HydroCAD Stormwater Modeling - Since 1986

Corrugated Metal Pipe Storage Calculations


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.

Click for complete self-training materialsMany 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 Metal Pipe (CMP)

A few of the many chamber optionsHydroCAD provides a full range of chamber definitions for modeling CMP, including round and arch-pipe cross-sections.  These definitions can be viewed and compared on the chamber report screen, as shown here.

Each CMP definition specifies the internal dimensions and volume of the pipe.  Due to the small corrugation depth for CMP (typically 1 inch or less), the corrugations are not considered when evaluating the displacement, and the outside dimensions and displacement are set equal to the internal values, causing the storage and displacement to be slightly underestimated.  If greater accuracy is needed for a specific application please contact

The CMP chamber definitions were added in HydroCAD 9.1.  For earlier versions, please obtain a program update. For further details read about chamber modeling and the HydroCAD chamber wizard.


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