Design detail

• Pipe sizing (in which hydraulic gradient is reported if the pipe is surcharged)

• Open channels of either a generalized trapezoidal shape or a more complex cross-section defined graphically and modified with up to 50 co-ordinate pairs.

• Hydrograph flood routing in part-full pipes or open channels.

• Detention ponds including a variety of tools for computing depth-discharge and depth-storage curves for a variety of outflow control devices and pond geometries.

• Exfiltration trenches with multiple perforated and non-perforated pipes.

• Diversion structures for separation of hydrograph components (e.g. major and minor).

• Culverts including storage routing

• Cascade lets you route the current inflow hydrograph through a short cascade of
  storage cells
   

The above detailed design tools are available at all points in the developmetn of the drainage network.

PIPES

You can design a pipe to carry the peak flow of the current Inflow hydrograph. If no hydrograph has been calculated you can specify a desired constant flow.

For the peak flow you will be shown a table of diameters, gradients and average velocities which represent a feasible design. You can either choose one of these diameter-gradient pairs by double clicking on a row in the table or you can enter explicit values for diameter and gradient.

MIDUSS carries out a uniform flow analysis and reports the actual and relative depth, velocity, pipe capacity and also the critical depth. You can experiment by changing either the pipe roughness (i.e. the Manning 'n') or the diameter or gradient and press the [Design] button to see the results.

There is more pipe information in our FAQ section.  Click here.

CHANNELS

MIDUSS lets you design channels with two types of cross-section to carry the current peak flow in the Inflow hydrograph.   If no hydrograph has been calculated you can enter a constant flow value.

The cross-section can be:

1. A general trapezoidal shape defined by a base width and left and right sideslopes.
2. An arbitrary shape defined by up to 50 pairs of coordinates.

In both cases a table of depth, gradient, velocity values is displayed which represent feasible designs. You can select from this list by double clicking on a row of the table or you can specify a total depth and gradient explicitly.

Pressing the [Design] button causes a uniform flow analysis to display the uniform flow depth, critical depth, average velocity and channel capacity.

You can experiment with alternative schemes until satisfied. Pressing the [Accept] button saves the current design.

An arbitrary cross sectin can be drawn with the mouse pointer and the coordinates iof the selected points are shown automatically in a grid.  These coordinates can be edited to refinen the drawing.  If the length dX of a segment is altered all the points to the right are adjusted automatically.

There is more channel information in our FAQ section.  Click here.

ROUTING

Once a drainage conduit has been designed - either a pipe or channel - you can route the Inflow hydrograph through a reach of specified length to obtain the Outflow hydrograph at the downstream end.

For each conduit design MIDUSS adjusts the time step and reach length to acceptable sub-multiples in order to ensure numerical stability in the routing process.  You are advised of these changes but need not take any action.

The result of the routing operation is displayed in both graphical and tabular form. 

When an outflow hydrograph has been created by some routing operation you may choose from two possible courses of action. Either the outflow can be copied to the inflow array in order to continue to the next downstream link, or the outflow may be stored at a junction node to be combined with other flows at a confluence point.

DETENTION POND

See a short 90 second demo on detention pond design.  Click here.

MIDUSS helps you to design a detention pond to achieve a desired reduction in the peak flow of a hydrograph.  

The current peak flow and the total volume of the inflow hydrograph are reported and you are prompted to specify the desired peak outflow. MIDUSS estimates the maximum storage requirement to achieve this.

The storage routing through the pond requires a table of values defining the outflow discharge and the storage volume corresponding to a range of stage or depth levels. You can enter this data directly into the grid if you wish, but it is usually easier to use some of the features of the Pond command to automate this process.

The outflow control can be designed using multiple orifices and weir controls. The Stage - Storage values can be estimated for different types of storage facility. These may be a multi-stage pond with an idealized rectangular plan shape and different side slopes in each stage; one or more "super-pipes" or oversized storm sewers; wedge storage formed on graded parking lots; or a combination of these types of storage.

Rooftop storage can also be modelled to simulate controlled flow from the roof of a commercial development.

Following use of the ROUTE command you can experiment by changing any of the flow or storage data until the desired result is obtained.

There is more pond information in our FAQ section.  Click here.

[Please click to expand this screen image.]

EXFILTRATION TRENCH

The Trench command lets you proportion an exfiltration trench to provide underground storage for flow peak attenuation and also to promote return of runoff to the groundwater. 

The trench usually consists of a trench of roughly trapezoidal cross-section filled with clear stone with a voids ratio of around 40% and with one or more perforated pipes to distribute the inflow along the length of the trench.

The exfiltration trench splits the inflow hydrograph into two components. One of these is the flow which infiltrates into the ground water; the balance of the inflow is transmitted as an outflow hydrograph. Obviously an exfiltration trench requires reasonable porosity of the soil and a water table below the trench invert.

The design involves several steps including definition of the trench and soil characteristics, definition of the number, size and type of pipes in the trench and description of the outflow control device comprising orifice and weir controls as used in the Pond command.

The outflow control devices are similar to those used in the detention Pond command. Water from the inflow hydrograph enters the stone fill through one or more perforated pipes running the length of the trench. The trench may also have a conventional, un-perforated storm sewer between the manholes to convey the Outflow. The positioning of the various pipes in the trench can be defined graphically using the Trench pipes window. The diameter and type (perforated or non-perforated) can be specified and the location set by dragging the pipe to the desired position or by editing the numerical data in a grid. During the drag and drop procedure the current pipe cover is shown to assist in ensuring adequate clearance.

DIVERSION DEVICE

A diversion structure allows the inflow hydrograph to be split into two separate components, the outflow hydrograph and the diverted flow hydrograph. 

Below a user-specified threshold flow all of the inflow will be transmitted to the outflow hydrograph. When the inflow exceeds the threshold value, the excess is divided in proportion to a specified fraction.  For example, if the inflow is 25 cfs and the thresh-hold is 5 cfs so the excess flow is 20 cfs.  Now if the capture fraction is F = 0.8 this means that 80% of the excess flow is diverted and the diverted flow will be 16 cfs and the outflow will be 9 cfs.

Instead of specifying the diverted fraction F you can define this implicitly by specifying the desired peak outflow. MIDUSS will then work out the necessary fraction to be diverted.

The diverted flow hydrograph is written to a file so that it may be recovered at a later time and used to design the necessary conduit or channel.

Use of the diversion command is the only instance in which the topology of the network changes from a tree to a circuited network.

There is more diversion information in our FAQ section.  Click here.

Culvert

The Culvert command lets you model the behaviour of a culvert under various conditions of flow.

Because of the many variables involved, the process is largely one of trial and error and MIDUSS does not suggest initial feasible values for the design.

Culvert design can be carried out for either steady, (i.e. time invariant) flow or for an inflow
hydrograph. When inflow is in the form of a hydrograph the hydraulic design can be followed by
a routing process that shows the attenuation of the inflow hydrograph caused by ponding that
occurs upstream of the embankment. In such cases the peak outflow from the barrel will be less
than the peak inflow and you can refine the barrel design for the reduced flow if desired.

Your Culvert design can be preceded by a Channel design with either a trapezoidal or
complex cross-section. When this is done the cross-sectional shape of the channel is ‘inherited’ by
the culvert design and used to describe the flow cross-section upstream of the culvert. If the
inflow is a flow hydrograph, a channel design may be followed by a Channel routing process from
which the channel outflow forms the inflow to the culvert.

The culvert is assumed to be located below a sag point in a highway embankment that will form
an overflow weir in the event that the barrel flow capacity is sufficiently surcharged. Flow
separation between barrel and weir flow is assumed to be recombined downstream of the barrel.
The cross-section of the barrel conduit may be a circular pipe, a rectangular box, a horizontal or
vertical ellipse or a pipe arch. Multiple barrels may be used but cross-section and other hydraulic
parameters are assumed to be the same for all barrels.

Casacde

The Cascade command lets you route the current inflow hydrograph through a short cascade of
storage cells formed from a variety of cross-sectional shapes such as pipes, rectangular boxes,
horizontal and vertical elliptical pipes and pipe arch sections.

The storage may be provided by a ‘superpipe’ or oversized storm sewer with a modest slope and a reach length limited to 100 - 150m (330 - 500 ft). Two reaches of superpipe can be used in series.

Each chamber is horizontal with a specified length, width, height and invert elevation.

The outflow control from each chamber is assumed to be an orifice of specified diameter and
coefficient of contraction with the orifice invert equal to the bottom of the upstream chamber.

You can specify a pipe, box or any of three special pipe sections (e.g. horiz elliptical, vert elliptical or pipe arch).  If you select the special pipes a drop-down list lets you browse through a set of commercially available sizes. These are shown in metric or imperial sizes depending on the choice of units.

If a cell is surcharged, the data box containing the Height is highlighted to warn you that more
storage or a larger orifice is required.

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