Description

1) Updates and Advancements in 2-Dimensional Modeling Processes: FEMA’s 2D IPT
Laura Algeo, FEMA HQ, laura.algeo@fema.dhs.gov
Co-presenters: Geoff Uhlemann, geoff.uhlemann@mbakerintl.com; Andy Bonner, andrew.bonner@aecom.com

Abstract: In 2019, FEMA initiated an Integrated Project Team (IPT) to focus on 2D floodway mapping issues. Several short-term issues were discussed and solved for with updates made to guidance in the 2020/2021 cycle. In June 2022, FEMA expanded this IPT to find solutions to various questions and issues associated more generally with 2D modeling as a whole. The IPT is focusing on the use of 2D meshes for flood modeling and how to ensure our products can be consistent between 1D and 2D modeling techniques. As our roles in supporting resilient communisis change over time, so does the delivery of our program, the IPT is also investigating potential changes in what and how we deliver flood hazard data to be successful in the future. The IPT consists of over 100 members across the public and private sectors. Members formed subgroups to focus on finding solutions related to flood products, maps, hydrology, hydraulics, regulatory requirements, and model setup. This presentation will review the 2D IPT, changes already suggested and approved, and the progress made to-date.

2) HEC-RAS Junction, What’s Your Function
Kevin Donnelly, PE, CFM, PMP, GISP, Stantec, kevin.donnelly@stantec.com
Co-presenters: None

Abstract: The Junction feature of HEC-RAS is typically not utilized for 1D steady flow on FEMA hydraulic studies. A stream junction is a location where two or more streams come together or split apart. The Junction in HEC-RAS links two reaches together and applies the backwater of one stream upon the other. When this function is not used, FEMA Mapping Partners typically model two streams separately, each assuming a normal depth based upon the channel slope at the downstream end of the stream. After the modeling is completed, the Mapping Partner manually assigns the main stem water surface elevation to the tributary for each flood profile (10%, 4%, 2%, 1%, 0.2%, 1%+) for each tributary cross section where the main stems water surface elevation exceeds that of the tributary for that flood profile. The incorrect application of backwater is one of the main errors FEMA Mapping Partners make in the development of FIS studies and has led to revised Preliminary Map Products needing to be issued, resulting in significant delays in completion of RiskMap projects. In order to avoid these mapping errors and delays, FEMA Mapping Partners spend significant amounts of effort assigning and checking backwater and water surface elevations at confluences. This presentation will look at the FEMA Guidance that prevents the wider use of this function and the consequences of its use/non-use. We will explore questions such as, are the assumptions in the FEMA Guidance appropriate and/or worthwhile? Why is this an option built into HEC-RAS by the USACE, if it is not technically credible? The presentation will compare results of studies modeled with and without Junctions to see if there are significant differences in the end results and if considerations such as relative stream size, terrain, or level of hydraulic analysis should impact the choice to use this function.

3) Exploring Mesh Considerations for Basin Scale 2D Hydrologic Modeling with HEC-RAS 6.3.1
Mark Forest, PE, CFM, HDR, mark.forest@hdrinc.com
Co-presenters: None

Abstract: As FEMA is working on the development of Guidelines and Standards for 2D modeling solutions within the NFIP, the FEMA 2D IPT Committee, Hydrology Subgroup has been evaluating the influence of grid size and other model mesh refinements on result sensitivity. The initial findings indicated grid size sensitivity. This presentation will explore the factors that are important for accurate and consistent watershed scale rain-on-mesh simulations to avoid grid size sensitivity. The use of rain-on-mesh solutions is the rapidly evolving approach to performing hydrologic analysis on watersheds of various sizes and topographic complexity. Rain-on-grid solutions provide a more physically based approach for modelling the flow accumulation from the watershed as an option to unit hydrographs and simulates rainfall excess with spatially and temporally specific inputs of rainfall, initial abstraction and infiltration parameters. Rain-on-mesh simulations also provide an opportunity to perform the hydrologic and hydraulic computations simultaneously and identify flood hazards throughout the watershed. However, there are modeling considerations to consider when attempting to use the results for both hydrology and hydraulics that will be explored in this presentation as well. This testing was performed using HEC-RAS 6.3.1. The goal of the testing was to identify key principles necessary to get more consistent results with variation in grid size.