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Tutorial 1: Channels

Task
Run DSM2 for a steady boundary condition flow and salinity (EC-electrical conductivity) simulation for a simple straight channel grid.

Skills Gained

  • Get started with DSM2
  • Creating channels
  • Establishing initial and boundary conditions

The purpose of this tutorial is twofold: to get a start with the DSM2 model and to get practice setting up channels. We will set up a simple channel-only grid with simple constant boundary conditions and run both HYDRO and QUAL. We will look at two formats for entering cross-section geometry (the new DSM2 single file format and CSDP [Cross Section Development Program] format) and familiarize ourselves with the echo output file that gives you a single-file complete record of all the input data used in a DSM2 module.

For the tutorial, the channels have the following configuration and specifications:

Channel Configuration

Figure 1 - Simple channel configuration and specifications.

Note that there are two cross-section geometries labeled A and B, which will be specified later in this tutorial. In all the channels except Channel 5, the cross-sections have been assigned at the midpoint of the channel. In Channel 5, the cross-sections are assigned at fractions 0.2 and 0.8 of the length of the channel measured from the upstream end. The DSM2 grid map includes arrows pointing from upstream to downstream, indicating the positive direction of flow.

Overview of DSM2 Channel Cross Sections

DSM2 assumes a piecewise linear cross-sectional bathymetry. Width, area, and wetted perimeter are tabulated according to elevation. Each elevation lists the data (width) or cumulative data (wetted perimeter and area) below the given elevation. Anything above the top elevation is extrapolated using a slope given by a global scalar called levee_slope.

Piecewise Linear Bathymetry
Figure 2: Piecewise linear bathymetry

For instance, for a cross-section halfway downstream in a fictitious channel 123, the five layers of a cross-section with elevations given by Figure 2 might be tabulated as follows:

XSECT_LAYER
CHAN_NO DIST ELEV AREA WIDTH WET_PERIM
123 0.5 -14.6 0.0 0.0 0.0
123 0.5 -9.2 216.0 80.0 102.5
123 0.5 -4.0 736.0 120.0 111.0
123 0.5 9.5 2410.0 160.0 142.3
123 0.5 12.0 3028.5 162.0 148.0

The above table is in the single-file DSM2 cross-section format. An analogous table is produced by the Cross Section Development Program (CSDP). We will practice using both in the tutorial. The parameter levee_slope is seldom changed from its standard value of 0.33.

The following steps will instruct you on how to create the channels, give them very simple boundary conditions, and run the model.

  1. Open the hydro input file and add parameters:

    1. Use a text editor like Notepad++, Textpad, or Emacs.
    2. Navigate to the ${DSM2_home}\tutorial\simple\t1_channels directory.
    3. Open the hydro.inp file.

  2. In HYDRO, add the Scalar Runtime information:

    1. Locate the SCALAR table in hydro.inp.

    2. Add the following scalars:

      SCALAR
NAME VALUE
run_start_date 01JAN1992
run_end_date 01MAR1992
run_start_time 0000
run_end_time 0000
temp_dir c:/temp
title "TUTORIAL SIMULATION ${DSM2MODIFIER}"
warn_unchecked false
END

    3. Ensure temp_dir points to a location with ample disk space.

  3. In HYDRO, add Channel information:

    • Add a CHANNEL table with channel details.
    • Add an XSECT_LAYER table for cross-section geometry.

    Example for Channel 1:

CHANNEL
CHAN_NO LENGTH MANNING DISPERSION UPNODE DOWNNODE
1 10000 0.035 10 1 2
END

XSECT_LAYER
CHAN_NO DIST ELEV AREA WIDTH WET_PERIM
1 0.5 -24.0 0.0 40.0 40.0
1 0.5 0.0 960.0 80.0 102.5
1 0.5 20.0 2640.0 160.0 192.0
END

Repeat for other channels as needed.

  1. In HYDRO, set the Boundary information:
  2. Add BOUNDARY_FLOW and BOUNDARY_STAGE tables for upstream and downstream boundaries.

Example:

``` BOUNDARY_FLOW NAME NODE SIGN FILLIN FILE PATH upstream_flow 1 1 last constant 200. END

BOUNDARY_STAGE NAME NODE FILLIN FILE PATH downstream_stage 7 last constant 0.0 END ```

  1. In HYDRO, set the Initial Conditions:
  2. Add a CHANNEL_IC table to set initial stage and flow for each channel.

Example:

CHANNEL_IC CHAN_NO DISTANCE STAGE FLOW 1 0 0.0 0.0 1 length 0.0 0.0 END

  1. In HYDRO, specify the Output Locations:
  2. Add an OUTPUT_CHANNEL table to request output data.

Example:

OUTPUT_CHANNEL NAME CHAN_NO DISTANCE VARIABLE INTERVAL PERIOD_OP FILE bnd_1 1 0 flow 15min inst output.dss END

  1. In QUAL, add the Scalar Runtime information:
  2. Add runtime scalars to qual.inp.

Example:

SCALAR NAME VALUE run_start_date 02JAN1992 run_end_date 01MAR1992 run_start_time 0000 run_end_time 0000 temp_dir c:/temp END

  1. In QUAL, set the Boundary Concentration information:
  2. Add a NODE_CONCENTRATION table for upstream and downstream boundaries.

Example:

NODE_CONCENTRATION NAME NODE_NO VARIABLE FILLIN FILE PATH upstream_flow 1 ec last constant 200. downstream_stage 7 ec last constant 30000. END

  1. In QUAL, specify Output Locations:
  2. Add an OUTPUT_CHANNEL table for concentration data.

Example:

OUTPUT_CHANNEL NAME CHAN_NO DISTANCE VARIABLE INTERVAL PERIOD_OP FILE bnd_1 1 0 ec 15min inst ${QUALOUTDSSFILE} END

  1. Run HYDRO and QUAL:

    • Run hydro hydro.inp and qual qual.inp from the command line.
    • Verify the output in output.dss.

  2. CSDP style cross-sections:

    • Use hydro_csdp.inp to reference external cross-section files.

    Example:

    XSECT CHAN_NO DIST FILE 1 0.5 xsect_a.txt END

  3. Rerun HYDRO and compare cross-section formats.

  4. Run HYDRO using echoed input.

Brain Teasers 1. What is the actual delta-x between computational points for each of the subreaches (channels 1-6)? 2. Why is the requested dx the minimum spatial step for each reach? Wouldn't you want to impose a maximum on how big dx can be? 3. Change the bottom elevation of one of the cross-sections by lowering it 5ft. How does this affect the simulation?