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April 8, 2013

Lab 6

Analysis of Open Channel Flow: Specific Energy Curve

Summary

In this lab, the flow rate over a broad crested weir is analyzed. First the flow rate is calculated by the use of the venturimeter. Two trails are performed to determine the specific energy curve for a given flow rate. In each trial, the height of water before and over the weir is measured. Two specific energy curves for each trail are graphed. Introduction

Weirs are structures built across open channels to measure the volumetric rate of water flow. The direction of the flow is perpendicular to the crest of a measurement weir. Weir has many advantages such as capability of precisely measuring a wide range of flows, easy to construct, tends to provide more accurate rating compared to flumes and orifices, can be portable or adjustable, most floating debris tend to pass over the structure. Open channel flow is a very important concept in the study of hydraulics. Weirs are known by the shape of their opening or notch. These openings can be either sharp or broad-crested. Based on the Burnoulli’s equation, the specific energy E in an open channel with respect to channel bottom is defined by equation 1. E = y + v22g Eq. 1

Where E is the specific Energy in distance (L), v is the velocity in (L/T), y is the depth of flow in (L), and g is the acceleration of gravity in (L/T2). Based on this equation, the energy before the weir is equal to the energy over the weir plus the height of the weir as shown in equation 2. E1 = E2 + Δz Eq. 2

From the known discharge, the velocity of the flow can be calculated at different heights using the equation 3. V1 = q/y1 Eq. 3

To graph the specific energy curve, the critical depth flow and the specific energy at critical points need to be known, and are calculated from the equations 4 and 5. yc = (q2./g)1/3 Eq.4

Ec = 32 yc Eq.5

Other values of E and y should be calculated to graph the specific...