Open Channel Flow Monitoring 2

[gambit219]

I have a Cal Trans owned Trap Channel that drains into my project site. The trap channel is continuously conveying nuisance water. Are there any insitu methods of determining the flow rate of said nuisance water? My employer does not want to spend money, so ultrasonic level transducers are out and I can not install weirs as Cal Trans will not approve. Any ideas? I thought possibly installing a 2 x 4 square frame the width of the channel. Measure time to fill box. It won’t be the most accurate but I only need an idea, not exact measurements and I could be in and out no need to get Cal Trans involved. lol

 

[GoldDredger]

Any way you can determine the drainage area upstream? The rational formula comes to mind.

As far as the flow rate that would be dependent upon the size of storm and intensity would it not?

Regarding the box idea, I suppose if you wanted to run out to the site every time it rains. Then when it fills up that first time the test is over until you drain it.

CalTrans should have as-builts that might give you an idea how much area is draining.

 

[gambit219]

Ignore the first idea, I don't know waht I was thinking. If I measure the high water level of the nuisance water and the slope of the trap channel over a given length would it be reasonable to say I could apply Bernouli's (sp?) equation at that point to predict flow rate?

 

[gbam]

gambit219;
I recommend using Manning's equation with two surveyed Xsecs and high water marks. You can approximate the Sf by the slope of the high water marks and average the conveyance and back calculate the flow rate.

 

[gambit219]

The trap is owned by Cal Trans but the flow comes from the city storm drain system. I know the 10 year and 100 year storm event flows from the city master drain plan but we want to quantify the nuisance water that no one anticipated during the development of the city. The hydro study for the city is broken up per development and hasn't been updated since 2002.

 

[gambit219]

Wow I am off my game today, gbam, I was referring to Mannings when I said Bernouli's. I know the dimensions of the trap from plans I obtained from the city master drain plan. Knowing hw mark will allow P to be calculated. I think that does it. Thanks for your help guys, sorry for the absent mindedness today. Appreciate the patience and help.

 

[cvg]

how about a more low tech approach. unless this water is quite deep, go down into the channel (with your boots or waders on) and visually assess the flow. estimate the approximate flow velocity using a stop watch or the second hand on your watch to get the speed in feet per second. use a tape measure to measure the approx depth at several locations. then measure the approximate flow width. Estimate the flow cross sectional area and then use Q = VA. This will be way more accurate than rational method and much quicker and perhaps just as accurate as any type of temporary 2x4 timber weir structure.

 

[JedClampett]

Or throw a rubber ducky in the flow and time it floating down the channel. You don't have to get wet and you can expense a rubber ducky.

 

[gambit219]

Thanks for the ideas. We actually decided to do a combination idea of Mannings and a paper boat. It was assumed the paper boat was light enough to represent the velocity and the average cross sectional area between the two points the boat was measured was used to calc Q. Then we used Mannings as well to try and verify our field observation. Surprisingly they were very similar. Mannings equation yeilded a slightly larger Q. Thanks for the help.

 

[bimr]

Why don't you just make a temporary weir structure and remove it when finished.