Nature Of Marine winds, Waves and Swells
over West African Coasts – Case study of
Victoria Island beach in Lagos, Nigeria
By
S. O. Gbuyiro and E. Olaniyan
Marine Unit, Nigeria Meteorological Agency,
Lagos - Nigeria
INTRODUCTION
• SEA STATE IS CONTROLLED BY ATMOSPHERIC
WINDS WHICH COME IN CONTACT WITH THE
OCEAN SURFACE.
• WIND IS THE PRIMARY CAUSE OF OCEAN WAVES
DUE TO INTERRACTION BETWEEN ATMOSPHERE
AND SEA SURFACE .
• WIND WAVES COMPRISE OF A SMALL PORTION OF
THE TOTAL OCEAN WAVE SPECTRUM WITH TYPICAL
WAVE PERIODS OF ABOUT 1 SEC TO 20SECONDS.
TYPES OF WAVES AND THEIR
SOURCES OF GENERATION
• INTERNAL GRAVITY WAVES - CAUSED BY WIND
STRESS . ** THE COMMONEST SINCE NEARLY ALL
OF OUR OCEAN INTERACTION OCCUR AT THE
OCEAN SURFACE.
• SEICHES – STANDING WAVES CONSISTING OF
TWO PROGRESSIVE WAVES TRAVELLING IN THE
OPPOSITE DIRECTIONS.
• TSUNAMIES- CAUSED BY EARTHQUAKES.
• TIDES - CAUSED BY GRAVITATIONAL PULL BY
THE MOON AND SUN ON OCEAN WATER AND
CENTRIPETAL FORCES PRODUCED BY A
ROTATING EARTH.
WAVES CHARACTERISTICS
• BASIN GEOMETRY INVOLVING
-- BASING LENGTH
-- WATER DEPTH
• WAVE LENGTH (L) * WAVE DEPTH (D)
• WAVE PERIOD (P)
• WATER DEPTH TO WAVE LENGTH RATIO (D/ L)
• WIND SPEED * WIND DURATION
• FETCH AREA / FETCH DISTANCE
IMPACT OF WAVES
• TRANSPORTATION & SHIPPING INVOLVING BOATS AND SHIPS.
• ENGINEERING WORKS: COASTAL
STRUCTURES
• WATER RECREATIONAL ACTIVITIES
ALONG THE COASTAL ZONES.
• FLOODING AND EROSION ALONG
COASTAL SETTLEMENTS
WHY THE STUDY
• ROLE OF ST HELENA HIGH PRESSURE SYSTEMS.
• TO IDENTIFY THE CAUSE OF OCEAN SURGES
DURING THE WINTER OR DRY SEASON.
• CONSTANT VICTORIAL ISLAND BEACH EROSION
AND FLOODING DUE TO STORM AND OCEAN
SURGES WHICH IS A THREAT TO ECONOMIC
ACTIVITIES (12000 KM2 , 6 MILLION POPULATION
Width of beach (m)
& US 3 BILLION SPENT ON SAND FILLING YEARLY.
1991
140
120
100
80
60
40
20
0
1994
F ig 1 .L o c a t io n s b e a c h w id t h fro m 1 9 9 1 t 0 1 9 9 4 in V ic t o ria Is la n d
Maroko
Ehotel
IMB
SLO
fghouse
Palace
niomr
Npayad
L o c a t io n s
STUDY AREA
• The Nigeria’s coastal zone which also suffer from the coastal
problems mentioned above lies within latitudes 4º 10’ to 6º
20’N and longitudes 2º 45’ to 8º 35’E spanning about 850 km of
low-lying coastline.
• The coastal zone can be divided into four distinct geomorphic
parts. These consist of
• (1)
The barrier-lagoon system stretching for almost 250km
from the Nigerian/Benin border
• (2) The Mahin transgressive Mud coast stretching for almost
75km.
• (3) The Niger delta covering almost 20,000 square kilometres is
the second largest delta in the world spanning a coastline of
almost 450km.
• (4) The Strand coast east of the Niger delta stretches for about
85km from the Imo river eastwards to the Nigerian/Cameroon
boundary.
Fig2ShowingtheNigeriacoast(a)andenlargedbox(b)VictoriaIsland
DATA AND METHODOLOGY
• Daily Winds, for 1998 to 2002 winter months
(November - March) at 10m level over the
Atlantic Ocean, between lat.10oS - 20oS and long
10oE - 20oW, were extracted from the general
circulation model (GCM) of METEO-FRANCE.
• The extracted winds between lat. 10oS - 20oS and
long 0o - 10oE, served as input data for a
parametric wave model. This model has been
adapted
FROM
LYON
(1994)
AND
WHITFORD(2001) to suit local conditions.
• The out put variables (wave height, wave period
and swells etc) were then obtained.
RESULTS
a. Winds from the fetch area (lat. 10 degree South - 20
degree South and long. 0 –10 degree East) generally lie
between 7-20 knots in strength. The weakest and
strongest winds were observed in January and March.
(Figs; 3a&b )
b. The corresponding generated significant wave heights
were less than 0.3 meters and more than 2.2 meters with
period ranging between 2.0 and 7.5 seconds. It takes the
lowest generated wave about 10-12 days and the highest
3-4 days to reach the coast as swells. (fig 4a&b). And
c. Swells of about 0.47 meters in height resulting from
winds of about 10 knots grazed the coast frequently
during the period
a. Rate of erosion from 1998 –2002 was observed to be
27m/year.
RESULTS
16
14
20
18
10
Winds(kts)
16
8
6
14
12
10
8
4
D ays
F i g . 3 a . T im e s e r i e s o f t h e
w in d f o r 2 0 0 1
F ig . 3 b . T im e s e rie s s o f th e fe tc h w in d fo r 2 0 0 2
fe t c h
1 .8
H sm
sw e ll
H sm
s w e ll
1 .6
2 .5
1 .2
2
Wave/swell (m)
1 .4
1
0 .8
0 .6
1 .5
1
0 .5
0 .4
0
D ays
F ig .4 b . G e n e r a te d w a v e a n d th e
c o rre s s p o n d in g c o a s ta l s w e lls fo r 2 0 0 2
26-Mar
12-Mar
26-Feb
12-Feb
26-Mar
12-Mar
26-Feb
12-Feb
29-Jan
15-Jan
1-Jan
D ays
F i g . 4 a . F e t c h s i g n i fi c a n t w a ve a n d
t h e c o r r e s s p o n d i n g c o a s t a l s w e l l s fo r
2001
29-Jan
0
15-Jan
1-Jan
0 .2
26-Mar
12-Mar
26-Feb
12-Feb
29-Jan
15-Jan
1-Jan
26-Mar
12-Mar
26-Feb
12-Feb
29-Jan
15-Jan
1-Jan
6
D ays
Wave/Swell (m)
Winds (kts)
12
20
25
18
20
16
Numbers
Numbers
14
12
10
15
10
8
5
6
4
0
W in d s (kn ts )
Fig 5b . Th e freq u en cy o f o ccu ren ce o f
th e win d 2001 - 2002 p erio d
17.8
15
13.3
12
Fig 5a. Th e freq u en cy o f o ccu ren ce o f
th e win d fo r 2001 p erio d
11.3
15
13.3
11.7
10
7.78
5
win d (kn ts )
10.4
0
8.75
7.22
2
CONCLUSION
This type of temporal observation in the marine
wind and associated coastal swells from the
generated wave can be important in the
determination of surge occurrence in the unusual
months. With the continuous coastal erosion caused
primarily by the wave and the inundation of the
coast during ocean surges in winter, this work will
contribute to integrated coastal management.
Thank You