Designing SWMM detention ponds involves key elements: storage units, orifices, and weirs. These components are detailed below:

1. Storage Units: Storage units in SWMM are nodes, distinct from Example 2’s junction nodes. They utilize a Storage Curve to describe volume, allow specification of an Evaporation Factor, and necessitate defining a Maximum Depth.

  • Storage Curve: This curve outlines a storage unit’s shape by detailing how surface area changes with water depth. SWMM integrates the curve to calculate stored volume based on depth. It can be defined as a functional equation or tabular data (area-depth pairs).
  • Evaporation Factor: An Evaporation Factor = 1 facilitates surface evaporation, using evaporation data from SWMM’s Climatology Editor. A default 0 value means no evaporation consideration.
  • Maximum Depth: Defining a maximum depth for storage units is crucial to avoid assuming a default depth of zero. If undefined, even with a storage curve or conduit connection, the model assumes zero depth. If the curve’s highest depth is below the maximum, the last curve area extends outward.

2. Orifices: SWMM’s orifice-type link models outlets on the side or bottom of storage units. Connecting an upstream node (storage unit) to a downstream junction via a conduit, orifices require specifications such as height above the unit’s bottom (invert offset), type (side or bottom), geometry (rectangle or circle with dimensions), and hydraulic properties (discharge coefficient, flap gate presence/absence for backflow prevention).

3. Weirs: SWMM’s weir-type link represents overflow structures at a storage unit’s top. Similar to orifices, the upstream node is the storage unit, while the downstream node connects to a conduit. Weir attributes encompass height above the unit’s bottom, type (transverse, V-notch, or trapezoidal), geometry, and hydraulic properties (discharge coefficients, end contractions, flap gate presence/absence for backflow prevention).

Within VOSWMM, the option to convert nodes or links into alternative node and conduit types is available. This conversion can be accomplished by adhering to the following instructions:

Convert a Node to a Storage Unit

1. Right-click the target node and select “Convert to…” from the pop-up menu.
2. Opt for “Storage Unit” from the ensuing sub-menu.

3. Click on the Properties for the new storage unit, assigning a new name (e.g. SU1), inheriting the node’s invert elevation and maximum depth.

4. Configure additional storage unit properties, like the Storage Curve, as necessary.

For conduit-to-orifice conversion:

1. Right-click the conduit connected downstream of the storage unit, choose “Convert to…” from the pop-up menu.
2. Select “Orifice” from the sub-menu.

3. Open the Property Editor for the orifice, specifying dimensions, invert offset, and discharge coefficient.

To convert an outfall to a node:

1. When a storage unit (SU1) requires multiple orifices, direct orifice connection to an outfall (O1) isn’t feasible. In this scenario, the outfall must transform into a node (O1), necessitating the creation of a new outfall (O2).
2. Apply the same conversion process as mentioned earlier to transform the outfall into a node.