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Harnessing the Power of Water
Robin Saunders
Mechanical Engineer
Contents:
Economics
Environmental Considerations
• Water Rights
• Measuring Available Water Power
Measuring Flow
Measuring Head
Calculating Power
• Dams
• Large and Small Flows
• Channels, Sluices and Pipes
Channel Flow
Pipe Flow
• Water Wheels and Water Turbines
• Waterwheels
Undershot Wheel
Poncelet Wheel
Breast Wheel
Overshot Wheel
• Water Turbines
Michell Turbine (Banki Turbine)
• Pelton Wheels
Reaction Turbines: Francis Wheel, Kaplan Turbine, Propeller (and others)
• Power Transmission
Glossary
Most everyone has witnessed the destruction caused by torrential floods, the subtleties of weathering or erosion, the power of wave motion, the strength and mystique of grand rivers, or the gentleness and swiftness of small streams. The power of water has the capacity for destruction and useful work.
Essentially water power is a form of solar energy. The sun begins the hydrologic cycle by evaporating water from lakes and oceans and then heating the air. The hot air then rises over the water carrying moisture with it to the land. The cycle continues when the water falls as precipitation onto the land and the potential energy of the water is dissipated as the water rushes and meanders its way back to the lakes and oceans.
The potential of water at an elevation above sea level is one of the "purest" forms of energy available. It is amost pollution free (when not contaminated) and can provide power without producing waste residuals. It is relatively easy to control and produces a high efficiency. From 80% to 90% of controlled water energy can usually be converted to useful work. This is dramatic when compared with the 25% to 45% efficiencies of solar, chemical and thermal energy systems. As a result, large and small rivers around teh world have been dammed and waterwheels and water turbines installed to capture the energy of water.
Here, we will be concerned with small hydro-plants that can service the needs of individuals and small communities. In many cases even very small streams can be harnessed to produce power. The power that can be developed at a site is calculated as the rate of flow of water (measured in cubic feet per minute) multiplied by the "head" or vertical distance (measured in feet) the water drops in a given distance. It is these two quantities which must first be measured to see if they are adequate to develop a hydro-plant.
Most hydro-power installations will require the construction of a dam. A dam can increase the reliability and power available from a stream. It can also provide a means by which to regulate the flow of water and can add to the elevatoin of the water (by making it deeper) thus providing greater head to operate the wheel or turbine.
Both waterwheels and turbines deliever their power as torque on a shaft. Pulleys, belts, chains or gear boxes are connected to the shaft to deliver power to such things as grinding wheels, compressors, pumps or electric generators.
WATERWHEEL INSTALLATION
Waterwheels are the old-style, large diameter, slow turning devices that are driven by the velocity of the weight of the water. Because they are slow turning they are more useful for producing mechnical power for grinding and pumping than electrical power. The mechanical power can be connected by pulleys and belts to drive saws, lathes, drill presses and other tools. Waterwheels can provide a small amount of electric power (up to 10 HP) but it involves complex and expensive "gearing up" to produce the necessary speeds to actuate an electric generator.
Water turbines are preferable for producing electricity from the stream. Turbines generally are small diameter, high rpm devices that are driven by water under pressure (through a pipe or nozzle). When coupled with a generator, even relatively small turbines can provide electricity for most homestead needs.
Economics
Perhaps the greatest stumbling block to utilizing water power will be cost. Purchasing a manufactured waterwheel or turbine will obviously be more expensive than building your own. (In the Reviews/Water section a few companies and their relative costs are reviewed.) However, only a few of the many different types of waterwheels and turbines should be attempted by the home-builder. This includes most of the lower technology waterwheels but only one of the turbines. The Banki Turbine can with some time and perhaps some technical assistance be home-built. The Pelton Wheel and the Reaction Turbine are probably best left to the manufacturer. A home-built unit could be inefficient as well as downright dangerous. The cost of the waterwheel or turbine is only a portion of the over-all cost of installing a hydro-power system. Building dams also costs money. here, the expense can be small if you use indigenous materials but if concrete and steel reinforcing are used the cost can be much greater. Then comes the expense for pipe or sluice to carry the water to the wheel or turbine. Depending upon the amount of flow and the distance involved this could be a very minor expense or a considerable one.
WATER TURBINE INSTALLATION
Probably the largest expense besides the wheel or turbine itself will be the gears or pulleys, the shafting and the electrical generator. A 1 KW generator should be obtainable for under $200 new and considerably less if surplus or used. A new 10 KW unit should start in the $800 range. The pulleys, drive belts and chains or gears need to "gear up" the power of the waterwheel or turbine should be rather inexpensive.
The cheapest electrical generating hydro-power system that could be installed, dam, generator, turbine and all would cost a minimum of $500, and more likely $1,000. On the other end of the scale, a manufactured turbine, with dam and piping being constructed by the owner could cost from $3,000 to $10,000.
It is difficult to determine the long range cost of building or buying a hydro-plant, as opposed to hooking up with the local utility company. At present is it probably cheaper to go with the utility. But as the age of cheap utilities and the promise of something-for-nothing from nuclear power is disappearing, building an independent power source will become a more economical investment.
Environment Considerations
Waterwheels and turbines in and of themselves have a negligible effect on the environment. However, the damming of a river or stream, a necessity with most installations, has an important and sometimes irrevocable effect upon the long-term ecological balance of that particular environment. Certainly dams can create a better environment for some animals and plants, and they can and do prevent natural disasters such as floods and severe erosion. But it is important to know that by building a dam, you are also creating a pond or lake where a stream or river used to exist; that you are flooding an already existing river ecosystem, encouraging the accumulation of silt, and perhaps providing a breeding ground for mosquitoes. The resultiing pond or lake behind a dam also usually raises the water table behind the dam (as a result of seepage) and lowers it below the dam. Innumerable other changes are effected by the construction of a dam, and it is generally fair to say that the larger the dam the greater the changes. It is therefore of primary importance to foresee the ecological impact of installing a hydro-plant, and if necessary, to forego that particular site plan or the entire project.
Water Rights
Water is subject to a complex array of laws and regulations concerning its use. Wherever you may settle, some rules and regulations about water will surely follow. This is particularly true in the west and southwest states where arid conditions mean that almost all water that flows needs to be used by someone. Generally in the U.S. there are two types of water rights associated with property: Riparian rights and Appropriative rights.
RIPARIAN RIGHTS were originally brought to the United States through English common law. These laws were easily adapted as water laws for the eastern and midwestern U.S., where the supply of water was similar to that in England. Riparian rights are usually defined in the following way: The owner of any land that contains or is adjacent to a water course has a common law right to "reasonable use" of the natural flow. This right is shared equally by all other properties along the water course. And each is subject to the equal rights of all other riparian properties, or in some states, the higher priority of upstream users. This "reasonable use" means sharing the available water for domestic use, and then using excess water for irrigation of commercial crops or watering livestock.
The usual distinction between riparian and appropriative users is the concept of equality among riparian users as compared to the time priority "first come, first served" among appropriative users. APPROPRIATIVE RIGHTS evolved in the arid portions of the country where little rainfall and periods of drought forced a different tactic for controlling water use. In these places, water that is used for other than domestic purposes has to be claimed through a legal process to establish appropriation use. This means that the first person to make a claim on the water can have the exclusive use of as much as he/she needs; later settlers up and down stream have the right to the use or claim of any water left over. The theory is that water is so limited that there is not enough to share.
Appropriative rights are the exclusive law in Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, New Mexico and Alaska. The states neighboring this drier region have a mixture of both riparian and appropriative rights; Washington, Oregon, California, North and South Dakota, Nebraska, Kansas and Texas.
The usual procedure for appropriative rights (assuming there is enough extra water available) requires public notice and perhaps a public hearing. If your claim is challenged, and it is important enough to warrant going to battle over the issue, a lawyer may be necessary (be sure to find one who knows about these matters; not many do).
Some older properties have mining or manufacturing claims that are listed by use of the water, rather than by a specific quantity of water appropriated. These may be listed as simply the amount of water required to operate a certain number of machines, or for certain processes that require water for washing or sluicing. In some areas these rights may still be on the books and tied to the property rights. To properly research these rights is a job for an historian (this should be part of the title search when purchasing a piece of property).
The local water districts and state water resource departments each has its own set of rules and rituals to obey in obtaining a dependable supply of water. Study them carefully; in many cases they can be circumvented though it is usually best to follow the proper procedures and establish a legal claim for your supply of water.
Once the claim has been filed, you must show ability to exercise the claim for appropriation (similar to mining and homesteading claims). This is usually done by filing a statement with the state's Department of Water Resources, of quantities and schedules of water removed. It may even be wise to file a report of how much is removed under the riparian rights; this can insure the supply in case someone upstream should attempt to cut it off.
Any local soil conservationist or Resources Conservation District (an agency of the U.S. Department of Agriculture) can usually be helpful in supplying information and assistance, not only concerning water rights problems, but also about dams, reservoirs and soils in the area. In addition, you should check with your local authorities and possibly file plans for your dam with them.
Water rights laws are some of the most involved around. There have been volumes of court discussions concerning the relative priorities of public, private, industrial, irrigation, hydro-electric, mining and navigational needs. Not only are they complicated, but water rights are often highly localized...differing considerably from state to state, county to county and city to city. In California, for example, there are such additional rights, besides the riparian and appropriative as: ground water, prescriptive, developed, surface, consumer, Pueblo, spring water, combinations, governmental, and used water. The rule then should be to investigate the laws and the local regulations before planning use of any water, either for irrigation, general water supply, or power.
Measuring Available Water Power
Assuming that you have overcome any legal, economic or environmental problems on your property you can now begin planning your hydro-power plant by calculating how much power is available from your stream. The amount of horse-power possible is determined by; (1) what quantities of water are available (the flow), and (2) what the drop or change in elevation (the head) along the water course is.
Measuring Flow
The volume of water flowing is found by measuring the capcity of the stream bed and the flow rate of the stream. Accurate measurements of the water flow are important for decisions about the size and type of water power installation. You will want to know; (a) normal flow and (b) minimum flow. If a unit is built just on the basis of normal flow measurements, it may be inefficient or even useless during times of low flow. If it is built just on the basis of minimum flow measurements, which usually occur during the late summer, the unit may produce considerably less power than possible. All water courses have a variation of flow. There are often daily as well as seasonal differences. The more measurements you make throughout the year, the better estimate you will have of what the water flow really is. Once you know the stream's varying flows, a system can be built to operate with these flows.
The Small Stream
[insert equation 1]
The easiest way to measure the flow rate of a small creek or stream with a capacity of less than one cubic foot per second, is to build a temporary dam in the stream. Channel all of the water flow into a pipe or trough and catch it in a bucket of known volume. Measure the time it takes to fill the bucket and use Eq. 1 to find the flow rate.
The Medium Stream: The Weir Method
[insert equation 2]
The weir method is used for measuring flows of medium streams with a capacity of more than one cubic foot per second. Basically a kind of water meter, a weir is usually a rectangular notch of definite dimensions, located in the center of a small dam. By measuring the (1) depth of water going over the weir and referring to standard tables, and measuring (2) the weir width, the volume of flow can be accurately calculated.
In order for standard tables to apply. the weir must be constructed of standard proportions. Before you build the dam, measure the depth of the stream at the site. The depth of the weir notch "H" (see Fig. 1A) must equal this. The weir notch must be located in the center of the weir dam (see Fig. 1B) with its lower edge at least a foot above the downstream water level. The opening must have a width at least three times its height (3H), and larger if possible. The notch will then be large enough for the water to pass through easily to measure the large flows, and yet not too large so that it cannot also accurately measure the small flows.
The three edges of the opening should be cut or filed on a 45° slant downstream, to produce a sharp edge on the upstream side of the weir. The sharp edges keep the water from becoming turbulent as it spills over the weir, so that measurements will be accurate.
[insert fig 1A]
The weir dam need not be permanent, although if left in, it is convenient for continuously monitoring stream flow. The temporary dam can be made simply from logs, tongue and groove lumber, scrap iron, or the like. The dam must be perpendicular to the flow of the stream. And, when the weir is installed, be certain that the sides are cut perpendicular to the bottom, and that the bottom is perfectly level
All water must flow through the weir opening, so any leakage through the sides and bottom of the dam must be sealed off. Side and downstream leakage can be stopped with planks extended into the banks and below the bed of the stream. Upstream leakage can be sealed off with clay or sheet plastic.
[insert fig 1B]
The accepted method of measuring the water depth over the weir notch (in lieu of putting a ruler in the weir notch) is to drive a stake in a spot accessible from the bank and at least four feet upstream from the weir. The reason for placing the stake at this distance is that the level of the water begins to fall as it nears the weir, where the water forms a crest. Four feet is a safe distance away from the weir to avoid measuring this lower water.
Pound the stake down until the top of the stake is exactly level with the bottom of the weir opening. A level can be established by placing a plank between the bottom of the weir opening and the stake, and using a carpenter's level. To measure the "head" (depth) of water flowing over the weir, allow the stream to reach its maximum flow through the weir, and place a ruler on the stake. Then directly measure the depth in feet of water over the stake.
Having measured the depth of the water, refer to the Flow Rate Weir Table (Table 1) for the flow rate for that depth labelled "H" for Head. The flow rate in Table 1 is given in cubic feet per second for each foot of width of the weir. It is necessary to multiply the flow rate by the width of the weir in feet, to find the actual flow rate. For example: If the depth (H) of the water is 1 foot then the given flow rate from Table 1 is 3.26. To find the flow over a weir that is 4 feet wide, multiply 4 × 3.26 which equals 13.04 cubic feet per second.
The Large Stream: Float Method
The following method is not as accurate as the previous two. It is impractical to dam a larger stream and measure it for preliminary study, but with large amounts of water, a precise measurement is probably not so important.
[insert table 1]
[insert equation 3]
Choose a length of stream that is fairly straight, with sides approximately parallel, at least 30 feet long (the longer the better), that has a relatively smooth and unobstructed bottom. Stake out a point at each end of the length, and erect posts on each side of the bank at these points. Connect the two upstream posts by a level wire or rope (use a carpenter's line level). Proceed the same way with the downstream posts (see Fig. 2)
Divide the stream into at least five equal sections along the wires (the more sections, the better), and measure the water depth for each section. Then average the depth figures by adding each value and dividing by the number of values. For example, if you have 7 readings of equal width, add depth0 + depth1 + depth2 + depth3 + depth4 + depth5 + depth6 + depth7 and divide by 7. Since Fig. 2 shows d0 and d7 at the edge of the stream, their depths are zero, they are not included in the calculation, and the sum of the values is divided by 5. For other situations, Eq. 4 should be used.
[insert fig 2]
[insert equation 4]
Now to find the stream's cross-sectional area, multiply the average value of the depth times the stream's width (the length of the wire or rope as in Eq. 4.)
Remember that you are trying to find the average area of a section of the stream, so you must take the value of (A) for each station, add them together and divide by two.
Your next step is to measure the stream’s velocity in order to determine O (the stream's flow rate). Make a float of light wood, or use a bottle that will ride awash. A pennant can be put on the float so that its progress can be followed easily. Now set the float adrift, in the middle of the stream, upstream from the first wire. Time its progress down the stream with a stop watch, beginning just when the float passes the first wire, and stopping just as it passes the second wire. Since the water does not flow as fast on the bottom as it does on the surface, you must multiply your calculations by a coefficient to give you a more accurate estimate of the stream's velocity.
[insert equation 5]
Having determined the area and velocity of the stream, Eq. 6 gives the flow rate of the stream.
[insert equation 6]
Measuring Head
The "head" or height of fall of the water determines what kind of waterwheel or turbine you will choose. Commonly, this distance is measured in feet, Head produces a pressure... water pressure. Basically the weight of the water at a given head exerts a pressure that is proportional to that head...the greater the head the greater the pressure. Pressure is Usually measured in pounds of force per square inch (psi). Pipes, fittings, valves and turbines may be rated either in head or psi. The relationship between the two if you need to know head, and the equipment is rated in psi and vice versa, is:
[insert equations]
An elevation of head will produce the same pressure, no matter what the volume or quantity of water is in that distance. If the head or depth of water is 20 feet the pressure of that water will be (20 × .433) = 8.7 psi whether there is a whole reservoir of water 20 feet deep or a 2 inch pipe filled with water 20 feet high.
You can get a rough estimate of head on your land from a detailed topographic map. The U.S. Geological Survey prints topographic maps of the entire U.S., with elevation contour intervals of 40'. They are available directly from the USGS (see page 68) or from some local sporting goods stores. These maps are useful for making note of particularly choice sites. However, since they only give an approximation of slopes and elevations, for a more accurate measure of head it will be necessary to take a level survey.
Level surveying is a relatively easy process. A good description of a Poorman's survey using a carpenter's level can be found in Cloudburst (see page 66). Also included in that publication is a critique that recommends the all-purpose hand level. The hand level is a metal sight tube with a plain glass cover at each end and a prism, cross hair, and spirit level inside the tube. In most cases the hand level will be the simplest and least expensive way to go. More expensive and elaborate instruments are available (a surveying transit or a surveyor's level) but they will probably not be needed for these basic measurements.
Measuring the vertical difference in elevation between point A and point B.
1. Basic Eyeball Method with Handlevel: How High is Your Eye?
Measure the distance standing upright from the bottom of your feet to the middle of your eye. Stand at the lower point (point B) and sight through the level at the top of an object or at the ground next to an object so you know where to go next. Go stand at that spot and sight again, always working towards point A. With a pole in the ground at point A the elevation of the final sighting can be marked and subtracted from the total. By simply multiplying the elevation of the eye by the number of sightings (minus the extra elevation in the last sighting) the vertical elevation change from point A to B is determined.
2. With surveying Rod and Handlevel
Attach a tape measure to a pole with the zero end at the bottom. Have one person hold the rod while another sights through the handlevel. This does require some conversation between the "rod-man" and the "instrument man" about where the "level point" is on the rod and what the value of that point is.
The instrument man must tell the rod man where the level point is on the rod (indicated with a pointing finger) and the rod man must read off and write down the value on the tape of that point. The instrument man stays put while the rod man moves on towards point B to some notable Point where a new reading is taken. Then the instrument man "leap-frogs" downhill past the rod for a new reading and so forth. Add up the differences of readings for each time the instrument man moves past the rod man; the total of these differences will be the elevation change from point A to point B.
The head is then the change in elevation between point A and point B. It is desirable to obtain the greatest amount of head possible. This can be accomplished in several ways. First, you can choose a site for the turbine Where the greatest drop in the stream occurs in the shortest distance. Secondly, the amount of head can be increased with the construction of a dam (see Dams). Most hydro-power systems will require a dam anyway, and the higher the dam can be built, the greater the head will be. Finally, the head may be increased by the use of a channel or sluice. The channel or sluice will be downstream from the dam to carry the water to a place where a steeper drop occurs. Other determining factors must be considered in trying to realize the greatest amount of head. The most basic are cost, property lines, construction laws, soil condition and pond area in back of the dam.
Calculating Power
We can define power as the ability to do work. In order to determine if the hydro-power installation will meet your needs, the amount of available power must be calculated. The amount of this power is proportional to the head available and the flow rate of the water. Waterwheels and water turbines generate mechanical power (however, turbines are usually hooked up to an electrical generator, and the mechanical power generates electrical power). The mechanical power is usually measured in horsepower.
[insert equation 7]
Eq. 7 illustrates the theoretical horsepower available from the head and flow of a stream. Eq. 8 below refines the calculation of this available power. There are losses in the amount of head due to friction in the channel or pipe which carries water from the dam to the wheel or turbine. Thus, actual head is the amount of head loss (due to friction) subtracted from the total head available. A discussion of the head loss in pipes and canals can be found in the Channels, Sluices and Pipes section.
Another loss factor that Eq. 8 accounts for is the efficiency of the devices (turbine, generator, and any mechanical connection between the two: belts, gears, chains, pulleys) used to harness the power. In the case of water power, efficiency is an indicator of the conversion performance of the machinery used to harness the water power. It is usually measured as a percentage. Each machine or device will have its own efficiency percentage, and they must be multiplied together in order to obtain the over-all efficiency:
[insert equation 8]
Dams
In developing water power, dams are needed for three functions: (1) to divert the stream flow to the waterwheel or turbine, (2) to store the energy of the flowing stream, and (3) to raise the water level (head) to increase the available power. In addition to being used to help develop power, a dam may be useful in providing a pond for watering livestock, for fire protection or for irrigation needs. However, keep in mind the possibility of ecological harm involved in damming a stream, and forego the construction if necessary.
There are four basic criteria for deciding possible sites for the dam and powerhouse: (1) the ease of building the dam, considering the width of the stream and the stability of the soil; (2) maximizing the amount of possible storage volume behind the dam without damaging the ecological balance; (3) minimizing the distance to a good powerhouse site in order to lower the difficulty and the expense of moving the water; and at the same time (4) finding a place where the greatest amount of head is available.
Once a site has been chosen the size and type of dam needed will largely depend upon the stream course and surroundings as well as your needs for power. An assessment must be made of basic requirements for power (both mechanical and electrical) and some estimate of future needs. This could be as simple as adding up the power needs for lighting and refrigeration, but could also include assessing needs for machinery, power tools, or appliances. Essentially then, the size and type of dam will depend upon the size and type of waterwheel or turbine that will be needed to fill power needs, and also upon the flow rate of the stream, the head available, the local restrictions on size and permanency, and the money available.
Diversion Dam
When there is a creek with a continuous flow and a natural drop that together will provide sufficient power for your needs, a small diversion dam will suffice. This can simply be a log placed up against some projecting rocks, with rocks, gravel and earth placed upstream to stop the underflow. Even a temporary dam of a few rows of sandbags will serve well (at least until the first flood comes) and can be cheaply and easily replaced. All that is necessary is a sufficient dam to divert the water into a sluice or pipe intake which carries the water to the turbine (see Channels, Sluices and Pipes).
Small diversion dams have the advantage of easily "washing out" during large flows thus preventing possible damage downstream which the washing out of a larger more permanent dam might cause. Also, diversion dams may be useful for running some of the stream's water into an off-stream storage location.
[insert fig 3]
The need for storage is one of the dam's primary functions, but during times of large flow or flood can cause potentially dangerous situations. Normally the water would be stored directly behind the dam in the stream's course. However, if a diversion were used to divert the water to a side canyon or hollow, the need for storage would be met, while avoiding the danger of a flood washout. When large flows occurred the diversion dam could either be easily taken down or allowed to wash out by the stream's force; both situations leaving the off-stream storage facility full and intact. When the large flow subsided, the dam could simply be rebuilt.
Low Dams of Simple Construction
These can be built by adding to the diversion dam's structure. Instead of one log, use several stacked together log cabin style, or like a corn-crib - hence the term "crib dam." The crib dam (see Fig. 3) consists of green logs or heavier timbers stacked perpendicular to each other, spaced about 2 or 3 feet apart. These should be spiked together where they cross, and the spaces in between filled with rocks and gravel. The upstream side, especially the base, should be covered with planks or sheets of plastic to prevent leakage, and then further covered with earth or clay to seal the edges. Priming planks should be driven into the soil approximately two to three feet deep at the upstream face to limit the seepage under the dam (particularly on porous soils). Priming planks are wooden boards, preferably tongue and groove, with one end cut to a point on one edge. They are driven into the soil so that the long pointed side is placed next to the board that was previously driven. Then as each successive board is driven into the soil it is forced up snug against the preceding board as a result of the angle of the bottom cut.
[insert image]
The downstream face of the dam must be protected from erosion or undercutting wherever water will spill over. This is most important at a time of large flows! The spillways can be made of concrete, lumber, or simply a pile of rocks large enough to withstand the continual flow. Crib dams can be built with the lower cross-timbers extended out to form a series of small water cascades downstream. Each cross-timber step should be at least as wide as it is tall.
[insert fig 4]
Earth-Fill Dam
This is the cheapest kind of dam if earth moving equipment is available. Sometimes these can be small gravel dams (under 5 feet) that can wash out with each season's flood, and can be rebuilt when necessary. For larger earth dams (in California anything more than 6 feet high or 5 acre feet or storage) a registered civil engineer will have to be consulted, and some soil studies should be made to determine the method of construction. For more information on the structure, placement, and suitability of earth fill dams, see the USDA Handbook No. 387, "Ponds for Water Supply and Recreation," page 67.
Plank Board Dam
Much like a plank-board overflow spillway, a small dam can be constructed from wooden planks supported by posts set in a concrete foundation (see Fig. 4). The posts can be wood 4 × 4's (or larger) with steel channel or angle-iron attached to the sides, or the posts could be steel {beams set directly in the foundation. The wooden planks can be dropped into the steel slots to form as much of a dam as is needed (up to the height of the posts), The upstream face of a plank board dam will often need to be sealed with plastic sheeting to prevent leakage. The planks (2 × 6 or larger) can be either added or removed to vary the height of the dam and can be completely removed during the flood season.
Rock Masonry Dam