HydroCAD- Modified Rational Method
HydroCAD- Modified Rational Method
(OP)
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.
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.





RE: HydroCAD- Modified Rational Method
For complete details on Rational Method, including the rise/fall factors, please see www.hydrocad.net/rational.htm
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: HydroCAD- Modified Rational Method
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.
RE: HydroCAD- Modified Rational Method
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?
RE: HydroCAD- Modified Rational Method
As for the critical duration, HydroCAD has a special analysis to determine the critical duration. For details see 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
www.hydrocad.net
RE: HydroCAD- Modified Rational Method
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.
RE: HydroCAD- Modified Rational Method
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
www.hydrocad.net
RE: HydroCAD- Modified Rational Method
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.
RE: HydroCAD- Modified Rational Method
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: HydroCAD- Modified Rational Method
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,...
RE: HydroCAD- Modified Rational Method
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
www.hydrocad.net