This is an update and correction to the December 24 post, “Weirs – Planning, Building, & Measuring Flows“. In that post explaining the essentials of installing a weir box, I had said to excavate the pad 4” deep and fill with base rock. It should have said, excavate 8″. I’m sure you already figured out why: the weir bottom is about 4″ thick, so the weir floor would have been above the bottom of the ditch. By excavating 8″, and then filling with 4″ and compacting a level pad of 3/4″-minus road base, when the weir is placed the floor of the weir will be level with the bottom of the ditch. That way, the weir is not too high, where flow will undercut
the base, and it is not too low, requiring extra boards to get a still pool upstream of the weir. The weir box in this photo is set – all it needs is for the water to be shut off, sides backfilled, and boards put in for easy measurement.
The important factor in figuring out where the weir gets placed along the ditch, is that the ditch needs to be straight upstream of the weir box. You can see in the photo above that the weir is located in a straight section of the ditch. When the box is placed in alignment with the straight ditch, the approaching water does not have to make a turn. Water going around a bend rolls toward the outside of the bend, and rolling or turbulent water might give a false reading of depth over the weir boards.
How long does the straight section of ditch have to be? The wider the weir, the longer the length of the ditch has to be straight. For a 1.0 foot-wide (1.0′) weir, which would pass a maximum of 1.0 cubic feet per second (cfs) if it worked as a suppressed weir, the minimum distance should be about 10′. For a wide weir box of 6.0′, the upstream distance should be 70′ or 80′.
How high do the boards have to be to provide an essentially still pool upstream of the weir? Remember the rule that the static head going over a weir, or the height of water that climbs up a 1/2″ engineering ruler held face-on to the flow, should be a maximum of 0.45′. A suppressed weir, with the flow width going from wall to wall as it goes over the weir, has to be 3 times that 0.45′, or 1.35′. 2 2″ x 8″ boards stacked up will get this height. If the weir is contracted, or cut into the board, then the board height only has to be twice the static head, or 0.90′. A 2″ x 12″ would take care of this. However, to be sure, never use less than 2 2″ x 8″ boards.
One more thing – the weir has to keep from collecting dirt or sand behind the boards. That means the boards may have to be lifted up every so often so the sediment can flush out. Weeds have to be kept down all around the weir so they don’t affect the flow of water. In the same way, sticks and grass have to be kept off the tops of the boards for the weir to work correctly.
Where can you find all this information yourself? As always, check the bible for measuring flows, the USBR Water Measurement Manual.
That’s enough for now, more to come soon! Have a great week and I hope it rains today where you are.