Pond-Free 102 Chapter 1
Calculating the Volume of Gravel-Filled Reservoirs
There’s an easy way to figure the reservoir size for your water features with streams. The key is to figure out how much water you have to pump to the top of the water feature before the first drops start to flow back into the reservoir at the bottom. Let's call this 'water in motion' the Dynamic Volume of the feature. The easiest way to calculate this Dynamic Volume is a simple calculation of stream Length times Width times Depth. Take the average of each and multiply to find the volume in cubic feet. For the least amount of maintenance and topping off, and the coolest water to keep evaporation losses as low as possible, you’ll want a generous reserve volume and plenty of water over the pump when the feature is running. The Reservoir Volume should be at least three times the Dynamic Volume, that amount you need to get the feature circulating. That way, when the feature is running, the reservoir will still be 2/3 full with 1/3 empty in reserve to store rain or irrigation water. However, that’s only part of the story. The volume in the reservoir needs to be at least three times what the feature needs to get running, but what happens to the reservoir itself if you’re going to backfill around the Vault with gravel? Well, according to my friend and mentor Bill Hoffman, of Pond Supplies of Ohio, who actually tested five different sizes of gravel (Thanks Bill!), the gravel in the reservoir will occupy about 2/3 of the space, leaving only 1/3 available for water, regardless of the size of the gravel. So backfilling with gravel forces you to excavate about 3 times as much total volume to hold the water you need, because you’ll lose two thirds when you put all that gravel back in. You’ll need at least 3x3 or nine times the Dynamic Volume for a graveled reservoir. And that’s before you figure the cost for the gravel and the labor to pick up, move and install all that stone. Ouch! The situation gets even worse with shallow reservoirs and tall, vertical pumps. You may need to ensure that the depth in the reservoir only drops by a quarter to keep the top of the pump submerged or the circuitry under the cap of the pump will overheat and fail prematurely. With shallow reservoirs and tall, vertical pumps, you might need TWELVE times the Dynamic Volume for a graveled reservoir. Obviously, pond-free features with large streams require a different strategy.
There’s an easy way to figure the reservoir size for your water features with streams. The key is to figure out how much water you have to pump to the top of the water feature before the first drops start to flow back into the reservoir at the bottom. Let's call this 'water in motion' the Dynamic Volume of the feature. The easiest way to calculate this Dynamic Volume is a simple calculation of stream Length times Width times Depth. Take the average of each and multiply to find the volume in cubic feet. For the least amount of maintenance and topping off, and the coolest water to keep evaporation losses as low as possible, you’ll want a generous reserve volume and plenty of water over the pump when the feature is running. The Reservoir Volume should be at least three times the Dynamic Volume, that amount you need to get the feature circulating. That way, when the feature is running, the reservoir will still be 2/3 full with 1/3 empty in reserve to store rain or irrigation water. However, that’s only part of the story. The volume in the reservoir needs to be at least three times what the feature needs to get running, but what happens to the reservoir itself if you’re going to backfill around the Vault with gravel? Well, according to my friend and mentor Bill Hoffman, of Pond Supplies of Ohio, who actually tested five different sizes of gravel (Thanks Bill!), the gravel in the reservoir will occupy about 2/3 of the space, leaving only 1/3 available for water, regardless of the size of the gravel. So backfilling with gravel forces you to excavate about 3 times as much total volume to hold the water you need, because you’ll lose two thirds when you put all that gravel back in. You’ll need at least 3x3 or nine times the Dynamic Volume for a graveled reservoir. And that’s before you figure the cost for the gravel and the labor to pick up, move and install all that stone. Ouch! The situation gets even worse with shallow reservoirs and tall, vertical pumps. You may need to ensure that the depth in the reservoir only drops by a quarter to keep the top of the pump submerged or the circuitry under the cap of the pump will overheat and fail prematurely. With shallow reservoirs and tall, vertical pumps, you might need TWELVE times the Dynamic Volume for a graveled reservoir. Obviously, pond-free features with large streams require a different strategy.
