SWMM 5 Pump Curve Head/Flow Curves (1)

Subject:  The Pump flow is based on the lookup table you enter for the pump (Figure 1).  At each iteration during each time step of the solution SWMM 5 will look up the flow for the pump based on the current control variable across the pump.  The control variable for the pump can be one of four variables:

1.   The volume of the upstream wet well,

2.   The depth of water at the upstream node or inlet node without interpolation between data points,

3.   The downstream water surface elevation across the pump minus the upstream water surface elevation, and

4.   The depth of water at the upstream node or inlet node with interpolation between data points.

 The pump summary table in the rpt file will tell you how often the pump was used, the maximum flow, the average flow, the total volume of the pump, the power usage and the percent of the time off the entered pump curve.  You can also plot the pump flow versus the inlet depth to see how often the pump was off the pump curve (Figure 2).

Figure 1:  Plot of Head and Flow for Pump PUMP1@82309e-15009e over time along with the input and output table for the pump.

Figure 2:  Plot of Head versus Flow for Pump PUMP1@82309e-15009e

Average Number of Node Iterations

Subject:  SWMM 5 will iterate for the new node depth at each time for a minimum of 2 iterations to a maximum of 8 iterations based on the Node Continuity equation.  If you plot the average number of iterations over time then typically the number of iterations go up as the Inflow increases.  The nodes with the most iterations changes over time as the peak flow moves through the network as shown in this plan view.  The iterations used during the simulation is a function of the node stop tolerance which has a default value of 0.005 feet in SWMM 5.

Link Area Types in SWMM 5

Note:  There are 7 Link flow classification classes that are used to assign the area of the link to the upstream and downstream nodes of the link.  The classes used during the simulation of the model are shown in the Link Classification Table in the RPT Report File.  The supercritical class is the same as the subcritical assignment.  The supercritical is a class of subcritical with a Froude number over 1.

Class

Description

Link Area Assignment

      0      Dry conduit

1/2 Upstream and 1/2 Downstream Node

      1      Upstream end is dry

1/2 Downstream Node

      2      Downstream end is dry

1/2 Upstream Node

      3      Sub-critical flow

1/2 Upstream and 1/2 Downstream Node

      4      Super-critical flow

1/2 Upstream and 1/2 Downstream Node

      5      Free-fall at upstream end

1/2 Downstream Node

      6      Free-fall at downstream end

1/2 Upstream Node

Orifice Critical Depth for Separating Weir Flow from Orifice Flow for Bottom Outlet Orifices

Note:  Orifice Critical Depth for Separating Weir Flow from Orifice Flow for Bottom Outlet Orifices

 The Critical height is the opening where weir flow turns into orifice flow. It equals (Co/Cw)*(Area/Length) where Co is the orifice coeff., Cw is the weir coeff/sqrt(2g), Area is the area of the opening, and Length = circumference of the opening. For a basic sharp crested weir, Cw = 0.414.  All of the units are based on the internal SWMM 5 units of American Standard.

 For a circular orifice the Critical Height is:

 Critical Height = Orifice Discharge Coefficient / 0.414 * Orifice Opening / 4

 For a rectangular orifice the Critical Height is:

 Critical Height = Orifice Discharge Coefficient / 0.414 * (Orifice Opening*Width) / (2.0*(Orifice Opening+Width))

 The Orifice Critical Depth changes dynamically as the orifice is opening and closing for a bottom outlet orifice.  The critical depth separating the orifice weir flow from orifice flow for a side outlet orifice is the height of the orifice.

The Three Depths in a Link in SWMM 5

Note: An explanation of the three depths in a Link in SWMM 5 and a plot of the upstream, middle and downstream link depth. The middle depth is an average of the upstream and downstream link depths. The plot of the variable depth or the middle depth is always between the upstream and downstream depths. All three depths are used in the computation of the St. Venant Flow in SWMM 5. The upstream area is a function of the upstream depth and the downstream area is a function of the downstream depth.


The dq4 term in dynamic.c uses the area upstream (a1) and area downstream (a2), the midpoint velocity, the sigma factor (a function of the link Froude number), the link length and the time step or

 

dq4 = Time Step * Velocity * Velocity * (a2 – a1) / Link Length * Sigma

the dq3 term in dynamic.c uses the current midpoint area (a function of the midpoint depth), the sigma factor and the midpoint velocity

 

dq3 = 2 * Velocity * ( Amid(current iteration) – Amid (last time step) * Sigma