Sunday, March 9, 2014

The Minimum Surface Area in SWMM 5 - Meaning and Usage

The Minimum Surface Area in SWMM 5 - Meaning and Usage

The minimum surface area in SWMM 5 is intended for manholes that have a gap between the Node Invert Elevation and the Lowest Connecting Link Invert but it also has other uses in simulation.  If there is a gap the minimum surface area is used to prevent a divide by a zero surface area in the node continuity equation.  In the case of very short links where the area of the links added to the node surface area is less than minimum surface area then the area used is the minimum surface area.  If you want the SWMM5 engine to essentially ignore the minimum surface area then set the area to a small value in the Dynamic Wave Tab of the Simulation Options Dialog (Figure 1).   This applies equally to the both Manholes and Storage nodes as shown in Figure 1.  The Surface area for the Storage Node is set equal to the Minimum Surface Area. 

1. Storage Nodes and Manholes use the same Node Continuity Equation until the Highest Link Soffit is reached and then the Manhole uses the Surcharge Equation
2. Storage Nodes and Manholes both use the Minimum Node Surface Area

Figure 1.  The Meaning of the Minimum Surface Area in SWMM 5

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