C.5.4 Portable Flumes

A properly installed small Montana flume (Figures C-32 and C-33), which is a truncated version of a Parshall flume, can provide a convenient method of making accurate measurements of small streams and ditches. The common sizes (i.e. throat dimensions) for portable use are 76.2 mm (3 inches) and 152 mm (6 inches). Commercially available versions of the Parshall and Montana flumes are typically constructed of fiberglass; they may also be built of plywood or sheet metal) provided the dimensions shown in Table C-11 and Figure C-35 are followed precisely.

Installation. When installing either type of flume, the crest should be used as an index. Careful leveling is necessary in both the longitudinal and transverse directions if standard discharge tables are to be used. In addition, a Montana flume should be used only under free flow conditions, i.e. where the maximum submergence limit (50% for 3 inches, and 60% for 6 inches) will not be exceeded. A submerged flow condition will exist if the tailwater level divided by the flume gauge water level exceeds the percentages shown. Under free flow conditions a phenomenon known as the hydraulic jump forms and is a certain indication of free flow conditions. When a standing wave occurs downstream from the flume, submergence may be indicated.

Note: When portable flumes are used to collect individual discharge measurements, the upstream backwater effect caused by the installation should be allowed to stabilize prior to obtaining the final gauge reading. This may take some time in watercourses with low gradients. In addition, ensure that any temporary changes to channel geometry due to the installation or removal of the flume does not affect the operation of a permanent reference gauge.

Figure C-32. Three-inch Montana flume set in temporary silt "dam".

Note clamped-on gauge plate.

Figure C-33. Six-inch Montana flume set in permanent concrete sack structure.

Note angled approach at the entrance and attached gauge. This gauge is used when a small V-notch weir plate is inserted across the flume entrance to measure flows <4.5 L/s (h _ 0.06 m). See Figure C-34.

Figure C-34. Small V-notch weir plate inserted across flume entrance.

Table C-11. Dimensions for fabricating a Montana flume.

W	A	2/3A	B	C	D	E	F	G	R	Max.	Max.	Min.
	mm	mm	mm	mm	mm	mm	mm	mm	mm	Head mm	Free flow disch. L/s
3 in. or 76 mm	467	311	457	609	259	457	152	552	-	0.330	32	0.77
6 in. or 152 mm	621	414	610	915	397	610	305	800	406	0.457	110	1.5
9 in. or 229 mm	879	587	864	1169	575	762	305	952	406	0.610	251	2.5
12 in. or 305 mm	1372	914	1343	1953	845	914	610	1219	508	0.760	455	3.3

Figure C-35. Various throat widths for the Montana flume.

Improved Designs. The flow characteristics of these flumes can be improved by adding rounded, or angled, entrances in the form of bolt-on additions to the standard design. These improvements will smooth out the turbulence that will otherwise occur in the vicinity of the gauging point when the water level in the flume exceeds 50% of capacity.

C.5.5 Volumetric Measurement Equipment

Volumetric measurement is used to carry out fast, accurate measurement of flow. The sites chosen for such measurements are usually limited to the exits of culverts or below cascades with a clear confined nap.

In many cases culvert exits are too close to the streambed to catch the discharge in a suitably sized container. To convert these awkward sites to measurable ones may be accomplished by installing a 2- or 3-m length of plastic pipe in a sandbag and sheet plastic headwall. Setting the pipe to a level grade will often provide sufficient elevation above the streambed at the downstream end. In the absence of suitable piping, custom-made flumes can be constructed on site using 1x6-inch stock.

Volumetric containers should be calibrated by commercial standard weigh scales, stopwatches should be water resistant and rubber cased.