	Applies To
	Product(s):	SewerGEMS, SewerGEMS, StormCAD, Civilstorm
	Version(s):	08.11.XX.XX and higher
	Area:	Modeling

Problem

How are ponds and pond outlet structures modeled with the different storm/sewer products and numerical solvers?

How can storage / detention be modeled with pumps and/or outlet structures / emergency overflow?

Background

The pond element can be used to model any storage (detention) facility, with outflow from either a pump or an outlet structure (weir, orifice, riser, etc). The wetwell element can alternatively be used to model typical pumped storage. If you need to direct overflow to a different location, the pond element can be used with a pond outlet set as a weir.

Note: the implicit solver considers the storage volume in manhole and catchbasin subsurface structures, but the impact is typically minimal. The Explicit (SWMM) solver does not consider this storage as manholes are treated as junction nodes in SWMM. So, any significant storage effects should be modeled as a pond or wetwell.

Ponds and pond outlets are treated differently, depending on the active numerical solver that you are using and whether or not you have designated that the pond outlet has a control structure.

Here are the available numerical solvers for each product:

StormCAD: GVF Rational
SewerCAD: GVF Convex
CivilStorm: GVF Rational, Implicit Dynamic, Explicit Dynamic (SWMM)
SewerGEMS, GVF Rational, GVF Convex, Implicit Dynamic, Explicit Dynamic (SWMM)

To connect a pond to a pond outlet, select the pond from the pond outlet's "upstream pond" field - you will see a dashed line connecting from the outlet to the centroid of the pond. (see below)

Here is the option of using a control structure on the pond. When set to "Yes", a Composite Outlet Structure can be used to calculate outflow based on water surface elevation.

Solution

Below documents the behavior of ponds and pond outlets for each combination of numerical solver and "Has Control Structure?" setting.

Note that the Explicit (SWMM) solver is recommended when modeling ponds and outlets, especially multiple ponds. Set the Routing Step calculation option to 1 second as a start.

Solver: Implicit Dynamic and Explicit (SWMM) Dynamic (available in CivilStorm and SewerGEMS):

Pond with no control structure

If there is no control structure used ("Has Control Structure" = "False"), then the Pond outflow is based on the hydraulics of the adjacent link (conduit or channel element downstream of the pond outlet node). The dynamic solvers calculate the flow restriction based on the size of the conduit or channel. For best results, using a control structure or headwall is suggested.

Pond with control structure or Headwall

When using a control structure ("Has Control Structure" = "True"), the Pond outflow is based on the composite outlet structure's Elevation-Flow-Tailwater (EQT) table. The setup and components of the Composite Outlet Structure determine the pond outflow for a given headwater and tailwater, creating this rating table. A separate rating table exists for each tailwater increment. This family of rating curves is called the EQT table (Elevation-Flow-Tailwater). Using the EQT table along with the pond's water surface elevation and downstream tailwater elevation, the outflow is calculated.

Note: in the properties of the Composite Outlet structure, ensure that the "Tailwater Type" is set to "Interconnected ponds" and that the "Downstream Element ID" is set to "<Automatic>" to ensure that the pond outflow properly senses the downstream hydraulic grade. Unexpected results can occur otherwise.

See more in the section "ICPM in SewerGEMS and CivilStorm" in this article: How does the Interconnected Pond Modeling (ICPM) algorithm work

Headwall

When using a headwall in place of the pond outlet node, outflow from the pond is based on the culvert EQT table. See more on headwalls here.

Note: You can have multiple Outlets downstream of a pond. Each one will "look" at the pond's water surface elevation along with its EQT table.

Pond with pump

To model a pumped pond, insert the pump node elements and connect them to the pond by way of a pond outlet node with "has control structure?" set to False. See: Modeling Pumps with Ponds

Pond Design tools

The PondMaker tool can be used to help size a pond based on predevelopment criteria. See: Using PondMaker

Video demonstration / training

See:

Solver: GVF-Rational (Available in StormCAD, CivilStorm and SewerGEMS):

StormCAD is a steady state (peak flow) modeling software and thus will not calculate Pond Volume hydrographs. Because the StormCAD product uses the same database format as the other Bentley storm and sewer products, other elements more traditionally used with dynamic solvers (in SewerGEMS and CivilStorm) are exposed and usable in StormCAD. This includes the pond and outlet elements. These elements are not native to StormCAD and are modeled differently. To see and lay out a pond or pond outlet, you may need to click the arrow at the bottom of the vertical layout toolbar and choose to add it.

Pond with no outlet structure

If there is no control structure used ("Has Control Structure" = "False"), then the pond is essentially treated as a manhole node and Flow in = Flow out. All fixed and rational flows pass through the pond without any change. In profiles, if the downstream pipe's calculated start HGL (based on backwater calculations) is lower than the user specified initial pond elevation, the initial pond elevation is used as the pond hydraulic grade line and for backwater calculation of the upstream elements.

Note: Pond Infiltration is ignored in the GVF- Rational solver.
Note: Multiple downstream pond outlets (split flow) are not allowed.

Pond with outlet structure

When using a control structure ("Has Control Structure" = "True"), pond outflow is based on the selected composite outlet structure rating table in conjunction with the pond "initial elevation". In other words, the program takes the specific initial pond elevation and looks it up on the outlet rating table to determine the pond outflow. Once the pond outflow is determined it becomes a fixed flow (system additional flow). All other upstream fixed and rational flows are terminated at the pond outlet and the new fixed outlet flow will carry on to the downstream calculations. In other words, if you had a fixed flow coming into your pond from an upstream conduit, that flow would not be included in the outflow from the pond. The outflow from the pond is exclusively determined from the user defined initial elevation you enter. Also note that even inflows that are entered directly in the pond are excluded from the outflow calculations when using an outlet structure in a pond.

If you do not enter an Initial Elevation higher than the pond bottom, you may see zero flow out of the pond. For peak flow design, you may consider an initial elevation near the top of the pond, for a conservative outflow.

In profiles, if the downstream pipe's calculated start HGL (based on backwater calculations) is lower than the user specified initial pond elevation, the initial pond elevation is used as the pond hydraulic grade line and for backwater calculation of the upstream elements.

Note: Multiple downstream pond outlets (split flow) are not allowed.

Solver: GVF Convex (Available in SewerCAD and SewerGEMS):

Pond without Outlet Structure - EPS

If there is no control structure used ("Has Control Structure" = "False"), then the pond is essentially treated as a manhole node and Flow in = Flow out. The upstream hydrograph (including Catchments flows) is routed downstream without any change.

Note: Multiple downstream pond outlets (split flow) are not allowed.
Note: Pond Infiltration is ignored with the GVF-Convex solver when the pond is not connected to an outlet structure.

Pond without Outlet Structure - Steady State

In steady state with no control structure, pond calculations are the same as EPS (flow in = flow out). The difference is that Catchment flows are ignored during Steady State (including an automated design run).

Note: Multiple downstream pond outlets (split flow) are not allowed.

Pond with Outlet Structure - EPS

When using a control structure ("Has Control Structure" = "True") in EPS, pond outflow is based on the selected composite outlet structure rating table in conjunction with the pond "initial elevation". In other words, the program takes the specific initial pond elevation and looks it up on the outlet rating table to determine the initial pond outflow. During the EPS, the pond may fill or drain based on the outflow and upstream inflow, and the outflow will adjust accordingly from the rating table.

Note: Multiple downstream pond outlets (split flow) are not allowed.
Note: Pond infiltration is supported in EPS.

Pond with Outlet Structure - Steady State

When using a control structure ("Has Control Structure" = "True") in Steady State, pond outflow is based on the selected composite outlet structure rating table in conjunction with the pond "initial elevation". In other words, the program takes the specific initial pond elevation and looks it up on the outlet rating table to determine the pond outflow. If the initial pond elevation is higher than the one calculated by the outlet pipe profile calculation then the initial elevation is set as the pond HGL. If you select Invert as initial pond elevation type, then the GVF engine picks the initial HGL based on higher elevation value between pond invert and outlet control elevation.

Note: Multiple downstream pond outlets (split flow) are not allowed.
Note: Catchment runoff is ignored during Steady State.

Troubleshooting

Composite outlet structure with a downstream culvert

If you are have a culvert downstream of a Pond Outlet Structure element, all flow from the individual outlet structures will be forced through the culvert. The culvert size may impact the rating table for the composite outlet structure. To better see and understand the rating table in cases like this, consider including the culvert as an outlet structure, setting the Downstream ID for the appropriate outlet structures as the culvert.

Unstable or incorrect results in the rating table

First, make sure that the input for the composite outlet structure is accurate. Invalid input data will skew the results.

If there are results in the rating table that appear to be inaccurate, you can also try adjusting the settings in the "Outlet Structure (Convergence Tolerances)" in the Composite Outlet Structure manager. By increasing the maximum iterations, the program will be able to attempt additional trials to find a stable solution. Adjusting the minimum and maximum tolerances for headwater, tailwater, and flow can also help.

It is not typical that adjusting the tolerances will be necessary.

Error when computing rating table: "Array dimensions exceeded supported range"

Depending on the version you are running, there is a hard limit of 18 outlet structures for a composite outlet structure. In a patch for version 10.04.00.158 of the storm-sewer products, this limit is increased to 100. Later releases will also include this new limit.

Warning User Notification: "Tailwater elevation is higher than pond outlet's initial elevation...."

Please see separate article: "Tailwater elevation is higher than pond outlet's initial elevation...."