- Background Of The Problem You Are Working In A Water Engineering Consulting Company As A Graduate Civil Engineer Your C 1 (78.46 KiB) Viewed 23 times
Torrents River Sluice gate 3rd Reach: Trapezoidal earth channel 4th Reach: Rectangular earth channel Redfish Reservoir Bigpond Dam 1* Reach: Rectangular 2nd Reach: Circular concrete (gunite) RCP channel channel Direct connection (freefall) Figure 1. Schematic of the irrigation channel system (top view).
Redfish Reservoir 1* Reach: 2nd Reach: Rectangular Circular concrete RCP (gunite) channel, channel 34 Reach: Trapezoidal earth channel H1 4" Reach: Rectangular earth channel H2 Chainage (m) 0 100 300 500 1500 1700 2500 3000 Bigpond Dam Figure 2. Schematic of the irrigation channel system (side view).
Q1 You are tasked to provide information to the client on how the opening of the sluice gate should be set to achieve the designed flow in the channel during normal conditions and prevent flooding during storm events. The sluice gate used in the rectangular concrete channel has a width same to that of the channel, as shown in Figure Q1. The sluice gate is installed close to the reservoir outlet. The maximum depth of the channel bank upstream of the sluice gate is 3.5 m and that downstream of the gate is 2.5 m. Under the normal reservoir operation condition, water surface level in the Redfish reservoir is 2.0 m higher than the bottom of the channel (i.e. Y1 = 2.0 m). During storm events, the reservoir level can be up to 3.0 m above the bottom of the channel i.e. Y1 = 3.0). It is known that the energy loss when water passing through the sluice gate is 10% of the velocity head of the flow just underneath the gate (cross-section 3). A designed flow rate of 3.0 m/s is to be achieved in the channel under normal reservoir operation conditions. Frictional loss due to wall sheer stress can be neglected due to the short distance considered in this scenario. To be able to achieve a well justified recommendation, you take the following steps: (i) Establish an energy equation between a point at the surface of the reservoir and the point just below the sluice gate. Determine the opening height of the sluice gate yz that would result in the designed flow rate under the normal reservoir operation condition. If multiple solutions are obtained, determine which one should be adopted. (6 marks) (ii) Based on the results from part (i) and taken into consideration of practical constrains, the tentative sluice gate opening y3 is determined as 0.25 m. Under the normal operation condition, yı is 2 m and less than the channel bank depth downstream of the sluice gate, such that there is no flooding risk. Considering that the reservoir level would be higher (y1 = 3 m) during storm events, you want to estimate the flow condition downstream of the sluice gate for the maximum depth scenario. Based on the information obtained so far, determine the flow rate, velocity, Froude number and flow regime for the flow through the sluice gate under the maximum reservoir depth scenario. (9 marks) (iii) Based on the information obtained, for relative reservoir level y1 = 3.0 m and the sluice gate opening y3 = 0.25 m, determine whether a hydraulic jump is likely to occur downstream of the sluice gate. If so, estimate the depth of flow downstream of the jump and the associated energy loss. (10 marks)
Top of channel bank Top of channel bank 3.5 m 2.5 m Possible hydraulic jump Reservoir Rectangular channel yi y2 y4 1 23 Figure Q1 Side view of the sluice gate used for flow control
Q2 The client wants to know the maximum flow capacity and the corresponding flow condition of the channel system (in particular, normal depth and critical depth). This would be useful for developing flood mitigation plans. The ageing of the channel should be considered since it delivers raw water and ageing can occur not long after the commission. For the circular channel, as per the convention, the design maximum flow is the full pipe flow without pressurisation. For other types of channels, a minimum freeboard of 0.3 m is required (i.e. the distance between the surface of the water and the top of the channel bank needs to be 0.3 m or more). The velocity should not be greater than 3.0 m/s under the maximum design flow. To determine the maximum flow capacity of the channel system under aged condition, you take the following steps: (i) (ii) (iii) Find the suitable design values of the Manning's coefficient n for each section of the channel from credible references. Clearly specify how you find the information such that your colleague or the client can check if needed. Justify the values selected. (5 marks) Using the Manning's equation, determine the maximum allowable normal flow rate and corresponding velocity for each individual channel section (except for the first horizontal section). Demonstrate the calculation and tabulate the results for all the channel sections. If the velocity is higher than the allowable maximum of 3 m/s, make adjustment to the maximum normal flow accordingly. Summarise results in Table 02-1. (8 marks) Based on the maximum allowable normal flow for individual sections as determined in part (ii), determine the maximum allowable flow for the whole channel system, and explain it to the client. (2 marks) Based on information obtained previously and other practical considerations, the client would like to set the maximum allowable flow for the whole channel system as 4.80 m/s. Based on this value, determine the corresponding normal depths and critical depths for all the relevant channel sections. Demonstrate the calculation and tabulate the results for all the channel sections using Tabel 02-2. (6 marks) Make a schematic of the side view of the channel (use Figure 2). Based on the normal depth and critical depth results obtained in part (iv), draw the normal depth line (NDL) and the critical depth line (CDL), respectively. Label the values of NDL and CDL. (4 marks) (iv) (v)