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Cork Institute of Technology
Bachelor of Engineering (Honours) in Structural Engineering - Stage 3
(CSTRU_8_Y3)
Summer 2008
Water Engineering
(Time: 3 Hours)
Instructions Answer five questions in total, a minimum of Two questions from each section. Use separate answer books for each section. All questions carry equal marks. Programmable calculators are not permitted.
Examiners: Dr. J Harrington Mr. L O’Driscoll Prof. P. O’Donoghue Mr P. Anthony
SECTION A
Note: Attachments including equations 1(a) A water channel is V-shaped with each side making an angle of 45 degrees to the vertical. Calculate the volume of water passing per second when the depth of water in the channel is 0.5 m and the slope of the channel is 1 in 250. Take the Chezy Coefficient C as
- What would be the depth of water in the channel to pass a flood condition of three times this volume per second if both the slope and the value of C were unaltered? (10 marks) (b) Discuss the issues associated with increased flood risk in rivers and coastal areas. (5 marks)
(c) Salt is used in a freshwater environment to determine the river flow rate using dilution gauging. The salt injection rate is 1 litre/sec at a concentration of 100 mg/l. The background salt concentration was measured as 1 mg/l. The steady state concentration downstream was measured as 5 mg/litre. Determine the river flow rate and also the % error in the river flow rate if the background concentration is ignored. (5 marks)
2(a) Briefly discuss: (i) Suspended Load and Bed Load (4 marks) (ii) Threshold of Motion (3 marks)
(b) A wide river with a flow depth of 3m, a mean velocity of 1 m/sec and a Chezy coefficient of 50 m1/2^ /sec contains sediments of density 2650kg/m^3. A sample of the suspended sediment load taken at mid depth shows a mean diameter of 0.02mm with a concentration of 200 mg/l. The viscosity of water may be taken as 1.13 x 10 -3^ N sec/m^2. Establish the suspended sediment profile as a function of depth (at 0.5m intervals) given a k value of 0.4. Plot these results. Also calculate the suspended load per metre width assuming a homogenous concentration distribution. (9 marks)
(c) If the river in (b) is 250m wide and discharges into a downstream reservoir of capacity 1.5 x 10^7 m^3 , find the design life expectancy of this reservoir assuming the sediment to have a porosity of 0.3. (4 marks)
3(a) Discuss the following: (i) EU Water Framework Directive (4 marks) (ii) The outbreak of Cryptosporidium in the Mullingar, Co. Westmeath Water Supply System in 2002 (6 marks)
(b) A 9.15 m thick layer of sandy soil overlies an impermeable rock. Ground water level is at a depth of 1.22 m below the top of the soil. Water was pumped out of the soil from a central well at a rate of 5680 kg/min and the drawdown of the water table was noted in two observation wells. These two wells were on a radial line from the centre of the main well at distances of 3.05 m and 30.5 m. During pumping the water level in the well nearest the pump was 4.57 m below ground level and in the furthest well was 2.13 m below ground level. Estimate the K value of the soil. Comment on the type of aquifer involved and the assumptions inherent in this analysis. (10 marks)
4(a) Discuss the use of ‘traditional’ sedimentation tanks, high rate clarifiers and Dissolved Air Flotation in water treatment, with the aid of diagrams. (8 marks)
(b) A water treatment plant built in Ireland in the 1970’s supplies a community with a population equivalent of 25,000 with water. It receives its supply from a nearby lake which now suffers eutrophication problems. Estimate the demand from the community. Outline the likely treatment process within the plant, size the main elements for the treatment process and quantify the daily amount of any chemicals used. (8 marks)
B. Eng. in Structural Engineering – Year 3
Water Engineering
Attachment to Section A – Equations
Open Channel Flow
Chezy Eqn.: v = C mS 0
Manning Eqn.: 01 /^2
2 / 3 v = mn S
Sediment Transport
Vertical Suspended Sediment Profile: (( )) ku^ *
v r
r r
s Y y
Y y y
y C
C (^)
− = −
Shear Velocity: u (^) * = gmS 0
Stoke’s Law: 18 (^ ρμ ρ)
(^2) − vs = gd p
Well Hydraulics
1 2 2 22 1
log ( ) r
r h h
K Q
e
SECTION B
B1 A new storm-water sewer is to be designed as shown in Figure B1. The lengths of the pipes and the catchments are shown in Table B1.
Design the sewer in Figure B1, for a return period of 5 years. Indicate the pipe diameters & pipe slopes using the attached Table B1. The downstream end of pipes 1.03 and 2. are to be designed to connect with an interceptor sewer at MH 6. The invert level of this interceptor sewer at MH 6 is 20.000mOD. The new connecting sewers 1.03 and 2. must have a downstream invert level higher than this.
Design Assumptions
- Assume the ground falls uniformly from manhole to manhole.
- A minimum cover of 1.0m is allowed between cover level, (CL), and top of pipe. Invert Level, (IL) = CL – 1.0m – diameter of pipe.
- Assume Time of Entry of 4 minutes.
- For each pipe, calculate the time of flow from header manhole and add to time of entry, round this value up to nearest ½ minute.
- Use the Dillon equation to establish rain intensity, I = 152.4 T (^) p 0.2^ / t 0. I is rain intensity in mm/hr T (^) p is the return period t is the storm duration in minutes.
- Use the Modified Rational method to establish flow, (^) Q = 2.78 Cv.Cr .I. A Where: Q is the flow in L/s, I is rain intensity in mm/hr and A is the impermeable area in Ha. PR = 80%; (^) Cr = 1.
- Minimum and maximum velocities are 0.5m/s and 3 m/s respectively.
- For pipeline design, see Attachments B1: Colebrook White Chart with Ks = 0. and pipe design sheet.
(20 marks)
Cork Institute of Technology
Table B1PipeNr
Pipelength(m)
Fall(m)
Slope(m/m)
Diamete
r (mm)
Velocit
y (m/s)
Timeof Entry(mins)
Timeof Flow(mins)
Timeof Conc(mins)
ImpArea(ha)
TotalImpArea(ha)
Rain(mm/hr)
Q (L/s)
Capacity(L/s)
Upstream IL
(mOD)
D/stream IL
(mOD)
75
60
55
60
80
Cork Institute of Technology
B2 A treatment plant is proposed for a town with a population equivalent (PE) of 80,000. Assume for this plant that 1PE equates to 60g BOD/day and 220L/PE/day. Wastewater is to be treated to the 25/35 standard. Suspended solids concentration at the inlet to the works is 220 mg SS/L. Design a conventional activated sludge system wastewater treatment plant, using the design parameters given in Figure B2. The treatment plant will also include a primary settlement stage. (a) Calculate the plan area and depth of the primary sedimentation tanks and the primary sludge production rate, given the following design parameters:
- Average overflow rate in the range 1-2 m^3 /m^2 /hr
- Peak hourly overflow rate = 4m^3 /m^2 /hr
- Average hydraulic retention time, 2.5 hours
- BOD removal rate = 25%
- Suspended solids removal rate 50%
- Waste sludge concentration 2% solids (4 marks) (b) Calculate daily primary sludge production (tonnes/day) (2 marks) (c) Calculate the plan area of the aeration tanks, given the following design parameters:
- Sludge depth 4m
- MLSS 3000mg/L (4 marks) (d) Design the plan area and depth of the secondary clarifiers, given:
- Average upward flow velocity in the range 0.6-1.3 m^3 /m^2 /hr
- Peak hourly upward flow velocity 2 m^3 /m^2 /hr
- Average hydraulic retention time 3 hours (2 marks) (e) Calculate the average volume (m^3 /day) of sludge to be wasted from the system each day, given:
- Qw = MLSS(VA + V (^) C ) / Sludge Age * S (^) W Where: V (^) A = Aeration tank volume (m^3 ); V (^) C = clarifier volume (m^3 ); SW = WAS suspended solids (mg/L)
- Average sludge age: 10 days
- Waste sludge concentration: 1.5% solids (2 marks) (f) Calculate the total weight (kg/day) of dry solids produced in the entire treatment plant each day. (3 marks)
B3 In a sewerage scheme for a town of 40,000 person equivalents, (PE), a pump station is employed to convey the wastewater to the treatment works by rising main. The static lift is 27m and the length of rising main is 2500m. Three dry weather flows, (3DWF), are pumped forward. The wastewater conveyed to the pump station is from a separate foul sewer. (a) Calculate the total friction losses in the rising mains for each pipeline for each of the rising main options listed. (2 marks) (b) Size the pump motor for each of the rising main options listed. (4 marks) (c) Calculate the annualised cost of each option. Establish which is the most economic pump and rising main option. (10 marks) (d) Describe how the pumping station is designed to cater for the diurnal variation in inflow to the pumping station from the urban area. Use graphs and/or sketches to support your answer. (4 marks)
Available rising main diameter: 450mm, 525mm or 600mm. Assumptions: 1 PE = 220L/day ks = 1. Pump Power required = {Q(m^3 /hr) x H (m)}/125 kW Ignore standby pumps in economic analysis. Allow for 0.5m of station loss. Capital Costs: Rising main: 450 mm dia. = €300/m; 525 mm dia. = €350/m; 600 dia. = €400/m Pumps: 100kW - €80 000; 50 kW - €50 000; 30 kW - €40 000; 5 kW - €10 000. Pump Station: 80 % of capital cost of pumps Running Costs: Cost of Capital: R = P{(1+r) Nr}/{(1+r) N^ - 1} Where: P is the capitalised amount of annual payments R with return on investment r over N years. Use 5% return on investment over 10 years Annual maintenance costs = 7% of capital cost of pumps. Cost of electricity = €0.1/kWh Attachments: Colebrook White Chart with ks = 1.
B4(a) Discuss the following rainfall relationships, and how they impact upon any hydrological analysis of a rural catchment: (i) Intensity-duration-frequency (ii) Depth-Area-Duration (6 marks)
B4(b) Describe the primary factors that influence the overall water-balance in a predominantly rural catchment. Explain how changes to these factors can influence the river hydrograph at the downstream end of the catchment. (8 marks)
B4(c) Describe how the infiltration capacity of the soil can influence the runoff from a greenfield catchment. (^) (3 marks)
Describe how the Dunne and Horton runoff mechanisms explain surface runoff. (3 marks)
