Description

1) The Saga of Gould Wash: A Challenging 2D Floodway Case Study
Matt Chaney, PE, CFM, AECOM, matt.chaney@aecom.com
Co-presenters: None

Abstract: An improperly modeled effective floodway set up a cascade of challenges during a Risk MAP update in Washington County, Utah. This presentation will recount the trials of identifying flood risk on Gould Wash, a unique study riddled with technical modeling and mapping challenges, conflicting FEMA guidance, and difficult decisions for community officials. The base flood model run resulted in a significant area of shallow flooding and multiple split flows in the overbanks. The presence of an effective floodway left the project team faced with either delineating a floodway through shallow flooding zones or downgrading the study and removing the floodway, both of which conflict with FEMA guidance. The solution was to present two potential (FEMA Region 8 approved) options to community officials, allowing them to choose how the floodway would be mapped through their community. Each option had challenging and unique floodplain administration implications that even the most experienced floodplain administrators find difficult to navigate. This presentation will discuss proper enforcement of floodplain ordinances for “regulatory” and “administrative” floodways, and obtaining community buy-in. We will also dip our toes in the technical details of 2D floodways such as split flows versus floodway islands, different evaluation line surcharge averaging methods, and a behind-the-scenes look at the iterative development of a complex 2D floodway.

2) FEMA 2D Modeling with Floodways: A case study - City of Ithaca, New York
Inger Sarappo, GISP, CFM, Stantec, inger.sarappo@stantec.com
Co-presenters: Patrick Dobbs, patrick.dobbs@stantec.com

Abstract: This presentation will be a case study of a complex 2D HEC-RAS Model with Floodways done for the City of Ithaca, NY. The model ended up being split into 3 models in order to stabilize it, which needed to be run in a series. 1D was used to model channels and floodways and 2D was used to model the overbank areas. A USGS model was our starting point and there was a levee in the area. This will be the first countywide digital study of the county and extra outreach was completed to prepare the communities for the upcoming preliminary issuance as we are flooding a large portion of the City of Ithaca. This is a complicated model and project with issued solved and we want to share our insights.

3) Going Regulatory with 2D Modeling in Kansas
Joanna Rohlf, GISP, CFM, Kansas Department of Agriculture, Division of Water Resources, joanna.rohlf@ks.gov
Co-presenters: Ben Rufenacht, PE, CFM, WSP, ben.rufenacht@wsp.com

Abstract: The Kansas Department of Agriculture (KDA) began using 2D HEC-RAS modeling widespread with FY17 work for Zone A and Base Level Engineering (BLE) mapping. Through the use of 2D modeling in BLE, Kansas has funded and nearly completed BLE projects that cover areas of the state with little to no flood risk information and for areas where the information is out-of-date. As a result of these efforts, the goal was to create best available data for areas with no flood risk information, update outdated mapping, look for areas of mitigation technical assistance needs, as well as to identify and provide countywide regulatory maps where needed. Most Zone A mapping in the state has moved forward with 2D modeling, however most Zone AE with Floodway had continued as 1D modeling. This presentation will discuss the Kansas 2D Statewide Initiative, and where 2D floodways are underway. This presentation will feature specific challenges when tying-in 1D Steady-State and 2D model areas and detail the move to modeling floodways in 2D. It will scrutinize the pro’s and con’s related to model data size, use in Letters of Map Revision and development projects, and accuracy of the methods. Finally, this presentation will preview the path forward with continued 2D modeling in Kansas.