the gambia coastal erosion case studies - yt2012.pdf

7/28/2019 The Gambia Coastal Erosion Case Studies - YT2012.pdf

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Chapter Four Case Studies

Scope of the chapter

Seven case studies have been prepared for the beaches along the GB coastline. The case study

involved taking photographs, conducting interviews with the locals and sketching a beach plan.

The beach plan was used to record the distance between the coastline and the nearest structures

which were mostly buildings and roads. All case study sites were selected with economic,

cultural and historic hierarchy being considered. A questionnaire was carried out to record how

far forward the coastline has shifted. The main points noted from the case study were to

characterize the coastal geomorphology of the GB coastline. The overall case study has been

divided into two areas; The City of Banjul and the Kombos.

Procedure and Equipment

The site visit gave the Author the opportunity to meet many locals who have spent a

considerable amount of years living or working by the beach, non-more so than the owner of

Radio Syd who kept a series of photographs of the damage caused by the rising tides. A brief

sketch of each coastline visited was followed by taking a record of distances between the

coastline and the nearest structure. This information was then annotated on the beach plan

sketches. Distances between high tide and low tide were also recorded using an analogue meter

counter (Fig 4.1 and 4.2) which underwent a simple instrument check. A tape with a known

distance was repeatedly measured and compared for accuracy. This instrument is capable of

recording readings to the nearest centimetre. A photo file has been compiled of photos taken

during the site visit which was integral to carrying out a reconnaissance for the selected case

studies. (See Appendix F) All images within the case studies are by the Author.

Figure 4.1 Meter counter with analogue

dial. (Red switch resets to zero for a new

reading)

Source Author

Figure 4.2 Meter

Counter/Trundle

Wheel

Source Author


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Case Studies

Banjul Case Studies

The first observation upon approaching the beach was the difference in perceptive wind and

wave strength. There appeared to be considerably less torrential currents along Bund Road,

adjacent the gentler River Gambia, than some of the more erosive waves experienced along

Radio Syd to Lands office. From the beaches visited and measured during the beach surveys,

Radio Syd recorded the maximum distance from structure to high water mark with an average

just over 120 metres while the shortest distance was at Bund Road which was less than five

metres from the high water mark. (Table 4.1) The average beach stretch distance reclaimed by

Haskoning in 2004 is 150m wide.

Figure 4.3 Map of Banjul-Kombo coastal highway pinpointing the beaches visited in Banjul. After

JICA (2002)Table 4.1 Distances to coastline

Beach Location Distance (m) Measured from;

Lands Office 30.25 Department of planning fence line

Radio Syd 126.10 Radio Syd fence line

Albert Craft Market 12.28 Rip Rap boulders protecting craft workshops

GPA Complex 17.15 Complex fence to shoreline

Bund Road 2.00 Rip Rap defences were too dangerous to measureSource Author


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4.1 Radio Syd - Lands OfficeThe beach behind Lands Office is situated between the east by the West African Examination

Council and the Muslim and Christian Cemetery on the west. This beach is presently

experiencing severe erosion with a creek forming just one hundred meters away from the Radio

Syd building. According to NEA reports (2006) this area had a previous erosion rate of up to

4m/year. This beach has been identified as a site of historic and cultural importance because of

the close vicinity of the Muslim and Christian Cemetery. The close proximity of public services

such as the West African Education Council and the Lands and Planning office add more weight

to this sites socio-economic importance as both education and property business rely heavily on

these institutes.

Beach nourishment was completed on this beach stretch in 2004 by an international Dutch

company Royal Haskoning. This was preceded by a feasibility study which concluded on the

historic and projected erosion rates over a twenty year period. The 2010 NEA report confirmed

that up to 120m of beach stretch was reclaimed along the Radio Syd beach site as a result of the

Haskoning Project (2004). Within this study the Radio Syd/Lagoon measured seventy nine

metres from the high water mark. During the site visit which took place in January 2012 the

maximum distance recorded from the Lands Office fence line to the high water mark was thirty

metres. Notably there is great cause for concern if erosion of this area remains at a constant rate

of 4m/year. After the Beach Replenishment Project a creek has formed behind Radio Syd beach

creating the Radio Syd lagoon.

Prior to the Beach Replenishment Project the main coastal defences were hard defences in the

form of Rhum Palm groins driven into the beach and tied together by wooden boards. These

ancient groins, built in the colonial days, were located close to the Mens Lodge a few hundred

metres east from Radio Syd. They have protected the city for decades but were replaced in 2004

due to negligence and poor maintenance. Four historic photographs of damaged infrastructure,

including the devastated old Scouts Headquarters, have been included in Appendix F

highlighting the inadequacy of this type of coastal defence. These images provide visual

understanding of the proximity of the Radio Syd building to the coastline in 2000 and can be

compared with photographs taken during the recent site visits of January 2012. (See Banjul Case

Study).

Along the beach close to the Cemetery and Radio Syd many trees have been ravaged by

torrential waters as shown in figure 4.1.2. The Cemetery and graves closest to the coastline were

washed away prior to the Haskoning Project. Figure 4.1.1 illustrates the Lands and Survey

Department fence line and a trail track left by the metre counter. According to the owner of

Radio Syd heavy winds during March and November increase the rate of erosion surrounding

beaches.


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Case Studies

Figure 4.4 Case Study Sites visited in Banjul. The map above identifies the beach sites in Banjul

which were visited and documented.After UK-Gambia (1981)

Figure 4.1.1 (left)

Sandy beach just

behind the Lands

and Survey fence

line.

Metre counter

track is visible in

the foreground. Figure 4.1.2 (above) Trees which have

been affected from erosive waves.


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4.2 Albert Craft Market Banjul to Barra Terminal (GPA Wharf)This beach is one of great economic importance. The Banjul to Barra Terminal is the most direct

route joining the north and western divisions of the country. The beach in this area has a north

south orientation at Banjul Point. This region is of particular interest thanks to the flurry of

activity that engulfs the neighbouring Albert Market; the citys main market. Two other human

activities which are predominant along this beach are local-scale fishing and petty craft workers

located just behind the main Albert Market.

One of the main landmarks of the country, the State House, is situated just a few hundred metres

from the beach behind Albert Market. Due to rigid security a case study could not be completed

of the State House, however in January 2010 the NEA produced a reported which stated that the

State House fence line measured just fifty one metres away from the coastline. (NEA, 2010)

According to measurements recorded in January 2012 the maximum distance recorded from the

nearby Iceman ice factory, to the high water mark was less than ten metres.

The main types of coastal defences visible on these shores are big boulder stones and tyres

driven into to ground as illustrated below (Fig 4.2.1). According to the craft workers

interviewed, some of the big boulders have been brought in from Radio Syd and others date back

to colonial times when large boulders were shipped in from Sierra Leone. The rocks guarding

the coastline at this point have been identified by the local craft workers and fishermen as basalt

stone. These large boulders have been complimented by concrete blocks, sourced from building

debris of a warehouse previously owned by the Public Works Department; according to

individuals interviewed the boulders were transported from the old warehouse site in the early

quarter of 2004 during the Haskoning Project.

Besides boulders and rocks which form Rip Rap defences there

are lines of used tyres which have been placed in an attempt to

create a flood barrier for the neighbouring craft workshops. (Fig

4.2.1) Some remains of Rhum Palm trees and concrete groinswere also encountered during the site visit. (See Appendix F)

Notably most of the rock armour boulders placed here were

previously used as Rip Rap defences for Radio Syd. From

discussions with the locals it has been learnt that these boulders

were taken from a building which was almost engulfed by the

rising tides in 2000. Consequently they are less effective in

protecting from further erosion.4.2.1 Tyres used to

create a flood barrier to

protect the flanking craft

workshops.


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4.3 Gambia Ports Authority ComplexThe Gambia Ports Authority Complex is based in the Banjul Dockyard, locally known as the Half

Die area. It begins from the end of Bund Road and continues all the way towards the Banjul-

Barra Terminal. The complex is the main port harbour for the country where the GPA controls

all port activities on a commercial scale. This makes this site a key area of economic importance.

No beach replenishment was carried out in this area as there is very little beach left due to the

abundance of port activities, including the Banjul to Barra ferry and the use of several

commercial ships to transport goods from the main Dockyard. A large area of reclaimed land has

been observed from GIS results indicating that the 13,800 metres squared of land was reclaimed

between 1984 and 2002 however from 2004 to 2012 there has been an increase in erosion.

After observing the bathymetric map provided by the GPA, it could be noted that the water

depths closer to the ports are considerably greater than those elsewhere along the River Gambia

mouth. In comparison an earlier bathymetric map, available within the TAMs (1999) Coastal

Protection Study, reveals the water depths have increased significantly with deeper waters

moving south along the Gambian River. Whereas previously the water was only 100m deep the

updated 2012 bathymetric survey map indicates areas where water is more than two hundred

metres in depth. It could be suggested that this is due to the GPA relocation from Banjul Point

southwards towards Half Die.

4.4 Bund RoadBund road is a major route for heavy goods vehicles (HGV). This road, bordered by mangroves

and water on both sides, connects the Banjul Serrekunda highway with the Gambia Ports

Authority Complex. Its close proximity to the ports explains why this road has become one of the

most important commercial highways enabling the HGVs to bypass heavy traffic further into the

city.

4.3.1 Waterinside the GPA

complex.

According to

maps dating back

to the 1980s this

entire area is

likely to be

reclaimed land.

Source Author


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Erosion along this highway appears minimal in the Pre Haskoning period with a significant

increase in land eroded during the Post Haskoning period. Results suggest that this area

accreted over 16,000 metres squared of land between 1984 and 2002. The distance between the

coastline and the road was no more than two metres at the time of the site visit. Along the side of

the road are Rip Rap defences and mangroves which have begun to show signs of retreat. The

land area most affected within this region is the Tanbi complex; a wetland site of international

importance. (See Appendix B Ramsar Sites)

To summarise the findings at this site, the main areas affected by erosion are the Tanbi

Wetlands. Historically this area has accreted land, however more recently the land has been

eroding. In order to ensure the comfort of passengers and the safe transfer of goods via this

commercial highway the current state of the road needs to assessed and given adequate

protection. Should this not occur it could result in a rise in the number of HGVs using the main

Banjul road which could further deteriorate the condition of the Banjul-Serrekunda highway.

Currently the road is in poor condition (fig 4.4.1) and requires significant rehabilitation.

According to local fishermen interviewed along this coast higher tides create erosive waves

which is compounded by heavy rains and this is when erosion is at its peak. During this period

water seeps onto the road which further deteriorates the condition of Bund Road.

Kombo St. Mary Case Studies

Three sites have been covered in the Kombo St. Marys region. These are Fajara, Bakau Fishing

Centre and Cape Point. From this list only Cape Point has been replenished while the remaining

two are beaches with cliffs facing the coast. These beaches are closer to the Atlantic Ocean where

the waves are much stronger and more erosive than those within the river estuary. In previous

feasibility studies along the Kombo coastline erosion was found to be at a reasonable rate of less

than one metre per year.

4.4.1 Bund Road has

been reduced to a dirt

track due to frequent

flooding and high HGV

usage.


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Figure 4.5 Case study map of the sites visited in the Kombo St Marys division.After JICA (2002)

4.5 Fajara (Golf Course to Hotels and Apartments)The beach from Bakau Fishing Complex heading east towards Fajara Golf Course is fronted with

numerous cliffs. (See Fig 4.5.1) The cliffs in this area appear to be formed from red sandstone

rocks. The beaches from Bakau to Fajara are littered with lots of cliff top debris which are

scattered along the coast. Cliff failure is easily visible along this coastline with a series of

forward-sloping cliffs towards residential areas. The structures nearest the coast are the beach

gardens and Seaview Apartments. The high walls of concrete facing the coast protect the

apartments from the onrushing waves at high tides. The furthest distance from structure to

shoreline35m

To the east, behind the cliffs close to Fajara Golf Course, are a cluster of beach bars and towards

Bakau there are apartments, tourist resorts and residential properties. This beach area is alsohome to a number of properties of government officials, the Medical Research Council and the

British High Commission, highlighting the importance the case study.

Figure 4.5.1 Forward -

sloping cliffs along the

beach of Fajara. The

scoured cliff is easily

visible on the right of

the photo.


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4.6 Bakau Fishing CentreBakau Fishing centre is at the heart of the Bakau community. The Bakau local market, where

women engage in the selling of the fish caught by the fishermen, is located close to the Fishing

centre. This fish landing site is the second largest fish commercial site, after Gunjur, with respect

to the amount of tonnage landed. However the fish landing site has since been run down

revealing an unsafe jetty and a lack of the cold storage facilities. This can have an adverse effect

on the fishermen which in turn threatens the level of commercial activity and socio-economic

stability of Bakau market.

Coastal erosion is a complex issue to resolve especially when lives and properties are at risk.

Take for example the village of Maplleton in North East Yorkshire of the UK. In 1991 this coastal

town implemented a two million pound coastal protection scheme. Since the completion of the

rock armour and groin systems at this beach site erosion has been halted and presently the cliffs

are no longer at great risk. However beaches further south are being starved of their natural

sediment supply exacerbating erosion on the coastline further south. Unfortunately for home

owners further south of Maplleton the reduction of sediment supply earmarked this coast as one

of Europes fastest eroding coast at a speed of ten metres a year. (BBC News, 2012) This account

is very similar to the case study of Gambias Cape Point and Bakau Fishing centre.

The rock armour groins and coastal defences along Cape Point are most likely starving the beach

further south of its natural sediment supply. Since the Haskoning Project the cliffs and sediments

along Bakau beach have been eroding at an alarming rate. The beach stretch at Bakau hasformed a concave shape and is bounded by cliffs on both sides. This concave shape indicates a

retreating beach. The coastal profile along this expanse of beach is interrupted by slopes and

cliffs. The Bakau fishing depot is sandwiched between two narrow cliffs. The nearest structure to

the coastline is the jetty which was constructed in 1960. According to a worker at the cold

storage facility, Mr Ngan Bahoum, the coastline was as far back as the jetty front when it was

completed. He also commented that when tides and currents are highest the water reaches the

fishing depot.

4.6.1 Image of the

commercial fish and

boat landing site at

Bakau.


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4.7 Cape PointThis beach stretch is shaped much like a cape giving it the name Cape Point. It is the northern

most point in the western division of the country and its pointed cape shape pinches the

entrance between the Atlantic and the Gambian River. This particular beach area is well known

as a quiet tourist venue away from the livelier and entertainment-packed Sene-Gambia strip.

Several hotels are clustered around this preferred tourist location such as the Cape Point Hotel,

Sun Beach and Ocean Bay hotels. The UN headquarters is also located within this area. This

tourist resort centre is bounded by Bakau on the west and Old Jeshwang on the east. Between

Cape Point and Jeshwang is the Cape Creek, a well-known bird watching site. Not far from Cape

Creek is located the distinguished Abuko Nature Reserve; a dedicated site for wildlife

conservation. This site has been selected as one of economic and environmental importance.

On the west of Cape Point the concave shaped coastlines from Cape Point to Bakau can be easily

observed from the maps collected. This coastline shape is very common with beaches

experiencing severe erosion. Observing figure 3.1 shows that sediment deposition is in a south-

west direction from Cape Point towards Kotu. Therefore beaches further south of Cape Point

depend on sediment supply from the River Gambia which can be reduced by coastal defences.

On the east of Cape Point are large rock amour groins with some measuring 150 meters long.

Before the Haskoning Beach Project this area was one of the sites most affected by the rising

tides and consequently suffered severe erosion.

A Creek has developed between Cape Point and Old Jeshwang which has become an entry point

for water into the Kombos and Old Jeshwang. After observing the 1993 aerial photographs along

with various maps the influence that this creek has on erosion requires further investigation.

The creek has spread eastward over the years and slowly eroding land close to the Serrekunda-

Banjul highway. Observations from four Terralook satellite images relay the fact that the Cape

Creek began expanding between 1990 and 2000. By 2006 the creek had expanded much more

claiming more land between Cape Point and Old Jeshwang.

4.6.2 Cliffs

neighbouring the

fishing depot have

begun erodingresulting in cliff

failure.

Concave beach

shape.


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South of Cape Point is a sand spit forming a barrier in front of the coastline. This area is most

affected by the rising tides. In 2004 five huge rock amour groins were erected at this site. (See

Appendix F) Since then the groins have been accumulating sand deposit accreting 1,200 square

metres of land. However surrounding regions have observed erosion. (See Fig. 5.7) Currently

Cape Point is recovering from the sediments trapped alongshore via the Gambian River. Notably,

since groins trap sediments alongshore they can starve adjacent beaches from their sediment

supply needed for natural beach replenishment.

4.7.1 ACCC ProjectBenchmark used during

beach replenishment and

groins construction.

All case study

photographs

were taken by

the Author


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Chapter Five Results and Discussion

GIS Results

The results of this project were generated using ArcGIS and Microsoft Excel. ArcGIS 10 was used

to digitise erosion and accretion polygon shapes from two different coastlines. The resulting

polygon areas where then tabulated on Excel to give the total area of land eroded/accreted for a

particular beach site. Thirteen beach sites where covered in Banjul and three were covered in

the Kombos. (See Appendix D for Erosion and Accretion Tables) Using Time Series Analysis the

results generated gave estimates of the amount of land eroded/accreted before and after the

2004 Haskoning Beach Project which, since its completion, changed the GB coastline.

Two main periods considered for this study are from 1984 to 2002, represented by Erosion 1

and Accretion 1 maps, and from 2004 to 2012 illustrated in Erosion 2 and Accretion 2 maps.

From the seven coastlines digitised for this study a coastline evolution diagram has been

produced for Banjul and the Kombos to give a visual of how the coastline has evolved over

twenty eight years. (See Figure 5.1 and 5.2) Coastline evolution maps show how the most

significant change occurred at beaches along the south-western part of Cape Point where Cape

Point Creek has been formed. With time this creek has expanded allowing more water to flood

the Kombos threatening the BakauJeshwang road which connects with the Banjul Serrekunda

highway.

Some beaches along the GB coastline have observed erosion when previously they indicated

accretion. This has been demonstrated in the comparison of Pre and Post Haskoning erosion and

accretion trends. Prior to the Haskoning Beach Project the beaches at Cape Point were eroding

while the beach at Bakau was accreting. Post Haskoning results indicate that this trend has been

reversed since 2004. The main cause of this change could be attributed to the groins located at

Cape Point which block the flow of sedimentary deposits moving from east to west along the

River Gambia. (See Figure 3.1) The sediment supply of beaches dictates the rate at which

beaches are naturally replenished while blocking the flow of sediments can have adverse effects

on beaches further west of Cape Point.

The changes of erosion and accretion can be observed in figures 5.1 and 5.2 which illustrate the

manner in which the coastline has changed over the twenty eight year observance period. Banjul

has steadily accumulated more land especially along the Gambia Port Authority Complex while

the mangrove wetlands have lost land. Two main sand spits have been identified at Toll Point

and Denton Bridge. These regions constantly accumulate sedimentary deposits due to

converging river currents from the River Gambia and the Creek at Denton Bridge.

the gambia coastal erosion case studies - yt2012.pdf

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