The simplest measurement device to install is a weir. Vertical side walls with board slots, or channel iron bolted onto the concrete, hold 2″ x 4″ boards. The boards need to be stacked 12″ to 18″ high, and the channel depth needs to be 24″ or deeper. Here is a 3′ wide weir with a 12″ board height:
But, what if the ditch is less than 24″ deep? The ditch shown below is about 18″ deep, and it must convey flow that comes near to the top of its banks. The ditch here is about 10′ wide and is passing 20 cfs:
The solution was to install a Watchman Replogle flume that is nearly as wide as the ditch – 8′ wide. It is installed on a concrete base so it stays level and does not tip. The flume floor is 3″ above the bottom of the ditch, and the maximum depth upstream of the ramp is 15″. In other words, the top of the flume as the same elevation as the top of the bank:
The flume is conveying about 12 cfs in these photos. It does so with a total accuracy better than +/- 10%.
The maximum ditch flow is 30 cfs. Will the flume convey that maximum flow, without overtopping? The photo below shows the flume conveying a flow of 30 cfs. Yes, it still measures flow accurately, without overtopping.
Here is the same size of flume in a different location, at a lower flow. Even with the low flow, the depth of water is the same all the way across the flume:
Here is another version of the same flume. The ditch here has plenty of depth, so overtopping was never an issue. So, why is a low-head flume installed here? This ditch is very flat and there is a possibility of restricting the flow into the ditch as it enters from a low pipe. Measuring the flow this way ensures accurate measurement without flooding the incoming pipe:
