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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
1
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.
2
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
3
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
4
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.
5
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).
6
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)
7
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)
8
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)
9
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)
10
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)
11
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
12
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
13
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”.
14
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.
16
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.
17
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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