Rainfall Measurement Systems in Mauritius and IDF Curves

D O N E BY:

KEERODHUR KHEVIN – 0813537

DOMUN AL-AFTAB – 0811969

THIELAMAY DOURAIVEN - 0810731

KUREEMUN YAASIR – 0812354

GOWREEA RAVIRAJ - 0814048

Rainfall Measurement Systems in Mauritius and

IDF curves

INTRODUCTION

‘In order to measure the effect of precipitation, it is

necessary to measure the precipitation, and to find out its

distribution at various places on the earth.’ (Garg, 2005,

p.107)

As stated by Garg above, rainfall measurement must be done

at different places, since the amount of precipitation varies

from place to place and different period in order to get an

idea of the amount the rainfall (in mm or the intensity) fallen

all over the country and at specific places. Rainfall Measurement System in Mauritius & IDF Curve18th Nov 2011

INSTRUMENTS USED TO MEASURE RAINFALL

Three types of instruments that the local Meteorological Station uses:

Raingauge

Tipping Bucket

Automatic Weather Station

Rainfall is monitored at 250 stations spread over the island. 220 stations

operated by the Sugar Estates are measuring the total daily rainfall. 30

continuous recording stations provide data about intensity and duration of

rainfall. Rainfall Measurement System in Mauritius & IDF Curve18th Nov 2011

RAINGAUGE

Pictures taken at the local Meteorological Station

Rainfall Measurement System in Mauritius & IDF Curve18th Nov 2011

Meteorological instrument for determining the depth of precipitation in mm that occurs over a unit area (usually one m2) and thus measuring rainfall amount in mm

RAINGAUGE


Advantage

Cheap compared with other instrument

No cost associated with the maintenance

Small in size and easy to install

Disadvantage

Must be maintained daily

Spare glass bottle must be kept

Data can be biased during heavy rainfall

RAINGAUGE


TIPPING BUCKET



A tipping bucket rain gauge is a meteorological device that measures the amount of precipitation, or rain that has fallen. It is one of the most common tools used to measure rainfall.

TIPPING BUCKET


Advantage

Most popular recording raingauges used and generally

providing relatively accurate point estimates of precipitation,

especially for low to intermediate intensity

Disadvantage

It may underestimate the quantity of rainfall especially during

the heavy rains or extreme snowfall.

It suffer errors due to both catching and counting inaccuracies

( i.e. losses through strong winds, wetting, drop splashing, dirt

and evaporation)

TIPPING BUCKET


AUTOMATIC WEATHER STATION



An Automatic Weather Station (AWS) is an automated version

of the traditional weather station, either to save human

labour or to enable measurements from remote areas.

An AWS will typically consist of a weather-proof enclosure

containing the data logger, rechargeable battery, telemetry

and the meteorological sensors with an attached solar panel

or wind turbine and mounted upon a mast.

There are 22 AWS installed at different places in Mauritius



Advantage

AWS is more consistent in their measurement

AWS provides data at a significantly greater frequency (some provide

data every minute)

AWS provides data in all weather, day and night, 365 days per year

AWS can be installed in sparsely populated areas

Disadvantage

AWS require a large capital investment

AWS are less flexible than human observers



METHODS OF AVERAGING RAINFALL DATA

There are three methods of averaging rainfall date:

Arithmetic average

Theissen polygons

Isohyetal method

Although nowadays, most of these calculations are

done with computer mapping programs, it is still

useful to understand these methods.Rainfall Measurement System in Mauritius & IDF Curve18th Nov 2011

THEISSEN METHOD FOR MAPPING RAINFALL

This involves determining the area of influence for each

stativariation. It is easier than the isohyetal method but

less accurate

How to create Theissen on, rather than assuming a

straight-line polygons?

Locate all rainfall stations.

Form a series of triangles by connecting nearby stations.

Erect perpendicular bisectors on each of these lines and extend

them to the intersect with other bisectors, thus forming a series of

irregular polygonsRainfall Measurement System in Mauritius & IDF Curve18th Nov 2011

Theissen Polygon Network

THEISSEN METHOD FOR MAPPING RAINFALL


ISOHYETAL METHOD

The most basic method of representing the spatial

distribution. This is generally the most accurate method but

is also the most laborious.

How to draw isohyet lines?

Locate all rainfall stations on a base map and record the rainfall amount.

Draw isohyets (lines of equal rainfall).

Calculate the mean precipitation for the area corresponding to each

isohyets.

Calculate the fraction of catchment area under isohyet, multiply by the

mean precipitation for that area and sum to get the catchment average.


ISOHYETAL METHOD


RAINFALL ANALYSIS

There are three common methods to analyse

rainfall:

Storm event analysis

Intensity-duration-frequency(IDF) curves

Probable maximum precipitation method


INTENSITY DURATION FREQUENCY CURVE

What is an IDF curve?

An IDF curve is a tool that characterizes an area’s rainfall

pattern.

It is a graphical representation of the probability that a given

average rainfall intensity will occur.


How to create an IDF curve?

IDF curves are created by analyzing years of rainfall records.

IDF curves are developed as follows:

Using a long term rainfall record for each specified rainfall

duration(5, 10, 15, 60, 90, 120)

The annual maximum rainfall depth are extracted from period of

record. This results from one depth value for each year of record.

A frequency analysis is recorded on the annual series.

The Intensity and duration points are plotted for selected

frequencies.



Interpreting an IDF curve

Intensity, duration and frequency are the parameters

that make up the axes of the graph of IDF curves

Each plotted lines in the graph represents rainfall

events with the same probability of occurrence in a

range of duration.



Characteristic of an IDF curve

A single curve represents data from several storm

events likely from different years.

IDF curves do not represent time histories of real

storms. Intensity are averages over indicated durations.

IDF curves provide the average intensity for specific

duration and frequency and is very commonly used for

the design of hydraulic structures.



Application of IDF curves in the Engineering field.

For design of hydraulic structures such as gutters, ditches, standard

culverts, bridges-design for river flow passage.

For estimating discharge using the rational method.

Qmax = CIA

Where Qmax = design flood (m3/s)

C = runoff coefficient between 0 and 1.0

A = catchment area (Km2)

I = mean intensity of rainfall in mm/h during the time of

concentration



THANKING YOU FOR YOUR KIND

ATTENTION


Rainfall Measurement Systems in Mauritius and IDF Curves

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