Modeling Storm Sewers
How 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.
Is 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.
