53:071 Hydraulics & HydrologyProject #2

Flow Measurement with a Weir

Li-Chuan Chen, Marian Muste, Gokhan Kirkil, and Allen Bradley

Problem Statement

The Iowa DNR plans to return farmland to a native tallgrass prairie landscape as an experiment.

Problem Statement

A small ephemeral stream drains the site. The Iowa DNR wishes to find out how the runoff from the site changes over time.

Objective

Design a weir flow control structure to continuously measure discharge at the site.

Principle

levelDownstream

Upstream level

H

P

W eircrest

W eir

Nappe

V = 2gh

L 4H

dhh

V h gh 2

Principle

n

A

H

kh

dhhhbgdAhVQ

0

)(2)(

)log()log()log()log( khnkhQ n

Slope

Intercept

Apparatus — Weirs

2'-0"

1'-0"

2'-0" 2'-0"

6" 6"10 3/4"

90.0°

60.0° =

=

W E IR E XP ERIM ENT

Rectangular Contracted W eir

90 Triangular Weiro

60 Triangular Weiro

WEIR EXPERIMENT

Procedures1. Insert the weir of the desired shape.2. Record the reference point of the weir.3. Set the appropriate discharge in the flume

and record the water level difference on the differential manometer.

4. Record the water surface elevation upstream of the weir.

5. Repeat Step 3 and 4 for four more discharges.

6. Repeat Step 1 to 5 for one more weir shapes.

Data Sheet

Discharge Triangular 90

h(ft)

Q(cfs)

Ref. Point (ft)

W.S. Elev.(ft)

Weir Head(ft)

         

         

         

         

         

Analysis

Determine the discharge Q in the flume (using the side-contraction meter) and the head H on the weirs.Using several measured Q-H pairs, plot log Q versus log H.From the best-fit line to the experimental points, determine the kexp (the intercept) and nexp (the slope).

Compare kexp and nexp with values indicated in the literature.

Sample Result

H (ft) 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1 .0

Q (

cfs)

0 .4

0 .5

0 .6

0 .7

0 .8

0 .9

1 .0

90 deg . w e ir Q 90 = 2 .60H 2.42

60 deg . w e ir Q 60 = 1 .56H 2.58

R ectangula r w eir Q rectangular = 3 .47H 1.47

Site Information

Measure flows up to the highwater mark

Engineering Analysis

Recommend a weir geometry and provide the head-discharge relationship.Provide profile and cross-section specs for the weir site (AutoCAD sketch). Estimate uncertainty of the discharge measurement (in ± cfs) for two cases Estimate of the extent of ponding behind the weir for the design flood conditions (water surface profile)Provide the client (the Iowa DNR Restoration Project Manager) with a concise report that answers the project objectives.