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Note
that the perc rate listed in the above table is determined by multiplying the 5
iph by the wetted surface area at each respective elevation. This is a
simplified approach for illustration purposes only and there are of course many
other methods to develop the relationship between stage and discharge rate or
even time and discharge rate to account for percolation from a pond.
The
model for
this system consists of only 2 nodes and a single link. The following general steps
are required:
1.
Create
2 nodes named "POND" and "GWSINK" (groundwater sink).
Remember, ICPR always requires at least 2 nodes. "POND"
should be a stage-area node type and "GWSINK"
should be a time-stage node. The initial stage for "POND" is 104
feet which is 4 feet above the bottom of the pond. Remember, our goal is to
determine the drawdown time for 4 feet of water in the pond. The stage at node "GWSINK"
is arbitrarily set to elevation 50 feet in this example. We are assuming
that the elevation of the groundwater table will not influence the
percolation rate. This is true for well drained soils with water tables
substantially below the bottom of the pond. A more rigorous geotechnical
investigation might be necessary for your particular application.
2.
Create
an operating table and name it "PERC".
Note that the operating table is a "Type: rating
curve" with a "Function: U/S Stage
vs. Discharge". The stage-discharge data developed in the table
above is loaded into the operating table. ICPR will not do anything with
this operating table until it is referenced with a specific link.
3.
Create
a rating curve link and name it "L-PERC".
Note that this link establishes connectivity between nodes "POND"
and "GWSINK". Also note that the operating table (called "PERC")
is specified under the "NAME" column in the link template. ICPR
uses the operating table data specified in step 2 above to move water from
node "POND" to node "GWSINK". The "ELEV. ON"
is set to 100.01 feet which is slightly higher than the pond bottom. When
the water level reaches this elevation, ICPR will use the operating table
"PERC" to determine the flow rate from node "POND" to
node "GWSINK". It will continue using the operating table data
until the water level in "POND" drops to the "ELEV. OFF"
setting which in this case is 100.01 feet.
4.
Set
up the simulation parameters. This is the
control data for ICPR. In this example, we are telling ICPR to conduct a 36
hour simulation. Note that no hydrographs have been assigned to node
"POND" because we are simply determining the time it takes for the
pond to percolate 4 feet of water. We are not performing a classical pond
routing in this case. Therefore, it is not necessary to perform any
hydrologic computations.
5.
After
the simulation is executed, the report manager
(see
menu selection) can be used to
graphically determine the drawdown time for
this example. As noted in the figure, approximately 10 hours is required for
4 feet of water to pass through the bottom of this pond at a perc rate of 5
iph. Outflow
(see
report form) and volume
(see
report form) graphs can also be viewed.
Click
HERE for a copy of the ICPR input files for this example.
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