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PONDS USER MANUAL
THEORY
The Simplified Analytical Method was developed to compute the time for recovery of
retention ponds or exfiltration trenches in water table aquifers. The assumptions of the
theory are listed below:
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The receiving aquifer system is idealized as a laterally infinite, single-layered,
homogenous, isotropic water table aquifer of uniform thickness, with a
horizontal pre-loading phreatic surface.
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The three dimensional shape of the pond is assumed to be that of a
rectangular trench.
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The pond fills up suddenly with the treatment volume (i.e., a slug loading).
This assumption is consistent with SJRWMD criteria which does not allow for
ground water or surface water discharge during the storm event.
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The moving zone of saturation (or transitory ground water mound) is idealized
as a series of triangular prisms adjacent to the pond perimeter. At the corners
of the rectangular pond, the triangular prisms assume the shape of a quadrant
of a solid cone. The lateral extent of the mound (or radius of influence)
increases as recovery progresses.
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From the law of conservation of mass, the volume of water which infiltrates
out of the pond/trench is equal to the volume of water in soil storage in the
triangular saturated prism at any instant.
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Darcy's Law is the governing equation for saturated ground water flow.
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Dupuit-Forchheimer assumptions are applicable; i.e.,
1.
Flow is considered to be purely horizontal
2.
Flow is assumed to be uniformly distributed with depth
The Dupuit-Forchheimer theory loses accuracy if the depth of the
impermeable layer increases, because of the increased importance of vertical
flow. Bouwer (1969) found that the Dupuit-Forchheimer theory gave
reasonable results if the distance of the impermeable layer below the pond
bottom was not more than twice the width of the water level in the pond.
August 7, 1995