Pre 2014 Blogs

Subject:  Output Statstics Manager to find negative flows in InfoSWMM

Output Statstics Manager to find negative flows with these parameters:

1.       Pipe Features

2.       Use a Domain with your force mains

3.       Select Flow

4.       Event Dependent

5.       Total – NOT Mean or Peak to  find the negative and positive flows

6.       Large NEGATIVE Flow Threshold

7.       Large NEGATIVE Volume Threshold

8.       Zero for Interevent Time to pick up all values

9.       You will get a table that shows you the minimun flows, and a histogram of the flows

Note:  The Keep and Dampen options and their effect on the four main terms of the St Venant equation. 

The four terms are are used in the new flow for a time step of Qnew:

Qnew = (Qold – dq2 + dq3 + dq4) / ( 1 + dq1)

when the force main or gravity main is full dq3 and dq4 are zero and  Qnew = (Qold – dq2) / ( 1 + dq1)

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

where Sigma is a function of the Froude Number and the Keep, Dampen and Ignore Inertial Term Options.  Keep sets Sigma to 1 always and Dampen set Sigma based on the Froude number, Ignore sets Sigma to 0 all  of the time during the simulation

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

dq1 = Time Step * RoughFactor / Rwtd^1.333 * |Velocity|

The weighted area (Awtd) is used in the dq2 term of the St. Venant equation:

dq2 = Time Step * Awtd * (Head Downstream – Head Upstream) / Link Length or

dq2 = Time Step * Awtd * (Head Downstream – Head Upstream) / Link Length

ormally, dq1 (Friction Loss / Maroon in the Graph) balances dq2 (Water Surface Slope Term or Green in the Graph) but often for links with a large difference between upstream and  downstream depths dq4 (Red in the Graph) can have a significant value.  If dq4 or dq3 are important then the depth of water to increases to pass the same flow using the Keep option over the Ignore.   If you have a link with a Froude number near or over 1.0 (Supercritical) then using Keep or Dampen  for the Options may result in depth differences.   The effect of Keep is to increase the “loss” terms in the St Venant Equation.   The effect of Dampen and Ignore is to decrease the sum of the “loss” terms in the St. Venant Solution and lower the simulated depth.

A surcharged node in SWMM 5 uses this point iteration equation (Figure 1):

dY/dt = dQ / The sum of the Connecting Link values of  dQ/dH

where Y is the depth in the node, dt is the time step, H is the head across the link (downstream – upstream), dQ is the net inflow into the node and dQ/dH is the derivative with respect to H of the link  St Venant equation.  If you are trying to calibrate the surcharged node depth, the main calibration variables are the time step and the link  roughness:

1.   Mannings’s N

2.   Hazen-Williams or

3.   Darcy-Weisbach

The link roughness is part of the term dq1 in the St Venant solution and the other loss terms are included in the term dq5.  You can adjust the roughness of the surcharged link  to affect the node surcharge depth.

Figure 1.  The Node Surcharge Equation is a function of the net inflow and the sum of the term dQ/dH in all connecting links. Generally, as you increase the roughness the value of dQ/dH increases and the denominator of the term dY/dt = dQ/dQdH increases.

Figure 2.  The value of dQ/dH in a link as the roughness of the link increases.

If you cannot open ArcMap, InfoSewer or InfoSWMM at all and get a fatal Esri error the problem may be the file normal.mxt

To remove the startup file in Windows XP for Arc GIS 10 go to the directory C:\Documents and Settings\Your Name\Application Data\ESRI\Desktop10.0\ArcMap\Templates and delete the file Normal.mxt.   You then reopen Arc Map and the normal.mxt file will be recreated and smaller.  You will have to reset the ArcMap toolbars to better control InfoSewer and InfoSWMM.   

In SWMM 5 only the Kinematic Wave solution allows a flow divider at a node to divide the Inflow to node to two  downstream  links, but you can use the Inflow/Outflow Outlet type in InfoSWMM to divide the inflow based on a Inflow/Outflow Diversion Table (Figure 1).  For example, in InfoSWMM it is possible to have two downstream links from a Node that are Outlet types Inflow/Outflow so that the low flow goes down one link and the high flow goes down the other link (Figure 2 and Figure 3).   The low flow and the high flow  link  use different diversion tables in which the tables are constructed so that the flow is positive in one link and zero in the other to a dividing flow value and then zero and positive for the same two links after the dividing flow value ( 5 cfs in the example).

Figure 1.  Types of OUTLETS in InfoSWMM and SWMM 5

Figure 2.  Example low flow and high flow Outlet Links to divide the total  inflow at the upstream node at 5 cfs.

Figure 3.   The flow is divided into the low and high flow links at the dividing flow of 5 cfs.

Your network data usually has a number of different attributes that describe the features it represents (Figure 3). While you’ll commonly use one of the attributes to symbolize the

data—for example, showing one quantity in the InfoSWMM Map Display —you may sometimes want to use more than one.   One way to show multiple attributes in InfoSWMM is to copy layers and then use the Layer Properties to color, map or otherwise display the multivariable data (Figure 1).  For example, Figure 2 shows the important Subcatchment parameters of Slope, Imperviousness and Width as graduated colors, dots and a pie shape, respectively.

Figure 1.  Use the Symbology Tab to select the attribute you want to show and the way to show the attribute.

Figure 2.   The Subcatchment slope is shown in graduated colors, the percent impervious in scattered dots a a measels map and the Subcatchment Width is shown in a pie graph with the size of the pie a function  of the total  width.

Figure 3.  Physical Data Estimated from a DEM using the Subcatchment Manager in InfoSWMM.

The Subcatchment Manager of InfoSWMM will  help calculate most of the  physical parameters associated with a Watershed or Subcatchment in SWMM 5 from a Digital Elevation Data (Step 1).  The Subcatchments slope is estimated from a slope raster (Step 2) and the Slope Calculator (Step 4). The created watershed area are calculated using the command Update DB from Map (Step 6) along with the Subcatchment Width (Step 3) and the Impervious Area (Step 5).   The physical parameters estimated from the DEM are shown in Figure 1.

Figure 1.  Physical Data Estimated from a DEM using the Subcatchment Manager in InfoSWMM.

Step 1.  Use the command Create Flow Stream to create a Flow Stream for the DTM or DEM that can be used later.

Step 2.   Create a Slope Raster from the DEM for later usage in the Slope Calculator.

Step 3.   Calculate the Width of the Subcatchment using one of five methods.

Step 4.   Calculate the Slope in percent from the Slope Raster created in Step 2.

Step 5.   Populate the Impervious area percentage using a Parcel shape file and the Created Subcatchments.

Step 6.   Use Arc Map to calculate the area of the Subcatchments using the command Update DB from Map and the following Operation Flags.

The Subcatchment Manager of InfoSWMM will  help calculate most of the  physical parameters associated with a Watershed or Subcatchment in SWMM 5 from a Digital Elevation Data (Step 1).  The Subcatchments area created from a Flow Direction Raster (Step 2) and a Flow Accumulation Raster (Step 4) after filling in any Sinks in the DEM (Step 3).  The created watersheds (Step 5).   The physical parameters estimated from the DEM are shown in Figure 1.

Figure 1.  Physical Data Estimated from a DEM using the Subcatchment Manager in InfoSWMM.

Step 1.  Find, Create or Otherwise Locate a TIN, DEM or DTM for the project area with elevation data that you can  use with the InfoSWMM Subcatchment Manager.

Step 2.   Create a Flow Direction Raster using the Watershed Command.

Step 3.   Check to see if there are Sinks in the Elevation Data that have to be filled using the Filled Sink Command.

Step 4.   Create a Flow Accumulation Raster

Step 5.   Create the Watersheds from the Flow Direction and Flow Accumulation Rasters.

If you have a large network and especially if you are doing continuous simulation then you want to have many tools for helping you understand the network and the simulation results.  

v  In InfoSWMM and H2OMAP SWMM you can have a Base Network with many differenct Child Scenaio generations.  A Child can be either based on the Base Scenario of a different generation Child Scenario.

v  Facility Manager allows you to make inactive and active sets areas of your network, which makes simulating larger and smaller models a snap to do in InfoSWMM.   Run Manager lets you control which areas of the model  network gets save to the  binary graphics file (Figure 1).  

v  The Process Control in  Run Manager (similar to the process control in SWMM 5) allows the modeler to control  which processes are simulation to  help in her model  calibration.

Figure 1.  Scenarios, Facility Manager and Run Manger Options.

Figure 2.  Run Manager Process Controls.

The RDII method in InfoSewer is similar to the RDII or RTK  method in  InfoSWMM with some differences.    The RTK data for triangles 1, 2 and 3 are defined in the Unit Hydrograph but instead of individual R values, the overall R is set and the Percent R1,  R2 and R3 are defined based on the total  R.  R3 is calculated internally as 100 – R1 – R2.   Each loading manhole with RDII flow has a total  area, a hyetograph and a Unit Hydrograph.  The hyetograph has to be set at multiples of the unit hydrograph, so you can define the time or X columns with integers and then use the Block Edit command to change X to minutes by multiplying  by the Unit Hydrograph time (Figure 1).   You can use only one component if you set R1 or R2 to 100 percent or R3 to 100 percent by setting R1 and R2 to 0 percent (Figure 2).  The overall area of the Unit Hydrograph is divided amongst the loading manhole using the Subbasin Area (Figure 3).   The storm flows generated can be viewed using a Group Graph (Figure 4).

Figure 1.   Hyetograph Curve for the RDII Unit Hydrograph

Figure 2.  The Unit Hydrograph is defined for various values of R, R1,  R2, T1,  T2,  T3, K1,  K2 and  K3.

Figure 3.  The Unit Hydrograph and Hyetograph are tied to a particular loading manhole using a Subbasin Area.

Figure 4.  The Unit Hydrographs that are generated can be viewed using a Group loading Manhole Graph.  The R1, R2 and R3 have only one triangle.

The maximum flow limit for a link applies to the kinematic wave and the dynamic wave solution.   The inflow to the link  in the kinematic wave solution is limited (Figure 1) but the calculated link flow is limited in the dynamic wave solution after the link flow (Figure 2):

1.       Is checked using the Culvert Inlet Equations (optional)

2.      The normal flow equation is checked (internally optional depending on the Normal flow options) and

3.      The Picard iteration solution under relaxation parameter (always 0.5) is applied (Figure 3).

Figure 1.  Kinematic Wave Solution Limits the Inflow to  the Link Maximum limit.

Figure 2. Dynamic Wave Solution link  flow limit.

Figure 3.  The Link  flow in the dynamic wave solution has three checks at each iteration in a time step.

If the slope of a link  is negative and the solution  is dynamic wave then the following data will be switched in link.c in SWMM 5.  All upstream data for the  link  is switched to the downstream end of the link  and  vice versa.   The means that if the flow  is from the original upstream node to the downstream node the flow  will  be negative in the output of  SWMM 5. 

Negative flow in SWMM 5 means:

1.   The link has an adverse or negative slope,

2.   The link  has reverse flow if the link slope is positive.

A storage node in SWMM 5 can have either a functional form or a tabular depth/area table.  The area functional form of a storage node is:

Area           =      A * Depth^B + C  and the Volume has the form in  node.c of the SWMM 5 of

Volume     =      A/(B+1)*Depth^(1+B) + C*Depth

For example if C is 25 square meters, A is 20 and the exponent B is 0.5 we get the following values for area and volume and you can also plot a Scatter Plot of Volume vs Depth in SWMM 5 (Figure 1).

Table 1.  Area and Volume for a Storage Node in SWMM 5.

Figure 1.  You can use a Scatter Graph in SWMM 5 to show the relationship between Volume and Depth.

SWMM 5 uses the method of Successive under-relaxation to solve the Node Continuity Equation and the Link Momentum/Continuity Equation for a time step.  The dynamic wave solution in dynwave.c will use up to 8 iterations to reach convergence before moving onto the next time step.  The differences between the link flows and node depths are typically small (in a non pumping system) and normally converge within a few iterations unless you are using too large a time step.  The number of iterations is a minimum of two with the 1^st iteration NOT using the under-relaxation parameter omega. The solution method can be term successive approximation, fixed iteration or Picard Iteration, fixed-point combinatory, iterated function and Lambda Calculus. In computer science, iterated functions occur as a special case of recursive functions, which in turn anchor the study of such broad topics as lambda calculus, or narrower ones, such as the denotational semantics

 of computer programs (http://en.wikipedia.org/wiki/Iterated_function). 

In the SWMM 5 application of this various named iteration process there are three main concepts for starting, iterating and stopping the iteration process during one time step:

·         The 1^st guess of the new node depth or link flow is the current link flow (Figure 3) and the new estimated node depths and link flows are used at each iteration to estimate the new time step depth or flow.  For example, in the node depth (H) equation dH/dt = dQ/A the value of dQ or the change in flow and the value of A or Area is updated at each iteration based on the last iteration’s value of all node depths and link flows.  

·         A bound or a bracket on each node depth or link flow iteration value is used by averaging the last iteration value with the new iteration value.  This places a boundary on how fast a node depth or link flow can change per iteration – it is always ½ of the change during the iteration (Figure 1).  

·         The Stopping Tolerance (Figure 2) determines how many iterations it takes to reach convergence and move out of the iteration process for this time step to the next time step.

Figure 1.  Under relaxation with an omega value of ½ is done on iterations 2 through a possible 8 in SWMM 5. This is not done for iteration 1.

Figure 2.  if the change in the Node Depth is less than the stopping tolerance in SWMM 5 the node is considered converged.  The stopping tolerance has a default value of 0.005 feet in SWMM 5.0.022. 

Figure 3.  The differences between the link flows and node depths are typically small (in a non pumping system) and normally converge within a few iterations unless you are using too large a time step.  The number of iterations is a minimum of two with the 1^st iteration NOT using the under-relaxation parameter omega.