I spend a fair bit of time camped in the forests of central Victoria: if possible I try to camp by a dam for a supply of non potable water - shower, washing up etc.
Dams seem to vary; a few have excellent water and are usually close to full all year around, others seem to be like a large puddle which dries up in hot weather.
I am guessing the former are fed by a spring and the latter are rain catchments? If so why don't the former overflow?
As you can tell this is a subject (one amongst many) I know stuff all about... but I'll bet you guys do....
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You may have a similar problem to NSW, people illegally pumping water.
Or could be water is pumped into those dams at a different time of year for storage when precipitation is high.
We stayed at Wellington Dam in WA last year. It was low. It was once used for drinking water but became too toxic due to land clearing. So the water is only used for non potable purposes.
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Spring fed dams (I have three on my property can be a bit fickle in times of prolonged droughts as we have recently experienced. As their water supply must first of all come from rain soaking into the earth/rocks it may take years for them to recover their flow. They do overflow if overly full, the perfect dam has a large enough surface area to evaporate/ soak in at a similar rate to the inflow.
When building a dam we must do a calculation (Guess) what volumes are flowing in, how much do we want to extract. We line our dams with bentonite clay as a waterproofing layer (Why most dam owners dislike yabbies boring into the walls). When money is no object (Cubbie Station) dams are lined with Hypalon (a Du Pont elastomer) as a waterproof membrane.
Care must be taken to provide an overflow that will not undercut the dam wall/s elsewise the dam may collapse in overfull situations.
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Maybe if you name the particular dams folk here may be able to help a little more, we used to live in north central Vic and the dams there were mostly either for irrigation or town supply withextra booster ( my word ) backup for times of low rainfall.
Specifically thinking of the Goulburn Valley/ Mitchell Shire areas.