NATIONAL EXAMINATIONS -MAY 2002

98 -CIV -B4

Engineering Hydrology

3 Hours Duration

Notes:

I. If doubt exists as to interpretation of any question, the candidate is urged to submit a clearstatement of any assumptions made with the answer paper.

2. Candidates may use one of two calculators, a Casio FX-991 or a Sharp EL-540.This is a closed book examination.

Equations, graph paper and any other required material are provided with thisexamination.

3,

4. Any five questions constitute a complete examination. Only the first five questions asthey appear in your examination book will be marked.

5. All questions cany equal weight.

Each part of a question carries equal weight.6.

7. This examination paper consists of eight pages, including this cover page and two pagesof probability paper. (The horizontal scale of the probability paper is in percent.)

Page 1 of 8

Briefly answer each of the following (5 to 10 lines each), using sketches whereappropriate.

1.

1)

2)

3)

4)

5)6)

2.

18

5

1610

1415

1025

12

20

835

645

425

25

Time (hours) OFlow (m3/s) 5Calculate the 4 hour unit hydro graph for the watershed using hydro graph ordinates of

m3/s for the times listed.I)

Calculate the direct runoff in mm from the watershed, and the runoff coefficient for the

watershed2)

Calculate the runoffhydrograph that would have resulted if the storm lasted 8 hours

instead of 4 hours.3)

A storage facility is to be built to reduce peak flows from this watershed to 25 m3ls.Estimate the volume required for this facility. State any assumptions used.4)

98 -CIV -B4, May 2002 Page 3 of 8

3. The following annual floods were recorded at a river gauging site.

Year Flow Year Flow Year Flow Year Flow

3.14.4 6 11 4.3 16 7.3

2 3.3 7 6.5 12 4.7 17 3.5

3 7.0 8 5.5 13 11.0 18 5.6

4 8.3 9 4.0 14 6.0 19 5.0

5 2.3 10 9.0 15 2.8

a) Using probability paper, detennine the 2,5, 10 and 100 year return period floods.

b) What is the probability that the annual flood will be larger than 11 m31s at least once inthe next 5 years?

c) A fishing camp operator is proposing to upgrade his dock near this gauge, and hasdetennined that his facilities could withstand floods up to 11 m3/s. He has alsodetennined that if a flood greater than this value occurs once in a 5 year period the facilitycan be repaired and the operation will be profitable over the 5 year period. If a largerflood occurs, the operator will lose money. What is the probability that the operator willbe profitable in a given 5 year period?

Plotting position: p = m / (n+ 1 )m = rank (from largest to smallest)

Risk(R): R= l-(l-p)DProbability ofExceedance: p = I/ Tr Tr = Return Period (years)

p = probability of exceedancen = number of years ofrecord

98 -CIV -B4, May 2002 Page 4 of 8

4. A dam at a lake outlet is being considered for rehabilitation to provide storage forhydroelectric power development.

The lake has an area of 10 krn2. The outlet of the dam is controlled by stop logs, and theelevation of the bottom of the stop log bays is 100 m. The total width of the stop log baysis 10 m. The 100 year design stonn inflow hydrograph to the lake is provided below.

Time

period

(hours)

Inflow Timeperiod(hours)

[nf1ow Timeperiod(hours)

Inflow Time

period

(hours)

Inflow

(m3/s) (m3/s) (rn3/s) (m3/s)0-5 5 20-25 240 40-45 130 60-65 25

5-10 10 25-30 255 45-50 90 65-70 15

50 30-35 225 50-55 6010-15 70-75 10

55-6015-20 160 35-40 180 40 75-80 5

Assuming no initial outflow from the lake, and an initial stop log elevation of 105 m,detern1ine the outflow hydrograph from the dam and the maximum elevation the lake willreach for the 100 year inflow hydrograph. Assume weir flow over the stop logs, and thatthe lake can be considered to have vertical sides.

Weir flow equation: Q = C L h3/2 whereQ = discharge (m3/s),C = weir coefficient (use 1.5),L = weir length (m), andh = head over the weir (m)

Continuity equation I -0 = ~S / ~t, whereI = average inflow over time period (m3/s),O = average outflow over time period (m3/s),~S = change in storage over time period (m3), and~t = time period (sec).

98 -CIV -B4, May 2002 Page 5 of 8

5. The following hydrograph was measured at the upstream end of a river reach.

a) Calculate the hydro graph at the end of the reach using the Muskingum routing method,given that the parameters K and x were determined to be K = 12 hours and x= 0.2. TheMuskingum routing equations are:

02 = Co 12 + C1 II + C2 01 , whereCo = ( -Kx + 0.5 ~t) / (K -Kx + 0.5~t)C1 = ( Kx + 0.5 ~t) / (K -Kx + 0.5!:1t)C2 = (K -Kx + 0.5 ~t) / (K -Kx + 0.5~t)Co + C1 + C2 = 1II' 12 = Inflows to reach at times 1 and 2 respectively01 , O2 = Outflows from reach at times 1 and 2 respectively~t = time step

b) How would you detemline the hydro graph at a point two thirds of the way downstreamfor the same inflow hydro graph. No calculations are required, but explain your answer intenns of the parameters used for the Muskingum routing method.

c) What would the peak discharge at the end of the reach be using the time lag routingmethod?

6. a)

porositypiezometric surfacestorage coefficienttransmissi vi tyspecific yieldhydraulic conductivity

2)