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Article 39: Water Returns - Capacity, by Jeff Richardson

Posted by Jeff Richardson on

Return Capacity: This can be a bit tricky.  What is the capacity of water in the return while the pumps run (or when off)?  Why is this important?  There are a number of reasons.  The water flowing through your return is the flow coming from your pump’s discharge.  Firstly, will the return be able to accommodate the flow without overwhelming it causing water to overtop the return and leak from your pond system?  Or conversely, is the return too big?  If you have x-flow of gallons suddenly spread out over a 4’ wide stream bed, it might be a shallow trickle instead of a deep creek like you envisioned.  This may also affect the water more susceptible to temperature changes. Further, if you have multiple returns, you may be causing one or all the returns to have an anemic flow.  Next, you need to consider the water in motion.  When water is pumped from the intake side, that pool or reservoir’s surface will lower.  Will this be a problem?  The pumps may cause the intake pool, cistern, housing or reservoir to remove water faster than it is replenished depending how long it takes for the return water to make it back to the intakes. This could cause a pump to run dry.  Lastly, does the return hold water?  When the pumps stop, does the return empty of all water back to the main pool, cistern or reservoir?  It is generally a bad idea to have small, stagnant pools or water which will be susceptible to evaporation, temperature changes, mosquitoes, or chemically changed.  Further it could be a danger if fish can swim into a stream that once the pumps stop, the fish are confined to one of these tiny pools.  They would lose their oxygen quickly and be easy pickings for predators.  However, if neither fish are a concern nor the balance of the water, then ensure you include this return holding capacity in your overall systems total gallons as a separate entry.

Designing Water in Motion: First, there is a minimum.  You want to filter all the water in your pond system about once an hour is a good rule of thumb to keep your water happy.  Next is visual flow.  I have seen many beautiful natural falls living here in the PNW.  One in particular stands out. A fan waterfall is where the water spreads out across a cliff face with the water spreading out wider at the bottom.  It seemed to be excessive amounts of water to make such a spectacular waterfall – but this was deceiving.  In the case of this fall, I found that it was fed by a tiny amount of water at the crest.  How?  The water was spread across a very thin layer over large surface which you could see.  Depth or volume of water in a stream or falls isn’t necessarily the be-all, end-all.  In Article 8 – Sizing Components, it gave a guideline for “apparent” visual flow as follows: For every 1’ width of waterfall or streambed, the feature needs 500 GPH for each increment of flow aesthetics as follows: 500 GPH: Trickling, 1000 GPH: Gentle, 1500 GPH: Average, 2000 GPH: Rigorous, and 2500 GPH: Turbulent.  Streams will likely need more water flow depending on the depth you wish to transit through the stream.  This brings up volume.  Remember, a gallon of water fits inside a 6-1/8” cube.  You can quickly get an idea if you wanted a 4’ wide stream that is 6” deep and 10’ long – that’s 160 gallons just to fill the return!  Lastly is how quickly you want the water to transit through the return.  Gravity is the key player here.  The steeper the course or bed, the faster water will travel or drop.  Of course you can speed and slow the water traveling through the return using slope, width and depth to control the speed faster or slower throughout the return.

Calculating Water in Motion: The first thing to calculate is how much water is being discharged in gallons per minute.  In my case, it was easy.  My settling tank that houses my filters has both of my pumps discharging into the bottom of it.  150 gallons is to the brim of the tank, but I have installed spillways through the side of the tank near the top.  So naturally, the tank holds less than 150 gallons.  Using my water meter, I filled the tank with a hose until it reached the spillways, stopping as the first drops of water flowed through a spillway which came out to be 110 gallons.  I emptied the tank and used a stop watch function on my phone turning on one pump and timing how long it took to fill 110 gallons.  I emptied the tank again and did the same thing for my second pump.  I did it a third time with both pumps.  Divide time by gallons – this is the Gallons per Second (GPS).  To calculate how quickly water transits through your return is a bit more subjective.  You can use a leaf, or something small that will float to get an idea how fast the water is moving from beginning to end.  Perform this task five times, throwing out the fastest and slowest times and average the three remaining to get an answer.  Armed with the discharge GPS and how long it takes for water to transit the return should give you an idea on how much water is in motion within the return (and thus missing from your main pool or reservoir).  You can even calculate how much the pool has lowered.  This will be much harder if you have multiple returns – like two stream pathways.  The best you can do is measure the cross section of each return bed and calculate a ratio of sizes.  If nearly the same size, 50% to each, or if one is twice as large, than a third is going to the smaller, and two-thirds to the larger.

Example: When I created my return, I fabricated a 12’ long, 2’ wide and 5’ high return as a fast-moving, turbulent waterfall.  Together my pumps send 100 gallons per minute down the waterfall course (50GPM per pump and 1-2/3 GPS overall).  The water only takes about 3 seconds to fall from crest to plunge pool (thus it has 5 gallons in motion).  My filter tank rises by about 25 gallons as well.  This means that there is approximately 30 gallons removed from my pond while the pumps are running.  My main pool lowers by ¼” across its entire surface.  When the pumps are both off, the waterfall course is empty (all water returns to the pool).  Thus, the waterfall does not count towards the total pond system capacity in gallons (as all the water is transient).


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