HydroCAD Stormwater Modeling - Since 1986

Modeling a roof or roof drains

To model a roof requires consideration of two separate issues:  The runoff from rainfall, and in the case of a flat roof, the possible storage and retention of water.

For all roofs, the runoff should be modeled using a subcatchment of suitable area, curve number, and time-of-concentration.  A curve number of 98 is commonly used for impervious surfaces, such as pavement or roofing.  The time-of-concentration may be evaluated by several methods, although using a value of zero is suitable for many situations.

If there is any possibility of a roof retaining or detaining water, then it should be modeled as a subcatchment (as described above) flowing into a suitable "pond."   The pond's stage-storage characteristics are easily determined by using the roof's area at several depths of inundation.

The stage-discharge curve can usually be modeled with one of HydroCAD's standard outlet devices, such as a horizontal orifice or grate.  If there are several identical outlets placed at the same elevation, these can be modeled with a single device by using an appropriate discharge multiplier.  In cases where the downspout inlet is the point of control, the downspout itself may not require direct inclusion in the model.

A simple approach for modeling a flat roof that may detain water:

1) Use a single subcatchment with CN=98 and Tc=0 to model the roof runoff.

2) Route the subcatchment into a pond.

3) Define the available pond storage by entering surface areas at two or more elevations.

4) Define a horizontal orifice for the pond's outlet.

5) Set the desired rainfall parameters and view the results.

Modeling a roof garden

The previous discussion assumes a curve number of 98 for an impervious roof surface.  To allow for the infiltration and retention effects of a roof garden, the CN value can be estimated by using the SCS equation for potential maximum retention.  Details here.

Also read about modeling rain gardens. (bioretention areas)

Handling the discharge

If the roof discharges into an underground storage and infiltration area (such as a drywell or chamber system), this is typically modeled as a pond with infiltration.

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