Minimizing Roadway Embankment Damage from Flooding (2016)

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68 CHAPTER NINE DESIGN CONSIDERATIONS the section “Modes of Failure” in chapter eight, the most effective approach to choose the design flood is the risk approach. Targeting a risk of 0.001 fatalities per year and $1,000 in the United States per year may be acceptable tar- get risk values. The calculation of the design flood is based on the equation R PoE C (9.1) FIGURE 109 Recommended limiting values for erosion resistance of plain and reinforced grass (Hewlett et al. 1987). Where R is the risk, PoE the probability of failure or exceedance, and C the value of the consequence. This equa- tion can be used with the target values of R mentioned previ- ously and the value of C obtained by evaluating the likely consequences of a failure of the embankment in terms of fatalities and cost. With R and C known, one can back-cal- culate the required PoE. Note that two values of the PoE will be obtained, one for fatalities and one for cost. The lower of the two values is used for further calculations. Once the PoE is known, the following equation can be used to find the return period Tr of the design flood, which will satisfy the target risk. 11 1 Y Y r PoE T (9.2) INTRODUCTION Roadway embankments do not play a flood control role; they may or may not be designed to resist flooding. If they are to be designed to resist flooding, helpful design steps specified in this chapter are consistent with best practice. This best practice is based on the national survey results, the available literature, and information from other fields such as levee and dam design. The purpose of the design is to minimize the probability of occurrence of the modes of failure identified in chapter two. These modes of failure are listed here: 1. Overtopping 2. Seepage through the roadway embankment 3. Seepage under the roadway embankment 4. Wave erosion 5. Softening by saturation 6. Lateral sliding 7. Culvert problems 8. Loss of pavement 9. Rapid drawdown. The chart in Figure 109 summarizes the steps that may be followed to design against flooding for all the failure modes. Note that in several instances the current knowledge is lim- ited and engineering judgment as well as prior experience will always be extremely valuable in the design process. This chapter proceeds by discussing each one of the design steps in more details. CHOOSE THE DESIGN FLOOD This is the step where the flood return period Tr, also called recurrence interval RI or even flood frequency Ff, is cho- sen. This is the most important step as it controls many of the subsequent calculations and decisions. As explained in

69 Where PoEy is the target PoE coming from Equation 9.1 with y being the number of years for the design life of the embankment. Now that the return period of the flood is known, the design can proceed for each one of the failure modes identified in Figure 109. OVERTOPPING Calculate Discharge Q This is done by using the regression equation available locally and for the return period identified in the previous section if there is no gage station close to the location being studied. If there is a USGS gage station at that location, the gage record is analyzed to identify the discharge, which corresponds to the return period identified in the previous section. Calculate Water Depth Once the discharge Q is known, the water depth in the river next to the embankment can be obtained by using a software program such as HER-RAS. The input includes the river cross section along with the roughness of the river bottom and the discharge. The water depth is output. Compare Water Level and Embankment Height If the water level is lower than the embankment height, no overtopping will occur for the design flood, and that part of the design is complete. If the water level is higher than the embankment height, overtopping will occur and the next step needs to be taken. Calculate Maximum Velocity vmax The maximum velocity is the velocity that occurs at the bot- tom of the downstream slope after overtopping takes place. This velocity can be conservatively estimated by max 2v gh (9.3) Where g is the acceleration from gravity and h is the verti- cal distance over which the water is falling on the downstream slope of the embankment. A closer and less conservative esti- mate of vmax is given by the equations in the chapter four sec- tion “Hydrological Methods and Considerations.” Compare Maximum Velocity and Critical Velocity The value of vmax is then compared with the value of the critical velocity vcrit for the soil on the downstream slope. This critical velocity can be estimated by using Figure 66 in chapter five. If high-quality grass satisfying the crite- ria in “Overtopping of Embankments” in chapter five is in place, the critical velocity value may be increased. If vmax is lower than vcrit, then no erosion will occur during over- topping and the design is complete. If vmax is higher than vcrit, then a countermeasure is needed (see chapter 10). SEEPAGE THROUGH THE EMBANKMENT Draw Flow Net A flow net is drawn through the embankment to represent the flow lines and equipotential of the flow. Calculate the Exit Gradient ie This is done by calculating the hydraulic gradient in the small- est flow field on the exit face of the flow net (Briaud 2013). Calculate the Critical Gradient ic This is done by using the equation sat w c w i (9.4) Where γsat is the saturated unit weight of the soil in the embankment and γw is the unit weight of water. The critical hydraulic gradient value is often around 1. Compare the Exit and Critical Gradients If ie is less than ic divided by an appropriate factor of safety (2?), then the seepage through the embankment is unlikely to create backward erosion and loss of effective stress at the exit face. If ie is approaching ic or is larger than ic, then a countermeasure is necessary. Check for Internal Erosion and Piping The design for this issue is not very precise and strong engi- neering judgment and experience is favored over a math- ematical approach. One design aide for internal erosion is Figure 79 in chapter five. SEEPAGE UNDER THE EMBANKMENT The steps for this design issue are the same as the steps for seepage through the embankment listed above, except that in Equation 9.4, γsat is the saturated unit weight of the soil under the embankment rather than of the embankment. WAVE EROSION Waves can overtop the embankment at regular intervals during a storm without the embankment being subjected to

70 constant sheet flow overtopping. In this case, the mean water level is lower than the embankment height but the waves can still overtop the crest. This case is similar to the sheet flow case except that the time over which the flow takes place is limited. The example in chapter five, “Example Process for Calculating Erosion Depth,” shows how the calculations might proceed. In the end, the most effective approach is to select and design the proper countermeasure. SOFTENING BY SATURATION Check the Sensitivity of the Soil to an Increase in Water Content This problem results from the loss of stiffness of the embank- ment soil as it becomes saturated by the water standing and flowing next to the embankment. This loss of stiffness can be checked by performing some Proctor compaction tests and obtaining the soil modulus as a function of the water content. If the soil shows little decrease in modulus with increasing water content, then this part of the design is com- plete. If, however, the soil exhibits a curve such as the one of Figure 8 in chapter two, then the soil is quite sensitive to an increase in water content. Improve Soil A soil less sensitive to water content increase must be chosen for the embankment. Load Test Pavement After Flood In case of uncertainty, the pavement can be load tested to check that the displacement under load is still acceptable before reopening the roadway embankment to traffic after the flood. LATERAL SLIDING Calculate the Water Push The horizontal push P per unit length of embankment exerted by the water on the embankment is calculated from 21 2 w P h (9.5) Where γw is the unit weight of water and h is the water depth. Calculate the Embankment Resistance The horizontal resistance of the embankment at the level of the bottom of the water is a result of the friction that exists between the embankment and the soil underneath the embankment. This friction force is calculated by first calculating the shear strength of the interface between the embankment and the soil underneath it. This shear strength is equal to the mean vertical effective stress at the interface between the embankment and the soil below times the fric- tion coefficient tanφ. That shear strength is then multiplied by the width of the embankment to obtain the maximum resistance R per unit length of embankment. Calculate the Factor of Safety Against Sliding The factor of safety against sliding is given by the ratio RF P (9.6) If this factor of safety is satisfactory, sliding is unlikely. If this factor of safety is not large enough, countermeasures need to be implemented. CULVERTS Check Hydraulic Capacity This is done by using the design discharge to calculate how much water will be directed through the culvert during the flood. If the culvert is not big enough, a second culvert may have to be installed or a bigger culvert is needed. Erosion Around the Culvert The boundary between the culvert and the surrounding soil is sensitive to erosion. It is important to ensure that good compaction is achieved at the boundary between the culvert and the surrounding soil. PAVEMENTS The best way to prevent partial or total pavement loss is to design an appropriate countermeasure (see chapter 10). In general, no unified guidelines have been adopted by all state DOTs to prevent pavement damage. Yet a number of practices gathered from the survey results are included herein: • Using rockfill for the subbase to avoid scour/erosion (Arkansas Practice) • Using black base, graded aggregate base, and underd- rain (Florida) • Allowing for a freeboard of 2 ft at design flood for a culvert (Idaho) • Using recycled asphalt pavement, geogrid with vegeta- tion, riprap, or cap clay on the slope, and paving the slope (Minnesota)

71 • Armoring the slope (Nevada) • Keeping the subbase above design flood level (Tennessee). Rapid Drawdown This condition is related to the case where the flood recedes relatively quickly but the low permeability of the embank- ment soil causes the water stress in the embankment to be retained. In this situation, the soil strength is low and the water pressure is no longer there to help support the upstream slope. This is the rapid drawdown case in slope stability analysis. Calculate Water Stress in the Upstream Slope This is done by using the results from the “Seepage Through the Embankment” section in this chapter. The flow net is used to calculate the water stress in the upstream embank- ment slope. Perform Slope Stability Analysis The water stress distribution is part of the input in a slope stability analysis. The output of this analysis is a factor of safety FS against the sliding of the slope. If FS is satisfac- tory, rapid drawdown is not a problem. If FS is too low, a countermeasure is necessary. SUMMARY This chapter outlines useful design considerations based on the information gathered through the case examples, survey, and literature review. First the design flood is selected, then the approaches that could be adopted in the mitigation of each of the failure modes identified in chapter two is pre- sented. The next chapter presents the countermeasures and maintenance solutions that would further minimize the roadway embankment damage from flooding.