Overfilled Storage
What Happens
If the defined pond storage becomes completely full, the routing will
normally continue by applying additional head (pressure) to the outlet devices.
In essence, the pond is treated as a closed volume without any overflow or
temporary
storage. HydroCAD will always generate a warning message to alert you to
this condition. In addition, you may see an exceptionally high water
surface elevation. This represents the head (pressure) that is required
for the pond to discharge the inflow hydrograph without utilizing any additional
storage.
What
to do
If the storage volume is physically closed (like a tank), and you intend it
to operate under pressure, then no further action is required. However, if
any additional storage is available inside the pond at the reported peak elevation, the model
must be adjusted in order to accurately define this volume:
(1) In the case of a custom storage definition for an open-air pond,
one or more additional stages should be added. This typically involves
defining the surface area at additional contour elevations. Include only
the area (volume) that lies within the sides of the pond and the outlet devices.
You should also have at least one defined stage above the
highest outlet device.
(2) If the primary storage is defined using one of the common shapes
(such as an underground vault or prefab chamber), then additional storage definition(s)
should be added to describe the overflow volume. This might include
secondary storage chambers, a feeding pipe, or even above-ground areas that would be inundated
when the primary storage overflows, such as a parking lot.
(3) When using a weir outlet, be sure to define the temporary pond storage above
the crest elevation, representing the additional volume that will be temporarily detained
behind the weir. Make sure you define enough storage to suppress
the "overfilled storage" message. This applies to all outlet devices, but
the oversight is most common with a weir.
In each case, you don't need to "make the pond bigger"
or enter "fictional" storage. Just supply complete information about the available storage so that an
accurate routing can be performed. For the weir shown in this photo, you
must specify the storage to at least the 3.60 foot WSE shown on the
gauge. Without this information, additional
head will be applied to complete the routing, rather than using the actual
storage available in the pond.
After adding the extra storage, re-check the peak elevation and make sure
this value is reasonable. If the value seems too high, you're probably
omitting storage and/or overflow devices that would store and/or discharge the
excess volume.
Frequently Asked Questions
What if I don't want the water to go that high?
You still have to provide additional storage information in order to
accurately assess how high the water will actually rise. After you provide
the storage information and get an accurate routing, then you can adjust the
model to reduce the peak elevation.
Why do I need enter storage data above a weir crest?
In order for water flow over a weir, the water surface elevation must exceed
the crest elevation. And that head, however slight, corresponds to a
specific volume of water stored in the pond. For example, if you have 1/10
foot of head over a weir with an average pond surface area of 10 acres, you have
1 acre-foot of temporary storage above the crest. This volume contributes to
the pond's detention effects, and must be specified in order to get an
accurate routing.
How do I calculate storage above the weir crest?
Construct an imaginary vertical wall above the weir and describe the volume
on the upstream side of the wall. This is the storage volume being
controlled by the weir. Do not include any volume below the
weir, since this is not part of the pond's level pool.
What if the water overflows in several places?
Define each overflow as a separate weir. Or use a custom weir to define
a more complex weir crest. Define the storage volume that is controlled by
the weirs, using an imaginary wall above each weir to define the edge of the
pond.
What if the water overflows evenly on all sides, like a depression or an
open tank?
Define the entire perimeter as a weir, or use a horizontal orifice with the
weir-flow option enabled. Define the storage to some point above
the crest, using an imaginary wall above the crest to define the edge of the
storage volume. For an overflowing tank with vertical walls, this can be
done by enter a storage height that is slightly greater than the tank height.
e.g. If the tank is 10 feet high, your would set the weir invert at 10 feet, but
set the top of the storage at 11 feet. This allows the volume to be
properly calculated as the tank overflows.
For additional information, read about pond storage
calculations.
Note: HydroCAD-6 (and earlier) considered all storage
definitions to be open to the air, and would automatically extrapolate
from the storage curves whenever they where overfilled. This allowed an
approximate routing to be performed until exact storage data was supplied.
In contrast, HydroCAD-7 (and later) considers each storage volume to be
closed, and never exceeds the defined storage. This allows HydroCAD to
model compound storage arrangements and pressurized ponds. Note that in
the absence of a storage warning, pond storage calculations are comparable in
all versions. For details see SSB 104.
