CEME2003: Civil Engineering Hydraulics - Design Project: PHM Water Supply Expansion - Engineering Assignment Help

Assignment Task:

1 Organisation 

This is an individual assessment. You are encouraged to discuss your ideas, understanding and approaches with your peers, but your work must be your own. The design is structured using a more open approach in comparison to the group project, where determining a feasible design is relatively straightforward, but the emphasis is on the preliminary investigation to build your understanding of the system to inform your design. The submission is an individual design report. 

2 Introduction 

Some time has passed since the construction of the Praminent Hollow Mine (PHM) water supply system, and the advent of a new agricultural development has necessitated an expansion of the current supply system to accommodate this. The proposed expansion will involve the construction of a channel system that delivers water from the service tank located at chainage 2000 (Figure 1) to feed water into a downstream reservoir for irrigation storage.

The channel system, depicted in Figure 2, is to involve two components: (i) a concrete inlet section that takes water from the service tank, at super-critical flow, and delivers it to the channel at sub-

critical flow; and (ii) a lined channel that delivers flow from the inlet section to the irrigation reservoir. These design is detail further in the following.

3 Design Outline 

The objective of the design is to deliver a design flow rate of 75 L/s from the service tank (chainage 2000, elevation 81.5 m above datum – EL 81.5 m) to a downstream reservoir located a distance of 200 m from the tank down a hill-side of slope 0.005. The water surface elevation of the downstream reservoir is 80.3 m. The design criteria, options and costing is outlined in the following. 

3.1 Channel inlet 

A schematic of the channel inlet is given in Figure 3. The channel inlet is to consist of a concrete structure that receives a super critical discharge from the feeder tank, and contains a hydraulic jump before releasing the flow into the channel. The inlet consists of: 

• A feeder tank that receives a controlled flow from the service storage tank (controlled by the flow control valve to ensure a constant water surface level in the feeder tank), and discharges to the inlet through a sluice gate; 

• A rectangular inlet section of channel containing a hydraulic jump, where at the upstream point the sluice gate controls the super-critical flow into the section, and at the downstream point the flow exits as sub-critical flow into the expander; and 

• An expander section that smoothly changes in cross-sectional shape from the rectangular inlet to the channel downstream. 

3.1.1 Design Variables 

Design variables that require determination are: 

• For the feeder tank: 

- Water surface elevation in the feeder tank (tank floor elevation is EL 81 m); - Height of the sluice gate above the tank floor; 

• For the rectangular inlet section: 

- Height, width and length of the section (the sluice gate is the full width of the section); 

• For the expander section 

- Height, width and length of the section 

3.1.2 Design Criteria 

The inlet section must satisfy the following design criteria 

• The rectangular inlet section must be long enough to contain the hydraulic jump, and discharge sub-critical flow into the expander. The height of this section must be 200 mm above the highest water surface level. 

• The expander section provides a smooth internal surface that transitions from the upstream rectangular shape to the downstream channel shape. 

• The sluice-gate height must be designed so as to enable the sequent depth to be of a sufficient height so as to match the back-water profile from the channel (after flowing through the expander) at the design flow rate. 

3.1.3 Analysis approach: 

The following are important considerations in designing this section: 

• The super-critical velocity under the sluice gate outlet is a function of the water height in the feeder tank. Consider the depth of the jet under the sluice gate to be 80% of the height of the sluice gate opening (due to the vena contracta effect). 

• Assume that the toe of the jump occurs 1 m downstream from the sluice gate and that the total jump length is six and a half times the depth of the downstream point of the jump. The rectangular inlet section needs to be sized to be sufficiently deep and long to entirely contain the jump. Friction losses within this section can be ignored. 

• The expander section delivers the flow from the upstream hydraulic jump to the downstream channel: 

- The expander section is to be a quarter of the length of the rectangular inlet section. - The flow depth at the upstream point matches the sequent depth from the hydraulic jump, and the downstream point matches the depth determined from the back-water profile in the channel. - Assume that losses are negligible within this section. That is, the specific energy at the upstream and downstream points is equal. 

3.1.4 Design Options and Costing 

The feeder tank and sluice gate are installed, and only the rectangular inlet section and expander require costing. These sections are constructed from 150 mm thick reinforced concrete slabs (i.e. walls and base can be considered as “slabs”), and cost $500 per m2 for materials and installation. 

3.2 Channel 

The channel is to deliver the design flow from the channel inlet (channel base elevation at inlet is EL 81 m) to a reservoir (water surface elevation EL 80.3 m ) a ground surface distance of 200 m down a hill of slope 0.005. The channel base is 0.5 m below the ground surface, but the channel sides can be built up above ground surface to the required height. 

3.2.1 Design Variables 

The design variables that require determination are: 

• The channel shape type and cross-sectional dimensions; 

• The channel lining material. 

3.2.2 Design criteria 

The design criteria for the channel is as follows: 

• The channel must provide the design flow at the required water surface elevation at the channel outlet (this corresponds to a depth of 0.3 m in the channel); 

• The channel height must contain the entire flow depth throughout the channel length, with an added free-board of 0.15 m above the maximum depth of the flow. 

3.2.3 Analysis approach 

The following are important design considerations for the channel: 

• The aim within this part is to determine the channel design that minimises the channel cost but meets the design criteria. 

• The backwater curve within the channel is to be solved using the gradually varied flow equations (i.e. use wither the direct-step or standard-step methods). 

• The flow within the channel is to be sub-critical, meaning that the backwater curve within the channel is controlled by the downstream conditions. 

• The backwater curve controls the sequent-depth of the jump in the channel inlet. 

3.2.4 Design Options and Costing 

The channel shapes that can be selected are: triangular, trapezoidal, or semi-circular. Any parameters for these shapes can be selected (e.g. side slopes for the triangular and trapezoidal sections). The channels are constructed by excavation to dig the main part of the channel, where the sides are built- up by earth compaction where required (i.e. where the channel height is above the earth height). Relevant earthwork costs are: 

• The cost of excavation is $600 per cubic metre of soil removed. 

• The cost of raising the channel sides above ground level are $40 per 100 mm raised, per metre of channel length1. 

The channel is to be lined with gravel to stabilise against erosion. Two options for gravel are available: 

• A finer grade gravel, with a Manning’s n of n = 0.015, and a cost of $80 per square metre of coverage; 

• A coarser grade gravel, with a Manning’s n of n = 0.045, and a cost of $50 per square metre of coverage; 

4 Report Structure 

The report is to be structured as follows: 

• Introduction - outline of problem and statement of objectives; 

• Design Analysis – this section of the report contains the preliminary design considerations that are used to substantiate the final design. This section could involve: 

o At least two alternative designs (different channel materials and shapes), and a cost comparison to determine the final design; or

o A more systematic investigation into each design component, accounting for the interdependencies, to determine the most cost effective overall design. 

• Final Design – this section is used to detail your final design. Show drawings/tables/figures presenting the design decisions. Show also the water surface throughout the system, outlining important characteristics such as: surface profile type; critical depth; normal depth. 

• Appendix – include all relevant material in an appendix. Make sure that the results in the report are substantiated by the calculations in the appendix. Where spreadsheet calculations 

1 That is, if the channel depth is to be 700 mm, this requires a raising of the channel bank height by 200 mm (as the channel is only 0.5 m below ground level). This means that for each metre of channel length, this bank raising will cost $80. are used, include an explanatory page which indicates the equations and processes used. Include hand calculations of the final design. 

5 Extension Task 

As an extension task, analyse the sensitivity of the final design. A sensitivity analysis involves the perturbation of select system parameters to see how the system responds. For example, the design would be considered highly sensitive if the flow downstream of the inlet broke its banks for only a small change in the sluice gate height. 

Systematically explore what happens to the performance of the design (i.e. does it still satisfy the design criteria) as key parameters are increased (or decreased) by 10% of their initial value. Key parameters to consider may be: the water height in the feeder tank; the sluice gate height; the Manning’s n for the channel lining. 

6 Assessment 

Marks will be awarded for clarity of report structure, and the breadth and depth of work for the preliminary design/design analysis sections. The assessment rubic will be released shortly. 

 

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