What are people’s opinions on the use of waterway=dam to map “dams” on rural properties?
In my mind, if an object is tagged as a dam I expect it to made of engineered concrete (or maybe stone or earth) and designed by a civi engineer. I’d probably expect it to be on a river, and not an intermittent stream or a gully on a hillside.
Where I noticed this happening was on the western edge of Canberra. It may happen elsewhere, but this is just an opinon gathering exercise.
But regardless, Tag:waterway=dam - OpenStreetMap Wiki says “A barrier built across a river or stream to impound the water.” In these cases there’s nothing “built” however the dam/reservoir likely was created by building up the earth to in effect create a dam wall.
You could add material=soil to the waterway=dam, that would help set them apart from the concrete dam walls.
Thanks Andrew - most farm water storage is not located on a river or stream, but a gully. So the wiki is correct for larger waterways, but maybe not for intermittent waterways or depressions in the landscape that rarely flow?
Pretty sure I have never mapped a “large” dam, but many a farm/roadside reservoir I have eyeballed. To add to the mix I have also done some (concrete etc) weir walls. To my thinking the reservoir itself is the body of water and the “dam” or “weir” is (only) a more complex civil works structure seen on larger works. I would never place a “wall” on a smaller farm structure. May also be an implied permanence situation. Farm sites can breach or be made redundant in a short time.
The artificial lake created by the water impounded behind the dam wall (commonly called a ‘dam’ in Australia) should be tagged as natural=water + water=reservoir or water=pond as appropriate.
That page also takes the view that the “dam” is the barrier and the contained water is the reservoir.
I agree with @AntBurnett that this type of dam is not usually across a stream. Often not even a gully. They often simply cut into the water table so it’s more similar to a big open well rather than something to catch rain.
Anyhow, I think care should be taken in discerning whether these are engineered or constructed. Some of these farm type dams are collossal constructions. The walls are composed of a heavy clay soil. They require maintenance from time to time to clear out the silty mud that accumulates. They’re not simply a ditch for the water to be in, but they’re also not engineered from concrete.
IMO, the water should be tagged according to the direction from the wiki, and the dam wall should be tagged at the discretion of the mapper with reference to it’s size or significance.
The other thing I would consider is if the object has a clearly defined spillway. Its usually a sign that its been engineered more than just a farmer pushing up a wall to collect some runoff. In that case I would be tempted to add the dam, and maybe even the spillway.
The vast majority of farm ‘dams’ I have added have simply been marked as reservoirs,
I’m dropping in from the US but it’s extremely common for mappers to map small earthworks that impound water as waterway=dam. The addition of high resolution LIDAR imagery makes it easy to see if it’s just a hole or there’s a built up portion along one side.
I’m also from the US. I wanted to post some examples of dams I’ve looked at, if that’s at all helpful. These three seem like earthworks with a pipe running under or through them. Maybe they should have a tag to differentiate from concrete, masonry, and metal dams? The American west has some huge ones.
To me, the Australian “dam” definitely fits more as water=reservoir or water=pond. Normally most of our terminology fits well with the very British based terminology OSM uses, but this is definitely one of the outliers.
Having grown up in western NSW, I am intimately familiar with the term “dam” or “tank” being used to describe the earthen excavations intended to capture water for use by livestock. (Whether or not they have good yabbies can be a source of pride among land owners and family parties, usually with the teenagers wading in to the “tank” as the harvesters :-))
As discussed earlier, they typically depend on rainfall in the nearby area to flow across the ground and into the “tank”. While a watercourse may exist, it would be best described as intermittent, and might only be visible as a shallow depression when standing there and observing closely, so wouldn’t warrant mapping as a watercourse.
Having lived in other countries, such as the US, a “dry pond” would be a largely similar feature in an urban setting using mid-Atlantic terminology.
Thanks donmac703 for the fast reply - I think I sidetracked my own topic
There seems to be consensus that the water behind (or within in outback NSW) such an embankment, generally known as a farm dam, can be water=reservoir or water=pond.
My question is should we be using man_made=embankment instead of waterway=dam for the section of earth that has been pushed by bulldozer to form the “wall”?
When the wall that holds back the water is mapped as waterway=dam, the rendering is a black line at the scale below which I think overemphasises such an insignificant feature as a farm dam (compared to Scrivener Dam).
The two highlighted dams below form large reservoirs - the farm dams do not. But they have the same tag combination.
I think man_made=embankment is appropriate for situations where someone has used a dozer to push up a bank, but waterway=dam is more appropriate for a reinforced concrete construction.
However, I think it’s a mistake to suggest that anything which is not concrete is not waterway=dam. If there is an actual waterway feeding it, or if it’s constructed from rocks or clay brought to this location, or if it’s large enough to service more than livestock in the immediate vicinity, then it’s more likely to be waterway=dam.
To clarify my explanation…
I used the term “earthen excavation” deliberately.
The material that has been excavated is simply formed into “mounds” that surround the excavation.
The water level in the tank will never exceed the nearby ground level (else how could the water drain into the “tank”?); so the mounds of earth surrounding the “tank” are simply spoil from the excavation and are not a “wall” with respect to the water contained in it although they probably form some sort of windbreak to reduce evaporation.
To my mind, a reservoir is a much bigger thing than a pond, and is possibly being used for irrigation purposes or human consumption (as part of a larger “system”) instead of just livestock (and native fauna) that may wander by.
Further, the size of a “farm dam” is much more similar to a “pond” in other parts of the world.
This seems to be a widely held misconception about farm dams, so maybe this is a community service announcement.
A farm dam is, like all earth-fill dams, a carefully engineered structure. They need to be:
sited appropriately
correctly sized including the type and capacity of the spillway
built from material that has the required physical properties
constructed in a manner that ensures that the required moisture and compaction levels are met
You should also contact your local council because in some places a farm dam requires development and or building approval. Your state or territory government may also have limits and required approvals.
In general, don’t try building your own farm dam without first consulting an appropriately qualified expert.
According to the wiki an embankment is a raised bank to carry a road, railway, or canal across a low-lying or wet area and a dam is a barrier built across a river or stream to impound the water. What we are mapping here is much closer to a dam than an embankment.
