Description

1) Goldilocks and the Three Urban Flood Models: How to size it Just Right!
Elise Ibendahl, PE, CFM, PMP, F.ASCE, Jacobs, elise.ibendahl@jacobs.com
Co-presenters: Gavin Lewis, g.lewis@fathom.global

Abstract: The summer of 2022 continued the trend of increasing urban flood frequency due to extreme storm events. Urban areas are increasingly impacted by heavy rainfall, even when an open body of water is nowhere in sight. Yet urban flood potential and the interaction of pluvial risk with fluvial and coastal inundation areas is not widely mapped in the United States. Consequently, urban flood potential is not fully understood by the public and many stakeholders, and populations are left vulnerable to flood exposure. Increased impervious, deferred maintenance of catch basins, undersized sewer systems, sediment and debris in sewers, and too few catch basins are some of the major contributing factors to urban flooding. Historic practices of filling natural waterways without safe relief pathways have reduced overland flow conveyance opportunities, and historic practices such as redlining have increased the economically disparate impacts of urban flooding on our most vulnerable populations. Reduced physics modeling approaches, such as Fathom’s dataset, quickly increase our understanding of urban flood potential by providing a first look at inundation risk for a range of extreme events with current and escalated climate conditions. This presentation will include case studies where “starter maps” like Fathom and FEMA have been coupled with historical flood complaint records to focus1D/2D modeling, planning, and mitigation prioritization efforts to a “just right” modeling approach and extent. Knowing where pluvial overland flow paths are likely to cause flood impacts allows our study teams to focus detailed 1D/2D modeling efforts that integrate pipe networks and surface overland flow. This in turn allows a more detailed understanding of overland flow paths and critical facilities at risk of flood damage and repetitive losses with refined 2D model extents. This modeling approach also allows for efficient 2D hydraulic modeling and an optimized balance between model resolution and runtime.

2) Clearing Hurdles After Harvey: HCFCD Tool Adapts FEMA Profiles for Unsteady Models
Mat Leclair, PE, CFM, CPESC, Freese and Nichols, mat.leclair@freese.com
Co-presenters: Todd Ward, Todd.Ward@hcfcd.hctx.net

Abstract: Harris County Flood Control District’s (HCFCD) MAAPnext program faced a dilemma: a square peg (new and better data) and a round hole (a traditionally used, but incompatible program). What to do … brute force the square peg through the round hole … or create a square hole? Motivated to simplify the process, a new tool was created that works more seamlessly with the MAAPnext models and direction of future H&H for FEMA mapping efforts, is faster, more efficient, more user friendly. As a result of several flooding events in the Houston region in 2015, 2016, and Hurricane Harvey in 2017, the HCFCD, in partnership with FEMA, initiated the MAAPnext program - a countywide flood hazard assessment using the latest technology and data to produce the most comprehensive and complete set of flood hazard maps and information. A significant change from the previous remapping effort in the early 2000s is the switch from steady-state to unsteady-state 1D/2D hydraulic modeling to better capture flood risk in flat terrain along the Texas gulf coast. As is typical of large remapping studies, there was a subsequent update to the Flood Insurance Study (FIS), and notably, new flood profiles were required. However, an obstacle emerged – the traditional approach to FIS profiles (FEMA’s RASPLOT program) only functions with steady-state hydraulic models while the new MAAPnext models are unsteady hydraulic models. Freese and Nichols developed a standalone tool to be used by all MAAPnext consultants with the overall goal of replicating RASPLOT output with better efficiency. The tool ensures consistency across a large user base and considers future users in the process with HCFCD’s LOMR Delegation program. The presentation will focus on critical items within its development highlighting improvements to functionality, usability, and speed while maintaining consistency and standardization across many people and consulting engineering firms.

3) Grays Creek Watershed Masterplan with HEC-RAS 2-D Models
Yangbin Tong, PE, CFM, Yunxiang Fan, Quality Engineering & Surveying, LLC, ytong@qesla.com
Co-presenters: Jacob Murry, EI, Quality Engineering & Surveying, jmurrey@qesla.com

Abstract: With the release of HEC-RAS Version 5 in 2016, the application of two-dimensional hydraulic models has become increasingly common. However, after decades of one-dimensional models being the focus of watershed modeling, there is ample work to be done in converting these models from one-dimensional to two-dimensional. This was the focus of a Quality Engineering & Surveying project for a local drainage district in Livingston Parish, Louisiana. For 18.67 linear miles, Gray’s Creek meanders throughout Livingston Parish, draining twenty-eight square miles of land, including most of the city of Denham Springs. In 2017, Quality Engineering & Surveying developed a watershed master plan for the Gray’s Creek watershed based upon a 1-D, steady state model. However, after more than half a decade of updates to and adoption of HEC-RAS two-dimensional modeling, Quality Engineering & Surveying recognized the need for a two-dimensional update to the model. In the case of Gray’s Creek, a 2-D model was imperative to allow for a better understanding of the broad, flat floodplain of the channel. This presentation will focus on the reasons that Quality Engineering & Surveying and the drainage district moved forward with the conversion of the Gray’s Creek watershed model, and the lessons learned through the process.