Minimizing Roadway Embankment Damage from Flooding (2016)

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xi Note: Many of the photographs, figures, and tables in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions. Pavements, 75 Stream Stabilization, 75 Summary, 75 76 CHAPTER ELEVEN STATE OF RESEARCH AND CONCLUSIONS Introduction, 76 Ongoing Studies, 76 Future Research Needs, 76 Conclusions, 77 Units, 78 80 GLOSSARY 81 REFERENCES 86 BIBLIOGRAPHY 92 APPENDIX A SURVEY QUESTIONNAIRE

SUMMARY MINIMIZING ROADWAY EMBANKMENT DAMAGE FROM FLOODING Roadway embankment damage from flooding is a shared concern among all U.S. states. Aside from the financial burden the states and federal government face to repair the damage, the preparation of an adequate design is a challenging task. This synthesis highlights major issues and design components in the absence of standard guidance specific to this topic. The information presented in the synthesis is based on a review of the related literature, a survey of current practice, and a series of telephone interviews. The probable failure mechanisms are identified and possible design approaches and repair countermeasures are highlighted. The study presents a comparison between roadway embankments and levees to empha- size that embankments are not designed as flood control structures. The differences between riverine and coastal flood mechanisms are also stated because they impact the most effective way to minimize roadway embankment damage resulting from flooding. The common failure mechanisms in coastal and riverine environments identified by the survey results are overtopping, seepage (through seepage and underseepage), piping, wave action, softening by saturation, and lateral sliding over the foundation soil. Pavement fail- ures and culvert-related failures are also outlined. Examples of failures and repair tech- niques are illustrated through 14 case examples gathered from six states that contributed information through the survey and follow-up interviews. Based on the case examples, different failure modes were identified and different solu- tions were described. The effectiveness of the adopted solutions is usually dependent on the site conditions. A protection technique that proves successful at one site is not necessarily the adequate solution at other sites. To arrive at an adequate design, the following factors are generally considered: hydrologic and hydraulic factors, geological and geotechnical factors, legal and funding aspects, and risk. General observations and guiding principles are listed. Hydrologic studies are essential in estimating the magnitude of the expected floods and in selecting a design flood. This information is used in nearly every aspect of the design. Hydrographs give information about the variation of the flow versus time. The use of hydrographs instead of peak flows can lead to more advanced analyses. Hydraulic methods use the hydrologic data to give an estimate of the water surface elevation, the overtopping height if overtopping is predicted, and the water velocity. Major geological considerations include the subsurface conditions, topography, floodplain and meandering potential, ero- sion and deposition, and basin characteristics and channel dimensions. Such considerations are essential in identifying the expected sources of damage at an early stage. Geotechnical calculations are crucial in designing against anticipated failure modes. Embankments are made of soil; therefore, identifying the characteristics of the embankment materials and their impact on the behavior of the embankment during flooding is very important. Key characteristics include the erodibility of the embankment materials, material properties (strength and permeability), and culvert- and pavement-related considerations.

2 In addition to the technical aspects previously described, the decision-making process is influenced by legal, regulatory, and funding aspects. After a failure, all decisions about the type of repairs as well as whether the same design will be repeated or whether betterments will be sought (temporary versus permanent, changing stream course, raising the freeboard) are bound by funding constraints, time constraints, constraints from interaction with other agencies (e.g., the U.S. Army Corps of Engineers, the Federal Emergency Management Agency, or EPA), and, in some cases, community constraints. Available design methods to mitigate the impact of flooding on roadway embankments include the following: 1. Choosing the design flood 2. Overtopping 3. Seepage through the embankment 4. Seepage under the embankment 5. Wave erosion 6. Softening by saturation 7. Lateral sliding 8. Culverts 9. Pavements 10. Rapid drawdown. The main countermeasures adopted by the surveyed department of transportation (DOT) engineers are identified, but their use is often best suited to a particular applica- tion. In the case of construction of a new roadway embankment, a key design element is the selection of an adequate roadway location, for which a thorough understanding of site characteristics and constraints (e.g., geology, geotechnical characteristics, stream hydraulic characteristics and meandering potential, upstream and downstream condi- tions, construction activities, and stormwater management facilities) is needed. In case of an old problematic site, it is essential to understand the key issue leading to recurring or aggravated damage. Possible issues include an increase in the severity of flooding events and the effects of stream instability caused by changes in upstream or downstream condi- tions (including man-made activities). By identifying all the major issues, an adequate design approach can be adopted. Such approaches would include one or a combination of the following options: relocating the embankment, stabilizing the stream, or designing for failure modes. The following protection techniques are commonly used to mitigate erosion derived from flooding mechanisms: 1. Vegetation 2. Riprap and geotextiles 3. Gabions 4. Articulated concrete blocks

3 5. Concrete lining 6. Paving. The location and extent of the protection depends on the anticipated flow mechanisms. It is essential to couple the design process and the use of countermeasures with engi- neering judgment, while keeping in mind all the issues outlined in this synthesis. It is also important to recognize that no design is foolproof and that the probability of failure is not zero. Consequently, it is also important to evaluate the probability of failure and the value of the consequence in terms of lives and economic loss. Ideally, it is the combination of the probability of failure and the value of the consequence or risk that can best guide design decisions, such as the selection of the design flood. For this purpose, future development of research relevant to risk factors and the level of risk accepted would be beneficial. Future research studies are also needed to produce optimum design results. Such stud- ies include relevant failure and success case studies, the impact of geomorphologic and geologic factors, efficient protection systems at different velocities of the water flow, guid- ance on the design and installation of culverts, the role of planning and management in the project success, and relevant software kits.

4 vey questionnaire is included in Appendix A. An outline of the report is provided herein. Chapter two discusses the common roadway embank- ment failure modes identified in the survey results and out- lines possible mitigation measures. Chapter three presents examples of failure modes based on survey responses. It also summarizes the adopted protection and mitigation measures that were both successful and unsuccessful based on the lim- ited information provided. Important concepts and consider- ations relevant to hydraulic and geotechnical disciplines are outlined in chapters four and five, respectively. A discussion of the hydrologic and geologic factors and their relevance to design are included. A brief discussion of the legal, regula- tory, and funding aspects is presented in chapter six. Chapter seven describes current probability and risk practices and elaborates on relevant useful concepts. The survey results are summarized and presented in chapter eight. The design steps that would minimize flood damage are compiled in chapter nine. Possible mitigation and maintenance measures are discussed in chapter 10. Finally, chapter 11 high- lights ongoing research topics and future research needs. ROADWAY EMBANKMENTS VERSUS LEVEES The distinction between roadway embankments, levees, and other flood control structures was presented in a September 10, 2008, FHWA memo. This memo was issued in response to levee certification initiatives and to the “inadvertent or incor- rect” designation of some roadway embankments as levees or other flood control structures that occurred during updates to Federal Emergency Management Agency (FEMA) maps. FEMA identifies flood risk areas through flood insurance studies. These studies are a part of the National Flood Insur- ance Program in participating counties. However, such stud- ies were not carried out until much of the nation’s highway system was already constructed. As a result, some roadway embankments are located in flood plains. Roadway embank- ments and levees (Figure 1) are made up of soils and may look similar at first glance. However, they are generally different in many aspects, including the agencies responsible for issuing the relevant design document or guidelines, their purpose, the level of geotechnical analysis required, their design features, and the slopes and the fill materials used in their construction. CHAPTER ONE INTRODUCTION INTRODUCTION Roadway (highway) embankment damage caused by flood- ing in coastal and riverine environments is one of the cur- rent challenges faced by the United States. Aside from the economic hardships that result from repairing the damage caused by flooding, design difficulties related to the type and extent of protection against anticipated damage are commonly faced by departments of transportation (DOTs). Avoiding or at least minimizing embankment damage dur- ing flooding requires an understanding of the interaction between various geotechnical and hydraulic factors during extreme events. Relevant literature that describes such fac- tors was collected to compile related technical information. DOT engineers were surveyed to identify the state of prac- tice and to gather associated case examples. Follow-up inter- views were conducted to complement that information. The findings are presented in this report. Based on the findings, the extent of flood damage var- ies between the surveyed states, and so do the design approaches and remedial measures adopted by differ- ent DOTs. The components that would be considered in the decision-making process are highlighted throughout this report. In this introduction chapter, the adopted study approach is presented. Then the general differences between roadway embankments and levees are outlined, to avoid the false assumption that roadway embankments could function as levees. Next, the differences between riverine and coastal mechanisms are defined. STUDY APPROACH This study is based on a review of available literature and the information gathered from the survey responses and follow-up interviews with selected DOT engineers. This review is not intended to be a comprehensive study of all the relevant literature; rather it provides insights into the differ- ent aspects of the problem being studied. The information included pertains to studies carried out by such agencies as FHWA, U.S. Army Corps of Engineers (USACE), and NCHRP, in addition to some local and international tech- nical literature. Information obtained from case examples mentioned in the survey responses and from the follow-up interviews are presented to reflect current practice. The sur-

5 This section highlights major features of such embankments versus those of levees. The relevant documents and guidelines available to aid in the design of embankments and levees are presented herein. Such agencies as AASHTO and FHWA provide guidelines for the design of riverine encroachments. FHWA and USACE provide guidelines for coastal highway design. USACE has extensive technical guidelines widely used for levee design. The purpose of a roadway embankment is to safely carry traffic as a part of the highway network. A levee, on the other hand, is a flood control structure designed to protect urban or agricultural areas from the anticipated design floods (USACE 2000). Although roadway embankments that lie within the floodplain are prone to similar hydraulic forces as levees during flooding events, they are not generally designed to retain water during flooding. As a result, the level of analysis required varies between the two structures. In the case of road- way embankments, the level of analysis is determined by high- way location and project requirements. In the case of levees, the analysis is more specific and focuses on preventing the overflow of water to the dry side of the levee up to design flood levels. As a result, the design features, adopted slopes, and fill material used in construction vary between the two structures. For example, levees sometimes feature impermeable cores while roadway embankments do not. Also, levee embankment slopes are generally flatter than those used in roadway embankments (Figure 1). In addition, gradation is specifically considered in levee design, to prevent seepage and erosion. Table 1 presents a comparison between road- way embankments and levees. Because roadway embankments could lie in riverine or coastal environments, the differences between riverine and coastal mechanisms and flooding duration are presented in the next section. COASTAL VERSUS RIVERINE EMBANKMENTS Roadway embankments within the floodplain of a river are termed riverine embankments. These embankments are subjected to hydraulic forces from flooding rivers or run- offs that occur during flooding events (Figure 2a). Coastal roadway embankments are raised earth structures located in coastal environments and on which the pavement struc- ture is constructed (Figure 2b). These embankments are subjected to coastal processes that include wave action and surges resulting from hurricanes. The terminology shown in Figure 2 will be adopted throughout the text. The difference between the riverine and coastal mechanisms and flood- ing durations are presented herein. Additionally, important design parameters for embankment design in both environ- ments are highlighted. TABLE 1 COMPARISON BETWEEN ROADWAY EMBANKMENTS AND LEVEES Roadway Embankments Levees Agencies That Issue Relevant Documents/ Guidelines AASHTO/FHWA USACE Purpose Carry traffic as a part of the highway network Flood controlling/retain- ing structure Level of Analysis Dependent on highway location and level of geo- technical analysis required Specific focused type of analysis Design Features No internal impervious core-homogeneous materials Sometimes internal impervious core (zoned) Freeboard varies based on considerations related to state requirements and/or regulatory agency require- ments (chapter six) Freeboard included Wider crest to accommo- date anticipated traffic (multiple lanes) Narrow crest (about 3 m) Slopes Steeper slopes: 1H to 1V, 2H to 1V, 3H to 1V, and 4H to 1V Relatively gentler slopes: 5H to 1V and 6H to 1V Fill Material Could be homogeneous embankment consisting of relatively similar/uniform material or heterogeneous depending on available mate- rial (especially for local sys- tems such as county roads) An inner core (imper- meable materials) is introduced Gradation generally not designed as a barrier against water Gradation selected to prevent seepage, piping, and infiltration FIGURE 1 Roadway embankment versus levee typical sections.

6 Coastal Versus Riverine Mechanisms Coastal environments exhibit flow conditions in which fac- tors such as tides, wind-generated waves, and storm surges play an essential role. Waves and tides represent an unsteady, periodic, pulsating, and intermittent flow component. Surges present a relatively steady motion for a short time with respect to inland flow discharges. The design water surface elevation mainly involves the estimation of the tidal water elevations and the wave heights. The tidal and wave changes are not typically sensitive to the presence of an embankment. Rivers, on the other hand, are relatively more sensitive to the presence of an encroachment or an embankment. For example, changes of conditions downstream could affect the upstream conditions. This might alter erosion and deposition patterns or jeopardize the river stability. On the other hand, rivers commonly show relatively con- stant flow discharge over a certain period of time. In other words, rivers generally do not exhibit sudden changes of flow overnight. Also, river waves do not generate run-up distances along the riverine embankment upstream slope as large as their coastal counterparts do along the seaside slope of a coastal embankment. Further information can be found in references that include PDDM Manual (FHWA 2014), HEC-25 (Douglass and Krolak 2008), USACE (Hughes 2008), Chen and Anderson (1987), and Clopper and Chen (1988). A summary of the differences between coastal and riverine overtopping mechanisms is presented in Table 2. Coastal Versus Riverine Flooding Duration River floods may last for days, and sometimes weeks. The survey results revealed that Minnesota, for example, experi- enced overtopping during the Red River flood of spring 2011 that lasted 3 weeks. Based on information gathered in a pre- vious study by Chen and Anderson (1987), sites in Arkansas and Missouri (December 1982 flood), Wyoming and Colo- rado (May 1983 flood), Arizona (September 1983 flood), and Wyoming (August 1985 flood) experienced overtopping durations that ranged from 9 hours to more than 3 days. As revealed in case examples from Minnesota and Colorado in chapter three of this report, one factor that affects the duration of flooding is the area’s topography. In Minnesota, longer flooding (and possibly inundation) durations occur in flatter terrains (Oslo) than in steeper terrains (Ada). In cases of steep terrains, such as those in Colorado, shorter duration and higher velocities are anticipated. TABLE 2 COMPARISON BETWEEN COASTAL AND RIVERINE OVERTOPPING MECHANISMS Characteristics Coastal Environments Riverine Environments Nature Unsteady discharge/ periodic/pulsing/inter- mittent flow Steady discharge/continu- ous flow Occurrence Wave overtopping can occur even if still-water level ≤ embankment height Overtopping occurs if headwater level ≥ embank- ment height Duration Relatively short (hours) Relatively long (days) Velocities Peak velocities sus- tained only for a brief time Peak velocities could last for prolonged durations Erosion Mechanisms Intermittent and consis- tent with periodic/inter- mittent flow Continuous erosion process that decreases with steady decrease of velocity Erosive Capability Wave action more prominent Long flow duration FIGURE 2 Typical roadway embankment section in (a) riverine and (b) coastal environments.

7 Coastal highways, on the other hand, may be subjected to hurricanes and the associated ocean surges and waves that may overtop an embankment for a relatively shorter period; that is, a couple of hours. The resulting damage, however, could be greater than that of rivers because of the higher velocities encountered. Design Parameters The main parameters involved in the hydraulic design cal- culations of roadway embankments are the peak discharge, expected flood height, and hydrograph (discharge as a func- tion of time). In coastal environments, wave parameters such as the run-up elevation and the significant wave height are also included. The hydrologic and hydraulic parameters are described in chapter four. From a geotechnical perspective, if no protection systems are used, the key parameters are related to soil type, com- paction level, soil strength, and erodibility. When slopes are protected, the embankment material is designed to support the weight of the applied protection system. Precautions related to the type and extent of protection are typically taken to prevent leeching of the underlying material through the surface protection. These precautions are discussed fur- ther in chapters five and 10. The design process would ideally include the following: • Selection of a design height (including freeboard) and the slopes of the roadway embankment • Estimation of the overtopping duration and the depth of overtopping measured from the embankment crest if the embankment is designed to overtop • Velocity calculations to select and design an adequate protection system • Evaluation of embankment damage given an overtop- ping duration. SUMMARY In this chapter, the study approach was outlined. The differ- ences between embankments and levees were discussed. A comparison between riverine and coastal roadway embank- ments was also presented. The next chapter will explore the different failure modes that generally occur in embankments. As discussed in later chapters, it is important to match the failure modes that frequently occur with the practices avail- able in order to mitigate their effect.