation drainage area, Bed Bed Stage of channel Number of Hori impact (num at pier of high tial number highway, mate pro channel constric piers in zon Verti Left Right point ber of (number flows scour bank bank piers) index (fig-1) date visited rial tection evolution tion channel tal cal Total (ft) of piers) (degrees)

Chickasaw County

19 Little Wapsipinicon 3 1 0 1 1 0 0 0 1 1 0 0 0 0 8.0 River near New (sand) (bed not (VI (8) (1) (0) (0) (0) (flu (flu (>100) (0) (0) (0) Hampton (05420650), protected) Restabili- vial) vial) 95.0 mi2- zation) U.S. Highway 18. May 12, 1992

20 East Fork Wapsipinicon 3 3 0 0 1 0 0 0 1 1 3 0 0 0 12.0 River near New (sand) (bed not (VI (0) (2) (0) (0) (0) (flu (flu (10) (0) (0) (0) Hampton (05420690), protected, Restabili- vial) vial) 30.3 mi2, both banks zation) U.S. Highway 63, protected) May 12, 1992

21 Little Cedar River near 3 3 0 0 1 0.67 0 0 0 0 3 2 0 2 14.7 (40) (none) (none) (0) (2) (0) (35) CO Ionia (05458000), (sand) (bed not (VI (0) (2) (5) (2) o o 306 mi2, protected, Restabili- c County Road B57, both banks zation) 3 protected) a May 12, 1992 CD X CD Clarke County r* s 22 South White Breast Creek 3 1 3 0 0 0 0 0 1 1 0 0 0 0 9.0 near Osceola (sand) (bed not (V (0) (0) (0) (0) (0) (flu- (flu- (MOO) (0) (0) (0) I (05487600), protected) Aggra vial) vial) X dation) CO 28.0mi2, 3* County Road R53, i February 27, 1992

CD Clay County

23 Ocheyedan River near 3.5 1 0 0 1 0 0 0 1 1 0 1 0 0 8.5 Spencer (06605000), (allu- (bed not (VI (0) (1) (0) (0) (0) (flu- (flu- (>100) (1) (0) (10) 426 mi2, vium) protected) Restabili- vial) vial) County Road M38, zation) April 28,1992 00 Table 4 . Potential-scour index at selecteej highway biridges in Iowa—Corltinued

T3 Index components o CD 3 Percentage of Bank erosion Proxim E Stream name and blockage by debris (type) ity of CO river Pier Mass Angle of o Site vicinity (USGS station Percent o meander skew wasting approach Poten c identifi number), age of cation drainage area, Bed Bed channel Number of Hori- impact (num at pier of high tial > Stage of CO Left Right point ber of (number flows scour CO numbei highway, mate pro channel constric piers in zon- Verti CD bank bank piers) of piers) (degrees) index CO («g-i) date visited rial tection evolution tion channel tal cal Total (ft) CO 3 CD 3 Clay County—Continued to 3 24 Willow Creek near 3.5 1 0 0 1 0 0 0 1 1 3 1 0 1 12.5 a Cornell (06605750), (allu (bed not (VI (0) (0) (flu (flu (0) (1) (0) (15) m (0) (1) (0) CO r* 78.6 mi2, vium) protected) Restabili- vial) vial) 3 U.S. Highway 71, zation) Q» CD* April 28, 1992 CO o Clayton County 3 to X 25 Turkey River at Garber 3.5 1 3 0 2 0 0 0 1 1 0 0 0 0 11.5 3 c (05412500), (allu (bed not (V (0) (3) (0) (0) (0) (flu (flu O100) (0) (0) (0) 3 1,545 mi2, vium) protected) Aggra vial) vial) CO o County Road C43, dation) o c "* February 6, 1992 to CO Dallas County CD CD O 26 South Raccoon River at 3.5 1 3 0 1 0 0 0 1 2 0 0 0 0 11.5 I Redheld (05484000), (allu (bed not (V (0) (1) (1) (5) (0) (flu (mass (MOO) (0) (0) (0) CD 994 mi2, vium) protected) Aggra vial) wast a CO County road, dation) ing) CD CO April 15, 1992

3 27 Raccoon River at Van 3.5 2 3 2 1 0.67 0.67 0.33 2 2 3 2 0 2 24.2 o Meter (05484500), (allu (bed not (V (35) (2) (40) (40) (16) (mass (mass (0) (2) (0) (30) 0) 3,441 mi2, vium) protected, Aggra wast wast County Road R16, right bank dation) ing) ing) April 16, 1992 protected)

Davis County

28 Fox River at 3 2 3 1 0 0 0 0 1 1 0 0 0 0 11.0 Bloomfield (05494300), (sand) (bed not (V (10 est.) (0) (0) (0) (0) (flu (flu (>100) (0) (0) (0) 87.7 mi2, protected, Aggra vial) vial) County road, right bank dation) February 3, 1992 protected) Table 4. Potential-scour index at selected highway bridges in Iowa—Continued

Index components

Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of Site vicinity (USGS station Percent - river Pier Mass Angle of identifi number), age of meander skew wasting approach Poten cation drainage area, Bed Bed Stage of channel Number of Hori- impact (num at pier of high tial number highway, mate pro channel constric piers in zon- Verti Left Right point ber of (number flows scour (fig.i) date visited rial tection evolution tion channel tal cal Total bank bank (ft) piers) of piers) (degrees) index

Decatur County

29 Elk Creek near Decatur 3 3 3 0 0 0 0 0 1 1 0 0 0 0 11.0 City (06897950), (sand) (bed not (V (0) (0) (0) (0) (0) (flu (flu (>100) (0) (0) (0) 52.5mi2, protected, Aggra vial) vial) County road, both banks dation) February 28, 1992 protected)

30 Thompson River at 3.5 1 3 0 1 0 0 0 1 1 0 1 0 2.5 14.0 Davis City (06898000), (allu (bed not (V (0 est.) (1) (0) (0) (0) (flu (flu (>100) (1) (0) (45) 701 mi2, vium) protected) Aggra vial) vial) U.S. Highway 69, dation) February 28, 1992 •a 31 Weldon River near 3 2 3 0 1 0 0 0 2 1 0 1 0 2.5 15.5 1 Leon (06898400), (sand) (bed not (V (0) (1) (0) (0) (0) (mass (flu (>100) (1) (0) (45) & 104mi2, protected, Aggra wast vial)

CO County Road J48, left bank dation) ing) ft o February 28, 1992 protected) c

3 Delaware County a. CD X CD 32 Plum Creek at 3 1 3 0 0 0 0 0 1 1 0 0 0 0 9.0 i Earlville (05417530), (sand) (bed not (V (0) (0) (0) (0) (0) (flu (flu (>100) (0) (0) (0) CD 41.1 mi2, protected) Aggra vial) vial) O U.S. Highway 20, dation) I February 6, 1992 X CO 3* Dubuque County

iCD i 3 1 0 0 0 0 1 1 0 0 0 0 7.0 CD 33 Little Maquoketa River 0 (bed (bed not (V (19) (0) (0) (0) (0) (flu (flu (>100) (0) (0) (0) a near Durango CO rock) Aggra vial) vial) CD (05414500), protected) CO 130mi2, dation) 3 County road, O February 6,1992 i en Table 4 © . Potential-scour index at selected highway bridges in Iowa—Continued

•0 Index components o CD 3 Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of CO ft Site vicinity (USGS station Percent river Pier Mass Angle of o Poten c identifi number), age of meander skew wasting approach cation Bed Number of Hori impact (num at pier of high tial > drainage area, Bed Stage of channel CO scour CO numbei highway, mate pro channel constric piers in zon Verti Left Right point ber of (number flows CD index CO («g. 1) date visited rial tection evolution tion channel tal cal Total bank bank (ft) piers) of piers) (degrees) CO 3 (D 3 Emmet County

01 3 34 Des Moines River at 3.5 2 0 0 1 0.33 0 0 0 0 0 0 0 0 6.8 a Estherville (05476500), (allu (bed not (10) (none) (none) (>I00) (0) (0) (0) m (VI (0) (2) (2) (0) CO 1,372 mi2, vium) protected, Restabili- i* State Highway 9, right bank zation) CD CD April 30, 1992 protected) CO O Floyd County 3 0) X 35 Cedar River at Charles 1 2 0 0 1 0 0 0 0 - 0 0 0 0 0 4.0 3 c City (05457700), (cob (bed not (VI (0) (2) (0) (0) (0) (none) (none) (>100) (0) (0) (0) 3 1,054 mi2, bles) protected, Restabili- CO o U.S. Highway 18, left bank zation) o c May 13, 1992 protected) 0> CO Fremont County

CD ft 0 0 1 11.0 ST 36 Waubonsie Creek near 4 2 1 0 0 0 0 0 0 0 3 a Bartlett (06806000), (silt/ (bed not (II (0) (0) (0) (0) (0) (none) (none) (0) (0) (0) (20) CD -% 30.4 mi2, clay) protected, Con a CO County Road J10, right bank structed) o CO March 24, 1992 protected) 3 37 West Nishnabotna 3.5 2 3 0 1 0 0.67 0 2 2 3 1 3 0 21.2 I River at Randolph (allu (bed not (V (0) (1) (5) (50) (3) (mass (mass (0) (1) (1) (10) (06808500), vium) protected, Aggra wast wast 1,326 mi2, right bank dation) ing) ing) State Highway 184, protected) March 24, 1992

38 Nishnabotna River above 3.5 1 3 0 1 0 0.33 0 1 1 3 1 0 1 15.8 Hamburg (06810000), (allu (bed not (V (0) (1) (5) (10) (1) (flu (flu (0) (1) (0) (15) 2,806 mi2, vium) protected) Aggra vial) vial) US Highway 275, dation) March 24, 1992 Table 4. Potential-scour index at selected highway bridges in Iowa—Continued

Index components

Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of Site vicinity (USGS station Percent • river Pier Mass Angle of identifi number), age of meander skew wasting approach Poten cation drainage area, Bed Bed Stage of channel Number of Hori- impact (num at pier of high tial number highway, mate pro channel constric piers in zon- Verti Left Right point ber of (number flows scour («g. 1) date visited rial tection evolution tion channel tal cal Total bank bank (ft) piers) of piers) (degrees) index

Greene County

39 North Raccoon River near 3.5 2 2 1 1 0 0 0 1 1 3 2 0 1 17.5 Jefferson (05482500), (allu (bed not (HI (18) (2) (0) (0) (0) (flu (flu (0) (2) (0) (20) 1,619 mi2, vium) protected, Degra vial) vial) State Highway 4, right bank dation) April 9, 1992 protected)

40 Hardin Creek near 3.5 1 0 0 1 0 0 0 1 1 1 0 0 0 8.5 Farlin (05482900), (allu (bed not (VI (0) (1) (0) (0) (0) (flu (flu (70) (0) (0) (0) 101 mi2, vium) protected) Restabili- vial) vial) County road, zation) April 4,1992

O Grundy County CD to 41 Black Hawk Creek at 3.5 2 2 0 1 0 0 0 1 1 0 0 0 0 10.5 T* CO Grundy Center (allu (bed not (HI (0) (1) (0) (0) (0) (flu (flu (>100) (0) (0) (0) o o vium) protected, Degra vial) vial) c (05463090), ^ 56.9 mi2. right bank dation) 5" O. State Highway 14, protected) CD X April 14, 1992 CD £ Guthrie County 8 42 Middle Raccoon River 3.5 1 2 0 0 0 0 0 1 1 3 0 0 1 12.5 I near Bayard (allu (bed not (0) (0) (0) (0) (0) (flu (flu (0) (0) (0) (15) X (HI CO (05483450), vium) protected) Degra vial) vial) 3" 375mi2, dation) i «< State Highway 25, CD April 6,1992 o. ca Middle Raccoon River at 3.5 2 3 0 1 0 0 0 1 1 3 2 0 2.5 19.0 CD 43 CO Panora (05483600), (allu (bed not (V (0) (2) (0) (0) (0) (flu (flu (0) (2) (0) (45) 3 440 mi2, vium) protected, Aggra vial) vial) O County road, left bank dation^ i April 6, 1992 protected) Table 4. Potential-scour index at selected highway bridges in Iowa—Continued

Index components

Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of (O Percent river Pier Mass Angle of a Site vicinity (USGS station o meander skew wasting approach Poten c identifi number), age of cation Bed channel Number of Horl- impact (num at pier of high tial > drainage area, Bed Stage of CO number constric Left Right point ber of (number flows scour CO highway, mate pro channel piers in zon- Verti- CD bank bank piers) of piers) (degrees) index CO («g. 1) date visited rial tection evolution tion channel tal cat Total (ft) CO 3 CD 3 Hamilton County CO CD 44 Mud Lake Drainage 3.5 2 0 1 1 0 0 0 0 2 1 0 0 0 10.5 Ditch 71 at Jewell (allu (bed not (VI (1) (0) (0) (0) (none) (mass (60) (0) (0) (0) m (ID ce (05469860), vium) protected, Restabili- wast 3 65.4 mi", right bank zation) ing) U.S. Highway 69, protected) April 15, 1992

45 Boone River near 3.5 2 0 0 1 0 0 0 1 1 2 1 0 0 11.5 Webster City (allu (bed not (VI (0) (1) (1) (3) (0) (flu (flu (40) (1) (0) (5) (05481000), vium) protected, Restabili- vial) vial) 844 mi2. left bank zation) State Highway 17, protected) April 27, 1992

Hancock County

46 West Branch Iowa River 3.5 2 0 1 1 0 0 0 1 1 0 0 0 0 9.5 near Klemme (allu (bed not (VI (18) (1) (0) (0) (0) (flu (flu (>100) (0) (0) (0) CD (05448500), vium) protected, Restabili- vial) vial) a 112 mi2, left bank zation)

Index components

Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of Site vicinity (USGS station Percent river Pier Mass Angle of identifi number), age of meander skew wasting approach Poten cation drainage area, Bed Bed Stage of channel Number of Hori- impact (num at pier of high tial number highway, mate pro channel constric piers in zon- Verti Left Right point ber of (number flows scour («g. 1) date visited rial tection evolution tion channel tal cal Total bank bank (ft) piers) of piers) (degrees) index

Harrison County

2 48 Soldier River at Pisgah 3.5 1 4 0 0 0 0 0 1 0 0 0 0 11.5 (06608500), (allu (bed not (IV (0) (0) (0) (0) (0) (flu (mass (>100) (0) (0) (5) 407mi2, vium) protected) Thresh vial) wast County Road F20, old) ing) April 7, 1992

49 Boyer River at Logan 3.5 3 4 I 1 0 0 0 2 2 0 0 3 0 19.5 (06609500), (allu (bed not (IV (9) (1) (3) (4) (0) (mass (mass (>100) (0) (1) (0) 871 mi2, vium) protected, Thresh wast wast U.S. Highway 30, both banks old) ing) ing) April 7, 1992 protected)

Henry County

50 Cedar Creek near 3.5 1 3 0 0 0 0 0 2 2 0 0 0 0 11.5 to Oakland Mills (allu (bed not (V (0) (0) (0) (0) (0) (mass (mass (MOO) (0) (0) (0) o o (05473400), vium) protected) Aggra wast wast c "1 530 mi2, dation) ing) ing) 3 Q. County Road H46, CD X December 11, 1992

CO 51 Big Creek near Mount 3.5 1 3 0 0 0 0 0 2 2 0 0 6 0 17.5 CD Pleasant (05473500), (allu (bed not (V (0 est.) (0) (0) (0) (0) (mass (mass (>100) (0) (2) (0) vium) protected) Aggra wast wast CD 106 mi2, Q. County road, dation) ing) ing) I December 11, 1992 CO zr i Howard County «< OD 52 Wapsipinicon River near 3 1 0 0 0 0 0 0 1 1 0 0 0 0 6.0 a to (sand) (bed not (VI (5) (0) (0) (0) (0) (flu (flu (>100) (0) (0) (0) CD Elma (05420560), CO 95.2 mi2, protected) Restabili- vial) vial) County Road B17, zation) May 13,1992 Table 4. Potential-scour index at selected highway bridges in Iowa—Continued

Index components

Percentage of Bank erosion Proxim Stream name and blockage by debris (type) ity of Site vicinity (USGS station Percent river Pier Mass Angle of identifi number), age of meander skew wasting approach Poten cation drainage area, Bed Bed Stage of channel Number of Hori- impact (num at pier of high tial numbei highway, mate pro channel constric piers in zon- Verti Left Right point ber of (number flows scour (fig. 1) date visited rial tection evolution tion channel tal cal Total bank bank (ft) piers) of piers) (degrees) index

Howard County—Continued

53 Little Wapsipinicon 3 2 0 1 0 0 0 0 1 1 3 0 0 2.5 13.5 River near Elma (sand) (bed not (VI (13) (0) (0) (0) (0) (flu (flu (20) (0) (0) (45) (05420640), protected, Restabili- vial) vial) 37.3 mi", left bank zation) County Road B17, protected) May 13, 1992