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Frequently asked
questions


Ponds

V2
= applies to MIDUSS Version 2 (current release)
M98 = applies to MIDUSS 98
The pond does require
a great deal of data. However MIDUSS contains many tools to
help you obtain this data automatically. The first thing you
should enter in the pond design is the target outflow. This
will provide you with an estimate of the volume of storage
that will be required. The next item you should define is the
number of stages and typically this will be an odd number
implying an even number of depth increments. The minimum and
maximum water levels represent the total depth of the pond and
can be expressed in terms of an actual elevation or an
arbitrary depth depending on your choice.
The table is available
for direct data entry if you chose to do this. However it is
much simpler to use some of the tools which are available in
the storage geometry menu or the outflow control menu.
The outflow control is
a device that will describe the relationship between the level
in the pond and the rate of outflow from the pond. Typically
an output control will comprise an orifice which controls the
low flow conditions and a weir which will pass the larger
flows for an extreme storm event.
The setup grid levels
option is included as a convenience to recalculate the levels
at the different elevations of the pond. You do not need to
use this.

[
M98
][
V2
]
In the pond design the
storage geometry
menu contains items for a rectangular pond. My pond is going to
have a rather arbitrary geometry. Can I use this?
The rectangular pond
represents an idealized shape that is defined by a set of
contours that are nested rectangles. You can usually get a
good approximation to the distribution of storage throughout
the depth of the pond by using this feature.
The term superpipe is
a description of an oversized storm sewer which is intended to
provide underground storage.

[
M98
][
V2
]
The storage geometry menu
item in the pond design command shows something called wedge
storage. What is this for?
Wedge storage is the
name used to define storage that might occur on a graded
parking lot in which ponding is encouraged in the vicinity of
the catch basins. In fact the term "wedge" may be a misnomer
as the shape of the storage volume will more likely be that of
an inverted cone or pyramid with a circular or elliptical base
at the top water level.
In the pond design
command there is an item for rooftop storage. When you use
this you will see a table that requires five pieces of
information to be entered. You can click on the up arrow of
the spin button and MIDUSS will display some typical values
for the current peak inflow. MIDUSS assumes that only 75% of
the roof area is available for storage with the rest used for
roof structures. The drainage from the roof is provided
through a set of roof drains that drain a contributing area of
approximately 500 sq.m or 5000 sq ft. Each of these roof
drains will have a flow capacity that depends on the depth at
the roof drain. You can also define the slope of the roof in
the vicinity of the roof drain.

[
M98
][
V2
]
I wanted to design rooftop
storage but I
was unable to enter data in the table. The reason seems to be a
message saying "Roof not 100% impervious." What has happened?
Before you can define
rooftop storage you must use the catchment command to define
the area of the roof that will generate the runoff. The
message you have described has been generated by MIDUSS to
warn you that last catchment you designed had a significant
amount of pervious area. This is normally not the case and
this message is simply for your own protection.

[
M98
][
V2
]
I have managed to design a
pond and I want
to accept this design. However I notice a check box labelled
"Keep all design data." What does this do?
When you press the
[Accept] command button the information will be copied to the
output file. If you do not check this box, the next time you
use the pond design it will start with the default values that
MIDUSS generates. If you want to refine the pond design or you
anticipate that you will need to design a second pond with
similar parameters you can check the "Keep all design data"
box and the next time you use the pond design it will start up
with the same information as your current design.

[
M98
][
V2
]
I want to model a pond
which has an irregular shape. I can get data from an AutoCAD
drawing. Do I have to calculate the storage volumes by hand and
enter them in the 'Volume' column of the grid?
You can do this but
there is an easier way that you can try. It can be shown that
the equivalent aspect ratio R = length/breadth is related to
the perimeter P and area A of any plan section of the pond
(i.e. base, top water level or in between). The relationship
is given by:
f(R) = 4R + 4/R + 8
= P^2/A
You can get values of P and A at the base of the pond and work
out the corresponding value of R. Then use the
GeometryRectangular menu command with this value of R as the
aspect ratio for the base. MIDUSS will work out the reducing
aspect ratio at higher elevations. At elevations where the
sideslope changes fairly abruptly you can check the value or R
from data taken from the AutoCAD drawing or contour plan and
enter a preferred value o aspect ratio.
You can even model
multicell ponds in this way. At elevations below the berm
which forms the cells the value of R will be quite large (e.g.
6 or even 10). Above the top of the berm, the aspect ratio
will reduce due to the abrupt reduction of the perimeter P.



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