FINAL REPORT

CIVIL ENGINEERING INTERNATIONAL INTERNSHIP (2009)

THE UNIVERSITY OF WESTERN ONTARIO

IN PARTNERSHIP WITH WARDFA, AND THE MINISTRY OF AGRICULTURE OF THE GAMBIA

AUGUST 7TH, 2009

BY JORDAN ATHERTON AND ADAM CROOKES

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EXECUTIVE SUMMARY

This report attempts to describe the undertakings of Soil and Water Management Services

(SWMS) of The Ministry of Agriculture (MOA) of The Gambia. It was prepared by civil

engineering students Jordan Atherton and Adam Crookes from The University of Western

Ontario (UWO) who took part in an internship with SWMS during the summer of 2009.

SWMS has been tasked with carrying out several externally funded projects. They have played

an integral role in implementing the 1997-2004 Lowland Agriculture Development Project

(LADEP), and since 2006, the Participatory Integrated Watershed Management Project

(PIWAMP). SWMS participation in these two projects has resulted in many achievements but

also some shortcomings.

Land use change and a transition away from traditional farming practices have resulted in

increased soil erosion in The Gambia and West Africa in general. Erosion leads to a loss of soil

fertility and ultimately degradation; that if left unchecked will lead to desertification. SWMS

works to slow this erosion process and conserve Gambia’s irreplaceable soil resources. Their

primary tactic is the construction of storm water runoff diversions (often called bunds).

Many observations were made by the engineering students throughout their summer

placement. Small adjustments are proposed in the areas of diversion design, diversion layout,

project management, communication, and collaboration. The hope of the students is that

SWMS will consider the observations made, and if appropriate, adopt some of the proposals

presented.

Alternative soil conservation methods are suggested as topics for further research. They have a

high potential to improve the soil conservation efforts of SWMS. The two conservation

methods proposed are Vegetative Hedgerows and Conservation Agriculture, which

encompasses no-till farming, organic soil cover, and diverse crop rotations.

Agriculture is the largest sector of the Gambian economy and plays an immense role in the

development of the country. The soil conservation work performed by SWMS is critical to

ensuring sustainable agricultural production, food security, and sustainable development. A

continued partnership between UWO and SWMS will greatly benefit both organizations.

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TABLE OF CONTENTS

Executive Summary .....................................................................................................................1

List of Figures ..............................................................................................................................3

List of Tables ...............................................................................................................................3

Introduction ................................................................................................................................4

Purpose ...................................................................................................................................4

SWMS..................................................................................................................................5

UWO Faculty .......................................................................................................................5

IDI Students .........................................................................................................................5

Background .................................................................................................................................5

Geography ..............................................................................................................................5

Climate and Weather ..............................................................................................................6

Soil Degradation ......................................................................................................................6

Land Use Change .................................................................................................................7

Erosion ................................................................................................................................8

Soil and Water Conservation in the Gambia ............................................................................9

History .................................................................................................................................9

Current and Recent SWMS Projects ...................................................................................10

Water Diversions ...................................................................................................................11

Planning and Management .......................................................................................................14

Diversion Layout....................................................................................................................14

Construction Sequence .........................................................................................................15

Equipment Management .......................................................................................................16

Summary ...............................................................................................................................16

Communication and Collaboration ............................................................................................16

Summary ...............................................................................................................................17

Design Alterations .....................................................................................................................18

Diversion Design....................................................................................................................18

Vegetation for Stabilization of Diversions and Channels ........................................................19

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Summary ...............................................................................................................................19

Alternative Conservation Methods ............................................................................................20

Vegetative Hedgerows ..........................................................................................................20

Conservation Agriculture .......................................................................................................21

Conclusions ...............................................................................................................................22

References ................................................................................................................................23

LIST OF FIGURES

Figure 1: Rainfall (July, August, and September). (Matsuura & Willmott, 2007) ...........................6

Figure 2: Development of forest cover from 1956 to 1993 (The Government of the Gambia,

2000) ..........................................................................................................................................7

Figure 3: Erosion in Bulok the Gambia (July 11, 2009) .................................................................8

Figure 4: Rill erosion in Sare Alpha, The Gambia (July 23, 2009) ..................................................8

Figure 5: Gully formed on main village road in Bulock, The Gambia (June 2, 2009) ......................9

Figure 6: Diversion cross Section (Land slope is exaggerated) ....................................................12

Figure 7: Map of Diversions in Sare Alpha, prepared by Erik Brohaugh (2009) ...........................12

Figure 8: Dike relocation in Bulock (June 2, 2009)......................................................................14

Figure 9: Failure of Bund in Sare Alpha (July 23, 2009) ..............................................................15

Figure 10: Diversion showing emergency spillway .....................................................................18

Figure 11: Drainage through the use of (a) earth bunds and (b) vegetative hedgerows

(Grimshaw, 1993) ......................................................................................................................20

Figure 12: Vetiver grass (Grimshaw, 1993) ................................................................................21

LIST OF TABLES

Table 1: Land use in the Gambia from 1980 to 1993 (The Government of the Gambia, 2000) .....7

Table 2: LADEP accomplishments of SWMU (ADB, 2007)...........................................................11

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INTRODUCTION

During the summer of 2009, three students from the Civil Engineering and International

Development Program at The University of Western Ontario (UWO) undertook an internship

with the Soil and Water Management Unit (SWMU) of the Department of State for Agriculture

(DOSA) in the West African country of The Gambia. Recently, the Gambian government made

some changes to their departments, part of this was changing the name DOSA to the Ministry

of Agriculture (MOA) and SWMU to Soil and Water Management Services (SWMS). Therefore,

SWMU is used when discussing past projects and SWMS is used when talking about the present

in this report.

UWO students participating in the internship at SWMS were Adam Crookes, Jordan Atherton

and Lindsay Christink. Supervising the three students at SWMS was Kebba Manka.

Unfortunately, Ms. Christink left midway through the internship due to prior commitments in

Malawi.

Arranging the internship on the Canadian side were UWO professors Dr. Fred Keenan and Dr.

Tim Newson as well as the International Development coordinator for UWO Stephanie

Laurence. On the Gambian side, Alpha Jallow from the West African Rural Development

Facilitators Association (WARDFA) and the West African Community Development Training

Center (WACD-TC) assisted in arranging placements in The Gambia. Mr. Jallow was the

student’s main supervisor while in the Gambia and provided the key transitional and logistical

support upon their arrival.

Students were taken to SWMS work sites across the country, where they observed a complete

cross section of the organization’s work. After the first two weeks with SWMS, the students

took a three week break to attend classes at WACD-TC, under Mr. Jallow. Upon returning to

SWMS, the students took a proactive approach and initiated a research report on SWMS.

PURPOSE

The purpose of this report is to provide written documentation of the UWO Civil Engineering

and International Development Internship (IDI) with SWMS from May to August 2009. The

report aims to illustrate the soil conservation work of SWMS and to share thoughts on possible

modifications to some of SWMS’s methods and techniques.

The three intended targets of the report are SWMS, the Civil Engineering and International

Development Department of UWO, and future UWO IDI students. Preparation of the report has

also allowed the authors to gather and generate ideas during its development.

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SWMS

The report includes observations of the activities of SWMS and a discussion of the techniques

and methods used. Also included are activities that the students feel could be more heavily

focused on and general discussion on a variety of subjects. It is the students’ hope that this

report will bring forth valuable ideas for SWMS to further consider, and perhaps implement.

UWO FACULTY

Another aim of the report is to bring to the attention of UWO faculty the work undertaken by

the students at SWMS. It is the hope of the students, that through this report, UWO will

understand the value of a continued partnership with SWMS as there is an abundance of work

available to further establish the partnership between the two organizations.

IDI STUDENTS

Future UWO IDI students who will work with SWMS could also benefit from this report. The

authors feel as though future participants would appreciate receiving as much information as

possible prior to their departure. The structure and operation of SWMS, a Gambian

government agency, is vastly different than one would be familiar with in Canada. Simple tasks

in Canada, such as obtaining data, can be a rather difficult process in The Gambia, requiring a

great deal of patience. The authors hope that this report can aid future IDI students in

understanding The Gambian working culture and better prepare them for the internship.

BACKGROUND

Agriculture is the driving force behind the Gambian economy, employing 78% of the population

and accounting for 27.8% of the GDP. Agriculture products make up 90% of all exports,

groundnut and peanut products making up 70% of this figure (FAO , 2005). The previous

statistics illustrate the importance of agriculture to the livelihoods of the Gambian people and

that agriculture should be an integral component of any development work in the country.

Many farmers in the Gambia rely on subsistence farming. Despite the importance of agriculture

in The Gambia, about half of all food must be imported to satisfy the population’s nutritional

needs (FAO , 2005).

GEOGRAPHY

The main geographic feature of The Gambia is The River Gambia which splits the country in two

as it winds its way from Guinea through eastern Senegal and then the entire length of The

Gambia. Topographically the country is divided into uplands and lowlands (river flood plain)

which dictate agricultural land use as well. In the lowlands, rice is typically grown with rotations

of vegetables. Upland, the primary crops grown are groundnut and early millet along with other

cereals (National Climate Committee, 2005).

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Salinity of The River Gambia plays a large role in determining what crops can be grown in the

lowlands. Tidal influences cause a saline front to move up river as far as 250 km from Banjul

during the dry season, and to be pushed back by increased river flow to within 50-80 km of

Banjul during the rainy season.

CLIMATE AND WEATHER

The Gambia is located in the Sahelian climate region of Western Africa. Rainfall in the country is

highly variable both inter-annually and inter-seasonally and has been decreasing over the last

half century (Figure 1). Intense storms and dry periods during the rainy season are not

uncommon. The unpredictable and variable nature of rainfall, coupled with increased food

demand due to population growth has forced farmers to change traditional practices. Certain

farming practices such as monoculture and intensive farming (National Climate Committee,

2005) destroy soil fertility, increase runoff, and increase the susceptibility for the soil to erode.

The sporadic nature of rainfall makes it virtually

impossible to predict. Currently, there exists no

reliable network of meteorological stations able

to accurately monitor country wide rainfall,

making it difficult for farmers to determine

appropriate crops and the time at which

planting should begin. There has been a rough

trend of reduced annual rainfall but in no way

has it stabilized. It is highly possible for above

average rainfall one year to but there is a

stronger tendency for drought.

SOIL DEGRADATION

According to the FAO, soil degradation is “the sum of the geologic, climatic, biological and

human factors which lead to the degradation of the physical, chemical and biological potential

of lands in arid and semi-arid zones, and endanger biodiversity and survival of human

communities.” (FAO, 1993)

The Gambia faces the serious problem of soil degradation and without maintaining the vitality

of soil it is only a matter of time before desertification takes root.

150

200

250

300

350

400

1960 1970 1980 1990 2000 2010

AV

ERA

GE

MO

NTH

LY P

REC

IP (

mm

)

YEAR

RAINFALL

FIGURE 1: RAINFALL (JULY, AUGUST, AND SEPTEMBER).

(MATSUURA & WILLMOTT, 2007)

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LAND USE CHANGE

The increasing food demands of a growing population have resulted in significant land use

change in The Gambia. More land is used for agricultural production to meet the demand.

Traditional farming practices have also changed. Land was traditionally cleared using the ‘slash

and burn’ technique, cultivated for a few years, followed by a long fallow period lasting many

years (FAO, 1983). Soil fertility was restored during the fallow period, ensuring sustainable use

of soil resources. With increased food demand, farmers have been forced to eliminate this long

fallow period by cultivating land every year.

Despite the transition from short

duration cultivation and long fallow

periods to yearly cultivation, the

traditional slash and burn technique

is still employed. Land use has

changed, but sustainable farming

practices have not been widely

adopted. A consequence of this is

increased top soil loss through

erosion, resulting in declining soil

fertility. Table 1 shows the land use

change in The Gambia from 1980 to

1993.

TABLE 1: LAND USE IN THE GAMBIA FROM 1980 TO 1993 (THE GOVERNMENT OF THE GAMBIA, 2000)

Land Use Category 1980 1993 Change

(ha) (%) (ha) (%) (ha) (%) Woodland 14,400 1.3 12,000 1.1 -2,400 -.2 Savannah woodland 121,600 10.7 88,800 7.8 -32,800 -2.9 Tree & shrub savannah 280,400 24.8 360,800 31.9 80,400 7.1 Sub-Total: Total Forest Cover 416,400 36.8 461,600 40.8 45,200 4.0 Agriculture with trees 84,000 7.4 85,200 7.5 1,200 0.1 Agriculture with no trees 226,400 20 241,200 21.3 14,800 1.3 Fallow area 138,800 12.3 89,200 7.9 -49,600 -4.4 Mangroves 68,000 6.0 59,600 5.3 -8,400 -0.7 Others 198,800 17.6 195,600 17.3 -3,200 -0.3 Total 1,132,400 100 1,132,400 100

0

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

1946 1968 1980 1993

Po

pu

lati

on

Den

sity

(p

erso

n/k

m2)

Per

cen

t o

f To

tal L

and

Forest

Population Density

FIGURE 2: DEVELOPMENT OF FOREST COVER FROM 1956 TO 1993 (THE GOVERNMENT OF THE GAMBIA, 2000)

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FIGURE 3: EROSION IN BULOK THE GAMBIA (JULY 11, 2009)

EROSION

Erosion occurs in two forms,

geologic and accelerated.

Geologic erosion is the normal

erosion process of land in its

natural environment with man

playing no part. Accelerated

erosion is an increased form of

geologic erosion caused by

human activities in altering

natural cover and soil conditions.

There are three types of

accelerated erosion: sheet

erosion, rill erosion and gullies.

(FAO, 1965)

SHEET EROSION

When a layer of top soil is uniformly removed from a flat land surface, it is called sheet erosion.

There are two process of sheet erosion the first caused by raindrops detaching soil particles,

the second is the transport of the detached soil particles. The force of rainfall has a significant

effect when it makes contact with soil, moving particles large distances. Once the rainfall has

exceeded the amount of water that can infiltrate into the ground, water begins to flow over the

land surface, transporting the detached soil particles. The flowing water with the detached soil

particles causes further erosion on the surface layer of the land. (FAO, 1965)

RILL EROSION

More common than sheet erosion is rill

erosion. Rill erosion occurs because of the

irregular topographic nature of a farmed land;

occurring when rainwater accumulates in

depressions, eventually finding the path of

least resistance and flowing through, causing

the formation of rills. Unlike sheet erosion, the

particle detachment mechanism is mainly the

increased energy of the flowing water. The

problem is exasperated by intense storms.

Once the rills are formed, erosion begins to cut

into the subsoil. (FAO, 1965)

FIGURE 4: RILL EROSION IN SARE ALPHA, THE

GAMBIA (JULY 23, 2009)

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GULLYING

Sheet and rill erosion lead to the

formation of gullies. Unlike rills, gullies

cannot be corrected by tillage due to the

severity of its cutting into the soil.

Gullies develop in areas where rills are

not repaired in sufficient time or where

there exists a natural land depression in

which runoff water accumulates.

Depressions can be caused by the

frequent use of a path or road by

machinery or livestock. (FAO, 1965)

SOIL AND WATER CONSERVATION IN THE GAMBIA

In The Gambia, soil and water conservation work began following the British LAND Resource

Study of The Gambia, with the creation of the Soil and Water Management Unit (SWMU), a

section of DOSA. Prior to SWMU’s creation, soil and water conservation was only dealt with by

legislation, which unfortunately was never enforced.

HISTORY

In 1978, the United States Agency for International Development (USAID), with the United

States Department for Agriculture's Soil Conservation Services (USDA/SCS), began a ten year

relationship with SWMU, to build their capacity in soil and water conservation. Three experts

from USDA/SCS were sent to The Gambia to initiate "data collection and select and train

potential Gambian candidates for further training in various disciplines relating to Soil and

Water Conservation." (Fye, 1988)

1981 saw the first group of Gambians sent to Nigeria and the United States to obtain further

education in their respective conservation disciplines. They began returning in 1983 and were

integrated into SWMU, beginning soil conservation projects in 1984 with an anti-salt/water-

retention dam built at Foni Jarrol in the Western Division. The next five years was dedicated to

SWMU perfecting their skills and establishing the unit as the national authority on Soil and

Water Conservation. (Fye, 1988)

In 1988, setbacks in training and the integration of trainees caused a three year extension in

building capacity of SWMU. The same year saw the reorganization of The Department of

Agriculture where SWMU was placed under the newly created Department of Agriculture

FIGURE 5: GULLY FORMED ON MAIN VILLAGE ROAD IN BULOCK, THE

GAMBIA (JUNE 2, 2009)

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Technical Services. This led to the Swampland Rehabilitation Project, funded by the German

Agency for Technical Cooperation (GTE) and run by the Ministry of Water Resources, to be

integrated with SWMU. GTE continued providing support for two years. (Fye, 1988)

Upon establishing themselves as a legitimate unit of the Department of Agriculture, SWMU

was given the mandate of : (1) halting and reversing environmental deterioration due to the

inadequacy of traditional cultivation practices; (2) increasing food, forage, wood and cash

crops; (3) reducing susceptibility to drought and other weather variations; (4) developing the

institutional and material services capabilities to deliver educational, technical and material

services to rural population in Soil and Water Management. Since then SWMU has been at the

forefront of Soil and Water Conservation and has gained the respect and admiration of the

government. (Fye, 1988)

CURRENT AND RECENT SWMS PROJECTS

SWMS is the main body responsible for implementing The Participatory Integrated Watershed

Management Project (PIWAMP) funded by the African Development Bank (ADB) and the

International Fund for Agriculture Development (IFAD). PIWAMP began in 2006 and is an eight

year project based on the success of the 1997-2004 Lowland Agriculture Development Project

(LADEP). (PIWAMP, 2009)

LOWLAND AGRICULTURE DEVELOPMENT PROJECT (LADEP)

LADEP was the first phase of a twenty year program for sustainable community-driven

reclamation and development of lowland areas to improve traditional rice production funded

by ADB, IFAD and the Gambian government. Its purpose was to increase traditional rice

production of the lowlands by 12,500 tons per annum by the end of the project’s eight year

duration. (ADB, 2007)

SWMU involvement was the construction of lowland retention structures, mainly dikes and

spillways, preventing saltwater intrusion and retaining moisture on rice fields. SWMU played a

vital role in the implementation of the project. The construction completed by SWMU was

beyond the expectations of the appraised targets. SWMU carried out construction projects on

274 sites, 17% more than the anticipated target. SWMU constructed 5,475 metres of spillways

(60% of appraisal target); 165.09 kilometres of dikes (375% of appraisal target); opened up

access to 4,926 ha of previously unused rice production land (132% of appraisal targets); and

upgraded 11,301 metres of dikes (604% of appraisal targets). (ADB, 2007)

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TABLE 2: LADEP ACCOMPLISHMENTS OF SWMU (ADB, 2007)

Target Achieved %

Sites 234 274 117%

Spillways (m) 9,150 5,475 60%

Dykes (km) 40-80 165.09 375%

Opened up access (ha) 3,735 4,926 132%

Dyke upgrade (m) 1,870 11,201 604%

PARTICIPATORY INTEGRATED WATERSHED MANAGEMENT PROJECT (PIWAMP)

PIWAMP is the second phase of the 20 year ADB and IFAD financed project. PIWAMP is based

on the lessons learned from the 2002 Mid-Term Review (MTR) of LADEP, keeping the

community-driven development approach. Two important ideas developed from the LADEP

MTR were the importance of taking into account the entire watershed, and the role irregular

climatic conditions play on food security.

LADEP was solely focused on developing lowland rice production, failing to take into account

the role of upland erosion. PIWAMP, therefore, was created to focus on the watershed of the

entire country, both upland and lowland. A component of the project is dedicated to

Watershed Management, focusing on lowland schemes, swamp access, upland management

and conservation farming, and agriculture development. Another addition was the planned

use of vetiver grass to stabilize earth bunds, though this has yet to be incorporated.

SWMS is responsible for the construction of 120 kilometres of causeways, 3,008 metres of

bridges, 720 kilometres of contour bunds, 80,000 metres of dikes, 480 kilometres of inter-

village roads and 2,400 gully plugs. (PIWAMP, 2009)

WATER DIVERSIONS

The primary method implemented by SWMS to control upland erosion and prevent flooding is

the use of storm water diversions. They are typically used when a village is suffering from

severe erosion by storm water runoff. The main roads through the village are often heavily

eroded. A road can be more than one metre below the original ground surface as a result of

erosion (Figure 5). This also causes exposure and undercutting of building foundations, making

the building more susceptible to collapse (Figure 3).

There are two main components in a diversion project; the diversions themselves and large

channels or waterways that carry the diverted flow. The principal behind the diversions is to

capture the runoff water from the high ground (often farmland) and divert it away from the

village into a stable waterway. The diversion has two features, a channel and a dike, or

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embankment (Figure 6). The term bund is used interchangeably with diversion in this report as

it is a common term used by SWMS.

FIGURE 6: DIVERSION CROSS SECTION (LAND SLOPE IS EXAGGERATED)

A number of bunds are placed upland to protect a village and farmland. Peak flow is reduced in

the diversion channels and the distance the runoff has to flow is decreased. As a result, runoff

does not achieve a high velocity and erode the fertile top soil from farm land. Each bund is

designed to have a gentle slope towards a natural waterway where all the flow collects (Figure

7). In many cases the natural waterways where the flow is diverted are badly scoured so

gabions are placed to capture and deposit sediment, slowly repairing the gully.

FIGURE 7: MAP OF DIVERSIONS IN SARE ALPHA, PREPARED BY ERIK BROHAUGH (2009)

Diversions are laid out and marked by a surveying team using a level and rod. The desired slope

of the diversion channel is approximately 0.5% (5 cm decrease in elevation per 10 m of channel

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length). A slope that is too gentle will cause any sediment in the runoff to be deposited in the

channel and a slope that is too steep will cause scouring in the diversion channel. After

surveying is complete, a large grader makes several passes to form a channel on the upslope

side and a dike on the down slope side. Design standards call for vegetation to be established

on the diversions immediately following construction and before any flow is carried but this is

not a common practice of SWMS at this time.

The majority of site visits by the students were upland areas where erosion was the primary

concern. Focus is placed in this report on erosion and the ways it is controlled by SWMS

(primarily storm water diversions) as well as alternative methods for controlling erosion. More

details on other activities carried out by SWMS can be found in an earlier document entitled

“Fact-Finding Report”.

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PLANNING AND MANAGEMENT

DIVERSION LAYOUT

Diversions are currently laid out in the field by a survey team, with limited use of maps and

topographic details. Several projects were observed that required modification after

construction had already begun due to inaccurate surveying or poor location planning. During a

site visit to Bulock, a bund location had to be changed after it was already under construction.

The land owners did not understand where the bund was planned to be, and were unhappy

with its placement. It had to be moved to a location where the slope of the channel was steeper

than ideal. This could have been prevented if drawings or maps were made showing the

planned bund locations before construction. The land owners or farmers need to be aware of

where the bunds will be and a map or drawing could show them.

FIGURE 8: DIKE RELOCATION IN BULOCK (JUNE 2, 2009)

Another more extreme example was observed in Sare Alpha, one of the larger diversion

projects. The first in the series of bunds had an extremely large catchment area (Figure 7). In

addition to this large catchment area, there were depressions in the diversion channels where

water would not flow well towards the main waterway. These factors led to the failure of the

first bund. Each bund after the first failed in the same fashion and runoff reached the village

(Figure 9).

Mapping the location of diversions before construction would enable the designer to realize

that a bund would have a very large catchment area and peak flow rate. The designer could

either design the diversion with a higher capacity or add more diversions up slope.

15

FIGURE 9: FAILURE OF BUND IN SARE ALPHA (JULY 23, 2009)

Detailed planning of the layout of diversions can save time and expense by eliminating the need

to fix mistakes after construction has finished. The use of topographic maps would aid in this

process, or if possible, Geographic Information Software (GIS). Investment in developing

SWMS’s GIS capabilities by acquiring software, data, and training a technician may be a

worthwhile capacity building initiative. Global Positioning System (GPS) units are currently used

by SWMS to map projects, but only after they are complete. Mapping a site before construction

ensures that the diversion layout is satisfactory.

CONSTRUCTION SEQUENCE

Creating a work plan before beginning construction of a project is standard practice when it

comes to project management in most organizations. The work plan is essentially a list of tasks

that need to be completed, and the order in which they should be completed. It illustrates what

tasks must be completed before a certain task can begin.

When constructing water diversions, the waterway to which they divert runoff should have

adequate capacity and be stabilized against erosion. Several SWMS diversion sites do not have

a stable channel with established vegetation in place before flow is diverted during the rainy

season. These waterways are susceptible to scouring. If the waterway is a deep gully and

requires gabions to trap sediment and repair the gully, they should also be placed before runoff

is diverted. It is extremely important that a channel be stabilized against scouring before flow is

diverted to it.

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EQUIPMENT MANAGEMENT

Managing construction equipment efficiently can greatly decrease construction time and costs,

allowing for more projects to be undertaken. Inefficient management of heavy equipment was

observed during road construction. Gravel was the main construction material which had to be

transported from a quarry, by tractors with trailers, to the site where it was spread by a grader.

The grader was forced to wait for the gravel loads to arrive and could quickly spread them, only

to wait for the next tractor. This is a large problem because PIWAMP has only two graders for

the entire country so many project sites must wait as the grader is used for long periods at

other locations.

The most obvious solution to this problem is to get more tractors, or have them carry larger

loads to bring gravel faster. However, this may not be possible due to economic constraints, the

availability of operators, or other reasons. A better solution would be to have the tractors travel

ahead to sites where a road is to be constructed while the grader is being used at another site

where tractors are no longer needed. Tractors could begin transporting gravel to these sites

and stockpiling it there. When the grader arrives, there would be a large amount of gravel on

site ready to be spread, so it would not be waiting as long.

SUMMARY

Planning and management helps to reduce mistakes and improve efficiency. Their importance

can be realized in any project. SWMS could improve its planning and managing in these ways:

Utilization of topographic maps and data, or GIS and GPS if capacity is built. These tools allow for detailed mapping of diversion locations, ensuring a satisfactory design.

Establishing a construction sequence. Main waterways would be constructed and stabilized against erosion before carrying diverted storm runoff.

Improving efficiency of heavy construction equipment. Graders spend less time idle and are more productive as a result.

COMMUNICATION AND COLLABORATION

Communication and transparency is critical in any government organization. Data and

information should be produced and willingly exchanged. Efforts to improve inter-agency

communication could be undertaken. The National Environment Agency (NEA) and Department

of Lands and Surveys (DL&S) have many resources which could assist the efforts of SWMS,

particularly GIS capabilities and data.

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Currently, the NEA does not share the GIS data they have collected from other agencies with

SWMS because SWMS lacks GIS capabilities. GIS data is also not distributed to persons or

organizations outside of the government by the NEA. The NEA can produce maps from their

collection of GIS data for a fee, price varies depending on the organization you belong to, but

they lack the appropriate software to properly analyze their data.

The DL&S has digital topographic data points, provided to them by the Japanese International

Cooperation Agency, available for a fee of 10,000 dalasi (about $400.00 CAD).

Lack of documentation regarding SWMS’s construction of projects is a large barrier, particularly

surveying details. Common problems observed were diversion channels flowing backwards or

flowing too slowly, usually due to surveying issues. Waypoints made along a diversion line

should be recorded and checked to prevent inevitable mistakes. No survey team can be perfect

all the time. Special care should be given to natural depressions as water is likely to pool at

these locations. Introduction of an approval process after a survey has been finished and before

construction begins could be beneficial. Documentation is also useful when trying to determine

what happened when projects do not go as planned.

UWO, SWMS and WARDFA need to establish an improved communication network so that all

parties are better prepared for the internship. Students returning from internship must provide

as detailed information as possible to potential interns so they are more knowledgeable and

capable of making a greater contribution. Returning students should also maintain contact with

SWMS and WARDFA so they can continue to develop and transfer knowledge even after the

internship is complete. In essence, the internship should be looked at as a yearlong

commitment rather than just a summer experience.

SUMMARY

Communication and collaboration could be strengthened by considering:

Improvement of inter-agency communication. Increases capacity by sharing resources with other organizations such as the NEA and DL&S.

Documentation of all projects. Information is available to more people and potential problems are discovered before construction begins.

Strengthening the partnership between UWO and SWMS. Students are better prepared to contribute during their internship. Information and knowledge are willingly transferred.

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DESIGN ALTERATIONS

DIVERSION DESIGN

The design of any flood control measure can never be completely fail safe, especially when

there are economic constraints. Whether designed for a 10-year, a 25-year, or even a 100-year

return period, eventually a storm will exceed the capacity of the flood control measure.

Recognizing this fact and designing to minimize the damage caused by a storm that exceeds the

design capacity is essential when constructing a flood control structure. Standard diversion

designs are site-specific. An acceptable design storm is selected, such as a 10-year, 24-hour

storm. The diversion channel capacity is designed to handle the peak flow that would result

from the selected design storm. The consequences of less frequent storms are also considered

in order to minimize damage.

During our site visits we observed several cases where bunds had failed or were susceptible to

fail. The failure mechanism was overtopping either due to capacity-exceeding flows or poor

construction. Once one bund failed, the others down grade of it were also likely to fail due to

the increased flow they experienced. It could be described as a domino effect. The most

obvious case of failure observed was in Sare Alpha. The first bund had such a large catchment

area that it was overtopped during a storm and failed. The domino effect began and caused all

the bunds to fail.

Bunds that are likely to experience flow rates above their design capacity could be fitted with

an emergency spillway to minimize the damage during a heavy storm. The spillway would help

prevent the bund from overtopping by carrying the excess flow through a stable outlet such as

a weir or a culvert. Not all flow would be diverted away from the village but damage to bunds

would be lessened. The simplest approach would be the installation of a culvert below the crest

of the bund. Corrugated steel pipe covered with soil, or a weir constructed of concrete, rip rap,

asphalt, etc could be used for the emergency spillway. The flow from the spillway could be

carried by a stable waterway and prevent the bunds from failing.

FIGURE 10: DIVERSION SHOWING EMERGENCY SPILLWAY

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VEGETATION FOR STABILIZATION OF DIVERSIONS AND CHANNELS

The purpose of the diversion system is to reduce soil erosion as much as possible, both in the

farm land and in the village. Diversions reduce the distance that runoff has to travel across farm

land, thereby reducing its velocity and ability to erode the soil on farm lands. However, flow is

being concentrated into diversion channels and eventually the main waterway. Concentrated

flow has much more erosive power so it should be carried only in a stable channel. Very few

diversions or waterways that were vegetated were observed, many were nearly completely

bare soil. These channels are likely to scour.

Scouring of a diversion can lead to its failure, especially when sections of the channel flow at a

low enough velocity for sediment picked up through scouring to be deposited. When enough

sediment is deposited, a blockage forms and runoff could be impeded so much that it overtops

the bund and causes it to fail. A vegetated channel is less likely to scour because the vegetation

reduces the sediment load of the runoff.

Main waterways can also be stabilized by vegetation to prevent scouring. Flow is even more

concentrated in these channels making them more likely to scour. Scouring increases sediment

load in runoff which increases its ability to erode and can have negative effects on river

ecosystems once the runoff reaches the river.

Creating stable diversions and waterways should be incorporated into the construction of any

diversion project. Intercepting runoff to reduce erosion of farm lands is not useful if that runoff

is to be carried by a channel that will also erode. Without creating stable channels, the problem

of erosion of farm land is essentially exchanged for the problem of erosion of channels.

Nurseries containing vetiver grass exist all over The Gambia; it just needs to be transplanted to

the sites. Vetiver can likely only be used to stabilize the dike as its long blades would impede

flow in a channel. Shorter grasses that grow naturally can be used for channel linings.

Temporary stabilization with materials such as jute fibre or straw may be necessary until

permanent vegetation can be established.

SUMMARY

A well designed project is one that performs the intended results with minimum cost and

maintenance and last for its expected lifetime. These design changes could produce projects

that have these qualities:

Designing for an acceptable storm frequency, while still considering extreme events. Reduces construction costs and the severity of damage from intense storms.

Incorporating vegetation as an important design consideration. Creates stable channels, increasing the project life time and reducing erosion.

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ALTERNATIVE CONSERVATION METHODS

This section briefly introduces topics that could benefit SWMS through further research.

Possible researchers include SWMS personnel, current UWO IDIs, and future UWO IDIs. The

focus is on alternative methods that reduce soil erosion and storm water runoff while

enhancing soil fertility which are less construction intensive and can be implemented on a

smaller scale.

VEGETATIVE HEDGEROWS

An alternative technique in preventing sheet and rill erosion is the use of vegetative

hedgerows. Hedgerows have the advantage of using the natural environment to combat the

effects of erosion by allowing for runoff water to drain naturally (Figure 11). The technology is

simple, requiring no heavy construction equipment allowing farmers to have an active part in

their construction.

FIGURE 11: DRAINAGE THROUGH THE USE OF (A) EARTH BUNDS AND (B) VEGETATIVE HEDGEROWS (GRIMSHAW, 1993)

Vetiver grass, known scientifically as vetiveria zizanioides and in the Wolof language as Sep and

Tiep, is a dense and resilient grass found naturally in swamplands of the Gambia. It is the ideal

vegetation to use for hedgerows. The plant is sterile outside of its native habitat, has roots that

can extend 3 metres into the ground, and grass blades that are 8 millimetres wide and 75

centimetres long. Its most attractive feature is its ability to grow in almost any soil conditions

and its resilience to relatively extreme environment events.

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FIGURE 12: VETIVER GRASS (GRIMSHAW, 1993)

For vetiver grass to effectively mitigate erosion of farmlands, a series of hedgerows are planted

along the contour of the land. It takes anywhere from one to three years before the hedgerows

have matured and have the capacity to alleviate the effects of runoff caused by torrential

storms. The grass blades thickness and length cause it to trap sediment and costly fertilizer,

and reduce the velocity of the runoff water. This saves the valuable top soil and allows runoff

water to infiltrate into the farmland. The roots of the grass provide stability to the subsurface,

preventing the formation of rills and gullies. (Grimshaw, 1993)

Vetiver grass is an amazing resource and should be utilized to its full potential. It can be

implemented in a wide range of conservation efforts. Every few months the hedgerows must be

pruned, the cuttings can then be used as cover material for crops.

Restoration of gullies is another potential use of vetiver hedgerows. Their roots stabilize the

subsurface and their leaves capture the sediment in runoff water.

CONSERVATION AGRICULTURE

Conservation agriculture (CA) is a farming method that is strongly supported by the Food and

Agriculture Organization of The United Nations (FAO) worldwide. It is essentially a collection of

three farming techniques that improve soil fertility and reduce erosion (FAO, 2008):

1) Minimum soil disturbance through mechanical methods (minimum tillage)

2) Permanent organic cover of soil

3) Diverse crop rotation

Any of the above farming techniques improves soil fertility and decreases the soil’s

susceptibility to erosion. They could be used individually or in unison for the maximum benefit.

CA has been adopted in many places around the world with successful results, particularly in

South America (Derpsch, 2005). Success in other parts of the world certainly does not mean CA

22

is a system that Gambian farmers should adopt. There are several barriers that need to be

addressed before even considering implementing CA in The Gambia. The initial cost associated

with new tools required is a large concern for low income subsistence farmers. Another

concern for Gambia is incorporating livestock into the CA system. Most cattle are free range,

and would be likely to eat the soil cover.

CONCLUSIONS

Although many observations have been made within this report, the students would like to

stress the importance of two issues: vegetation for stabilization of diversion and waterways,

and more time and effort on the planning and design phase of each project.

New methods and ways of thinking are constantly evolving in the field of soil and water

conservation. Keeping current and open to alternative conservation methods through active

research or reading the latest publications will allow SWMS to improve the services they

provide to the Gambian agriculture sector. Some research areas for consideration may be no-till

farming, organic soil cover, and vegetative hedgerows.

Agriculture is arguably the most important sector in The Gambia and SWMS’s efforts are

invaluable to farmers. The continual implementation of soil and water conservation practices

will play a vital role in the development of agriculture. In turn, this will provide the country

with food security, allowing them to end their reliance on food imports. Great potential exists

for a long lasting, mutually benefitting partnership between UWO and SWMS where

information and knowledge are developed and shared.

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FAO . (2005). The Gambia. Retrieved June 27, 2009, from AQUASTAT:

http://www.fao.org/nr/water/aquastat/countries/gambia/index.stm

FAO. (2008). Conservation Agriculture. Retrieved July 15, 2009, from FAO:

http://www.fao.org/ag/ca/

FAO. (1983). Integrating crops and livestock in West Africa. Rome.

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