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Flood Analysis:
Hydrologic Principles
&
Analysis.
Charles YearwoodDrainage Unit, Ministry of Public Works
Sept 2007
Research interest: Hydrologic data collection; Early warning systems; & Integrated flood management
Presentation Content
Introduction
Objective
Theory
Data Collection
Analysis
Introduction Over the past few years the disharmony
between human use systems and natural systems has resulted in repeated flooding in the Caribbean, eg:
Jamaica (1979, 1984, 1985, 1986, 1988, 2004, 2005),
Barbados (1970, 1984, 1986, 1988, 1998, 2004),
Guyana (1996, 2005); and
Trinidad and Tobago (1988, 1993, 1998, 1999)
Recently disastrous flooding in Haiti and the Dominican Republic, Trinidad and Tobago, and Barbados reinforces this reality.
Guyana 2005
Jamaica Nov 2004Barbados 2004
Uses & Users of Flood Analysis
Conduct Flood assessments
Hazard maps
Vulnerability assessments
Risk assessment
Develop Flood mitigation, prevention, preparedness and flood
reduction measures to the flood risk
Set criteria and guidelines for sound risk management four types of
users:
Town planners
Emergency managers
Community groups
Private sector
Propose of Flood Analysis
Watercourse/River
planning
Flood discharge
Water resource
Low flow rate
Flood forecasting
River discharge in several
hours in advance (e.g. 3
hours, 6 hours)
Flood runoff analysis
Peak discharge
Hydrograph
Long-term runoff
analysis
Low flow rate
Objective of Flood Analysis
Identify factors affecting runoff from watersheds
Determine areal rainfall estimation from point data
Understand the cause effect relation between rainfall and stage/runoff
Development of runoff hydrographs from rainfall,
Rainfall probability analysis,
Design storm,
Estimation techniques for runoff hydrographs
Methods of controlling runoff.
Flooding in the Caribbean often
result from one of the following:
Severe rainfall events
Hurricanes, tropical storms between June to November
Riverine flooding – excess runoff to gullies, streams,
rivers
Flash flooding High intensity - short/long duration rainfall events;
Prolonged rainfall events
Why Flooding Occurs?
High elevation:short reach (gullies lengths);
Coincidence of high tide with rainfall events;
Development pressures;
Dense development on flat lands;
Inadequate drainage systems and structures; and/or
High sand build-up at the outfall;
Flood Issues
Inundation of properties;
Lost of Business;
Soil loss & erosion;
Degradation beaches & reefs;
Publics’ nuisance and inconvenience;
Increased development and assoc. runoff;
Disruption to traffic flows; utilities & businesses
Illegal dumping;
Theory – Hydrological Principles
Hydrological processes in agricultural watersheds
Rainfall-Runoff in Watersheds
Loss(retention in
the soil）
Groundwater
flow
Unsaturated
flow
Depression storage
Overland flow
Interception
Rainfall (unity)
River
discharge
Infiltration in the Soil
Time(hr）
Infiltration
rate
（mm/hr）
Rainfall
Potential infiltration
Actual infiltration
Infiltration rate decreases as time passes.
Effective Rainfall
Rainfall loss
due to infiltration and local storage
Rai
nfa
ll
Saturation
Time
Factors affecting runoff magnitude
Scale of features, vegetation
Total rainfall, rainfall intensity, duration, spatial distribution
Soil wetness, initial flow rate, evaporation, air temperature,
season
Land-use
Procedure of flood runoff analysis
Investigation of flood data
1)Method of flood runoff calculation
Calculation of flood runoff
2)Division of catchment
3)Verification of runoff model and allowable errors
Procedure of flood runoff analysis
Investigation of flood record
1)Preparation of lists of existing flood data
Check records of rainfall, water level, discharge at
observation stations within the area to be studied.
2)Determination of floods to be analyzed and
floods to be estimated based on the records in (1)
Floods to be analyzed are used for the parameter
calibration and should have sufficient information on the
measured discharge hydrograph. It is recommended to
use three events for small, medium and large floods.
Floods to be estimated are the floods for which the
hydrograph must be estimated by a certain method.
Procedure of flood runoff analysis
Investigation of flood record
Missing records of rainfall should be supplemented with
the records at other rain gauge stations based on the
correlation analysis.
4)Calculation of average rainfall depth in watershed
If there are sufficient number of rain gauge stations:
1)Arithmetic mean method
2)Thiessen polygon method
3)Isohyetal method
If there are insufficient rain gauge stations:
Representative coefficient method
3)Supplementing records for missing observed
values of rainfall
Hydrological Data Analysis:Barbados Case Study
Watershed studies
Rainfall: elevation relationships
Wells’ Infiltration
Watercourse Surveys
Rainfall:Stage
Rainfall:Runoff
Storage (check dam/retention pond) Monitoring
Flood Observations and Surveys
Rainfall:Elevation Relationships
Comparative Rainfall:Elevation Between
Indian Ground and Indian Ground
0
100
200
300
400
500
600
Jun-
04
Aug-0
4
Oct
-04
Dec-04
Feb-0
5
Apr-0
5
Jun-
05
Aug-0
5
Oct
-05
Dec-05
Feb-0
6
Apr-0
6
Jun-
06
Monthly Rainfall
Rai
nfa
ll (
mm
)
Indian Ground at 262m
The Rock at 160m
2 per. Mov. Avg. (The Rock at 160m)
2 per. Mov. Avg. (Indian Ground at 262m)
Well Infiltration
Lascelles Terr Well infiltration
0.600
0.800
1.000
1.200
1.400
1.600
1 12 23 34 45 56 67 78 89 100 111 122 133 144
Time (mins)
He
igh
t (m
m)
13-Jan
21-Jan
23-Jan
1-Feb
3-Feb
12-Feb
Lascelles Well Infiltration
0.000
2.000
4.000
6.000
8.000
10.000
12.000
0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00
Time
Wa
ter
De
pth
in
We
ll
0.000
0.200
0.400
0.600
0.800
1.000
1.200
1.400
Wa
ter
De
pth
in
Cu
lve
rt
Well (Culvert) Inflows H Well Outflows H
Portvale Rainfall:Stage
Relationship Study
Cross section C (Ch 20)
106.000
108.000
110.000
112.000
114.000
116.000
-11.3
-9.7
-3.6
-1.4 0
1.9
5.3
7.8
11.8
Offset Chainage (m)
He
igh
t (m
)
Instrument Location
Cross Section Survey
Brucevale Valley – Rainfall: Stage
Relationship
East Coast BridgeTri-O-Path Bridge
Hway 2 & Lakes Jct BridgeDark Hole WC
Portvale Lag time Analysis (14Nov 2004)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
11:50 13:30 15:10 16:50 18:30 20:10 21:50 23:30
Time of Day
Sta
ge
(m
)
0
20
40
60
80
100
120
140
Ra
infa
ll (m
m)
Portvale R'fall WC Stage Jamestown Stage Dr H
Infiltration at Richmond Hill
0.000
20.000
40.000
60.000
80.000
100.000
120.000
140.000
160.000
320 325 330 335 340 345 350 355 400 405 410 415 420 425 430 435 440 445
Time
Cu
bic
me
tre
s
Infiltration
Time W.L. Reduce
Level
Volume Time W.L. Reduce
Level
Volume Time W.L. Reduc
e Level
Volum
e235 -254.8 73,930 320 1245 75,430 143.79 405 83375,018
66.84
240 357.2 74,542 325 1233 75,418 141.14 410 76774,952 57.01
245 590.5 74,776 330 1203 75,388 134.51 415 69074,875 45.88
250 788.0 74,973 335 1162 75,347 125.45 420 60974,794 34.16
255 953.0 75,138 340 1114 75,299 104.13 425 52374,708 24.17
300 1072.0 75,257 345 1064 75,249 103.87 430 43174,616 16.22
305 1155.0 75,340 350 1012 75,197 95.53 435 35074,535 9.22
310 1212.0 75,397 355 957 75,142 86.71 440 273.874,459 5.28
315 1241.0 75,426 400 895 75,080 76.77 445 197.6 74,383 3.57
Monitoring Richmond Hill Check Dam
overall rate of 101.49m3/hr
or 1.69m3/min
Flood Observations
Channel Flows
Inudations Nuisance flooding
Flood Surveys
Registering flood levels,
Recording level damages, and
Losses
Flood Hazard Map Production
Legend
contours
5yrFldLine
Updated Roads
20yrFldProofArea1996
20yrFldLine
100yrFldLine
Nov2004FldLine
/
From 1996 Barbados Drainage Study
C:\Drainage Unit -Server\GIS Files\Drainage Features\FloodPlainMapping
Created by Pernel Brewster
100-years Rainfall/Storm Surge events
Source: Coastal Hazard Project 2007
Thank-you
Questions?
Appreciation: to Drainage Unit’s staff who braved flood conditions to provide flood data
