Experiments on Urban Flooding Caused by a Levee BreachLindsey Ann LaRocque, M.ASCE1; Mohamed Elkholy, M.ASCE2; M. Hanif Chaudhry, F.ASCE3;

and Jasim Imran, M.ASCE4

Abstract: Breached levees often cause extensive flood damage to urban and residential areas; there has been considerable interest in thenumerical modeling of flooding of urban areas in recent years. However, comprehensive data that describe the complex flow field inresidential areas are not available for validating the numerical models or for determining the extent of such damage. To fill this gap, resultsare presented of experiments on a scale model used as a test setup that examined the flow field in an urban area attributable to flooding causedby a levee breach. Steady state flow depths and flow velocity were measured by a point gauge and by an array of ultrasonic velocity profilers,respectively. In addition, the surface velocities were recorded in the entire flooded area by using a digital particle tracking velocimetrytechnique. The details of these measurements, limited flow data, and the observed flow characteristics are presented in this study. Full datasets are provided on the Internet. The results of these investigations show that closely spaced buildings act like a single obstacle with acommon wake zone, and there is little flow between these buildings. The results also show that the strongest flow is in the prebreach directionand that the local topography plays an important role in distributing the flow in the flooded area. The complete data set may be utilized tovalidate numerical models of free surface flow.DOI: 10.1061/(ASCE)HY.1943-7900.0000754.© 2013 American Society of Civil Engineers.

CE Database subject headings: Three-dimensional flow; Urban areas; Floods; Levees and dikes; Failures.

Author keywords: Digital particle tracking velocimetry; Levee breach; Path lines; Trajectories; Ultrasonic velocity profilers.

Introduction

Levees and flood walls are constructed to protect major cities from50 or 100 year design floods. However, these protection structuresmay fail during historic floods and cause fatalities and extensiveproperty damages. A few examples of such floods are the 1993Mississippi flood that affected vast areas in the Midwest, the2010 flood in Pakistan, the 2011 flood in Eastern Australia, andmany major floods in Europe in recent years. All of these causedsignificant damages in urban and rural areas.

Hurricane Katrina, one of the most detrimental storms ofthe United States, reached the Louisiana and Mississippi coastson August 29, 2005. The storm impacted approximately93,000 mi2, flooded approximately 80% of New Orleans, Louisi-ana (Kunreuther 2006), completely destroyed nearly 300,000homes, and resulted in approximately $67 billion in damage(Caraballo-Nadal et al. 2006), of which approximately $25 billionoccurred in the New Orleans metropolitan area. The city of New

Orleans is situated in Orleans Parish along the eastern edge of theMississippi River. Much of the surface elevation of the area isgenerally near and below the sea level. Continuous pumping ofgroundwater and levees surrounding the city is required to keepthe water from the Gulf from entering the city. A 400 ft(122 m) wide levee failure occurred at the east side of the 17thStreet Canal, near the Hammond Highway Bridge. A section ofthe levee was displaced approximately 50 ft (15 m) east of its origi-nal location as a result of its failure. The displacement of the flood-wall created ditches parallel and perpendicular to the breach. The17th Street levee breach massively contributed to the overall flood-ing of the Metropolitan Orleans East Bank protected basin; morethan half of the loss of life, and a similar fraction of the overalldamages, occurred in this heavily populated basin. Floodwaterscontinued to stream through this breach for two and a half days(Seed et al. 2008). Map of the Orleans Parish and East Bank,and an aerial photo of the canal and flooded residential area afterthe levee was breached are shown in Fig. 1 [United States ArmyCorps of Engineers (USACE) 2007].

The USACE (2007) report provides a detailed forensic analysisof the performance of New Orleans and the Southeast LouisianaHurricane Protection System during Hurricane Katrina. The workconducted by the task committee has resulted in several technicalpapers focusing on the causes of levee failure (Briaud et al. 2008;Seed et al. 2008; Ubilla et al. 2008). Many works involve damageassessment resulting from Katrina (van de Lindt et al. 2007; Francoet al. 2010; Link 2010). However, no detailed analysis has beenpresented of the flow field caused by levee breach flooding.

Flooding in urban residential areas represent three-dimensional(3D) flows in which buildings often act as obstacles, leading tohydrodynamic forces due to stagnation pressure, lateral shear, andflow separation similar to that observed around surface mountedcubes in a wind tunnel experiment (Baskaran and Stathopoulos1989; Paterson and Apelt 1989; Cowan 1997; Murakami andMochida 1989; He 1997; Mufuta 2001; Calhoun et al. 2004).The presence of buildings also leads to increased water depth

1Graduate student, Dept. of Civil and Environmental Engineering,Univ. of South Carolina, 300 Main St., Columbia, SC 29208. E-mail:larocqul@cec.sc.edu

2Postdoctoral Fellow, Dept. of Civil and Environmental Engineering,Univ. of South Carolina, 300 Main St., Columbia, SC 29208. E-mail:elkholy@email.sc.edu

3Mr. and Mrs. Irwin B. Kahn Professor, Dept. of Civil and Environmen-tal Engineering, Univ. of South Carolina, 300 Main St., Columbia, SC29208. E-mail: chaudhry@cec.sc.edu

4Distinguished Professor, Dept. of Civil and Environmental Engi-neering, Univ. of South Carolina, 300 Main St., Columbia, SC 29208(corresponding author). E-mail: imran@cec.sc.edu

Note. This manuscript was submitted on January 20, 2012; approved onMarch 14, 2013; published online on March 16, 2013. Discussion periodopen until February 1, 2014; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Hydraulic Engineer-ing, Vol. 139, No. 9, September 1, 2013. © ASCE, ISSN 0733-9429/2013/9-960-973/$25.00.

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and regions of high velocity. For emergency planning, there hasbeen a strong interest in the analysis of urban flooding in recentyears (Nania et al. 2004; Testa et al. 2007; Soares-Frazao et al.2008; Sanders et al. 2008; Petaccia et al. 2010; Velickovic et al.2010). Testa (2007) measured water levels in a physical modelof a city block as a result of a flash flood. Soares-Frazao and Zech(2008) measured both water level and surface velocity in an ideal-ized city block as a result of a dam break flow. They also simulatedtheir experiment using a shallow water model. Liang et al. (2007)applied a two-dimensional (2D) depth-averaged model to study theinteraction between buildings and flood flows, in which the build-ings were modeled as porous media. Similarly, Soares-Frazao andZech (2008), Sanders et al. (2008), Petaccia et al. (2010), andVelickovic et al. (2010) incorporated a porosity term in their shal-low water models.

Cheng et al. (2003) applied the large eddy simulation (LES) andthe k-ε model on a matrix of cubes in a wind tunnel. They foundthat the LES model provided better results with the experimentaldata than the k-ε model. Lien and Yee (2004) applied two high-resolution Reynolds-averaged Navier-Stokes (RANS) flow modelsto study wind flow through a complex array of 3D buildings.

Compared to wind flow around buildings, modeling an open chan-nel flow is more challenging because of the presence of the freesurface. In the context of numerical modeling that resolves verticalflow structure, the free surface is tracked by several methods,e.g., marker and cell, volume of fluid, and level set (Hirt andNicohls 1981; Yue et al. 2003; LaRocque et al. 2013). Data fromthe present study will be a valuable resource for validating theseapproaches for accurately simulating complex free surface flowfields. In addition, the data can be useful for calibrating and iden-tifying the limitations of shallow water models to the case of urbanflooding.

The objectives of the work are: (1) to study the flow patternthrough visualization and measurements of flooding on a scalephysical model of a flooded residential area, and (2) to providea comprehensive data set on complex free surface flow in an urbanarea for validating numerical models. The flow field on the eastside of the 17th Street Canal levee breach in New Orleans isstudied on a 1∶50 scale model of the area, marked approximatelyby the dashed lines in Fig. 1(b). Unlike previous works (Soares-Frazao and Zech 2008), the present study focuses on steadyflow around buildings with an irregular pattern and a natural

Fig. 1. (a) Map of the Orleans Parish showing the locations of breached levees (USACE 2007); (b) aerial photo of the 17th Street levee breach, withthe dashed line showing the area considered for modeling (photo courtesy of NOAA)

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topography of the canal, breach, and flooded area that is repro-duced in the laboratory to resemble the real case. The experimentcombines different measurement techniques to measure and visu-alize the flow field.

Experimental Setup

The experiments were conducted on a 1∶50 scale model of thearea surrounding the 17th Street Canal breach. A schematic ofthe model is shown in Fig. 2(a). The setup was originally builtto study procedures for the closure of the 17th Street Canal breachin New Orleans (Sattar et al. 2008; Elkholy and Chaudhry 2009).The model–prototype relations are based on the Froude similitudeand the geometric scale was chosen to minimize the effects of vis-cosity and surface tension in the model. For a discharge of 57 L=sin the physical model, the Reynolds numbers of the flow equal162,000 at the entrance of the canal and 74,000 at the breach sec-tion. Data used to build the hydraulic model, including topographyand bathymetry, were taken from the interagency performanceevaluation taskforce (IPET) report (USACE 2007). The hydraulicmodel included a 570 m (1,870 ft) section of the 17th Street Canalsouth of the Old Hammond Highway Bridge and a small part of theneighborhood, 3.172 ha (7.84 acres), in the vicinity of the breachwhere major flooding occurred (Sattar et al. 2008). The topographyof the breach area and the neighborhood were carefully reproduced.The nonerodible topography of the canal, breach, and flooded areawere reproduced by using a mixture of cement, sand, and zonolite.Details of the construction of the physical model were reported bySattar et al. (2008). A movable grid was built over the model to helpwith the positioning of the probes, with cell size of 15.2 × 15.2 cm,with half of that near the levee. The origin of the coordinates was atthe northwest corner, as shown in the figure. The model of the res-idential area was built on a raised platform to have free fall in all thethree directions at the outflow boundaries, as indicated by the smallarrows in the figure. A total of 22 blocks representing houses werespaced throughout the model.

Along with the Orleans Canal and South London Avenue Canal,the 17th Street Canal is used for pumping storm water to LakePonchartrain. These canals are closed at one end, as shown inFig. 1(a). According to the time history estimated in the IPET report(USACE 2007), the flow coming from the upstream pumping sta-tion was less than 10% of the breach flow and most of the flowcame from Lake Ponchartrain. In the present model, the openend of the canal is considered to be the upstream. With the otherend of the canal kept closed, the entire flow of the canal comingfrom the lake entered the residential area through the breached sec-tion The houses on the west side of the neighborhood, i.e., nearestto the levee breach, were colored gray and those on the east sidewere colored black for easy identification. In Fig. 2(a), the housesare numbered according to the color. The shaded areas around someof the buildings indicate locations of 2D velocity measurements.The upstream section (north) contained buildings G1–G6 andB1–B7 and the downstream (south) section contained buildingsG7–G10 and B8–B12; the G and B stand for gray and black houses,respectively.

Fig. 2(b) shows the bed elevation of the model. The crest level ofthe breach was the datum. The topography of the model shows sig-nificant variation in the elevation, especially near the breach, wherea ditch was eroded by the breaching. The ditch ran mostly parallelto the breach and had an eastward extension on the southern side.By using a nonerodible bed in the model, it is assumed that theflooded area did not experience further erosion during the sustainedflow following the levee breach.

Experimental Procedure

Flow depth was measured by using a point gauge with an accuracyof 0.1 mm. Ultrasonic velocity profilers (UVPs) were used formeasuring the flow field around model buildings and along thebreach. The noninvasiveness of the probe and its small footprintallows for the measurement of velocity in very shallow depthsand in tightly spaced locations. A digital particle tracking velocim-etry (DPTV) technique involving the motion of hollow plastic ballswas applied to measure surface velocities, overall flow pattern, andflow distribution in different areas of the model. In addition to thevelocity readings, long-exposure still photos were taken duringthe experiment for visualization at various locations throughoutthe model.

Water Depth Measurement

A point gauge was used to measure water depth at the grid points(Fig. 2). Although the flow was steady, there was fluctuation in thewater surface elevation close to the breach. At these grid points,data from repeated measurements were averaged. The point gaugemeasurement was checked against limited ultrasonic measurementsmade along several transects in the north–south and east–westdirections by using a Baumer ultrasonic distance profiler with anaccuracy of �0.5 mm. Comparison between the point gaugeand Baumer data showed a close match with relative root-meansquared error (RRMSE) ranging from 0.006 to 0.017 for differenttransects.

Velocity was measured by using an array of UVPs. The princi-ple of the UVP is based on the detection of ultrasonic pulses re-flected by particles suspended in the fluid (Met-Flow 2002).The velocity of the reflector is evaluated from the instantaneousDoppler shift frequency according to the following relation:

vc¼ fd

2foð1Þ

where v = velocity component along the transducers axis; fd =Doppler shift; c = speed of sound in the liquid (m=s); and fo =transmitting frequency (Met-Flow 2002). The manufacturer of theUVP reported a maximum error of 5% for velocity measurementsfor the maximum detectable length of 0.75 m in water (Met-Flow2002). The sampling duration for velocity measurement was 51.2 sfor each probe.

Velocity profiles in the breach were obtained along a line par-allel to the breach located 1.3 m away from the west levee. The tipof the UVP probe was placed close to the surface of the water andreadings were taken every 15 cm for 14 locations along the lengthof the breach. The probes were secured at a 45° angle facing thenorth direction for the resultant velocity in the x-z plane and facingthe west direction for the resultant velocity in the y-z plane.

For the flow field around an individual building or a cluster ofbuildings, the velocity was measured in both the x and y-directionsat a distance of 20 mm above the bed. The axis is shown in Fig. 2.When the velocity in the x-direction was measured, the probes werefacing in the opposite direction to the flow, so they did not interferewith the flow. For the measurement in the y-direction, the probeswere facing the building. Each house or house cluster had differentdimensions; therefore, the number of measurement sections wasnot always the same. For the building cluster B1 and B2, theUVP velocity measurement could not be conducted because thewater level at this location was less than 1 cm. To obtain qualityreadings for velocity, it is recommended to use particles with adiameter greater than the wavelength of the emitted ultrasound

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Fig. 2. (a) Schematic description of the model setup; (b) contour map of the bed elevation in (mm)

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burst divided by 4ðd ≥ λ4Þ (Met-Flow 2002). For an emitting fre-

quency of 4 MHz, a minimum of 93 μm in diameter is required(Met-Flow 2002). Crushed walnut shells with grain sizes of90–120 μm and density of 1,200 kg=m3 were used for seeding.

The raw data were extracted from each run and each timesequence was independently reviewed. The sampling time was setat 50 ms. Six probes were placed 1.5 cm apart and parallel to eachother. The probes measured the x-velocity component along a lineand could be rotated 90° to measure the y-velocity component.A combination of the results of two measuring lines that werenot parallel was used to determine the position and two componentsof the velocity at the point (Kikura et al. 1998).

Surface Velocity Measurement

To measure the surface velocity, software based on DPTV tech-nique was developed in-house and used to track the motion ofhollow plastic balls. The software segmented the particles fromthe background, located the centroid of each particle to the nearestsubpixel, and plotted the trajectory of the particle. Segmenting theballs from the background depended on choosing an appropriatevalue for the threshold. For an image fðx; yÞ, the segmented imagegðx; yÞ was computed as

gðx; yÞ ¼�1 if fðx; yÞ ≥ T0 if fðx; yÞ < T

ð2Þ

where T = threshold value. The result of segmentation was a binaryimage in which the particle was marked as 1 and the backgroundwas marked as 0. The value of the threshold T was individuallychosen from each image by preprocessing the image fðx; yÞ to re-move the uneven background illumination and applying the globalthreshold to the preprocessed image. Once the particle was seg-mented, a subpixel accuracy test was performed to determine thecentroid of the each particle. A first-order approximation of thevelocity of the particle was computed by subtracting the particleposition from two successive frames and dividing by the frameperiod of the camera.

The motion of 10-mm hollow plastic balls was recorded by aSony DCR-SR47 mounted on a fixed bracket above the model.The model bed was painted white and the balls were painted blackfor ease in identification and tracking. The balls were dropped atthe canal entrance and collected by using a mesh net at the exit ofthe model. The video camera was placed above the model at ninedifferent locations in sequence to track the motion of the balls in theentire model. At each location, the covered area was 2.44 × 1.3 m.

The surface velocity is calculated by dividing the distance theball moved from one frame to the next by the time interval betweenthe frames. Measurements of the surface velocity were especiallyuseful in areas where the UVP could not be used because of shallowwater depth. Additionally, the surface velocity measurementsprovided a close approximation to the actual flow pattern in theentire model.

Visualization

Photos were taken for flow visualization with the aid of charcoaland walnut shell mixtures released at various locations in themodel, to complement the flow pattern revealed from the trajecto-ries of the plastic balls. Seven individual building clusters werephotographed. A Canon SX110 camera was placed 2 m abovethe model at various locations. The camera was set with an expo-sure time of 0.3 s, which was appropriate for the lighting in thelaboratory.

Repeatability

To ensure that the flow entering the residential model was steady,and that the experiment was repeatable, multiple velocity measure-ments were made along the breach and the velocity profiles werecompared. Figs. 3(a and b) show the velocity profiles at two differ-ent grid locations in the ditch (scour hole area) for two differenttrials from separate runs for a flow rate of 95 L=s. Figs. 3(c and d)show the same comparison as a scatter plot. The plots show a closematch between the trials, confirming the steadiness of the flow andthe reliability of UVP as a velocity measurement instrument. Thesmall differences observed in these plots were likely associatedwith random fluctuations in the flow field as a result of the complextopography at the measurement locations.

Results

This section first presents a general description of the flowfield obtained by using DPTV and water depth measurements.The velocity profiles along the breach measured by using UVPare presented, followed by detailed velocity field around individualbuildings and building clusters, along with path lines and long-exposure images. Based on a calibration curve developed bySattar et al. (2008) and the high water mark (USACE 2007), theestimated maximum breach flow rate at the prototype scale is1,061 m3=s. The IPET report (USACE 2007) also provides anestimated peak discharge of 821.2 m3=s. The tests were conductedfor the flow rate of 57 L=s, which corresponds to a flow rate of1,000 m3=s in the prototype. A limited number of measurementswere made with a flow rate of 95 L=s to study the consistency ofthe flow field under different flow rates. Unless otherwise specified,the results presented in the following correspond to a flow rateof 57 L=s.

Description of the Flow Field

Fig. 4 shows the water depth in the model under steady flow con-ditions. There were two high areas in the vicinity of the ditch: oneon the north side of the eastward breach and one on the west side ofBuildings G4 and G5. The water depths on these high areas weresmall. Also, the water depths near the houses on the east side of themodel were shallower than those near the houses on the west sidebecause the houses acted as barriers to the flow.

Fig. 5(a) shows the path lines obtained from the trajectories ofplastic balls in the entire model. This plot shows several distinctflow regions: (1) uniform flow in the canal up to Grid X9; (2) 2Dflow toward the breach with a strong y-component from Grids X10to X27; (3) a dead recirculating zone attributable to the closed gateat the downstream end of the canal spanning from Grid X16 toX35; (4) a complex flow field in the urban area with flow movingprimarily in east, north, and south directions. The focus of thisstudy is in the flooded urban area. Several important observationscan be made from the path lines in the urban area: (1) closelyspaced Buildings G2–G5 and Buildings G7–G10 acted like a singleobstacle to the flow; (2) zones of small velocities and recirculationoccurred near the tip of the upstream eastern levee, and at somelocations between the buildings; (3) the flow toward the northeastcorner was small; (4) buildings close to the breach, i.e., G1–G9,were impacted more by the flow, and the dominant mode of hydro-dynamic force was in the form of shear; (5) zones of no flow werein the area bounded by Grids X12–X18 and Y8–Y10 and thatbounded by the grid X26–X27 and Y9–Y11 because of highelevation.

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Fig. 3. Comparison of measured velocity in the ditch between two different trials at the following grid locations: (a) X21–Y9; (b) X22–Y9;(c) X21–Y9; (d) X22–Y9

Fig. 4. Contour map of the water depth in (mm) for a flow rate of 57 L=s

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To illustrate the effect of buildings on the breach flow distribu-tion in the model, an experiment was run by removing the buildingsfrom the model. The surface velocity was measured in the canal andthe flooded area close to the breach. Fig. 5(b) shows the path linesfor this case. In the absence of significant topographic variation inthe model urban area, the breach flow would be in the southeastdirection. However, similar to Fig. 5(a), this figure shows that partof the flow turned north.

The surface velocity map obtained from PTV measurements isshown in Fig. 6. The highest velocity (∼1 m=s) occurred in theurban area, not in the canal. Throughout the urban area, manyhigh velocity regions can be observed. However, the flow pri-marily accelerated around Building Cluster G7–G10. The velocityclose to the gray houses was higher than that near the blackhouses, which were farther from the breach. Low velocity zonescan be observed on topographic highs and in wake regions

B4 B12E

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Fig. 5. Path lines of the plastic balls for the flow rate of 57 L=s: (a) with buildings; (b) without buildings

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located near downstream faces of several buildings. The velocityin the y-direction was dominant along the entire crest of the leveebreach.

The distribution of outflow along different sides of the floodedarea was estimated using the trapezoidal rule, by multiplyingthe measured surface velocity by the measured water depth.Figs. 5(a and b) show numbered sections marked by dashed lineswhere the rates of outflow are calculated. Table 1 shows the per-centages of the flow rate along with the standard deviation foreach marked section. For this case, the flow rate on the south side(Sections 7 and 8) was much higher than that on the opposite side(Sections 1 and 2), which is expected as a result of the inertia of thebreach outflow. Almost half of the breach flow was directed towardthe south side of the neighborhood. The northeast side had the low-est flow rate (Sections 2 and 3). The reason for the high flow rate(19%) on the southeastern side was the presence of the ditch thatcaused some of the flow to this side. The overall flow rates throughthe north (Sections 1 and 2), east (Sections 3 to 6), and south sides(Sections 7 and 8) of the model were 18� 1, 41� 4, and 41� 4%,respectively. Similar outflow rates were observed for the case with-out buildings. The overall flow rates through the north (Sections 1),east (Sections 2), and south sides (Sections 3) shown in Fig. 5(b)of the model were 17� 2, 41� 4, and 42� 0.1%, respectively.According to these results, the topography of the model is the pri-mary reason for distributing the flow rates, whereas the buildingsplayed a role in redistributing the flow within the flooded area.

Velocity Profiles near the Breach

Velocity profiles in the ditch close to the breach for grid locations(X12,Y9)–(X25,Y9) were obtained by a UVP probe. Fig. 7 showsthe velocity profiles along the probe direction on the x-z and y-zplanes, along with the water surface elevation. The figure showsthat the resultant velocity in the x-z plane at most of the locationsis higher than that in the y-z plane because of the inertia of the flowleaving the canal through the breach and the north–south orienta-tion of the ditch. Because of the 3D nature of the flow field in the

ditch, the velocity profiles at several locations strongly deviatedfrom the typical logarithmic profile of steady uniform open channelflow. In particular, the resultant velocity in the y-z plane is zero overan extended distance from the bed at several locations. The subcriti-cal response of the flow to the topographic variation is clearlyshown in this figure because the water surface variation is out ofphase with the bed perturbation. The surface velocity at each loca-tion is superimposed on the figure as a black dot, which closelyagrees with the velocity profile obtained from UVP measurements.

Flow Field around Buildings

One important advantage of using UVP is that the probe measuresvelocity along a line, instead of a point. The relatively small size ofthe probe and the small footprint were helpful for measuring veloc-ities around most of the buildings. Because the water depths aroundthe buildings were relatively shallow, these were measured in ahorizontal plane 2 cm above the bed and the 2D measurementswere combined to construct the velocity vectors. The data werecomplemented by images obtained from long-exposure still imagesand path lines obtained from videos of the motion of hollow plasticballs. The flow field around individual buildings and buildingclusters is discussed in the following paragraphs.

Figs. 8(a and b) show the velocity vectors and path lines, respec-tively, superimposed on a still photo around Building G1. Thisbuilding is located near the exit of the flooded area on the upstreamside. The velocity vectors show that the flow was parallel to thebuilding wall on the west and east sides. The photo and path linesalso display the same trend. UVP measurements were not made onthe south side of the building because the water depth was less than2 cm. The flow visualization also shows low velocity regions onboth the north and south sides of this building. The north sideis the wake region of Building G1 and the south side is the wakeregion of Buildings G2–G4, both showing flow circulation. Despitesufficient gap between Buildings G2 and G4, the flow was stagnantbetween these buildings.

Figs. 8(c and d) show the velocity vectors around Building Clus-ter G2–G4. The flow approached the southwest corner of BuildingG4 at an angle of approximately 45° and split around the corner.The majority of the flow passed along the west side of the cluster,while part of the flow moved parallel to the south side of BuildingG4. Some of the flow entered the narrow spaces between buildings,but did not exit with the same flow intensity as that on the east side,as indicated by the velocity vectors and buildup of coal tracers.

Table 1. Distribution of Outflow

Section 1 2 3 4 5 6 7 8

Qbreach (%) 13 5 3 3 19 15 24 18SD (σ) 3.4 1.5 1.1 1.3 0.1 4.2 2.1 2.1

Fig. 6. Magnitude of surface velocity for the flow rate of 57 L=s

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UVP measurements were not made on the south side of BuildingG4 because of its close proximity to Building G5 and G6. However,from the path lines, it is clear that the flow in this region was domi-nantly in the y-direction, with higher intensity near the wall.

Figs. 8(e and f) show the velocity vectors and flow pattern, re-spectively, around Buildings G5 and G6. The approaching flowstagnated on the west faces of the building. The velocity vectorsindicate that the diverted flow was strongest near the south faceof Building G6. The visualizations also corroborate this. Flow cir-culation can be observed on the north and east sides of BuildingG5, whereas reduced flow can be observed on the east side ofBuilding G6.

UVP measurements were made around Buildings G7–G10for two different flow rates of 57 and 95 L=s to study the consis-tency of the flow field under different flow rates. A comparison ofthe velocity vectors for these flow rates is shown in Figs. 9(a and b).The figure shows that the orientations of the vectors are identicalin the two cases, although the magnitudes differ. The average uvelocity levels on the west (levee) side of the buildings are52.6 and 70.4 cm=s, respectively, for the flow rates of 57 and95 L=s. However, the average magnitude of the same is muchsmaller on the east (far) side, with values of 17.3 and23.5 cm=s, respectively.

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Fig. 7. Velocity readings measured along the length of the levee breach (Axis Y9) from Grids X12 to X25: (a) resultant velocity in x-z plane;(b) resultant velocity in y-z plane

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On the levee side, the flow direction remained mostly parallel tothe buildings, with water entering the area between the buildings.On the far side, the flow remained mostly separated from the build-ings (also shown in Figs. 5 and 6).

Figs. 10(a and b), show the velocity vectors and flow patternaround Buildings B3–B7, respectively. The presence of these build-ings caused large disturbances to the flow coming from the leveeside. Building B7 was directly on the path of the flow and the path

Fig. 8. Flow around upstream gray buildings G1–G6 for flow rate of 57 L=s (images by authors): (a) Building G1; (b) photo and surface trajectories;(c) Buildings G2–G4; (d) photo and surface trajectories; (e) Buildings G5–G6; (f) photo and surface trajectories

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lines show a classic case of flow around a cube. Part of the flowpassed between Buildings B6 and B7 and between B5 and B6, butmost of the flow was along the south side of B7.

Figs. 11(a and b) show the velocity vectors, flow path lines andvisualization near Buildings B8 and B9. Buildings B8 and B9 werelocated on the downstream or south side of the flooded area. Theflow approached Building B8 from the west side and was redirectedto the north and south sides. Similar to Building B7, most of thewest face of this building was directly on the flow path, and thus,experienced stagnation pressure. The velocity was much higher onthe north side of Buildings B8 and B9 than on the south side be-cause of the influence of the eastward arm of the ditch.

Velocity vectors and flow path lines imposed on an image aroundBuildings B10, B11, and B12 are shown in Figs. 11(c and d). Themajority of the flow entered the northwest corner of B10 and wasredirected to the north and west sides. Velocity could not be mea-sured on the south side of the Building B10 because the water wastoo shallow.

Discussion

Experiments by others on dam-break flows have shown thatduring a flood, the severe transient phase is followed by a slow

Fig. 10. Velocity vectors 0.02 m from the bottom and flow path lines around buildings B3–B7 (image by authors): (a) Buildings B3–B7; (b) photoand surface trajectories

Fig. 9. Velocity vectors measured at 0.02 m from the bottom around Buildings G7 to G10 for both 57 and 95 L=s: (a) Buildings G7–G10 for 57 L=s;(b) Buildings G7–G10 for 95 L=s

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transient that is, in some aspects, close to steady flow conditions(Soares-Frazao and Zech 2008). The present study was conductedunder a steady flow condition. As such, the severe transient phaseof the flooding was not captured. However, the observations canbe helpful for guiding emergency response at the time of floods,especially for levee breach closure. The amount of breach dis-charge flowing to the south side was almost twice that flowingto the north side in the present case. The houses also acted as afirst barrier to slow down the flow and increase the water level onthe levee side of the flooded area. The low velocity regions suchas areas between buildings or the upstream side of the breach canbe the first targeted for placing sandbags to reduce the velocity ofthe breach outflow, making it easier to eventually close thebreach.

In this study, pressure forces on the building were not measured.However, the measured flow field can be utilized to estimate andcompare the hydrostatic and hydrodynamic pressure forces on thebuildings. Hydrostatic forces occur when slow floodwater comesinto contact with a building. Hydrodynamic forces are induced

by moving floodwater or storm surges (Caraballo-Nadal et al.2006). These forces are a function of floodwater velocity and build-ing geometry. The hydrodynamic pressure forces on some of thebuildings were calculated by using the normal velocity averagedover the impacted face of the building. The following relations wereused:

FD ¼ 0.5CDρU2A ð3Þ

FS ¼1

2γhA ð4Þ

where FD = hydrodynamic force acting on the building face; FS =hydrostatic force acting on the building face; U = average surfacevelocity in the normal direction; h = average depth near the build-ing face; CD = drag coefficient, which typically ranges from 1.25 to2.0 (FEMA 1986). Here, a value of 1.5 was selected for CD. Basedon measurements and visualization, only a limited number of build-ing faces directly obstructed the flow, and thus, experienced hydro-dynamic pressure forces. These are houses labeled G5, G6, B6, B7,

Fig. 11. Velocity vectors 0.02 m from the bottom and flow path lines around Buildings B8–B12 for 57 L=s (images by authors); (a) BuildingsB8–B9; (b) photo and surface trajectories; (c) Buildings B10–B12; (d) photo and surface trajectories

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B8 and G7. Table 2 shows the calculated values of FD and the front(west), FSf, and back (east), FSb.

Buildings B8 and G7 were on the path of the dominant flowdirection and experienced the highest amount of hydrodynamicand hydrostatic forces. Although Buildings B6 and B7 were inclose proximity, the dynamic force on B7 was almost four timesthat on B6. Building B7 also had a higher hydrostatic force as aresult of its proximity to the eastward branch of the ditch, witha bed elevation 7.0 cm lower than the average bed level. The ratioof the dynamic force to the static force is very small, with an aver-age value of 1.6%, which indicates that once the initial flood wavepasses, the static force may be more important than the dynamicforce.

Recent flooding in New Jersey and New York caused by stormSandy in late October of 2012, and the resulting damage, tends toconfirm the findings of this research concerning flow distribution inurban areas. The present and relevant future studies will be helpfulfor emergency preparedness and for first responders.

Conclusions

This experimental investigation provided a detailed description ofthe flow field on a scale model of the area near the 17th Street Canallevee breach in New Orleans, Louisiana. A data set for the flowfield in the flooded urban area with irregular building patternsand a complex topography has been compiled. The measurementtechniques and the important flow features are described. Velocityprofiles along the breach, flow path lines, and visualization with theaid of tracers reveal a complex flow field in the model urban area.Wakes behind the buildings, zones of low velocity with recir-culation, and flow separation at building corners were observed.Closely spaced building clusters acted similar to single obstruc-tions, with a common wake zone and little flow in the area betweenthe buildings. Only a few building faces were on the direct path ofthe flow. The depth also varied considerably between the backs andfronts of buildings. The experiment was conducted under roughturbulent flow. Therefore, the results can be upscaled. However, inthe absence of prototype flow data, the result should be interpretedas a general description of the flow field, not a precise representa-tion. The data set is intended for the verification of mathematicalmodels for urban areas and for numerical models of flow with freesurface variation. The data may be downloaded from http://pire.ce.sc.edu.

Acknowledgments

Funding from National Science Foundation under the PIRE pro-gram, Grant No. OISE 0730246 is gratefully acknowledged. CyrusRiahi-Nehzad provided assistance during the measurements. Threeanonymous reviewers and the Associate Editor provided helpfulcomments.

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