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Module 6 Introduction to Flood
estimation and modelling Ir Dr Kelvin Kuok
Faculty of Engineering, Computing & Science Room E613
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
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
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
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.
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
12 hr Duration, 2 year ARI Intensity, I12h,2y
72 hr Duration, 2 year ARI Intensity, I72h,2y
1 hr Duration, 50 year ARI Intensity, I1h,50y
12 hr Duration, 50 year ARI Intensity, I12h,50y
72 hr Duration, 50 year ARI Intensity, I72h,50y
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