Outlet structure help

[csevers]

I'm designing a dry extended detention pond and I'm having difficulty meeting my water quality requirements. What techniques do you use to to decrease the initial drawdown rate and subsequently increase drawdown rate. Basically, I need to reduce the rate the first 50% of my WQv is released and increase the rate that the last 50% is released. My WQv is 1200 CF (I know it's a rediculously small basin, but they insist). I have a WQ outlet at 886, and a flood control outlet at 886.5. I don't have any room to work with as far as elevation goes, I'm stuck between 886 and 886.55 (WQv stage is 886.55).

 

[SMIAH]

If I get it right, You could try to use a double orifice(s) system?

Maybe one with a Vortex Flow Regulator.

 

[gbam]

Staged outflow structure like SMIAH stated. Small weir notch low with larger one higher up the wall.

 

[beej67]

I think you guys have it backwards. He's asking for a way to reduce the initial discharge, and increase the low head discharge. I'm not sure why.

One neat trick they use on sediment ponds in South Carolina is to put the orifice on a float. Basically, they hang a pipe under a couple of pontoons that has an orifice a small distance below the surface, and then connect the pipe to the bottom of the outlet control structure with a rigid pipe that has two variable bends in it. It flattens your stage-discharge relationship, making it a constant because the head on the floating orifice is always a constant. Neat trick, but it requires having a funny looking floating barge in your pond that goes up and down with stage.

I must ask, why? What's driving you to care what the flow rates are for different portions of your WQV drawdown? I'm not sure I've seen a reg before that cares, nor an application where it mattered.

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

http://www.campbellcivil.com

 

[psmart]

Not sure if this is what you're looking for, but there are number of companies that make a floating orifice/weir/skimmer similar to what beej67 describes. These are designed to maintain a constant discharge at any WSE. Another approach is a vortex valve, which can actually reduce the discharge at high head.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[csevers]

Thanks for the suggestions. I've chalked up the problems I was having to limitations of hydrocad. Yes, my initial outlet structure was a water quality orifice with a larger flood control orifice above it. With that setup I wanted to reduce the rate of release at high head and increase the rate of release at lower head. The only non-mechancical outlet I could think of would be some sort of inverted v-notch weir.

As some people have noted, a skimmer device could be used, although those are typical of temporary basins and not recommended for permanent structures.

The requirement I was trying to meet is based on time to drain. It stated that less than 1/2 of the water quality volume shall drain in the first 1/3 of the time to drain the complete water quality volume.

My next question is what software do you guys use for routing calculations, pond design, etc.?
I've been using hydrocad, although it is somewhat limited. I am also familiar with Bentley Pondpack and Autodesk Hydraflow.

 

[beej67]

Where is the project? Curious, don't think I've heard that reg before. Do they allow you to include exfiltration as part of your drawdown? You can get pretty linear discharge rates out of that.

(my experience is primarily with HydroCAD, which I like, XP-SWMM, which I'm indifferent about, and Hydraflow, which I hate)

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

http://www.campbellcivil.com

 

[csevers]

I'm in Ohio. Our soils are clay, so no I do not include exfiltration.

Here's my problem with HydroCAD:

Only applies when routing a drainage area into a pond with outlet structure.

I double click the pond to bring up my results, then I pull up the table to see what's actually going on. Everything looks good, untill I scroll down to where the hydrograph should end. It doesn't end, that's the problem. For example, my model's saying it takes 13 days to drain 1500 cf through a 1" orifice with an average head of 1'. That's totally incorrect, I don't even need to do any calcs to tell you it won't take that long. One thing I am noticing is that hydroCAD is saying it takes something like 8 days to drain the last ~20 cf from the basin (I am looking at the "storage" colume of my hydrograph report table, but the elevation column works too). If you're looking at the elevation column you will notice that it takes an absurd amount of time to drain the last fraction of the basin. I realize why it's doing this (1/2 head for each time step iteration makes the drain time go to infinity for an infinately small time step) but how do I work around this? I obviously cannot use a table (or any calculations) that do not exactly agree with my report.

 

[psmart]

As the head decreases so does the discharge, so it can take a very long time to completely empty the pond through a 1" orifice. There could also be other issues, such as a base flow, that will extend the draw-down time. If you want to post the file (or email to HydroCAD support) I'll be glad to have a look. But please don't discount the results until you get another opinion.

PS: Make sure there are no warning messages, which will flag most routing problems.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

Using the "average" head is meaningless, because the decrease in WSE is not linear. You have to do the routing calcs (or integrate the discharge vs time) to determine the volume and WSE at any given time. Since the head is decreasing asymptotically, it will take an infinite time to empty 100% of the pond volume, which is what your HydroCAD model is (correctly) telling you.

This points out a common problem with many WQV regulations: Since it can take a very long time for a pond to empty completely, the WQV requirement should be written in terms of discharging a high percentage of the volume (say 95%). Attempting to measure the discharge time for 100% volume is problematic, as you have experienced.

To further illustrate the challenge, consider these seemingly insignificant variations:

1) Place the orifice slightly below the bottom of the pond. Even 0.01 feet below the storage will ensure a finite draw-down time, because the head never drops below 0.01 feet until the entire volume has discharged.

2) Sloping the bottom of the pond slightly (rather than using a flat bottom) has a similar effect, maintaining a larger head for very small volumes.

Of course, if your orifice is above the bottom of the pond you cannot employ either of these adjustments and you will need to accept the reality of an infinite time to empty 100% of the volume.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

At a very fundamental level, all routing procedures never truly "end." It's like the game where you take a step half way between you and a wall, and then take another step half way between you and the wall, and another step half way, and you never get there. That's not particular to one software, it should show up in all softwares if they're doing it right.

Peter points out an easy trick if you have the fall in your system to do it. Set the orifice in a rip rap apron a foot below the bottom of the pond. That guarantees your head never approaches zero. I've done this before on dry retention ponds in Florida. We dropped in a precast flared end section a foot below the bottom of the pond, graded to it, and sleeved it with an orifice, then bolted a trash rack in front of it. Orifice never had less than a foot of head on it, so you never get the tail action you're talking about.



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

http://www.campbellcivil.com

 

[csevers]

Unfortunately, I rarely have the fall to do what you're talking about (Ohio's super flat), although I really like the idea you are proposing. I'm sure I'll use it in the future.

I understand the routing time will approach infinity, but then how can I possibly meet requirements without using the trick beej67 talked about? I agree the regulations should be written to 90% or 95% of total water quality volume, but unfortunately it's not.

How do I specify a sloped bottom pond in HydroCAD? I typically use the stage-area volume calculation method and just use the area of my pond contours.

Also, I do have error messages related to minimum time steps, but I can't seem to get any better results by no matter what I do with the time step and I'm dealing with really short Tc's (like 10 minutes).

I've uploaded a stripped down version of my model.

 

[psmart]

To create a sloped bottom with custom storage, start with a very small (or zero) area for the first stage, like this.

886.00 0 sf
886.10 3026 sf
887.00 4009 sf
888.00 5117 sf

With this subtle change, the pond empties to 20 cf in just 85 hours instead of 156.

The time step warning is the result of your 1 hour time step. Using the default of 0.05 hours will correct the problem. Please click the warning message for complete details. I also suggest that you leave "Fast hydrograph plots" selected. This will improve the visual quality of the plots without any effect on the calculations.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[csevers]

Thanks so much. That is a great trick with the 0 area at invert el.

I don't know how my time step got switched to 1, I had it at .01 previously.