JFE
© International Science Press (India)
JOURNAL OF FLOOD ENGINEERING ISSN : 0976-6219
3(1-2) Jan. – Dec. 2012; Pp. 93–113
THE EFFECTS OF DEVELOPMENT ACTIVITIES ON THE
OCCURRENCE OF FLOODS IN THE MBABANECITY:
SWAZILAND
J. I. Matondo1, A. Salam1*, C. Ndzabandzaba2 & J. Nobert2
2
Department of Geography, Environmental Science and Planning P/B-4, University of Swaziland,
Kwaluesni, Swaziland, E-mail: *
2
Department of Water Resources Engineering, University of Dar es Salam,Tanzania
ABSTRACT:Urbanization and expansion of structural developments within the urban area of Mbabane
are challenges that require a rapid response in terms of flood-damage mitigation measures. There have
been several cases of flash floods in Mbabane mostly resulting from heavy downpour and a poor drainage
system, thus causing in loss of property and goods. In this study, the effects of development activities on
the occurrence of floods in Mbabane City were investigated using two set of orthophotomaps one for
1972 (Scenario 1) and the other for 2003 (Scenario 2). Impervious areas were computed and quantified to
determine the hydrological response of the catchment due to development activities. The study used two
approaches, where the amount of runoff in two scenarios were estimated (i) using the runoff coefficient
method and (ii) a runoff simulation model (SWMM). Before the simulation was run, some parameters of
the model were computed using physical characteristics while others were estimated using secondary
data. Results thus obtained indicate that increasing development activities were responsible for the
occurrence of flash floods in Mbabane City and primarily due to percent increase of imperviousness
which had increased in 2003 compared to 1972. It was also determined that the generated amount of
volume (of runoff) from Scenario 2 far exceeded that from Scenario 1.Therefore, it is recommended that
the drainage system in the Mbabane city be upgraded, properly re-routed and cleaned regularly while the
Mbabane River should be channelized to accommodate excess runoff or alternatively a gauging station
together with a flood-warning system be installed to reduce loss of life and property in the city.
Keywords: Catchment, Flash flood, Impervious areas, Rainfall-runoff modeling, Runoff coefficient,
Surface runoff, Urbanization
1. INTRODUCTION
Over the years, Mbabane City, a riverside township in the north-eastern areas of Swaziland has
been experiencing extensive development coupled with high urban growth hence increasing
of impervious areas within its watershed. In addition to this, heavy rains in this area have been
causing frequent floods with roads in many places submerged for several hours and the road
traffic coming to a halt, taking the people at large off guard and bringing in considerable
damage to public and private properties. The scientific debates on possible changes in runoff/
flood regimes induced by climate and land-use changes have been lively in the recent decades
as these can influence the water cycle dramatically, and in turn the magnitude of flood peaks
and volumes (Ranzi et al., 2002).
, 94 / JOURNAL OF FLOOD ENGINEERING (JFE)
The changes made to an area by construction of a concentration of buildings have a direct
effect on its surface hydrology (Shaw, 1996). The covering of the land surface by a large
proportion of impervious material means that a much larger proportion of any rainfall forms
immediate runoff and any slope of the land also greatly enhances the runoff response of a
paved area (Shaw, 1996). Mbabane, is at risk due to surface water flooding and as well as
riverine flooding. Flowing through the city centre is the Mbabane River, whose capacity to
convey water is being altered resulting in increased stage corresponding to a given discharge.
In particular, structures that encroach into the floodplain, such as bridges, can increase upstream
flooding by narrowing the width of the channel and increasing the channel’s resistance to flow.
As a result, the water is at a higher stage as it flows past the obstruction, creating a backwater
that will inundate a larger area upstream (U.S. Geological Survey, 2005).
The World Meteorological Organization and Global Water Partnership (2008) assert that
floods are a consequence of natural hydro-meteorological phenomenon, combined with their
interaction with the catchment characteristics. Flood-damage mitigation embraces methods
for combating the effects of excess water in streams (Linsley et al. 1992). It is important to
note that little can be done to control floods either economically or physically. Therefore,
according to Linsley et al. (1992), absolute control over floods is not feasible.
There have been several cases of flash floods in Mbabane mostly resulting from heavy
downpour and poor drainage system whose operational capacities could not cope with excessive
surface runoff (Bicon Consulting Engineers, 2008). The most recent flood event in Mbabane
occurred on the17th February 2006. It was preceded by another flood event which occurred on
the 17th March 2005 and another one which occurred on the 28 th November 2003 (Bicon
Consulting Engineers, 2008).
The progressive transformation of the watershed from rural to urban land uses have
highlighted the need to understand the consequences of urban developmental activities on
flooding of the Mbabane urban area. Since there is limited data on Mbabane River it is not
easy to predict the occurrence of floods. Therefore, there is need to mitigate the flood-damage
which needs proper identification of the effects of development activities on the occurrence of
floods for an effective flood-damage mitigation/flood control measure.
1.1 Causes of Flooding
The World Meteorological Organization and Global Water Partnership (2008) assert that a
flood as an extreme event/hazard is caused by natural forces or a combination of natural forces
and human influences. Nelson (2010) contends that whenever humans modify the landscape in
any way changes are to be expected in the way water drains from the land. Unless careful
consideration is given to the possible drainage consequences, such landscape modifications
can result in higher incidence of flooding. Development on floodplains should therefore be
undertaken only with great care. Existing developments that have enhanced flooding problems
are often costly to fix.
, THE EFFECTS OF DEVELOPMENT ACTIVITIES ON THE OCCURRENCE OF FLOODS... / 95
1.2 Effects of Urbanization on the Hydologic System
There are changes in land and water use when urbanization takes place. These changes have
effects on the local water system. During initiation of urbanization, coupled with change in
land use, there is often removal of trees and vegetation. Houses are built, some with sewers
and some with septic tanks. The effect is more storm runoff and erosion because there is less
vegetation to slow water as it runs down hills. More sediment is washed in to streams. Flooding
can occur because water-drainage patterns are changed (U.S. Geological Survey, 2010).
1.3 Hydraulic Effects from Changes to Stream Channel and Flood Plains
Development along stream channels and floodplains can alter the capacity of a channel to
convey water and can increase the height of the water surface (stage) corresponding to a given
discharge. In particular, structures that encroach on the floodplain, such as bridges, can increase
upstream flooding by narrowing the width of the channel and increasing the channel’s resistance
to flow. As a result, the water is at a higher stage as it flows past the obstruction, creating a
backwater that will inundate a larger area upstream (U.S. Geological survey, 2005).
1.4 Effects of Urban Development of Flood Discharge and Frequency
Common consequences of urban development are increased peak discharge and frequency of
floods. Typically, the annual maximum discharge of a stream will increase as urban development
occurs, although the increase is sometimes masked by substantial year-to-year variation in
storms (Konrad, 2005). Urbanization also affects the availability of sediment supplies and
stream channel morphology. Knighton (1984) noted that available sediment sources can increase
during urban construction and then decrease because of the increased stability of the urban
landscape. Also, stream channel morphology adjusts to the increase in frequency of channel
forming flows. Channel adjustments are often characterized by enlargement including: widening
and down cutting of the streambed.
1.5 Managem, Ent Strategies
Mangarellaand Peter(2002) reviewed many different management programs in the USA that
have been implemented nationwide. Typically, these programs focus on implementing Best
Management Practices (BMP’s) to maintain natural functions of the hydrologic and geomorphic
processes, protect riparian corridors, and integrate storm water control measures into
development to mitigate expected impacts. Some suggested actions include: maintaining the
natural rainfall-runoff ratios, protecting hydrological sensitive areas, sediment sources, and
sensitive habitat areas, minimize and hydraulically disconnect impervious areas, such as rooftops,
rain gutters, parking lots and roads, minimize topography changes and soil compaction, cluster
development in less sensitive areas, integrate flood control and water quality structure into the
landscape, and stream channel day lighting (Alayande and Agunwamba, 2010). Therefore
this study was designed to investigate the effects of development activities on the hydrology
and thus the occurrence of floods in Mbabane City, by the comparison of two scenarios (before
extensive and after extensive urbanization).
© International Science Press (India)
JOURNAL OF FLOOD ENGINEERING ISSN : 0976-6219
3(1-2) Jan. – Dec. 2012; Pp. 93–113
THE EFFECTS OF DEVELOPMENT ACTIVITIES ON THE
OCCURRENCE OF FLOODS IN THE MBABANECITY:
SWAZILAND
J. I. Matondo1, A. Salam1*, C. Ndzabandzaba2 & J. Nobert2
2
Department of Geography, Environmental Science and Planning P/B-4, University of Swaziland,
Kwaluesni, Swaziland, E-mail: *
2
Department of Water Resources Engineering, University of Dar es Salam,Tanzania
ABSTRACT:Urbanization and expansion of structural developments within the urban area of Mbabane
are challenges that require a rapid response in terms of flood-damage mitigation measures. There have
been several cases of flash floods in Mbabane mostly resulting from heavy downpour and a poor drainage
system, thus causing in loss of property and goods. In this study, the effects of development activities on
the occurrence of floods in Mbabane City were investigated using two set of orthophotomaps one for
1972 (Scenario 1) and the other for 2003 (Scenario 2). Impervious areas were computed and quantified to
determine the hydrological response of the catchment due to development activities. The study used two
approaches, where the amount of runoff in two scenarios were estimated (i) using the runoff coefficient
method and (ii) a runoff simulation model (SWMM). Before the simulation was run, some parameters of
the model were computed using physical characteristics while others were estimated using secondary
data. Results thus obtained indicate that increasing development activities were responsible for the
occurrence of flash floods in Mbabane City and primarily due to percent increase of imperviousness
which had increased in 2003 compared to 1972. It was also determined that the generated amount of
volume (of runoff) from Scenario 2 far exceeded that from Scenario 1.Therefore, it is recommended that
the drainage system in the Mbabane city be upgraded, properly re-routed and cleaned regularly while the
Mbabane River should be channelized to accommodate excess runoff or alternatively a gauging station
together with a flood-warning system be installed to reduce loss of life and property in the city.
Keywords: Catchment, Flash flood, Impervious areas, Rainfall-runoff modeling, Runoff coefficient,
Surface runoff, Urbanization
1. INTRODUCTION
Over the years, Mbabane City, a riverside township in the north-eastern areas of Swaziland has
been experiencing extensive development coupled with high urban growth hence increasing
of impervious areas within its watershed. In addition to this, heavy rains in this area have been
causing frequent floods with roads in many places submerged for several hours and the road
traffic coming to a halt, taking the people at large off guard and bringing in considerable
damage to public and private properties. The scientific debates on possible changes in runoff/
flood regimes induced by climate and land-use changes have been lively in the recent decades
as these can influence the water cycle dramatically, and in turn the magnitude of flood peaks
and volumes (Ranzi et al., 2002).
, 94 / JOURNAL OF FLOOD ENGINEERING (JFE)
The changes made to an area by construction of a concentration of buildings have a direct
effect on its surface hydrology (Shaw, 1996). The covering of the land surface by a large
proportion of impervious material means that a much larger proportion of any rainfall forms
immediate runoff and any slope of the land also greatly enhances the runoff response of a
paved area (Shaw, 1996). Mbabane, is at risk due to surface water flooding and as well as
riverine flooding. Flowing through the city centre is the Mbabane River, whose capacity to
convey water is being altered resulting in increased stage corresponding to a given discharge.
In particular, structures that encroach into the floodplain, such as bridges, can increase upstream
flooding by narrowing the width of the channel and increasing the channel’s resistance to flow.
As a result, the water is at a higher stage as it flows past the obstruction, creating a backwater
that will inundate a larger area upstream (U.S. Geological Survey, 2005).
The World Meteorological Organization and Global Water Partnership (2008) assert that
floods are a consequence of natural hydro-meteorological phenomenon, combined with their
interaction with the catchment characteristics. Flood-damage mitigation embraces methods
for combating the effects of excess water in streams (Linsley et al. 1992). It is important to
note that little can be done to control floods either economically or physically. Therefore,
according to Linsley et al. (1992), absolute control over floods is not feasible.
There have been several cases of flash floods in Mbabane mostly resulting from heavy
downpour and poor drainage system whose operational capacities could not cope with excessive
surface runoff (Bicon Consulting Engineers, 2008). The most recent flood event in Mbabane
occurred on the17th February 2006. It was preceded by another flood event which occurred on
the 17th March 2005 and another one which occurred on the 28 th November 2003 (Bicon
Consulting Engineers, 2008).
The progressive transformation of the watershed from rural to urban land uses have
highlighted the need to understand the consequences of urban developmental activities on
flooding of the Mbabane urban area. Since there is limited data on Mbabane River it is not
easy to predict the occurrence of floods. Therefore, there is need to mitigate the flood-damage
which needs proper identification of the effects of development activities on the occurrence of
floods for an effective flood-damage mitigation/flood control measure.
1.1 Causes of Flooding
The World Meteorological Organization and Global Water Partnership (2008) assert that a
flood as an extreme event/hazard is caused by natural forces or a combination of natural forces
and human influences. Nelson (2010) contends that whenever humans modify the landscape in
any way changes are to be expected in the way water drains from the land. Unless careful
consideration is given to the possible drainage consequences, such landscape modifications
can result in higher incidence of flooding. Development on floodplains should therefore be
undertaken only with great care. Existing developments that have enhanced flooding problems
are often costly to fix.
, THE EFFECTS OF DEVELOPMENT ACTIVITIES ON THE OCCURRENCE OF FLOODS... / 95
1.2 Effects of Urbanization on the Hydologic System
There are changes in land and water use when urbanization takes place. These changes have
effects on the local water system. During initiation of urbanization, coupled with change in
land use, there is often removal of trees and vegetation. Houses are built, some with sewers
and some with septic tanks. The effect is more storm runoff and erosion because there is less
vegetation to slow water as it runs down hills. More sediment is washed in to streams. Flooding
can occur because water-drainage patterns are changed (U.S. Geological Survey, 2010).
1.3 Hydraulic Effects from Changes to Stream Channel and Flood Plains
Development along stream channels and floodplains can alter the capacity of a channel to
convey water and can increase the height of the water surface (stage) corresponding to a given
discharge. In particular, structures that encroach on the floodplain, such as bridges, can increase
upstream flooding by narrowing the width of the channel and increasing the channel’s resistance
to flow. As a result, the water is at a higher stage as it flows past the obstruction, creating a
backwater that will inundate a larger area upstream (U.S. Geological survey, 2005).
1.4 Effects of Urban Development of Flood Discharge and Frequency
Common consequences of urban development are increased peak discharge and frequency of
floods. Typically, the annual maximum discharge of a stream will increase as urban development
occurs, although the increase is sometimes masked by substantial year-to-year variation in
storms (Konrad, 2005). Urbanization also affects the availability of sediment supplies and
stream channel morphology. Knighton (1984) noted that available sediment sources can increase
during urban construction and then decrease because of the increased stability of the urban
landscape. Also, stream channel morphology adjusts to the increase in frequency of channel
forming flows. Channel adjustments are often characterized by enlargement including: widening
and down cutting of the streambed.
1.5 Managem, Ent Strategies
Mangarellaand Peter(2002) reviewed many different management programs in the USA that
have been implemented nationwide. Typically, these programs focus on implementing Best
Management Practices (BMP’s) to maintain natural functions of the hydrologic and geomorphic
processes, protect riparian corridors, and integrate storm water control measures into
development to mitigate expected impacts. Some suggested actions include: maintaining the
natural rainfall-runoff ratios, protecting hydrological sensitive areas, sediment sources, and
sensitive habitat areas, minimize and hydraulically disconnect impervious areas, such as rooftops,
rain gutters, parking lots and roads, minimize topography changes and soil compaction, cluster
development in less sensitive areas, integrate flood control and water quality structure into the
landscape, and stream channel day lighting (Alayande and Agunwamba, 2010). Therefore
this study was designed to investigate the effects of development activities on the hydrology
and thus the occurrence of floods in Mbabane City, by the comparison of two scenarios (before
extensive and after extensive urbanization).
