Role of Wetlands in Flood mitigation: The Zambezi wetlands
case study
Amon Murwira 1, Elisha Madamombe2 and Karin. S. Schmidt-Murwira3
Abstract
The role of riverine wetlands in mitigating flooding in areas downstream of their location, especially during
times of above average rainfall and runoff has been hypothesized in ecological and hydrological literature.
The main basis of this hypothesis is that a wetland behaves like a sponge that absorbs water from upstream
and from local rainfall and then releases it slowly to areas downstream thereby reducing the likelihood of a
flood downstream. However, to the best of our knowledge this hypothesis has not been tested
quantitatively, particularly in Southern Africa. In this study, we quantify the extent to which the Zambezi
wetlands, specifically the Eastern Caprivi wetland contributed to flood mitigation in the Zambezi
catchment, in areas downstream of this wetland during the April 2004 flood event. The results indicated the
eastern Caprivi wetland played a significant role in mitigating floods downstream of it. The evidence was
manifold. Firstly, the results indicated that the mean rates of change, i.e., increase and decline in water
levels at Katima Mulilo were significantly higher (p < 0.05) than those recorded at Victoria Falls indicating
that the Victoria Falls experiences regulated flow due to the wetland releasing water slowly. Secondly, the
wetland area/volume derived from remote sensing imagery was significantly (p < 0.05) related to the water
level measurements made at Katima Mulilo, further giving evidence that the wetland was absorbing water
from upstream. The wetland area/volume derived from remote sensing imagery was significantly (p < 0.05)
related to the water level measurements, as well as the rates change over time in water levels made at
Victoria Falls, indicating the role of the wetland in regulating flow thereby reducing the risk of flooding
downstream. The findings in this study reinforce the need to preserve wetlands as a means to reduce the
risk of flooding downstream.
1
University of Zimbabwe, Department of Geography and Environmental Science, P.O. Bo MP167, Mount Pleasant, Harare,
Zimbabwe. mailto:murwira@arts.uz.ac.zw
2
Zimbabwe National Water Authority (ZINWA), Harare, Zimbabwe. mailto:hycos@mweb.co.zw
3
mailto:murwira@arts.uz.ac.zw
Introduction
The role of riverine wetlands in mitigating flooding in areas downstream of their location,
especially during times of above average rainfall and runoff has been hypothesized in
ecological and hydrological literature. The main basis of this hypothesis is that a wetland
behaves like a sponge that absorb water from upstream and from rainfall and then
releases it slowly to areas downstream thereby reducing the likelihood of a flood
downstream (Smith R.L. and Smith T.M., 2000). However, to the best of our knowledge
this hypothesis has not been tested quantitatively, at least within the Southern African
Development Community (SADC) region. We feel that the paucity of work that tests this
hypothesis may lead to wetland management decisions that lack a sound scientific basis.
In effect, this may compromise the sustainable management of riverine wetlands.
Consequently, understanding the role of wetlands in flood mitigation quantitatively is
critical.
In this study, the extent to which the Zambezi wetlands, specifically the Eastern
Caprivi wetland contributed to flood mitigation in the Zambezi catchment, in areas
downstream of this wetland during the April 2004 flood event is quantified. This study
was motivated by the fact that hydrographical data for Katima Mulilo (upstream of the
wetland) and Victoria Falls (downstream of the wetland) before, during and after the
flood event of 2004 showed a fascinating pattern. Particularly, it was observed that the
water levels, as well as the rates of change in the water levels measured at Katima Mulilo
were higher compared with the same parameters measured at Victoria Falls. This led to
the hypothesis that the Eastern Caprivi wetland significantly contributed to flood
mitigation in areas downstream of it.
Consequently, this study is based on the Eastern Caprivi wetland system that
lies between Katima Mulilo and Victoria Falls. This centre of the Eastern Caprivi wetland
is defined by the geographical coordinates 17 30’ 00”S and 24 30’ 00”E. The area of the
wetland was estimated to be 11, 200 km2 in 1985 (Chenje, 2000). The threats to this
wetland include reduced flows due to road building and agriculture, among others
(Chenje, 2000). The role of this wetland in flood mitigation has not been investigated
quantitatively. Fig. 1 illustrates the location and the elevation details of the immediate
surroundings of the wetland.
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Figure 1: The location of the study area and the elevation details of the immediate
surroundings of the wetland.
This paper presents the results of a research project in which the key objective
was to quantify the extent to which the Eastern Caprivi wetland contributed to flood
mitigation in the Zambezi catchment, in areas downstream of this wetland during the
April 2004 flood event. Three specific objectives were set as a means to achieve this
main objective.
The first specific objective was to test whether the rates of change in the water
levels at Katima Mulilo and Victoria Falls are significantly different.
The second specific objective was to use remote sensing imagery and a digital
elevation model (DEM) to quantify the area and volume of water in the eastern Caprivi
wetland and its distribution in space over the period before, during and after the flood
event.
In order to link the measurements at the gauging stations with the satellite
measurements, the third objective was to determine whether the area and volume of water
measured in the wetland was significantly related with water levels measured at both
Katima Mulilo and Victoria Falls.
Methods
Hydrological analysis
Hydrological data was obtained from the Division of Hydrology in the Ministry of Water
Affairs in Namibia for the Katima Mulilo and from the Zimbabwe National Water
Authority for the Victoria Falls. The hydrological data analysis involved both exploratory
and confirmatory data analysis.
Exploratory data analysis started by the construction and visual inspection of
hydrographs that show variations in water levels (measured in metres) at Katima Mulilo
(i.e., upstream of the eastern Caprivi wetland) and at Victoria Falls (i.e., downstream of
the eastern Caprivi wetland) before, during and after the flood event. This step was meant
to determine whether there were any observable differences between the distribution
patterns of water levels upstream and downstream of the wetland before, during after the
flood event. Data was also tested for normality based on the Kolmogorov-Smirnov test as
a basis for selecting appropriate statistical tests (i.e., when data is normally distributed,
powerful parametric tests are performed as opposed to robust non parametric tests that
are performed on non-normally distributed data). We found that the data did not
significantly (p > 0.05) deviate from a normal curve. We took this as an important step
towards the testing of hypotheses related to the water flow regulation by the wetland.
Exploratory hydrological data analysis also involved the calculation of the rate of
change in water levels (in metres per day) measured at Katima Mulilo and Victoria Falls,
followed by graphically plotting it to enable visual inspection. Rate of change of water
levels were placed into two groups, i.e., data reflecting the rising limb of water levels up
to the peak of the flood event and data reflecting the decrease in water levels after the
peak of the flood event.
Confirmatory hydrological data analysis involved the use of a Student t-test
(t-test), to find out whether the mean rate of change (with respect to both increase and
decrease) in water levels at Katima Mulilo were significantly higher than respective
means of increase and decrease at Victoria Falls during the flood event.
Data obtained at the gauging station, ZPG25 at Victoria Falls in Zimbabwe was
used in this analysis. This station is the one gauging water flow immediately after the
eastern Caprivi wetland (approximately 25 km downstream of the wetland)
Remote sensing and digital elevation model (DEM) for water area and
volume modelling
The Moderate Resolution Imaging Spectroradiometer (MODIS) was used for mapping
the area under water in the wetland. MODIS is the key instrument aboard the Terra and
Aqua satellites. It is an optical scanner that views the Earth in 36 channels with spatial
resolution ranging from 250 meters to 1 kilometer (http://modis.gsfc.nasa.gov/). In this
study, we used band 1 and band 2 for mapping the area extent of water before, during and
after the flood event. The spatial resolution of band 1 and 2 is 250m at nadir and the
bands record radiance between 620-670nm (visible red) and 841-876nm (near infrared)
respectively, thereby making these bands suitable for mapping water.
Also, their
moderate spatial resolution of 250 m was deemed suitable for mapping this large wetland
(fig. 2). Terra's orbit around the Earth is timed so that it passes from north to south across
the equator in the morning, while Aqua passes south to north over the equator in the
afternoon. Terra MODIS and Aqua MODIS are viewing the entire Earth's surface every 1
to 2 days.
MODIS images were downloaded from the National Atmospheric and Space
Administration via the Internet at the Department of Geography and Environmental
Science of the University of Zimbabwe. Only, cloud free imagery was downloaded. All
in all, 38 cloud free images were downloaded. These were acquired between the 2 nd of
February 2004 (before the flood) and the 26th of July 2004 (after the flood). The imagery
was geographically referenced. This facilitated the analysis and the imagery was coregistered to the other ancillary data, i.e. the maps of Victoria Falls and Katima Mulilo as
well as the DEM. The MODIS images were classified digitally using processing imageprocessing software. A hybrid approach was used to determine the area covered by water
from the imagery. Firstly, the ISODATA clustering algorithm was used to separate four
classes that were determined after spectrally and visually inspecting the imagery.
Secondly, the water class was determined from the four classes based on expert
knowledge. Finally, the area under water was calculated in a Geographic Information
system (GIS).
A DEM was used to delimit the potential wetland area below the elevation of the
Katima Mulilo gauging station. The DEM was derived from the 90 m Shuttle Radar
Terrain Mission data (http://edcsgs9.cr.usgs.gov/). A logical computer algorithm
determining the areas with an elevation (altitude) below that of the Katima Mulilo
gauging station was implemented. The result was a potential wetland (fig. 2a).
As a preamble to calculating water volume, the depth at every 90 m pixel was
calculated via two steps. Firstly, pixels with the highest altitude upstream of the wetland
and pixels with the lowest altitude downstream of the wetland were extracted from the
altitude map representing the potential wetland area. Next, a plane trend surface was
modeled between the highest and lowest pixels so as to form a plane that behaves like a
lid on the potential wetland. Finally, the elevations of the potential wetland map were
subtracted from the modeled plane trend surface, resulting in a depth map representing
depth at every point in the wetland (fig. 2b).
Next, the maps of water area variations in the study area over time were plotted.
In addition, wetland water the area/ volume against time so as to reveal the patterns
before, during and after the flood event were also plotted. A polynomial was fitted so as
to describe the shape of the water area and volume changes over time. The graphs were
plotted together with the changes in water levels at Katima Mulilo and Victoria Falls so
as to describe any similarities and differences.
Figure 2: Maps of (a) elevation above sea level (in metres), as well as (b) the estimated
depth (in metres) of the Caprivi potential wetland. Note that the lower places in (a) have
higher depths (b).
Relationship between wetland area/volume and the water levels
upstream and downstream of the wetland
Regression analysis was used to test the nature and significance of the relationship
between water levels (metres) measured at Katima Mulilo and the area (m2) and volume
(m3) of the water observed in the wetland. Finally, regression analysis was also used to
test the hypothesis whether the variations in the water area and volume observed in the
wetland were significantly related to the water levels measured at Victoria Falls. In the
latter case, it is the water area and volume that was used as independent variables while
the water levels measured at Victoria Falls was the dependent variable. This is because
Victoria Falls is downstream of the wetland, thereby being affected by the wetland
upstream.
Results
Hydrological data analysis
It was observed that the hydrograph of Katima Mulilo shows higher water levels than the
ones for Victoria Falls during the study period (fig. 3). It can also be observed that the
hydrographs have a similar shape. However, the Victoria Falls hydrographs show a more
depressed shape.
Figure 3: The variations in water levels at Katima Mulilo and at the two measuring
stations at Victoria Falls (ZPG25 upstream and ZPG26 further downstream).
Fig. 4 illustrates the results of the calculated the rate of change (in m/day) in water
levels in the Katima Mulilo and Victoria Falls. Fig. 4a shows that there were higher rates
of change on the Katima Mulilo graph compared with the rates of changes at Victoria
Falls. Formal t-test results show that the mean rate of change at Katima Mulilo for both
increasing or rising limb (0.0.43 m/day, fig. 4b) and the decreasing or falling limb
(0.06 m/day, fig. 4c) was significantly higher (p < 0.05) than the respective mean rates of
change (i.e., 0.01m /day and 0.008 m /day) at Victoria Falls. The mean rate of increase in
water level at Katima Mulilo is 4 times higher than the mean rate of increase in water
levels at Victoria Falls. On the other hand, the mean rate of decrease in water levels at
Katima Mulilo is 7.5 times higher than the mean rate of increase in water levels at
Victoria Falls.
Figure 4. The (a) rate of change (m/day) of water levels at Katima Mulilo and Victoria
Falls over the study period and the significantly (p < 0.05) different mean rates of change
for (b) the rising limb building up to the flood peak and the (c) falling limb after the flood
peak. Note that the falling the rates of change for the falling limb compared as absolute
values. The dashed line centred at zero on the y-axis in fig 5a represents the zero rate of
change.
Variations in the wetland water area/volume in space and time
Maps illustrating the changes in the area under water in the Eastern Caprivi wetland are
shown in fig. 5 to 10. The area of the wetland has little coverage of water on the 36 th day
of 2004 and this increases and reaches its peak by the 94th to the 112th day (fig. 5 and 6)
after which the area under water gradually decreases until it becomes low again on the
208th day of 2004.
Figure 11 illustrates a comparison of the hydrographs at Katima Mulilo and
Victoria Falls with the wetland water area/volume plots that we measured using MODIS
imagery in combination with a DEM.
It can be observed that the shape of the
hydrographs, particularly the shape of the hydrograph at Katima Mulilo corresponds
closely with the measured wetland water area/volume plots that are fitted with a 5th order
polynomial.
Figure 5: Changes in the area under water from the 36th day to the 94th day of 2004
Figure 6: Changes in the area under water from the 100th day to the 112th day of 2004
Figure 7: Changes in the area under water from the 116th day to the 125th day of 2004
Figure 8: Changes in the area under water from the 126th day to the 134th day of 2004
Figure 9: Changes in the area under water from the 135th day to the 148th day of 2004
Figure 10: Changes in the area under water from the 150th day to the 208th day of 2004
Figure 11: (a) The hydrographs at Katima Mulilo and Victoria Falls and the wetland (b)
the water area and (c) the wetland water volume measured from MODIS imagery in
combination with a DEM.
Relationship between wetland area/volume and the water levels
upstream and downstream of the wetland
There was a significant positive asymptotic (p < 0.05) relationship between the water
levels measured at Katima Mulilo and the wetland water area/volume measured in the
wetland over the duration of the study period (fig. 12). The curve tends to flatten when
water levels at Katima Mulilo reach approximately 5 m.
The water levels at Katima
Mulilo explained up to 97 % and 93 % of the variance in the area and volume of water
contained in the wetland respectively.
Figure 12: Significant (p < 0.05) relationship between water level measured at Katima
Mulilo (upstream of the wetland) and the wetland water (a) area and (b) volume.
The results also show a significant positive relationship (p < 0.05) between the
wetland water area/ volume and the water levels measured at Victoria Falls (ZPG25)
(fig. 13). It can be observed that the fitted curve tends to flatten when the water area and
volume in the wetland reach approximately 2 billion square metres and 18 billion cubic
metres respectively (fig. 13). The water area and volume in the eastern Caprivi wetland
explained up to 90 % and 85 % of the variance in the water levels measured at Victoria
Falls in Zimbabwe (fig 13).
Figure 13: Significant (p < 0.05) relationship between the wetland water (a) area and (b)
volume and water level measured at Victoria Falls
(ZPG 25) (downstream of the
wetland)
Discussion
So, to what extent do wetlands contribute to flood mitigation? The results indicated that
the eastern Caprivi wetland played a significant role in mitigating floods downstream of
the wetland. This is because the results provided evidence that the wetland absorbed the
high water levels at Katima Mulilo that in turn resulted in lower water levels being
recorded at Victoria Falls (fig. 3). In addition, the results indicated a significantly higher
(p < 0.05) mean in the rate of change in water levels (m/day) at Katima Mulilo compared
with the mean rate of change in the water level at Victoria Falls (fig. 4).
The results also demonstrated that water levels at Katima Mulilo significantly
explained (p< 0.05) 97 % and 93 % of the variance in water area and water volume in the
wetland (fig. 12). However, a relatively low coefficient of determination (r2 = 0.93) for
the water volume/water level function (fig. 12b) compared with the water area/water
level function (r2 = 0.97) may be due to the fact that volume was derived as a second
order measure compared with area that was directly derived from a satellite image
classification. Both the water area and water volume responded to the increase in water
levels positively and asymptotically with the curve flattening at about 5 m after which
there is little or no increase in the water area/volume with additional increases in water
levels. This asymptotic part of the relationship between water levels and water
area/volume indicates that the wetland has approached maximum capacity and excess
water is then freely flowing through the wetland system. Therefore, it can be deduced that
the eastern Caprivi wetland played a significant role in absorbing water from upstream
during the flood event.
Having investigated how the water area and volume in the eastern Caprivi
wetland responded to water levels at Katima Mulilo, whether the water area and volume
in the wetland in turn significantly explained the variance in the water levels at Victoria
Falls was next investigated. The findings indicated that the area under water, as well as
the volume of water in the Eastern Caprivi significantly (p < 0.05) and respectively
explained 90 % and 85 % of the variance in the recorded water levels at the Victoria
Falls. Again, relatively lower coefficient of determination (r2 = 0.85) for the water
volume/water level function (fig. 12b) compared with the water area/water level function
(r2 = 0.90) may be due to the fact that volume was derived as a second order measure
compared with area that was directly derived from a satellite image classification. The
asymptotic part of the relationship between water levels and water area/volume indicates
that the wetland regulates the flow of water passing downstream to Victoria Falls.
Specifically, when water area and volume in the wetland is in excess of approximately
2 billion square metres and 18 billion cubic metres (fig. 13), the water levels at Victoria
Falls tend to increase slowly. In effect, it can be deduce that the eastern Caprivi wetland
played a significant role in regulating the flow of water downstream thereby preventing
the propagation of potentially destructive flood downstream during the 2004 flood event.
Thus, these findings provide evidence for the need to protect the integrity of the wetland
system, in order to maintain this hydrological function for the purpose of dissipating
flood propagation.
Conclusions
The key objective in this study was to quantify the extent to which the Zambezi wetlands,
specifically the Eastern Caprivi wetland contributed to flood mitigation in the Zambezi
catchment, in areas downstream of this wetland during the April 2004 flood event. The
specific objectives were (1) to test whether the rates of change in the water levels at
Katima Mulilo and Victoria Falls were significantly different thereby indicating some
effect of the wetland on the water flows, (2) to use remote sensing imagery and a digital
elevation model (DEM) to quantify the area and volume of water in the eastern Caprivi
wetland and its distribution in space over the period before, during and after the flood
event and (3) to determine whether the area and volume of water measured in the wetland
was significantly related with water levels measured at both Katima Mulilo and Victoria
Falls. Therefore, some conclusions can be drawn from this study:

The mean rates of change, i.e., increase and decline in water levels at Katima
Mulilo were significantly higher than those recorded at Victoria Falls indicating
that the Victoria Falls experiences regulated flow due to the wetland.

The wetland area/volume derived from remote sensing imagery were significantly
related to the water level measurements made at Katima Mulilo, further giving
evidence that the wetland was absorbing water.

The wetland area/volume derived from remote sensing imagery were significantly
related to the water level measurements made at Victoria Falls, indicating the role
of the wetland in regulating flow thereby reducing the risk of flooding
downstream.

The findings in this study reinforce the need to preserve wetlands as a means to
reduce the risk of flooding downstream.
Acknowledgements
This research was funded by the IUCN Regional Office for Southern Africa.We wish to
thank Mr. Lenka Thamae for being closely involved in the project and for reviewing and
editing this report. We also wish to thank the Zimbabwe National Water Authority and
the Namibian Ministry of Water Affairs, Hydrology Division for providing hydrological
data for Victoria Falls and Katima Mulilo respectively
References
CHENJE, M., 2000. The State of the environment: Zambezi Basin 2000. Harare.
SADC/IUCN/ZRA/SARDC.
SMITH R.L. and SMITH T.M., 2000. Elements of Ecology. Benjamin/Cummings Science
Publishing, San Francisco.