Module 6_Itroduction to Flood Estmation and Modeling (2015)

Module 6 Introduction to Flood

estimation and modelling Ir Dr Kelvin Kuok

Faculty of Engineering, Computing & Science Room E613

[email protected]

CVE30001 – Urban Water Resources

2015

Flood Modelling Estimation Main Focus for Civil Engineers Modelling Rainfall – Runoff Processes – Complex Computer Models

• Spatial and Temporal Variation in Characteristics

• Event Model vs Continuous Simulation

• Model Whole Hydrograph – Simple Analysis Methods • Peak Runoff Estimation – rational method - Probabilistic or Statistical Estimate

• Statistical Analysis from Observed Floods

Peak Runoff

Runoff Generation

Flood Modelling Process

Estimation of Specific Floods also Important Input data - Actual Rainfall Events Calibration of Models etc.

Requires Deterministic Approach Model Describes a Mechanism

Initial and Boundary Conditions include catchment area, landuse, hydraulic structures, river length, river cross section, riverbed slope etc.

Must Model all Conditions

Why Estimate Runoff from Rainfall?

Often no streamflow data available at site Analysis of other data not warranted Cost for estimation small Important for nation – flood prevention Need best possible design methods Methods widely accepted for urban drainage design

Analysis of Rainfall Data Predicting Runoff from Rainfall Require Statistical Rainfall Distribution Flood Frequency Analysis

Discharge vs Frequency Only

Rainfall Frequency Analysis Duration Also Important

Short Duration – High Average Intensity Long Duration – Low Average Intensity

Definition of Failure Hydraulic Structures are Designed for a Specific Frequency of

Exceedence Risk Based Approach Failure Occurs when a Larger Event Occurs

NOT Collapse or Destruction of Structure Should Consider Structural Failure as Part of Design Process

Failure !!!!!! Building Vs Drainage System

Risk and Failure Level of Acceptable Risk

Depends on Service Level Expected by Community

Minor Structures Small Cost for Failure ($ and Life)

Frequent Failure Acceptable eg: Kerb and Gutter Flooding Once every Year

Major Structures Large Cost for Failure ($ and Life)

Infrequent Failure Acceptable eg: Flooding of House Once every 100 Years

Risk and Failure – acceptable?

Risk and Failure– acceptable?

Designing for Failure

Design for a Reduction of the Effects of Surcharging Provide for Passage of Floods that Exceed the Design

Flood Minimum Social, Physical and Environmental Damage

eg: Smart Tunnel, Storage pond.

Smart Tunnel KL

Storage Pond

Probability in Flood Estimation

Flood Estimation Process Risk Based Approach

Exceedance Probability Probability that a Magnitude is Exceeded for Specific Flood Value

Generally Adopt Year as Time Period

Annual Exceedance Probability, AEP The Probability that an Event Magnitude is Exceeded Once,

or More than Once in a Year

AEP = 0 Never Occurs

AEP = 1 Always Occurs

0 < AEP < 1

Often Defined as a Percentage

Average Recurrence Interval, ARI The Average Period Between Years in which an Event

Magnitude is Exceeded Once, or More than Once Avoid Use of Return Period

Implies Specific Period of Recurrence Average Implies Statistical Estimate

An Average Based on Many Observations

ARI = 1/AEP

Hydrological Data Rainfall

Daily Volumes (mm/day) Instantaneous Intensity (mm/hr)

Hyetograph (Intensity v’s Time)

Evaporation Stream Flow Data

Time Period Depends on Use Daily or Monthly Volumes for Water Resource Management

Stream Gauging / Rating Curve

Reliability of Data Accuracy May be an Issue

Failure of Automatic Equipment etc

Extrapolation of Rating Curves 50% of NSW Gauging Stations are Gauged for Floods that are

Less than 20% of the Maximum Recorded Event Reliability of Extreme Event Prediction

Selection of Design Flood Risk Based Approach Urban Drainage Systems

Major and Minor Drainage System Minor System

High Design AEP (1 to 5 year ARI) Low Cost Includes Underground Pipes and Some Open Channels

Major System Low Design AEP (50 to 100 year ARI) High Cost Includes Major Drainage Reserves and Floodways

Selection of Design Flood Maintenance or Low Flow

1 year ARI

Non Surcharging 1 year to 100 years ARI

Controlled Surcharge (Little to No Damage) 20 years to 200 years ARI

Surcharging with Appreciable Damage 50 years to 500 years ARI

Catastrophic Failure- a sudden and total failure of a system where recovery is impossible

ARI > > 100 years

Flood Frequency Analysis Statistical Analysis of Observed Floods

Determine AEP v’s Flood Magnitude

A Valid Analysis of the Data Data Should Constitute a Random Sample of Independent Values

from Homogeneous Population. Different Events Due to Same Weather Pattern

Extract Discrete Values (Continuous Record) Maximum Event Runoff (m3/s)

Fit Distribution No Theoretically Correct Distribution

Flood Frequency Analysis Partial Series Analysis

Analyse All Floods Whose Magnitude Exceeds a Defined Value Should Have At Least as Many Floods as Years of Record Must be Independent Events

Annual Series Analysis Analyse Maximum Discharge in Each Year of Record

Must be Independent Events

Rainfall IFD Data Use with Rational Method

Converts Rainfall Frequency to Flood Frequency

Define Frequency of Event ARI or AEP

Need to Know Appropriate Rainfall Duration Depends on Catchment Size

Runoff Coefficient Based on Catchment Characteristics

Intensity Duration Frequency (IFD) Curves Values can be read-off from the curve to obtain rainfall

intensity (rate) for different durations and return period Determine rainfall intensity in Australia

Reference for Australian Practice Australian Rainfall and Runoff, (1987)

Republished in 2000. Engineers Australia (Various Authors) Chapter 5, (Book 4) Pages 95 to 99 and Pages 103 to 104

IFD Generation Procedure Select Standard Parameters from Maps

Data Derived from Statistical Analysis Isopleth Curves Covering Australia

Substitute Parameters Into Standard Equations AUSIFD Software

MS Windows Based Software

Generate Rainfall Intensity versus Duration for Specific ARI’s 1 year to 100 years ARI

Standard Parameters Maps 1 to 6

1 hr Duration, 2 year ARI Intensity, I1h,2y






Map 7 Average Regional Skewness, G

Map 8 Geographic Factor, F2

Map 9 Geographic Factor, F50

Definitions IDF – Intensity duration frequency ARI – Average recurrence interval YiD - Log-normal rainfall intensity for ARI of years (Y) and

duration (D) YID – Log Pearson Type 3 (LP3) rainfall intensity for ARI of

years (Y) and duration (D)

Basic durations 6 min, 1, 12 and 72 hours Basic ARIs – 2 and 50 years Determination of IDF curve should begin with basic

durations and ARIs. Other durations and ARIs (standard) can then obtain Best demonstrated though an example of an area such as

Melbourne

Melbourne Example To determine intensity (mm/hr ) for the following storms in

Melbourne 100 I 6m ; 1 I 2 ; 50 I 30m

Seven major steps are required

Step 1- Read-off 6 basic values from ARR(87). 14 regions have been divided for Australia Melbourne is on maps 1.8-6.8 (region 8) 2i 1 = 18.9mm/hr; 2i 12 = 3.81mm/hr; (38.1) 2i 72 = 1.13 mm/hr (11.3) 50i 1= 38.7 mm/hr; 50i 12 = 7.09mm/hr; (70.9) 50i 72 = 2.21 mm/hr (22.1)

N.B. 12 and 72 hr must be scaled by a factor of 10

Solution:

Step 2 – obtain short duration factor for 6 minutes intensities F2 and F50 are shown as contour lines and are read from ARR

(87) (maps 8 and 9) F2 = 4.29; F50 = 14.95 for Melbourne Obtain 2i 6m and 50i 6m from the following relationship: 2i 6m = F2 (2i 1 )0.9 ⇒ 4.29 (18.9) 0.9 = 60.43mm/hr 50i 6m=F50(50i 1)0.6 ⇒ 14.95 (38.7) 0.6 =134.05 mm/hr

Step 3 – Obtain skewness value (G) To convert LN2 values (obtain from step 1) to LP3 G = 0.36 for Melbourne (obtain from Map 7c of ARR (87) So far, 8 values of basic ARI have been obtained, and are

required to plot on the ARI interpolation/extrapolation diagram

Step 5 – Convert LN2 to LP3 value Draw a horizontal line through G value Read the Basic ARI and durations (through G line)

Y/D 6m 1 12*

72*

2 58.5 18.2 3.71 1.1 50 146 42 7.55 2.38

Scale down by 10

Step 6 – Read standard ARI and evaluate 1 year ARI. Standard ARIs are 1, 2, 5, 10, 20, 50 and 100 years. ARI, 2 and 50 √ ARI, 5, 10, and 20 read value from plot ARI, 1 from

1ID = 0.885x2ID / [1+0.4046 log10 (1.13x50ID / 2ID)] ……………(Eq - 1.1)

Summary of Melbourne ARI intensities

Y/D 6m 1 12* 72* 1 43.8 13.8 2.86 0.84 2 58.5 18.2 3.71 1.1 5 80 24.3 4.71 1.44 10 96 28.6 5.4 1.66 20 116 34 6.3 1.96 50 146 42 7.55 2.38 100 170 48.3 8.55 2.72

Obtain from equation 1.1

* scale down by 10

Step 7 – Plot all ARI values on duration interpolation diagram (IFD curve)

Join points with straight line Straight line can be extended to D=5min. Read off required intensities for Melbourne

100 I 6m ; 1 I 2 ; 50 I 30m

170 mm/hr

9 mm/hr 65 mm/hr

Map 1.8 (1 hr duration- 2 yr ARI)

Map 2.8 (12 hr duration-2 yr ARI)

Map 3.8 (72 hr duration – 2yr ARI)

Map 4.8 (1 hr duration- 50yr ARI)

Map 5.8 (12 hr duration- 50yr ARI)

Map 6.8 (72 hr duration-50yr ARI)

Map 8 (F2)

Map 9 (F50)

Skewness, G

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Module 6_Itroduction to Flood Estmation and Modeling (2015)

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