US Army Corps of Engineers Trinity Floodway Report Appendix G

8/14/2019 US Army Corps of Engineers Trinity Floodway Report Appendix G

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APPENDIX G

FLOOD 2007GEOTECHNICAL TRIP REPORT


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CESWF-EC-DS 07 July 2007BRANCH/1703

MEMORANDUM FOR RECORD

SUBJECT: Dallas Floodway, Trip Report for Inspections on 3 & 5 July 2007

1. GENERAL INFORMATION. On 2 July 2007, the USACE PM for the Dallas Floodwayproject, Mr. Gene Rice, informed me that he had been notified by the City of Dallas thatseveral slides had developed along the Dallas Floodway levees. Since I had a tripscheduled the next day to observe damage to the Dallas Floodway Lower Chain of Wetlands

construction project, I told Mr. Rice that I would stop by and inspect the slides at the leveesas well.

At 11:30 on 3 July 2007, I met with the Dallas Flood Control Division (FCD) Manager, Mr.David Garcia; and the Assistant Superintendent of the Street Service Department, Mr. KellyHigh. Mr. Garcia and Mr. High confirmed that there were slides of the levees along the ElmFork and along the main stem of the Trinity River and proceeded to take me to each of theeleven slides they had identified. Figure 1 shows the locations of these slides.


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CESWF-EC-DSSUBJECT: Dallas Floodway, Trinity River Levee Slide Inspection Trip Report

2. CHARACTERIZATION of SLIDES.

a. Elm Fork. Five shallow slides are located along the Elm Fork of the Trinity River. Eachof these slides is located on the wet side of the levee. The most significant slide, Slide #1(see Photos 1 and 2), is located upstream of the railroad bridge. Approximately 170-feetlong and as much as 10-feet deep, the toe of Slide #1 exits below the river surface(approximate Elevation 405 at the time of the inspection).

Photo 1, 01 July 2007, Slide 1. The City of Dallas Flood Control District works to maketemporary repairs to the most serious of the levee slides. Select backfill mixed withapproximately 8 to 10% lime (by volume) was mixed offsite and then placed at the top ofthe scarp by backhoe. (Photo courtesy of the Dallas Flood Control District)


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Photo 2a, 03 July 2007, Slide 1. This photowas taken less than 24 hours after the repairsshown in Photo 1 were made. Placement of the

fill material on top of the already failed soilsadded weight to the failed mass and drove itfurther downslope. The top of the fill materialused for the attempted repair is approximately 3-feet below the levee crest.

Photo 2b, 03 July 2007, Slide 1. View of theleft side of the scarp looking upslope. Theentire slide was covered with black plastic after

the inspection to limit infiltra tion of additionalpredicted rainfall.

Photo 3, 01 July 2007. Aerial view ofSlides 2 (background) and 3 (foreground).Both slides are shallow and exit below theriver level. (Photo courtesy of the DallasFlood Control District)


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Photo 4a, 01 July 2007, Slide 3. Aerial view ofSlide #3. (Photo courtesy of the Dallas FloodControl District)

Photo 4b, 01 July 2007, Slide 4. Aerial viewof Slide #4, located just upstream of the StateRoute #183 Bridge. (Photo cour tesy of the

Dallas Flood Control District)

b. Main Stem Trinity River, West Levee. Five slides are located on the West Levee alongthe main stem of the Trinity River. Each of these slides is located on the wet side of thelevee. As on the Elm Fork, these slides are shallow (all are less than 9-feet deep) withwidths ranging from approximately 90 to 120-feet.

Photo 5a, 01 July 2007. Aerial view of Slides 6 Photo 5b, 03 July 2007, Slide 7. Close-up of


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Similar slides have occurred at the Coombs Gate location before, and it is reported that thelast slide repair included the drilling of a pier to provide support for the bridge. This will beverified before the repair of this slide is implemented. As shown in Photo 7, this slidethreatens the bridge abutment. If a deep foundation has not been provided, failure of thebridge could occur if the slide progresses.

Photo 6, 01 July 2007, Slide 10. Aerial view of the slide at the Coombs Gate structure.(Photo courtesy of the Dallas Flood Control District)


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Photo 7, 03 July 2007, Slide 10. Close-up of the slide at the Coombs Gate. The fai luresurface of the slide intersects the bridge seat. It is reported that a pier wasincorporated into the bridge foundation the last time a slide in this area was repaired


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3. TRIP FINDINGS.

a. Project Conditions. In spite of the numerous shallow slides that have developed, the

levees are in good condition overall. Due to heavy precipitation over the past severalmonths, the uppermost foot of the levee surface (side slopes and crest) is saturated and verysoft. This makes access for inspection and maintenance difficult and dangerous, and willlikely contribute to the development of more shallow slides along the floodway.

b. Directions to FCD. Attempted repairs made the day before the inspection had alreadyfailed (see Photo 1) so we requested that the FCD cease to make temporary repairs by

Photo 8, 03 July 2007, Slide 11. Slide #11 is located on the East Levee. Thepartially filled sump is in the foreground, with the main stem Trinity River in thebackground. (Photo courtesy of the Dallas Flood Control District)


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The FCD agreed to this request. In a follow-up call to David Garcia the morning of 4 July2007 to check on the condition of the project, Mr. Garcia confirmed that they hadsuccessfully completed or were in the process of completing:

vegetation removal with weed-eaters (50% complete)

covering the slides with plastic (80% complete)

locating a potential borrow source (completed: 5,000 CY of borrow was located at aconstruction site in nearby Seagoville)

daily inspections (ongoing)

c. Aerial Surveillance. The levees were last mowed in May 2007. Heavy rains since thenhave accelerated vegetation growth and made it unsafe to mow again. The high vegetationmakes ground inspection of the project and identification of slides very difficult, so the FCD iscommended for coordinating with the Dallas Police Department Helicopter Unit to provideaerial surveillance during this time.

d. USACE Follow-UpInspection. On 5 July2007, Mr. Les Perrin(CESWF-EC-DG), Mr.Mark Sissom (CESWF-OD) and the undersignedmade a brief inspectionof the floodway,observing conditions atSlide #4 (see Photo 9),

and the Coombs Gateslide, Slide #10 (seePhotos 6 and 7). Duringthe inspection, the FCDcrew reported that twoadditional slides haddeveloped along the ElmFork of the Trinity,approximately 200 yards

downstream of Slide #4(see Figure 3). We wereunable to inspect thesenew slides due to heavyvegetation growth.

Photo 9, 05 July 2007, Slide 4. The FCD had done a good jobcovering up the slides with plastic to stop further infiltration fromheavy rains that moved into the area the morning of 5 July. Totalreported rainfall on this date was 0.88-inch.


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While the overall performance of the modified levees has been good, hundreds of shallowinfinite slope failures requiring repair have developed in the highly plastic clay embankments.Records maintained by the Dallas Flood Control District (FCD) show that more than 300such slides have been repaired since 1966, so the current slides are neither unprecedentednor unexpected. Most of these slides have occurred in the same general areas that slidesare currently being observed: on the East Levee along the Elm Fork and on the West Levee.It is interesting to note that nineteen shallow slides are clearly visible in the Google Earthsatellite imagery (see Figure 4). That imagery is believed to have been obtained during2004, another year in which precipitation was substantially above normal.

b. Comparison of the Current High Water Event with Previous High Water Events.Since the Dallas levees were modified by USACE in the 1950s, the Flood of Record forthe Dallas Floodway occurred on 3 May 1990 when a discharge of 82,300 cfs wasrecorded. Although flood levels were high, the maximum flood height for this event,Elevation 415.1 (gauge height of 47.1-feet), was more than 14-feet below the design crestelevation. The Flood of Record has an estimated probability of annual occurrence of 0.022,roughly equivalent to a 45-year storm. Photo 10 shows the Flood of Record height relativeto the top of the East Levee, while Photo 11 shows the flood height existing along the East

and West Levees during this current flood event. The location of the jail is annotated in bothphotos for reference purposes.

Jail


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Jail

Photo 11, 01 July 2007. View is looking downstream along the mainstem Trinity River.The river level at this time was approximately Elevation 404.5 (gauge height of 36.5-feet), more than10-feet below the record high river stage shown in Photo 10. The current event is less than a 10-year storm. (Photo cour tesy of the Dallas Flood Control District)

5. INSPECTION INFORMATION.

a Weather During Inspections.

1. 3 July 2007. During the initial site inspection, the weather was humid and overcast,

with occasional sunshine and a temperature of approximately 85. There was noprecipitation during the site visit.


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c. Gauge Readings During Inspections. Gauge readings from the Commerce StreetGauge at the beginning and end of each inspection were obtained from the USGS website(http://waterdata.usgs.gov/nwis) and are provided in Table 1. These readings are availablein real-time, and had not been corrected by the USGS at the time of this report. Plots of theuncorrected data since 15 June 2007 are provided as Figure 2.

Date / Time Gauge Reading Elevation (NGVD29) Discharge (CFS)

03 July 2007 / 11:30 36.42 404.44 23,800

03 July 2007 / 16:30 36.48 404.50 24,000

05 July 2007 / 10:30 36.24 404.26 23,200

05 July 2007 / 13:00 36.35 404.37 23,600

Table 1. Uncorrected tabular Commerce St. gauge data (source:http://waterdata.usgs.gov/nwis)


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d. Photographs. Although high vegetation obscured most of the slides, making it difficult toobtain good record photos from the ground, a RICOH camera with GPS was used todocument and map the location of each slide. Additionally, the Dallas Flood Control Districtprovided aerial photographs for our use. Aerial and land photos are available on the swf-fs1bgis server:

(\\Swf-fs1bgis\userdirs\Geotechnical Section\Photos\Civil Projects\Dallas Floodway).

6. CONCLUSIONS.

a. Although the number of slides is substantial, none of the slides is considered to present ahigh risk of failure of the Floodway at this time. The most serious slide (Slide #1) is locatedon the Elm Fork. This slide was ~170' long at the time of the inspection, and wasapproximately 10' deep. The failure surface intersected the embankment crest, and exitedbelow the water surface. Because the cross-section of the levee in this area is slightly widerdue to the location of the levee access road, the loss of a portion of the crest is not criticalunless the river level rises to SPF level (more than 25-feet above the current river stage).

Under normal conditions, the slides that have developed along the Floodway would beconsidered a recurring maintenance issue; however, due to the continuing heavyprecipitation, the situation is considered to be serious. It is not, however, dangerous orcritical at this time. CESWF will continue to assist the Dallas Flood Control District inmonitoring the floodway and will provide guidance for additional corrective actions ifwarranted.

b. Additional shallow slides on the levees should be expected.

c. Repairs of the slide at the Coombs Gate should include evaluation of the foundation ofthe service bridge.

Anita Branch, PECivil EngineerCESWF-EC-DG


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Dallas Floodway, Trinity River Levee Slide Inspection Trip Report Page 13 of 14Figure 3. Larger View of the Slide Location Map presented in Figure 1 Showing Location of New Slides

ApproximateLocation ofNew Slides


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Dallas Floodway, Trinity River Levee Slide Inspection Trip Report Page 14 of 14Figure 4. Location of 19 Slides Clearly Evident on the Google Earth Satellite Imagery

US Army Corps of Engineers Trinity Floodway Report Appendix G

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