Being Civil

RESULTS FROM MULTIPLE STORM ANALYSIS

I had a user questioning – “My SSA model runs fine, however it would not provide me any reports; all the reporting tools are grayed out, the time series plot does not contain any elements and the profile plot does not show HGL & EGL and no reports in the table below. The Excel Report contains only the input data.”

Figure 1. All reporting tools are grayed out after the analysis.

There are two options to obtain the results from Multiple Storm Analysis in SSA. The first option is to select the required design storm for the program to generate the reports immediately after performing the analysis (likewise performing a single storm analysis). One has to select the require design storm in the “Load output file:” at the bottom for the Storm Selection tab as shown in the image below.

Figure 2. Selecting the design storm to be loaded after the analysis.

The second option is to manually load the solution file for the required design storm you wish to analyze. Below is the pictorial instruction on loading the solution file within the Time Series Plot.

Figure 3. Manually selecting the solution file to be loaded for analysis.

I hope these little tips save your precious time wondering what is going on with the program. Keep tunning in for more.

ANALYZING HGL WITH TAIL WATER CONDITION

I would like to show a very simple yet commonly mistaken procedure committed by many SSA users with tail water condition at the outfall of the system. Below is a screenshot of a profile plot with fixed water elevation at the outfall, however the profile plot shows a different HGL where one would expect the HGL to match the fixed water elevation i.e. 980 ft. for this model.

Figure 1. Profile Plot for tail water at the outfall @ 980 ft. using Kinematic wave routing.

The reason for the above profile plot reporting a Max HGL at outfall as 977.40 ft. is due to Kinematic wave routing used for the analysis which provides a non-linear reservoir formulation for channels and pipes, including translation and attenuation effects that assume the water surface is parallel to the invert slope. It does not account for backwater effects or flow reversals.

The key to analyzing any tail water, flow reversal or looped network is to analyze with Hydrodynamic routing method. It solves the complete St. Venant equations throughout the drainage network and includes modeling of backwater effects, flow reversal, surcharging, looped connections, pressure flow, tidal outfalls, and interconnected ponds.  

Notice the change in the Max HGL for the same profile plot when using Hydrodynamic routing method. The Max HGL reports at 980 ft. at the outfall that match the Tail water elevation as expected.

Figure 2. Profile Plot for tail water at the outfall @ 980 ft. using Hydrodynamic routing.

I hope this helps. Stay tunned for more tips to come.

 

 

 

 

 

Important aspects to define inlets in the model

In SSA, I would like to discuss few things about the inlets which are very important while setting up the inlets in the model. If we miss those parameters and run the analysis it will pop up an analysis errors.

On grade inlets setup in the model always require two outlet links with positive slope. At least, one storm water sewer pipe and a bypass gutter link must be defined. If the Roadway/gutter bypass link, option is undefined we are inviting trouble, so it is important this option should be defined.

Junction Losses

While working as a product Support Specialist in BOSS, I encountered a technical issue related to Junction losses as the client mentioned that StormNET (earlier name of SSA) is not computing the loss of energy correctly in the junctions. Normally if we take a look at the situation junction, head losses are encountered in going from one pipe to another through a node. It is represented as being proportional to the velocity head of the outlet pipe and the energy is approximated by using the mentioned equation below.

Why Do I See Difference in Time Series Plot And Output Results / Analysis Summary?

A lot of times, SSA users find that there is a slight difference in the results seen in the Time Series Plots and the numbers seen within the Analysis Summary in the elements Property dialog as shown below:

Figure 1.0 Different results seen in the Time Series Plot and Analysis Summary

The Time Steps section within the Analysis Options is the key to resolve this difference seen in the results. It establishes the time step length to be used for the runoff computations, routing computations, and results reporting. The Reporting and Routing Time Step is the most important thing for network analysis.

SSA uses a default Reporting time step of 5 minutes and Routing time step of 30 seconds. In most cases users tend to use the default and define only the Start and End Dates to perform the analysis. A very important thing to note here is that the Output Report or Analysis Summary within the element properties windows shows the results based on every routing time step whereas the Time Series Plot is generated on the Reporting time steps defined. This is where the difference emanates.

Different output for the same model in SSA and SNET

On Friday, I got a model which was set up in StormNET and as the model is run in SSA, got different output results for the hydraulic section, in comparison to the StormNET output report, even though the model was not changed and all the modeling parameters were same. First and foremost thing which came to my mind was the change in the EPA SWMM engine has caused the difference in the results.

Latest SSA version is using EPA SWMM version 5.0.021 in compare to the SNET using EPA SWMM version 5.0.014. However, I still had questions in my mind that what could be the other factors which contributed significantly for the different results for the links and the nodes in the two output reports. Though, SNET was not installed on my system. I  took the help of Matt Anderson to compare the EPA SWMM 5.0.014 results with SSA. In the discussion came to know that apart from the change in EPA SWMM engine the culvert equation and the storage node floods (HGL above the node storage curve) contributed to the difference in the output results for SSA and SNET. I would like to share the data presented by Matt Anderson related to the corrections to the EPA SWMM engine between 5.0.014 (SNET) and 5.0.021 (SSA 2012).

Defining Storage Curve Data in SSA

As I was about to log out for the day, I was contacted by a customer who indicated that the analysis was not affected by the size or depth of the stormwater detention pond that they entered. 

Well, the solution boils down to the definition of the storage curve data. The confusion arises when a user who had select Elevation from Project Options window for Elevation type and defines the Storage Curve data for the pond as an Elevation vs. Area. A quick and careful look at the Storage data type indicates that it requires Depth vs. Area as shown in the figure below: 
 Figure 1. Storage data of a pond defined in Depth vs. Area

One must take special care in defining the Depth column as and offset depth from the invert of the pond irrespective of the Elevation type selected in the Project Options dialog box as shown in the figure above.

You may want to refer to another blog post Using Civil 3D Stage Storage information in SSA by Matt Anderson for the possible workflow to incorporate Civil 3D stage storage information into the storage curve data in SSA.

SSA considers each stage as a horizontal section of prism and uses the traditional “Average-Area” method to determine the volume that is provided by the equation: 

V = (A1 + A2) h/2  

Where

V = volume at the given stage (ft^3 or m^3)

h = height or depth of the section (ft or m) which is defined in the Depth column in the Storage Curve data type  

A1 = area of the bottom section (ft^2 or m^2)

A2 = area of top section (ft^2 or m^2)

The volume at any stage is the cumulative volume of the pond. In the example noted above in Figure 1, at the depth of 4 feet, the pond contains 750 cubic feet of water.

Lengthening Time Step

I'm Anurag Thakur. I am a Science graduate who started my Civil Engineering career in 2007 by joining BOSS international as an application Engineer. In BOSS International, since 2007, I supported their Hydrology and Hydraulic software, StormNET; now called as Autodesk Storm and Sanitary Analysis and RiverCAD Pro. Later in 2010, I joined Autodesk, Inc as a consultant for Autodesk products (Storm and Sanitary Analysis and RiverCAD Pro) and on April 01, 2011 joined Autodesk, Inc as a Product Support Specialist.

Now, the topics related to Autodesk Storm and Sanitary Analysis (SSA) will be available under the Hydrology link on the top of the page, as mentioned by my colleague Matt in the Blog introduction to Autodesk Storm and Sanitary Analysis. I will start with post, Lengthening Time Step.

I have seen it number of times where the model goes crazy when being modeled with the hydrodynamic routing and we start having doubts about hydrodynamic wave calculations. Specially it happens for a model, in which the conveyance links length may vary too much .  However, under such circumstances, the rule of the thumb is to set up a lengthening time step. Details on Lengthening time step are available in SSA user guide. This is a time step, in seconds, used to artificially lengthen channel and pipe links so they meet the current stability criteria under full-flow conditions. It means the travel time of a wave will not be smaller than the specified conduit lengthening step. With the decrease in the value, fewer channel and pipe links will require lengthening. A zero value for Lengthening time step means that no channel and pipe links will be lengthened (i.e., will be using their defined length values). However the insuitability of the results could be reflected in the Analysis results, and in the Time Series Plots.

  

 
 

If the model appears to be unstable then increasing this value will tend to make the model more stable. Most of the time, while troubleshooting such issues, setting up the lengthening time step resolved the hydrodynamic wave calcs. The user could try using a value of 60 secs and check if the model results become more stable.  If the model results are stable, try reducing this value to the minimum possible while keeping the model stable. On the other hand, if the model results are not stable, then continue to this value up to a maximum of 300 (5 minutes). If the mode is not stable at this increased value, then there is something else causing the model instability.

I hope you find this useful, "Enjoy your hydrology, stay tuned for more".