SWMM 5 Arrow Direction Question

Subject:   SWMM 5 Arrow Direction Question 

 A question often asked is what happens to the flow in a SWMM 5 link if the downstream and upstream node names are entered in reverse.    The flow will be exactly the same as if the nodes were entered in the right order but the flow in the link will be negative.  As long as the inflow to the model is at the same node in both alternate models the node depth, the link upstream depths and upstream cross sectional areas will be the same.  For example, the flow out of Nodes 80408A and 80408 will be the same but the flow out of 80408 will be negative and the flow out of 80408A will be positive.

SWMM 5 and InfoSWMM Time Step Guide

Subject:   SWMM 5 and InfoSWMM Time Step Guide

 If you use a variable time step in SWMM 5 or InfoSWMM/H2OMAP SWMM it is hard to gauge the proper value of the conduit lengthening.  You want to use a value that does not increase the volume of the network yet does increase the length of the shortest links so you can use a longer time step.  A good approximation to the time step that you want to use is shown in the image.  

 The Time Step Guide in seconds is Link Length / [Velocity + sqrt(g*Maximum Depth)] with the assumption that the velocity at maximum depth is about the value of the wave celerity for closed links or sqrt(g*Maximum Depth).  Normally (unless pumps are involved) the average time step used during the simulation is a good gauge of the time to use for the simulation.  For example, in this model run the time step used is 13 seconds which is about the conduit lengthening time step of 20 seconds * adjustment factor of 0.75

Siphon Simulation in SWMM 5 and InfoSWMM

Subject:  Siphon Simulation in SWMM 5 and InfoSWMM

 A Siphon is simulated in SWMM 5 and InfoSWMM using the basic node and link data and downstream boundary condition:

 1.   Inflow can be time series, dry weather flow pattern, wet weather inflow or Subcatchment Runoff,

2.   The boundary condition can be either a free outfall, fixed or time series,

3.   The node invert, node maximum depth and node surcharge depth are defined by the user or network,

4.   The link lengths, diameters, link offset depths upstream and downstream are defined by the user of the network,

5.   The node depths, link flows, link depths and link cross sectional areas are calculated at each time based on the node continuity equation and the link momentum and continuity equation.  The link flows are a function of the friction loss, head difference across the link and the difference in the cross sectional areas of the link.

6.   In the particular model the Inflow at node MH1 fills up the MH1 depth which causes the links downstream to start flowing – the head difference across the links drives the flow up and over the siphon.

3 Types of Subcatchment Flow in SWMM 5

Subject:   3 Types of Subcatchment Flow in SWMM 5

 

There are three types of Subcatchment flow in SWMM 5 (Figure's 1 and 2)

 

1.   Impervious area with depression storage in which the runoff from the precipitation is delayed due to the depression storage.  Evaporation occurs based on the depth of water in the subarea of the Subcatchment.

2.   Impervious area without depression storage in which the runoff from precipitation is NOT delayed.  Evaporation does occur based on the depth of water in the subarea of the Subcatchment.

3.   Pervious area with depression storage in which the runoff from the precipitation is delayed due to the depression storage.  Evaporation and Infiltration occurs based on the depth of water in the subarea of the Subcatchment.

 

 

Figure 1.  Runoff from the 3 Types of Subcatchment Subareas

Figure 2: Subcatchment SubArea Types