Pre 2014 Blogs

The groundwater component of SWMM 5 is found in the gwater.c code.  It (as is all of SWMM 5) is excellently written in small functions by Lew Rossman of the EPA during the SWMM 5 development process.  However, code being code sometimes it is easier to see how the code is functioning.  This blog or note tries to show the mass balance local function updateMassBal

The groundwater component consists of groundwater data (gw in the equation) and aquifer data (a) in the equation.  The equation for the groundwater mass balance is shown in Figure 1.   The infiltration, evaporation occur only over the pervious area but the percolation out the bottom of the aquifer occurs over the whole Subcatchment.

Figure 1.  Groundwater Mass Balance

The groundwater component of SWMM 5 is found in the gwater.c code.  It (as is all of SWMM 5) is excellently written in small functions by Lew Rossman of the EPA during the SWMM 5 development process.  However, code being code sometimes it is easier to see how the code is functioning.  This blog or note tries to show that function. 

The groundwater component consists of groundwater data (gw in the equation) and aquifer data (a) in the equation.  The equation for the groundwater volume is shown in Figure 1.   The volume is the water content (theta) times the upper depth and the porosity of the aquifer times the lower depth (Figure 2).

Figure 1.  Groundwater Volume Calculations

Figure 2.  Lower and Upper Depth of the Groundwater Compartrment

Subject: Hierarchy of Your Network in InfoSWMM and H2OMAP SWMM

In both InfoSWMM and H2OMAP SWMM you can run a subset of the network by using the Facility Manager to make part of the network inactive and not solved.  You can make the output files smaller if you are performing a continuous simulation and save only the results of All, the Domain Only or a Selection Set to the graphical output file (Figure 1).   Figure 2 shows a few ways to query, view, graph and perform statistics for the model run.

Figure 1.  Options for saving the Active Network Data to the Graphical Output Data Set.

Figure 2.  Output View, Query and Graphical Options.

Subject:  Advanced SWMM 5 import into InfoSWMM and H2OMAP SWMM

Subject:   Import of Sections from SWMM 5 into InfoSWMM and H2oMAP SWMM

A very useful hidden feature of the import SWMM 5 to InfoSWMM and H2OMAP SWMM is the ability to import all of the data or just one section.  For example, you can import the LID data, DWF patterns, control rules, pollutants, transects and other data that is transferable between different networks.

 

SWMM 5 has three loss terms available for each link:  Entrance, Exit and Other losses.  The Entrance loss uses the upstream link velocity, the  Other loss uses the center link velocity and the Exit loss uses the downstream link velocity.  The general form of the loss term in the St. Venant equation is K*V^2/2g Table 1 shows the loss in feet of head for various combinations of velocity and K value.  If you want to  simulate a little loss of head at each node then a small value of K should be used otherwise the cumulative loss in the whole networks will be many feet of head.

  Loss Term units equals K * V^2/2g = ft/sec * ft/sec * sec^2/ft = ft

Table 1:  Loss in feet of head for various combinations of velocity and K values.

The newer option for SWMM 5 culverts uses three culvert classifications and associated equations to compute the inlet controlled flow into a culvert using the FHWA (1985) equations.  The culvert code in the culvert.c code of SWMM 5 uses:

1.   Two Equations for Unsubmerged culvert flow,

2.   One Equation for the Transition flow, and

3.   One Equation for Submerged flow.

Subject:  Elevation Interpolation from a Contour in H2OMAP SWMM

The node invert elevation or the node maximum depth can be interpolated if you use the Elevation Interpolation Tool in H2OMAP SWMM.

Steps	Action
1.   Make a Contour Plot of the Node Invert Elevations.
2.   The Created Contours are now a layer in H2OMAP SWMM.
3.   Recreate the Invert Elevation from the Contour by using the Value Field and Interpolate Field.
4.   You can  also estimate the Maximum Depth of the Node from the Contour and the known Node Invert Elevation.

 Variable Time Step in SWMM 5

v  The goal of the link lengthening in SWMM 5 it to meet the CFL time step condition for the full link depth and full link velocity at the chosen lengthening time step.  If the link does not meet the CFL condition then this means the time step needed is smaller than your selected lengthening time step.  SWMM 5 will make an hydraulically equivalent longer link with a smaller roughness but the same full flow velocity as the shorter link.

v  If you are running a simulation in which all of the pipes are exactly full – no surcharge in any pipe – and the variable time step then there would be no need for SWMM 5 to use anything other than the minimum of the routing or lengthening time step.  However, since most real networks have a mixture of partial flow, surcharged flow and pressure flow, the actual time step depth, velocity/Froude Number is different than the assumed full depth and full flow velocity.  For example, the depth can be higher at one end of the pipe and the velocity higher than full flow velocity due to the water surface slope being higher than the bed slope.  The only way SWMM 5 can now satisfy the CFL time step condition since the modified length is fixed is to lower the variable time step.

Subject:   Reading the Output of Older SWMM 5 versions in Newer SWMM 5 Versions

It is very easy to read the output graphs and output text file from older versions of SWMM 5 in newer versions of SWMM 5 as long as the rules are followed:

1.   You need to have the RPT file for the InputFileName or InputFileName.RPT

2.   You need to have the OUT file for the InputFileName or InputFileName.OUT

3.   The File Size for InputFileName.RPT is greater than 0

4.   The Run Status for InputFileName.OUT is true based on the tests in CheckRunStatus

Figure 1.   The RPT File or OUT File is not saved unless you 1^st save the Current Simulation Results.

Figure 2.   The binary output file of SWMM 5.0.013 in SWMM 5.0.022

This note attempts to explain both how the head upstream and the head downstream of a weir in SWMM 5 is calculated compared to the weir crest elevation and also to explain how the weir is presented in the HGL plot of SWMm 5.  There has been confusion in the past concering how the weir is shown compared to the actual weir calculations.  The node head is calculated obviously at both ends of the weir but the head over the weir is always based on H1-Crest or H2-Crest (Figure 1) and hence the weir should look flat – to the weir the downstream head is important but NOT the downstream node invert so the weir really is flat and should look flat in the HGL Profile across the weir (Figure 2).    The crest elevation is always relative to the upstream node invert elevation NOT the downstream node invert elevaation

Figure 1.  How the Head across a Weir is calculated in SWMM 5

Figure 2.   HGL Profile across a Weir in SWMM 5.0.022.  The black line should be shown flat.

The time step actually used during the simulation is related to this velocity and the safety adjustment factor.  The larger the safety adjustment factor the larger the mean time step listed in the Routing Time Step Suggestion.

An oft requested feature in SWMM 5 is the ability to better simulate a variable speed pump.   The basic feature we are trying to model is multiple pumps between two nodes, one pump curve for all of the pumps and the ability to turn on and turn off the pumps based on either the head or depth at a Wet Well (Figure 1).  You can turn on or off the pumps Pump1, Pump2 and Pump3 based on the depth at the Wet Well but this feature is stepwise linear and usually uses three pump curves.  A better way to simulate this feature is to use the SWMM 5 Real Time Rules (RTC) to simulate the Pump setting based on a control curve.  

The Pump flow at any time step is the Pump Flow estimated from the Pump Curve (Figure 2) * The Pump Setting (Figure 3)

Each of the three pumps has a different Control Curve (Figure’s 4, 5 and 6, respectively) which turns on or turns off the Pump based on a range of Wet Well Depths.  The overall effect is that the total flow summing all three pumps together is smoother (Figure 7 and Figure 8) and the user can simulate different pump speeds based on the same pump curve depending on which pump is currently on.

Figure 1.   Example RTC Rules and VSP Pumps in a SWMM 5 model.

Figure 2.  The Pump Curve Used for all 3 Pumps

Figure 3.  The Pump Setting for all Three Pumps

Figure 4.   Pump Control Curve for Pump 1.  The Pump has a Setting of ¼ between 0.5 and 3 feet at the node Wet Well and zero otherwise.

Figure 5.   Pump Control Curve for Pump 2.  The Pump has a Setting of 1/2 between 3 and 5 feet at the node Wet Well and zero otherwise.

Figure 6.   Pump Control Curve for Pump 3.  The Pump has a Setting of 1 above 5 feet at the node Wet Well and zero otherwise.

Figure 7.  The Flow in all 3 Pumps.

Figure 8.  The total flow from all three Pumps to the downstream node.

A new Map Display feature in SWMM 5.0.022 is the LID Usage parameter which shows you whether a Subcatchment has LID’s or not.   You use it by using Map Display and choosing LID Usage as the Map Display.  LID - Low Impact Development.