Description: A pond is an artificially constructed, open-to-atmosphere body of water supported structurally by earth and filled with run-off, spring flow, and/or water diverted into it from elsewhere. They are built by either constructing a dam wall below an existing water collection point or excavating a depression, and more typically a combination of both. When appropriate, the soil excavated to create the storage volume is utilized to construct the dam wall that creates the impoundment area. Constructed ponds are generally sealed by compacting soil layers with sufficient clay content to create a barrier to prevent the water from infiltrating deeper.

• Ponds vs. Lakes: Ponds are shallower than lakes, such that the entirety of the water body exists in the photic zone – meaning light penetrates all the way to the pond bottom and plants are thus able to grow throughout the entire water column. Lakes are deeper, and have both a photic and aphotic zone. The aphotic zone lacks sufficient light to sustain plant growth, and is generally cooler than the photic zone.

Function(s): Ponds and lakes perform a TON of beneficial functions within a larger ecosystem, and depending on your context there are many different reasons why you might want to design one (or many) into your landscape. Below is a short list of primary functions performed by ponds.

• Water Resilience: Ponds store large quantities of water for a relatively low price that can be used as primary or back up sources for; drinking and other potable use-cases (with appropriate filtration – such as a slow-sand charcoal biofilter); irrigation and fertigation of cropland, pastures, gardens, paddies; and active fire suppression and fire buffers.
• Food Production: Ponds can be shaped and fitted with appropriate gates, channels, benches and habitat to become incredible food production ecosystems.
  • Aquaculture for production of aquatic organisms such as fish, crayfish, shrimp, mollusks, turtles and more.
  • Raft gardens for production of terrestrial crops with enhanced protection from terrestrial pests, enhanced light availability, optimum hydration etc.
  • Littoral zone cultivation of semi-aquatic crops – taro, rice, arrowhead tubers – basically anything that loves to grow in the shallow, inundated edges of a water body.
• Climate Moderation: Large water bodies moderate the climate immediately around them.
  • Water’s surface is highly reflective. If a pond is situated below a food forest on a south-facing slope (in the Northern hemisphere) the light reflected from the pond will increase the light available to the fruit trees. If a home is situated here, it will have extra solar energy reflected onto it at certain times of the year.
  • Winds blowing across the surface of a pond will humidify the local air column. This can create cooler temperatures in downwind zones on warm days, and conversely, create warmer temperatures in downwind zones on cooler days. Water has a high specific-heat capacity and acts as a buffer against extreme temperature swings.
  • Water is an excellent thermal mass – planting tender trees along the edges of a pond is one way to moderate temperatures experienced by the tree and potentially improve production, especially if the tree in question is otherwise planted to the edge of its normal climate range.
• Recreation: Ponds are beautiful! Large bodies of still water create a sense of calm in people, and invite a reflective, introspective state. This is so broadly true of people that Christopher Alexander has numerous patterns that speak directly to the incorporation of still water into human environments in his book A Pattern Language (25 – Access To Water; 59 – Quiet Backs, 64 – Pools and Streams; 71 – Still Water). Ponds provide opportunities for swimming, fishing, throwing the ball for your dogs, floating quietly on a raft or canoe, and just sitting and enjoying the day.
• Ecosystem Enhancement: Ponds create a lot of overlapping edges in a landscape. Edges between different ecotones are where the greatest productivity and biodiversity are found.
  • The edge between the water and the land; terrestrial vs aquatic. Ponds provide excellent opportunities for terrestrial species to get a drink of water, and will quickly become wildlife hotspots wherever they are built.
    • There will also be a band of soil around the pond that wicks up extra moisture via capillary action, making it capable of supporting a diverse array of plant, animal, insect, fungal and microbial species that would not otherwise make it in the drier upland soils and would be too water-logged to live in the littoral zone of the pond. This belt of terrestrial habitat around the water’s edge is generally referred to as the riparian zone.
  • The edge between litoral and limnetic (see Pond Zones image above) – litoral being the area of the pond where the bottom receives enough light to support plant growth (the photic zone), and limnetic zones being the area of the pond where the bottom is deeper and receives too little light to support plant growth (the aphotic zone).
  • Ponds also exhibit temperature stratification within their water columns, which creates yet another edge between habitats capable of supporting different species of flora and fauna.
  • Ponds that are hydrologically connected to the ground around them (lined with clay or native soil, not an impermeable liner or polymer) will function to hydrate the surrounding landscape and recharge aquifers, if slowly.
    • Ponds can be designed to maximize infiltration into groundwater (aquifer) or soil storage, in which case they are typically referred to as infiltration basins.

Types Of Dams

There are many different types of dams suitable to creating ponds in a variety of different topographical contexts to serve many different use cases. This section describes the most common types of dams used to create functional broadacre water systems. Think of this list as a starter pallet of forms for open-body water storage across varied terrain.

*click to jump to specific dam type

Saddle Dams

Usually the highest available storages in the landscape, saddle dams are located on saddles or dips in the skyline profile of ridges or hills. Saddle ponds can be fully excavated (below grade) or built up (above grade, similar to a Turkey Nest or Ring Dam, and may have dam walls on one or many sides and can be oblong, circular or “shark egg” shaped depending on the topography of the saddle, desired storage characteristics and catchment opportunities.

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Ridgepoint Dams

Built on descending ridgelines, ridgepoint dams generally straddle the dividing line between watersheds (the imaginary line bisecting contour that runs down the ridgecrest). Ridgepoint dams are generally shaped like a cloven hoof, boomerang or oblong crescent as they wrap around the ridgecrest. They can be excavated (below grade) or walled in by earth banks (above grade). Ridgepoint dams tend to have a higher cost per volume stored than other dam types because of the necessarily long dam wall and relatively steeper grade behind it (a low ratio of dam wall length: length of water dammed).

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Keypoint Dams

Located in the valleys of secondary streams in humid landscapes, keypoint dams are installed such that the high water line of the pond will sits at the keypoint – the location in the valley where its primary shape goes from convex to concave (i.e. where the valley bottom begins to flatten out and the width between contour lines begins to increase). This location in the valley profile is often (though not always) an advantageous location in the landscape to store water from a cost per volume stored perspective, especially where long, relatively flat valleys narrow suddenly.

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Contour Dams

Contour dams are built on slopes between the ridgecrest and valley bottom. The dam walls can range from convex to concave. The dam wall is constructed to follow contour. Contour dams have a relatively high cost per volume stored because they cannot back up water very far and the dam wall must be constructed around three sides of the impoundment. They are often used for aquaculture, irrigation, and flood control in semi-arid climates. Contour dams can generally be built wherever the grade is 8% or less, and are generally incorporated into a linked series of ponds and water harvesting elements connecting valley bottoms to ridges.

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Valley / Barrier Dams

Valley / barrier dams are constructed in valley bottoms across perennial or seasonal drainages. Because valley / barrier dam impoundments are subject to prolonged or constant inflows, they must be carefully designed and constructed, with large well-armored spillways, substantial and well-compacted keyways, trickle pipes to handle normal discharge flows, and fish ladders to maintain seasonal migration patterns for aquatic life. Because they see prolonged or constant inflows of water, valley / barrier dams are often used for small-scale hydropower generation. They can also be used for irrigation if located high enough above the valley floor.

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Turkey’s Nest / Ring Dams

Located in the valleys of secondary stream in humid landscapes, keypoint dams are installed such that the high water line of the pond will sits at the keypoint – the location in the valley where its primary shape goes from convex to concave (i.e. where it begins to flatten out and the widths between contour lines begins to increase). This location in the valley profile is often (though not always) an advantageous location in the landscape to store water from a cost per volume storage perspective, especially where long, relatively flat valleys narrow suddenly.

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Check Dams

Check dams are not intended to create storages, but rather to regulate or direct stream-flows. They are often used to create small elevation drops in a waterway to pump water (as with a hydraulic ram pump), generate power (water wheel), to divert stream flow into contour earthworks (swales) or irrigation canals, and to buffer and slow flood flows. Check dams that are primarily designed to buffer flood flows are often fitted with a base pipe or a fixed opening that will regulate downstream flow while backing up the flood crest behind the dam to decrease the amplitude and extend the duration of the flood crest (de-energize the flood flow). Such an opening at the base will allow the check dam to self-clean by scouring any silt built up behind the wall, keeping it free of siltation.

In drylands, check dams are frequently built across incised drainages without a base pipe, but are instead constructed of knitted stone so that they leak. This allows water to pass through while creating a sediment basin on the upstream side of the dam, which increases moisture persistence during the long dry season and reduces flood energy during the wet season. These sediment pockets ultimately become valuable growing grounds for crops that appreciate additional moisture compared to what they would receive on upland landscapes.

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Context-Specific Design Criteria for Ponds & Dams

Ponds are generally a good fit for a specific context when:

• The climate provides ample precipitation to fill the water body based on its catchment area.
• The amount of earth moved to create the water body is at a minimum relative to the amount of water held.
  • A helpful way to guage this and compare potential dam sites based on cost per volume of water stored is to compare the ratio between the length of the dam wall and the length of water that will back up behind it. The higher the ratio, the more water is stored per yard of earth moved.
• Annual evaporation is less than annual mean rainfall. (Evaporation < Rainfall)
  • Storing water in open-to-atmosphere storage such as ponds makes sense where evaporation losses are low enough that the storage would remain full enough for long enough to perform its design function (irrigating pasture, watering livestock, providing recreation, fertigation, potable water etc.) before the next resupply.
• There is natural and consistent inbound flow that outweighs demand (year-round rainfall patterns, high-producing and consistent supply).
• The pond can be located above the end-use points. This enables gravity-pressurized flow to desired points of use.
• Soil content has enough clay to create a seal, or specific clay layers are available and affordable to create a seal.

Ponds may not be a good fit for a specific context when:

• The climate has inconsistent rainfall, such as arid desert or Mediterranean, and other climates with a short seasonal concentration of rain followed by a long dry season. You can still make a pond, you’ll just have a dry hole for most of the year.
• There is a low amount of inbound flow (small catchment area or little run-off) that would lead to the water body going dry for portions of the year.
• Annual evaporation is greater than mean rainfall. (Evaporation > Rainfall)
  • In ‘evaporation over rainfall’ climates storing water in an open-to-atmosphere storage such as a pond incurs high water losses due to evaporation. Depending on the design function of the pond, this may or may not be acceptable (i.e. a small garden pond set in a shady, wind-protected corner for pure aesthetic enjoyment vs. a pond whose primary function is to water livestock on open rangeland).
    • In arid climates, ponds with large catchment areas composed of fed by run-off or groundwater with high TDS (total dissolved solids) counts are prone to salt build up in the impoundment due to the high evaporation rates. Salty water can be harmful to livestock and wildlife and is generally not suitable for irrigating crops.
  • Exceptions prove the rule of course – suitable locations with perennial inflows from streams, seeps or springs can be developed into ponds to enhance the local ecology even when evaporation losses exceed rainfall. Ephemeral ponds that stay wet for only a portion of the year also provide valuable riparian habitat, and often host an array of rare and unique species.
• The only suitable location is below the required end-use points.
  • It may still be advantageous for other reasons to install the pond, however an additional and ongoing energy cost will be incurred to deliver the water to where it is needed (pumping, hauling etc).
• Soils lack sufficient clay content to create an effective seal.

General Design & Installation Considerations for Ponds

• DESIGN
  • The design of the pond depends on its intended use. Before bringing in heavy machinery, get clear on why you want a pond and what functions you need it to perform.
  • Multi-purpose ponds seldom fulfill all of their intended uses.
  • Types of Man-Made Ponds and Where to Put Them.
    • Storage Ponds: Open, earth-supported tanks. Should be situated well out of flood plains. For highest water quality, they should have an elevated rim around the entire perimeter so that no uncontrolled run-off enters the impoundment. Storage ponds are typically installed when water quality must be high, and are often lined with EPDM or concrete.
    • Living Ponds: Look like natural ponds. Should also be located out of flood plains. Typically have associated structures that increase functional catchment area and/or allow diversion/drainage (swales, armored drains, trickle pipes, lock pipes, syphons).
    • Run-off Harvesting Ponds: Seasonal ponds that collect run-off that would otherwise be lost and allow it to infiltrate slowly int the soil (i.e. an extra large infiltration basin).
  • Pond Size
    • Pond should be no bigger than necessary for intended use to save on construction costs.
      • Smaller ponds have fewer problems with wind-driven wave erosion on banks.
      • Larger ponds store more water at lower per unit cost.
  • Pond Depth: Ideal pond depth is a function of intended use and local climate conditions.
    • For Water Storage: Deeper is better – at least 12′ and up to a maximum of ~20′.
    • For Living Ponds: Depth should be between 8-15′ in at least 25% of the basin. In general, colder climates should have deeper ponds, up to a maximum of ~15′.
      • Beyond 15′ of depth there may be a deep zone without oxygen, which if the pond “turns over” – i.e. the water layers swap position due to a change in relative temperature – can mix with upper layers and kill fish and other aquatic life.
    • For Run-off Harvesting Ponds: Typically shallow, max 5-6′ deep. Shallower is better especially if they are in water courses to minimize risk of catastrophic failure.
• SITING: The criteria below should be evaluated when assessing locations in the landscape for constructing ponds and dams.
  • Soil type: What type of soils are located within the proposed basin and dam wall footprints? What is the clay content? Generally looking for 20-40% clay content to be able to create dam walls and pond bottoms that can be sealed with heavy machinery.
  • Grade behind dam: The shallower the grade, the further water will be backed up per unit of dam height. Locations with shallower grades behind the dam will create larger storage volumes than locations with steeper grades for the same amount of soil moved.
  • Downstream safety: What are the downstream risks of a dam failure?
  • Height above use points: Is the storage located high enough to move water with gravity and at requisite pressures and flow rates to desired end-use points?
  • Available catchment / diversion: There must be ample catchment area to fill and supply the pond basin for it’s designed function(s).
    • Annual rainfall distribution, intensity, amount and the nature of the soils constituting the contributing watershed must all be taken into account to decide what size catchment area will be required to serve a given pond storage volume.
• DAM WALL CONSTRUCTION
  • Key
    • The key / keyway / key trench is a trench dug in the center of the dam wall footprint deeper than the bottom of the dam wall (generally deeper than the bottom of the storage, though not always). This trench is then backfilled with compacted lifts of soil with good clay content to create a compacted, subterranean cut-off wall to impede subsurface flows from moving underneath the dam.
      • Recommended for all walls 6’+ in height.
      • Depth of Key: At minimum, 2-3′ deep, proportionally deeper for larger dams.
  • Wall
    • Should have a 1:3 inside slope and a 1:2.5 to a 1:2 slope outside at the steepest.
    • The freeboard (distance between the height of the spillway and the top of the dam wall) should be 3 feet.
    • Build the wall in 8-10″ lifts of soils, compacting each lift with proper moisture levels and a heavy, tracked machine (dozer, excavator, sheeps foot roller etc).
    • In ponds large enough to experience significant wave action generated by winds, line the wall with tightly-knit rock to prevent wave damage to the dam wall at and below the water line.

• INFLOW FILTRATION
  • Constructing sediment traps – small basins designed to create standing pools of water and allow any carried bedload to fall out of the water column before emptying into the larger storage – is recommended anytime there is a concentrated flow entering a pond. This sediment trap will clean the water entering the pond, which will decrease or eliminate the frequency of dredging required to maintain its volume. Sediment traps should be sited for easy access with hand tools or machinery (depending on scale) to allow for seasonal removal of sediments building up. These sediments (effectively the building blocks of flood plain soil) can be incorporated into other growing systems throughout the landscape.
• OVERFLOW DISCHARGE
  • All ponds MUST have at least an emergency level-sill spillway built away from the dam wall to safely discharge overflow during heavy precipitation events.
    • For ponds with constant or prolonged overflow (those fed by perennial streams, seeps, or springs or roof catchments that generate large volumes of run-off), trickle pipes should be installed to keep spillways dry and in good condition for use with heavy outflows only.
  • The spillway should always be located away from the dam wall, and NEVER on it. Ensure that any water spilled from the spillway will not find its way back towards the toe of the dam before flowing down-watershed.
• DRAINAGE
  • There should be some way for the entire pond to be drained. A lock pipe built into the bottom of the dam wall, outfitted with a monk to adjust standing water level (can also function as a trickle pipe), or a syphon (preferred option for highest dam wall integrity).
• SEALING THE POND
  • Ponds built in locations with 20-40% clay content of soil can be sealed with the heavy machinery used in construction.
  • For ponds that lack adequate clay content, there are a number of other ways to seal the pond bottom and/or a leaky dam wall:
    • Gleying: a 6-9″ layer of fresh green manure or sappy, wet, green plant material is lain down across the leaky zone or the entire pond bottom and interior dam wall face, and covered with a plastic sheet, earth, thick wet clay, thick wet paper or cardboard and allowed to ferment anaerobically for 2-10 days or until completely digested (length of fermentation will depend on climate factors). Gleying produces a bacterial slime which permanently seals soil, sand and small gravels from water penetration. Once fermentation is complete, the pond is filled or pumped full, and the plastic or paper layers can be removed.
      • Gleying can also be done by penning animals (cattle or hogs) into the pond basin to concentrate their manuring and hoof action / wallowing. This method is capable of sealing ponds even in sandy or gravelly soils. See Mike Newby’s thread on Permies for a well-documented example of gleying a pond with pigs in nearly pure gravel and stony soils.
      • Bentonite Clay: Also known as sodium bentonite or driller’s mud, is a fine, super-expansive clay that can be added to an empty or full pond to help seal leaks. If the pond is empty or capable of being drained below the level of the known leak area, the area to be treated should be excavated 4-6” below the surface, and a blanket layer of bentonite applied at 1-2 pounds per square foot. This area should then be covered again with the excavated soil and compacted. Alternately, if the leak area is not as well defined, the pond bottom can have bentonite applied at 1-3 pounds per square foot in the suspected leaky area and the bentonite can be rototilled into the top 3-5″ of the soil. Bentonite can be used to seal entire ponds, but this gets expensive very quickly.
      • Barite: When it is not practical to drain a pond below the level of the leak, barite can be applied by pouring it into the water over the suspected leak area. Barite is a mineral substance about twice as heavy as bentonite clay that is often used in oil drilling and mining operations to form gel-like plugs on earthen impoundment dams. It is a naturally occurring mineral, however precautions should be observed when applying it, as the dust contains high amounts of silica which can irritate lungs and eyes. Barite is effective for stopping seepage loss, however it will not plug larger holes made by tree roots or burrowing animals.
      • Clay: Expensive to bring in, however if a clay deposit is located nearby, it can be applied in lifts 9-12″ thick above trouble spots or to the entirety of the pond bottom to create a seal.
      • Impermeable Membranes: Typically made of EPDM, these membranes are effective at creating a sealed basin. Liners should be underlined and overlined with geotextile fabric with at least 6″ of sand or soil placed on top of that to minimize the risk of puncture. Pond liners come with a number of disadvantages; 1) they are expensive, 2) a single hole in the membrane will drain the entire pond, 3) they are subject to photo-degradation over time, and have a limited lifespan, 4) they prevent hydrological connection of the pond basin with the adjacent landscape – this means no vegetated riparian buffer and no groundwater recharge to the parent aquifer. Pond depths within the photic zone will not vegetated unless overlain with soil for aquatic plants to root into, and then plants will have to be managed carefully to ensure that no species with roots capable of puncturing the liner take root. Bottom Line: avoid artificial liners if possible to maximize ecological benefit and minimize future maintenance costs.

• POND MAINTENANCE
  • Dam walls and levees must be protected from tunneling and diging creatures (on the aquatic side muskrats, crayfish, beavers, and on the dry side gophers, ground squirrels, groundhogs etc).
  • Aquatic plants needs to be controlled. Steeper sides will generally discourage over-establishment of aggressive aquatic “weed” species like Typha spp. Seasonally lowering the water level and removing dead or weak plants will limit excess organic matter build up in the water body that could lead to eutrophication (algal blooms, anaerobic water etc).
  • Ponds may require occasional dredging. Limit the potential for sedimentation by installing sediment traps at inflow points and maintaining a filter strip of perennial vegetation all around the pond edge that contributes run-off. Also, consider using a syphon or lock pipe to scour out accumulated bottom sediment in place of draining the pond and dredging it with heavy equipment (killing most of your desirable biology in the process).
  • Prevent woody or otherwise tap-rooted plants from establishing on the dam wall. Roots penetrating the dam wall will raise the phreatic surface, creating opportunities for piping and animal burrowing along dead root lines and, in the case of trees on the dam face, risk of blow-over induced gouging of the dam wall. Non-woody plants with shallow, mat-forming roots (bamboo) are ideal for planting on dam walls.

Resources for Continued Learning about Ponds & Dams

Description: Tanks and cisterns are enclosed containers designed to store a specific volume of water. A cistern is a tank without an outlet at the bottom. The design considerations for each are quite similar, and the terms will be used interchangeably throughout this post.

A well-designed tank offers nearly complete control of storage conditions, including:

• valve-controlled inlets and outlets
• minimal or no evaporative losses
• shade from direct sunlight to prevent the growth of algae
• exclusion of mosquitoes and other vermin
• security against leakage
• water temperature (in some cases, depending on tank location)

Function(s): Storage of potable, high-quality water for domestic supply, fire suppression, and emergency use. Because the cost-per-volume-stored is so much greater in tanks than earthen ponds, tank storages are typically prioritized for storing high-quality water that has more potential use cases than water in open-surface storages.

Context-Specific Design Criteria for Tanks & Cisterns

Tanks & Cisterns are generally a good fit for a specific context when:

• You want more security than a direct connection to your source can provide (i.e. a pipe stuck into a spring flowing directly to your house = little to no buffer supply).
• The yield of the source cannot directly provide for peak demand (this is frequently the case with lower-yielding sources, such as springs)
• The yield of the source is less than required for firefighting (fire fighting equipment requires both high pressure and high flow to function properly – tanks are one of the best ways to achieve both of these criteria).
• The source is less secure (has more points of failure) than water stored in a tank (i.e. if the source requires pumping to the tank, while the tank can gravity flow to points of use).
• The distance to the source is so far that it is more economical to use a smaller diameter pipe and collect only a portion of peak source flow and store it in a tank much closer to end-use points than to run a large diameter pipe that can accommodate enough flow to meet peak demands at end-use points.

Tanks & Cisterns may not be a good fit for a specific context when:

• The yield of the source exceeds peak demand (i.e. storage not necessary to meet peak flow requirements). This includes emergency use cases such as firefighting.
• Source is intermittent enough that the size of tank required to store enough water to meet usage requirements between resupply events is too great. In such situation aquifer storage is the only cost -effective means to store enough water to meet usage between resupply events.
• The source is located very close to points of use and available in amounts that exceed peak demand.
• The primary use is for irrigation. Infiltration and soil storage should be optimized first to meet irrigation needs, and only then should supplemental storage options for bulk water be considered.

General Design & Installation Considerations for Tanks & Cisterns

• Tank Components Overview
  • Inlet: Where water flow into the tank.
    • If multiple sources feed the same tank, each source should have its own inlet.
    • If the water source is a well, or otherwise located below the tank, the inlet must have an air gap and should be set at least one full pipe diameter higher than the overflow when discharging at fully capacity in order to avoid siphoning through the inlet line and possible contaminating the well.
    • If the water source is gravity-flow water, the inlet can be located anywhere – the inlet water cannot flow back uphill and will not pose a contamination risk to the source.
  • Outlet: The pipe that distributes water from the tank.
    • Outlet pipes should be as close to the bottom of the tank as possible without being so log that it sucks settled muck off the bottom. This maximizes storage capacity and allows for settling to clarify the water.
    • Create an emergency reserve (for fire fighting or other purpose) by installing a low outlet for reserve and a middle outlet for normal use.
    • Each outlet should have a shut off valve.
  • Service Access: A way to get inside the tank for inspection, maintenance and cleaning.
    • An opening 2′ wide will allow an adult-sized body to get in and out of the tank.
    • Tall tanks should have an integrated ladder for convenience and safety. This reduces the chance of introducing contamination from an external ladder.
  • Drain: How to get the sludge, last remaining water and wash water out after cleaning a tank.
    • Especially important if water source contains sediment – a good drain can make sediment removal and cleaning much easier, and thus more likely to happen regularly, which equates to better tank condition and higher-quality water.
    • Tank pads or floors should be sloped ever so slightly (.5% to 1%) towards the drain to facilitate complete drainage of the tank by gravity flow.
      • Floors can be retrofitted to have a slope by pouring a concrete floor inside a flat-bottomed tank.
    • Tanks will drain much faster and more completely with a sump (low area) around the drain to facilitate flow towars the exit.
    • Drain pipes should be a minimum of two pipe sizes larger than the inlet to facilitate efficient draining.
  • Overflow mechanism: The overflow establishes the maximum water level and carries away excess water.
    • Should be located as high as possible while leaving room for an air gap between the max water level and the inlet level.
    • Should be large enough to accommodate full flow from all inlets and sources. High pressure inlets (such as from a well pump) will require larger overflow pipes.
  • Vermin-exclusion: All points of ingress to the storage tank should exclude insects, rodents, and birds.
    • Entrance points should be blocked with mosquito net, welded wire mess, close valves, check valves, water seals or a forceful outflow of water.
    • Water traps (like a P-trap in typical sink plumbing) will stop aquatic insects like mosquitoes from gaining entry, but it will not stop rats.
  • Air venting: To prevent a vacuum from developing
    • If air can’t exit the tank, water can’t get in.
    • Air vents should be screened against rodents and insects.
    • Air vents should be designed to prevent direct or reflected light from shining on the stored water.
    • Vents should preclude roof runoff from entering the tank.
    • Vents don’t need to be huge – just enough to allow full inflow from the inlets.
  • Sunscreen: To keep direct and indirect light from shining on the water.
    • Almost all water has enough residual nutrient or contaminants to fuel algal growth – if sunlight is present.

• SITING:
  • The location of your water tank will largely determine:
    • Which parts of your land can be supplied with tank water by gravity flow
    • The amount of pressure at any given point in the system
    • The length and cost of pipe runs, control-wire runs and line-of-sight if using radio-controlled pumps or valves
    • How visible your tank will be in amongst the surrounding landscape
    • Vulnerability of the tanks and all connection plumbing and piping to hazards such as falling trees, landslides, falling rocks, and animal impacts
    • The size of tank that is feasible to build (tighter spaces = smaller tanks)
    • Ease of construction and maintenance access
  • When siting your water tank(s), take into consideration:
    • Elevation: Whenever possible, locate your tank above desired end-use points at such a height that gravity will supply all the pressure required for the desired use(s). The differential in height between your tank outlet location and the desired end use point will determine how much pressure will be available at that use point (minus losses due to pipe friction as well). Site tanks as high as they need to be to supply required pressures, but not higher. This will save on piping costs, eliminate the need for pressure regulators and, if you are pumping up to the tank, reduce the lifetime electrical bill for pumping requirements (unless of course you are using freely available energy to pump your water – such as a ram pump, river-powered Archimedes screw pump, or bunyip pump.
      • At 62^oF, one foot of water = .433 pounds per square inch (psi). See the table below for some helpful psi/ft.hd. calculations.
      • NOTE: In general, if your tank is 100′ (10 stories, ~43 psi) above your use point, appliances like your washing machine, RO water purifier and on-demand hot water heater will start to work properly. At 230′ (23 stories, ~ 99 psi) a fire hose will function optimally. Remember, appliances like tubs and sinks need flow, not necessarily high pressure. If available pressure based on available tank locations will be low, use larger diameter pipes to connect to these appliances to increase available flow.
    • If no elevated tank sites are available to produce the pressures your require, you can always construct a water tower. Water is very heavy at 8.34 lbs/gallon – ensure that any water tower is properly engineered to withstand designed loads.

• • Slope and Soil Stability: Stable and solid ground is key for the stability and security of your tank. Water tanks have a relatively low load per unit area – the human foot can easily place much higher point loads on the soil – however it is the aggregate load placed upon a large, contiguous area of soil by a water tank that can degrade a tank pad and send your tank down into the valley below.
    • If installing your tank on a slope (very common), install the tank on the cut portion of the excavation, not the fill. For very large tanks, or any tank that has to be placed on fill soil, it is best to consult an engineer.
  • Aesthetics / Visibility: Water tanks can stick out like sore thumbs in the landscape, especially when their profile breaks the natural silhouette of the ridge line or horizon. To preserve views, keep water tanks below the ridge crest or horizon line – this can save money on pumping or piping as well. If you have to put your tank on top of the hill, consider a partially or completely buried tank, ring it with trees or other vegetation, and/or make it a viewing deck to enjoy the long views provided.
    • Pros vs. Cons for Buried Tanks
      • Pros: less obstrusive, cooler, completely sunblocked, more secure against accidental drainage, considerable frost protection.
      • Cons: can’t install a gravity drain – unless installed on a steep hill, usually requires a pump to get the water out, design is structurally more challenging (tank must be engineered to resist the forces of the earth pushing in as well as the water pushing out), limits choice of materials to thos that don’t corrode, can be a safety hazard (easier to fall in if you don’t first have to climb on top), increased contamination risk from surrounding surface run-off or soil seepage, inspection, repair and replacement are more challenging.
  • Security: Location your tank downstream from any hazards – rivers that flood, rockfall or landslide areas, large trees that can fall and break lines – that lie between you and your water source. The fewer hazards between your tank and you, the more water secure you will be.
• SIZING YOUR TANK: The size of your storage tank is one of the main factors that will determine under what circumstances you find yourself without water and for how long, and how much your storage system will cost.
  • Sizing For Security: Size your tank volume according to your desired degree of water security within your budget and other limiting factors.
    • List your reasons for wanting water storage, as they will drive the calculation of tank size:
      • You want more security than a direct connection to your source can provide (i.e. a pipe stuck into a spring flowing directly to your house).
      • The yield of the source cannot directly provide for peak demand (this is frequently the case with lower-yielding sources, such as springs)
      • The yield of the source is less than required for firefighting (fire fighting equipment requires both high pressure and high flow to function properly – tanks are one of the best ways to achieve both of these criteria).
      • The source is less secure than water stored in a tank (i.e. if the source requires pumping to the tank, while the tank can gravity flow to points of use).
      • The distance to the source is so far that it is more economical to use a smaller diameter pipe and collect only a portion of peak source flow and store it in a tank much closer to end-use points than to run a large diameter pipe that can accommodate enough flow to meet peak demands at end-use points.
    • In general, the more storage you have the longer you can go following an interruption in supply, and the more water secure you will be.
  • Different Contexts For Sizing Your Tank
    • When Peak Demand Exceeds Flow From Source
      • Water use is not evenly spread throughout the day. The vast majority of use occurs during 10-12 hours of the day, within which even smaller windows of time account for the majority of use.
      • In this situation, water provided by the source during low-demand periods (e.g. overnight) can be stored for use during high-demand periods (e.g. evenings or mornings).
    • Limited Water Supply With Scheduled Use
      • Size the tank cover production from the source during times without water use.
    • Sizing a Tank to Cover Use During Supply Interruption
      • Consider what is likely to jeopardize your supply and for how long, and build storage that will cover typical, or even rationed use, for the anticipated duration of the interruption.
        • NOTE: If you are aware that your water supply has been interrupted, it is easy to stretch reserves quite a bit longer through conservation – the key is being alerted to when your supply is interrupted! For this reason it can be advantageous to install two different outlets at the bottom of the tank – the standard supply outlet, and a bit lower down, the emergency reserve outlet. If your standard supply outlet stops yielding water, then you know that you have whatever that emergency reserve volume is left in your tank and that there has been a supply interruption.
          • Also – if facing yearlong or multi-year drought, the only storage large enough to cover use during these long periods in the vast majority of cases will be aquifer storage.
    • Production Is Intermittent (e.g., harvesting rainwater)
      • Size tank to cover the maximum accumulated deficit between production and consumption.
        • To do this, create a simple chart:
          • Plot a bar graph of average runoff from your roof catchment area(s) by month.
          • Plot a cumulative runoff, by adding each monthly figure to all the previous ones.
          • Draw a line representing mean daily use that accumulates by day, over the course of an entire year.
          • In order to never run out of water, the mean daily consumption line must never cross the cumulative runoff line. If it does, this is where you’d be out of water.
    • Sizing A Tank For Firefighting
      • Tanks for firefighting come with all manner of legal requirements and regulations that must be met to ensure their performance when it matters most. Specific requirements will differ depending on your jurisdiction. Below is the general equation used to size fire storage tanks:
        • SSR = NFF + MDC – PC – ES – SS – FDS, where…
          • SSR = Storage Supply Required
          • NFF = Needed Fire Flow
          • MDC = Maximum Daily Consumption
          • PC = Production Capacity (of the water source)
          • ES = Emergency Supply (water that can be brought into the system from connections with other systems)
          • SS = Suction Supply (the supply that can be taken from nearby ponds or open-surface water bodies during teh fire)
          • FDS = Fire Department Supply (water that can be brought in by fire trucks)
  • Structural Integrity Of Your Tank
    • Larger tanks = bigger structural engineering issues.
      • A 10,000 gallon tank requires serious consideration of the loads that will be operating on it and the tank pad
      • Any tank above 30,000 gallons should be professionally engineered.

• TANK SHAPE
  • Tank shape determines how the material will resist the applied forces. Shape effects:
    • materials efficiency (how much material it takes to contain a given volume of water)
    • structural efficiency (how easily the tank material can resist the loads applied to it)
    • how much elevation (pressure) is lost between the top of the tank and the bottom
    • ease of fabrication
    • ease of fitting a given volume of water into a specific location
  • Generally speaking, square or rectangular tank shapes are less efficient structurally and in their use of material than cylindrical or round tanks.
    • An egg shape uses the least material to enclose the most water, and is most structurally efficient – it just doesn’t stand up well on its own.
  • Classic tank shape – cylindrical, about as wide as it is tall, with a domed roof and flat bottom, is a good combination of structural and material efficiency, and is the best choice in most cases. Some exceptions:
    • If you have very little fall: In low pressure situations when each foot of head matters, use tanks that are wide and short so you lose less height between the inlet and outlet while keeping the same storage volume.
    • If the tank is pressurized: A sphere or cylindrical tank with rounded ends is most structurally efficient.
    • Tanks designed to be buried: These tanks will often have unique shapes to resist the pressure of the earth pressing in on the tank in addition to the water pressing out.
    • Tower tanks: Because water tower tanks do not need to be buried or sit on the ground, they are often engineered in materially efficient egg or sphere shapes.
    • In tight spaces: Tall cylinders and slimline tanks have a small footprint relative to their storage volume.
  • Visit a site like Tank-Depot.com to get an idea for the many different tank shapes, sizes and forms available for different applications.
• TANK MATERIALS (from Art Ludwig’s Water Storage – Tanks, Cisterns, Aquifers and Ponds – pgs. 39-48)
  • The following materials are known to be hazardous and should be avoided:
    • PVC exposed to sunlight.
    • Pre-1997 PVC – was made with more of the leachable plasticizers.
    • PVC trash cans – plasticizers, again.
    • Pre-1980 tank coatings – these include coal tare and lead-based paint – good for stopping corrosion but not so good for us.
    • Lead pipe and pre-1987 lead-soldered copper pipe – pre-’87 solders were up to 50% lead, now they are typically just 2%.
    • Western Red Cedar – the compounds that make it rot-proof are toxic if ingested.
    • Fly ash in concrete – leaches out when exposed to acidic water (which rainwater is naturally).
  • Ferrocement
    • Nearly the same durability and strength as concrete at a fraction of the materials use, with complete flexibility in shape.
    • Ferrocement tanks are constructed from a grid, typically made of a rebar cage with overlapping steel mesh, onto which a sand/cement mix is applied. The resulting wall is typically only 1.5-3″ thick and is quite strong, particularly if it is curved.
    • Ferrocement tanks cannot be moved – they are built to fit a given site.
    • Ferrocement tanks are labor intensive, and you’ll typically have to DIY it as they aren’t very well known.
    • Ferrocement tanks should be built using NSF 61 certified cement and cement sealers. There is little concern about leaching after it has cured, which is mostly complete within 30 days.
  • Galvanized Steel
    • High strength, medium durability, fire resistant, good transportability.
    • Thicker metal is better. Corrugation generally indicated thinner metal.
    • Corrosion will eventually win, an the tank will have to be replaced.
    • Any damage to the galvanized zinc layer will accelerate tank degradation.
  • Rock & Mortar
    • Uncommon in the industrialized world. Attractive option where labor is cheap, rock is accessible nearby, and a small bit of inevitable leakage will not be a problem. They are not a good choice for seismically active areas.
  • Brick
    • Good for small, square tanks. Much easier to work with than natural rock, and requires less material. Insides are plastered just like ferrocement.
  • Clay
    • Heavy and brittle, but beautiful, clay tanks are excellent for storing drinking water.
  • Wood
    • Common in the past. Wood expands when wet, steel hoops are used to contain the expansion and thus create self-sealing joints. Most commonly made of redwood, cedar or oak.
    • Main limitation is that if the water level drops and the boards, particularly the floor, dries out, the tank may not seal again properly and the tank will cease to function. Cedar tanks are not suitable for potable water storage.
  • Plastic
    • Low cost, lightweight, impervious. Good choice for small- and medium-sized tanks for residences and farms. They are not available in very large sizes.
    • HDPE (high-density polyethylene) is best. Avoid PVC. Tanks should be kept shaded regardless.
    • EPDM (ethylene propylene diene monomer) is the best artificial pond liner – a synthetic rubber that is heat-resistant and stable in the presence of ozone and UV light. This material is able to stretch without tearing (to a degree). EPDM is generally considered to be inert, and is not known to pose a leaching risk
  • Fiberglass (Glass Fiber-Reinforced Polyester, GRP)
    • Fiberglass tanks are very strong, lightweight, and non-corrosive. Stronger and more expensive than HDPE, and generally considered higher quality. Superior to HDPE for underground tanks due to its high strength. Nasty solvents are used in the resin to make fiberglass, and leaching is a concern.
  • Epoxy-Coated Steel or Concrete
    • Good choice for a large, durable tank, often used in large municipal tanks. There is some concern about leaching from the epoxy coatings (which should meet the aforementioned NSF 61 standard for potable water).
  • Masonry in and over Plastic
    • Plastic tank with ferrocement or masonry exterior. This provides complete UV protection and better protection against puncture for the plastic tank. Masonry can be dry stacked rock or stucco lain over a chicken wire mesh. Masonry should be added around the outside with a full tank of warm water so that the tank and not the masonry take the tension load.
  • Galvanized Steel with Plastic Membrane
    • Enhances the life span of lightweight galvanized tank. Trade-off is losing the flexibility to add inlets and outlets, reducing repairability and maintainability, and increased difficulty of getting a good drain.
• FOOTINGS, PADS, & FLOORS
  • General footing/tank pad consideration that apply to all tanks:
    • The earth underneath the tank should be well-compacted and free of sharp or large stones.
    • The surface of the tank pad should drain away from the tank in all directions.
    • Tanks on benches cut into a hillside should be resting entirely on firm, undisturbed (cut) soil rather than on loose soil from the excavation (fill).
  • Steel Tanks
    • should be set on a bed of gravel to slow corrosion of the bottom of the tank by keeping it dry. The earth underneath the gravel should be shaped to drain away from the tank. Coarse gravel is preferred to promote rapid drying.
  • Plastic / Fiberglass Tanks
    • Can be set directly on the surface of rock-free soil.
    • If rocks are present or surface is uneven, a layer of compacted sand or pea gravel that won’t wash away (retained in place by a wooden frame or large stones). Coarse gravel is not recommended because plastic is not strong enough to bridge the larger gaps between this kind of aggregate.
    • For tanks which exert 800 lbs/ square foot or more of force, a concrete footing with steel support stands is recommended to prevent movement in wind or seismic activity.
  • Ferrocement Tanks
    • Floors can be poured directly onto firm soil free of rocks. If too unevent, smooth with a layer of tamped sand.
    • Tank walls should be slightly below grade to prevent water from scouring the soil around the edge and potentially causing a crack to form.
  • Plastic, Fiberglass, and Ferrocement Tanks
    • Can be partially buried – assuming the tank is capable of withstanding the pressure of the soil on the outside without collapsing.

Resources for Continued Learning about Tanks & Cisterns

Description: An aquifer is an underground reservoir of water where water has saturated all of the space between soil particles. Aquifers have a definite level, just like a lake. Water can be withdrawn from aquifers via wells, springs, artesian wells or horizontal wells. Aquifers recharge very slowly – typically at rates of 0.1 to 0.3% per year. Good management is essential to avoid overdraft.

Types of Aquifers

• Perched Aquifer: An aquifer “perched” on top of a confining (impermeable) layer, with air in the soil space below it.
• Artesian Well (or spring): A well or spring pressurized by an aquifer confined above and below by impermeable layers.
• Gravity Spring: A spring which drains directly from saturated soil space above it, with no confinginlayer that could contain pressure were one to plug the spring.
• Fissured aquifer: Groundwater formation with bulk flow of water through cracks in bedrock.
• Fossil aquifers: A term used to describe ancient aquifers that were filled in the distant past and have no appreciable means of recharge. Such aquifers are similar to crude oil deposits – they are a mined resource, and once they are used up, they are gone “forever”, at least as far as human civilization is concerned.

The quality and quantity of water in aquifers depends on the wider community. Aquifers are vulnerable to contamination from the myriad of modern wastes that seep down through the soil. Living soils are effective filters against biological pathogens, however artificial, man-made chemical toxins can pass right through this living filter (often degrading or destroying its function) and into groundwater storages.

Function(s): Storage of water between soil particles, or in cracks between rocks.

Context-Specific Design Criteria for Aquifer Storage

Aquifer Storage are generally a good fit for a specific context when:

• Aquifer storage is the only cost-effective water storage option when the period between resupply events (rainfall) reaches a year or multiple years in length.

Aquifer storage may not be a good fit for a specific context when:

• Water in the aquifer is known to be contaminated with chemical pollutants, heavy metals etc. Diverting good water into a polluted storage will only create more polluted water.
• Aquifer is “uncontained” – i.e. any water added to the aquifer is unlikely to be available when needed because it will flow away from the point of recharge. This is a major part of the tragedy of the commons when it comes to relying upon shared aquifers – any one single recharge won’t change the local situation enough to make it worthwhile – instead it takes a community-wide effort, which requires proper incentives, awareness and action.

General Design & Installation Considerations for Aquifer Storage

• Ways to Increase the Amount of Water in Your Aquifer
  • withdraw less
  • increase rainfall infiltration coefficient (the percentage of rain that soaks into the aquifer)
  • detain water in infiltration basins, swales, and unlined ponds.
  • infiltrate water through creek beds and river beds
  • inject water into wells
• Avoid leaving wells uncapped, as this is a potential entry point for contaminants, pollutants, dead animals, feces etc to enter the groundwater supply.
• In locations where septic effluent is generated, treat the waste water using an aerobic aquatic system to kill anaerobic micro-organisms prior to discharging the water into the soil. Wherever possible, treat effluent using living roots and in the upper layers of the soil where aerobic biology can break down potential toxins and kill any anaerobic pathogens before commingling with existing groundwater.

Resources for Continued Learning about Aquifers