116
1 Introduction
Land use changes continue to pose severe threat to natural hydrological process of
wetlands in urban Lilongwe (FAO 2013). In peri-urban wetland catchment areas,
unprecedented changes in land use practices can alter hydrological processes;
wetland development should therefore be monitored. Research has demonstrated that
land use and cover change and its impacts on the existing environment should be
investigated and monitored in a systematic manner in order to prevent flooding
(Hadjimitsis 2010). Land use and land cover changes affect hydrological processes
in general and increase flood risks (Khan 2005; Brath et al. 2006). Furthermore,
because hydrological behavior of wetlands is inter-related with the catchment’s topologic, meteorological, climatic, and biological factors, such unprecedented land use
change through deforestation and construction of buildings can trigger off sequences
of floods due to changes in the hydrological regime of the catchment (Hadjimitsis 2010).
It is also understood that human pressures on natural resources tend to exceed
regenerative capacities of the natural processes, leading to their degradation (Munthali
and Murayama 2011). For accurate monitoring of such pressures, natural resources
managers recognize the use of high-resolution images to quantitate the rates at which
changes occur. In most cases, satellite-acquired image data is recommended for land
use and cover analysis. According to Hadjimitsis (2010), satellite imagery provides a
synoptic overview of large regions, recorded with a standardized monitoring system.
Measurements used for predicting channel characteristics are needed to decide
wetland management approaches in the face of land use changes. In particular, discharge is needed for planning flood control and designing engineering structures,
including bridges and road culverts (Brooks et al. 2013). Flooding occurs when
streamflow discharge exceeds the capacity of the channel or stream (Brooks et al.
2013). Field records of streamflow are either monitored using installed weirs or
using velocity-area method (Brassington 1998). Since weirs are expensive to install
and maintain, hydrologists prefer the latter, which uses flotation method to calculate
water velocities along a control section and a steel tape to measure length and width
of the channel at right angles to the direction of the flow to determine cross- sectional
area. The product of cross-sectional area and velocity gives streamflow discharge.
In theory, this method tends to overestimate streamflow discharge since surface
velocity is significantly greater than average velocity. Therefore, estimated values
are corrected with a factor of 0.75 (Brassington 1998).
Another critical aspect of gauging is selecting a control section, part of the stream
for which a rating curve
1
is developed. The criteria for selecting a control section are
that it must be stable, must have a sufficient depth for obtaining velocity measurements even at the lowest of streamflow, and should be located in a straight reach free
from turbulent flow (Brooks et al. 2013). Such conditions are based on the wetland’s
natural configuration. Therefore, a reconnaissance field survey is required before
gauging can begin.
1 Graph of stage against discharge
A. K. Mkulama et al.
1 Introduction
Land use changes continue to pose severe threat to natural hydrological process of
wetlands in urban Lilongwe (FAO 2013). In peri-urban wetland catchment areas,
unprecedented changes in land use practices can alter hydrological processes;
wetland development should therefore be monitored. Research has demonstrated that
land use and cover change and its impacts on the existing environment should be
investigated and monitored in a systematic manner in order to prevent flooding
(Hadjimitsis 2010). Land use and land cover changes affect hydrological processes
in general and increase flood risks (Khan 2005; Brath et al. 2006). Furthermore,
because hydrological behavior of wetlands is inter-related with the catchment’s topologic, meteorological, climatic, and biological factors, such unprecedented land use
change through deforestation and construction of buildings can trigger off sequences
of floods due to changes in the hydrological regime of the catchment (Hadjimitsis 2010).
It is also understood that human pressures on natural resources tend to exceed
regenerative capacities of the natural processes, leading to their degradation (Munthali
and Murayama 2011). For accurate monitoring of such pressures, natural resources
managers recognize the use of high-resolution images to quantitate the rates at which
changes occur. In most cases, satellite-acquired image data is recommended for land
use and cover analysis. According to Hadjimitsis (2010), satellite imagery provides a
synoptic overview of large regions, recorded with a standardized monitoring system.
Measurements used for predicting channel characteristics are needed to decide
wetland management approaches in the face of land use changes. In particular, discharge is needed for planning flood control and designing engineering structures,
including bridges and road culverts (Brooks et al. 2013). Flooding occurs when
streamflow discharge exceeds the capacity of the channel or stream (Brooks et al.
2013). Field records of streamflow are either monitored using installed weirs or
using velocity-area method (Brassington 1998). Since weirs are expensive to install
and maintain, hydrologists prefer the latter, which uses flotation method to calculate
water velocities along a control section and a steel tape to measure length and width
of the channel at right angles to the direction of the flow to determine cross- sectional
area. The product of cross-sectional area and velocity gives streamflow discharge.
In theory, this method tends to overestimate streamflow discharge since surface
velocity is significantly greater than average velocity. Therefore, estimated values
are corrected with a factor of 0.75 (Brassington 1998).
Another critical aspect of gauging is selecting a control section, part of the stream
for which a rating curve
1
is developed. The criteria for selecting a control section are
that it must be stable, must have a sufficient depth for obtaining velocity measurements even at the lowest of streamflow, and should be located in a straight reach free
from turbulent flow (Brooks et al. 2013). Such conditions are based on the wetland’s
natural configuration. Therefore, a reconnaissance field survey is required before
gauging can begin.
1 Graph of stage against discharge
A. K. Mkulama et al.