Proceedings of the 2023 Mississippi Water Resources Conference

Year: 2023 Authors: Carlisle A.

The presentation will review several case studies in which the hydraulic analysis and design was improved using two-dimensional hydraulic modeling. The Standard for Hydraulic Modeling is a One-Dimensional Modeling Routine developed by USACE Hydrologic Engineering Center (HEC-RAS, HEC River Analysis System). The model computes water surface elevations (WSE) by solving the energy equation using a standard step method between cross sections, using inputs such as geometry (cross section data), roughness Parameters (Manning "N" values), boundary conditions, and discharge/flow data (steady or unsteady). The model predicts average WSE and velocity for the cross section, main channel, and both overbanks. Certain site-specific conditions can present challenges when using 1D modeling, including a) project areas with flood flows that vary spatially, b) floodplain encroachments with skewed alignments to overall floodplain, c) Modeling multiple openings within one floodplain, and d) assessing and quantifying lateral discharge. When these constraints arise, 2D modeling can improve the modeling and subsequent design. The 2Dhydraulic models include the generation of a mesh with variable number of elements over the project domain, and computes hydraulic parameters at each element node using a finite volume approach. (HEC-RAS 2D, SRH-2D, FLO-2D, Others). The 2D models aide with the challenging constraints mentioned above. Other advantages include assessing the impact of flood storage in project domain, and the impact of floodplain encroachment ineffective flow area zones (useful in bridge scour / approach XS estimation). 2D models utilize similar inputs as 1D, including discharge data, boundary conditions, terrain (geometry), and roughness Parameters (Manning's "n"). Within 2D models, the terrain and roughness inputs are included for the entire project domain (floodplain) instead of at specific cross section locations (1D). For terrain, utilize available LiDAR elevation data and merge with field survey topographic data. This creates a mesh with elements to simulate actual field conditions. or roughness parameters, the National Land Cover Datasets can be used for large project domains or digitize roughness polygons for smaller domains. It is noted that 1D models are still utilized to obtain boundary conditions for the 2D analyses. In this presentation, we will review four case studies to demonstrate examples of each application below and discuss how 2D capabilities improved the hydraulic modeling at each Case Study.

1. Subject areas with flood flows that vary spatially.
2. Floodplain encroachments with skewed alignments to overall floodplain.
3. Modeling multiple openings with floodplain.
4. Assessing and quantifying lateral discharge.