HydroCAD® Stormwater Modeling - Since 1986
How do I ... ?
HydroCAD is a collection of powerful hydrology and hydraulics tools which must be combined and applied by a qualified engineer. Since design objectives and local requirements vary greatly, it's impossible for us to provide simple "how to" instructions for most projects. However, we can offer these general guidelines for setting up your model. You will also find detailed information on many specific issues on our support page.
1) You must use one or more subcatchments to simulate the rainfall runoff from your site. Remember to include all surface areas, including roof tops and pond surfaces, otherwise the rain falling on these areas will not be included in the model.
2) In some cases, you may want to model your stormwater management system (ponds and reaches) first, and then create the subcatchments as required to represent the inflow areas. The number of subcatchments can vary greatly, depending on the complexity of the routing and analysis you need to perform. The number of subcatchments is not directly related to the land area.
3) You must use a pond for any area that will detain water. This includes not only detention ponds, but also flooding in parking lots, underground stormwater chambers, and any volume where water may accumulate. Read about pond storage and pond sizing.
4) Consider each pipe carefully to determine how (or if) it should be included in the model. There are several ways to model a pipe, depending on its role in the system.
5) Use a reach only for modeling normal open-channel flow. In most cases, a reach should not be used to model a pipe. (See above.) Many models will not use a reach at all. In many cases you can route the subcatchments directly to your pond(s).
6) A catch basin may be modeled as a pond with zero storage. This lets you analyze the behavior of outlet control devices (typically a culvert), and determine the peak elevation (HGL) at the CB. Since the outflow will be the same as the inflow (no storage = no attenuation), the primary objective of modeling a CB is to determine the tailwater elevation for inflowing nodes. Otherwise, you may not need to include the CB in your model. Read about storm sewers.
7) Use a link for special situations, such as entering a pre-defined hydrograph or setting a custom tailwater.
8) Add flows together by routing them to any reach, pond, or link. You don't need to describe the node - just route the inflows. To see details on the hydrograph summation, right-click the tabular hydrograph and select "Inflows".
9) Review and resolve all warning messages! Most problems and support questions are the result of overlooked warnings. Even with a "failed" system you need to resolve the warnings in order to know exactly how the system is failing. Remember to click on any message for complete details, and follow the links for further information. This will resolve 99% of all support questions.
10) Select the appropriate routing procedure ASAP. Changing the routing procedure after the model is fully developed will make any problems harder to isolate. In the absence of any tailwater warnings, you can use the default Storage-Indication method. If there are tailwater warnings, you may need to use the Dynamic Storage-Indication or Sim-Route procedures.
11) Start small, build gradually, and check results as you go. Especially for complex projects, it's best to make basic decisions (runoff method, routing procedures, etc) with just a few nodes, while you can easily alter your modeling approach. Examine the reports carefully and make sure you understand everything. Resolve any issues ASAP, before the model becomes more complex. This will ensure the best results in the least time.
12) Keep the model simple! Determine the exact objectives of your model and use the minimum number of nodes required to perform the analysis. This means that some details may not appear in the HydroCAD model at all. If it doesn't store water or create a tailwater condition for an incoming node, you probably don't need it in the model. Remember that each HydroCAD node represents a hydrograph routing operation, and may not correspond one-to-one with structures on the site.
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