Us Army Corps Of Engineers Trinity Floodway Report Appendix G
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APPENDIX G FLOOD 2007 GEOTECHNICAL TRIP REPORT
CESWF-EC-DS
07 July 2007 BRANCH/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 Floodway project, Mr. Gene Rice, informed me that he had been notified by the City of Dallas that several slides had developed along the Dallas Floodway levees. Since I had a trip scheduled 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 levees as 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. Kelly High. Mr. Garcia and Mr. High confirmed that there were slides of the levees along the Elm Fork and along the main stem of the Trinity River and proceeded to take me to each of the eleven slides they had identified. Figure 1 shows the locations of these slides.
Figure 1. GOOGLE EARTH aerial view with locations of slides observed on 3 July 2007.
CESWF-EC-DS SUBJECT: 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. Each of 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-feet long 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 make temporary repairs to the most serious of the levee slides. Select backfill mixed with approximately 8 to 10% lime (by volume) was mixed offsite and then placed at the top of the scarp by backhoe. (Photo courtesy of the Dallas Flood Control District)
Due to the access road down into the floodway, the levee cross-section in the area of Slide #1 is slightly larger, so even though this slide removed a small portion of the levee’s crest (approximately 1-foot), the reduction in the crest width is not considered critical.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Photo 2a, 03 July 2007, Slide 1. This photo was taken less than 24 hours after the repairs shown in Photo 1 were made. Placement of the fill material on top of the already failed soils added weight to the failed mass and drove it further downslope. The top of the fill material used for the attempted repair is approximately 3feet below the levee crest.
Photo 2b, 03 July 2007, Slide 1. View of the left side of the scarp looking upslope. The entire slide was covered with black plastic after the inspection to limit infiltration of additional predicted rainfall.
Photo 3, 01 July 2007. Aerial view of Slides 2 (background) and 3 (foreground). Both slides are shallow and exit below the river level. (Photo courtesy of the Dallas Flood Control District)
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Photo 4a, 01 July 2007, Slide 3. Aerial view of Slide #3. (Photo courtesy of the Dallas Flood Control District)
Photo 4b, 01 July 2007, Slide 4. Aerial view of Slide #4, located just upstream of the State Route #183 Bridge. (Photo courtesy of the Dallas Flood Control District)
b. Main Stem Trinity River, West Levee. Five slides are located on the West Levee along the main stem of the Trinity River. Each of these slides is located on the wet side of the levee. As on the Elm Fork, these slides are shallow (all are less than 9-feet deep) with widths ranging from approximately 90 to 120-feet.
Photo 5a, 01 July 2007. Aerial view of Slides 6 (right), 7 (center) and 8 (left). (Photo courtesy of the Dallas Flood Control District)
Photo 5b, 03 July 2007, Slide 7. Close-up of the top of the scarp for Slide #7. This scarp is approximately 90-feet wide and 7-feet deep.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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)
Similar slides have occurred at the Coombs Gate location before, and it is reported that the last slide repair included the drilling of a pier to provide support for the bridge. This will be verified before the repair of this slide is implemented. As shown in Photo 7, this slide threatens the bridge abutment. If a deep foundation has not been provided, failure of the bridge could occur if the slide progresses.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Photo 7, 03 July 2007, Slide 10. Close-up of the slide at the Coombs Gate. The failure surface of the slide intersects the bridge seat. It is reported that a pier was incorporated into the bridge foundation the last time a slide in this area was repaired. This will be verified before implementation of repairs to this slide.
c. Main Stem Trinity River, East Levee. Slide #11 is located on the East Levee. Unlike the other slides, this slide was located on the dry side of the levee, inside the partially filled sump area.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Photo 8, 03 July 2007, Slide 11. Slide #11 is located on the East Levee. The partially filled sump is in the foreground, with the main stem Trinity River in the background. (Photo courtesy of the Dallas Flood Control District)
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 several months, the uppermost foot of the levee surface (side slopes and crest) is saturated and very soft. This makes access for inspection and maintenance difficult and dangerous, and will likely contribute to the development of more shallow slides along the floodway. b. Directions to FCD. Attempted repairs made the day before the inspection had already failed (see Photo 1), so we requested that the FCD cease to make temporary repairs by adding fill to the failure surface, and that they: • • • •
use weed-eaters to cut vegetation in the slide areas to improve ground surveillance cover the slides with plastic to prevent further infiltration of rainfall locate a potential borrow source in the event that the situation continues to worsen so that additional fill can be added to the dry slide of the levee if warranted make daily inspections of the slide areas and dry side toe to see if seepage problems are developing, with additional inspections after any heavy downpours
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 7 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report The FCD agreed to this request. In a follow-up call to David Garcia the morning of 4 July 2007 to check on the condition of the project, Mr. Garcia confirmed that they had successfully 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 a construction site in nearby Seagoville) daily inspections (ongoing)
c. Aerial Surveillance. The levees were last mowed in May 2007. Heavy rains since then have accelerated vegetation growth and made it unsafe to mow again. The high vegetation makes ground inspection of the project and identification of slides very difficult, so the FCD is commended for coordinating with the Dallas Police Department Helicopter Unit to provide aerial surveillance during this time.
Photo 9, 05 July 2007, Slide 4. The FCD had done a good job covering up the slides with plastic to stop further infiltration from heavy rains that moved into the area the morning of 5 July. Total reported rainfall on this date was 0.88-inch.
d. USACE Follow-Up Inspection. On 5 July 2007, Mr. Les Perrin (CESWF-EC-DG), Mr. Mark Sissom (CESWFOD) and the undersigned made a brief inspection of the floodway, observing conditions at Slide #4 (see Photo 9), and the Coombs Gate slide, Slide #10 (see Photos 6 and 7). During the inspection, the FCD crew reported that two additional slides had developed along the Elm Fork of the Trinity, approximately 200 yards downstream of Slide #4 (see Figure 3). We were unable to inspect these new slides due to heavy vegetation growth.
4. HISTORICAL PERSPECTIVE. a. Levees. Originally constructed by the City of Dallas in the 1930’s with 2.5H:1V side slopes, and a maximum height of 35-feet; the levee system was modified by USACE in the late 1950’s by flattening the side slopes from between 3H:1V to 4H:1V, and increasing the crest width from 6 to 16-feet.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 8 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report While the overall performance of the modified levees has been good, hundreds of shallow infinite 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 300 such slides have been repaired since 1966, so the current slides are neither unprecedented nor unexpected. Most of these slides have occurred in the same general areas that slides are 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 Earth satellite imagery (see Figure 4). That imagery is believed to have been obtained during 2004, 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 1950’s, the Flood of Record for the Dallas Floodway occurred on 3 May 1990 when a discharge of 82,300 cfs was recorded. 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 crest elevation. 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 relative to 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 both photos for reference purposes.
Jail
Photo 10. Photo was taken during the modern Flood of Record, 3 May 1990. View is of the downtown area along the mainstem Trinity River, looking to the North-Northwest. Upstream is to the left. The maximum flood height for this event was Elevation 415.1 (gauge height of 47.1-feet), roughly equivalent to a ’45-year’ storm.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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 than 10-feet below the record high river stage shown in Photo 10. The current event is less than a ’10year’ storm. (Photo courtesy 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 no precipitation during the site visit. 2. 5 July 2007. Weather during the follow-up site visit was overcast with heavy rain. The temperature was approximately 85°. b. Antecedent Conditions. Thus far this year, precipitation for 2007 is nearly 13-inches above normal. After two years of serious drought in 2005 and 2006, this amount of rainfall is more than sufficient to fill cracks that developed in the levee embankments during the drought period. This makes it likely that shallow slides will continue to develop throughout the Floodway.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 10 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report c. Gauge Readings During Inspections. Gauge readings from the Commerce Street Gauge 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 available in real-time, and had not been corrected by the USGS at the time of this report. Plots of the uncorrected 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)
Figure 2. Uncorrected (i.e. ‘provisional’) gauge data from the Commerce Street Gauge (source: http://waterdata.usgs.gov/nwis).
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report d. Photographs. Although high vegetation obscured most of the slides, making it difficult to obtain good record photos from the ground, a RICOH camera with GPS was used to document and map the location of each slide. Additionally, the Dallas Flood Control District provided aerial photographs for our use. Aerial and land photos are available on the swffs1bgis 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 a high risk of failure of the Floodway at this time. The most serious slide (Slide #1) is located on the Elm Fork. This slide was ~170' long at the time of the inspection, and was approximately 10' deep. The failure surface intersected the embankment crest, and exited below the water surface. Because the cross-section of the levee in this area is slightly wider due to the location of the levee access road, the loss of a portion of the crest is not critical unless 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 be considered a recurring maintenance issue; however, due to the continuing heavy precipitation, the situation is considered to be serious. It is not, however, dangerous or critical at this time. CESWF will continue to assist the Dallas Flood Control District in monitoring the floodway and will provide guidance for additional corrective actions if warranted. 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 of the service bridge.
Anita Branch, PE Civil Engineer CESWF-EC-DG
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 12 of 14
Approximate Location of New Slides
Figure 3. Larger View of the Slide Location Map presented in Figure 1 Showing Location of New Slides Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 13 of 14
Figure 4. Location of 19 Slides Clearly Evident on the Google Earth Satellite Imagery Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 14 of 14
CESWF-EC-DS
07 July 2007 BRANCH/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 Floodway project, Mr. Gene Rice, informed me that he had been notified by the City of Dallas that several slides had developed along the Dallas Floodway levees. Since I had a trip scheduled 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 levees as 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. Kelly High. Mr. Garcia and Mr. High confirmed that there were slides of the levees along the Elm Fork and along the main stem of the Trinity River and proceeded to take me to each of the eleven slides they had identified. Figure 1 shows the locations of these slides.
Figure 1. GOOGLE EARTH aerial view with locations of slides observed on 3 July 2007.
CESWF-EC-DS SUBJECT: 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. Each of 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-feet long 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 make temporary repairs to the most serious of the levee slides. Select backfill mixed with approximately 8 to 10% lime (by volume) was mixed offsite and then placed at the top of the scarp by backhoe. (Photo courtesy of the Dallas Flood Control District)
Due to the access road down into the floodway, the levee cross-section in the area of Slide #1 is slightly larger, so even though this slide removed a small portion of the levee’s crest (approximately 1-foot), the reduction in the crest width is not considered critical.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 2 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Photo 2a, 03 July 2007, Slide 1. This photo was taken less than 24 hours after the repairs shown in Photo 1 were made. Placement of the fill material on top of the already failed soils added weight to the failed mass and drove it further downslope. The top of the fill material used for the attempted repair is approximately 3feet below the levee crest.
Photo 2b, 03 July 2007, Slide 1. View of the left side of the scarp looking upslope. The entire slide was covered with black plastic after the inspection to limit infiltration of additional predicted rainfall.
Photo 3, 01 July 2007. Aerial view of Slides 2 (background) and 3 (foreground). Both slides are shallow and exit below the river level. (Photo courtesy of the Dallas Flood Control District)
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Photo 4a, 01 July 2007, Slide 3. Aerial view of Slide #3. (Photo courtesy of the Dallas Flood Control District)
Photo 4b, 01 July 2007, Slide 4. Aerial view of Slide #4, located just upstream of the State Route #183 Bridge. (Photo courtesy of the Dallas Flood Control District)
b. Main Stem Trinity River, West Levee. Five slides are located on the West Levee along the main stem of the Trinity River. Each of these slides is located on the wet side of the levee. As on the Elm Fork, these slides are shallow (all are less than 9-feet deep) with widths ranging from approximately 90 to 120-feet.
Photo 5a, 01 July 2007. Aerial view of Slides 6 (right), 7 (center) and 8 (left). (Photo courtesy of the Dallas Flood Control District)
Photo 5b, 03 July 2007, Slide 7. Close-up of the top of the scarp for Slide #7. This scarp is approximately 90-feet wide and 7-feet deep.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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)
Similar slides have occurred at the Coombs Gate location before, and it is reported that the last slide repair included the drilling of a pier to provide support for the bridge. This will be verified before the repair of this slide is implemented. As shown in Photo 7, this slide threatens the bridge abutment. If a deep foundation has not been provided, failure of the bridge could occur if the slide progresses.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 5 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Photo 7, 03 July 2007, Slide 10. Close-up of the slide at the Coombs Gate. The failure surface of the slide intersects the bridge seat. It is reported that a pier was incorporated into the bridge foundation the last time a slide in this area was repaired. This will be verified before implementation of repairs to this slide.
c. Main Stem Trinity River, East Levee. Slide #11 is located on the East Levee. Unlike the other slides, this slide was located on the dry side of the levee, inside the partially filled sump area.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 6 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Photo 8, 03 July 2007, Slide 11. Slide #11 is located on the East Levee. The partially filled sump is in the foreground, with the main stem Trinity River in the background. (Photo courtesy of the Dallas Flood Control District)
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 several months, the uppermost foot of the levee surface (side slopes and crest) is saturated and very soft. This makes access for inspection and maintenance difficult and dangerous, and will likely contribute to the development of more shallow slides along the floodway. b. Directions to FCD. Attempted repairs made the day before the inspection had already failed (see Photo 1), so we requested that the FCD cease to make temporary repairs by adding fill to the failure surface, and that they: • • • •
use weed-eaters to cut vegetation in the slide areas to improve ground surveillance cover the slides with plastic to prevent further infiltration of rainfall locate a potential borrow source in the event that the situation continues to worsen so that additional fill can be added to the dry slide of the levee if warranted make daily inspections of the slide areas and dry side toe to see if seepage problems are developing, with additional inspections after any heavy downpours
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 7 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report The FCD agreed to this request. In a follow-up call to David Garcia the morning of 4 July 2007 to check on the condition of the project, Mr. Garcia confirmed that they had successfully 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 a construction site in nearby Seagoville) daily inspections (ongoing)
c. Aerial Surveillance. The levees were last mowed in May 2007. Heavy rains since then have accelerated vegetation growth and made it unsafe to mow again. The high vegetation makes ground inspection of the project and identification of slides very difficult, so the FCD is commended for coordinating with the Dallas Police Department Helicopter Unit to provide aerial surveillance during this time.
Photo 9, 05 July 2007, Slide 4. The FCD had done a good job covering up the slides with plastic to stop further infiltration from heavy rains that moved into the area the morning of 5 July. Total reported rainfall on this date was 0.88-inch.
d. USACE Follow-Up Inspection. On 5 July 2007, Mr. Les Perrin (CESWF-EC-DG), Mr. Mark Sissom (CESWFOD) and the undersigned made a brief inspection of the floodway, observing conditions at Slide #4 (see Photo 9), and the Coombs Gate slide, Slide #10 (see Photos 6 and 7). During the inspection, the FCD crew reported that two additional slides had developed along the Elm Fork of the Trinity, approximately 200 yards downstream of Slide #4 (see Figure 3). We were unable to inspect these new slides due to heavy vegetation growth.
4. HISTORICAL PERSPECTIVE. a. Levees. Originally constructed by the City of Dallas in the 1930’s with 2.5H:1V side slopes, and a maximum height of 35-feet; the levee system was modified by USACE in the late 1950’s by flattening the side slopes from between 3H:1V to 4H:1V, and increasing the crest width from 6 to 16-feet.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 8 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report While the overall performance of the modified levees has been good, hundreds of shallow infinite 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 300 such slides have been repaired since 1966, so the current slides are neither unprecedented nor unexpected. Most of these slides have occurred in the same general areas that slides are 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 Earth satellite imagery (see Figure 4). That imagery is believed to have been obtained during 2004, 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 1950’s, the Flood of Record for the Dallas Floodway occurred on 3 May 1990 when a discharge of 82,300 cfs was recorded. 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 crest elevation. 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 relative to 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 both photos for reference purposes.
Jail
Photo 10. Photo was taken during the modern Flood of Record, 3 May 1990. View is of the downtown area along the mainstem Trinity River, looking to the North-Northwest. Upstream is to the left. The maximum flood height for this event was Elevation 415.1 (gauge height of 47.1-feet), roughly equivalent to a ’45-year’ storm.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 9 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
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 than 10-feet below the record high river stage shown in Photo 10. The current event is less than a ’10year’ storm. (Photo courtesy 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 no precipitation during the site visit. 2. 5 July 2007. Weather during the follow-up site visit was overcast with heavy rain. The temperature was approximately 85°. b. Antecedent Conditions. Thus far this year, precipitation for 2007 is nearly 13-inches above normal. After two years of serious drought in 2005 and 2006, this amount of rainfall is more than sufficient to fill cracks that developed in the levee embankments during the drought period. This makes it likely that shallow slides will continue to develop throughout the Floodway.
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 10 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report c. Gauge Readings During Inspections. Gauge readings from the Commerce Street Gauge 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 available in real-time, and had not been corrected by the USGS at the time of this report. Plots of the uncorrected 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)
Figure 2. Uncorrected (i.e. ‘provisional’) gauge data from the Commerce Street Gauge (source: http://waterdata.usgs.gov/nwis).
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 11 of 14
CESWF-EC-DS SUBJECT: Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report d. Photographs. Although high vegetation obscured most of the slides, making it difficult to obtain good record photos from the ground, a RICOH camera with GPS was used to document and map the location of each slide. Additionally, the Dallas Flood Control District provided aerial photographs for our use. Aerial and land photos are available on the swffs1bgis 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 a high risk of failure of the Floodway at this time. The most serious slide (Slide #1) is located on the Elm Fork. This slide was ~170' long at the time of the inspection, and was approximately 10' deep. The failure surface intersected the embankment crest, and exited below the water surface. Because the cross-section of the levee in this area is slightly wider due to the location of the levee access road, the loss of a portion of the crest is not critical unless 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 be considered a recurring maintenance issue; however, due to the continuing heavy precipitation, the situation is considered to be serious. It is not, however, dangerous or critical at this time. CESWF will continue to assist the Dallas Flood Control District in monitoring the floodway and will provide guidance for additional corrective actions if warranted. 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 of the service bridge.
Anita Branch, PE Civil Engineer CESWF-EC-DG
Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 12 of 14
Approximate Location of New Slides
Figure 3. Larger View of the Slide Location Map presented in Figure 1 Showing Location of New Slides Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 13 of 14
Figure 4. Location of 19 Slides Clearly Evident on the Google Earth Satellite Imagery Dallas Floodway, Trinity River – Levee Slide Inspection Trip Report
Page 14 of 14
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