Background

According to the National Oceanic and Atmospheric Administration, the number and cost of weather and climate-related disasters are increasing in the United States due to a combination of increased exposure, vulnerability, and the fact that climate change is increasing the frequency of extreme events. The increased frequency of hurricanes and severe storm events are requiring state departments of transportation (DOTs) to consider how to anticipate, plan for, and adapt to these changing conditions. In addition, DOTs are considering how to withstand, respond to, and recover more rapidly when disruptions occur. For these reasons, more complex hydrologic modeling that allows for scenario testing and impact assessment is needed.

Distributed rainfall-runoff methods are models that use physical equations to describe rainfall patterns and water movement to create flow rates. Distributed rainfall-runoff models are hydrologic models that simulate runoff production, transport, and accumulation in waterways. These models split the watershed into small elements that are used to calculate and track infiltration and movement of runoff on the ground by using processed based equations.

Increased computing power and modeling efficiency have made distributed rainfall-runoff models more cost effective for engineering projects. Because distributed models are process based, they also have more flexibility than statistical methods because they are not restricted to historical data. As a result, there has been increased interest among engineering practitioners in using distributed rainfall-runoff models to create more resilient designs. However, there has been little documented guidance on applying modern distributed rainfall-runoff models to help engineers use them in the highway design process.

Objective

The objective of this synthesis is to document state DOT use of distributed rainfall-runoff models. The synthesis will focus on the use of distributed rainfall-runoff methods for hydrologic analyses for the planning, design, and operation of bridges and roadway projects.

Information to be gathered regarding state DOT use of distributed rainfall-runoff models includes (but is not limited to):
• The extent to which state DOTs are using these models;
• Types of projects for which the models are used (e.g., design, long-range planning projects, vulnerability assessments);
• Factors that determine whether the models are applicable (e.g., watershed basin size, watershed characteristics, calibration information availability, complexity of transportation project, future climate event scenarios, funding, continuous versus event based simulation, etc.);
• Distributed rainfall-runoff modeling software packages used (e.g., HEC-RAS 2D, HEC-HMS);
• Modeling techniques used (e.g., grid sizes, parameterization of hydrologic processes such as groundwater infiltration and sheet flow, modeling hydraulic control structures, calibration techniques, etc.);
• 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 distributed rainfall-runoff models;
• 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).

Information will be gathered through a literature review, a survey of state DOTs, and follow-up interviews with selected agencies for the development of case examples. Information gaps and suggestions for research to address those gaps will be identified.

Status

Completed report can be found at NCHRP Synthesis Report 602.