Corrugated Plastic Pipe Storage Calculations
Background
Pipe flow (conveyance) is generally modeled in HydroCAD by entering the appropriate shape and interior dimensions of the pipe. This allows HydroCAD to model virtually any pipe, regardless of design or manufacturer, without any special customization of the software. Details here.
Many pipe products can also be used for stormwater
storage, such as detention or retention applications. For pipe with a
smooth interior, the "Pipe Storage" option can be used to model the exact volume
based on the pipe shape and interior dimensions. However, certain types of
pipe, such as corrugated metal or plastic, may provide additional storage in the
corrugations, which is more difficult to model.
Certain installations may also use perforated pipe, with supplemental storage provided by the stone backfill. For these systems, the outside dimensions of the pipe must also be known, so that the stone displacement and storage can be properly calculated. For pipes with a smooth exterior (such as concrete), this can be handled by setting the appropriate wall thickness on the pipe storage screen. Again, pipes with a corrugated exterior present an additional challenge.
In order to provide optimum modeling for all types of pipe storage, HydroCAD includes chamber definitions for many common pipe products and sizes. Each chamber definition provides complete details for a specific type and size of pipe, including it's precise internal storage and exterior displacement. Each chamber definition also provides basic installation and layout parameters, allowing the automated layout of pipe storage systems using the HydroCAD chamber wizard.
Corrugated Plastic Pipe
HydroCAD provides a wide range of chamber definitions for modeling corrugated plastic pipe (CPP):
Single-Wall (Type C) pipe is defined by a set of generic "CPP"
definitions. Each definition specifies the inside diameter of the pipe,
outside diameter, and the approximate corrugation profile. Based on this
information, HydroCAD is able to calculate the
total internal storage of the pipe. Since the actual plastic wall has a
negligible volume, the displaced volume is considered to be the same as the
interior volume.
CPP data up to 24" ID has been added in HydroCAD 10.0 build 4. The HydroCAD CPP dimensions are taken from the Design Manual of the Plastics Pipe Institute. Applications over 24" diameter will generally use double-wall pipe, as described below.
Double-Wall (Type S) pipe differs from Type C by the presence of a
smooth internal lining. This creates a closed space between the lining and
the corrugations, so the internal storage can be calculated as a basic cylinder.
However, the displacement is determined by the outside diameter and the
corrugations. In order accommodate the unique profile used by different
manufacturer's, HydroCAD provides specific chamber definitions for Type S pipe
from ADS, Baughman,
Haviland, Lane, and Prinsco.
Modeling Notes
When using chamber definitions in HydroCAD, the invert elevation refers to the outside (bottom) of the pipe or chamber, rather than the inside. This facilitates the placement of the chamber on a stone bed, and avoids the need to make manual adjustments for the bottom (wall) thickness. HydroCAD will automatically add the appropriate bottom thickness as required for each chamber definition, so all you need to do is set the bottom elevation. And the chamber wizard makes this even easier, because the chamber elevation is set automatically based on the depth of the stone bed.
For further details read about chamber modeling and the HydroCAD chamber wizard.