HydroCAD- Modified Rational Method

[Sdk444]

Hello, I'm trying to adjust to HydroCAD coming from Hydraflow Hydrographs. I'm having difficulty setting up a modified rational method hydrograph. The entire owner's manual has about one sentence on it regarding rise/fall factors.

So I went to calculation settings> choose rational method> rise/fall factor 1/2 respectively (as explained in the help menu). After this I'm looking for somewhere to input my target flow so I can ultimately get the trapezoidal hydrograph. Or if there's something I'm missing to view a trapezoidal graph I'd be happy to hear suggestions. Thank you.

 

[psmart]

The purpose of HydroCAD is to calculate the flow, not just to reproduce a predetermined flow. So you won't find anywhere to enter a "target flow". If you must use a predetermined flow you can use a link to enter a hydrograph manually, but otherwise you would use the built-in runoff calculations.

For complete details on Rational Method, including the rise/fall factors, please see

http://www.hydrocad.net/rational.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Sdk444]

Thanks for your response. I see now I can use the Link icon to create a manual hydrograph. But like I could do in Hydraflow, isn't there a similar window where I can input my target flows to have the program give me SDF's? I use these SDF's to mutliply my Tc by to direct me to use the proper duration storms for determining the critical storm duration. I've attached a screenshot of the window I'm referring to from Hydraflow.

 

[TerryScan]

Sdk444,

I have never used the funcion, so I can't speak to it....but did you read the section "How do I determine the critical duration?" in the link that Peter provided above?

 

[psmart]

Where are you getting the target flows? Why not let HydroCAD generate the runoff hydrograph as it was designed to do? Using a link is the hard way to do this. It will result in a fixed hydrograph that cannot respond to changes in the rainfall or the watershed. When you use a subcatchment HydroCAD automatically generates the runoff hydrograph based on rainfall and watershed parameters.

As for the critical duration, HydroCAD has a special analysis to determine the critical duration. For details see

http://www.hydrocad.net/rational.htm

Please, try the program as it was intended and you'll find it works much better.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[LincolnPE]

...to determine the duration that causes the highest peak flow...

The Help in HydroCAD is a bit confusing...it states the above for the definition of critical duration for the Modified Rational Method.

The critical duration in the Modified Rational method however, is the duration that creates the greatest Volume of runoff for the catchment area.

 

[psmart]

The critical duration in HydroCAD is the duration that creates the highest discharge rate from a subcatchment or pond pond. For a subcatchment, this occurs when the duration is equal to the Tc. For a pond, the highest discharge corresponds to the highest water surface elevation and storage in the pond. (Assuming that any tailwater is constant).

In contrast, the total runoff volume from the catchment area will continue to increase with the rainfall duration. Have a look at any IDF curve and you'll see that total depth increases monotonically with duration. i.e. a longer storm always contains more volume than a shorter one. The average intensity is lower, but the volume is greater.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[LincolnPE]

...The critical duration in HydroCAD is the duration that creates the highest discharge rate from a subcatchment or pond pond...

The critical duration for the Modified Rational Method should be the duration that creates the maximum storage volume for a catchment given an allowable release rate.

It's not a peak discharge method, it's a storage volume approximation method.

 

[psmart]

When HydroCAD evaluates the critical duration for the pond, the software is actually maximizing the WSE and storage. For most applications (constant tailwater, no active outlets, etc) this is the same condition that produces the maximum pond discharge.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[LincolnPE]

...Where are you getting the target flows?...o

The target flow the OP mentions is the maximum allowable release rate. This is the Modified Rational Method I am familiar with that computes a critical duration that will produce the maximum required storage for a given release rate (target). It is used in Hydraflow as the OP mentions.

The Help in HydroCAD discusses the Modified Rational Method as simply being a manipulation of limb factors?

From the Help:

...Variations of the Rational method (often called the Modified Rational method), may use different rise and fall factors, which can be set directly on the Settings|Calculation screen...

For the Modified Rational Method, below is not always the case as stated in the Help:

..For a single subcatchment, the critical duration is equal to the Tc,...

 

[psmart]

..For a single subcatchment, the critical duration is equal to the Tc,...

This assumes a rise factor of one. For other rise factors the maximum runoff rate is achieved when the rainfall duration is equal to the Tc times the rise factor.

Peter Smart
HydroCAD Software

http://www.hydrocad.net