By John Hicks, Landscape Architect
The pond should have a minimum depth to meet the proposed use requirements, and also to offset normal seepage and evaporation losses. Such losses vary across the continent amidst different climatic zones, soil types, and also from year to year.
For instance, the eastern United States along with the Province of Ontario require a recommended minimum depth of 1.8 to 2.1 metres (six to seven feet). This parameter holds true all the way south to the panhandle of Florida, and west beyond the Mississippi Valley. In the central United States, it increases to between 2.4 and three metres (eight to 10 feet) and up to 4.3 metres (14 feet) in the arid western States. Deeper ponds are required where a year-round water supply is needed or where seepage losses exceed 75 millimetres (three inches) per month. Fish ponds, dealt with in later chapters, should be at least 3.7 to 4.6 m (12 to 15 feet) deep in part of the pond to assure survival over winter, and also to maintain a colder area in summer, particularly for trout species. 
Deep ponds over 4.6 metres (15 feet) may also be suitable for fish if a bottom draw-off spillway device allows the de-oxygenated bottom water to be removed. Wildlife ponds need only 1.2 to 1.5 metres (four to five feet) of depth with low sloping banks which encourage vegetation and weed growth. Oxygenation in these ponds is not a great concern.
Building a pond is a major undertaking. Be aware that multi-purpose ponds rarely fulfill all of their intended uses. The use will often determine the depth, the bed and bank profiles, and what aquatic plant species should be planted in it. In the case of wildlife ponds, the bed or bottom of the pond or lake will provide a variety of micro-habitats, depending on its degree of irregular topography (flat bottoms producing little or no variety). The bank profile will also determine the rate of colonization of the aquatic plants chosen. The nature of the edge (shape of the pond) is important for breeding water birds. A highly indented shoreline will produce more nesting opportunities than a smooth, round shoreline. Likewise, fish ponds should have some indentations, even peninsulas, for fishing stations while providing opportunities for fish nesting habitat.
Kidney-shaped ponds are normally the most successful, with the longest reach of the pond oriented in a north-south direction to minimize wave action and erosion of the banks. They have the highest appeal to pond owners. Swimming ponds, however, should be free of indentations or promontories that will inhibit circulation and trap pockets of untreated water. Various site restrictions such as a narrow valley, or the preservation of trees, etc., may direct the design towards a triangular, irregular, or even round shape. Square ponds are often the result of bulldozer excavation prevalent in many farm pond situations. These appear very unnatural and are difficult to manage with corners that resist circulation, warm up excessively and invite weed infestations.
Knowing how to manage your pond will require knowledge of its surface area and volume.
Many algae reduction techniques, chemical applications, and fish stocking programs will also require knowledge of pond volume and surface area. If you consult your local fish and wildlife department, or conservation authority, a field officer may be available to assist you with this calculation. However, you can achieve a very close approximation on your own using the formulas that follow.
Surface area calculation
Rectangular Ponds
If your pond is rectangular or square, the surface area is simply length x width. You can usually regard an irregular-shaped pond as a rectangle or square, computing the area from straight boundary lines that approximate the actual shorelines. If your units are in square feet, divide by 43,560 to arrive at acres. If you want your result in hectares, then use length x width in metres to obtain square metres, and divide by 10,000 to get hectares.
(2.5 acres = 1 hectare). If you do not have a metric rule, you can convert from square feet to square metres by multiplying by 0.0929, then divide by 10,000 to convert to hectares.
Round Ponds
If your pond is circular, or nearly so, you can measure the total perimeter of the pond by tape (in sections) or by pace, if your pace is accurate. This is the circumference of the pond. Square the number obtained and divide by 547,390 to arrive at the surface area in acres.
This formula will hold true for ponds that are close to round, but the more egg-shaped the pond is, the more errors will be introduced in the computation.
Determining the depth of the pond
The easiest way to accomplish this measurement involves making soundings in a regular fashion over the entire pond surface. This task is best done by boat, using a weighted rope that is marked off in one foot (or one-third metre) intervals and lowered to the bottom of the pond or lake. Averaging at least 15 of these readings will yield a close approximation of the average depth of your pond.
Pond volume calculations
A) In the situation where soundings have been taken on a built pond, the volume is simply the surface area multiplied by the average depth (the result of the 15 readings average suggested above). The acre-feet parameter was often useful in the past to estimate chemical treatment dosage or fish stocking capacity. Easily arrived at if you take your soundings in feet. (If in metres, convert to feet (x 0.3048) and multiply by acreage.
Volume in acre-feet = Surface area in acres x average Depth in feet
B) A simple formula for determining reservoir capacity utilizes the maximum depth of water in feet and area in acres, computing the final volume in gallons. The accuracy of this formula is questionable, but for a rough estimate of gallons it might prove useful.
Volume in gallons = 100,000 A x D
Where A = acres, D = Maximum depth in feet
C) An accurate prismoidal formula to determine cubic yards of excavation required for a rectangular pond requires calculating the excavation mid-depth and bottom depth.
Volume in Cubic Yards = [(TL x Tw) + 4(ML x Mw) + (BL x Bw)] x D divided by 27
Where (TL x Tw) = length x width of excavation at ground surface in square feet, (MLx Mw) = length x width of excavation at mid-depth point in square feet, (BL x Bw) = length x width of excavation at bottom of pond in square feet, D = average depth of pond in feet, 27 = conversion factor (cubic feet to cubic yards), V = volume of excavation in cubic yards
The most difficult factors to obtain in this formula are Bottom length and width, and the Mid-point length and width.
To calculate these, one needs to know the side slope ratio at either end of the longitudinal section of the excavation, and the side slope ratios on both sides. For instance, a side slope ratio of 2:1 (a one-foot rise for every two feet horizontal), will produce a two-foot distance for every one foot of depth. A pond of 12-foot depth and 2:1 side slope would create a slope occupying 2 x 12 feet = 24 feet. Similarly, a side slope of 4:1 would create a slope occupying 4 x 12 feet = 48 feet. These two slopes are subtracted from the length of the excavation at ground surface (or top of the pond) to yield the Bottom length and width. The Mid-point length and width uses the same exact slope ratios, but with the depth “D” cut in half (since it’s the mid-point).
This estimate will serve primarily as a basis for soliciting bids from a contractor who will want to know the volume of earth he has to remove. Refer to figure 3.4 in Chapter 3 for Pond Storage Capacity Table
Types of ponds
Bypass ponds
Highly recommended by departments of fisheries, fish & wildlife agencies and conservation authorities, these ponds do not block the movement of fish in a nearby stream. They also do not raise stream temperatures as much as in-stream (or on-stream) ponds. They reduce the opportunity for stream erosion and flooding damage, and if the excavated materials are kept within the flood plain, the exercise in creating the pond is essentially a balanced cut-and-fill operation. Most regulatory agencies require that all work done within the flood plain is by nature a balanced cut-and-fill, thereby not changing the reservoir capacity (flood capacity) of the stream in that location. In-stream ponds become full of silt, eventually from continuous sedimentation due to the reduction in stream velocity through the body of the pond, whereas the bypass type allows some measure of control—particularly during periods of high stream sedimentation from heavy rains, spring melt etc.
The pond site must be chosen carefully so that the pond can receive and discharge water from the adjacent stream/river. This means that attention must be paid to the existing grades, and it may be that in a low gradient stream, the outlet pipe from the pond will have to be very long to attain the necessary gravity drain. A distinct advantage of the bypass pond is the owner’s ability to shut the inflow pipe off during periods of high-flow and high-silt in the supply stream, saving the bypass type of pond from siltation and possibly contamination.
Fig. 5.3 A typical bermed ByPass pond
In-stream ponds created by damming up natural, permanently flowing watercourses are not approved by most land management or water management agencies, and will likely be removed and the owner charged if undertaken without authority. Even the intermittent watercourses are to be avoided since they often provide critical habitat for many wildlife species, and offer insufficient flow for a permanent pond.
Inlet and outlet controls for bypass ponds
Check Dams
In some cases, it will be advisable to install a low height check-dam to increase the water level at the inlet pipe end, but the installation of such a structure creates a small on-stream pond, which places the installation outside the recommended policy of the water management authority. Basically, the damming up of rivers or streams invites flood damage, erosion, and warm-water infusion to downstream habitats, even though there are other beneficial advantages such as fish habitat creation, water conservation, and ground water supplementation—at least at the pond location.
The choice of pond site is critical since it is not normally feasible to construct a bypass pond if the stream banks are excessively steep or high. The lower the stream is below the surrounding grade, the deeper the pond excavation will have to be to reach the water level predetermined by the inlet pipe level in the stream.
The inlet pipe
The recommended size for the inlet pipe is generally no more than one-third of the average summer flow of the stream. The inlet pipe should be raised above the stream bottom to avoid silt transfer into the pond, and also to avoid draw-down of the stream during periods of low flow worsening its drought condition. The pipe should not intrude excessively into the stream channel and the entrance must be protected with water-washed boulders and stone to prevent erosion at this vulnerable point. A concrete headwall, if constructed, will likely require approvals and such a structure will most probably be discouraged by the water management authority. A grating must be placed over the inlet pipe at the stream end to prevent fish from entering or escaping from the pond. This prevents any invasive species or oriental varieties from getting into the stream should a careless landowner stock the pond with an inappropriate fish species (an illegal activity).
Larger, deeper ponds may require several options for flow control such as a slide gate, a butterfly gate, or a concrete/wood headwall
The outlet pipe
The outlet pipe location is just as critical as the inlet because it controls the water level in the pond, and can also serve as a bottom draw-off device (to usher cool water downstream). The capacity of the outlet pipe should be at least 50 per cent larger than the inlet pipe, to carry excess flow from rainfall, runoff, or spring flood conditions. If water conservation is required in the pond, the outlet can be equipped with a valve or gate mechanism. To guarantee a cold water effluent from the pond, a flexible hose/pipe could be run from the mouth of the outlet pipe to the bottom of the pond, weighed down with rock so that it remains on the bottom. Deeper, larger ponds may require a drop inlet type spillway with stop logs or heavy planks which also aid in controlling the water level in the pond.
The outlet pipe should not project excessively into the stream and must also be protected with boulders and other natural erosion control methods (plantings, fascines, etc.)
Isolated dugout ponds
Dug-out or simple excavated ponds are the easiest to build, most often in flat or nearly flat terrain. They usually have a moderate to slow rate of recovery following water removal for irrigation, fire protection, etc., but if constructed deep enough, exposing a minimum water surface area, they overcome evaporation losses quickly. Compared to off-stream bypass ponds, and embankment ponds, they are relatively flood-safe. Two types of excavated ponds exist: one fed largely by surface runoff, and the other fed by groundwater springs or aquifers. Many dugout ponds are replenished by both.
Locating an isolated dugout pond
Often dugout ponds are situated in a drainage way or beside one, where runoff can be diverted into the pond. It is wise not to choose the lowest elevation in a natural drainage way or depression to allow excess water in the pond to flow out and away from the pond during storms or just from natural topping-up by rainfall. As pointed out in earlier chapters, it is not wise to locate a dugout pond in a marsh, fen, or bog, displacing valuable wildlife habitat and inviting an algae or emergent aquatic weed invasion from the former ecosystem.
Test pits
To ascertain ground water supply, aquifer or surface run-off potential to fill the pond, test pits should be dug in your chosen location beforehand. The water level rising in the test pits after a day or two will indicate the expected water level in the future pond. The difference in level between the surface of the water in the pit and the ground surface indicates how much overburden will have to be removed before the contractor reaches the potential surface of water in the pond. Check the recovery rate in the test pits by pumping them out and letting them refill with water. A slow rate of recovery renders the pond rather useless for irrigation or water-taking, since the rate of fresh ground water inflow will be low. This could deter you from digging an expensive hole in the ground resulting in a disappointing low volume of stagnant water. Dig the test pits in mid-summer, to avoid false impressions resulting from a spring high water table usually attributed to snow melt and runoff.
The depth of soil to be excavated above the eventual surface of the pond just increases the construction costs and doesn’t contribute to the eventual planned depth of the pond.
The pond “banks” should not exceed six feet in depth for aesthetic and practical purposes, since their gradient will make access to the water surface difficult to reach unless the banks are sloped back considerably, again increasing excavation and earth removal costs. Steep pond banks will also encourage erosion and siltation without erosion protection.
Soils
Fine-textured clays and silty clays, if extending below the proposed pond bottom, will usually assure a reliable pond site. The same soil types will exhibit the best impervious qualities, avoiding seepage losses. Sandy clays reaching adequate pond depths are also suitable. Limestone bedrock, however, will exhibit a high degree of seepage with cracks, and crevices, as will coarse textured sands or sandy gravel mixtures exhibiting an open structure. Ponds with such permeable bottoms will have to be lined with rubber or butyl liners to retain water which is an expensive undertaking and not very natural. Take a good look around your vicinity for other finished ponds, check the soil type, runoff situation, and depths, to get an indication of your soil’s suitability.
Outlet/overflow pipes
In some dugout ponds, contained within an embankment on sloping lands, an overflow/outlet system should be installed. This will prevent erosion and possible loss of the embankment in the event of a storm, fast snow melt, or extensive rainfall. Burying long steel pipes on a slight angle in the embankment through to its downstream side will allow excess water to trickle out. Three pipes, from four to six inches diameter, will be sufficient. The centre pipe (outlet pipe) should be placed with its mouth at the proposed pond water level, which is usually about two feet below the finished grade of the embankment.
The other two pipes, acting as emergency “spillway” pipes, should be placed about one foot below finished grade of the embankment. They will normally be dry and high above the pond surface, never needed until a storm event. The outlet pipe has three functions:
• maintains a continuous water outlet from the pond
• forestalls the possibility of the water level from reaching the higher elevation of the overflow pipes
• provokes a water intake from the aquifer(s) in the pond bottom, adding cool, fresh water to the pond
Cut the inlet end of the outlet pipe on an angle to form a hood, and install anti-seep collars if installed on a large pond.
The inlet end of the outlet pipe should be screened to retain fish in the pond. The easiest way to do this is to install a wire basket arrangement around the pipe. The top and bottom of the basket is left open, and a hole is cut in the side of the basket made at the exact diameter of the outlet pipe. The basket is fitted over the pipe inlet, the pipe protruding into the basket through the pre-cut hole, while the open bottom of the basket is thrust into the sloping pond bottom. The open top of the basket allows cleaning of the inlet while preventing debris and pond weeds from infesting the inlet.
A long one-inch-diameter plumbing pipe, cut to fit the length of the outlet pipe plus a foot or two, can be thrust up the pipe from the downstream side of the embankment to clean it out.
Excavation, berming, and distribution of material
The use of excavated earth in a floodplain environment may be a necessity depending upon its characteristics. Muck and peat will not make good material for berming. They are easily eroded away by wind and water, and muck could easily be eutrophic which could add harmful bacteria to the floodplain. If the soil excavated is suitable, it can be used for berming the bypass pond shown in Fig. 5.3. It should be placed in layers about one foot thick (0.3 metres), each layer compacted after deposition. The foot or base of berm should be at least 10 feet (0.3 metres) away from the stream or river source. Berm slopes should not be any steeper than 2:1 (one foot rise for every two foot run, for both erosion control and mowing purposes). Should the normal pond level be higher than the water surface in a nearby stream or river, the berm should be constructed in a high percentage clay soil. Certainly the berm core must be of clay construction to avoid any rupture and leakage into the stream.
Overall considerations:
• no more than one-third of the normal stream flow should be diverted through the inlet pipe. It is not good stewardship to take water from a stream during periods of drought or low flow.
• the owner should inspect the pond condition regularly, particularly after a heavy rainfall or flood condition, to relieve the pond of excess volume or to remove blockages from the inlet or outlet pipes.
• grass and weeds on the berm containing the pond should be cut, but only to within 15 feet of water’s edge (maintaining a natural filter and buffer for the littoral zone habitat).
Embankment ponds (ponds with dams)
Detailed site investigation and engineering surveys
Having located the best site for the pond and its dam (embankment), the location should be surveyed with a dumpy level, or with laser level to determine the existing grades and the extent of flooding which might occur. If you are not familiar with survey techniques it will be necessary to hire an engineer or engineering technologist to do this for you. The usual method consists of running a “profile” of the centre-line of the dam and earth spillway from which offsets are taken to estimate existing ground levels within the confines of the proposed pond structure. This step will allow an estimation of the pond capacity. (Refer back to Chapter 4 “Pond Volume Calculations” to see a simple way of calculating approximate pond capacity).
For larger ponds and reservoirs, a complete topographical survey may be needed. This involves running a “line of profile levels” along the centreline of the proposed dam with offset levels up both sides of the containment and beyond the proposed top of dam and earth spillway locations. Similarly, a profile survey should be undertaken along the centreline of the earth spillway. All significant changes in slope should be noted no more than 50 feet apart. These measurements will determine the volume of earth needed to build the dam, establishing the height of the earth spillway and “trickle tube.”
The survey is started from a point on the upstream end that is well below the estimated water surface elevation and existing elevations are determined on this line all the way downstream to a point where water is to be discharged without damage to the dam structure. A bench mark or (reference elevation point) is usually located which will remain undisturbed through the construction process. The bench mark is used as a constant reference for existing and proposed elevations.
Soil studies
The choice of pond site is dependent upon the ability of the soils to hold water. Clays and silty sands are superior, although sandy clays may be satisfactory. Coarse textured sands and sand-gravel mixtures are too pervious and unsuitable. The lack of impervious material can be corrected by sealing with a well-graded, coarse grained soil containing at least 20 per cent clay or adding bentonite (fine textured colloidal clay), thoroughly compacted. Limestone bedrock sites are particularly permeable with crevices, and channels below the upper soil horizon that are not visible on the surface. An inspection of surrounding rural and farm ponds in the area may reveal the soil and sub-soil characteristics, avoiding soil-boring tests.
Dam core and foundation requirements
The dam foundation must be stable and provide resistance to the pressure of a contained reservoir. In this respect it is essential to make soil borings at the proposed dam site. The natural undisturbed banks at the wings of the dam should also be investigated for structural characteristics. If a dam is to be built on rock, boring studies need to done to determine the rock thickness and any fissures or seams through which water can drain.
A “cutoff core” of impervious material should be installed under the proposed dam, or on the upstream face of the dam, composed of a leak-resistant material. Fine textured silts and clays are not stable enough for larger dams illustrated in this chapter. Some relief against water pressure can be achieved by flattening the side slopes of some dams, although this will likely require more fill and regrading. The owner must remove all mucks, peats, and organic matter soils from the dam foundation area. Suitable dam foundation materials possess both stability and imperviousness. A good mixture of course and fine extured soils seem to provide both criteria. These include:
• gravel – sand – clay mixtures
• gravel – sand – silt mixtures
• sand – clay mixtures
• sand – silt mixtures
These same soils prescribed for dam foundations are usually suitable for fill material. Organic silts and clays which exhibit the characteristic of slippage or “slumping” should not be utilized in the foundation work. The optimal specifications for an earthfill dam requires material ranging from small gravel or coarse sand, to fine sand-plus-clay with 20 per cent by weight of clay particles. Stay away from soils with a higher percentage of gravel or coarse sand that are pervious, allowing seepage through the dam. Too high a clay content will also result in swelling when wet, and shrinking on drying. This situation will result in dangerous cracks and potential leakage points.
Some foundation soil conditions will require extensive construction methods not justified for smaller ponds and lakes. The best foundation situation consists of a thick layer of impervious consolidated clay or sand-clay. Removal of all existing topsoil will provide a suitable bond with the clay or sand-clay subsoil. If there is no impervious clay layer at an economical depth, an engineer should be consulted to design the dam since reinforcing the foundation soils will be required to prevent seepage and failure. Similarly, a soil profile consisting of highly plastic clay or similar unconsolidated soil will require more investigation and design for stability. Dams built on bedrock conditions will require anchoring the structure into the rock and most often involve an entire cement structure to prevent slippage or seepage.
Cutoff trench
In situations where alluvial deposits of various pervious sands and gravels exist at the surface, with rock or clay at a greater depth, the possibility of seepage cannot be overlooked. A “cutoff” is needed to adhere to the rock or clay stratum and join with the base of the dam. A compacted, high percentage clay material is what is most often used. A trench is dug along the centerline of the dam down into the impervious layer, extending well into the proposed side abutments. The trench bottom should be no less than eight feet wide with a 1:1 slope. Filling the trench with successive layers of sandy-clay or clay material and compacting each layer will provide a good “core trench.” Keeping the layers moist while compacting is essential.
Top width
Dams less than 10 feet high require a top width of about eight feet, but as the height of the dam increases, this width also increases. If vehicular access is required across the dam, the top width should be at least 14 feet, which will require a much wider base of the dam to maintain the same side slopes.
Side slopes
The percent slope will depend upon both the stability of the fill and the foundation materials. Side slopes can be steeper with a more stable fill, but unstable materials will require a much reduced percent slope – up to 4:1 slope.
Freeboard
The added height of dam above the water surface is called “freeboard.” This added height of structure prevents wave action (produced by winds or floodwaters) from overtopping the dam, particularly if the emergency spillway cannot entirely handle the flow. Heights of freeboard should be as follows:
Longer ponds will require the services of an engineer to calculate freeboard, the pond or lake dependent on a much larger watershed.
Settlement of fill
The earth dam should be constructed somewhat higher than the design specified in order to allow for settlement of fill. If the fill process involves compacting in successive layers under good moisture conditions, there will not be an appreciable amount of settlement in the dam. Most uncompacted foundations have a settlement range from one to six per cent, and the settlement rate for a compacted, layered dam will be about five per cent of the designed dam height. This means that the dam must be built five per cent higher in order to properly contain the final design water level.
Earth fill
After the design has been established, the number of cubic yards (cubic metres) of fill must be estimated to establish the cost of the project. The total estimate of volume will include the allowance for settlement, the volume required to backfill the cutoff trench, and all other holes or depressions in the foundation. There is an efficient method in place to calculate the volume of earth fill called “the sum of end area method” published by the Soil Conservation Service, Agricultural Handbook No. 387.
Plans and specifications
An accurate plan for the pond or lake should illustrate all existing and proposed elevations with dimensions of the dam and cutoff trench. The location of the drop inlet pipe should be shown along with a detailed drawing of its construction complete with dimensions. The plan provides an instrument for obtaining accurate bidding by contractors, and a document to refer to as work progresses, verifying that work specified is being done properly. Sometimes, by hiring a knowledgeable contractor, a simple contour plan of the proposed pond will suffice, but for larger ponds with more complex dam structures, it will be necessary to obtain the services of an engineer to arrive at a more detailed plan.
Staking out the pond
Prior to construction, the proposed waterline should be staked out by a level survey along with the areas to be cleared of trees, etc. This usually involves the dam site, the spillway location, and the total area to be impounded. Stakes for clearing must be placed about 15 feet outside the waterline. Locating the dam involves setting stakes on its centreline about every 50 feet. Following this, the fill and slope stakes should be placed upstream and downstream from the centreline stakes to locate the points where side slopes meet the ground surface, and to locate the outer limits of construction. Locating the earth spillway is done by first staking the spillway centreline, then setting the “cut” and “slope” stakes along the existing ground surface where the spillway side slopes will meet it. The drop inlet “trickle tube” location must be located later along its proposed course once the dam foundation construction is complete enough. The drop inlet should be located on firm ground, preferably supported with a poured concrete footing.
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