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International Masters Programme in

Urban Water and Sanitation

Examination SE04 Urban Drainage and Sewerage Thursday 5 February, 2015 09.45 - 12.45 hrs

CLOSED BOOK

Please note the following:

1. This examination paper consists of 23 pages (including this front page). Please make

sure that your examination paper is complete before starting the exam.

2. Candidates are not permitted to bring any written and printed material into the

examination room.

3. Please keep your answers concise.

4. Submit the examination paper and your answer papers in the examination cover.

5. Write your student number and locker number on each page. Do not write your name on

the exam pages or answer sheets.

6. Clearly number the questions you answer.

7. Write in pen only: do not use pencil.

8. During the examination, please plan your time wisely; check the time regularly to reduce

any time pressure towards the end of the examination session.

9. The weight of question is indicated by the maximum number of points per question.

Good luck!

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Please note the following: 1. This exam has four parts.

Part Topic Mark

(%)

Time allocated

(min)

1 Rainfall and Wet Weather Flow 25 45

2 Introduction to Urban Drainage and Sewer

Systems & Hydraulics for Urban Drainage

20 30

3 Sewer processes 25 45

4 Design Urban Drainage and Pumping Stations 30 60

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Part 1 - Rainfall and Wet Weather Flows (25%)

Lecturer: Assela Pathirana

Time allocated for part 1 = 3/4hour

Note: Each of the questions in this part carries equal marks.

Question 1. Explain the process of formation of convective clouds.

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Question 2. You are given an hourly rainfall time series covering a period of 50 years. Explain the process you would follow to obtain estimates for hourly extreme rainfall with

return periods 5, 40 and 100 years. (Hint: You are not required to remember

equations, or to do sample calculations. Just explain the process clearly. You may

use diagrams.)

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Question 3. What is the Urban Heat Island? What is the impact it could have for storm drainage design?

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Part 2 - Introduction to Urban Drainage and Sewer Systems & Hydraulics for

Urban Drainage (20%) Lecturer: Michael Hammond

Time allocated for part 2 = 1/2 hour

Note: Useful formulae and Moody diagram are provided at the end of Part 2.

Question 4. Separate drainage systems have many environmental benefits. However, many cities in the developed countries (e.g. Tokyo, Delft, London..) have combined sewer systems.

What could be the reasons for this?

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Question 5. A 600mm diameter circular pipe, running full, with estimated absolute roughness height of 1.5mm is used to pump storm water over a length of 1500m, at a rate of 0.6m

3/s.

Kinematic viscosity, = 1.110-6m2/s.

Determine

a) The Reynolds number

b) The roughness value, , using an appropriate method c) The head loss over 1000m due to friction

d) The drop in pressure in the pipe (in Pa or kN/m2)

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Useful Formulae and Charts

Equation name Equation where

Reynold's number

vDRe

v is mean velocity (m/s)

D is pipe diameter (m)

is kinematic viscosity (use =1.1*10-6m

2/s)

fh head loss due to friction (m)

friction factor (no unit) L is pipe length (m) g is gravitational acceleration (m/s2)

(use 9.81m/s2)

sk is pipe wall roughness (m)

fS is hydraulic gradient or friction slope

( fh L ), (no unit)

C is runoff coefficient (no unit)

i is rainfall intensity in mm/hr A is catchment area in hectares

Darcy-Weisbach

g

v

D

Lh f

2

2

Colebrook-White

e

s

RD

k 51.2

7.310log2

1

If we replace from Darcy-

Weisbach equation

to Colebrook-

White

DgSDD

kDgSv

f

s

f2

51.2

7.310log22

Rational formula

360p

CiAQ

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Part 3 - Sewer processes (25%)

Lecturers: Jes Vollertsen, Asbjrn Nielsen and Natrcia Matias

Time allocated for Part 3 = 3/4hour

Consider a pressure main, in which hydrogen sulfide is produced in the biofilm with a constant

flux rate. No significant production takes place in the water phase. The dissolved oxygen

concentration and the sulfide concentration at the inlet to the pressure main are both 0. The pipe

length is 3500 m, and the volumetric flow rate of the wastewater is Q = 150 m3 h

1.

The sulfide flux from the biofilm into the wastewater can be estimated from Equation 1.

Equation 1: 0.5 200.005 ( 50) 1.03T Ca Sr COD

where,

ra Sulfide flux from the sewer biofilm (gS m-2

h-1

)

CODs Dissolved organic matter (g m-3

)

T Wastewater temperature (C)

The temperature of the wastewater is 17C and the dissolved organic matter concentration

(CODS) is 250 g m-3

.

Two different scenarios for transport of the wastewater shall be compared with respect to the

sulfide concentration in the wastewater phase at the outlet of the pipe. Transport will take

place in a pipe (pipe #1) with an inner diameter of 0.3 m and in a pipe (pipe #2) with an

inner diameter of 0.5 m.

Question 6. What is the volume-specific sulfide production rate for the two scenarios; i.e., expressed as gS m-3 h-1?

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Question 7. What are the hydraulic retention time and the resulting sulfide concentration in the wastewater at the outlet of the pressure main in the two scenarios?

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Question 8. Assuming the wastewater pH to be near neutral (pH 7), can we expect sulfide related problems in terms of odor and concrete corrosion in the two scenarios?

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Part 4 - Pumping stations and Urban Drainage Design (30%)

Lecturer: Bert van Duijl

Time allocated for Part 3 = 1 hour

The weight for question 1 and 2 are equal: each 15%

Note: No calculations are needed for the determination of the power; just plug in your

values in the formulae.

Question 9 Pumping stations

Two pumps have been installed for pumping wastewater and storm water:

a. pump no 2 with impeller size 300 mm operates for waste water only b. pump no 1 with impeller size 359 mm operates when storm water enters the sewer

system.

The characteristics of the pumps with the corresponding efficiencies are given in Figure 1.

Both pumps are pumping into the same delivery main (parallel installation)

The discharge level at the end of the rising main is 7 m above the highest water level in the sump

of the pumping station. The hydraulic losses in the rising main and in the pumping station are 4.0

m at a flow of 50 liters/second (or 216 m/hour).

a. Determine the head loss for the flows of 25 and 75 L/s.

Draw the system curve in the sketch (Figure 1).

b. How much is the discharge, the efficiency and the required power of the wastewater pump (pump no 2) ?

c. How much is the discharge of the selected pump at the lowest water level in the sump, which is 2 m below the highest water level? How much is the efficiency of the pump? Indicate in the sketch the working range of the pump between the lowest and highest water levels in the sump.

d. Would it be possible to operate both pumps at the same time in order to increase the capacity of the pumping station? Explain your answer?

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Figure 1 Pumping characteristics

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Question 10 Urban Drainage Design

Note: No calculations are needed; just plug in your values in the formulae.

Show hydraulic grade lines in the given sketch.

A storm water drain, diameter 1.2 m, length 1000 m, discharges into a river with a varying water

level. The ground- and invert levels at the upper end of the drain are +8.00 m and +5.30 m as

shown in Figure 2. The slope of the drain is 0.5 o/oo (1:2000).

Questions

I. Determine the discharge and velocity of flow in the drain when flowing full (100%) and at a free discharge into the river

II. Determine whether the drain is self-cleansing Note: The drain is self-cleansing when a tractive force of 2.4 N/m

2 is obtained when

running full.

III. How much is the velocity of flow and the water depth when the pipe is running 5% full. IV. Determine the maximum possible flow of the drain (see Figure 2).

Show the hydraulic grade line in the sketch (Figure 2).

Formulas

ASRkQ Str2/13/2

The roughness value is: k = 75 m1/3/s

Tractive force: = w g R S

Shields: min = f g d (g - w)

for sewers: f = 0.06 to keep relatively clean grit in suspension The density of the grit is 2650 kg/m3

The gravity force is 9.81 m2/s.

21

gQHN

Note: The partial flow diagram is given on separate sheet.

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Figure 2 Longitudinal section of storm water drain

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..

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Hydraulic characteristics for pipes, flowing partly full

based on the flow formula of Prandtl-Colebrook) Qfull , vfull = flow and velocity for pipe running full; D = diameter Filled section: Q, v = flow and velocity in partially filled section h = depth of flow, R = hydraulic radius; w = width of water surface

Q/ Qfull h/D v/vfull R/D w/D Q/ Qfull h/D v/vfull R/D w/D

0.001 0.023 0.17 0.0152 0.2998 0.41 0.445 0.95 0.2313 0.9939

0.002 0.032 0.21 0.0210 0.3520 0.42 0.451 0.96 0.2334 0.9952

0.005 0.049 0.28 0.0319 0.4317 0.43 0.458 0.96 0.2359 0.9965

0.01 0.068 0.34 0.0439 0.5035 0.44 0.464 0.97 0.2380 0.9974

0.015 0.083 0.38 0.0532 0.5518 0.45 0.470 0.97 0.2401 0.9982

0.02 0.095 0.41 0.0605 0.5864 0.46 0.476 0.98 0.2420 0.9988

0.03 0.116 0.46 0.0731 0.6404 0.47 0.482 0.99 0.2441 0.9994

0.04 0.134 0.50 0.0837 0.6813 0.48 0.488 0.99 0.2461 0.9997

0.05 0.149 0.54 0.0923 0.7122 0.49 0.494 1.00 0.2481 0.9999

0.06 0.163 0.57 0.1002 0.7387 0.50 0.500 1.00 0.2500 1.000

0.07 0.176 0.59 0.1075 0.7616 0.51 0.506 1.00 0.2519 0.9999

0.08 0.188 0.61 0.1141 0.7814 0.52 0.512 1.01 0.2538 0.9997

0.09 0.200 0.63 0.1206 0.8000 0.53 0.519 1.01 0.2559 0.9993

0.10 0.211 0.65 0.1265 0.8160 0.54 0.525 1.02 0.2577 0.9987

0.11 0.221 0.67 0.1317 0.8289 0.55 0.531 1.02 0.2595 0.9981

0.12 0.231 0.69 0.1369 0.8429 0.56 0.537 1.02 0.2612 0.9973

0.13 0.241 0.70 0.1421 0.8554 0.57 0.543 1.03 0.2629 0.9963

0.14 0.250 0.72 0.1466 0.8660 0.58 0.550 1.03 0.2649 0.9950

0.15 0.259 0.73 0.1511 0.8762 0.59 0.556 1.03 0.2665 0.9937

0.16 0.268 0.74 0.1556 0.8858 0.60 0.562 1.04 0.2681 0.9923

0.17 0.276 0.76 0.1595 0.8940 0.62 0.575 1.04 0.2715 0.9987

0.18 0.285 0.77 0.1638 0.9028 0.64 0.587 1.05 0.2745 0.9847

0.19 0.293 0.78 0.1676 0.9103 0.65 0.594 1.05 0.2762 0.9822

0.20 0.301 0.79 0.1714 0.9174 0.66 0.600 1.05 0.2776 0.9798

0.21 0-.309 0.80 0.1751 0.9242 0.68 0.613 1.06 0.2806 0.9741

0.22 0.316 0.81 0.1784 0.9298 0.70 0.626 1.06 0.2834 0.9677

0.23 0.324 0.82 0.1820 0.9360 0.72 0.640 1.07 0.2862 0.9600

0.24 0.331 0.83 0.1851 0.9411 0.74 0.653 1.07 0.2887 0.9520

0.25 0.339 0.84 0.1887 0.9465 0.75 0.660 1.07 0.2900 0.9474

0.26 0.346 0.85 0.1918 0.9514 0.76 0.667 1.07 0.2912 0.9426

0.27 0.353 0.86 0.1948 0.9558 0.78 0.682 1.07 0.2936 0.9314

0.28 0.360 0.86 0.1978 0.9600 0.80 0.697 1.07 0.2958 0.9191

0.29 0.367 0.87 0.2007 0.9640 0.82 0.713 1.08 0.2979 0.9047

0.30 0.374 0.88 0.2037 0.9677 0.84 0.729 1.07 0.2997 0.8890

0.31 0.381 0.89 0.2066 0.9713 0.85 0.738 1.07 0.3006 0.8794

0.32 o.387 0.89 0.2090 0.9741 0.86 0.747 1.07 0.3014 0.8695

0.33 0.394 0.90 0.2118 0.9773 0.88 0.766 1.07 0.3028 0.8467

0.34 0.401 0.91 0.2146 0.9802 0.90 0.786 1.07 0.3038 0.8203

0.35 0.407 0.92 0.2170 0.9802 0.92 0.808 1.06 0.3043 0.7877

0.36 0.414 0.92 0.2197 0.9851 0.94 0.834 1.05 0.3040 0.7442

0.37 0.420 0.93 0.2220 0.9871 0.95 0.849 1.05 0.3033 0.7161

0.38 0.426 0.93 0.2243 0.9890 0.96 0.865 1.04 0.3022 0.6834

0.39 0.433 0.94 0.2269 0.9910 0.98 0.905 1.03 0.2972 0.5864

0.40 0.439 0.95 0.2291 0.9925 1.00 1.000 1.00 0.2500 0.0000