Maximum Rainfall Intensity-duration Frequencies in Ghana

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REPUBLIC OF GHANA

GHANA METEOROLOGICAL SERVICESDEPARTMENT

DEPARTMENTAL NOTE No. 23

I

II

1\1AXI'MUM RAINFALt 'INTENSITY-DURATI'ON'., )

FREQ.UENCIES. IN GHANA

by.,

J. B. DANKWA

. r

Legon1974

r:

I i( 'I'i\' ., r :

REPUBUC OF GHANA

GHANA METEOROLOGICAL SERVICES DEPARTMENT

. DEPARTMENTAL NOTE No. 23

. , .

MAXIMUM RAINFALL INTENSITY-DURATIONFREQUENCIES IN GHANA

by

J. B. DANKWA, B. SC't M. Sc.

Legon1974

ACKNOWLEDGEMENTThe Author wishes to express his deepest appreciation t 0

all people" too numerous to mention, who helped in diverse ways to

make this publication possible, Mention is made of members 0 f

the Hydrometeorological section and the many students from the

Un iversity ol Science and Technology, Kumasi who computed most

of the figures during their vacation training.

Special mention must also be made of Mr. S. E. Tandoh, theSenior Meteorologist in charge of Climatological Section who dili·

gently read the manuscript and offered many useful suggestions.

FOREWORDTODAY water resources planning has become so sophistica-

ted that it can almost be called an academic discipline in itself

Like no ather subject it has brought together scientists from a va-

riety of fields notably civil engineering, hydrology, regional and

national planning and economics

Water resources planning takes into cognizance not only da-

ta requirements themselves but also the relationship between the

quality of available data on the one hand and the various p Ic nn inq

techniques on the other.hand

This publication is designed to assist .c ivi l engineers as

well as other interested users to arrive at suitable design criteria

for the many engineering projects such as dams, bridges, culverts,

and other forms of water resources developments that may be u n-

dertaken in the country.

It is hoped that the judicious use of this publication w i I I

lead to a better planning of projects for large,oScale and long.term

investments of the water resources in Ghana.

F. A. A. ACQUAAH

DIRECTOR OF METEOROLOGICAL SERVICES

INTRODUCTIONL 1 Suitable hydrologic design criteria for most river structuresare basic problems engineers encounter the world over, Primarly,

these design criteria are based on economic considerations. The

hydrologic problems include among other things such things as optimumstorage capacity of reservoirs, s p i IIway design for concrete s t r u c-tures, carrying capacities of chennels, culverts, channel imp r 0-

vement schemes, storm sewer systems etc, etc Optimum d esi gncriteria are obtained by balancing the cost of repairing damage oninfrequent occasion against cost of providing for larger design cri-teria. And for such economic analysis, flood frequency dot awnich are derived from precipitation data are ana lysed by s p e c ialstati stical tech n iq u e s ,

1.1.1 In specific cases u high degree of safety is required in thedesign criteria e.g earth filled dams where over toppling co nn 0 tbe tolerated. Under such conditions, the dam' would fail entirelya great.economic disaster! Here, the design criteria must be suchthat the spillway should be able to pass the largest ,floods that arelikely ever to occur. These extreme limits are arrived at by s p e-c ic ] statistical analysis und techniques as well as physical reason--ning.

1.2 Apart from netural causes such as earthquakes and I and-

slides, meteorological conditions are directly the causel effects ofmajor floods. For this reason such physical analyses start withmeteorological studies. The physical and statistical analyses in-volve estimation of upper limits of storm rainfall in which case theresulting estimates are referred to as "Probable Maximum Precipi-tation" and these become "Probale Maximum Floods" when con-

verted into flood flows by any of the suitable conventional me-thods

1.3 It must be pointed out that the final selection of design cr i-

feria for any structure involves economic, moral and even politi-cal considerations in addition to those of hydrologic nature Ashydrometeorologists, our job is to provide the data and analysis re·

qu i re d to permit an intelligent assessrnent of the flood potential of

the site in question. It is hoped that this note will contribute toan improvement in analysis procedure and pra,ctices in Ghana and

to a better understanding of the importance of hydrological a n a ly-sis in safe and efficient design of river structures.

PURPOSES OF STUDIES:

2.1 Over the past few years, engineers and other in te re s t e d par-

ties have shown an increasing interest for the frequency of hea vyrainfall for drainage design facilities,. Information on m o x im u m rain-

fa 11- in te n s ifi e s are scanty and hove existed in various forms in the de-

partment. The need to collect all these pieces of information to-gether into one complete form has never been felt as now when every

effort is being made to resuscitate the shattered economy of the

country.

2.2 For this reason, one common statistical technique has bee nused to analyse all the available rainfall data for the mcijor stations

of Ghana with sufficiently long and accurate records so that valuesderived from these analysis would have a common basis for com.p c ri s o n , And the purpose of the present analysis is to presen tthese data in a form which will find a ready use for all interestedparties.

BASIC DATA USED:3.1 There are at present eighteen (18) Synoptic Stations in Gh e ,na. These major stations are manned by meteorological person.nel hence the accuracy of elata can be guaranteed to a rreasonable

degree. The records of autographic guages at these synoptic sto-tions with s u Hl c l en tly long records extending for at least ten years;and over, formed the basis of the present analysis.

Extreme Value Analysis:The method of Gumbel on Extreme Value Fun c ti on s we r e

utilised in the present e nc ly s l s . It is recognized that other rne-thods exist for maximum rainfall intensity analysis but Gum bel'smethod is preferred for its simplicity and rigid statistical method.

3.2.1 Values for "return periods" of 5, 10, 15,20, 25,50, 100years for fourteen out of the eighteen synoptic stations were c o m-putee! using Gumbel's method. In this context, a value with are·turn period of five years for example is the amount that will be e q-

ualled or exceeded only once in every five years on the aver ag e

over a long period of time.

3.2.2 Statistical calculations of return period values accor dingto Gumbel, require a minimum of about ten years of record to pr o-duce realistic results for 10 years return values. For this rea·son, stations whose records fall short of 10 years were discardedin th e pre s e n tan a Iy s is.

:3 3.3 Partial Duration Series:

3.3.1 A series of annual maximum value is an array of data st-ving one maximum value for each successive 12. months period. As

only one maximum value is given for each, the second or third high·est value within these years are omitted, although s o-me may havebeen higher than the highest values in other years.

3.3.2 The partial duration series on the other hand is-a list of allvalues greater than a certain threshold value regardless of the timeof their occurence. Analysis based on data from a partial duro.

tion series would give values which will normally exceed the va·lues based on annual maximum series in magnitude and an extreme

8

value analysis should, therefore, be based on partial d u rat ionseries for best results, particularly for short return period va luesHowever, such a series is difficult to abstract from long per i o d s

of observations and is not readily dealt with by statistical the ory

3 4 Theory

The Gumbel estimating equation is

x + K (T n~ s,extreme rainfall to be estimated for a return period T

means of series of maximum annual values

K -Sx -T -n --

Frequency factor read from tables

Standard Deviation of Annual Extreme Rainfnll

Return Period in Years

number of values in a series of maximum annual values

Jn

~i == I

(xi-X)2n-l4. Analysis:

4.1 For each particular station, the daily autographic c hart s

were scaled using an appropriate scale, thus the maximum r a i n-fall v c lu e s for the various durations could easily be obtained. Foreach duration, there are specified thre s ho ld s below which val ues

are discarded.

These are:-TABLE I

DURATION OF0.2 0.4 0.7 1.0 2.0 3.0 6.0 12.0 24.0PERIODS IN HOURS

THRESHOLD 0.5 0.6 0.8 1.0 1.0 1.0 1.2 1.5 1.5VALUES INCHES

Thus ani y values which exceed these indicated thresholds, are

counted as significant in this context.

4.2 Ne)(t, for each duration the maximum value for each was

picked to form an annual series whose mean and standard d e v l c-ti o n s were computed by the usual methods, Gumbel's e qua.tion was then applied The length of records ranged from 1 5

years to 29 years,

9

2

345

6

7

NO. OFSTATION YEARS OF

RECORDS

HO 19

TAKORADI 19

SALTPOND 16

W A 15

ADA 15

NAVRONGO 18

AXIM 16

TABLE II

8

9

10

1 1

12

13

14

NO. OFSTATION YEARS OF

RECORDS

TAMALE 29

ACCRA 24

KETE-KRACHI 17

YEN DI 15

WENCHI 15

KUMASI 18

AKUSE 19

4.3 Graphical Representation of Maximum RainfallIntensities.4.3,1 The general distribution of rainfall intensities as obtained,by this method were plotted on 10g.log graph sheets with intensity

values as ordinates and duration as abscissa. A series of c on-

centric curves were obtained for return periods I'lf 5, 10,25,50 and

100 years.

4.3.2 Spatial Representation of Rainfall Intensity.F r e que n c yYolugs,lsopluvial lines were drawn for maximum rainfall values for

24 hour durations and for return periods of 5, 10, 25, 50, and 100years. Figs. (1·5)

,Also-, amounts for durations of 3,6 and 12 h 0 u r 0 f th e 5,10, 25, 50 and 100 years return period were expressed in per cent

of the 24 hour values for the corresponding return periods. Thesepercentages for the various return periods within each durationwere averaged and a mean percentage for each duration was -thu sdetermined. These were then presented in map forms. Fi gs (6.8)

RESULTS AND DISCUSSIONS

5.1 Genera lIy speaking, for all return periods, maximum r a in.fall values are greater in the southern half of the country than i r.the northern half. In the southern half maxim Jm 24.hour val u e s

decrease from SW towards NE CiS Far as northern parts of the Volto

Region from where they increase s eu th-e c s rwnrd s as far as to th eeastern coast line, There is a low region around the A c c r aPlains just below the Akwapim Scarp. See Figs. (1.5),

5.2 There is also a low area endosed by 4 inches isohyete inthe middle section of the country extending from Sunyani area asFar as the boundary between northern and upper regions and a Iso

exte nds eastwards beyond Tamale. Fig. 1. This low area per.s is t s throughout (Figs 1.5) and by the time one reaches the 100.year return period map, the endosed isohyete has "increased to 6,0in ch e s ,

10

5.3 This general trend follows the normal distribution of an·nual rainfall pattern, where amounts decrease from SW tow a r dsN E in the lower half' of the country as far as northern Volta Regionfrom where the isohyetes decrease south.eastwards to the easterncoast line.

5.4. Three maps of ls evp er c en+c l maps are attached Figs (6.8)they represent the 3.hour, the 6·hour and the 12·hour d u rat ion

Generally speaking between 70 and 90% of the maximum 24· hour

rainfall amounts fall within 3 hours and between 75 to alomost 90%fall within 6 hours, and over 90% fall within 12 hours th re u qh cu tthe whole country. This is in conformity with the general tre ndfound in most tropical areas where 90% of precipitation am 0 u nts

are realised within 10% of the time (Riehl).

5.5 Extensive figures for various return periods and differe nt

durations have been included. One would notice that the return p e-

r io d s for Navrongo, Wenchi, Axim and Accra differ slightly fro mthe others, Kumasi, Ho, Takoradi, Akuse, Saltpond and A d asho w return periods of 5, 10, 15, 25, 50 and 100 years. Tho s efor Navrongo, Wenchi and Axim are 5, 10, 20, 50 and 100 yea r S.·

The corresponding figures for Accra I are 5, 10, 25, 50 and100 years. It would have been desirable to have uniform retur nperiods but the nc n-un l Ie rm ity will in no way detract anything fromthe usefulness of these figures and graphs. Extrapolations caneasily be worked out if so desired.

SUMMARY:

6.1 The maximum rainfall intensity duration curv.es have bee nestablished for 14 stations with sufficiently long and ace u ra t erecords for Ghana. A common method (Gumbel's has bee nutilised so that records can easily be compared.

6.2 For return periods of 5, 10, 25, 50· and 100 years the m c x l ,mum amounts expected within 24 hou~s have been spatially p or·trayed. In addition, mean per centages of these amounts expe c .ted to fall within 3, 6 and 12 hours have been included. It ishoped that these curves and various maps will readily find ma n ypractical uses among consulting engineers and other interestedparties in divers ways especiall>: for design purposes.

11

REFERENCES:

1. U. Sp e rn s , 1963: Rainfall ln ten s lty-Du re tl cn Frequency Mapsfor Ontario

12

. 2. WMO No 233 T.P. Estimation of Maximum Floods TechnicalNote No. 98

3. Guide to Hydrometeorological PracticesW.M.O.No. 168 T.P82PP A9 • A.14

4. ASCE: Manuals of Engineering Practice No. 28 H y d rologyHandbook PP. 17 • 19 -

5. Gum bel, Eo J. 1958: S tat i 5 tic S 0 f Ext rem e S

MAXIMUM RAINFALL INTENSITIES AND RETURN PERIODS

(INTENSITIES IN INCHES/HOURS)

NAVRONGO

~

PERIonYEARS

DURA· 5 10 20 50 100TION

HO~JRS

0.2 4.35 4.85 5,35 5,95 6.45

0.4 3.88 ( 10 4 ..50 5.20 6.00

0.7 3.00 330 3,60 4.20 4,70

1.0 2.50 2,70 3,00 3,50 3,80

2.0 1.52 1.76 2,00 2,20 2.50

3.0 1.15 1.25 1040 1,65 1,80

6.0 0.65 0,76 0,86 0,99 1,09

12.0 0.36 0,42 0.48 0.56 0,61

24.0 0,19 0,22 0.26 0.30 a 34

W A

~

PERIODYEARS 5 10 15 25 50 100

LJUH.,"·TIOI

HOURS

0,2 5.90 6.35 7.10 7.65 8.35 9.10

04 4.30 4.80 5,08 5.43 5,90 6,800.7 2.99 3.30 4.49 3.70 4,40 5<00

0.1 2,41 2.61 2.88 3.09 3.50 4.00

2.0 1,48 1.60 1.70 2.00 2.22 2..43

3.0 1.09 1.14 1.25 1.50 1,66 1.83

6.0 0.59 0.64 0.70 0.83 0.92 1.02

12.0 0.32 0.34 0.40 0,44 0,50 0,5224.0 0.17 0.20 0.22 0.24 0.27 0,30

TAMALE

~

PERlODYEAR~ 5 10 15 25 50 100DURA·

TIONflOURS

0.2 5.10 5.65 6.00 6040 6,95 7,45t 0.4 3.93 4.40 4.68 5.20 6,00 6.40

0.7 3.19 3.64 3.80 4.00 4.64 5.001.0 2,48 2.84 3,04 3,29 3,63 3,962.0 1.40 1.59 1]0 1.84 2.10 2.40

3.0 0.99 1.12 1,20 1040 1.41 1.70

6.0 0.55 0.63 0.67 0.78 0.80 0.96

12,0 0.30 0.36 0.39 0.42 0047 0,51

24.0 0.16 0.18 0.20 0.22 0,24 0,27

YENDI

~

PERIODYEARS

DURA· 5 10 15 25 58 100TIONIi.OURS

0,2 5,50 6.20 6.60 7.20 7.80 8.40

0,4 4,50 5,08 5,38 6,20 6.60 7.40

0.7 3,61 3,93 4.30 4.90 5,10 5,60

1,0 3.05 3.41 3,61 3.86 4.20 4,53

2,0 1,74 1,99 2,13 2,31 2.54 2,77

3.0 1,20 1,36 i.ss 1,56 1.71 1.86

6,0 0.62 0.70 0.77 0,8. 089 0,96

12,0 0,33 0,37 0.39 0.41 0.44 0.4724,0 0,18 a 20 0.21 0,22 0,20 0.26

'II E N CHI

~

YEARS 5 10 20 50 100DL'R A•TIO"HOlR:'-

0.2 5,20 S,gO 6.20 6.80 7 20

0.4 0,15 4.60 5,01 5 S3 5,930,7 3.10 3,'31 3,l0 419 4A91.0 2.47 2.73 2.95 3 26 JA8

2.0 1.48 160 1.75 1.90 2 103,0 1,05 1. 18 1 3i 1,47 1 59

6,0 5.57 0,63 0.69 0.76 0,8612,0 0.29 0.33 Q,37 0.41 OA424.0 0.16 0.18 0.20 0,23 0.25

KETE· KRACHI

~

PERIODYEAR,S

5 10 15 25 50 100DURI\-TIONHOURS

0,2 5,35 6.00 6,35 6.30 7.40 8,050.4 4.85 5.45 5.78 6.20 6.78 7.200,7 3,80 4,30 4 70 5.00 5,30 6,001,0 3,00 3,51 3.80 4.00 4,31 4,802,0 1,62 1,88 2 02 219 2 42 2653.0 1.16 1.33 1.43 1,61 1.72 1.87

6,0 0,65 0.73 0,77 0,83 0.94 0.98

12.0 0,34 0.38 0.40 0.43 0.47 0,51

24.0 0.19 0.22 0.23 0.2~ 0,28 0.30

13

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1~1"""IJVIUI.1 ",""'~I ""'L..L. II" I '-.,,"". II~~ I"\I"V 1'\,,- I UI'\-I"IIII rL.I"IVU~

(INTENSITIES IN INCHES/HOURS)

KUMASI

~

PERIOI)YEARS

DURA- 5 10 15 25 50 100nONHOURS

0.2 5.40 6.05 6.45 6.95 7.55 8.20·0.4 4.65 5,00 5.65 6.10 6,70 7.300.7 3,69 4.10 4.5D 4.84 5.31 5.791.0 3.04 3.30 3.71 4.01 4.41 4.81

2.0 1.80 2. 10 2,37 2.55 2.90 3019

3.0 1.30 1,52 1.63 1.78 1.98 2.30

6.0 0.76 0,90 0.98 L08 1.21 1.35

12.0 0.41 0,49 0,54 0 e.60 0•.68 0.71

24.0 0021 0,.25 0.28 0.31 0.35 0.38

H 0

~

'PERIODYEARS 5 10 15 25 50 100

DURA-TlO~flOURS

0.2 5,30 5.50 5,95 6.55 6.80 7.40

004 4.32 4.90 4]0 5.22 5..62 6.00

0.7 3.30 3.65 3.50 4.11 4.44 4.78

1,0 2.58 2.88 2.BO 3.25 3.52 3.79

2.0 1.60 1.92 1.12 1.95 2.20 2.40

3.0 I 1. 15 1.31 1.30 1.50 1.65 1.816,0 I 0.66 0.77 0.;'4 0,83 0.99 1.09

12,0 0.34 0.38 0.42 0.46 0,51 0.5524.0 0.18 0.21 0,23 0.25 0.28 0.30

~ K USE

~

Pl1RIODEARS

5 10 15 25 50 100DURA-TIONHOURS

0,2 6.00 6,00 6.60 7,20 7.80 8,60

0.4 5.05 5,58 5.90 6,27 6.80 7.30

0.7 3,88 4.40 4.110 4.70 5,10 5.70

1. 0 3.06 3.39 3,57 3.80 4.20 4.70

2.0 1.84 2,06 2,19 2.35 2.70 2.90

3.0 1.38 1.59 1.70 1.85 2.03 2.24

6,0 0]8 0.92 0.94 1.06 1.15 1.26

12,0 0,3 0,46 050 0.55 0,62 0.68

24.0 0,21 0.24 0.27 0.29 0.33 0.37

A X I M

I~PERIODYEARS

DURA- 5 10 20 50 100TI0N.HOURS

0.2 5.40 6.15 6.75 7.65 8.450.4 4.60 5.15 5.85 6.55 7.23OJ 3.83 4.31 4.]9 5.39 5.841.0 3.16 3.50 4.00 4.50 5.002.0 2.15 2.43 262 3,00 3.~93.0 1.64 1.83 2.97 2,34 3.54

6.0 0.96 1.96 1.17 1.31 1.44

12.0 0.54 0.61 0.68 0.77 0.85

24,0 0.29 Ot33 0.37 004.1 OA5

TAKORADI

~

PERIODYEARS 5 10 15 25 50 100DURA·

TIONHOURS

0.2 4.95 5J5 6.25 6.80 7.60 8.35

0.4 4.48 5.20 5.63 6,15 6,85 7.55

0.7 3,64 4.20 4.51 4.51 5,44 5.97

1.0 3.10 3.86 3,90 4.20 4.50 5.00

2.0 2.02 2.32 2.49 2.71 2.99 3.27

3.0 3.46 1,67 1.78 1.93 2.12 2.29.6.0 0.82 0.94 1.00 1.09 1.19 1.30

12.0 0.46 0.52 0.56 0.61 0.68 0.7424,0 0.26 0.30 0.33 O.3~ 0.40 0,45

SALTPOND

~

PERIODYEARS

DURA- 5 10 15 25 50 100nONflOIJRS

0.2 7.00 8.75 9.6 10.7( 12.20 13.700.4 5.60 6.93 7.6'3 8.60 9.85 11.10

0.7 4016 5.10 5.63 6.30 7.17 8.06

1.0 3,35 4.09 4.50 5.03 5.72 6.41

2.0 1.93 2.35 2.58 2.87 3.26 3.64

3,0 1.39 1.68 1.85 2.06 2.33 2.61

6.0 0.76 0.91 1.00 1.10 1.24 1.38

12.0 0.41 0.49 0.54 0.60 0.68 0.76

24.0 0.21 0.25 0.28 0,31 0.35 0,39

14

12

i 8,45

i 17.231- 5.84I 5.00)J 3.;29I 3.54

I J 1.44, 0.85

OAS

5 100

) 8.35

,85 7.55

5.97

,50 5.00.. 1 3.27'ol2 2.29 •

c LQ 1.30I. I 0.74i.40 0.45

jf 100

100

MAXIMUM RAINFALL INTENSITIES AND RETURN PERIODS

(INTENSITIES IN INCHES/HOURS)

A C C R A

~

PERIODOUR YEARS 5 10 . 25 50 100TIONHOURS

0.2 5.00 5.5~ 6.50 7.10 7,750.4 3.90 4.60 5.20 5.80 6.40

0.7 2.93 3.37 3.91 4.33 4.74

1.0 2.46 2,83 3.31 3.66 4.01

2.0 1.49 1.75 2.08 2.33 2.58

3.0 1.15 1.30 1.50 1,72 1.90

6.0 0.62 0.77 0,94 1.07 1.13

12.0 0.34 0.42 0.52 0,60 0.6724.0 0.17 0.21 0.26 0.30 0,34

A D A

~

PERIODDURA- YEARS 5 10 15 25 50 100nON

HOURS

0.2 5.55 6,20 6.60 7.05 7,~5 8.400,4 5,10 5.70 6.00 6.50 7.20 8.20

0.7 4.4'1 5.00 5.40 6.00 6.50 7.20

1,0 3.59 4.30 4.70 5.20 5.80 6,40

2,0 2.54 3,00 3.26 3.58 4.02 4.45

3,0 1.81 2,14 2.32 2.55 2.86 3.16

6.0 0.99 1.14 1.26 1.34 1.49 1.6312.0 0,51 0.59 0.63 0,69 0,76 0,8424.0 0.26 0,30 0.32 0.35 0,38 0.42

. , 13.708!> 11.10

• "-> 8.06.I 6,41'0 __

326 3.64

2.61

.24 1.38

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MAXIMUM 24- HOUR RAINFALL 5 - YEAR RETURN PERIOD(Values in inches)

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I

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".

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II

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Fig.1

MAXIMUM 24-HOUR RAINFALL 10- YEAR RETURN PERIOD(Va,lues in inches)

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MAXIMUM 24-HOUR RAINFALL 2S-YEAR RETURN PERIOD(Values in inches)

" "' . ..--/, /) -1----___ :!-./( ( • I~'T.-r",rUiill~-...,:r- <, i . W.lew.le~ ..

U' 0 ) ~.ppo I· V.gab. V--j, 5·79 \

Wa ./0_'~'r~-...t---~\----t.-~-n-ina--~--t-D-UC-i.-.~-..~7i~-+I--.--~--~~~------~-~-K-.r-a-ga----4-----~\~· ~--------~~

,. .'_'.~ ~~ r\ ( ,.."''''''_''-''-'' .Wapuli~\. \."'.=K.A'1dl.----=-----t--- •Savelugu \...,y..na .O,boya <._J~'r-t-~~~--r--------+---------+--------~--------+---------~--~~:J---------+~".S.~·-· i '"TAMALE Vendi c./.) <,

I 5.18 5.38 ly0'/ .>()(\ Damongo Tama~ Parj Mand."" Bole ...___1-- ~'1-i_--~~~~~~~~i_------~~--------4---------4---------4-~~----~--------~~r

?\-~,,"w. ./:"~:~":~'.,..h.t v~7~\i \' $'':g. r·,"t-~--------~--~----~--+-----~----~~~------7L-/'·4-·--------~~------~~------~~-\, (, r· I / Kpandar ! \ -\ -"'~ / (, v;.'''-;,::;; - .-. L._-~) Nkwan~".., /'."/ / ny\(:- ./ Yeji~-,, '.'."--..-) ,-- ) ,,-'

~ \'_ • Banda. \".- .._..~ .,.,..-•..: Kinta£DpClO \ . /.1 . " ...•. ~ ... Dutukp~ne )~ "K.m0r' ., ( .., i\./ I I • "'-..r-l----=~~j---------l_--~L---i_~~~~~--------~====~--,r--------,r~------~_1r

a; /rl'·~ 4.98 / 1//~·Ab'e~SiV---- ~ ~~ i '<; Wenchiy ~ jeAtebubu I<.t~ .KOr3hi..,.. t\.' . ( oo s;~ - Ah:-lnansu

. Nko nza r : \ .....1>J'I-+!-i·--~--~----'-----::::-::l="7"::""·--~:'L/::""-->" ==.....'-=1. f=::.=:=::,......,..---.jl- ---.jl-+'..:",-w:.:.o:.:.r_.w•...or:.:.a-l+' -t---l".

> j Bere~ ••. <:-~t7..x;".. .,;.-'. ._.. .. • \, \~-''-.l( SUNVANI '.', Seko-·masi. Sabuso -. -··Qb~sum . \ Baglo ~- ) .._, "I .

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Fig.3

50-YEARinches)

24-HOUR RETURN PERIODMAXIMUM RAINFALL(Values Jn

)' 0'

", _..--J~'--+---,-.,---r---~:Rn""'>ofl".j \ .'~ • .ongo- a ~"'.

" \ Walewale ,.."" e Yagaba J

\ hereponi ill f./,:) t.,l'I_+_---''--_+--;;""'- __ + __ -+_-+ -!- -!- -!- ~.J~+_----+__Il0'

@lKaraga j

..+------\-----_J --1.Wapul; I.. 6,

'-,(-,1---t--~~ft----r_------~4f----------r_----------_t-----------r_--------~_t-----,_~,~,~~~,;__~------------~_1n*TAMALE Yend; .•••·)

5.77 5,82 \Zabzugu (

(j1--4_-~~~~~~--~~----------t_--~~--4_~------_t--------_4--------~~--------~~~

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p p E v o L TRu

7.0

11 Funst~appo

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• Wangasi-Turu• Bimbila

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S.64-6'I -.s,oWoraw.ora

GHANA

G u30'2' )0'

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Fig.4

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MAXIMUM 24- HOUR RAINFALL 100- YEAR RETURN PERIOD(Values in inches)

lJ',.--,-,. -:,"_' ". ,lJr' "T" lJ'r-_-i'-.,0'

U P PER V 0. L T A .I'\/"' 0 '-.,,," '-'-.

11.1--+_-,_~~+__:_----+-~~'=J'-'=jr=--~=--t-e_=,,~7=-.;:--::-r---!!;Ba!!W~k!!.u-1-t' ~----'--=·:-i_-----I--lil.I _----~~N~~~ro;n ••~o~._t __ --~·~/-'t_-------H),/ Tumu 1'--8,0

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" ~) _.. ,12..~~.....--' rv . " ..- .. _ .. _ .._.. .Wapuli i..,. J • Sav.lugu ••••,'In '.••••...;Ka'ib. J1.l\ 'j.' .D.boy' <.q~~--~~---t---------i----~----t----------r---------+--------~----~~~}----------l---l'-,i TAMALE Yend; C.r')

)i L----------L *6.36 6.26 \

V~ Zabzugu (

( Darnonge Tam. Per ji Mand.ar ole' • ("r__r----~~,,~~~~-------r---------+--------~\rl----------+--~~--~~------~J----------l---l~

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>- \_ »Baoda \'-"-/-/~~nt!mpo Kamo ';""'. (/' Yeii e-, , -, ·""X~~:._~/ DutukP~ne( i 0

"~~----~~,...,~_+----------~----------1-~~~~~~~------~~----~'r--+-----------+-+---------l--4rr-J.S.mpa ' V ·Ab~"Sil/ \

a;P' j :/ / \ \.'0 l 5. 87 Kete Krach;" ('Wench;. Atebubu 7 .24 "'" • . ClAha1nansu

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> ,/ Berekum' I * k~ ,.. · \,... ... '''')~I '<, Sekodu~SIIl Sabuso -'4Qbo5um ~ Baglo ~

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) ) oaso - -----r- --.... ! "\'\) • I ::.»> .\<Ukuom K~~~~ ~~ omp.t.. 7\( ..... -A~etifi /" ..J \

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,. . K ,,_ R <I".. r::- .

\V.Enchi .A ••nkr.ngw. "'~",'\.' ~,:•.~~.--ty~-~""'<;,~ ~ k<LLJf// ..J IO.'C;.\)t~, ','-- .-.-----..-~ "t /: .~ 1.-. !'." -s 0) '.\) • 0

__ " ,',--.-~ '-.. Nub..' '" -"", ,0 -. ~ \ ~ \ ~'" ~,,~ <?~,~_t---+_----1_----------+-~~--~~~---------+--~~----~~~~y~--1-~--~----L-----~--~L--llJ'\ Preste e V .i: ~v <P ""1)jJT:no•o ~I~Tarkwa 1•._, ~ '" ''l ~

~\

.• I---- ??~

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c I . l~~~~~~~"'DI r G UU Fe) , o>f <1/ -(

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Fig.5

MAXIMUM 3-HOUR RAINFALL IN % OF 24-HOUR VALUES

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Fig,6

MAXIMUM 6-HOUR RAINFALL IN % OF 24:-HOUR VALUES

J' D' 2'

U P P E R

.I./

TumuIi e Hani elawra

I.

-.""-- . /Navrongo e \- .

~6L&ATAN ~A* e Zuarungu

A

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Fig.7

/

MAXIMUM 12-HOUR RAINFALL IN Of. OF 24-HOUR VALUES

JO'

v o l T

o

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o

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, 0'

I .K~kuom(

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I.Enchi

GHANA

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Fig.8