Low Head Dams Created by Joshua Toebbe NOHS 2015

Low head dams A 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 dam Created by Joshua Toebbe NOHS 2015

Low head dams Low 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. Created by Joshua Toebbe NOHS 2015

Irrigation Even 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 run Weirs are relatively easy to install, and require little maintenance. Created by Joshua Toebbe NOHS 2015

Navigable water ways Water 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. Created by Joshua Toebbe NOHS 2015

Flow rate Created by Joshua Toebbe NOHS 2015

Flow rate example #1 A 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? Created by Joshua Toebbe NOHS 2015

Flow rate example #1 Created by Joshua Toebbe NOHS 2015

Flow rate practice #1 Created by Joshua Toebbe NOHS 2015

Flow rate practice #2 Created by Joshua Toebbe NOHS 2015

Low head dam fluid mechanics Now 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. Created by Joshua Toebbe NOHS 2015

Low head dam fluid mechanics On 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. Created by Joshua Toebbe NOHS 2015

Low head dam fluid mechanics This change in pressure (and flow velocity) creates some interesting affects on sedimentation and deposition. Created by Joshua Toebbe NOHS 2015

Low head dam dangers This 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…” Created by Joshua Toebbe NOHS 2015

Low head dam dangers However, 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) Created by Joshua Toebbe NOHS 2015

Low head dam dangers Hydraulic jumps occur in four types. (Fully developed hydraulic jumps) Case A (swept out) jump Case B (optimum) jump Created by Joshua Toebbe NOHS 2015

Low head dam dangers Case C (Submerged hydraulic jumps) jump Case D (also submerged, practically non-existent) jump Created by Joshua Toebbe NOHS 2015

Low head dam retrofitting Class C jumps can occur based on two processes Scour Deposition Created by Joshua Toebbe NOHS 2015

Low head dam retrofitting Retrofitting considerations Cost Energy dissipation Scour and deposition Safe passage of people and debris Variance in tail water height Fish passage Ice passage Created by Joshua Toebbe NOHS 2015

Low head dam retrofitting 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 spillwaySloping spillway Created by Joshua Toebbe NOHS 2015

Low head dam retrofitting A more economical and common solution is to fill in the scour hole and remove the deposition. Created by Joshua Toebbe NOHS 2015

The End Don’t forget to go with the flow… (I know, I know…) (and turn in your practice problems please) Created by Joshua Toebbe NOHS 2015

Sources Kern, E. (2013, August 19). Video Presentation: Submerged Hydraulic Jumps. Retrieved January 26, 2015, from Horton, R. (1907). Weir Experiments, Coefficients, and Formulas. Retrieved January 26, 2015, from Francis, V. (n.d.). Hydraulic jump, Low head dam installation, and course sediment transport. Retrieved January 26, 2015, from Created by Joshua Toebbe NOHS 2015