Flow out of a reservoir 5

[Kodo]

I'm working on a dam project and am trying to calculate the flow out of the dam thru a 400 foot section of pipe with the elevation of water in the dam 68 feet above the invert of the outlet structure. The effective diameter of the (non-circular) pipe is 4.437ft. The pipe is concrete, assuming a 0.004 value. When I calculate the friction loss in the pipe I get -2 ft of effective head. Anyone have any suggestions? or help so I can understand what is going on?
Thanks,

 

[francesca]

Are you using Manning's for your friction loss? If so, where did you get a 0.004 value for concrete from? You should take a look at the Lindberg PE reference manual for conditions of entry/exit control. I would guess that, due to the head loss, you have an exit control condition. Also, you do not mention the slope of the outlet pipe.

 

[jartgo]

I agree with francesca.
I'm not sure about taking a guess...I'd be hard pressed to pick one at 400 feet length, but I don't do that everyday. I can run it through a spreadsheet if no one else responds more satisfactorily.

I think you'd need a culvert made of teflon coated glass to get a Manning's n of 0.004, and then only in the first year.

 

[RWF7437]

I'm guessing but I think the original poster meant that the Slope of the outlet "pipe" is 0.004 ( not the Manning's "n"). He does not tell us what the actual shape is so we can't calculate the hydraulic radius, nor do we know the area. Equivalent diameter is an ambiguous term as used here. Also, we do not know the outlet condition or the inlet configuration.

In addition, we do not know whether or not the water level may be assumed to be "constant" or whether it may drop appreciably during the outflow event.

Without this information the problem, as stated cannot be solved.

try again and good luck Kodo

 

[bimr]

You are describing the flow condition of Culvert Flowing Full (No tailwater at outlet end).

a. Full Flow. The hydraulic condition in a culvert flowing full is called pressure flow. If the cross-sectional area of the culvert in pressure flow were increased, the flow area would expand. One condition which can create pressure flow in a culvert is the back pressure caused by a high downstream water surface elevation. A high upstream water surface elevation may also produce full flow (Figure I-11}. Regardless of the cause, the capacity of a culvert operating under pressure flow is affected by upstream and downstream conditions and by the hydraulic characteristics of the culvert.

http://www.cflhd.gov/design/hyd/hds5_03r.pdf

You have to do more than just estimate the flow in the conduit, you have to determine the effect of the upstream and downstream conditions.

Note that it is not possible to have negative head ("get -2 ft of effective head"}.

Reviewing the Federal Highway Administration publication should answer your questions.

http://www.cflhd.gov/design/hyd/hds5_03r.pdf

 

[tsgrue]

.

You indicate designing a 68+ feet high dam. The failure of such could result in serious damage to property and/or loss of life.

Please retain an engineer with expertise in this area to work with you. I also strongly suggest obtaining and using the following texts:

Design of Small Dams
US Bureau of Reclamation

Design Hydrology and Sedimentology for Small Catchments
CT Haan, BJ Barfield, JC Hayes

Gravity Dam Design
USACE EM 1110-2-2200

An Internet search engine will point you to these.

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[TerryScan]

It is funny how many different scenarios we can come up with when we are presented with an incomplete question and choose to make assumptions.

I did a pressure flow calculation with an older version of Haestad's Flowmaster. I assumed the pipe flat (68ft of head from inlet to outlet). If the 0.004 is the slope of the pipe, there will be a minor differnce in results. This assumes that "outlet structure" refers simply to the pipe at the bottom of the resevoir. Of course, if the "outlet structure" involves any kind of riser or other appurtanances, it's an entirely different scenario.

Hazen-Williams formula Discharge: 1,003 cfs
Darcy-Weisbach Formula Discharge: 953 cfs
Manning's formula Discharge: 846 cfs

 

[francesca]

Anybody else smell a homework problem?

 

[Kodo]

This was my first time to post. I apologize for not including a more detailed account of my question and all the variables/constants I've been using. And no this is not a homework problem. Some of you are a little brutal and I would ask that you request more information before making assumptions.

Situation: A dam with an elevation of 7957'max water surface level. The "drainage" structure (pipe)is a rectangular bottom 2.5' high by 4.5' wide with a 2.5' raduis top. The invert of the inlet of the pipe is at elevation 7894' and the invert of the out let of the pipe is at elevation 7889'. The pipe has two different slopes, for the first run of 196 ft s=0.005 and for the second run of 202 ft s=0.0125. The pipe is old concrete, I'm using a specific roughness of 0.004 ft.
I was using Darcy eqn for friction loss, not mannings.

I need to find the flowrate out of the structure at varying water surface elevations in the reservoir. This is in order to estimate if additional flow is needed for local fish.

 

[civilperson]

A little brutal is when life and property is lost due to engineering negligence. Comments about lack of specifics are love taps.

 

[cvg]

simplifying the pipe slope, making assumptions on the inlet conditions and the tailwater channel and simplifying the shape of the culvert and using HY8 I am getting no more than about 340 cfs through the pipe with water surface at the top of the dam.

 

[RWF7437]

My suggestions would be:

1. Make no assumptions you don't absolutely have to make.
2. Ask if someone associated with this dam has already done these calculations
3. If not ( which seems highly unlikely !) go to Step 4
4. Model this in HEC-RAS using the actual shape of the conduit; NOT the "equivalent pipe size". Note that HEC-RAS will tell you whether the outlet is flowing full, part full or under pressure based on the Laws of Conservation of Energy, Momentum and Mass. Do NOT use the Hazen William Equation which is only valid for water at normal temperatures, flowing full, with turbulent flow conditions throuhout a Circular pipe. It is suggested you use a Manning's n of at least 0.013 for the concrete conduit. You may want to try other "n" values if you have some way to field check your results.

5. Do a series of HEC-RAS steady flow runs starting from 0 cfs to 500 cfs in 50 cfs steps.
6. Plot the results. Plot flow vs headwater depth.
7. Based on this plot, expand the range of flows, if necessary, until the headwater depth reaches 68 Feet above the conduit invert or the dam is overtopped. Again plot all the data. This will give you what is called the "rating curve" for this outlet.

8. If at all possible, find a way to field check you results.
9. If at all possible, find a way to field check you results.
Good luck

 

[bimr]

You are describing the flow condition of Culvert Flowing Full (No tailwater at outlet end). Refer to page 35 of the Federal Highway Administration publication for a diagram that shows that condition. See the picture of page 5.

Outlet control flow conditions can be calculated based on energy balance. The total energy (HL) required to pass the flow through the culvert barrel is made up of the entrance loss (He), the friction losses through the barrel (Hf), and the exit loss (Ho). Other losses, including bend losses (Hb), losses at junctions (Hj), and loses at grates (Hg) should be included as appropriate. See page 32 of the Federal Highway Administration publication for the method used to calculate the flow.

http://www.cflhd.gov/design/hyd/hds5_03r.pdf

 

[chicopee]

You should try to do this problem by hand before using a spreadsheet. My result by using Bern.'s equation comes up with an estimate of 68.9 ft head loss in the pipe only. Average flow in the pipe at 30 ft/sec. Since roughness in pipe has a range, I used .05 ft as an average. Reynold's number at 2X10^-4 ft-ft/sec. k/Deq at .0113 and f=.038 (from Moody's diagram), Kentr.at .5. Delta P's at zero and V at reservoir level at zero. Only two "trial amd error" procedures were needed.

 

[RWF7437]

http://books.google.com/books?id=Mi...X-3&sig=Im51f1KgJiYqCDw_jHUJjnQW-FY#PPA678,M1

 

[msquared48]

I take it that since the structure is so old that no piezometers were installed at the inlet and outlet structures to help monitor the flow?

Mike McCann
McCann Engineering

 

[parrot77]

Flow through an oriface under a head of water is generally expressed from an empirical equasion, as a function of head height and oriface area. This equasion will vary significantly with the shape of the oriface. This will only give the flow into the concrete pipe. Then calculations need to evaluate the flow rate based on Colebrook White equasions

 

[Kodo]

Thanks everyone for your input. The dam is old 1920's no piezometers, flow is figured from downstream gaging station.

cvg: I'm insterested in seeing your calcs. Is there a way I can look at them?

 

[cvg]

I went back and looked at that and with a bit of refinement I am getting closer to 500 cfs. This is a very rough estimate, based on limited information and simplified greatly, but should at least be in the ballpark.

See the following: