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Resilient Design with Distributed Rainfall-Runoff Modeling (2023)

Chapter: Chapter 1 - Introduction

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Suggested Citation:"Chapter 1 - Introduction." 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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Suggested Citation:"Chapter 1 - Introduction." 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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Suggested Citation:"Chapter 1 - Introduction." 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.
×
Page 6
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Suggested Citation:"Chapter 1 - Introduction." 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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4 This chapter presents definitions that are used throughout the work, background information, the synthesis objectives, the synthesis approach, and the report organization. Definitions Certain key terms are defined that pertain to the scope of the synthesis. Additional terms are defined in the context of the relevant sections. • Curve Number (CN): A widely used hydrological parameter that predicts direct runoff or infil- tration from rainfall, presented in the Windows Technical Release 55 (WinTR-55) (Cronshey et al. 1986). Tabulated values of CN range from 46 to 98, with larger values associated with larger runoff volumes and smaller infiltration. The CN values depend on the Hydrologic Soil Group (HSG), the type of land use, and the antecedent moisture conditions. • Digital Elevation Models (DEMs): Computer files used in the context of geospatial applications that provide a representation of the bare ground topographic land surface that disregards natural or man-made features. However, more recent DEMs have been derived from light detec- tion and ranging (lidar) studies, which consider surface objects such as trees and buildings (USGS 1981; Humpal 2022; Stoker and Miller 2022). • Distributed Rainfall-Runoff Models (DRRMs): In this synthesis study, DRRMs include the dis- tributed or semi-distributed models used/defined by Sitterson et al. (2017), as shown in Fig- ure 1. These are two types of hydrological modeling tools that create a spatial discretization of a studied watershed. These tools enable the use of more detailed soil characteristics and pos- sibly physics-based formulations to describe hydrological processes. These models are more detailed in the subsequent chapters of this synthesis. • Hydrologic Soil Group (HSG): A classification of soil characteristics that is based on measured rainfall, runoff, and infiltrometer data (Musgrave 1955). Soil groups vary from Group A, with the smallest runoff potential, to Group D, with the highest runoff potential when thoroughly wet (Hawkins et al. 2017; NRCS 2017). HSGs influence the assigned CN in an area within a watershed. • Lumped Models: Models and approaches that group/average the characteristics of watersheds and rainfall to perform hydrological estimates, typically of peak flows. These include the Rational Method, regression equations, and gage data analysis. • Regression Equations: A methodology based on multiparameter statistical regression data that is used to estimate hydrological characteristics of streams, including peak flows for a given recurrence interval. The methodology applies existing stream information derived in gaged watersheds to be applied in ungaged watersheds to estimate flows based on land use, local geology, and catchment areas, among other factors. • Unit Hydrograph (UH) Method: Hydrological methods that apply mathematical procedures such as convolution to transform the excess rainfall into a direct runoff hydrograph for a C H A P T E R   1 Introduction

Introduction 5 catchment. Examples of methods using the UH approach include the Snyder (1938) method and the Soil Conservation Service (SCS) dimensionless UH (Cronshey et al. 1986). Background State departments of transportation (DOTs) are tasked with the planning, design, operation, and maintenance of transportation infrastructure considering a variety of stressors, including water-related stressors. With a future scenario of climate change, sea-level rise, and more intense and frequent storms, water-related stressors such as flooding, in-stream structure scour, and aggradation can worsen. Water quality impacts created by pollutants on stormwater runoff from roadways and bridges also create challenges that are met by state DOTs. Lumped hydrological modeling approaches have been applied successfully in the past to estimate peak flows to support the hydraulic design of conveyance facilities in roadways. Yet some of these lumped techniques, such as the Rational Method, might be less accurate and thus provide less reliability for design and operation decisions to overcome worsened water-related stressors. Distributed hydrological models are comparatively newer approaches that introduce spatial discretization of watersheds and enable higher accuracy and more detailed results. These models, called Distributed Rainfall-Runoff Models, or DRRMs, in this synthesis, are constructed with physical-based formulation in addition to semi-empirical ones. Either through the use of sub- catchments, hydrological response units, or gridded units, DRRMs perform necessary hydro- logical calculations at each spatial unit over time, which are integrated with routing processes within the watershed. DRRMs provide more insights on the hydrology of watersheds than do lumped approaches. Certain DRRM results include inundation maps, flow velocity maps, esti- mated shear stresses near hydraulic structures, interactions with shallow groundwater, and water quality simulations. Synthesis Objective This synthesis documents DOT practice on the use of DRRMs. The synthesis focused on the use of DRRMs for hydrologic analyses for the planning, design, and operation of bridges and roadway projects. This synthesis gathered the following information: • The extent to which state DOTs are using these distributed models; • Types of projects for which the models are used (e.g., design, long-range planning projects, vulnerability assessments); A. B. C. Figure 1. The spatial discretization in rainfall-runoff models. (A) Lumped model, (B) Semi-distributed model by sub-catchment, and (C) Distributed model by grid cell (Sitterson et al. 2017). For this synthesis, models using B and C are defined as DRRMs.

6 Resilient Design with Distributed Rainfall-Runoff Modeling • Factors that determine whether the models are applicable (e.g., watershed basin size, water- shed characteristics, calibration information availability, complexity of the transportation project, future climate event scenarios, funding, continuous versus event-based simulation); • Distributed rainfall-runoff modeling software packages used [e.g., Hydrologic Engineering Center-River Analysis System—two dimensions (HEC-RAS 2D), HEC-Hydraulic Modeling Software (HEC-HMS)]; • Modeling techniques used (e.g., grid sizes, parameterization of hydrologic processes such as infiltration and sheet flow, modeling hydraulic control structures, and calibration techniques); • Models’ data requirements and data sources (e.g., historic and future rainfall, topographic, calibration data); • Who is currently doing the modeling (e.g., in-house, consultants); • Documented state DOT guidance for DRRMs; • Documented state DOT assessment of costs and benefits regarding the use of distributed rainfall-runoff modeling techniques; and • Barriers to implementation (e.g., lack of in-house expertise, data availability). Study Approach and Report Organization To achieve the goals of the synthesis, a multipronged effort was performed to collect data and document the current use of DRRMs in state DOTs. The sources used for the data gathering in this synthesis included (1) a literature review, (2) a survey of state DOTs, and (3) interviews with selected DOT personnel detailing case examples in which DRRMs were applied in the context of transportation infrastructure. The literature review, presented in Chapter 2, was based on two main types of documents. The first group of documents was obtained through a search into each state transportation agency’s website for manuals and guidelines on hydrological design. A complete list of each of the state DOT’s hydrological design guidelines, including eventual references to DRRMs, was produced and is presented in Table 2. The second group of documents was obtained by researching the National Transportation Library (NTL) for publications over the past 20 years that make direct reference to DRRMs in the context of roadway transportation. Although not strict design guide- lines, the documents on the NTL provide valuable insight on the useful applications in which DRRMs were used for roadway-related studies. These studies included resiliency assessments of roadway infrastructure to flooding, sediment-related issues such as scour and aggradation, and the hydrologic and water quality impacts of stormwater runoff. Chapter 3 presents the results of the second component of the data gathering effort, based on the questionnaire sent to personnel in all 50 state DOTs who are in charge of hydrological studies. This survey was also sent to personnel in the District of Columbia and Puerto Rico transporta- tion agencies. The questionnaire comprised 20 questions that aimed to provide details on the status of DRRM use, as well as the factors that determine the use of DRRMs in hydrological design. The questions also provided details on the characteristics of DRRM implementation, the costs and benefits associated with the use of these tools, and the barriers for the imple- mentation of these models. In addition, the questionnaire was adapted to the DOTs that do not adopt DRRMs, in which case the questions addressed the factors and advantages determining the use of lumped approaches. The third and last component of data gathering involved the interview of personnel in state DOTs who are involved in hydrological studies that use DRRMs. These interviews were valuable because the details of selected projects that used distributed modeling were discussed. The inter- views provided detailed background and motivation for using DRRMs in projects in which the criticality, liability, greater uncertainties in estimates, and costs were major keys. These projects

Introduction 7 involved the estimates of flooding in roadways and propose approaches to improve roadway resil- ience and more precise estimates of peak flows for bridge scour computations, among other applications. The synthesis is completed with Chapter 5, which presents a summary of key findings of the literature review, questionnaire answers, and interviews. These findings include an evaluation of the applications in which DRRMs are most frequently applied, the barriers that were identified for a wider adoption of these modeling tools, and recommended future research linked to DRRM use in roadway context. After these chapters, Appendices A and B present a blank survey question- naire and a copy of the survey questions and results.

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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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