low head dams created by joshua toebbe nohs 2015

Fluid Mechanics Applied

Low Head Dams

Created by Joshua Toebbe NOHS 2015

Low head damsA low head dam, also known as weirs, are

small structures used to change the flow characteristics of a stream or river.

These structure span the full width of the waterway, and are deceptively complicated and dangerous.

Typical low head damCreated by Joshua Toebbe NOHS 2015

Low head damsLow head dams are used regularly to raise

water levels slightly for multiple purposes.Field irrigation is a common use of these

structures.Weirs also allow an easy way to calculate flow

rate.And can make waterways more navigable.


IrrigationEven when rainfall is lower than average, a

low head dam will raise the water level allowing for easy diversion to lower fields.

In the absence of a weir structure water must be pumped up hill.Pumps are expensive to maintain, and require

lots of energy to runWeirs are relatively easy to install, and require

little maintenance.


Navigable water waysWater ways with excessive elevation changes

can be very hard to navigate, even for experienced individuals.

This process can be done on a large scale, and incorporate a system of locks to help large vessels navigate the elevation changes. (This is done at the falls of the Ohio).

Or on a small scale, creating multiple steps which can be traversed by small vessels such as rafts, canoes, and kayaks.


Flow rateOften small waterways must be monitored by forestry and

biological departments.Weirs provide an easy way to monitor and calculate the flow

rate of small waterways.

(Francis formula)

While there are far better, and more exact equations to calculate the flow rate of weirs, this on is much simpler, and provides a decent approximation.


Flow rate example #1A horizontal weir has been installed in a

forestry research area to prevent the recolonization of evasive water parasites above the weir, limiting the area to which a chemical treatment must be applied, and measure the flow rate to determine the volume of chemical needed. (this is also done in the great lakes with Lamprey, parasitic organisms)

The water way is 2 meters wide with an average head above the weir of 3 cm, and has a coefficient of 1.9. What is its flow rate?


Flow rate example #1Using our approximation equation

And substituting in our values including an exponent of 3/2 for a horizontal structure.


Flow rate practice #1Considering that a flow rate greater than 2

cubic meters per second would be detrimental to downstream ecosystems, what head should the forestry agency watch for on the previous weir?

(practice problems will be collected at end of class)


Flow rate practice #2A v-notch weir (n=5/2) has a head of .5

meters and a width of 1.3 meters, what is the flow rate over the weir?


Low head dam fluid mechanicsNow lets take a closer look at what is actually

happening in the fluid pouring over a low head dam.

As the water pours over the dam it enters a fluid constriction. Thus it speed up.

This change in speed creates a drop in pressure.


Low head dam fluid mechanicsOn the down stream side of the weir the fluid

slows down and the pressure rises again.This rise in pressure is known as a hydraulic

jump, and creates an increase in water level.


Low head dam fluid mechanicsThis change in pressure (and flow velocity)

creates some interesting affects on sedimentation and deposition.

https://www.youtube.com/watch?v=AE771AdF5dM




Low head dam dangersThis change in pressure (and flow velocity)

also creates some very dangerous situations. A kayaker (as a random, non-specific hypothetical example) may see a small one or two foot drop in water and think: “no big deal, I can easily make it over that…”


Low head dam dangersHowever, the recirculating current at the

base of the hydraulic jump can be extremely fast, and easily trap individuals.

Unfortunately this happens routinely in states where large weir structures are common (Virginia is one such state)


Low head dam dangersHydraulic jumps occur in four types.(Fully developed hydraulic jumps)Case A (swept out) jump

Case B (optimum) jump


Low head dam dangers

Case C (Submerged hydraulic jumps) jump

Case D (also submerged, practically non-existent) jump


Low head dam retrofitting

Class C jumps can occur based on two processesScourDeposition



Retrofitting considerationsCostEnergy dissipationScour and depositionSafe passage of people and debrisVariance in tail water heightFish passageIce passage



Even if deposition mounds are removed and scour holes are filled in the pattern is likely to reoccur.

Instead more elaborate retrofitting options must be considered.Stepped spillway Sloping spillway



A more economical and common solution is to fill in the scour hole and remove the deposition.


The EndDon’t forget to go with the flow… (I know, I

know…)

(and turn in your practice problems please)


SourcesKern, E. (2013, August 19). Video Presentation:

Submerged Hydraulic Jumps. Retrieved January 26, 2015, from http://krcproject.groups.et.byu.net/

Horton, R. (1907). Weir Experiments, Coefficients, and Formulas. Retrieved January 26, 2015, from http://pubs.usgs.gov/wsp/0200/report/pdf

Francis, V. (n.d.). Hydraulic jump, Low head dam installation, and course sediment transport. Retrieved January 26, 2015, from http://m.youtube.com/watchv=aE771AdF5dM


http://krcproject.groups.et.byu.net/

http://krcproject.groups.et.byu.net/

http://pubs.usgs.gov/wsp/0200/report/pdf

http://m.youtube.com/watchv=aE771AdF5dM

low head dams created by joshua toebbe nohs 2015

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