HDPE pipe as principal spillway with 10' wide weir (3:1 side slopes) as emergency spillway

[BillsSis]

hello everyone,
I'm fairly new to the detention pond hydraulics concept but am understanding it so far. A question or two....I've read in some manuals (don't recall specific ones)or books that the user should calculate the weir and orifice equations. Is this for a riser pipe (or box) or is this for the horizontal pipe that would be considered the principal spillway (without riser)? Because the detention pond has both structures (principal and emergency spillways), can I use HEC-HMS to calculate the flow through the detention pond?
Oh, another question...I've also seen a 1' dimension from the emergency spillway. When I put in the elevation for the emergency spillway, do I put in the elevation at this 1' below emergency spillway or do I put in the actual elevation of the emergency spillway?

Thank you!!

 

[gbam]

Howdy BillsSis - Your outlet will be your design; either weir and/or orifice and/or culvert what ever you so desire. The key is that you have controlling hydraulic calculations to show the maximum head for a specific discharge. A typical outlet, that I design, is a culvert and weir. The culvert alone up to the elevation where the weir kicks in then its combined. In regard to the 1' dimension, that sounds like a freeboard requirement, we treat that below the weir elevation. We typically design our basin on the culvert outlet and the weir to handle the 100-year incluase the culvert gets clogged. Hope that helps.

 

[BillsSis]

Thank you Gbam...So how do I know to desire to use the orifice equation for the pipe or the weir equation for the pipe or just use the Manning's equation as for a typical culvert? Do I do all three calculations to see what has the highest amount of head at the outlet?

I must be making this way too hard, as I normally do. LOL I understand the rest of your message, just not the beginning.

Thanks again!
BillsSis

 

[gbam]

If the outlet works is a simple culvert with overflow weir, I would use FHWA's HDS-5 to develop a rating table for the culvert. Then for the same watersurface elevations as the culvert, I would develop a rating table for the weir (Weir Equation). The more complex outlet works the more analysis you need to do inorder to determin the control section of the stgructure. For instance if you have a culvert as the primary outlet, but, the inlet is a concrete structure with orifices then you would need to analyze the culvert section and the orifice sections seperate to determine if the culvert creates backwater on the orifice. Hope that helps.

 

[beej67]

If the culvert is steep enough, then the orifice of the culvert controls. If the culvert is flat enough, then the flow in the barrel by manning's equation might be less than the flow in the orifice, so it would be in "barrel control." Tailwater may have some effect. Once it stages over the weir then it's a compound outlet. It's actually a pretty complicated system. If you wanted a quick one-off way to develop a rating curve for your system to take a look at it, stick it into HY-8. Won't do variable tailwater, but it should give you some idea of what's going on.

I would think HMS does this, but the last time I worked in HMS I found it to be a little clunky, so using another software as a check vs its rating curve might be a pretty good idea.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[BillsSis]

Hi both Gbam and Beej67,

Thanks for both of your thoughts. The culvert is a 2.5' diameter HDPE pipe at a 4.5% slope from inlet to outlet. So, in anticipation of the results, I would think this structure's headwater would be greatest using the orifice equation. I will use the HY-8 program to give me a good idea of what I'm looking at, then use the HMS program.

thanks again! here goes nothing....

 

[cvg]

HY-8 is a good choice for the outlet pipe analysis and it will do variable tailwater. It doesn't really handle risers which usually function as a weir. You will also likely need to have a trash rack so make sure you include any losses through your rack, especially for the clogged condition. yes, you can use HMS, but I usually prefer to generate rating curves using other methods and to be able to include weirs and trash racks effects. than use HMS to route the hydrograph through the reservoir and spillways.

as far as the 1 foot below the emergency spillway, I assume that you want 1 foot of freeboard above the design water surface elevation before water begins to flow over it. Usually these are designed to handle flows larger than 100-year storm runoff.