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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2023. Resilient Design with Distributed Rainfall-Runoff Modeling. Washington, DC: The National Academies Press. doi: 10.17226/27051.
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Page 1
Page 2
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2023. Resilient Design with Distributed Rainfall-Runoff Modeling. Washington, DC: The National Academies Press. doi: 10.17226/27051.
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Page 2
Page 3
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2023. Resilient Design with Distributed Rainfall-Runoff Modeling. Washington, DC: The National Academies Press. doi: 10.17226/27051.
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1   The increased frequency of extreme rainfall events, inland and coastal flooding, and other water-related stressors poses challenges to roadway infrastructure. Initial approaches to quantify these stressors used lumped hydrological modeling approaches. These approaches provide rapid and convenient estimates of peak flows or simplified hydrographs associated with hypothetical or synthetical design storms. Lumped approaches rely on past hydrological data—for example, measured streamflows to develop regression equations and observed rain- fall for the intensity–duration–frequency (IDF) curves, which might not be representative of the increased stressors linked to climate change. Moreover, lumped approaches may group distinct watershed characteristics, which can impact the accuracy of hydrological predictions. The Distributed Rainfall-Runoff Model, referred to as DRRM, is a newer type of hydro- logic modeling approach that divides the watershed under study into smaller spatial elements. Within each of these elements, different hydrological processes are represented that enable a more detailed and accurate representation of runoff formation, transport, and accumulation. The formulation and implementation of DRRMs enable representative hydrological descrip- tions of watersheds, supporting effective decision making regarding hydrological risks. This synthesis documents state department of transportation (DOT) practice on the use of DRRMs. The synthesis identified state DOTs that have adopted DRRMs and the context in which these models are applied. The sources of information used in this synthesis included: (1) a review of the design guidelines for each state DOT about hydrological modeling, (2) a literature review of published studies that involve the use of DRRMs in the context of roadways, (3) a survey of state DOTs, and (4) follow-up interviews with selected state DOTs that detailed the application of DRRMs in the context of a roadway design or study. A survey was sent to 50 state DOTs and the District of Columbia and Puerto Rico trans- portation agencies. Forty-seven state DOTs and Puerto Rico responded to the questionnaire, corresponding to a response rate of 92%. The survey questionnaire was organized into five groups: (1) status of DRRM use in state DOTs, (2) factors determining the use of DRRMs in hydrological design by state DOTs, (3) characteristics of DRRM implementation within transportation agencies, (4) assessment of the costs and benefits of DRRMs, and (5) barriers for implementation of DRRMs. After the review of the questionnaire results, four DOTs were invited for a follow-up interview: North Carolina, Kentucky, California, and Texas. Key findings from the literature review, survey of DOTs, and case examples include the following: • The degree of DRRM adoption among state DOTs is still not as widespread as some other lumped approaches. The Rational Method and regression equations are referenced in 98% of the hydrological design guidelines adopted by 50 state DOTs. On the other hand, refer- ences to DRRMs appeared only in 54% of these design guidelines. Resilient Design with Distributed Rainfall-Runoff Modeling S U M M A R Y

2 Resilient Design with Distributed Rainfall-Runoff Modeling • One common application for hydrological techniques is estimating peak flows from hydro- logical events, such as culvert design. Lumped approaches can provide these peak esti- mates more conveniently, albeit less adaptable than DRRMs, considering factors such as climate change. • According to the review of the DOTs’ hydrological design guidelines, the most referenced DRRM was the Hydrologic Engineering Center Hydraulic Modeling Software (HEC-HMS) model, with a total of 24 states (48% of the guidelines). Other models referenced in the design guidelines included the HEC-1 model, Watershed Modeling System (WMS)-based models, Storm Water Management Model (SWMM), and a few commercial models. • Another type of model referenced in state DOT hydrological design guidelines was the WinTR-20 (42% of the guidelines) and the WinTR-55 (52% of the guidelines). • A literature review was performed in the available references in the National Transportation Library that involved roadway transportation and DRRMs. These documents are not design guidelines but indicate how DRRMs could be applied in studies involving transportation projects and hydrology. This review indicated three main types of studies in which DRRMs were used: (1) assessment of the vulnerability or the resiliency of transportation infrastruc- ture to flooding events, (2) evaluation of processes such as bridge scouring and aggradation and streambed mobility, and (3) the impact of stormwater runoff from roadways. • Sixteen responding state DOTs (33% of the respondents) indicated that they actively use DRRMs in their hydrological analysis. This number is less than the number of state DOTs that reference these models in their design guidelines. The survey also indicated that most agencies using DRRMs (63%) adopted DRRMs more than 15 years ago. • Eighty-eight percent of the respondents that use DRRMs (14 agencies) use private consul- tants in studies involving DRRMs, and 13 agencies use in-house engineers for this modeling tool. The top three types of guidance used in conjunction with DRRMs include federal design guidance (11 agencies), software user manuals (11 agencies), and software training materials (9 agencies). • The top factors and advantages determining the use of DRRMs that were reported in the questionnaire included (1) the size of the watershed being studied and its characteristics, (2) a better spatial representation that was achieved by the discretization of these models, and (3) a better representation of the rainfall loss, runoff transformation, and routing. • The top three applications for DRRMs in the context of state DOTs included (1) deter- mining peak flows and hydrographs, (2) designing hydraulic infrastructure, and (3) assess- ing roadway resiliency. • Concerning data requirements, most responding state DOTs that use DRRMs indicate the knowledge of the stream network (15 agencies) and digital elevation maps (13 agencies). Other data requirements included land use/land cover knowledge and soil properties data. • Regarding rainfall data used in DRRMs, the agencies that use DRRMs indicate that the two most commonly used are the design rainfall depth for different return periods (14 agen- cies) and the design storm rainfall distribution over time (8 agencies). The most predomi- nant source of rainfall data was synthetic rainfall, such as the Atlas 14 (13 agencies). The most predominant method to compute rainfall losses was based on the Soil Conservation Service (SCS) Curve Number (CN) (15 agencies). • Most responding state DOTs that use DRRMs apply the effective rainfall transformation method proposed by the SCS/Natural Resources Conservation Service (NRCS) (14 agen- cies). With regard to the parameter mostly used in DRRM sensitivity analysis, unit hydro- graph parameters were the most predominant (15 agencies). • The two most common components and metrics used to assess the costs and benefits of DRRMs included (1) the accurate design that reduced project costs (nine agencies) and (2) the importance/criticality of a project that requires improved hydrological simulations (nine agencies).

Summary 3   • Finally, the top three barriers reported by state DOTs to the use DRRMs included (1) the lack of training opportunities, (2) insufficient in-house expertise for operating models, and (3) the turnover of the workforce. • The criticality and costs of certain roadway designs, the need to consider more precisely the effects of extreme rain events, and diagnosed causes for flooding were cited by DOTs as motivations for applying DRRMs. The insight provided by DRRMs and the ability to reduce hydrologic uncertainties were cited as benefits of using DRRMs. However, imple- menting these models is time-consuming, and with limited staff, state DOTs often need to resort to external consultants to perform modeling tasks. • The interviewed DOTs reported that systematic training of the workforce on DRRMs and improved awareness of the costs and benefits associated with these models could expand the use of these tools in roadway operation and design. A suggested area for future research is how DRRMs can quantify and mitigate the impending hydrological stressors linked to climate change on roadways. The compilation of different case studies focused on the best practices for DRRM applications in flooding, scour esti- mates, and stormwater management in the context of roadways could be considered. These could be focused on roadways’ problems and how DRRMs are used to diagnose issues, propose alternative designs, and suggest mitigation strategies. Another potential research area is using different types of spatial rainfall data within these DRRMs, exploring their availability and effectiveness compared with point estimates.

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The increased frequency of extreme rainfall events, inland and coastal flooding, and other water-related stressors poses challenges to roadway infrastructure.

The TRB National Cooperative Highway Research Program's NCHRP Synthesis 602: Resilient Design with Distributed Rainfall-Runoff Modeling documents the practices of state departments of transportation on the use of DRRMs and identifies state DOTs that have adopted DRRMs and the context in which these models are applied.

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