COC Transport Documentation
COC Transport Extension (version 1.2)
COC Transport is an ArcView extension that enhances surface water modeling to include
concentration calculations. Most of the existing hydrology applications for ArcView are
currently focussed on delineating water drainage patterns using digital elevation models. These
applications calculate water flow based on the paths of steepest descent (i.e., water flows from
high head to low head). Some example applications include delineating streams and watersheds,
and preparing input files for external programs (e.g., HMS). COC Transport provides an
additional function for these models of computing downstream concentrations from known
inputs to the system. The conservation of mass principle requires that the sum of inputs to a
system must equal the total output. Assuming the losses in the system are minimal, the sum of
the input loads equals the total output load:
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Load (W) = Flow (Q) * Concentration (c) [1]
 (Qin*cin) = Qout*cout [2]
cout =  (Win)/Qout [3]
Therefore, if the inputs to the system are known, then a downstream concentration can be
calculated. The resulting concentration at each point downstream is a conservative
approximation because it does not account for any chemical decay or other losses, such as
sorption.
The extension can incorporate inputs from both point sources, such as a treatment plant outfall,
and area sources, such as runoff flow from farmland. The user has the option to create point
sources, area sources, or both. The user can also run the extension with a custom input theme.
Once the inputs are defined, the program will proceed through the steps necessary to calculate
concentrations. The result is a grid of concentrations, where each grid cell is treated as an outlet,
and the concentration is computed from equation 3.
The only requirements for the extension are Spatial Analyst, a flow direction grid, and
knowledge of the system inputs. The following sections detail the individual process steps. If
you have any questions, please contact Andrew Romanek at romanek@mail.utexas.edu.
Process Steps
1. Set Analysis Extent: This is a required step that determines the cell size and extent for all the
grids that will be created. The user needs to set the Analysis Extent and Analysis Cell Size to
Same as Flow Direction, where flow direction is the name of your flow direction grid. The
flow direction grid is chosen so that the flow and load grids will accumulate properly since
the flow direction grid is the foundation for the accumulation process. Once the analysis
extent has been set, it will remain the same and does not need to be changed unless a process
separate from the extension is run.
Note: Either one of the following two steps or both can be run depending on your system inputs.
The methods used to calculate concentrations vary depending on the type of sources you have.
Thus, many of the later process steps will ask for which type of sources are being used so that the
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COC Transport Documentation
extension knows what section of code to run. The user can skip the next two steps if he/she has
already created an input source theme. However, the input source theme must contain the
following three fields: Concentration, Flow, and Load.
2. Point Sources: Creates a shapefile of points, where each point has a predefined concentration
or load, and flow. The first part of the program simply creates the shapefile. The user must
then use the special blue diamond tool
and click on point source locations. The user has
the option to use either a concentration based or load based point source. The user must enter
a concentration [mg/L] if concentration based is chosen and a load [mg/d] if load based is
chosen. Both options also prompt the user for a location description and a flow [ft3/s]. If the
flow is a result of runoff, simply click on OK to proceed (the runoff flow will be incorporated
later). If a flow is given, then the program will calculate via equation 1, either concentration
or load, whichever parameter was not defined. Otherwise, the field records are left blank.
As many points as necessary can be added. When finished, simply reply No to the question,
"Would you like to add another point?"
3. Area Sources: Creates a shapefile of areas, where each area has a predefined concentration or
load, and flow. The procedure is identical to that for Point Sources, but the user must now
use the special green diamond tool
to create areas. The green diamond tool is similar to
the polygon tool, so each mouse click will add a vertex to the polygon. Double click on the
final vertex to complete the shape.
4. Concentration Grid: Creates a grid of concentrations from a previously created point source
theme, area source theme, or both a point and area source theme. The resulting grid will
contain no data cells everywhere except where there is a point or area. In these locations,
grid cell values are determined by the concentration value from the input theme(s). Note that
when using both point and area sources, a point source will always have precedent over an
area source. For example, if a point source lies within an area, the concentration grid will
have a value equal to the concentration of the area everywhere within the area except at the
point source, where the value will equal the concentration of the point source. The resulting
concentration grid will not include any concentrations that are the result of a load with runoff
flow. These loads will be incorporated in the Load Grid step.
5. Flow Grid: Creates a flow grid using flows from the input source theme(s) and runoff. The
flow grid values will be equal to the values from the runoff grid everywhere except where
there is a point or area source with a defined flow. In these areas, the flow grid will have
values equal to the flows defined in the input theme(s). The user can use a previously created
runoff grid (must have the same extent and cell size as the flow direction grid) or he/she has
the option to create a runoff grid [in/yr]. This runoff grid will have a user-defined, constant
value everywhere. Simpler models might not need to incorporate runoff. In these
circumstances, the runoff grid value can be set to 0.
6. Accumulated Flow Grid: Creates an accumulated flow grid using a flow direction grid. For
each grid cell, the program sums all of the upstream flow. The flow direction grid provides
the network of which cells are upstream of others.
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7. Load Grid: Creates a grid of loads from the concentration grid, flow or accumulated flow
grid, and input source theme(s). The program multiplies the concentration grid by a flow
grid to compute load. This grid is then merged with the point source theme, area source
theme, or both to create a load grid that has the value of the multiplied concentration and
flow grids everywhere except where there is a point or area with a predefined load. Thus,
this step incorporates those points or areas in which a load was defined with runoff flow. If
the input theme is a point source theme, the program multiplies the concentration grid by the
accumulated flow grid. However, if the input theme is an area source theme, the program
multiplies the concentration grid by the flow grid. The assumption is that for a point source,
the flow should be equal to any input flow plus any upstream flow (e.g., runoff). For
example, if a point source is a runoff sampling location, the flow should be equal to the
accumulated flow at that point, and the concentration should be equal to the value observed
at that point. For area sources, the concentration is assumed to be the same value throughout
the area, so the flow used in the calculation of load for each grid cell should simply be the
flow for that grid cell. To enhance your understanding of the methodology, envision a
drainage area. This drainage area could be modeled as an area source or as a point source at
the outlet.
8. Accumulated Load Grid: Creates an accumulated load grid using a flow direction grid. The
process is identical to that used in the Accumulated Flow Grid step except that the calculated
load grid is used.
9. Predicted Concentration Grid: Creates a predicted concentration grid from the accumulated
flow and load grids. The predicted concentration grid is computed by dividing the
accumulated load grid by the accumulated flow grid. Essentially, for each grid cell, the sum
of the loads to that point is divided by the sum of the flows to that point (equation 3).
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