Once you have entered your Line data you are ready to calculate results. This is initiated by selecting the Compute tab on the Ribbon menu and clicking the [Run] button.
Stormwater Studio offers a variety of options for computing your system. So before clicking Run, please review the options below. Please keep in mind that the primary purpose of computing is to obtain various hydraulic parameters, none of which can be found without a flow rate, Q.
Choose the Return Period
Hydrology options allow you to choose the return period as well as set the smallest Tc that will be used when accessing your IDF curves. If you are using Known Q’s exclusively, you may choose “None” for the Return Period. If you are employing the Rational method in your model, be sure to select a valid Return Period or you will get zero Qs for those Lines.
There are a variety of methods and options to use when computing the results. These options are selected from the Calculation Options category on the Ribbon Menu.
You have two primary calculation methods to choose.
1. Analysis and Design
This is the default setting and will be the one you use the most. Use for new or existing systems. This option will first compute the flows in the pipe system. It then analyzes and designs from the downstream up and will use any and all existing data for the calculations, i.e., pipe sizes and/or invert elevations input by you. Any data that has been set to zero, for design, will be designed by the program along the way.
Use this option when downstream constraints are important, when modeling existing systems or when correcting deficiencies in a system. Just remember this methods works from the downstream up and the Minimum Cover constraint will be ignored.
Balance Tc with HGL
When using the Rational method, this calculation option helps to eliminate conflicts between the time of concentration and the final hydraulic grade line. Traditionally, storm sewers are designed to flow full. This is acceptable because the pipes are sized and sloped to match a certain flow rate and velocity. Thus the area, A, of the pipe is assumed to be the full area.
When analyzing existing systems, however, the actual area and velocity is not known until the hydraulic grade line (HGL) is computed. The computed Q’s are based on assumed pipe velocities and thus an assumed Tc. The resulting HGL is based on these assumptions as well.
When the actual velocity is different from the assumed velocity, the computed Tc is incorrect and thus the computed Q and resulting HGL are incorrect. This option solves this problem by re-computing the hydraulic grade line (HGL) based on actual flow rates and actual Tc’s. To do this Stormwater Studio must compute multiple system iterations so that the computed Tc’s match those that were assumed with reasonable accuracy.
If you’re finding Lines in your results with extremely large System Tc’s and low Velocities, re-compute with this option checked ON.
Stormwater Studio first computes the HGL using Tc’s based on a full-flow velocity. It then computes the system a second time using Tc’s based on actual velocities. These new velocities are still incorrect because they are based on the original HGL calculation, but they are more accurate than those used on the first trial. As one would expect, several system iterations would cause the Q’s, Tc’s and resulting HGL to converge to correct values. Stormwater Studio uses three iterations.
If you’re finding Lines in your results with extremely large System Tc’s and low Velocities, re-compute with this option checked ON. Most likely you have a pipe assumed to be flowing full with a very small Q causing an extremely long pipe flow time, adding to the total Tc. Time = Pipe Length / Velocity.
2. Full Design
Use for new designs only. When using this option, Stormwater Studio resets all pipe sizes and invert elevations to zero and re-designs the entire system. It first sizes pipes based on the specified Minimum & Maximum Pipe Sizes and Minimum Velocity. It then computes the invert slopes.
Next it sets the invert elevations as high as possible but always below the Minimum Cover specified. At junctions, the outlet invert elevation is fixed by the lowest Surface Elevation. Note that this option requires all Lines to have Surface Elevations inputted.
Use the Full Design option when upstream constraints are important and when designing news systems. Just remember this method sets pipes working from the upstream down and adheres to the Minimum Cover constraint.
Follow Ground Surface
This option will force the pipe slopes to follow the ground surface rather than the theoretical slope produced by Manning’s equation. This reduces the usage of drop structures, minimizing excavation costs. Pipe slopes are designed not to exceed the Maximum Slope set in the Ribbon menu Pipe Design tab.
Check Inlet Control
This option will automatically compute the headwater, Hw, using HDS-5 methodology for all junctions that are designated as a Headwall, including those with overtopping weirs. It compares Hw with the computed HGL Junction using outlet control. The higher of the two will be used as the final HGL Junct. The outputted values will be appended with “ic” or “oc” representing Inlet Control and Outlet Control respectively.
Compute Supercritical Profile
Turn this on if you’d like the program to check for and compute a supercritical flow profile. (Only available for circular or rectangular pipes and open channels.) Learn more about supercritical profiles in the Computational Methods.
Junction Loss Options
The software provides three different methods to compute junction losses:
- HEC-22 – This will compute losses at all junctions (except for Headwalls optioned for Inlet Control) using the 3rd Edition of HEC-22. Entrance losses including consideration for inlet and outlet control, exit losses, and other adjustments based on benching, angles of incoming lines, and plunging flows are calculated.
- AASHTO – This option uses the 1991-based method developed by the American Association of State Highway and Transportation Officials. Like HEC-22, this method addresses entrance (expansion), exit (contraction), bend and plunging losses. Credit is also provided for structure shaping. When employing the AASHTO method, the software adds an additional dedicated report on the Reports tab that mimics VDOT LD-347 report.
- None – Use this option to skip the calculation of (minor) energy losses at upstream structures.
As described above in Analysis & Design, the program will fill in or design any pipe size or invert elevation that has been set to zero. These design options allow you to reset those items without having to individually edit them. In addition, It allows you to preserve the downstream invert at Outfalls. Use these options when employing Analysis & Design. Full Design resets them anyway.
For example, if your system is already designed from an invert elevation standpoint and you’d like to re-size the pipes, check “Reset Pipe Sizes” and re-compute using Analysis & Design.
Note that Design Options uncheck themselves after each use.
Accumulate Known Q’s
Known Qs can be introduced into any line of your system. This feature accumulates these known Qs as the program works downstream, if checked. If unchecked, known Q’s are not summed.
Use Inlet Captured Flows in System
Typically, inlet flow quantity calculations are done separately from system flows. In other words, most designers assume that all of the flows computed by the Rational method will enter the pipes. If checked, this option limits the pipe flows to those actually captured by the inlets. This option should typically be in the OFF position.
Important! If the junction is not an inlet (headwalls do qualify as inlets) it cannot capture inlet flows, resulting in a zero Q. If a Line has a Drainage Area, C and Inlet Time, its junction type must be an inlet when this option has been checked On. Also note that if the Total Q’s shown on your reports don’t match the corresponding Q = Cia, this option has most-likely been checked On.
Suppress Pipe Travel Time
Turn this feature on if you want the calculations to ignore travel times in pipes when computing Tc for the system.
Why are my Lines and Inlets colored yellow?
Stormwater Studio will change Line colors, highlighting any Line or Inlet to a bright yellow color, when certain criteria have not been satisfied and it’s corresponding Notification option has been turned On. For example, you can have any Line in your system highlighted whose flow rate is above the calculated Full-flow Capacity.
Keep in mind that it is usually okay for a Line to have a higher Q than its theoretical full-flow Q. It’s the final HGL and EGL that should determine if the Q is acceptable.
Other options include Velocity (Min and Max) and Slope as set in the Pipe Design tab and the Max Spread as set in the Inlet Design tab. When any of these criteria are violated, the Line or Inlet is highlighted. Max Spread only applies to Curb, Grate and Combination inlets.
To activate the various notifications, check or uncheck these options located on the Compute tab.