Hydraulic Evaluation and Determination of Requirements for Overtopping of Earth Non-Overflow Embankments Stuart Dobberpuhl, P.E. Senior Hydraulic Engineer, St Paul District

Introduction:

A hydraulic evaluation of the requiremnts for non-overtopping of earth non-overflow embankments was accomplished. The purpose of this study is to develop information to develop a plan of action prior to the 1998 spring runoff.

References and Experience:

The evaluation at this time considers the technical information and procedures used for the Waterways Experiment Station June 1964 Technical Report 2-650, entitled "Stability of Riprap and Discharge Characteristics, Overflow Embankments, Arkansas River, Arkansas." It also considers the experience of the Rock Island District with the 1993 flood event, the St. Paul District's experience with the Lock and Dam No. 3 spot dikes and the St. Paul District's experience with the Environmental Management Program earth embankments.

A. WES Technical Report 2-650: Figure 1 is a plot of approach energy elevation versus tailwater elevation based on Waterways Experiment Station Technical Report 2-650. Even though this plot was developed for embankments protected with riprap to determine stability, the same type of plots for earth embankments provides insight with regard to the stability. The riprap gradations used in the physical model tests are presented in Table 1.

Table 1

Riprap Gradations used in "Waterways Experiment Station June 1964 Technical Report 2-650, entitled "Stability of Riprap and Discharge Characteristics, Overflow Embankments, Arkansas River, Arkansas."
Percent Lighter	Gradation A		Gradation A1		Gradation B		Gradation C		Gradation
	Diameter in inches	Weight (pounds)	Diameter in inches	Weight (pounds)	Diameter in inches	Weight (pounds)	Diameter in inches	Weight (pounds)	Diameter in inches	Weight (pounds)
100	37.50	2620	24.00	687	16.00	204	16.00	204	16.00	204
50	16.20	211	15.80	196	6.90	16	5.30	7.40	4.40	4.23
15	8.00	25	8.00	25	3.50	2	1.60	0.20	1.50	0.17

1. General. Table 2 summarizes the evaluation of the performance of the earth non-overflow embankments in the Rock Island District. In summary, Lock and Dam No. 18 is the only site which is applicable for this evaluation. The earth non-overflow embankments in the reach from Lock and Dam No. 11 through 13 were not overtopped. For the reach from Lock and Dam No. 14 through 22, the embankments, with the exception of Lock and Dam 18, are either fortified with erosion protection or a earth non-overflow embankment is not part of the project.

2. Lock and Dam 18. The following is a summary of the information available for Lock and Dam No. 18:

1. Gage Related Information- is summarized on Figure 2 and Figure 3.

2. Water Surface Profiles - are shown on Figure 4 and Figure 5.

3. Plots of Head Differential, Stage and Discharge for the

   1993 flood are shown on Figure 6.

4. Plot of Head and Tailwater relative to the crest elevation of the earth non-overflow embankment is shown on Figure 7.

5. Plot of Head versus tailwater elevation relative to the crest for the daily values when the non-overflow embankment was overtopped is shown on Figure 8.

d. Lock and Dam No. 3 spot dikes. The St. Paul District has considerable experience with the Lock and Dam No. 3 project where earth embankments are overtopped about 15 percent of the time based on a duration curve. Figure 1 is a plot of approach energy elevation versus tailwater elevation using WES TR 2-650 procedures. The plot shows gradations tested in the physical model study in addition to plots of observed approach energy versus tailwater elevations for the period from 1959-1996. This plot was developed for the design condition which considers the total headloss at one embankment. Currently, there are three embankments in series that are overtopped with most of the headloss occurring at the downstream embankment. From Figure 1, it can be seen that:

1. Asymptote- A line with the approach energy elevation equal to the tailwater elevation forms an asymptote for the data.

2. 1993 barge accident- The conditions that existed at the time of the accident created the plot that goes through the critical point with an approach energy of 4.1 and a tailwater of -.8 feet. For this condition, the downstream rock and earth embankment sustained considerable damage. The embankment in the breach area was totally eroded. In addition, scour continued to create a scour hole with a depth about 30 feet below the normal water level for the Mississippi River at that location. WES TR-2-650 indicates that for the head-tailwater condition experienced, even the heavier gradation A₁ would be unstable. This is consistent with what was observed in the field.

3. Tailwater=2.0- For this tailwater condition, most of the data for Lock and Dam No. 3 falls between the approach energy elevation of 2.2 through 2.7 with one event at 3.5 feet. For the downstream embankment, the approach energy elevation would be on the order of 2.1 through 2.6 assuming that most of the headloss is at that embankment as was the case in 1993. From the experience with the downstream embankment or spot dike system, a approach energy elevation in the range of 2.1 - 2.6 can cause considerable damage to an earth embankment.

4. Other Tailwater Conditions- For other tailwater conditions, similar conclusions can be drawn. The only difference is that the relative relationship to the approach energy elevation is shifted.

e. Environmental Management Program- Embankment overtopping experiences associated with habitat projects summarized below are from Jon Hendrickson's 29 December 1997 Memorandum for Record, subject:"Embankment Overtopping Experiences Associated With Habitat Projects":

1. The following tables document conditions and effects of habitat project earth embankment overtopping during floods. Earth embankments constructed as part of habitat projects take on several forms including islands, closures structures, and dikes. Most of these projects have been constructed as part of the Upper Mississippi River Environmental Management Program (EMP).

Table 3

Pool 4, Peterson Lake Sand Closure	Construction Year	Overtopping Depth (feet)	Head Differential (feet)	Vegetative Stabilization	Scour Depth (feet below average water surface)
	1995	4 ft in 1996	0 to 0.5	Poor	4 to 15
Discussion: The three sand closure structures at the upstream end of the lake completely eroded away. In addition, scour occurred so that the post-flood channel depths at all three closures increased. This scour was most significant at channel 3 where a 15-foot scour hole developed in what had been a 5-foot deep channel.

Table 4

Pool 5, Sand Run Closures, Weaver Bottoms	Construction Year	Overtopping Depth (feet)	Head Differential (feet)	Vegetative Stabilization	Scour Depth (feet below average water surface)
	1986	3 feet in 1993 2 feet in 1994 4 feet in 1997	0 to 0.5	Good	No Scour, Structure was Stable
Discussion: Because of the relatively small head differential and the good vegetation cover, these structures were stable during the floods.

Table 5

Pool 5, MN-14 Closures, Weaver Bottoms.	Construction Year	Overtopping Depth (feet)	Head Differential (feet)	Vegetative Stabilization	Scour Depth (feet below average water surface)
	1987	3 feet in 1993 2 feet in 1994 4 feet in 1997	0.5 to 1.0	Variable	Up to 10 feet
Discussion: Vegetative stabilization on the structure was variable. In the areas where vegetation was sparse, significant erosion occurred, with up to 10 foot deep scour holes forming. In areas where good vegetative cover had been established, no erosion occurred. The perimeter of this structure had been stabilized with riprap, and this undoubtedly acted as a hard point, reducing structure erosion.

Table 6

Pool 5, Spring Lake Peninsula	Construction Year	Overtopping Depth (feet)	Head Differential (feet)	Vegetative Stabilization	Scour Depth (feet below average water surface)
	1994	2 feet in 1997	0 to 0.5	Good	No Scour,Structure was Stable
Discussion: Because of the relatively small head differential and the good vegetation cover, this structure was stable during the floods. Some erosion did occur on a road adjacent to and lower than the structure. The overtopping depth on the road was 4 feet.

3. Approach Energy versus tailwater- Figure 9 is a plot of approach energy versus tailwater for the Lock and Dam No. 18 earth embankment with the 1993 flood event. Figure 10 shows plots for the category 3 projects as defined in paragraph 4. Figure 11 shows the category 4 projects as defined in paragraph 5.

4. Classification of earth embankments- For the purpose of this evaluation, a classification system is proposed in Table 9.

Table 9

Classification of earth embankments subjected to overtopping
Category	Headloss Range(ft)	Minimum W50 Stone (lbs)	Description
1	0.0-0.2	1 or vegetation	Category 1 earth embankments or levees are defined as those embankments subjected to overtopping with an approach energy versus tailwater relationship that equates to a headloss less than 0.2 foot. Embankments or levees in this category are not likely to sustain significant damage if overtopped. With a tailwater of 4.0 feet, extrapolating the available data from TR2-650 indicates that a W50 stone size on the order of 2.9 inches or a 1 pound stone would be required to resist the erosion with an approach energy of 4.2 feet for the upper limit of headloss for this category. The W50 size for category 1 ranges from 0 to 1 pound.
2	0.2-0.5	2 or vegetation	Category 2 earth embankments or levees are defined as those embankments or levees subjected to overtopping with an approach energy versus tailwater relationship that equates to a headloss in the range of 0.2 feet to 0.5 feet. Earth embankments or levees in this category are in a transition between a low risk of sustaining significant damage to a high risk of sustaining significant damage if overtopped. Vegetation and/or materials of which the earth embankment or levee are constructed impact the risk of significant damage. With a tailwater of 4.0 feet, extrapolating the available data from TR2-650 indicates that a W50 stone size on the order of 3.2 inches or a 2 pound stone would be required to resist the erosion with an approach energy of 4.5 feet for the upper limit of headloss in this category. From this information, it can be seen that going from a headloss of .2 feet to .5 feet, will result in twice the stone weight required for the W50 size. The W50 size for category 2 ranges from 1 to 2 pounds.
3	0.5-2.0	7	Category 3 embankments are defined as those earth embankments subjected to overtopping with an approach energy versus tailwater relationship that equates to a headloss greater than 0.5 feet and less than 2 feet. Earth embankments or levees in this category would have a high risk of erosion.With a tailwater of 4.0 feet, extrapolating the available data from TR2-650 indicates that a W50 stone size on the order of 5.3 inches or a 7 pound stone would be required to resist the erosion with an approach energy of 6.0 feet for the upper limit of headloss in this category. The W50 size for category 3 ranges from 2 to 7 pounds.
4	2.0-3.0	210	Category 4 embankments are defined as those earth embankments subjected to overtopping with an approach energy versus tailwater relationship that equates to a headloss greater than 2 feet and less than 3 feet. Earth embankments or levees in this category would have a high risk of erosion. With a tailwater of 4.0 feet, extrapolating the available data from TR2-650 indicates that a W50 stone size on the order of 16.2 inches or a 210 pound stone would be required to resist the erosion with an approach energy of 6.0 feet for the upper limit of headloss in this category. The W50 size for category 4 ranges from 7 to 210 pounds.
5	> 3.0	> 210 or concrete	Category 5 embankments are defined as those earth embankments subjected to overtopping with an approach energy versus tailwater relationship that equates to a headloss greater than 3 feet. Earth embankments or levees in this category would have an extremely high risk of erosion.With a tailwater of 4.0 feet, extrapolating the available data from TR2-650 indicates that a W50 stone size greater than 210 pounds would be required to resist the erosion with an approach energy of 7.0 feet for the upper limit of headloss in this category. Because of the large stone size requirements for this category, a concrete structure may be more feasible than providing riprap for erosion protection.

5. Verification of Classification System- The verification of the classification system used for this evaluation is presented in Table 10.

Table 10

Verification of the Classification System for Earth Embankments subject to Overtopping
Project	Headloss in Feet	Category	Verifies Category Performance	Discussion
Pool 4- Peterson Lake Sand Closure	0.0-0.5	1 or 2	Yes	This project is most likely in category 2. Poor vegetation contributed to the erosion observed.
Pool 5- Sand Run Closures at Weaver Bottoms	0.0-0.2	1	Yes	Hydraulic information is not sufficient to determine whether this project should be in category 1 or 2. However, no scour was observed which fit with the definitions of category 1 or 2.
Pool 5- MN-14 Closures at Weaver Bottoms	0.5-1.0	3	Yes	Scour occurred to a depth 10 feet below the average water surface elevation.
Pool 5- Spring Lake Peninsula	0.0-0.2	1	Yes	Scour was not a problem.
Pool 8 Islands	0.2-0.5	2	Yes	Scour was not a problem.
Pool 9- Lansing Big Lake Closures	0.2-0.5	2	Yes	Erosion and scour occurred.
Lock and Dam No. 3 Upper- Corps Spot Dike System	<.2	1	Yes	Generally, erosion from overtopping is not a significant problem for the upper Corps Spot Dike System as long as the lower spot dike system is not breached.
Lock and Dam No. 3 Intermediate Spot Dike System	<.2	1	Yes	Generally, erosion from overtopping is not a significant problem for the Intermediate Spot Dike System as long as the lower spot dike system is not breached.
Lock and Dam No. 3 Lower Spot Dike System	0.5 Normal 4.8 Barge Accident	3 Normal 5 Barge Accident	Yes	Erosion has been a major problem for the lower spot dike system for normal conditions and an extreme problem for the barge accident scenario.
Lock and Dam 18	0.8-1.41	3	Yes	At Lock and Dam No. 18, the cost of repair was minor. However, the fact that breaching occurred to a depth of 5-6 feet demonstrates that breaching is a serious consideration. At Lock and Dam 18, additional information is needed with regard to the materials used to construct the embankment to gain further insight. Since the embankment was overtopped during the month of July, any vegetation would have had enough time to be well established prior to overtopping. A spring runoff event would occur before any vegetation would be established and, therefore, have a higher rate of erosion

6. Application of Classification System- The application of the classification system used for this evaluation is presented in Table 11.

Table 11

Application of Classification System to Locks and Dams 2-10 on the Mississippi River within the St. Paul District
Project	Headloss in Feet	Category	Discussion and Consequence of Overtopping
Lock and Dam No. 2 Earth Embankment	0.8-2.3(1969)	3 normal 4 (1969)	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching.
Lock and Dam No. 3 Earth Embankment- MN (Not Spot Dikes)	0.5	3	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching.
Lock and Dam No. 4 Earth Embankment	0.7	3	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching.
Lock and Dam No. 5 Earth Embankment	2.4 Upstream 0.9 Downstream	4	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching. In 1997, field surveys indicated that the pool was 1.5 feet higher at the upstream end of the embankment.
Lock and Dam No. 5A Earth Embankment	0.7 Upstream 0.5 Downstream	3	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching. In 1997, field surveys indicated that the pool was 0.2 feet higher at the upstream end of the embankment.
Lock and Dam No. 6 Earth Embankment	1.6	3	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching.
Lock and Dam No. 7 Earth Embankment	0.8	3	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching.
Lock and Dam No. 8 Earth Embankment	1.2 Upstream 0.7 Downstream	3	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching. In 1997, field surveys indicated that the pool was 0.5 feet higher at the upstream end of the embankment.
Lock and Dam No. 9 Earth Embankment	0.7	3	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching.
Lock and Dam No. 10 Earth Embankment	0.6	3	If overtopped by a spring flood, the earth embankment is likely to sustain considerable erosion and possible breaching.

¹ Source: USAED-St Paul Water Control Web Server; historic water surface profile tabulation for 1952, 1965, 1967, 1969 and 1993 events.

8. Options: At this time, the recommended plan has not been determined since the determination of the recommended plan will need to involve input from other disciplines in the District. However, some options available are discussed below:

• Do nothing- This is currently the selected option in the Rock Island District for the earth non-overflow embankments. The advantages and disadvantages for this alternative are as follows:
  • Advantages- This is the least cost alternative initially.

  • Disadvantages- No assurance can be made with regard to the stability of the earth embankment with regard to erosion. It is likely that erosion will occur. Based on the experience with other projects as presented in paragraph 2, it can be seen that there is a significant risk that the earth embankments will breach. Once a breach is initiated, the general tendency is for the flow to increase through the breach causing higher and higher scouring velocities with additional erosion occurring. This process was demonstrated during 1993 for the earth dike between Gantenbein Lake and the Mississippi River where a breach formed. Once the dike was breached, the scour accelerated in a 1-2 day time period to the point where a 30 foot scour hole formed where the embankment once existed.

• Raise non-overflow embankments. The advantages and disadvantages for this alternative are as follows:
  • Advantages- This alternative will prevent damages to the earth non-overflow embankments.

  • Disadvantages- This alternative requires a substantial effort making it quite costly.

• Provide erosion protection- The advantages and disadvantages for this alternative are as follows:
  • Advantages- This alternative will prevent damages to the earth non-overflow embankments. The costs for this alternative could be relatively low when compared to raising the embankment. It appears that a minimal riprap layer thickness with a filter material would be adequate to prevent erosion for most sites based on the category determination shown in Table 11 and the requirements for specific categories shown in Table 9. This would be needed on the crest and only the upper portion of the embankment from the crest to a point 5 to 10 feet below the crest on the downstream slope. The exceptions to the minimal requirements would be Lock and Dam No. 5 and, possibly, Lock and Dam No. 2 depending on which design condition is used.

  • Disadvantages- This alternative minimizes the disadvantages.

• Combination of Options A, B and/or C.- The advantages and disadvantages for this alternative are as follows:
  • Advantages- This alternative will prevent damages to the earth non-overflow embankments. It also allows for an overall optimization of the alternatives.

  • Disadvantages- This alternative minimizes the disadvantages.

                                       /signed/
                                       STUART V DOBBERPUHL, P.E.
                                       Senior Hydraulic Engineer
                                       Hydraulics Section
                                       Hydraulics & Hydrology Branch

Figure 1

Figure 2 Gage Related Information MI18 - L&D 18 OMNI data Stream Mississippi River Gage Zero 518.52 feet MSL (1912) Flood Stage 10.00 feet Record Stage 21.54 Date 07-10-93 Location Lat 40-53-07 Long 91-01-21 Drainage Area 113,600 square miles River Mile 410.5 Location of Gage Lock and Dam 18 Gladstone, IL.

Figure 3 Gage Related Information GLDI2- L&D 18 near Gladstone, Il Stream Mississippi River Gage Zero 518.52 feet MSL (1912) Flood Stage 10.00 feet Record Stage 21.54 Date 07-10-93 Location Lat 40-53-07 Long 91-01-21 Drainage Area 113,600 square miles River Mile 410.5 Location of Gage Lock and Dam 18 Gladstone, IL.

Figure 4

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Figure 9

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Figure 11

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