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Chasing Floods and Measuring Scour

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Chasing Floods and Measuring Scour 236 TRANSPORTATION RESEARCH RECORD 1290 Chasing Floods and Measuring Scour Roy E. TRENT AND MARK LANDERS ABSTRACT INCREASING DISCHARGE DECREASING DISCHARGE Field measurements of local scour at bridges during floods, and 1.,,.11~ . POOW 15 "' ... measurement of sedimentation processes in general, has been a 2 _ frustrating and unrewarding activity in the past. Surprisingly, iii!.. 0 10 little scour data has been collected in that time and only a small w . portion of the data have been taken during floods at bridges ~ 5 crossing natural streams. Recent efforts of U.S. Geological 8... ;; Survey (USGS) field crews to measure scour during floods for the 0 Federal Highway Administration and several State highway agencies, however, demonstrate that it can be done. This paper i -5 ~ outlines procedures used, insights and experiences gained, and some preliminary results obtained while chasing floods and -10 o 100 200 300 400 0 100 200 300 400 measuring scour during the May 1990 floods in the Southwest. WIDTH, IN FEET WIDTH, IN FEET Figure 1. Scour and deposition with flood flow for Colorado River at Lees BACKGROUND Ferry, Arizona. Scour data collected during flood conditions reinforces our knowledge that fluvial processes are dynamic. The high flow importance of measuring scour near the peak of the flood to velocities and turbulence that occurs during a flood causes improve our understanding of these processes. This is increased shear stress on the streambed and results in scour, particularly important when conducting scour inspections of erosion, and increased sediment transport capacity of streams. As bridges. flood flows recede, the bed load is redeposited and streambed Obviously, it cannot be safely concluded that a depression or elevations typically increase and scour holes in the streambed and deep hole measured around a pier after the flood represents the near structures refill. Figure 1 shows an example of bed scour maximum scour experienced in recent time. Accordingly, and fill for a 1956 flood on the Colorado River at Lees Ferry, inspection of bridges for scour, even shortly after a flood, may Arizona. Figure 2 shows another example of this process for the result in misleading and even dangerous conclusions. Perhaps, May 1990 flood on the Red River at the U.S. Highway 71 bridge coring or using geophysical subbottom profiling techniques, and in Arkansas. This process is characteristic of alluvial streams looking for undisturbed sediments, marles, or other parent having non<;ohesive sand or silt bed materials. This process is materials, may provide conclusions about the scour history of a less typical of rivers with cobbles or boulders or cohesive clay particular pier. Every pier, however, requires separate evaluation bed materials; however, if finer materials are also present, they because every pier has different scour potential and scour will fill local scour holes in the cohesive or large grain size resistance. Scour potential also varies laterally across the channel material during the flow recession. The process of channel scour as a function of channel curvature, flow concentration due to and fill is not linear with hydraulic factors because of the effects contraction or flow redirection, and many other site specific of bed load and other deterministic factors on scour. Bed load is parameters. Many variables affect the rate and depth of scour. itself a temporally and spatially dynamic process, so that The actual scour performance of a bridge during any given flood generalizations of the relation between scour and hydraulic factors event is best determined by real time measurements by fidd must be applied with care. The relation of bed load to discharge crews. will generally follow a clockwise, hysteretic curve for a given flow event. For live bed streams, this may mean initial filling of the general (and thus local scour) sections for a flow event. This DAT A REQUIREMENTS is illustrated in figure 3 for data collected on the South Altamaha River at I-95 near Brunswick, Georgia, and in figure 4 for San Different levels of data are required for different purposes. Juan River near Bluff, Utah. These and other data emphasize the Currently, a primary concern of bridge owners and major purpose 1 Office of Research and Development, Federal Highway Administration, 6300 Georgetown Pike, McLean, VA 22101- 2296 and United States Geological Survey, Water Resources Division MS 415, 12201 Sunrise Valley Drive, Res ton, VA 22092 Trent and Landers 237 T--------', ------------------------May 9, 1990: 262,000 els --------I • ..._ r"---... __ ..J ~ I - '~--, I ', I 270 \ I '~ I \ May 23, 1990: 64,100 els I I . ........ u .. ..... .. ...... ............. ... so . .. .. ... .. , 260 March 14, 1990: 51,500 els i z 0 ~ I tu 250 ...J UI 0 UI m ...J 240 UIz z <( :I: 0 230 I ~PIERS 220 ,__~.._~_._~........ ~-'-~-'-~-'-~~~--'~--''--~L-~"--~-'-~...o....~.....L..~~~......1 0 200 400 600 800 1000 1200 1400 1600 DISTANCE FROM NOTRH ABUTMENT, IN FEET Figure 2. Changes in channel geometry with flood discharge for the Red River at U.S. Highway 71 near Ibex, Arkansas. for measuring bridge scour is to verify local scour formulas or to surveys of study sites) and soon after the flood should be made. put practical constraints on the resulting scour estimates. A Uses for this level of data includes modeling the fluvial processes limited data set is required for these objectives. This paper for long-term performance of a river system, as well as the discusses procedures for obtaining the limited data set. Such data response of the channel to a single flow event. Simulation of includes and is an extension of data (stage, velocities, depths) channel response to measures like channelization, dredging, obtained from standard flood discharge measurement procedures aggregate mining, channel shortening, flow control, flood used by USGS. An important distinction exists between flood proofing, and width constrictions are now possible. These data discharge measurements and scour measurements: the limited will support improved technology and simulation models that are data set focuses measurements on and around the bridge easier to use and more reliable. Ultimately this and even more features that are typically avoided in standard discharge detailed data are needed to establish cause and effect relationships, measurements. Scour measurement adds details on geometry of for example, to relate the flow event to the sedimentation the river bottom near the bridge piers and abutments, including processes and relate that to river responses. cross sections on both sides of the bridge, photographs and For selected high risk bridges, where scour potential is known observations of flow conditions, and a judgement about the status to be a serious threat, scour measurements taken during major of bedload movement. USGS scour survey teams use a checklist flood events could become a part of the standard bridge inspection as a reminder of items to be included in a limited data set (figure reports. Only a very restricted inspection data set (local 5). Other critical data are site records on foundations, bed geometry near piers and abutments) would usually be required for material, channel and floodplain characteristics, and bridge safety inspections of bridges during floods. The methods hydraulics. These data are preferably obtained prior to measured prescribed here would be suitable for adoption and adaptation by flood events, but at many sites they can also be collected bridge inspection teams or highway maintenance crews for afterwards. making scour inspections during floods. The primary use for Another reason to collect data is to develop, calibrate, and such data would be to document conditions of foundation support verify sediment transport models and methods in order to simulate rather than to quantify or describe scour processes. flu vial processes--a detalled data set is required. Data required for this purpose would include flow, sediment, and bed load data in addition to scour depths and velocity data. This data must be PROCEDURES measured at the bridge and at sections well upstream and downstream several times over the flood hydrograph. In addition Procedures for collecting scour data are governed by the to collecting real time data during the flood, surveys of section requirements for the data set being collected, be that an bedforms and channel conditions before (during preliminary field inspection, limited, or detailed data set. The procedures i .. .! -10 § -15 I I ... .i· J 11 • ... I; m -30 -I -36 ·2 ...~ ~,_~~~~~~~~----~~~~~ ti 00 10 00 12 00 14 00 18.00 11.00 20.00 "'• •• IOO '" 1000 1200 CflO.I UCTION WN>nt. tN FEET ... PIER 12 -11 /' " ' -20 ' ' /\ ' -21 ' '-.., "' I PIER 12 I / 7•ir ·-\ \\ ' -22 \ k""' I , I ~- ,., , : -- ~ I ~ , ' I I , , r' ·, ·ZI \ ~~ I \ \ ' \·, /I I i... '. \ \ J I PIER 11 lil. -24 '. ' \::.» ) . -•'--~~~'--~----~'--~....... ~'-~----~'-~..___, 3 ,.. \ \;Co .. : 0.02 0 ,04 0. 01 0 .. u.oo to ll•••ure.-.nl I • FAC>t.OE N4AllEfl • I ... I 12:00 I .J 13:00 \ I ... 14:4& / " 171)6 I I _,. '. ·, ./ " ~ -27 ·,,.' ,. ~ ! ~ "''20 040 HO 700 720 740 /:; '" iii ,,,... __ CRO&.I l l.ClnOM WM> nt, ..,_ H .£T "" PIUI 12 ~ _,. / ................... .... ... -20 " I !" PIER 13 • ... "" """ ' -21 .,,. " ' ' " PIER'· 11 .,:11 _,. U. 02 0.03 0.04 O. OI O.OI 0.07 0 .01 0. 09 F..ou>E ...... Hfl ... •.! " ~E ii. 11 _,. § ; I ... ~ 12 ~ i ~ -27 ~ 1111E Of' llEAIUAEllEHT I 1.0 -21 J: ... """..... ~ ......... "' .. 14:41 ,..,., ,, .. ~ ... ... "' ... 'iC " ' .Jr ' ... ... .. '• ' ....... "u 02 0. 03 004 0.06 001 007 .... 000 FAOUJE Nl.lilHR Figure 3. Relation between kinetic energy (Froude number) of the approach flow, channel geometry, and local scour depth, for South Althamaha River at the upstream side ofl-95, near Darien, Georgia, on February 12, 1990. o; l OO .2: I I .s 200 ++~t~ci-ci-Cl-dl ~,-~-·-+- ' ' 11 EXPLANATION £ --· -0 DISCHARGE ~ 100 I Risi no FollinQ Width o; Depth ~ .S Velocity £ Load 0.
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