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Dave72

Rational Method interface/problems

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Hello,

I`ve been seeing some odd behavior from the rational method - IDF Computation dialogue boxes

1. When I switch to metric units, the change is not reflected in the "Input variables for IDF curves" dialogue box.

2. After entering values for IDF (i.e, 1:100 year storm values), when I close and reopen the project I have unusual numbers appearing. My original values are gone and replaced with IDF tables for all return intervals. Previously before closing I only had the values I entered with "No data computed yet" listed for the other user supplied data.

3. The Compute tc radial option has 3 variables. 2 are taken from the watershed data already calculated by WMS using DEM data.

Mannings I enter. Slope appears correct and stable. Length changes when I close the rational method diologue boxes and reopen. The number changes from 7000+ to 2165.04. It is also not clear if the length number is metric or imperial. I have not been able to find a description of this parameter in the tutorials / manuals.

I'm also curious how this rational method is related to the OC rational model and the Modrat model. Are there any links. Is there a manual for the WMS rational method included in the program?

Thanks for you usual good service,

Cheers,

Dave

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Dave,

Let me see if I can answer your questions:

1. It looks like the IDF curve/Intensity calculations are all done in US Customary units. Changing your units to metric only impacts the calculations in the main rational method dialog.

2. I tried entering some user-defined values and found a couple of problems. I will look at these problems, but if you could send the files you are having problems with to our tech support department, that would be helpful.

3. WMS is converting from feet to meters in the Time Computation Arc Attributes dialog. The equations that are used in WMS assume lengths are given in feet rather than meters, so if your model units are in meters, WMS converts from meters to feet to compute the travel time. More information on this is available here: http://xmswiki.com/x...s_from_Map_Data

The rational method is a method commonly used in the United States to compute a peak flow for small urban watersheds. It's probably the simplest hydrologic equation: Q=CiA, where the variables are defined in the WMS interface. The OC Rational method is a method that is specific to Orange County, California. It is based on the Rational equation but has some unique changes. There are tutorials and help pages that document some, but not all, of this model on http://xmswiki.com. The MODRAT method is another rational-based method that allows for a variable intensity in the basic rational equation as the storm progresses. It's commonly used in Los Angeles County, California. The Rational, OC Rational, and MODRAT models are all rational based models, but the results will be very different based on which one you use. They are normally used in urban areas.

Chris

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Hi Chris,

1. Okay

2. I will have to check confidentiality on this. I'll get back to you. Shouldn't be a problem.

3. So before closing, you are saying the units are being shown in feet. Then after closing and opening again they are shown in meters. From your link it appears that I need to manualy "create flow path arcs from selected points using the Node->Flow Arcs and Stream->Flow Arcs commands". However, I have not done this and a number is appearing in the "Length" box . This number, as noted, changes from 7000+ to 2165.04. Is this possibly a glitch because I did not define a flow arc? And if I don't define a flow arc, what is WMS using to populate this field. Is it a reliable number? (I am using a DEM coverage with basins defined).

Is the only manual for WMS Rational Method the Wiki? Where would I find the supporting equations. I ask because I would like to see how the manning number is being used. This variable is not used in the accepted method we use for the rational method in our region and I need to verify it before using it (or at least report/reference it). And I should probably also verify it by hand before using it.

A small additional question:

In the "Drainage Data Computation Options", the following can be computed:

10/85% Slope for the Max Flow Distance

Is it possible to display this slope using the "Drain Data Display Options" ?

Thanks again,

Dave

Edited by Dave72

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Dave,

You must have defined a time computation coverage when computing basin data, otherwise you would not be able to define the time computation arc attributes dialog. Selecting the option to define a time computation coverage creates a flow arc (a Tc arc) in a coverage automatically so you don't need to select the Node->Flow Arcs command. There is a large library of equations you can assign to a Tc arc in WMS. There's even a way to define a custom equation. You can choose any of these options if the equation with Manning's n value does not work for you. The equation being used is listed in the Time Computation Arc Attributes dialog.

Unfortunately, there is not an easy way of showing the 10-85% slope for max flow distance. The computed value is listed, in feet per mile, under the NSS method National Urban Equation. Just switch your model to NSS in the hydrologic modeling module. Go to a basin and select the National Urban equation in the list of available equations. The Channel slope (abbreviated CSL10_85) is the 10-85% slope for the basin. Be sure to switch your model back to the model you were using after you're done getting this value.

Chris

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Chris,

I am checking the recalculation and Tc is unchecked. I don't believe I selected anything past default when doing basin data. If it is on by default in the initial basin computation, then possibly. Other than that, I do not believe so. I am checking now if it is using max flow slope or max flow distance.

Speaking of which, I've done a search on the wiki and the pdf manuals and cannot find definitions for:

-Max Flow Length

-Max Flow Distance

-Max Flow Distance

-Max Flow Slope

Would you be able to shed some light on these?

Dave

Edit: I've done a quick check and the value being populated into the "Length" box on the" IDF computation" window (Compute tc) is the Maximum Flow Distance.

Lastly, is there a way to send you a message on these forums?

Edited by Dave72

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Dave,

The definitions are located on the wiki page here:

http://xmswiki.com/xms/WMS:Variables_Computed_by_WMS

The max flow length and the max flow distance are the same things and use the abbreviation MFD on the web page. The MFDS is the maximum flow slope. Both the distance and the slope are from a point that gives you the longest flow path in the watershed (in terms of distance) and includes both overland flow and stream flow distance/slope. You can email me at csmemoe@aquaveo.com if you need to send me a message.

Thanks,

Chris

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Thanks Chris,

... The equation being used is listed in the Time Computation Arc Attributes dialog.

I am still uncertain which formula is used for Compute tc in the "Rational Method - IDF computation" box.

I have the model set to "Rational". I changed the Time Computation Arc Attributes to the simplest I could find (New Mexico Shallow concentrated flow equation) that gives me 0.798hrs in the Time Computation Arc Attributes dialogue, and when I return to the Rational ยป Run Simulation and recalculate the intensity, the tc does not update to the 0.798hrs I saw in the other diaologue.

If there is a better way to work these out with you, let me know.

Thanks,

Dave

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Dave,

The rational method equation that computes the Tc and I in the IDF dialog uses a different equation than what's specified for the basin Tc value. You could copy/paste the Tc value computed for the basin into the rational method IDF dialog if you want, or you can use the rational method approach that's defined in the IDF dialog. The IDF approach iterates between the intensity equation shown in the dialog and the following equation until a solution is reached:

Tc = (0.93*L^0.6*n^0.6) / (i^0.4*S^0.3)

L = Watershed max flow Length (feet)

n = Manning's n

i = Precip Intensity (in/hr)

S = Watershed average basin Slope

Chris

Chris

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Perfect, thanks Chris. That Tc calculation is exactly what I needed to gain some confidence working in this model.

Do you know the source / author of that equation?

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Dave,

Just off the top of my head, the equation is probably from FHWA HDS 2. The link to this manual is here: http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=2&id=6

I can look into the source of the equation some more if you need more information or are not able to find it on the FHWA web site.

Chris

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