Meander Modeling 101 by Julia Delphia, DWR Northern Region Office

Transcription

1 Meander Modeling 101 by Julia Delphia, DWR Northern Region Office The following instructions are based upon a demonstration given by the Meander Model creator, Eric Larsen, on May 27, Larsen was using Matlab 2013b and the latest working version of the Meander Model (Meander or 8, not sure) that he had. Other information presented in this document was obtained from reports written by Larsen, from any ongoing communications with him, and from trial and error in trying to figure this out. The science of meander modeling will not be presented in this document. For more information on the science, refer to Eric Larsen s reports. The Meander Model is a work in progress. There are a few bugs, such as the legend on some of the figures is incorrect. Sometimes an error will show up on the Matlab interface, but the Meander Model still runs correctly. As Matlab versions change, sometimes the Meander Model code has to change. If the model code is from a different version of Matlab than you are currently running, the Meander Model may not completely run. If you run into any such issues, notify Eric Larsen. The Meander Model cannot predict the exact location of the river centerline in the future. What it can do is determine the trend of the river meander. File Structure The Meander Model does not run as many typical models do where the program resides in one location and the data and output resides in another location. Larsen copies the Project Folder with all the latest model code along with the any input files to a new folder and makes edits from there. Below is the basic file structure: Project Folder (named by modeler) o 00defaults.dat o Meander code files (filename.m) o Project Subfolder (named by modeler) Data files (other than 00defaults.dat, all other data files are here. Some are user created files and some are model created files) Backup Data Subfolders (Eric uses 01 Flow Data, 02 Channels, and 03 Erosion_fields, but you can use whatever you like as the model does not read data from here) Scenario Output Folders (helpful if named based upon which data files are used) Figure 1. File Structure Presented at Demo (not all files shown) Project Folder and Subfolders Files within Project Folder Files within Project Subfolder Project Subfolder Project Folder 1

2 Data As with any model, the Meander Model needs data. The data required for the model runs are: o 00defaults.dat o Stream Centerline o o Erodibility Surface Flow Parameters Daily flow Average channel width, depth, and slope for the modeled reach Grain Size Threshold and Bankful Flows These will be described in further detail below. Note: All units are in metric. 00defaults.dat The 00defaults.dat file is the only data file located under the main Project Folder. Before running the Meander Model, edit this file as needed with a text editor such as Word Pad. Figure 2 is an example 00defaults.dat file. As can been seen, the first four rows can be used as title information to define the project. The main thing that needs to be set is the Lastpj information. This must be set to the same name as the Project Subfolder. The other variable values can be left as shown below. Figure 2. 00defaults.dat File Project Subfolder Stream Centerline The stream centerline is defined by the 2 year flow event. Using aerial photography and the estimated width of the stream at the 2 year flow, the centerline of the channel is digitized from upstream to downstream for the full length of the reach being modeled. According to Larsen, it is important to digitize carefully as duplicate vertices or segments out of order can cause the program to crash. Ideally, you will have at least two known centerlines digitized of past years that can be used to calibrate the model. Once calibrated, the model can be run into the future for a specified number of years. The stream centerlines are saved as channel1.dat and channel2.dat within the Project Subfolder. The first three rows of the data file contain information to define the centerline (creator, source, coordinate system, etc). Row 4 must contain the year of the centerline. Rows 5 through the end of the file contain the coordinates of the centerline vertices as X, Y or Easting, Northing. The data values are tab delimited. Figure 3 2

3 is a screen capture of the first few rows of a centerline data file. In this example, the coordinates have been rounded to the nearest meter. Figure 3. Portion of a Steam Centerline File Larsen has a script that works in Arcview 3.3 that converts a GIS centerline shapefile into the data file format and prompts for the input of title rows. This script is not a viable option for most people. Following are two suggested methods for getting the centerline into the correct data format. ArcGIS: 1. Convert the line feature to a point feature shapefile or geodatabase feature (Data Management Tools > Features > Feature Vertices to Points). 2. Add X and Y values (Data Management Tools > Features > Add XY Coordinates). This creates and populates fields POINT_X, POINT_Y, and depending on line type, POINT_Z. 3. If a geodatabase feature was created, open the attribute table and export to a dbase table. 4. Open the *.dbf file with excel, edit it to have only the four cells of header data followed by the two columns of X, Y data, and save it to a new file in the correct data format (tab delimited txt file). 5. Rename the file from *.txt to *.dat. AutoCAD Map: 1. Import the shapefile into AutoCAD Map and then List the polyline. 2. Copy the coordinates of the polyline to an excel file, edit it to have only the four cells of header data followed by the two columns of X, Y data, and save it to a new file in the correct data format (tab delimited txt file). 3. Rename the file from *.txt to *.dat. Larsen also saves centerline data files in a backup data subfolder. Any number of centerlines files can be saved in the backup data subfolder. Before running a scenario, copy the desired centerlines to the Project Subfolder 3

4 and rename them as channel1.dat and channel2.dat. It is also important to delete the file riprap.dat when running scenarios with a new set of channel centerlines. When running a scenario, name the output file to represent which centerline file was used. Erodibility Surface The erodibility surface is a dataset that represents the erodibility potential of the area. It is typically created in GIS by combining a feature class of geology with a feature class of land cover, including revetment, and saving that as a raster dataset. The raster dataset is exported out as an ASCII text file, e0.asc, which is saved to the Project Subfolder. In Larsen s model, the raster dataset has a 30 meter cell size. The e0 values are dimensionless bank erodibility coefficients of the order Figure 4 is a screen capture of the top portion of an erodibility surface ASCII data file. The top 6 rows define the number of rows and columns in the dataset, the coordinate of the lower left corner of the dataset grid, the cell size, and the no data value. Following those 6 rows are the erodibility values. Figure 4. Portion of an Erodibility Surface ASCII Data File Originally, when the Meander Model code was set up, the erodibility data file contained the coefficient e0 the bigger the e0, the more the erosion potential. Later, the model code was changed such the data is now in terms of Fd, which is an inverse of e0, though not strictly 1/e0. Fd can be thought of like a particle size the bigger it is, the less things can erode. So even though the file is called e0.asc, the data values are actually Fd values, not e0 values. 4

5 Creation of Erodibility Surface with GIS To create the erodibility surface in GIS: 1. Obtain or create the geology and land cover feature classes or shapefiles these will typically be polygon features. There may be more than one file needed to define the complete geology and land cover. 2. Clip the feature classes or shapefiles (Analysis Tools > Extract > Clip) to the rectangular area of interest and save as new feature classes or shapefiles in UTM coordinates. 3. Use the data of those feature classes or shapefiles as is or edit the attribute table and add a field(s) that you will need to define erodibility. For instance, using the fields Lithology from geology and VegType from land cover might be enough information for determining erodibility. 4. Create the erodibility polygon feature class or shapefile (Analysis Tools > Overlay > Union). The input features are all the geology and land cover polygon datasets. The output feature class or shapefile will have polygons with both geology and land cover data. 5. Open the attribute table of the erodibility polygon feature class or shapefile and add an erodibility field. This field should be set as an integer field as it will hold values from 9999 to ####. 6. Populate the erodibility field with data based upon the geology and land cover data. In one of Larson s reports, he states that natural vegetation is given one value, agricultural lands are given another value, and constrained areas are given a value of 0. For geology, all areas are assumed erodible except for Qr (Riverbank formation), Qm (Modesto formation) and Qoc (Old channel deposits). Natural vegetation was assigned a value of 250 and agricultural lands were assigned a value of 85. Constrained areas have a value of Calibration is required to determine the final values. When I created an erodibility surface, I first classified the geology and land use with relative erodibility: non, low, moderate, and high. From that, I assigned erodibility values to the relative erodibility: 10000, 2000, 400, and 85, respectively. 7. Create the erodibility raster dataset (Conversion Tools > To Raster > Feature to Raster). Select the erodibility field as the field to use for assigning values to the raster. Set the output cell size (Larsen used 30m). 8. Create the erodibility ASCII file (Conversion Tools > From Raster > Raster to ASCII). The output file name is e0.asc (select asc as file type). Save this file in the Project Subfolder. Larsen also saves erodibility ASCII data files in a backup data subfolder. Any number of erodibility data files can be saved in the backup data subfolder. Before running a scenario, copy the desired erodibility file to the Project Subfolder and rename it as e0.asc. Also refer to the section below on the Erosion E0 Button. When running a scenario, name the output file to represent which erodibility file was used. 5

6 Flow Parameters The Meander Model assumes a rectangular flow channel, and uses Manning s equation for flow in cubic meters per second (cms) as follows: Q cms = (1/n)AR 2/3 S 1/2 Where: Q = flow, in cms n = Manning s roughness coefficient (n value) A = Area = B*H, in square meters R = hydraulics radius = A/P S = slope P = Wetter Perimeter = B+2*H, in meters (Manning s equation is not a function of grain size) The Meander Model uses the input parameters to calculate Manning s n value as shown below: n = (1/u) S 1/2 H 2/3 Where: u = velocity = Q/A, in meters per second Several flow parameters are required for modeling. These values are saved in the data file flow_params.dat. These data can be modified by changing the values within the Meander Model (menu: Parameters > Flow). As in most of the other data files, the first four rows are for title information. Data values start in position 14 of the subsequent lines. Figure 5 is a screen capture of a flow_params.dat file. Figure 5. flow_params.dat File I tried copying a data file with different values to the Project Subfolder and naming it flow_params.dat, but the Meander Model errored out. Does the information entered directly into the Meander Model get used in the model somewhere in addition to what is saved in and used by flow_params.dat? Larsen was not able to provide any explanation for this. Daily Flow The Meander Model can run either a constant daily flow or a variable daily flow hydrograph. The flows are in cubic meters per sec (cms). The constant daily flow is defined as Q in the flow_params.dat file and should represent the 2 year flow event. The variable flow hydrograph data is saved to a *.dat file that can be 6

7 named as desired by the modeler and be located anywhere (Larsen saves them in the 01 Flow Data backup subfolder). The format for the variable hydrograph data is a bit different than other data files in that there are no rows of title information. The file contains two columns of data: 1) date in numerical format and 2) flows in cms. The two columns are separated by a space. If you want to use a variable flow hydrograph in your scenario run, you will be prompted to browse to and select the desired file. Figure 6 is a screen capture of the first few lines of a variable flow hydrograph. Figure 6. Portion of Variable Hydrograph Data File Channel Width, Depth, and Slope The model also needs the reach average channel width and depth in meters, and water surface slope of the modeled reach. These values are relative to the 2 year flow event and are defined as H for Depth and B for Width of channel at floodplain elevation, and S for Slope in the flow_params.dat file (Figure 5 above). Ideally, a hydraulic model of the reach is available from which this data can be determined. The cross sectional area divided by the width of the channel will give the depth of the channel. The width and depth must be hydraulically consistent and can be checked with Manning s equation and adjusted if necessary. Mannings Eq: Q = 1/n A R 2/3 S 1/2 Grain Size Reach average median particle size of bed material in mm. Threshold and Bankfull Flows According to Larsen, FlowThresh is the lower threshold flow and is only used when running variable flow. Bankful flow is typically defined as a 1.5 to 2 year flow event. As discussed above, the centerlines are based on the 2 year flow event. In the Meander Model, Bankfull is the incorrect term this is actually the upper threshold and is never used. It really means overbank flow. 7

8 Running the Meander Model As stated above, the Meander Model was created with and runs in Matlab. Therefore, in order to run the Meander Model, first the user must run Matlab and have some basic familiarity with its interface (Figure 7). Below is the list of steps required to run the Meander Model. 1. Start Matlab 2. Browse to the Project Folder may need to add it to the Matlab path 3. Type meander at >> prompt and hit enter Figure 7. Matlab Window 2 3 If the programs runs successfully, two windows will pop up: Meander Tool (Figure 8) and Figure 1 (Figure 9). These are described in greater detail below. If the program does not run successfully, try to figure out what the problem might be based upon the error code. If you can t figure it out, contact Eric Larsen. According to Larsen, there are only a few places where the user typically makes changes to the program settings or data in the Meander Tool window. These are in shown in Bold in Figure 8 and are discussed in greater detail in the section below. The other values can typically be left at the default values and are not discussed. 8

9 Figure 8. Meander Tool Window and List of Menu Details and Buttons Menu Bar File Open New Project Save Output As Save Calibration As Save Curvature As Reload Meander Quit Options Display Defaults Parameters Computational Hydraulic Flow Smoothing Adjust Cf Erosion Algorithm Factors Cutoff Parameters Bend Parameters Write E0 (aka e0) Dates File Tools Plot E0 Field Calculate Sinuosity Compute Area Between Centerlines Write Script to Text File Buttons Adjust Parameters Erosion (E0) Make edits to e0 files Bank Height Riprap Eric Larson says this is buggy and to not use it. Set Calibration Area Run Model Calibration Prediction (grayed out) Scenario Run a scenario 9

10 Figure 9. Meander Model Figure 1 10

11 Meander Tool Commonly Changed Values This section shows the input windows of the commonly used tools along with some explanation. Set Display Options Defines what will display on Figure 3 of Scenario Run Maximum E0 = Maximum erodibility value that will display Maximum Relative Elevation = Pause time (seconds) = Check boxes to turn on display of data Set Defaults Defines default project and prediction year length Check boxes to enable cutoff routine and options 11

12 Set Computational Parameters These items can typically be left at the default values. If there are problems running the model, try unchecking Calculate Uf. Set Flow Parameters Input the values to define the flow parameters. Clicking OK saves to flow_params.dat. Discharge = Q Depth = H Width = B Slope = S Grain Size = Ds Flow threshold = lower flow threshold only used for variable flow Bankfull = Incorrect term really is overbank flow and is not used. 12

13 Adjust Cf Coefficient of Friction Eric Larsen suggests starting with a value of 1 and maybe double it or half it for calculations and calibration. Sinuosity Calculation Tool Adjust the beginning and end sliders to the desired location and click on the Calculate button to calculate the sinuosity of the reach. 13

14 Erosion E0 Button This section shows the input window of the Erosion E0 button along with some explanation. Refer to the Erodibility Surface Section above for an explanation about the E0 relationship to Fd. Using this tool can be a quick way to change Fd values when doing calibration. Erosion (E0) Erosion Resistance (Fd) Adjusting Tool Note: As the user moves the mouse cursor over the figure, the Easting, Northing, E0, and Fd values are indicated. This tool lets the user change the erosion resistance value by either clicking the Select All button or by drawing a polygon around an area and then entering the Fd value and clicking the Set Fd button. To draw a polygon around the area where the Fd will be changed, simply move the mouse cursor to the desired area, click the left mouse button and draw point to point. Finish the polygon by clicking the Close Poly button. There are also some buttons for modifying the Fd values Add, Multiply, and Reclassify from old to new. Clicking the Save button will save the data back to the e0.dat file. Clicking the Save As button will allow the user to save to a filename of their choice, typically to the backup folder. Clicking the Load From ASCII button allows the user to read in E0 data from the backup folder. Scenario Button To run the actual Meander Model, click the Scenario button. Clicking the Scenario button will prompt the user with the following: Which channel would you like to start from? Select Channel 1 or Channel 2 button. Start year is: ####. What year to predict to? Defaults to 50 years from start channel, but can be changed. 14

15 Specify Output Folder for Scenario. This will create a folder under the Project Subfolder with the given name. If running multiple scenarios, it is helpful to name the scenario folders to represent the data being used. Input variable hydrograph? Select Yes or No button. At this point, Figure 3 appears on the screen and displays the centerlines for each year of the analysis. Note: the legend is incorrect in this version of the meander model that I am running. It takes a little while to run, maybe a couple minutes or so. The Scenario Folder contains shape files and other output data files for every year of the model run. The shape files are by year in this format YEAR_bends.shp. Calibration Button Manual or Automated Automated 15

16 Just select avel, accepted these values, no variable hydro, it asked variable hydro a couple times then error. Calibration Calibration is the process of adjusting the input variables until the outcome closely matches a known event. In the case of the Meander Model, calibration is done from a known centerline to a centerline. For instance, if the user has digitized centerlines for 1952 and 1976, the user can run a scenario from channel 1 (1952) for 24 years and then can compare the output 1976_bends.shp to the 1976 centerline. Prediction Once the model is calibrated, then future predictions can be run. As stated at the beginning, the Meander Model cannot predict the exact location of the river centerline in the future, but it can determine the trend of the river meander. Cutoff Simulation 16

17 References Larsen, E. W. and S. E. Greco, Modeling Channel Management Impacts on River Migration: A Case Study of Woodson Bridge State Recreation Area, Sacramento River, California, USA, Environmental Management Vol. 30, No. 2, pp , Larsen, E. W., Sacramento River Ecological Flows Study: Meander Migration Modeling Final Report, Prepared for The Nature Conservancy, October Larsen, E. W., Modeling Meander Migration for Assessing Impacts and Benefits of Channel Management Scenarios, Middle Sacramento River, California, Prepared for US Army Corps of Engineers, Draft January