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Channels

Overview:

Channels are the fundamental objects of the DSM2 grid. The Channels table allows you to enter channel connectivity, parameters and geometry. Channels connectivity is defined by an upstream and downstream node numbers of the channels. Two child tables describe the locations and geometry of user-described cross-sections in the selected channel. Note that a default initial condition is required for every channel number in the DSM2 grid, and this is entered seperately in the Channel Initial Conditions table.

Tables:

CHANNEL

The CHANNEL table defines the connectivity, length, friction and dispersion characteristics of a channel.

Field Descriptions

CHAN_NOChannel number. This is the identifier of the channel, and corresponds to the number you typically see on a grid map.LENGTH (ft)Length of the channel reachMANNINGManning's n friction coefficient for the whole reach.DISPERSIONDimensional dispersion factor.UPNODENumber of the upstream node at which channel is connected.DOWNNODENumber of the downstream node at which channel is connected.

Table Info

Identifier:CHAN_NOParent Table:Table is parentInclude Block:GRID

XSECT

This table lists files where bathymetric cross-sections are specified by the user using the CSDP format. The table lists the fraction of the distance along the reach (from upstream to downstream) at which the user cross-section is located. These cross-sections will be interpolated by the model at computational points. Overspecification of geometry is a frequent source of user error/misconception, please see usage notes below. Also note that this style of input and the XSECT_LAYER "single file" format below should not be freely mixed for a given channel -- use one or the other.

Field Descriptions

CHAN_NOChannel number where cross-section is locatedDISTFraction of distance from upstream node to downstream node where cross-section is locatedFILECSDP-formatted file where cross-section geometry is defined.

Table Info

Identifier:CHAN_NO, DISTParent Table:CHANNELParent Identifier:CHAN_NOInclude Block:GRID

XSECT_LAYER

The Cross-Section Layer Table lists geometric information about each cross-section. This information is in the form of lookup tables of hydraulically important quantities such as area, width and wetted perimeter.

Field Descriptions

CHAN_NOChannel number in which cross-section is located.DISTFraction of distance from upstream node to downstream node where cross-section is locatedELEVElevation from bottom at which properties are known. The area, width, etc. apply to this elevation, and channel properties between elevations are linearly interpolated.AREAArea of channel from bottom to cross section(sq ft). Ignored if Area disagrees with the integral of WIDTH.WIDTHWidth of channel at top (ft).WET_PERIMWetted perimeter of channel at given elevation.

Table Info

Identifier:CHAN_NO, DIST, ELEVParent Table:CHANNELParent Identifier:CHAN_NOInclude Block:GRID

Examples:

CHANNEL with XSECT_LAYER cross-section

# This example shows channels using cross-sections
# In the XSECT_LAYER format. The benefit of this format
# is that the input can all be put in one file.
# This can be useful for archiving or echoing back input

# CHANNEL SPECS
CHANNEL
CHAN_NO LENGTH MANNING DISPERSION UPNODE DOWNNODE
1        15000   0.035        0.3      1        2 
2        15000   0.035        0.3      2        3
END

# This is a child table. Its rows must "link" to a parent
# using the parent id (CHAN_NO in this case).
# Note that two cross-sections are defined here,
# one in each channel, halfway downstream, with three
# layers each. 
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     91.22 
1        0.5  20.0 2640.0 160.0     133.6 
2        0.5 -24.0    0.0  40.0      40.0 
2        0.5   0.0  960.0  80.0     91.22 
2        0.5  20.0 2640.0 160.0     133.6 
END

CHANNEL with XSECT (csdp) cross-section

# This example shows channels using cross-sections
# In the XSECT format. The specification is not 
# complete -- we are really referring to 
# Cross-Section Development Program (CSDP) files
# which are in there own format.

# CHANNEL SPECS
CHANNEL
CHAN_NO LENGTH MANNING DISPERSION UPNODE DOWNNODE
1        15000   0.035        0.3      1        2 
2        15000   0.035        0.3      2        3
END

# This is a child table. It is an alternative to the 
# XSECT_LAYER table (the two can co-exist, but you 
# should not mix input for a channel). The FILE column
# points to a file that contains the real data which
# would normally come out of the CSDP or other tool.
XSECT
CHAN_NO DIST     FILE
1           0.5   1_0.50000.txt
2           0.5   2_0.50000.txt
END
  • All channels must have an initial condition and at least one cross-section.
  • Older versions of DSM2 had the notion of a "regular" cross-section (meaning rectangular). In the current DSM2 this is just a cross-section with two layers.
  • Users frequently overspecificy cross-sections, either by specifying more cross-sections longitudinally than the model can possibly use or by describing cross-sections vertically in such a way as to capture highly local features such as small constrictions, sills and undulations. DSM2 is commonly used with spatial resolution (delta x) of several thousand feet. You should only include features that are well resolved by this resolution, which means changes that persist over several miles. Even more importantly, you should avoid adjacent cross-sections with bottoms that vary greatly in elevation because they can cause odd behavior when cross-sections are interpolated to computation points. The bottoms layers of cross sections should represent the "overall" slope of the channel.