Description

1) Beyond the Bayous - The Harris County Urban Flood Risk Pilot Study
Todd Ward, PE, CFM, Harris County Flood Control District, todd.ward@hcfcd.org
Co-presenters: Ashley Poe, ashley.poe@freese.com; Brian Edmondson, brian.edmondson@freese.com

Abstract: As 2D modeling has become a mainstay in the H&H world helping to drive decision-making and support floodplain management, a new challenge has emerged associated with model maintenance – piecemeal terrain updates to reflect new development and projects completed after base lidar is flown. What standards, guidance, and considerations can help communities, regulatory agencies, engineers, and developers to anticipate potential issues, refine survey requirements, and plan for long-term model maintenance? In the forefront of navigating this challenge, the Harris County Flood Control District (HCFCD) acquired lidar in 2018 for the Modeling, Assessment, and Awareness Project (MAAPnext), a detailed county-wide floodplain study utilizing advanced watershed-wide 2D modeling techniques. Due to the long time span required to produce FEMA floodplain maps at such a scale, HCFCD encouraged and accepted the incorporation of terrain updates through 2021 before models and flood risk products were finalized to capture the ongoing rapid changes in the county since the lidar was flown, which would also ultimately minimize the need for future updates including LOMRs. As one of the fastest growing counties in the United States still recovering from Hurricane Harvey, Harris County experienced significant land development and construction of flood mitigation projects within this time period, resulting in diverse types and scales of terrain update requests. This presentation will share insights from reviewing and incorporating terrain updates for two of the most dynamic and rapidly developing watersheds (Addicks and Cypress Creek), including key points from a comprehensive guidance document that was developed to maintain model integrity after the MAAPnext models become effective. Specific topics include considerations for what submittal requirements should be based on community needs, common pitfalls and their consequences, and a streamlined process for creating Digital Elevation Models (DEMs) that can be incorporated in a HEC-RAS model terrain.

2) Integrated Pluvial and Fluvial Flooding Assessment of an Urban Watershed in Fayetteville, NC
Matthew Jones, PhD, PE, Hazen and Sawyer, mjones@hazenandsawyer.com
Co-presenters: Alicia Lanier, PE; AliciaLanier@FayettevilleNC.gov

Abstract: The Cross Creek watershed in Fayetteville, NC, traverses assorted land uses across the City, including the downtown area, before discharging to the Cape Fear River. The watershed has a history of pluvial and fluvial flooding, including notable flooding in the downtown area associated with Hurricane Florence in 2018. As part of a comprehensive city-wide stormwater master planning initiative, the City is studying the Cross Creek watershed to better understand flooding concerns and develop proposed solutions to provide drainage and flood relief. Existing breached and intact impoundments are distributed throughout the watershed, posing unique opportunities to mitigate flooding of the downstream downtown area through upstream impoundment retrofits. A linked and dynamic modeling approach was essential to fully understand the storm dynamics through the watershed and effectively evaluate proposed solutions, especially given the implications of hydrograph timing and watershed detention characteristics on downtown flooding. Storm drainage infrastructure was evaluated through the use of a 1D/2D Infoworks ICM model, connected with a 2D unsteady HEC-RAS model of Cross Creek and its main tributaries. The HEC-RAS model built upon the FEMA steady-state, 1D HEC-RAS model and 2D model results were compared to those reported by FEMA. Pluvial flooding concerns in this watershed are primarily addressed through pipe upsizing, necessitating a consideration of downstream impacts due to reduced attenuation. Best practices were established through the course of watershed master plan development to effectively consider interactions between the creek, impoundments, and network of pipes and surface channels across both the HEC-RAS and Infoworks ICM model. This presentation will demonstrate how recent developments in 2D modeling tools for pipe networks and open channels support comprehensive flooding evaluations and optimized solutions in complex watersheds where understanding the dynamic interaction of hydrologic and hydraulic elements is crucial to providing effective drainage and flood relief.

3) Easy as HEC: Case Studies in Maximizing New HEC-RAS Capabilities
Andrew Swynenberg, Freese and Nichols, Inc., andrew.swynenberg@freese.com
Co-presenters: Mark Pauls, PE, CFM; mark.pauls@freese.com; Gregory Mika, EIT, CFM; gregory.mika@freese.com

Abstract:In recent years, the timeline for new HEC-RAS releases has generally shifted from years to months. Since May 2021, the HEC team has moved from HEC-RAS v6.0 to v6.1, v6.2, and v6.3. Each release brings with it exciting new features that lead to new opportunities and new challenges. This presentation will provide specific examples of how Freese and Nichols, Inc. (FNI) modeling teams have quickly adapted approaches to take advantage of the latest HEC-RAS features, providing increased efficiency and precision to hydraulic model development. Entering data for 2D bridges is a notoriously tedious process. We’ll discuss how to navigate HEC-RAS text files in order to transfer 1D bridge data into 2D models to save time, improve quality, and leverage modeling that’s been previously completed. Hydrologic connectivity is critical in the context of rain-on-mesh 2D models. Defining hydraulic limitation in a hydrologic context can be a difficult skill to master without extensive experience. We’ll show geospatial and terrain processing techniques that help standardize and automate this process, saving time and improving model quality. Model output as model input. HEC-RAS has added two key features that can be used synergistically to approach model uncertainty and produce better calibration without parameter hacking.