One Day Workshop

“Reducing Errors in

Hydrologic and Hydraulic

Modelling of Drainage

System”

Introduction to Hydrology and Hydraulics

• Ir. Abd Jalil Hassan

Backwater Computation of Prismatic Channel

• Ir. Abd Jalil Hassan & Pn. Marhanis

Multiple Drainage Network Design

• Ir. Abd Jalil Hassan & Ir. Hambali

Pond Design Under Backwater Effect

• Ir Abd Jalil Hassan & En. Afizi

Online and Offline Pond Design and Impact on Low and High ARI’s

• Ir Abd Jalil Hassan & En. Azad

Questions & Discussion

Introduction to

Hydrology and Hydraulics

&

Backwater Computation of

Prismatic Channel

Abd Jalil Hassan Marhanis Zailan

August 2019

3

Introduction to

Hydrology and Hydraulics

Before development

New development

Flood

Flood downstream

Conventional approach

Pond

MSMA

The General Concept of the Hydrological Cycle

The hydrological cycle

is a closed system in

that water circulation

in the system always

remains within the

system.

The whole cycle is

driven by the excess

of incoming solar

radiation over outgoing

radiation.

The cycle consists of

these subsystems:

atmospheric, surface

runoff, subsurface

10

Rainfall Runoff Model

Rational Method

Modified Rational (Hydrograph)

Time Area Method

Unit Hydrograph Method

Etc

11

Rational Method

Q = CIA

o C= Runoff coefficient

o I = Rainfall Intensity

o A = Catchment area

MSMA - Only valid for small catchment

< 80 hectares

Hydrological Procedure No 5 –For rural catchments with

areas ranging from

3.9 to 186 km²

12

1.0

Runoff

Coeff

icie

nt,

C

Rainfall Intensity , I (mm/hr)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

190 200

2

1

7

6

5

4

3

8Impervious Roofs, Concrete

City Areas Full and Solidly Built Up

Urban Residential Fully Built Up with Limited Gardens

Surface Clay, Poor Paving, Sandstone Rock

Commercial & City Areas Closely Built Up

Semi Detached Houses on Bare Earth

Bare Earth, Earth with Sandstone Outcrops

Bare Loam, Suburban Residential with Gardens

Widely Detached Houses on Ordinary Loam

Suburban Fully Built Upon Sand Strata

Park Lawns and Meadows

Cultivated Fields with Good Growth

Sand Strata8

7

6

5

4

3

2

1

Time-Area Method

t

(a) Rainfall H istogram (b) C atchment Isochrones

2

t

3 t

4 t

Isochrones

A rea A1

A2

A4

A3

(c) Time-A rea C urv e (d) Runoff Hy drograph

Runoff

(m

3/s

)

T ime t

tq1

q2

q3

q5

q4

Rain

fall

inte

nsi

ty I

T ime t

I1

I2 I

3

0

I4

t 2 t 3 t 4 t

t

Cum

ula

tive A

rea

T ime t

0 t 2 t 3 t 4 t

13

Conceptual Time Area Method

Say a catchment area with 30 minutes Tc Divide it into 6 Isochrone of 5 minutes

1 2

3

4

6

5

Sample calculation

T1 = R1 x A1

T2 = R2 x A1 + R1 x A2

T3 = R3 x A1 + R2 x A2 + R1 x A3

T4 ………

Unit Hydrograph Method

SCS Unit Hydrograph

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5

t/Tp

q/qp

5.0

7.08.0

1900

9.0)/1000(100

S

CNLtc

16

Sample of IDF

Temporal Patern

Duration

(min)

No. of

Time

Periods

10 2 0.570 0.430 - - - - - - - - - -

15 3 0.320 0.500 0.180 - - - - - - - - -

30 6 0.160 0.250 0.330 0.090 0.110 0.060 - - - - - -

60 12 0.039 0.070 0.168 0.120 0.232 0.101 0.089 0.057 0.048 0.031 0.028 0.017

120 8 0.030 0.119 0.310 0.208 0.090 0.119 0.094 0.030 - - - -

180 6 0.060 0.220 0.340 0.220 0.120 0.040 - - - - - -

360 6 0.320 0.410 0.110 0.080 0.050 0.030 - - - - - -

Fraction of Rainfall in Each Time Period

30 minute Duration

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3 4 5 6

Time Period

15 min Duration

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3

Time Period

60 minute Duration

0.0

0.1

0.2

0.3

1 2 3 4 5 6 7 8 9 10 11 12

Time Period

120 minute Duration

0.0

0.1

0.2

0.3

0.4

0.5

1 2 3 4 5 6 7 8

Time Period

180 minute Duration

0.0

0.1

0.2

0.3

0.4

0.5

1 2 3 4 5 6

Time Period

360 minute Duration

0.0

0.1

0.2

0.3

0.4

0.5

1 2 3 4 5 6

Time Period

10 min Duration

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1 2

Time Period

18

Concepts and Principles in

Hydraulics

What is Hydraulics?

Study of how water moves

Deterministic based on mass

conservation and force balance

Uses principles of momentum and energy

transfer

Provides water levels, velocities, flow

rates

20

Open Channel Principles

Energy and Momentum

Type of Flow

States of Flow

Water Surface Profiles

21

Energy and Momentum

Energy is the “capacity” to do “work”

Kinetic energy (from speed)

Potential energy (from position)

Total Energy is conserved

Energy “losses” arise because some

energy types are ignored in analysis

22

Energy and Momentum

Momentum is mass times velocity

Changed by forces and impulses

Use Newton’s second law

Has magnitude and direction

Used to calculate forces on structures

Can be applied where energy “losses” are

large

23

Type of Flow

Uniform flow

depth remain the same along the channel

Non Uniform flow

Depth varies along the channel

Steady flow

Discharge remain constant at all time

Unsteady flow

Discharge varies with time

Uniform flow profile

Distance

Wate

r le

vel

Water surface is approximately parallel to average bed slope

Bed profile

Average bed slope

25

Uniform flow

Central to understanding of open channel

hydraulics

Energy “line”, water surface slope and

channel bed are all parallel

The depth is called “Normal Depth”

Several assumptions in the analysis

Rarely occurs in practice!

26

Calculating Uniform Flow

Assumptions are

steady flow

regular shape of cross-section

no change of velocity, depth or slope with

distance along channel

rate of “loss” of potential energy balances

work done against flow resistance - but ...

What is really happening?

27

Uniform flow equation

28

Manning Formula

Q = 1/ŋ A R2/3 S1/2

ŋ = Surface roughness

A = flow area

R = hydraulic radius ( A/P)

P = Wetted perimeter

S = water gradient / assume channel bed gradient

Hydraulic Radius

Represents the shape of the cross

section

Ratio of Area, A to Wetted Perimeter, P

R = A / P

Area A

P

29

What affects roughness?

Bed surface material

Channel irregularity

Channel alignment and sinuosity

Depth and discharge velocity

Vegetation and sediments

Altitude or gradient

30

Slope

Measure elevation change between outlet

and head of channel

The steeper the slope, the faster the

velocity of flow.

Sample calculation – Continuity Equation

Q = VA Where,

Q = the volumetric flow rate

A = the cross sectional area of flow

V = the mean velocity

If

V = 1.5m/s

A = 8m

Sample calculation – Manning Formula

b = 2

d

Q= (𝟏

𝒏)A𝑹𝟐/𝟑𝑺𝟏/𝟐

If,

Slope = 1: 1000

n = 0.012 (concrete drain)

A = b x d

P =b + 2d

R =A/P

= (bxd)/(b+2d)

Q = 1 x (bxd) x {(bxd)/(b+2d)}2/3 x S0.5

If Q = 4.5m3/s From trial and error it will be easy to look for the

solution

If Slope = 0

Q = ?

Classification of Flows

Flow States

Sub-critical

Slow and deep - low kinetic energy

Super-critical

Fast and shallow - high kinetic energy

Critical

Special, unique relation between velocity

and “mean” depth, y

Vc = (gy)1/2

35

Flow States

Critical depth yc when

Vc = V = (gyc)1/2

Sub-critical

y > yc

V < (gy)1/2

Super-critical

y < yc

V > (gy)1/2

36

Alternative Classification

Froude number

Fr = V/(g y)1/2

where V is velocity (m/s),

y is depth (m)

g is acceleration due to gravity (m/s2)

Fr < 1 subcritical flow

Fr = 1 critical flow

Fr > 1 supercritical flow

37

Normal and critical depths

Fr<1

Fr>1

Fr=1

Critical depth

Sub critical yn > yc

Supercritical yn < yc

38

Energy Function

Specific Energy E = y + V2/2g

Graph of E for a fixed discharge q

39

Transition - Hydraulic Jump

40

Water profiles

Gradually – Rapidly varied flow

Non-uniform Flow Profiles

42

43

St. Venant equation

0

0

fSSgx

yg

x

VV

t

V

t

A

x

Q

--- Continuity equation

--- Momentum equation

Local acceleration

Convective acceleration

Pressure Force

Gravity Force

Friction Force

Kinematic wave

Diffusion wave

Dynamic wave

44

Sample Model

Step 1

To build M2 curve

Step 2

To build M1 curve

Thank You

47