EXAM 2017, questions PDF

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PRIFYSGOL ABERTAWE SWANSEA UNIVERSITY College of Engineering SEMESTER 1 EXAMINATIONS

JANUARY 2017

EGA331/EG-M107

COASTAL PROCESSES AND ENGINEERING YEAR 3/YEAR 4 University calculators permitted only Translation dictionaries are not permitted, but an English dictionary may be borrowed from the invigilator on request.

Time allowed:

2 hours Answer THREE questions

Please insert any standard constants here:Closed Book SEE ADDITIONAL DATA SHEET PROVIDED Graph paper should be provided.

PLEASE NOTE THAT THIS EXAM PAPER IS PRINTED ON BOTH SIDES

TURN OVER Page 1 of 4

Q1 (a) Describe why wave transformation takes place when waves travel from deepwater to the shoreline. [5 marks] (b) A wave gauge mounted on a seaward end of a pier where the water depth is 6 m, measures a wave having wave height of 2.3 m and wave period of 8 sec. This wave is one of a train of waves that are travelling normal to the shore without refracting. (i) Determine the deepwater wave height and energy.

[5 marks]

(ii) Determine the magnitudes of the total pressure distribution at the seaward end of the pier at 1 m depth intervals across the water column. [12 marks] (ii) Where does the maximum pressure occur at this water depth? [3 marks] (TOTAL 25 MARKS)

Q2 (a) Describe why wave dispersion occurs. [5 marks] (b) Describe different types of wave breaking. [5 marks] (c) A wave has a measured height of 1.4 m and a period of 6 sec at a 6 m water depth. If this wave shoals on a 1:50 slope, determine the width of the surf zone and the wave height at breaking. Assume that the wave propagates in an orthogonal direction to the shoreline without refracting. [15 marks] (TOTAL 25 MARKS)

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Q3 (a)

Describe what is meant by the term ‘tidal rotation’ and explain what causes it. [4 marks]

(b)

The table below (Table Q3) summarises some incomplete information known about the tidal type and tidal constituents at three harbours. Use this information to complete the table. Estimate the mean spring and neap tidal ranges at each harbour. Amplitude (cms)

Harmonic M2 S2 K1 O1 Z0 Tidal ratio

Period (hours) 12.42 12.00 23.93 25.83 -

Harbour A

Harbour B

? 68 15 17 0

22 ? 35 26 50

3 4 70 68 -10

2.00

?

0.16

Harbour C

Table Q3: Tidal information at three harbours [9 marks]

(c)

A port is located in an estuary, upstream of a bridge base whose deck is at +20 mCD, (Chart Datum). There is a sandbar across the entrance of the estuary which has a drying height of +2 mCD. A ship wishing to enter the port has a draft of 4 m and requires a vertical clearance allowance of 10 m. The tide is semi-diurnal and exhibits a quasiresonant behaviour with the M2 tidal constituent amplified preferentially. The mean tide level is +7 mCD. The maximum, spring and neap tidal ranges are 14 m, 12 m and 8 m respectively. • Find the amplitudes of the tidal constituents M2 and S 2 , and use •

them to explain why the M2 constituent might be quasi-resonant. Draw a diagram showing LAT, HAT, MTL, MLWN, MLWS, MHWN, MHWS, the level of the base of the bridge and the level

•

of the top of the sand bar, specifying their numerical values relative to CD. What is the water level range that a ship can dock safely? [12 marks] (TOTAL 25 MARKS) TURN OVER Page 3 of 4

Q4 (a) What are the main causes of tsunamis? [2 marks] (b) One of the waves in a tsunami has a period of 50 minutes and a height of 0.4 m at a point in the ocean where the depth is 4000 m. Determine the celerity (phase speed) and wave length of this wave. The tsunami wave crest is parallel to the depth contours, which are themselves parallel to the shoreline. Determine the celerity, wavelength and height of the tsunami wave once it has propagated inshore where the still water depth is 8 m. (Use shallow water wave theory.) What process causes the change in wave height? [9 marks] (c) Explain what is meant by beach nourishment and outline the options for material placement and the techniques available to do so. [6 marks] (d) In the context of the 1-line beach model, the shoreline position, y(x,t), is measured relative to a reference line (the x-axis) and varies with x and with time t. A long straight shoreline is defined by y(x,0) = 0. A beach nourishment scheme results in the shoreline being advanced by a distance of 100m over a 1km long stretch of shoreline defined by |x| < 500 m. The coastal constant, K, has been estimated to be 106 m2/year. The solution of the one-line equation in this case may be written as:

y=

100   500 − x   500 + x   erf erf +     2   2 Kt   2 Kt  

where erf(z) is the error function and z is the argument. Using the approximation

erf ( z ) ≈

2  z3   z −  π 3 

derive an expression for the position of the beach in terms of powers of x and hence estimate the position of the centre of the nourishment, (x = 0), after 1, 5 and 10 years. What do you conclude from your results about the effectiveness of the nourishment? [8 marks]

(TOTAL 25 MARKS)

END OF PAPER

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