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Computation of Overdredged Material: CROSS SECTIONS AND VOLUMES vs TIN MODEL Computation of Overdredged Material: CROSS
SECTIONS AND VOLUMES vs TIN MODEL
By Pat Sanders
This definitely isnt going to make anybodys best-seller list, but its important to know if
you are paying (or being paid) based on the Philadelphia Methods available in
HYPACK®.
The Philadelphia Post-Dredge method computes the overdredged material--the amount
of material that has been removed beneath the overdepth template between the pre-
dredge and post-dredge surveys.
Limiting the DBL to Pre Dredge
Above Sub Depth Side Slope
CROSS SECTIONS AND VOLUMES
In the CROSS SECTIONS AND VOLUMES
program, there is an important setting that
affects the reported quantities.
Under the Graph Options Volume tab, about
midway down the form, you will see the option:
Limit DBL to Pre Dredge Above Sub Depth
Side Slope.
Thats a real mouthful. Lets take a look at
what this option does.








When the option is not checked, it computes any overdredged material across the
entire span of the profile. In the following graphic, all of the blue area would be reported
as Overdredged Material.
When the option is checked, it limits the computation of Overdredged Material to the
region where the Before Dredge profile intersects the toe lines.

The reasoning behind this feature was that a contractor should not be penalized for
material accumulating on the side slopes where the bottom is well beneath the
overdepth template.
This option is only available in CROSS SECTIONS AND VOLUMES (CSV).
TIN Model
In the TIN MODEL, the Philadelphia routine trims the TIN surface to the channels top of
bank lines. The result is similar to the option where Limit DBL to Pre Dredge Above
Sub Depth Side Slope is unchecked. Computing Overdredged Material
There is one more important difference between how CSV and the TIN MODEL
computes Overdredged Material. Take a look at the following graphic.

Cross Sections and Volumes
In the report of CSV, the Infill Material (shown in yellow) is being deducted from the
Overdredged Material (blue). What is presented in the report is a Net Overdredged
Material. If the amounts of Infill are large enough, the Overdredged Material can actually
be reported as a negative value!
TIN Model
In the report of the TIN MODEL, the Infill Material is not deducted from the Overdredged
Material. This can result in the TIN MODEL reporting significantly different amounts of
Overdredged Material.