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PONDS USER MANUAL
Figure D-1:
D-4
Conceptual Model of Refined Method of Retention Pond Recovery
UNSATURATED FLOW ALGORITHM
Unsaturated flow is computed on an incremental basis for each time step. If there is
discharge through the weir or orifice during unsaturated flow, the algorithm iteratively
solves for the stage elevation which satisfies the following volumetric balance:
Runoff Inflow = Discharge Through Weir + Unsaturated Infiltration + Increase in
Storage
Schematic D-1 shows the logic of this algorithm. This model assumes that unsaturated flow
is:
i)
ii)
iii)
iv)
only taking place through the bottom area of the pond at an infiltration rate
equal to Iv (ft/day) (note: the bottom area of the pond changes as the water
level changes),
purely vertical,
filling the soil voids directly beneath the water surface, and
complete once the soil voids beneath the pond are filled.
If the inflow rate is faster than the rate at which water can vertically infiltrate through the
effective pond bottom, water accumulates in the pond. If water accumulates in the pond
above the weir discharge elevation, discharge is modeled using the weir equations.
This is a relatively simple representation but it is one that is applicable to most cases where
the water table is not deep (say less than 5 feet below the pond bottom). For situations
where the water table is over 5 feet below the pond bottom, lateral flow can be significant
during the unsaturated flow regime but this representation will give conservative results.
The two input parameters for this module are the vertical infiltration rate (for unsaturated
conditions) and the maximum area available for vertical unsaturated infiltration.
August 7, 1995