Description

1) Florida flood hazard modeling and vulnerability assessments – an innovative climate change focused approach
Justin Gregory, PE, Jones Edmunds & Associates, jgregory@jonesedmunds.com
Co-presenters: Gavin Lewis, Fathom, g.lewis@fathom.global

Abstract: Hurricane Ian was a devasting example of Florida's flood vulnerability. Extreme coastal flooding paired with rainfall exceeding 20 inches resulted in $112billion in damages. Projected changes to sea level and extreme precipitation will continue to increase Florida's flood risk. Since 2021, the Resilient Florida program has provided over $40million in funding to support communities planning for future flood impacts through flood vulnerability assessments and adaptation. Assessing and planning for expected flood hazard changes are critical to making Florida more resilient. Traditional flood hazard mapping needed to support these flood vulnerability assessments can take years and cost millions. The models used to perform these assessments require frequent updates to account for changes in the landscape, availability of new high‐resolution LiDAR‐based digital elevation models, revised rainfall projections, and changes in boundary conditions. In addition, the different modeling platforms used to perform flood hazard assessments include inconsistencies that can affect the results of the vulnerability assessments. Fathom's US flood hazard maps provide national‐scale coverage of both current and future flood scenarios in unprecedented detail. This presentation will demonstrate how we can apply the latest results from Fathom to assess countywide vulnerability to future flood risk for communities and critical infrastructure. This innovative approach considers Florida’s most recent statewide LiDAR datasets and projected changes in extreme precipitation. Fathom's flood hazard maps for vulnerability assessments provides the potential for consistent analysis across the whole state, the evaluation of multiple future scenarios for inland and coastal flooding, the application of the latest research in flood modeling. Furthermore, this presentation will contrast this analysis with traditional flood modeling approaches to assessing future flood vulnerabilities.

2) Traveling to the Next Dimension of Flood Mitigation using 2-D HEC-RAS
Chris Shultz, PE, JEO Consulting Group, cshultz@jeo.com
Co-presenters: Becky Appleford, rappleford@jeo.com

Abstract: This presentation will offer a design modeling methodology case study highlighting the power of using 2-D modeling for flood mitigation actions. Design of large-scale flood risk reduction has historically been supported by 1-D hydraulic modeling. With the advancement of 2-D capabilities, when is it appropriate to re-analyze a project for residual risk? The Upper Prairie, Silver, and Moores (UPSM) Creek’s watersheds northwest of the city of Grand Island, NE were subject to repetitive and significant flooding resulting in structural flood impacts to the City. These historical flood impacts gave momentum to implementation of a multi-year, multi-phase flood risk reduction project consisting of upland dry dams, a large lowland detention cell, and a levee. Construction of the project was substantially completed in early 2019. In March 2019, a region wide flood event tested the UPSM project. Due to the size and scale of the flood event, significant overtopping between the various watersheds occurred leading to higher flows than those that were simulated by the 1-D models developed during design of the project. While the UPSM project performed well, the 2019 flood illuminated areas of residual risk that 1-D hydraulic modeling could not predict. By leveraging PDM funding for the hazard mitigation plan, a flood risk analysis task was added to better evaluate residual risk and to identify feasible mitigation solutions. Using the new capabilities of HEC-RAS, a rain-on-grid 2-D model was developed for the entire UPSM project region to accurately determine flood risk and mitigation actions for the region. Beyond the modeling updates, a Flood Alert project was created as a response to the 2019 floods, which increased monitoring and modeling capabilities for real flood events.

3) Urban Area Considerations for Flood Protection and Phased Implementation Apple Branch Watershed Danville VA
Chris Rogers, Timmons Group, Chris.Rogers@timmons.com
Co-presenters: None

Abstract: Apple Branch, a FEMA regulated stream, is a tributary to the Dan River located in the heart of Danville, Virginia. Structural impacts to existing structures and roadway flooding occur during high intensity short duration and tropical storm events in this urban watershed. Challenges in maintaining stormwater conveyance come from the steeply sloped watershed of Apple Branch entering a fully developed broad flood plain of the Dan River that has resulted in extensive flooding which extends to adjacent buildings, parking lots and a divided primary collector street paralleling the Dan River. Grant funds for the engineering study were provided through the Pre-Disaster Mitigation Grant Program administrated by the Virginia Department of Emergency Management providing 75% of the total project cost. The engineering study included an evaluation of existing conditions of watershed pipe systems, channels, flood extents and the development of potential solutions to reduce flooding impacts throughout the watershed. The presentation will cover a brief overview of pipe system and two-dimensional hydraulic modeling completed and will focus on proposed solutions developed including detention creation with property buyout, pipe system backflow prevention and pipe/culvert system improvements using a phased approach to accommodate City funding and associated time frames. This presentation will provide information regarding recommended proposed improvements and priority phasing to maximize benefits. Proposed improvements to be discussed will include adjustable detention wall height recommendations incorporating a permanent wall structure with a temporary wall extension to contain both existing culvert and future bridge replacement conditions. Benefits of partial solutions will be explored that provide some level of flood relief to adjacent property owners until the full extents of the improvements can be made.