Stream flow measurements are fundamental in water management and. Different streams and rivers require

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Andrea Chapman
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1 Stream Flow Measurements

2 Introduction Stream flow measurements are fundamental in water management and projects Different streams and rivers require appropriate flow measurement methods Weirs and Flumes Dilution Gauging Velocity Area Method

3 Weirs and Flumes Involves the installation of various types of weirs and flumes (V-notch weir, Parshall flume, H flume, etc) Suitable for small streams

4 Dilution Gauging Suitable for small streams with lots of boulders, aquatic growth, or other roughness elements Involves the addition of a volume of water (dyed), called tracer, of nown concentration to the stream of unnown discharge The diluted concentration is measured downstream (where concentration is assumed to be uniformly diluted) From the degree of dilution the unnown discharge is determined

Velocity Area Method Involves determining the

of the river is subdivided into smaller areas and

The total discharge is the summation of all

5 Velocity Area Method Involves determining the area of fflow of the stream as well as the average velocity of flow Returns a near instantaneous value of discharge The area of flow of the river is subdivided into smaller areas and velocities representative of these areas are taen. The total discharge is the summation of all discharges in the sub-areas Two types: Using current meters Using floats

6 Velocity Area Method - Procedure The first step is selecting a suitable cross section across the total width of the stream Select a straight reach where the streambed is uniform It should be relatively free of boulders and aquatic growth The flow should be uniform and free of eddies, dead water near bans, and excessive turbulence With a measuring tape, measure the width of the t a easu g tape, easu e t e dt o t e stream from ban to ban; measurement should be done perpendicular to the flow

7 Velocity Area Method - Procedure Subdivide the total width of the stream. Sources suggest that t verticals should be spaced such that t no subsection has more than 10% of the total discharge. At these points (verticals) velocities are measured (Each measured velocity is assumed to be the same from the point of measurement to halfway between this measurement and the adjacent measurements on either side**) The US-EPA suggests that at points <2.5ft deep, velocity is determined at 0.6 of the depth; for >2.5ft, velocity is measured at 0.2 and 0.8 of the depth Other sources suggest velocities should be measured at 0.2, 0.6, and 0.8 of the depth Alternative procedures say that velocity at 0.6 depth may be sufficient

8 Current Meters vs. Floats With current meters the velocity of water at the depths mentioned above can easily be obtained Is not influenced by wind Velocity is obtained from the reading (usually expressed in revolutions per second) of the meters; velocity of water is proportional to the angular velocity of the propeller Floats measure surface velocity of the river Influenced by wind To get the velocity, floats need to be released upstream and allowed to travel on a nown distance (to be done for each subsection) A relatively long, straight portion of the river is desirable Flow areas of the river at point of release and at the finish point downstream are required

9 Velocity Area Method - Computations The width of extent of each velocity is: w = ( d + 1 d ) ( d d 1 ) w w = = d d d d 1 = 1, 2,, n-1 d = distance from ban = 0; on the ban = n; on the other ban

10 Velocity Area Method - Computations The area of flow for each subsection is assumed to be rectangular and is therefore computed as the product of the width w and the depth h A = h w = 1, 2,, n-1. The areas of flow by either ban are assumed to be triangular 1 h + 1 = 0 A = w 2 2 A = h w = n.

11 Velocity Area Method - Computations To solve for the average velocity at each subsection: V 1 3 ( v + v + v ) or V = ( v + v v ) = If only two points are measured (at 0.2 and 0.8 of depth), simply get the average Velocity at 0.6 depth may be taen as representative of the sub-area** For floats, float velocity is determined first by dividing the distance traveled by the travel time. Mean flow velocity is obtained by applying conversion factors to the float velocity (usually varying from 085t 0.85 to 095) 0.95)

12 Data Sheet (for current meter) Distance from ban (d) Depth (h) Observatio nal Depth (d 0.2, d 0.6, d 0.8 ) Measured Velocity (V) Average Width (w) Area (A) Discharge (Q) 0 1 2

13 Data Sheet (for floats) Launch Site Distance from ban Finish Distance from ban Travel time Float Velocity Vertical Launch Finish Average Mean Crosssectiosection Cross- Crosssection Velocity

Remars (for current meters) It is preferred that measurements be done by

This provides the one doing the measurements more control over the

the operator is standing facing one of the bans, slightly downstream of

14 Remars (for current meters) It is preferred that measurements be done by wading (if conditions permit). This provides the one doing the measurements more control over the procedure than when done from cableways and bridges The best position of the operator is standing facing one of the bans, slightly downstream of the meter and an arm s length from it Rod should be ept vertical Meter should be parallel to the direction of the flow

15 Remars (for floats) The largest source of errors is the process of getting the average velocity across the with of the stream As wind affects floats on the surface, better measurement may be obtained using subsurface floats

Discharge. Discharge (Streamflow) is: Q = Velocity (L T -1 ) x Area (L 2 ) Units: L 3 T -1 e.g., m 3 s -1. Velocity. Area

Discharge. Discharge (Streamflow) is: Q = Velocity (L T -1 ) x Area (L 2 ) Units: L 3 T -1 e.g., m 3 s -1. Velocity. Area Discharge Discharge (Streamflow) is: Q = Velocity (L T -1 ) x Area (L 2 ) Units: L 3 T -1 e.g., m 3 s -1 Velocity Area Where is the average velocity?? 3 Source: Brooks et al., Hydrology and the Management