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
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