Minimizing Roadway Embankment Damage from Flooding (2016)

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76 CHAPTER ELEVEN STATE OF RESEARCH AND CONCLUSIONS Historical learning is essential to advancing our knowl- edge relevant to minimizing roadway embankment damage resulting from flooding. However, adequate documenting the flooding event occurrences and their effects is not com- monly done as a part of DOT practice. When embankments and pavements are damaged or fail as a result of flooding, restoring service to the facility and studying the effects of the measures adopted on the budget become the main concern. As a result, documentation of the case is not regarded as a priority. Noting that the embankment and pavement failures from inundation often occur months after the event itself, by the time the damage is inflicted, the event details are already forgotten. This calls for the development of documentation guidelines and the preservation of that documentation. Based on information from different case examples, knowledge of the geomorphologic and geologic setting is crucial to carry out adequate design for such projects. Under- standing of the global situation based on aerial photographs and studying the different factors involved before making decisions cannot be further stressed. As a result, it would be helpful to highlight the geological and geomorphological factors that are important, and the effect of different rele- vant conditions on the design. Also, it would be important to emphasize the benefit of involving geological and geotechni- cal considerations in early design stages. Also, changing the floodplain owing to meandering or to a sudden change in a river course is not uncommon. It would be helpful to inspect why rivers change their course and predict such change. Although general knowledge is available on the offered protection products, there is a shortage in documented expertise on the applicability and the success or failure of these systems. Also, it would be useful to establish border- lines between different types of protection systems through identifying at what point a costlier system can be adopted. Additionally, specific studies on available protection sys- tems such as grouted riprap versus ungrouted riprap would be helpful. In this regard, another point of research would be the impact of debris in the channel on the riprap sizing, especially when larger debris is being transported. It is common to see risk as one of the topics included in state requirements and federal guidelines and standards. Yet there is still no clear guidance on what level of risk is accept- able, and how to incorporate this into the design. Hence, it is INTRODUCTION Information relevant to minimizing roadway embankment damage from flooding is limited. Based on the data gath- ered from the survey, the number of relevant ongoing studies is limited as well. Throughout the synthesis, research gaps have been identified that are related to a number of aspects. The ongoing studies and the future research needs are high- lighted herein and the synthesis is concluded. ONGOING STUDIES Based on the survey results, three ongoing studies are rel- evant to the synthesis topic. Two of these projects are carried out for Minnesota Department of Transportation (MnDOT) and are related to design considerations for protection against overtopping and assessment of flashflood and related dam- age, maintenance, and the impact of climate change. The third study is carried out for NCHRP and addresses the case of scour at the base of retaining walls. The following are the references for these studies: • MnDOT—Design Considerations for Embankment Protection During Road Overtopping Events, University of Minnesota—project in progress; end date March 31, 2017. • MnDOT—Flash Flood Vulnerability and Adaptation Assessment Pilot Project, Philip Schaffner. This proj- ect includes investigation of slope failure, mainte- nance-identified historical overtopping, and assessed impacts of climate change on overtopping. • NCHRP—NCHRP Project 24-36: Scour at the Base of Retaining Walls, David Reynaud—in progress. FUTURE RESEARCH NEEDS The future research needs are compiled in this section based on the feedback received from DOT engineers through the survey responses and interviews. The research needs could be grouped under the following topics: histori- cal learning, geomorphic and geologic factors, protection systems, risk factors, guidelines for culverts and pavement considerations, factors related to management, and model- ing packages.

77 important to develop a risk-based methodology that could be adopted in the design. Because culverts are an important infrastructure in road- way embankments, standard clear guidance on the design and installation of culverts is essential. Different DOTs have different requirements for culverts and the case examples revealed several culvert-related failures in different locations. It is important to identify best practices applied in DOTs. Relevant to pavement, little guidance is generally available. It would be helpful to identify the effect of inundation on the pavement, how long it takes to regain its strength after satura- tion, and general preparation and installation guidelines. Aside from the technical aspect, management is a very important factor in a project success. Especially when fed- eral funds are used, it is important to finish the construction in time; otherwise, funding may be lost. Delays throughout the project may result from regulations imposed by different agencies (such as federal regulations, environmental regula- tions, and permitting regulations) that the engineer gradually becomes aware of throughout the project’s course. Because not all relevant regulations are identified at the beginning of the project, meeting the construction deadlines might be compromised and compliance with agency requirements become challenging. It is also essential to have clear guid- ance on the studies that would be required for each project at an early stage. This will aid in better management and result in optimized designs. Lastly, in order for design information to be useful for engineers, it would be helpful to present it for them as part of their software packages. This would facilitate its incor- poration into the design. An example is developing software to predict meandering potential and future location of the channel. Also, software for modeling embankment damage is long overdue. Modeling of damage is important for many reasons, such as to identify whether a system is feasible and to predict at what point the embankment would fail. CONCLUSIONS Minimizing roadway embankment damage from flooding requires knowledge of many disciplines. Understanding the project needs based on the site-specific conditions at an early stage of design, the limitations of the available methodolo- gies, the agencies involved and the relevant constraints on the decision-making process, the risk that can be accepted, and the relevant regulations on local, state, and federal levels are crucial for preparing adequate repair measures within the imposed time, monetary, and quality frame of work. It is essential to understand that sound engineering judg- ment is an indispensable component of every design deci- sion. Whereas this synthesis highlights some common practices and important design aspects, it is not a standard or a guideline. Rather, this synthesis is a first step toward understanding the components of this topic, employing available knowledge to produce better designs, and identify- ing the research gaps. In summary, roadway embankment damage from flood- ing is a shared concern among the 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 spe- cific to this topic. The information presented in the synthesis is based on a review of the related literature, a survey of cur- rent practice, and a series of telephone interviews. The prob- able failure mechanisms are identified and possible design approaches and repair countermeasures are highlighted. The study presents a comparison between roadway embankments and levees to emphasize that embankments are not designed as flood control structures. The differences between riverine and coastal flood mechanisms are also stated. The common failure mechanisms in coastal and riv- erine environments are identified as overtopping, seepage (through-seepage and underseepage), piping, wave action, softening by saturation, and lateral sliding on the foundation soil. Pavement failures and culvert-related failures are also outlined. Examples of failures and repair techniques are illus- trated through 14 case examples gathered from six states. As reflected by the case examples, variable state practices, and available literature, minimizing damage to roadway embankments can be tackled by altering the embankment design and slope protection techniques or altering the stream course, or both. A number of systems and approaches can be considered for the same project. The success of an approach is site-dependent because, as shown in the case examples, an approach that serves its intended design purpose at one site does not necessarily work at another site. Different failure modes were identified by the case exam- ples and different solutions were demonstrated. The effec- tiveness of the adopted solutions are generally dependent on the site conditions. A protection technique that would prove successful in one site condition is not necessarily the adequate solution for other site conditions. To arrive at an adequate design, the following factors should be considered: hydrologic and hydraulic factors, geological and geotech- nical factors, legal and funding aspects, and risk. General observations and guiding principles are listed. Hydrologic studies are essential in estimating the magni- tude 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

78 flows can lead to more advanced analyses. Hydraulic meth- ods use the hydrologic data to give an estimate of the water surface elevation, the overtopping height, and the water velocity. Major geological considerations include the sub- surface conditions, topography, floodplain and meandering potential, erosion 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; thus, the identification of the characteristics of the embankment materials and their impact on the behavior of the embankment during flooding is very important. Some key aspects include erodibility of the embankment materi- als, material properties (strength and permeability), and cul- vert- and pavement-related considerations. The decision-making process is not solely based on techni- cal aspects; it is also influenced by legal, regulatory, and fund- ing aspects. After a failure, decisions must be made related to the type of repairs and to whether the same design will be repeated or whether betterments will be sought (temporary versus permanent, changing stream course, raising the free- board). These decisions are all bound by funding constraints, time constraints, constraints from interaction with other agencies (such as the U.S. Army Corps of Engineers, Federal Emergency Management Agency, or Environmental Protec- tion Agency), and, in some cases, community constraints. Available design steps to mitigate the impact of flooding on roadway embankments are outlined. The steps 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 are identified and their use is associated with the most effective applications. They include the following: 1. Vegetation 2. Riprap and geotextiles 3. Gabions 4. Articulated concrete blocks 5. Paving. It is essential to couple the design process and the use of countermeasures with engineering 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 evalu- ate the probability of failure and the value of the consequence in terms of lives lost and economic loss. Ideally, it is the com- bination of the probability of failure and the value of the con- sequence or risk that can most effectively guide the decision. Finally, the ongoing and future research needs are outlined. UNITS Acceleration 9.81 m/s2 = 386.22 in./s2 = 32.185 ft/s2, Paris: g = 9.80665 m/s2, London: g = 3.2174 x 101 ft/s2 Area 1 m2 = 1.5500 x 103 in.2 = 1.0764 x 101 ft2 = 1.196 yd2 = 106 mm2 = 104 cm2 = 2.471 x 104 acres = 3.861 x 107 mi2 = 1.0000 x 10-4 hectares Bending stiffness 1 kN.m2 = 103 N.m2 = 106 kN.mm2 = 2.4198 x 103 lb.ft2 = 2.4198 kip.ft2 = 3.4845 x 102 kip.in.2 = 3.4845 x 105 lb.in.2 Coefficient of consolidation 1 m2/s = 3.1557 x 107 m2/yr = 104 cm2/s = 6x 105 cm2/min = 3.6 x 107 cm2 /h = 8.64 x 108 cm2/day= 2.628 x 1010 cm2/month = 3.1536 x 1011 cm2/year = 1.550 x 103 in.2/s = 4.0734 x 109 in2/month= 1.3392 x 108 in2 /day = 4.8881 x 1010 in.2/year = 9.3000 x 105 ft2/day = 2.8288 x 107 ft2/month = 3.3945 x 108 ft2/year Flow 1 m3/s = 106 cm3/s = 8.64 x 104 m3 /day = 8.64 x 1010 cm3/day = 3.5314 x 101 ft3/s = 3.0511 x 106 ft3/day Force 10 kN = 2.2481 x 103 lb = 2.2481 kip = 1.1240 t (short ton = 2,000 lb) = 1.0197 x 103 kg = 1.0197 x 106 g = 1.0197 T (metric ton= 1000 kg) = 109 dynes = 3.5969 x 104 ounces = 1.022 tl (long ton = 2200 lb) Force per unit length 1 kN/m = 6.8522 x 101 lb/ft = 6.8522 x 10-2 kip/ ft = 3.4261 x 10-2 t/ft = 1.0197 x 102 kg/m = 1.0197 x 10-1 T/m Length 1 m = 3.9370 x 101 in. = 3.2808 ft = 1.0936 yd = 1010 Angstrom = 106 microns = 103 mm = 102 cm = 10-3 km = 6.2137 x 10-4 mile = 5.3996 x 10- 4 nautical mile Moment or energy 1 kN.m = 7.3756 x l02 lb.ft = 7.3756 x 10-1 kip.ft = 3.6878 x 10-1 t.ft = 1.0197 x 103 g.cm = 1.0197 x 102 kg.m = 1.0197 x 10-1 T.m = 103 N.m = 103 Joule Moment of inertia 1 m4 = 2.4025 x 106 in4 = 1.1586 x 102 ft4 = 1.4304 yd4 = 108 cm4 = 1012 mm4

79 Moment per unit length 1 kN.m/m = 2.2481 x 102 lb.ft/ft = 2.2481 x 10-1 kip.ft/ft= 1.1240 x 10-1 t.ft/ft = 1.0197 x 102 kg.m/m = 1.0197 x 10-1 T.m/m Pressure 100 kPa = 102 kN/m2 = 1.4504 x 101 lb/in.2 = 2.0885 x 103 lb/ft2 = 1.4504 x 10-2 kip/in.2 = 2.0885 kip/ft2 = 1.0443 t/ft2 = 7.5006 x 101 cm of Hg (0 °C) = 1.0197 kg/cm2 = 1.0197 x 101 T/m2 = 9.8692 x 10-1 Atm = 3.3489 x 101 ft of H2O (60 °F) = 1.0000 bar = 106 dynes/cm2 Temperature °C = 5/9 (°F - 32), °K =°C + 273.15 Time 1 yr = 12 mo. = 365 day = 8,760 h = 5.256 x 105 min = 3.1536 x 107 s Unit weight, coefficient of subgrade reaction 10 kN/m3 = 6.3659 x 101 lb/ft3 = 3.6840 x 10-2 lb/in.3 = 1.0197 g/cm3 = 1.0197 T/m3 = 1.0197 x 103 kg/m3 Velocity or permeability 1 m/s = 3.6 km/h = 2.2369 mile/h = 6 x 101 m/ min = 102 cm/s = 3.15 x 107 m/yr = 1.9685 x 102 ft/min = 3.2808 ft/s = 1.0346 x 108ft/year = 2.8346 x 105 ft/day Volume 1 m3 = 6.1024 x 104 in.3 = 3.5315 x 101 ft3 = 1.3080 yd3 = 109 mm3 = 106 cm3 = 103 dm3 = 33814.02 ounces= 2113.38 pints (US) = 103 liter = 2.1997 x 102 gallon (UK) = 2.6417 x 102 gal- lon (US)