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Resilient Design with Distributed Rainfall-Runoff Modeling (2023)

Chapter: Chapter 5 - Summary of Findings

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Page 66
Suggested Citation:"Chapter 5 - Summary of Findings." 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 66
Page 67
Suggested Citation:"Chapter 5 - Summary of Findings." 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 67

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66 This synthesis objective is to document state DOT practice on the use of DRRMs. The infor- mation was gathered for the synthesis through a literature review, a survey of state DOTs that received 48 responses, and interviews with selected DOTs that provided valuable insights into their hydrological modeling practices. The findings of the survey in this synthesis are summarized below: • Lumped approaches, such as gage data analysis to determine peak flows, regression equations, and the Rational Method, are the most popular approaches used by DOTs in hydrological studies. • By comparison, DRRMs are not as frequently applied by state DOTs for hydrological studies. Whereas 54% of the design guidelines from DOTs reference DRRMs, only 30% of the trans- portation agencies that responded to the survey (16 out of 48) have reported using DRRMs in their modeling practices. Most of the use is linked to the determination of peak flows to support hydraulic designs. • Based on the literature review and the responses obtained in the survey, the DRRM tool that is most adopted across DOTs is HEC-HMS. This tool is cited in 48% of the DOT design guideline documentation. Other DRRM tools that were referenced in these hydrological design guide- lines were HEC-1, SWMM, and WMS-based tools. • The literature review on the NTL focusing DRRMs application in roadway infrastructure indi- cated a diverse application of these tools in a variety of water resources and environmental problems. The review covered studies in the last 20 years, and nearly half of the reports (47%) were published in the past 5 years. The DRRM tools most frequently used in these reports were HEC-HMS (56%), followed by SWMM (19%), and applications included the calculation of peak flows, flooding studies, and stormwater management. • Considering only the subset of the questionnaire respondents that use DRRMs (16 DOTs) HEC-HMS was adopted by 75% of these agencies. Other popular DRRM tools were EPA SWMM 5 (and its commercial implementations) and HEC-RAS 2-D. • Interviews with staff in selected state DOTs indicated that most hydrological studies and designs are performed with lumped approaches. The case example DOTs reported that DRRMs are used for critical applications such as the following: – To assess and recommend design decisions in roadways likely to be impacted by extreme hydrological events. – To determine the causes for flooding affecting roadways within watersheds and to propose various scenarios with countermeasures to prevent or mitigate flooding. – To reduce the uncertainty in hydrological estimates and enable higher accuracy in hydraulic computations of bridge scour. • A project or study involving DRRMs is typically developed either by in-house engineers or private consultants. Most of these applications apply synthetic rainfall distributions and use the CN as a means to account for rainfall losses. C H A P T E R   5 Summary of Findings

Summary of Findings 67 • The applications that were discussed in the interviews with DOT staff were all event-based simulations instead of continuous simulations. • Variables that were used in the calibration of DRRMs discussed in Chapter 4 included the roughness of overland and channel flow and parameters that regulate rainfall abstractions. The calibration aimed at the representation of reported high water marks and gaged peak flow, when these were available. Upon calibration, the models were accurate in representing the extreme hydrological conditions they were intended to represent. Barriers for DRRM Implementation in State DOTs The capabilities and versatility of DRRMs have improved in the past years, and the context in which they have been used in roadway design and assessments has consequently expanded. Yet, as indicated in this synthesis, a large portion of the state DOTs do not apply DRRMs routinely in roadway projects. Fatichi et al. (2016) pointed out that certain lumped models are easier to use, require less training and understanding of underlying physical processes, and often require much less data. Suggested Future Research Considering the information that was gathered about the state of practice regarding the use of DRRMs by state DOTs, the following research is suggested to widen the use of these models for transportation infrastructure: • The development of a guidance document focused on the application of DRRMs in the context of transportation infrastructure. Although the existing studies and reports applying DRRMs for transportation are growing, there has been no attempt to generalize the procedures beyond the context of case studies. This document could potentially include the following: – A thorough comparison between semi-distributed and gridded-based DRRMs for relevant problems in the context of roadways; – Exploration of hydrological conditions in which the advantages of DRRMs are fully explored and contrasted with lumped models; – Best practices for the development of DRRMs in roadway applications; – Determination of optimum data requirements to run DRRM applications; and – Cost–benefit assessments of using DRRMs to support hydrologic decision making for roadway design and operation. • An evaluation of the use of different types of distributed spatial rainfall data within DRRMs, particularly gridded rainfall, considering future climate scenarios. This research topic could include a discussion on different sources of spatially distributed rainfall and how much impact distributed spatial rainfall data have in DRRM predictions compared with lumped rain data. • A comparison between the application of event-based simulation using DRRMs versus continuous simulation in which multiple hydrological stressors could be considered over an extended time period. This research would, for instance, enable an evaluation of how a sequence of rain events could impact flooding due to reduced infiltration.

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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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