chap 8 hydrological forecasting (msma) 1213-1

8/12/2019 Chap 8 Hydrological Forecasting (MSMA) 1213-1

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CHAPTER 8

HYDROLOGICAL FORECASTING

(Flood/Flow Estimation)


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COURSE OUTCOME:

Estimatepeak discharge andproposeurban

drainage dimensions using MSMA(UrbanStormwater Management Manual for Malaysia)

and Probability Distribution.

Lesson Outcomes:

Estimate the peak discharge using MSMA

Propose dimension of drainage system Calculate and estimate the peak discharge

using probability distribution


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Introduction Hydrological forecasting is important to

estimateand manage all event related toflood.

Flood forecasting is the use of real time

precipitation and streamflow data in rainfall-runoff and streamflow routing modelsto

forecast flow rates and water levelsfor

periods ranging from a few hours to days

ahead, depending on the size of thewatershed or river basin.

Its can forecast the effects of urbanization

on runoff from undeveloped watershed.


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Flood Control

Structural Methods

Flood Control Reservoir Detention/Retention Pond

Dam

Channel Modification & Environmental Impacts

Swales

Diversion Channels, Levees & By Pass Channels

Increased Infiltration

To increase the amount of previous area wherever

possible

Example; Porous parking lots through the use of

concrete block or similar shapes laid such as water

can infiltrate trough the soil-filled center.


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Detention Pond


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Retention Pond


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Flood Control Structure

Placing control structures in a river system is a management technique used

to mitigate flooding associated with periods of heavy rainfall.

While this approach helps to reduce flooding in urban and agricultural areas,

flow patterns in the vicinity of a control structure can jeopardize its stability.

In the example to the right, local flow patterns have caused scour to occur in

the channel used to convey flood flows.

Dam


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Swale


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Nonstructural Methods

Flood proofing Flood warning mechanisms

Land use controls such as zoning &

development ordinances

Flood insurance programs

Flood preparedness activities

Public awareness & education programs


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Methods

There are several methods to estimate

flood or flow rate such as;

Empirical Formula Rational Method

Frequency Analysis

Normal Distribution Extreme Gumbel

Log Pearson Type III


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Empirical Formula

Q = CAn

Q = Flood discharge (m

3

/s)A = Area of catchment (km2)

n = Index (0.5 1.25)

C = Coefficient (wheather and catchment)

log Q = log C + n log A

need many catchment


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Rational Method

Calculate Peak Flow

Qp= CIA

Qp

= the peak runoff rate

C = the runoff coefficient (assumed to dimensionless)

I = The average rainfall intensity for a storm

with a duration equal to a critical periodof the time tc

A = the size of the drainage area

tc

= the time of concentration


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Rational Method

Useful for small, usually urban, watersheds(


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Runoff Coefficient

Coefficient that

represents the

fraction of runoff

to rainfall

Depends on typeof surface

Iowa DOT Design Manual, Chapter 4, The Rational Method


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Runoff Coefficient



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Runoff Coefficient



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Runoff Coefficient

When a drainage area has distinct partswith different C

Used weighted average

C = C1A1+ C2A2+ .. + CnAn

Ai


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Average Recurrence Interval, ARI(Design Event)

2 ARI-- Design of intakes and spread ofwater on pavement for primary highways andcity streets

10 ARI-- Design of intakes and spread ofwater on pavement for freeways andinterstate highways

50 ARI-- Design of subways (underpasses)

and sag vertical curves where storm sewerpipe is the only outlet 100 ARI-- Major storm check on all projects


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Intensity

Average intensity for a selectedfrequency and duration

Based on design event (i.e. 50-yearstorm)

Overdesign is costly

Underdesign may be inadequateDuration


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Time of Concentration (tc)

Time for water to flow fromhydraulically most distance point on the

watershed to the point of interest

tc= to+ td (MASMA)

to= time of overland flow

td= time of flow in drain


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Time of Concentration (tc)

Depends on:

Size and shape of drainage area

Type of surface Slope of drainage area

Rainfall intensity

Whether flow is entirely overland orwhether some is channelized


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Rational Method using MASMA

One of the most frequently used urbanhydrology methods in Malaysia (simplicity)

It gives satisfactory results for smallcatchment (up to 80 hectares)

Qy= C yI

tA

360 Q

y= y year ARI peak flow (m3/s)

C = dimensionless runoff coefficient

yI

t= y year ARI average rainfall intensity over time

of concentration, tc(mm/hr)

A = drainage area (ha)


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Assumption

1. The peak flow occurs when the entirecatchment is contributing to the flow

2. The rainfall intensity is the same over theentire catchment area

3. The rainfall intensity is uniform over a timeduration equal to the time of concentration, tc

4. The ARI of the computed peak flow is thesame as that of the rainfall intensity

i.e. A 5 year ARI rainfall intensity will producea 5 year ARI peak flow


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Not recommended

the catchment area is greater than 80hectares

ponding of stormwater in the catchment mightaffect peak discharge

the design and operation of large (and hencemore costly) drainage facilities is to be

undertaken, particularly if they involvestorage


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4. Determine average rainfall intensity,yIt

- calculate yItfor design ARI of y years and

durat ion t equal to the t ime of concentrat ion,

from IDF data for area of interest

- Refer equati on 13.1and 13.3, Table 13.A1and

13.3, Figu re 13.3

5. Estimate runoff coefficients

- estimate C values fo r each segm ent if there are

dif ferent land covers- Design Chart 14.3 and 14.4


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6. Calculate average runoff coefficientC

avg= C

iA

i

Ai

7. Calculate peak flow rate from equation

Qy= C yI

tA

360


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Example 1

Determine the design peak

for flow generated from a

minor drainage of medium

density residential area of

10 hectares in KualaLumpur. Assume 80 m of

overland flow followed by

400 m of flow in an open

drain. Catchment area

average slope = 0.5%. The

catchment is shown.

Catchmentarea 10 ha

Main drain

River


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Solution

Step 1: Determine tc

Step 2: Determine I and C

ln(I) = a + b ln t + c (ln t)2+ d (ln t)3 Eqn. 13.3

Pd= P30FD(P60 - P30) Eqn. 13.3

I = Pd/ d Eqn. 13.4

Step 3: Determine Qp


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Exercise 1

The catchments area in Melaka Townhas two different characteristics shown

in figure. Determine the total design

peak flow generated from minordrainage for the whole catchments.

Average velocity in open drain is 1.0

m/s and average slope is 1.0%.


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E i 2


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Exercise 2

The catchments in rural area at KualaPilah, Negeri Sembilan has two different

areas shown in figure. Determine the

design peak flow generated from minordrainage for Area A and major drainage

for Area B. The average velocity in

open drain is 1.0 m/s and the averageslope is 0.5%.


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Data for whole catchment

Overland flow = 100 m

Flow in Drain = 450 m

Low Density Residential

Surface: Poorly grassed

Area A = 5 ha

Medium Soil - Forest

Area B = 10 ha

Sandy Soil- Forest


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Most effective cross section


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The one that will have the greatest

capacity for a given slope, area androughness.

If these parameter constant, velocity will

be greatest when the wetted perimeteris smallest.

The most efficient (effective) is the most

economical. Semicircular smallest wetted

perimeter

M t ff ti ti


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The most efficient cross section happened

when:

Flow rate(Q) is maximum

Slope(S) constantso hydraulics radius(R) is

maximumand wetted perimeter(P) isminimum

dP = 0

dy


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Example 2

Using the design peak discharge from

Example 1, propose the dimensions of

rectangular drainage with freeboard which

is 5% from water depth.

E i 3


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Exercise 3

Using the design peak discharge from

Exercise 1, propose the dimensions ofrectangular drainage with freeboard

which is 5% from water depth.

chap 8 hydrological forecasting (msma) 1213-1

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