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# Modeling a 2 stage weir?

## Modeling a 2 stage weir?

(OP)
I have a two stage weir.

The first stage is designed to handle the 25 year event.
For purposes of discussion, let's say that this weir is a broad crested rectangular weir, 6 foot wide, at elevation 100.

The second stage is designed to handle the 100 year event. It is 12 foot wide, at elevation 102.00.

When setting up this situation which weir would be the primary weir and which is device #1?

What's the correct way to model this situation?

Thanks
Replies continue below

### RE: Modeling a 2 stage weir?

Device numbers are assigned by the program.  Their main purpose is to route one device through another device. (such as a drop structure - culvert = device #1, orifice/weir = device #2 which is routed (flows) through culvert (device #1))

The option for Primary and Secondary routing are to send flows in different directions, or to be able to separate 2 flows in reporting.  (Even when routing to the same location, I sometimes make my emergency spillway a secondary flow for reporting).

In your situation, I do not believe one weir actually will flow through the other in series, so one would route them to either Primary or Secondary, not to a device.

If the one weir is embeded within the other (the 6 ft. wier is in the base of the 12 ft. wier), one would need to fill the "rise" entry on a weir device to limit its usage (rise = 2 ft for el. 100.00-102.00).  However, I notice only sharp crested wiers allow for a "rise" entry.  If the wiers are embedded and you need to use broad-crested, you may need to create your own rating table.  (or enter the 102.00 weir is only 6 ft. wide? (12 ft- 6ft lower wier)).

Have I made this clearer or more confusing?

### RE: Modeling a 2 stage weir?

(OP)
The 6' weir is embedded in the bottom 12' weir.
Imagine a rectangular notch cut into the bottom of
the 12' weir.

If I understand you, your saying I should route the 6' weir to primary and route the 12' to secondary?

I'm now wonder if their is a better way to set up my outfall structure?  Make modeling easier?

### RE: Modeling a 2 stage weir?

Keep it simple!

Device#1 should be a 6' long sharp-crested weir.  Set the Rise=2'.  This avoids "overlapping" flows with the upper weir.  In effect, the lower weir will become an orifice when the head exceeds 2 feet.

Device#2 should be your 12' long weir.  Leave the default Rise=0 to prevent consideration of orifice flow.

In order to set the rise, the lower weir cannot be board-crested.  A BC weir uses an arbitrary coefficient-vs-head relationship which makes it hard to predict the orifice flow when the rise is exceeded.  (Folks tend to overuse the BC weir anyway.  The differences are minimal in most cases.)

Both weirs should be routed to Primary, since their flows are combined.  (Added together).  Other routing options are used ONLY for flow diversions or compound outlets (like a riser.)  For details see www.hydrocad.net/pond1.htm

ANOTHER OPTION is to use a custom weir/orifice, which allows you to "draw" the outline of your stepped weir as a single outlet device.

(OP)
I've got it.

Thank you

### RE: Modeling a 2 stage weir?

Thanks Peter!

I had not seen the custom weir/orifice option.  In what version did that get added?

I have used this configuration before, but always used 2 devices.

### RE: Modeling a 2 stage weir?

The custom weir/orifice is very handy, since it lets you model arbitrary weir or orifice shapes as a single outlet device.  For details and examples see "Custom Weir/Orifice" in HydroCAD help.

The custom weir/orifice was added in HydroCAD 7.1, which was released in May 2005.  For a list of other 7.1 enhancements see www.hydrocad.net/info71.htm

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