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Rainfall Settings

HydroCAD Rainfall Settings

Note: This page pertains specifically to rainfall settings used with the SCS/NRCS or SBUH runoff procedures.  These settings are not available when using the  Rational Method.

Setting the rainfall parameters

All rainfall parameters are configured on the Rainfall tab of the Settigns|Calculations screen, as shown at right.  This screen is used to define all the rainfall parameters in each HydroCAD project, as follows;

The storm type determines how the precipitation is distributed over time. The drop-down list shows the available storms. Additional information about the selected storm appears below its name.

The storm curve allows selection of a specific curve within the specified storm type. This option is active only when the selected storm type contains multiple curves.

The storm duration sets the length of the storm. Many storms will set a default duration, which should be used in most situations. Dimensionless storms will require the entry of a duration.

Setting back-to-back storms to a value greater than 1 will generate multiple identical storms.  The specified storm depth and duration will be automatically increased by this factor.

The AMC (Antecedent Moisture Condition) specifies the moisture level in the ground immediately prior to the storm. Use a value of "2" for "normal conditions".

The storm depth specifies the total precipitation that will occur during the selected storm. Some storms set a default depth, while others require manual entry of the appropriate design depth.

How are rainfall events defined in HydroCAD?

Although you could manually adjust each rainfall parameter as required, it's much easier to define a set of "rainfall events" which can automatically preset each of the rainfall parameters to the appropriate values.

Rainfall events can be defined by manually entering the values for each event.  But in most cases HydroCAD can automatically define all your events in a single operation.  Details here.

Sorting your rainfall events

Rainfall events are normally sorted alphabetically according to the event name.  But if you use certain special event names, such as "Water Quality" or "Historic Storm", you may prefer to sort the events by depth for a more logical presentation order.

Making changes to your events

If you need to change any of your rainfall parameters make sure you update each of the applicable events.  If you don't save your changes to a specific event, HydroCAD will automatically create a "Custom" rainfall event with the latest settings when you click OK.  If this is not what you intended, you can return to the Rainfall tab, delete the Custom event, and update the events.

Tip: You can use the "View All" to make changes to multiple rainfall events on a single screen.

Note: The "Custom" event and sorting options were added in HydroCAD-10 build 20.


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