Hydrology details
 

The hydrologic modelling methods used in MIDUSS are well recognized and very versatile. 

You can select from a choice of:

• Five single event storms (including custom Malaysia storms),

• Three rainfall abstraction models, and

• Four overland flow routing methods

This combination provides a wide range of modelling options.  This allows you to examine the sensitivity of results to the choice of algorithm – a feature appreciated equally by both professional engineers and teachers.

In addition to alternative methods for generating runoff from a catchment there are capabilities to add baseflow and to model a large, reasonably homogeneous catchment as a ‘lumped area’ without having to resort to unreasonable values for the overland routing parameters.

You can use either Metric or US units throughout MIDUSS.

If you have generated hydrographs using another software product, you can use the them in MIDUSS and takes advantage of the MIDUSS design capabilities you don't get with other packages.

The MIDUSS Version 2 Reference manual includes comprehensive engineering theory for all the available hydrology models.  In fact, the MIDUSS user manual has been used as a supplementary text book at many Universities and Colleges. 

Learn more about the MIDUSS hydrology features below.

Storm
Catchment
Lag & Route
Base flow
IUH Hydrograph

This lets you define a hydrograph based on a peak flow value and time
to peak applied to an Instantaneous Unit Hydrograph.

STORM

The Storm command allows you to define a rainfall hyetograph either of the synthetic, design type or a historic storm. 

This window shows one of 5 available options to specify a design storm.

Storms may also be defined by importing a hyetograph file in simple text format.

CATCHMENT

The Catchment command lets you define a single sub-catchment and computes the total overland flow hydrograph for the currently defined storm.  You can, of course,  combine an unlimited number of catchments within a drainage network.

The roughness, degree of imperviousness and surface slope of both the pervious and impervious fraction are defined in this command. The effective rainfall on these two fractions is computed and stored for future use. 

The runoff hydrographs from the pervious and impervious areas are computed separately and added to give the total runoff.

MIDUSS offers a choice between three different models for estimating infiltration and rainfall losses and four alternative methods for routing the overland flow. 

Rainfall loss can also be estimated using the simple runoff coefficient which is converted to a corresponding SCS CN for the current storm depth.  All rainfall loss methods can be used with any of the flow routing algorithmns with the exception of the SWMM runoff method*.

The Triangular SCS is a dynamic triangular response function in which time of concentration varies with the intensity of the effective rainfall

The Rectangular response function varies dynamically in the same manner as the triangular response.

The SWMM RUNOFF algorithm uses a stage-discharge relation based on the Manning equation coupled with a non-linear reservoir.  *Use of the SWMM routing method limits infiltration methods to either Horton or Green & Ampt. 

The Linear reservoir response function is defined by the impulse response of a single linear reservoir.  Use of this method is similar to the OTTHYMO procedure.

MIDUSS lets you compare methods and to examine the sensitivity of the resulting runoff hydrograph to the methods used. This flexibility means, however, that you must exercise some care and consistency in the selection of procedures and parameter values for a particular application.

This window shows the first of many options in the catchment command. The runoff is computed as the sum of the direct runoff hydrographs from the pervious and impervious fractions. These can be specified and computed from the appropriate tabs on this form.

LAG and ROUTE

This command helps you model the runoff from a very large sub-catchment without having to resort to specifying unrealistically long overland flow lengths.

The command computes the lag time in minutes of a hypothetical linear channel and linear reservoir through which the runoff hydrograph is routed. Typically this results in a smaller, delayed runoff peak flow.

Lag and Route is intended to simulate a very large catchment (>30 ha or 75 acres) using a hypothetical linear reservoir in series with a linear channel at the downstream end of the catchment. The lag of the two components is roughly 2/3 of the total travel time in the conduits from the most remote point in the drainage network to the outflow. The linear reservoir lag is roughly 2/3 of the total. These fractions are defined by an empirical curve built in to the program and which can be edited.

The travel time is dependant on the type of conduit, the slope, roughness and average flow. The reservoir and channel lags are computed and displayed but you can modify these as a special option.

The modified peak flow is shown on the form along with a graphical and tabular display.

BASE FLOW

This command lets you specify a constant positive value of base flow to be added to the current inflow hydrograph.

The direct runoff hydrograph computed by the Catchment command does not include any baseflow. This command lets you add an estimated baseflow to the current Inflow hydrograph. If some baseflow has been added previously, a negative value can be used as long as it does not result in a negative ordinate in the inflow hydrograph.

IUH HYDROGRAPH

This command provides a simple way to create an Inflow hydrograph with a user-specified peak
flow and time to peak (or duration) with a shape defined by a file containing the coordinates of a
pre-defined Instantaneous Unit Hydrograph. A file containing the SCS IUH curve is included with MIDUSS and you can easily prepare similar files to describe other IUH shapes.

You can enter a desired peak flow value and also specify either the time to peak in minutes or the
duration in minutes.