Numerical Hydraulics

Transcription

1 ETHZ, Fall 2017 Numerical Hydraulics Assignment 3 Comparison of two numerical solutions of river flow: use of Finite Elements (HEC-RAS) and Finite Volumes (BASEMENT)

2 1 Introduction In the course, two different numerical schemes were introduced to solve shallow water equations, namely finite differences and finite volumes. In practice there exist several codes to solve numerically water related problems either in 1D or 2D. In this assignment we will focus on two commonly used software platforms that are freely available. The first program is HEC-RAS which uses a Finite Differences related numerical scheme in combination with a digital elevation model; the second software, BASEMENT, is based on the Finite Volumes. HEC-RAS was developed at the Hydrologic Engineering Center (HEC), which is a division of the Institute for Water Resources (IWR), U.S. Army Corps of Engineers. More information can be found on the program webpage ( BASEMENT was developed at the VAW (Versuchsanstalt für Wasserbau und Glaziologie) at ETH Zurich. More information can be found on the program webpage ( Before a numerical simulation can be performed, the channel geometry, the channel properties, the hydrodynamics as well as the boundary and initial conditions must be provided. 1D simulations are usually based on defined cross-sections, whereas a computational grid is required for 2D simulations. 2 Objective The goal of this assignment is to interpret and compare the solutions obtained by the two different numerical schemes (finite differences in HEC-RAS and finite volumes in BASEMENT) using the same geometry and channel properties, first, for 1D and, second, for 2D simulations. The solutions of the HEC-RAS finite differences simulations will be provided, while the finite volumes BASEMENT solutions have to be computed. During the lecture, we will introduce and run the BASEMENT model together. With this, you will be able to evaluate the results of the four simulations and answer the specific questions given below. Please download and install the required software before the in-class exercise (installation documentation is provided on the webpage)! Note: The generation of the geometry as well as the detailed model setup is not part of this exercise. We will focus on the different numerical solutions. Some of you will complete the course River morphodynamic modelling during the next semester where you will put emphasis on these aspects. In the Flow lab, which is taking place during the second half of this semester, you will also use the software HEC-RAS and BASEMENT in more details. We, therefore, strongly recommend doing this assignment, as it provides you a first introduction into the work with these numerical programs. 1

3 2.1 Geometry of the river reach For the simulations, we will use a very simple straight river reach with double trapezoidal shaped crosssections (Fig. 1). After m, a 200 m long widening is present, as it often occurs in real channels, with continuous transition of the width over 100 m both on the upstream and downstream side. You will find that this widening induces a hydraulic jump. In addition, inside the widening a bridge pier is located having a width of 4 m and a length of 10 m. Fig. 1: Plan view of the used geometry for the 1D simulations (top), sketch of the double trapezoidal cross-section of the river reach (bottom). For both simulations (HEC-RAS and BASEMENT, 1D and 2D), we will use exactly the same geometry and the same boundary conditions. All four simulations are performed for a transient case where the flood wave shown in Fig. 2 propagates through the channel. We will focus on the numerical results after 30 minutes and 70 minutes where discharges are 200 m 3 /s and 300 m 3 /s, respectively. 2

4 Fig. 2: Flow hydrograph which is applied for the numerical simulations of assignment D Simulations The distance between each cross-section for the 1D simulations with BASEMENT and HEC-RAS is 5 m. The geometry file of Basement as well as the solution of the HEC-RAS simulation will be provided to you. You can have a look at the cross sections by opening the BASEMENT project and pressing the button Edit1-D Grid. This will open the BASEMENT editor. Click File open and choose the geometry file (xxx.bmg) D Simulations BASEMENT is using a triangular grid, while HEC-RAS is able to deal with arbitrary elements having up to 8 faces (i.e. polygonal cells). You can visualize the BASEMENT-grid in QGIS after importing a result file by checking the box Display Mesh in the Crayfish panel. The HEC-RAS-grid is provided to you as a shapefile. 3

5 3 Tasks 3.1 1D Simulations 1. Complete the Basement template NUM_HYD_Basement_1D_5m.bmc using the information in Tab. 1. Tab. 1: Additional information for the 1D Basement simulation. Block Tag Value PARALLEL TIMESTEP (upstream) (downstream) INITIAL OUTPUT Number_threads CFL 0.95 Total_run_time Maximum_time_step string file Check the number of cores of your computer; do not use all of them! s 1 s slope 4 string slope 4 upstream hydrograph Hydrograph.txt Downstream hqrelation backwater q_out 200 m 3 /s WSE_out Output_time_step Console_time_step 1.95 m 600 s 300 s 2. Export the results of the water surface elevation (WSE), the velocity and the Froude-number after 30 min (Q = 200 m 3 /s) and 70 min (Q = 300 m 3 /s). 4

6 3.2 2D Simulations 1. Complete the Basement template NUM_HYD_Basement_2D.bmc using the information in Tab. 2. Tab. 2: Additional information for the 2D Basement simulation. Block Tag Value PARALLEL Number_threads Check the number of cores of your computer; do not use all of them! CFL 0.95 TIMESTEP (upstream) Total_run_time Maximum_time_step String_name file s 1 s Inflow hydrograph Hydrograph.txt slope 4 (downstream) String_name Outflow zero_gradient INITIAL dry OUTPUT Special_Output (node_centered) Console_timestep Restart_time_step Format Values Output_time_step 300 s 1E32 s sms Depth, velocity, wse 600 s 2. Visualize the results of the Basement simulation in QGIS (review the document NUM_HYD_Ex3_Software_QGIS ). 3. Export the longitudinal profile of the water surface elevation and the velocity close to the bridge pier (use the shapefile Longitudinal_Profile ) after 30 min (Q = 200 m 3 /s) and 70 min (Q = 300 m 3 /s). The document NUM_HYD_Ex3_Software_QGIS contains an explanation how to export the results along a profile. 5

7 4 Tasks 1. Plot the water surface elevation and the velocity profiles of all four simulations as well as the Froude number of the 1D simulations. What do you observe? 2. Determine the minimum and the maximum flow depth of each simulation. Where can you observe the highest differences? 3. Evaluate the maximum and minimum flow depth along the bridge pier for all simulations. (The bridge pier starts at x = 1200 and ends at x = 1210). Explain the differences between the 1D and 2D simulations. 4. Create maps of the flow depths and the velocity fields of the results obtained by the 2D simulations after 30 and 70 minutes (The document NUM_HYD_Ex3_Software_QGIS includes a guidance how to create maps with QGIS. You can use the provided color ramp for the visualization). Which simulation result would you rate as most realistic and reliable for the design of flood protection measures? 6