HYDRAULIC MODELING AND FLOOD.pdf
- 1. This work is supported by the National Science Foundation’s Directorate for Education and Human Resources TUES-1245025, IUSE- 1612248, IUSE-1725347, and IUSE-1914915. Questions, contact education-AT-unavco.org HYDRAULIC MODELING AND FLOOD INUNDATION MAPPING USING HEC-RAS Dr. Venkatesh Merwade, Lyles School of Civil Engineering, Purdue University Version: 10/08/2018 by V. Merwade
- 2. • Hydraulic model: A hydraulic model is a mathematical representation of a water/sewer/storm system and is used to analyze the system’s hydraulic behavior. • Hydraulic modeling is frequently used to understand a hydraulic system’s behavior under different scenarios at different spatial and temporal scales. 2 Lab experiment Modeling Cost Time WHAT IS A HYDRAULIC MODEL?
- 3. DIFFERENT TYPES OF HYDRAULIC MODELS Hydraulic models can be categorized by its dimensionality 3 1D 2D 3D 1D/2D Flow is considered one dimensional (1D) both in channel and floodplain Flow is considered two dimensional (1D) in both channel and floodplain Combined 1D-2D. 1D in channel and 2D in floodplain Flow is considered three dimensional (3D) in both channel and floodplain
- 4. ONE DIMENSIONAL (1D) HYDRAULIC MODEL A 1D model assume flow in one direction – generally along the river. 4 1D Flow along longitudinal direction Flow along lateral direction is neglected EXAMPLES •HEC-RAS 1D (Hydraulic Engineering Center-River Analysis Service 1D Model) • MIKE 11 • SWMM (Storm and water management model) •HY8
- 5. HEC-RAS 1D It can be used for the following situations: ▪ Steady or unsteady riverine systems ▪ Flow primarily along one direction ▪ Minimal split flow 5 Developed by U.S Army Corps of Engineering River centerline, banks and cross- sections need to be defined. Calculations are conducted between different two contingent cross sections
- 6. GOVERNING EQUATIONS 6 1D hydraulic models compute cross-sectional average water surface elevation (WSE) and velocity at discrete cross-sections by solving a full version of 1D Saint-Venant equations using implicit finite difference method. A: cross-sectional area, Q: Discharge, S: frictional slope, z: water depth, x: distance along the flow, f: fraction to determine channel versus floodplain discharge, t: time
- 7. 1D PROFILE CALCULATIONS 7 1 2 Plan View Longitudinal view he: head loss, V: velocity, g: gravitational acceleration, L: reach length, a: velocity coefficient
- 8. LOSS IN ENERGY HEAD 8 𝑉𝐿𝑂𝐵 2 2𝑔 𝑉𝑅𝑂𝐵 2 2𝑔 𝑉 𝑐 2 2𝑔 a𝑉2 2𝑔 LLOB LROB LC C: contraction/expansion coefficient. Contraction occurs when downstream velocity head is higher and vice versa. Plan View Cross-sectional View
- 9. FLOW CONVEYANCE AND FRICTIONAL SLOPE 9 Computation of flow conveyance (K) and frictional slope (Sf) is based on Manning’s n values. Thus Manning’s n or roughness coefficient plays a critical role in hydraulic modeling.
- 10. PUTTING IT ALL TOGETHER 10 • Y1 is given. Assume Y2 • Based on Y1 and Y2, compute conveyance (K) and friction slope (Sf), and then get he. • Use he to compute Y2. • If the error between computed Y2 and assumed Y2 is greater than a specified tolerance (e.g., 0.01 ft), iterate Y2 until the error is within tolerance. • If the difference between computed Y2 and assume Y2 is within the specified tolerance, Y2 becomes Y1 and the computations move upstream.
- 11. DATA REQUIREMENTS • River Channel description – Length and slope the reach – Channel and floodplain roughness – Cross-section geometry • Boundary Conditions • Flow and/or stage data at upstream and downstream locations • Structure geometry – Bridges – Culverts – Weirs – Levees, etc
- 12. GETTING RIVER DESCRIPTION 12 A Digital Elevation Model (DEM) or Triangulated Irregular Network (TIN) is needed to extract cross-sections for HEC-RAS
- 13. CREATING GEOMETRY IN RAS MAPPER
- 14. GEOMETRY DATA PLAN VIEW River or stream Station number Junction Bank locations Cross-section
- 15. GEOMETRY DATA – CROSS SECTIONAL VIEW Station or distance along XS Elevation values along XS
- 16. STEADY FLOW DATA – UPSTREAM BOUNDARY CONDITION Flow value is specified at the upstream of each reach. Multiple values can be specified to create multiple profiles.
- 17. STEADY FLOW DATA – DOWNSTREAM BOUNDARY CONDITION Water depth (known water surface elevation, critical depth or normal depth) can be provided as downstream boundary for each reach
- 18. RUNNING SIMULATION AND VIEWING RESULTS Steady flow analysis editor Profile view of results Cross-sectional view XYZ view
- 20. HEC-RAS LAB • You are provided with a HEC-RAS model for Wabash-Tippecanoe confluence in West Lafayette, IN • Run the model for different return periods ranging from 2 to 500-year and create flood inundation maps using RAS Mapper • All instructions are provided in the handout