Coursework: Laboratory Classes COURSE MODULE (and code): Hydraulics and Hydrology (CL216) ASSIGNMENT TITLE: Channel Controls Lab Date set: Submission date: Lab sessions to take place as indicated on the lab timetable Individual lab reports to be submitted by 23:59, three weeks after your allocated lab slot Submission method: Electronic copy via Myplace only Penalty scheme for unauthorised late submission: It is important to note that if your submission is late, a penalty will be applied as detailed on the Civil and Environmental Engineering Undergraduate Homepage. Students can request an extension to their coursework submission deadline when they feel they have circumstances which are impacting their ability to meet the submission date. The length of an extension will be decided by delegated staff, however, extensions to the submission deadline will normally be limited to the date by which feedback on the assessment is returned to students. Coursework accepted following the approval of an extension will be assessed in the normal way. Students must submit their request for an extension as early as possible which must normally be before the coursework deadline. Students must be made aware that ongoing, longer term mitigating circumstances impacting their studies in general must be logged on PEGASUS in line with the Personal Circumstances and Academic Appeals Procedure. 1 Assessment This coursework is part of the Hydraulics class and is assessed for 15% of Semester 2/7.5% overall for CL216 (20 credits). Each student will attend the laboratory class and submit an individual laboratory report. The submission requirements are as stated below. All of the required calculations, tables, graphs, etc. should be submitted, preceded by at most 4 single pages of text on A4 paper, of font size 12 (minimum). Reports should be of a professional style and standard, with correct referencing. Please refer to the reference guide on Myplace. Lab Aim and Objectives To demonstrate and investigate the influence of a broad-crested weir and a constriction (a Venturi flume) on open-channel flow and to develop an appreciation of "total" and "specific" energy concepts. At the end of this assignment, a student should be able to: • record, analyse and interpret experimental results ● write a laboratory report • appreciate the concepts of total and specific energy do a range of calculations relating to specific and total energy Background Theory Broad-crested weirs and Venturi flumes are hydraulic structures that can be used to measure and/or control flow in channels. A local increase in bed level (hump) or reduction in width in the channel will increase the local velocity. The increase in velocity causes a reduction in depth of the flow. If the hump is high enough (i.e. a weir), the flow over it will increase in velocity until it becomes critical. Similarly, a constriction (lateral contraction) in a channel will increase the velocity and, for a sufficiently narrow channel, the velocity will become critical. Therefore, the depth of flow can be related to the volumetric flow rate for channel sections of known dimensions. 2 For a channel with a horizontal bed, or very small slope, Bernoulli's equation in heads form can be written as: H = z + p/pg+ v²/2g In which: H = Total Energy (m) z = elevation of channel bed above datum (m). p = the hydrostatic pressure of the flow (N/m² or Pa) p = density of the flow (kg/m³) g = gravitational acceleration (ms²²) v = mean channel velocity (ms-1) (1) The term "Specific Energy" may be used instead of “total energy” if the datum is the bottom of the channel. Thus, the specific energy is the total head or total energy per unit weight of the liquid with respect to the bottom of the channel. Assuming that the pressure is hydrostatic; h = z + p/pg Therefore, with the bed level as a datum, Bernoulli's equation becomes: E = h + v²/2g In this case E is the Specific Energy (m). (2) (3) For a rectangular channel of width B and with a flow rate of Q flowing along it, the flow rate per metre width, or specific flow rate, q (m²/s), is: q = Q/B (4) Therefore, the mean velocity is: = V=Q/A qB/hB = q/h (5) In which A = the cross-sectional area (m²) The Specific Energy equation (3) may therefore be written as: E = h + q²/2gh² 3 (6) Experimental Procedure The experiment involves the collection and analysis of data from two experimental flow controls. The first, considers flow within a channel of rectangular cross-section containing a broad crested weir (BCW). The second considers flow within a similar channel but which contains a constriction (a narrower section known as a Venturi). In each case, the variation of depth, velocity, total energy, and specific energy along the channel will be investigated. The experiments are broadly similar; hence it is suggested that you can work together in your lab groups to create the results tables, i.e one person can work on the BCW weir data, and another on the Venturi data. Data can then be shared between the group for subsequent analysis so that everyone has both sets of data. Your final report should contain results from both experiments, but must be an individual submission. Data Collection The following instructions will get you started with the data collection, however more detailed instructions on taking measurements and the appropriate tailgate heights to use will be provided in the lab session: 1. Measure the dimensions of the flume, i.e. all necessary dimensions that would allow an accurate plan and longitudinal profile of the flume to be drawn. To speed up the measurements it is recommended that you use two vernier depth gauges in each flume. In order that data from each are consistent be sure to set the datum bed reading to be the same value on each probe by setting both at the same point in the channel. In the BCW, take bed levels about every 200mm taking more readings where the levels change most rapidly. Less depth readings are needed in the venturi flume but more width readings should be taken to define the shape of the lateral contraction and expansion, again take width readings at 200mm intervals or more as needed to describe the profile. In both the BCW and venturi, you will need bed and width readings at locations where you measure the depth of flow. 2. Drop the tailgate to its lowest position, i.e. tailgate should not protrude above bed of flume. 3. Open the control valve to create the maximum discharge (Q) through the flume, measure the value by means of the flow gauge. Be sure you understand the units marked on the gauge and use m³s-1 in your calculations. 4. Measure the depth at salient points along the length of the channel such that the variations of depth with distance can be plotted, see figures below. Make sure you have 4 - a bed level at each location you measure depth, remember depth = (surface level – bed level). Always record both bed and surface levels. In the Venturi you will need a width measurement at each location you measure depth. 5. Raise the tailgate to form a stable hydraulic jump at some point downstream of the structure (see figures below). Measure depth at salient points along the channel. Included in these measurements make sure you measure the depths downstream and immediately upstream of the hydraulic jump on the horizontal section downstream of the weir or constriction. 6. Raise the tailgate to the point of control, i.e. to the point where any further increase would start to affect the water level upstream of the structure (see figures below). Measure depth at salient points along the channel. In trying to find the point of control, you may create a profile just below this level, and you may also measure and plot this profile. At the point of control, the weir or Venturi constriction is just about to stop controlling upstream water depths. That is, if the downstream water levels rise any further you will see a sudden increase in upstream water levels as the structure 'drowns'. The tailgate will then take over control as the weir becomes drowned, see step 7. This has a very practical significance; in a real flood water levels upstream of a structure would rise very suddenly as a structure that was in control becomes drowned. 7. Raise the tailgate in 2 further stages to a point where the structure is completely drowned, i.e. the water level is effectively constant along the channel (see figures below). Measure depth at salient points for each staged increase. 5