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HydroCAD Stormwater Modeling - Since 1986

Storm Sewers

Modeling Storm Sewers

Click for complete self-training materialsHow do I model a catch basin?

A catch-basin with negligible storage can often be modeled as a "zero-storage" pond.  This allows evaluation of the outlet control devices (usually a culvert) without consideration of storage effects.

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.

In cases with "significant" storage, such as above-ground storage that is used when the basin overflows, just include the storage information in the "pond" description. This allows a more accurate culvert analysis, including the effects of headwater and inlet losses.  Be sure to enter enough stage-storage data to prevent a storage exceeded warning.  Note that the outlet pipe is not part of the level-pool storage, and therefore should not be included as part of the pond storage.

Why is the peak elevation so high?

The peak water surface elevation represents the head that is required to push the inflow hydrograph through the outlet devices.  If this peak WSE exceeds the top of the catch basin, the CB may overflow through the grate, or above-grade ponding may occur.  In order to obtain accurate results in these cases you must include an appropriate overflow device and/or above-grade storage in the model.  Otherwise the head will continue to increase as if the CB were extended above-grade.

The pipe capacity isn't what I expected!

For a culvert outlet, HydroCAD performs a complete culvert analysis including headwater, inlet loss, frictional loss, and tailwater conditions.  In general, the result will be different than using Manning's equation alone.  For example, headwater above the crown (pressure flow) can exceed the Manning's capacity, while the entrance loss coefficient (Ke) can reduce the flow below the Manning's capacity.  When evaluating or designing pipe systems, you must understand the exact sizing criteria being used.

What about the grate?

Since HydroCAD deals only with outlet controls, any grate effects are modeled by treating the grate as an outlet device on the appropriate "pond":

1) If the grate acts primarily as an overflow device, then you could model it as weir outlet on the pond/catch-basin.  Routing the weir to the secondary discharge will generate a separate outflow hydrograph that can be routed independently.

2) If the grate restricts the flow into the catch-basin, you are essentially modeling an above-grade pond, controlled by the grate.  The effects of the culvert outlet can be included by setting up both devices as compound outlets on the same pond.  However, if you also have piped inflow from an upstream source, you would need to model the CB as a separate "pond" in order to combine the pipe flow downstream of the grate.  Otherwise, the grate will be incorrectly restricting the underground flow.

3) In many cases, the grate doesn't have a significant effect on the overall hydrograph routing, and therefore does not need to appear in the model.  In general, use the simplest model required to meet the goals of your analysis.

How do I connect one catch basin to the next?

In most cases, you can route the outflow of one catch basin directly to the next.  Use of an intermediate pipe reach is generally not recommended, since it prevents the upper CB from "seeing" the actual tailwater created by the next CB.  A reach is appropriate only for open-channel normal-flow governed strictly by Manning's equation.

What about a closed storm sewer?

A storm sewer can generally be modeled as a series of ponds with culvert outlets, as described above. However, if the culverts are not operating under free discharge, you will need to specify the appropriate tailwater, or use a tailwater-sensitive routing procedure, such as the Dynamic Storage-Indication method. (This and other features are available in the latest HydroCAD update.)

For earlier versions of HydroCAD, you may need to test the "worst case" scenarios, such as a minimum or maximum tailwater.  When using HydroCAD 5-6, specifying a differential tailwater will often be the best solution.

Note: In some cases, complex storm sewer systems may need to be modeled using specialized steady-state techniques.  There are pipe network programs designed specifically for this purpose, and they are commonly used to analyze complex systems operating with backwater or pressure-flow. However, these programs are typically based on the Rational Method, and since they are steady-state models, are unable to generate or route hydrographs.  Some projects may require both types of analysis.

Will HydroCAD calculate the HGL?

An HGL calculation often implies a steady-state analysis, rather than the time-varying (hydrograph) analysis being performed by HydroCAD.  However, HydroCAD will calculate the peak water surface elevation at each node in the system.

Click for complete self-training materialsIs there an easy way to model a pond fed by a storm sewer?

Rather than modeling the entire storm sewer, the system can sometimes be approximated by a subcatchment feeding a single pipe reach.  The subcatchment will provide a realistic runoff volume, while the reach will limit the outflow to the Manning's capacity of the pipe, delaying any excess until later in the storm.  (If the storm sewer discharge is already known, the pipe size can be adjusted to produce the predetermined flow.)  The reach outflow will be a flat-topped hydrograph suitable for pond routing. 

Also read about ponds, pipes, and tailwater.

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