Advanced Hydrodynamic Floodplain Analysis in North Albany, Oregon
DATE: Wednesday, March 25th, 2015
PLACE: Hawthorne Lucky Lab, 915 SE Hawthorne Blvd, Portland
TIME: 11:30 a.m. Networking, 12:00 – 1:00 p.m. Presentation
SPEAKER: Seth Jelen, PE, CFM, CWRE, AMEC Foster & Wheeler
The study examined the appropriateness of traditional, 1-dimensional modeling using HEC-RAS versus 2-dimensional modeling using FLO-2D and found that using both – in different portions of the study area – was the optimal course for complex floodplain analysis. The study also developed ways to use aerial imagery of historic flood inundation in conjunction with LiDAR ground elevations to estimate flood elevations for model calibration.
The study area includes several complex overflow paths and floodplains inside a four-mile meander of the Willamette River in Albany, Oregon. Most of the area is mapped as “floodplain fringe” of the river. Normally, fill and flow obstructions would be allowed and base flood elevations are set equal to those of the river. This study established that these paths were hydraulically separated from the river, located the overflow paths, and mapped floodways to protect against development and fill that would have increased flood elevations nearby to unacceptable levels. The advanced floodplain analysis used a complex network of 1-dimension reaches in HEC-RAS coupled with a large FLO-2D modeled area, all balanced to the existing river flood profile. A unique use of upstream water surface elevation profiles and custom pre- and post-processing of FLO-2D and HEC-RAS input and results were part of the study.
The study found that floodplain elevations in the Thornton Lake Overflow should be lower by as much as 5 feet, while flood elevations in places along the Willamette River should be more than 2 feet higher. The take-home lessons from this study are that traditional 1-dimension models may not always provide the right level of analysis, use of aerial imagery and LiDAR for estimating flood elevations is appropriate when better data are not available, and combining various modeling tools can help overcome limitations of more traditional approaches (such as typical boundary conditions) to improve the understanding of complex floodplain science.
About the speaker:
Seth Jelen has served in the field of water resources science and engineering, modeling, software development, and GIS analysis for over 24 years. His practice has focused on surface water including flow (both runoff and water supply prediction), flooding (hydrology, flood elevation, flood mitigation, and floodplain mapping and management), and water quality (in-stream and pollutant load modeling). He is an expert in hydrologic and hydraulic modeling in 1- and 2-dimensions and steady and unsteady/dynamic modes; water quality analysis; transport of sediment and pollutants in streams and off urban surfaces; and in custom application development using C++, Visual Basic and other languages. He has developed over two hundred custom applications to improve and extend GIS mapping / analysis and model pre- and post-processing. He is an expert user of HEC-RAS, HEC-HMS, FLO-2D; DELFT3D; EPA-SWMM; HSPF; and GIS and AutoCAD software. He authored the Flow Duration Design Model (FDDM), was lead programmer for the Simplified Particulate Transport Model (SIMPTM), and has co-authored a number of papers. He is currently a principal engineer with Amec Foster Wheeler.