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Some effects of the Tiga Dam on valleyside erosion in downstream reaches of the River Kano
Applied Geography (1984), 4, 321-332
Some effects of the Tiga Dam on valleyside erosion in downstream reaches of the River Kano E. A. Olofin Department of Geography, Nigeria
Bayer0 University, PMB 3011, Kano,
Abstract Some of the effects of a lower river stage on valleyside fluvial processes are examined in downstream reaches of the Tiga Dam in the Kano River basin. The focus is on valley-bottom gullies which responded clearly and quickly to the lower height of flow in the Kano River channel in these reaches. The characteristics of the valley-bottom gullies at a time before the construction of the Tiga Dam were compared with those of the same gullies when the Tiga Dam was in operation. It is clear that valley-bottom gullies along the Kano channel changed from the typical, savanna shallow types (pre-Tiga) to deeper ones (post-Tiga). For example, the depth of the gully mouth increased from 1.3 m to 2.02 m, the gullies lost their alluvial deposits on the gully floors and a dual-cycle gully shape was established. The incision was rapid and intense, lasting not more than three wet seasons. It is argued that the construction of major dams on savanna streams would usually escalate fluvial processes on the valleyside slopes downstream, and that the maintenance of perennial flow in the relevant river channel would shorten the relaxation time required for a new equilibrium to occur.
Introduction Chorley and Kennedy (1971) have argued that when man intervenes to produce operational changes in the distribution of energy and mass in natural systems, he invariably brings about changes in the equilibrium relationships which exist between the morphological and cascading variables of such systems. For example, a dam built across a river should be expected to upset any equilibrium already established between channel morphology and flow rCgime in that river basin since the flow rCgime is expected to change in stage, discharge and duration of flow. These changes in the flow rCgime, particularly that in stage, should be expected to affect fluvial processes on the valleyside slopes, in addition to their effects on channel morphology which have been discussed by Mrowka (1974) and Schumm (1977). This response occurs because the level of flow in the main channel constitutes the local base level for fluvial processes on the valleyside slopes. Thus a decrease in stream stage downstream of dams should be expected to reactivate or increase the rate of soil erosion on such slopes. Piest et al. (1975: 65-66), for example, contend that a ‘change in base grade’ is one of the factors that induce ‘the initiation of . valley-bottom gullies’. The investigations described in this paper were based on the premise that the level of flow in a savanna channel is the local base level for fluvial processes on the valleyside slopes. That this premise is valid in the study area is not in doubt. A 0143~h228/X4/040321-12 $03.00 0
19X4 Butterworth
& Co (Publishers) Ltd
study of one gully mouth in the River Chalawa basin before the construction of major dams within the basin illustrates the close relationship between flow in the main channel and tributary alluviation. The first local rains in May of the year in question destroyed the alluviated gully mouth which originally lay above the bed of the Chalawa channel. The presence of this higher mouth, termed ‘hanging gully mouth’ by Olofin (1980b), is itself evidence of the height of flow in the Chalawa channel at the time of the last gully discharge during the previous wet season. The first rains were abie to remove the hanging element because they generated gully discharge at a time when the flow in the channel was low and diverted away from the gully mouth was alluviated to the gully mouth. Subsequently, however, different heights throughout the wet season of that year in response to changing heights of flow in the Chalawa channel. Thus the stage in the main channel acts as a regulator for the processes of overland flow which link the rainfall cascade with the channel flow cascade. Since the level of channel flow is likely to decrease downstream of a dam, it may be expected that the construction of the Tiga Dam would encourage entrenchment of valleyside tributaries. For this reason, a study was made of valleyside processes in areas immediately downstream of the dam (Fig. I). Valley-bottom (low terrace) gullies are used in this paper to serve as an index of valleyside erosion. The relevant investigations were undertaken between 1976 and 1979 as part of a wider project (Olofin 1980b) aimed at assessing: 1, 2. 3.
the magnitude of changes in flow stage and discharge; the effects of the changes in stage on valleyside processes, and the effects of the changes in discharge and valleyside processes morphology, debris storage and vegetation system.
Aspects addition
of the third line of investigations to the main work itself.
are contained
in Olofin
on channel
(1980a)
in
The study area Locution The study area is shown in Fig. 1, which also indicates the location of the sample sites for both the study of gullies and that of channel changes. The study area consists of the lower courses of the Kano and Chalawa rivers shortly before they join to form River Hadejia. The stretch of the Kano River within the study area is about 60 km but only about 15 km of the Chalawa is included. Both the river basins lie entirely in the Sudan savanna zone, although the River Kano rises in a more humid region at the footslopes of the Jos Plateau. Ctimute The climate may be classified as a tropical dry-and-wei type. The annual mean rainfall is between 800 mm and 900 mm. Variations about the annual mean value are up to f 30 per cent. More than 300 mm of the rainfall is received in August alone, while the truly wet season lasts from June to September. However, it is usual to regard mid-May to mid-October as the wet season. Rainfall intensity is in the range of 4%60 mm hr-‘, but it is particularly high at the beginning and end of the wet season when rainfall is characterised by heavy storms whose average intensity is about X0 mm hr- ’ (Leow and Oioge 1981).
E. A. Oiofin NIGER
A
I
‘Kura
Kadawa p Irrigation
Explanation +
+
-0
-
@
Boundary of study area Boundary of control area
Settlement
% Site of channel cross profile
z=zz -------
Major
road
A!q
Minor
road
-++t++
e .b
Dams
and
Site of low Railway
terrace
gully
line EAOlSOT
Reservairs Figure 1. The location
study
of the study area.
323
324
Effects of the Tiga Dam on valleyside erosion
There are three main temperature seasons. A cool and dry season lasts from November to February, during which the mean monthly temperature is between 21 and 23°C with a diurnal range of 12-14°C. The Harmattan winds prevail at this time. This period is usually followed by a hot and dry season which lasts from March to mid-May. The mean monthly temperature during this period is in excess of 30°C and the daily range is up to 20°C. This is followed by the wet season which is warm, with mean monthly temperatures about 26°C and a diurnal range of about lo”C, rising to 13°C in September. Evapotranspiration is generally very high throughout the year since potential evapotranspiration is not less than 120 mm for any month. Rainfall may exceed potential evapotranspiration only in four months (June to September), but in dry years a positive water balance may occur only in July and August. The mean annual potential evapotranspiration is in excess of 1800 mm. Geology
and landforms
The study area is underlain by the rocks of the Basement Complex which consist of Older Granites and old metamorphic rocks of various descriptions, The ancient rocks have been overlain in many areas by moderately thick regolith derived from them and by a top layer of windblown material (wind drift) up to 2 m thick. Outcrops of the ancient rocks are common, but are irregularly distributed. Apart from these rock outcrops, a typical valleyside slope profile shows the following landform units: an upland plain, a higher terrace, a low terrace and the present flat-bedded river channel. Each of these units is separated from the next by a scarp about 2-3 m high, densely gullied and steeper than 60 degrees in slope angle. The main units themselves are gently inclined at about l-2 degrees, except at eroded zones where slope angles in the range of 34 degrees are encountered. It is the occurrence of the scarps that encourages rapid rainfall runoff and increases rates of soil erosion on the valleyside slopes. Gully erosion is particularly active on these scarps. Attention here is focused on gully erosion on the low terrace (valley bottom), for this part of the slope is most responsive to changes of flow in the main channel. It should be noted also that gully erosion provides a more measurable index of erosion than other forms of fluvial erosion. Dam construction
in the study area
The construction of dams in Kano State started in 1969 and reached the study area in 1970, when the Bagauda dam was initiated on a tributary of the River Kano. Between 1970 and 1974 three dams were constructed in the study area: two in the Kano basin (Bagauda, 197(&71 and Tiga, 1971-74) and one in the Chalawa basin (Karaye, 1972). The dam at Karaye, on a tributary of the Chalawa, controls a stream with a mean annual discharge of only 0.45 m’s_’ and accounts for only 2.1 per cent of the normal flow of the River Chalawa. The Bagauda, on a tributary of the River Kano, controls a stream with a mean annual discharge of 1.25 m3 ss’ which is about 3.5 per cent of the normal flow of the River Kano. The Tiga dam was the only major dam operating in the study area at the time of this study. Its construction was completed early in 1974 and impoundment started in that year’s wet season. The dam controls a mean annual discharge of approximately 3358 m3 for about 91 per cent of the mean annual flow of the river. It is s-‘, accounting located directly on the river and has an optimum storage capacity of 1.97 x 10” m3
E. A. Olofin
325
and a reservoir surface area, at this capacity, of 178.1 km2 (Olofin 1980b). Statistical analyses of stage and discharge have shown that both the Bagauda and Karaye dams have had negligible effects on the hydrology of their relevant basins, but the effects of the Tiga dam on the hydrology of the Kano river basin have been great, as is shown below. The effects of the Tip Dam on basin hydrology
The analyses of stage and discharge of the River Kano, downstream of the Tiga dam, have shown that the dam and its reservoir have caused a lower peak discharge, a lower peak stage and a longer duration of flow in these reaches. For example, the mean annual discharge in the pre-Tiga period stood at 37 m3 s-l in the Chiromawa gauging station (KS in Fig. l), but this decreased to 9-6 m3 s-’ in the post-Tiga period. The net increase in evapotranspiration as a direct result of the Tiga Reservoir has been computed (Olofin 1980b) to be about 7.6 m3 s-‘. This net loss to evapotranspiration should be seen as a part of the total effect of the dam and its reservoir on the environment of the study area. Thus in terms of total volume per water-year, an annual discharge of 1.17 x 10” m3 at the pre-Tiga time decreased to only 0.30 x lo9 m3 in the post-Tiga period. The relationship can be stated thus:
Qm+I = 0.26 Qm,
(1)
where t = period before the Tiga dam t+l = period after the Tiga dam Qm = mean discharge in cumecs The wet season mean discharge also decreased from 86 m3 s-l at time t to 13 m3 s -r at time t+l, but the dry season mean discharge over the same time span increased from about O-5 m3 s-’ to approximately 7.0 m3 s-‘. Thus the flow regime was changed from a seasonal to a perennial type. The peak stage in the wet season has also decreased very significantly by more than one metre, particularly in July and August, while the dry season stage increased by up to 60 cm. In contrast to these changes in the flow regime and quantity of discharge of the Kano River, the analyses of the discharge and stage of the Chalawa River, over the same period of study (1965-70 and 1974-79), have shown that the discharge and stage of the Chalawa River did not change significantly from the normal. Methodology
The aim of the research was to compare the pre-Tiga magnitudes of the investigated variables with those of the post-Tiga period. For this purpose, a combination of field, laboratory and statistical techniques was used to study twelve low terrace gullies along the Kano River channel (A, B, C, . . . L in Fig. 1) and four similar gullies along the Chalawa River channel (A, B, C, D). Existing records, literature and air photographs were utilized to estimate the magnitudes of the selected variables, in addition to actual field measurements, observations and interviews. The main focus was on the depth, shape, gully-mouth condition and gully-floor material which were believed to provide clear indices of erosion or deposition in various combinations. Field measurements and observations were made twice during each dry season,
326
Effects
of the Tiga Dam
on valleyside
erosion
E.A. Olofin
327
at the beginning and at the end of the season. A study visit was paid to each accessible site during each wet season from 1976 to 1979. In addition, pre-Tiga gully characteristics were traced back 12 years, principally through a set of air photographs taken in December 1963. Another set flown in December 1978 provided adequate post-Tiga reference. The 1963 set was enlarged to a scale of 15000 while the 1978 set was actually taken at that scale. The results of the various analyses undertaken on the data generated through the investigations are presented below. The effects of the Tiga Dam on valley-bottom gullies Typical savanna valley-bottom gullies
An analysis of the available data on the characteristics of valley-bottom (low terrace) gullies along the Kano River channel in pre-Tiga time shows that the pre-Tiga valley-bottom gullies along the river channel were typical savanna ones. The characteristics of a typical savanna valley-bottom gully are illustrated in Fig. 2 and Table 1. These characteristics are believed to represent a process-response system in which the inputs of water and debris from upper slopes and gully sides are approximately the same as the outputs of water and debris into the main channel. Such a condition, where the valley-bottom gully acts merely as a conduit for the cascading system without measurable modifications to its characteristics, represents an equilibrium state within a savanna environment. However, the equilibrium state is limited to the valley-bottom section of a valleyside gully. Table 1. Mean
GLllly site A
B C D Mean s.d.
depths of typical savanna v~llley-bottom River basin, 1979
Distance from confluence (km) 14.00 13.00 8#0 4*OO
gullies. Chaiawa
Mean depth of whole gully (metres)
Mean depth of gully mouth (metres)
Height of gully mouth above river bed (metres)
1.06 0.70 1.06 1.16
1.19 1.05 0.94 1.45
l-00 1.35 1.05 0.95
1.00 0.18
1.16 o-19
1.09 O-16
Note: ~ulIy-in-gully morphology is absent; there is thick sandy alluvium on the gully mouth and gully floor. Source: Olofin (1980h)
From these illustrations (and the data on the first-phase gullies in Table 2), it is clear that before the Tiga dam and reservoir came into operation the valley-bottom gullies along both channels were first-phase, single-cycle types which were about 3-4 m wide and l-2-1-3 m deep. The gully floors were flat and consisted of an accumulation of sandy alluvium 40-60 cm thick, while the gully mouths were higher than the beds of the river channels into which they discharged by up to one metre. The entire gully unit (floor and wall) was without vegetation cover, but this bare surface did not result in measurabIe changes in the mean dimensions of valley-bottom gullies in the study area, as shown by studies of the Chalawa gullies.
During the pre-Tiga period, it is believed that the peak height of flow (HF) in the River Kano channel was slightly higher than the level of gully floor at the time of gully-mouth alluviation. Gully discharge, baseflow from soil moisture and seepage from groundwater must have responded, at least in part, to the height of Row in the main channel. Field evidence has shown that the valley-bottom gullies along the Chalawa channel maintained these typical characteristics of a savanna valley-bottom gully throughout the time of study. Some of these characteristics are shown in Table 1. The valley-bottom gullies along the Chalawa channel maintained these characteristics because no major dam existed in the Chalawa basin at the time. It is interesting to note that since then these gullies have undergone significant changes in depth and shape because of the numerous dams that have been constructed in the basin, including a major one, scheduled to be completed in 1984, on the Chalawa River itself. By May 1983 the valley-bottom gullies along the Chalawa channel had reached the phase of maximum incision shown in Fig. 3 and Table 2 (whole gully). When the major dam (the Chalawa Gorge) is completed and operated as the Tiga, these gullies along the Chaiawa channel will become similar to the post-T&a ones along the Kano channel described below.
The mean dimensions and other characteristics of the valley-bottom gullies along the Kano channel in post-Tiga period are illustrated in Fig. 3 and Table 2. These characteristics are believed to represent a control system at the point of optimum response to a negative change in the height of flow in the main channel. In this study they represent the point, roughly January 1978, shortly before the establishment of vegetation at the gully mouths. Field observations have shown that initial and rapid incision followed the lowering of the stage in the Kano River channel, increasing the depth of gully mouth from l-4 m to 2.02 m and the shape from single-cycle (1) to dual-cycle (2), while the top width (W) remained unchanged at 3.5 m. Gully water and debris discharge changed temporarily from the equilibrium state (it), described earlier. to a definite increase (+) condition due to additional inputs through activated erosion and release of part of the water which pre-existed as, or would have replenished, the groundwater storage at a higher water table. As these data show, the post-Tiga valley-bottom gullies have generally lost most of their pre-Tiga, typical savanna characteristics. They have become two-phased in shape with terraces standing at the levet of the former gully floor; their mouths have been incised to the level of the now aggraded bed of the main channel, increasing their mean depth from about 1.3 to 2 m, and the thick sandy alluvium on the gully floor has been removed. The mean incision of ail the gullies in the reactivated reaches is about 54 cm while the mean gully mouth incision is about 64 cm deep. In some cases the gully mouths have been extended over the aggraded bed of the Kano channel to reach the new effective channel. Such extensions are cut into the bed to depths ranging from 20 to 30 cm. During the 1977-78 dry season, however, vegetati~~n started to invade the gully mouths from sections of the former bed no longer utilized by the Kane River discharge. Lateral erosion removed most of the gully terraces in the following wet season, and by May 1979 the valley-bottom gullies along the Kano channel had achieved a kind of new dynamic equilibrium with thick, stabilizing, covers of grasses and small shrubs, as well as creepers and reeds.
E.A. Olofin Table 2. Mean depths
Gully site’
of post-Tiga
Distance from confluence’ (in km)
gullies along the Kano River
valley-bottom channel, 1979
Mean depth of whole gully (metres)
Mean depth of lst-phase gully’ (metres)
Mean depth of 2nd-phase gullyd (metres)
0.82 2.00
0.33 0.49
The entire reactivated section of the gullies A’ 59.97 1.15 B 58.40 2.47 C D
46.00 42.00
2.45 2.87
1.90 1.68
0.50 1.00
E F
38.50 38.00
1.74 1.77
1.30 1.19
0.49 0.58
G H
24.00 15.00
2.20 1.50
1.65 1.05
0.49 0.46
I J
13.00 7.00
1.18 1.53
0.61 1.00
0.57 0.51
K L
4.00 3.00
1.54 1.50
1.03 1.03
0.52 0.48
Mean s.d.
1.83 0.49
1.27 0.42
0.54 0.15
The gully mouth A’ 59.97 B 58.40
1.37 2.53
1.02 2.00
0.35 0.53
C D
46.00 42.00
2.45 2.76
1.80 1.71
0.65 1.05
E F
38.50 38.00
2.08 2.20
1.40 1.55
0.68 0.65
G H
24.00 15.00
2.21 1.77
1.65 1.15
0.56 0.62
I J
13.00 7.00
1.71 1.75
1.11 1.18
0.60 0.57
K L
4.00 3.00
1.67 1.70
1.02 1.15
0.59 0.55
2.02 0.40
1.40 0.32
0.64 0.12
Mean s.d.
329
“The first of each pair is from the right bank, the other is from the left *The Tiga dam is 60 km from the Kano-Chalawa confluence ‘M-phase = first-phase section, or pre-Tiga mean depth “2nd-phase = second-phase section, or post-Tiga incision ‘There is a strong structural control here, being rocky Source: Olofin (1980b)
The depths of gullies at time t (pre-Tiga) and at time t-t1 (post-Tiga) were statistically examined for significant differences, using the Student t-test. First, the first-phase gullies along the Kano channel were compared with the gullies along the Chalawa channel. Next the depth of the whole (first- plus second-phase) gullies along the Kano was compared with that of the gullies along the Chalawa.
0
HR q Average W = Average
of gully of gully
Figure 3. The structure
depth width
CASCADE COMPONENTS -_D Q=Discharge &Bed load Qo=Run off M= Suspended Load REGULATINg OR CONTROLLING >A = Exceedrng angle of repose? >F*= Exceeding Hg = Height of Gully mouth above channel flow HF q Height of channel flow GE = Gullv erosion MORPHOLOGICAL ’ 0 $I ; ;;&;ob;turrly
UPPER SLOPE GULLY SYSTEM
+_ = Balance
of a post-Tip
I
of
I
Debris Water
E:
gully along the Kano River channel.
change
I
SYSTEM
Condition
GULLY
valley-bottom
+, - = Direction
I
rYES
LOW TERRACE
STREAM
CHANNEL
SYSTEM
E. A. Olofin
331
Comparisons were made initially using all the 12 gullies investigated along the Kano channel, and then repeated using only the five Kano sites nearest to the Chalawa samples. The results of these comparisons are shown in Table 3. These results show that the first-cycle (pre-Tiga) gullies along the Kano channel were identical in depth with the gullies along the Chalawa channel at the time of study. None of the Student t-values shows a significant difference, even at as low a probability as 0.7. However, the mean depth of the whole Kano gullies (both the five selected ones and all the 12 studied) are shown to be significantly different from the Chalawa gullies at probabilities of 0.01 and 0401 respectively (see D2 and t2 in Table 3). Table 3. Statistical
analyses
of the changes
Mean gully depth along the Chalawa (metres) s.d. x
in valley-bottom
Mean depth of lst-phase gully along the Kano (metres) ic s.d.
gullies in the study area (1979)
Mean whole gully depth along the Kano (metres) x s.d.
Differences Student D,’ DZh t (mean) values (m) (m) tI t2
Five selected Kano gullies and four Chalawa gullies’ Entire gully I.00 0.18 0.95 0.16 Gully mouth 1.16 0.19 1.12 0.06
1.45 1.72
0.14 0.04
0.05 0.04
0.45 0.56
0.39 0.36
All lwelve Kano gullies and four Chalawa gullies’ Entire gully 1.00 0.18 1.27 0.42 Gully mouth 1.00 0.19 1.40 0.32
1.83 2.02
0.49 0.41
0.27 0.24
0.83 0.84
1.65 164
“Difference ‘Difference ‘Degrees of “Differences ‘Degrees of ‘Differences
between mean depth of Chalawa between mean depth of Chalawa freedom = 7 significant at 0.01 level freedom = 14 significant at 0.001 level
gully and mean depth gully and mean depth
360d 5.05d 4.59f 5.15’
of Kano lst-phase gully (=t,) of whole Kano gully ( =t2)
The cause of the significant changes in the valley-bottom gullies along the Kano River has been traced to the hydrological changes in the basin consequent on the construction of the Tiga dam and reservoir. Thus the changes in the characteristics of the gullies reflect some of the effects of the Tiga dam and its reservoir on valleyside fluvial processes in the study area. As stated earlier, the changes occurred very rapidly. Active erosion lasted for about three wet seasons after the Tiga dam was established, the first wet season being the most active. This type of reaction means that the lowering of the base level (control input) almost immediately escalated the erosion of the valleyside slopes (output), achieving what may be classed as episodic erosion. The rapidity of adjustments in erosional processes to management control is not peculiar to this study. For example, Dvorak and Heinemann (1967) have reported that more than 98 per cent of the sediments eroded from a gully in Nebraska, USA, between April 1951 and April 1956 occurred during the first year. One implication of such rapid adjustments is that such activities could be missed entirely if studies were not undertaken within the first or second year of their initiation. Conclusion One conclusion to be drawn from this study is that major dams and reservoirs built on seasonal streams in a savanna environment will generate changes in the
332
Effects of the Tiga Dam on valleyside erosion
components of the process-response systems in downstream reaches. One of the process-response systems that would be affected is the valleyside gully which responds to the consequent lowering of the height of flow in the river channel. The erosional phase is, however, very short and intense. It is relevant to state here that apart from the reactivation of valley-bottom gullies, other forms of fluvial processes must have responded in similar fashion. Indeed, the responses would have been transmitted up through the system because gullies exert a major control over many other processes operating on the valleyside slopes. Finally, where a perennial flow is maintained in the river channel, a dynamic equilibrium state is soon achieved with a substantial vegetation cover, which helps to stabilize the gullies and other valleyside fluvial processes. Nonetheless, this dynamic equilibrium state is quite different from the normal one which could exist under an undisturbed seasonal regime. Acknowledgements I wish to thank Mr S. 0. Taiwo of the Department University, Kano, for copying the figures of this paper Olofin (1980b).
of Geography, from the original
Bayer0 ones in
References Chorley, R. J. and Kennedy, B. A. (1971) Physicalgeography: a systems approach. London: Prentice-Hall. Dvorak, V. 1. and Heinemann, H. C. (1967) Cooperative runoff and sediment investigations on Medicine Creek Watershed in Nebraska. US Department of Agriculture. Report, 41-130. Leow, K. S. and Ologe, K. 0. (1981) Rates of soil wash under a Savanna climate, Zaria, Northern Nigeria. In Summaries of papers (Kano, 24th Annual Conference of the Nigerian Geographical Association), 18-22. Mrowka, J. P. (1974) Man’s impact on stream regimen quality. In Perspectives on environment (I. R. Manners and M. W. Mikesell, eds), pp. 79-104. Association of American Geographers, Commission on College Geography, Publication No. 13. Olofin, E. A. (1980a) The effects of the Tiga Dam and reservoir on the flow and channel characteristics of River Kano in downstream areas, Kano State Nigeria. Paper presented at the Pre-Congress Symposium of the IHP-IGU, Tsukuba, Japan. Summary contained in Abstracts for the Symposium, 6-7. Olofin, E. A. (1980b) Some effects of the T&a Dam on the environment downstream in the Kuno River basin. Unpublished PhD thesis, Ahmadu Bello University, Zaria. Piest, R. F., Bradford, J. M. and Wyatt, G. (1975) Soil erosion and sediment transport from gullies. Journal of Hydraulics Division, American Society of Civil Engineers 101. 65-80. Schumm, S. A. (1977) The fluvial system. New York: Wiley. (Revised manuscript received 16 January iY84)
Some effects of the Tiga Dam on valleyside erosion in downstream reaches of the River Kano E. A. Olofin Department of Geography, Nigeria
Bayer0 University, PMB 3011, Kano,
Abstract Some of the effects of a lower river stage on valleyside fluvial processes are examined in downstream reaches of the Tiga Dam in the Kano River basin. The focus is on valley-bottom gullies which responded clearly and quickly to the lower height of flow in the Kano River channel in these reaches. The characteristics of the valley-bottom gullies at a time before the construction of the Tiga Dam were compared with those of the same gullies when the Tiga Dam was in operation. It is clear that valley-bottom gullies along the Kano channel changed from the typical, savanna shallow types (pre-Tiga) to deeper ones (post-Tiga). For example, the depth of the gully mouth increased from 1.3 m to 2.02 m, the gullies lost their alluvial deposits on the gully floors and a dual-cycle gully shape was established. The incision was rapid and intense, lasting not more than three wet seasons. It is argued that the construction of major dams on savanna streams would usually escalate fluvial processes on the valleyside slopes downstream, and that the maintenance of perennial flow in the relevant river channel would shorten the relaxation time required for a new equilibrium to occur.
Introduction Chorley and Kennedy (1971) have argued that when man intervenes to produce operational changes in the distribution of energy and mass in natural systems, he invariably brings about changes in the equilibrium relationships which exist between the morphological and cascading variables of such systems. For example, a dam built across a river should be expected to upset any equilibrium already established between channel morphology and flow rCgime in that river basin since the flow rCgime is expected to change in stage, discharge and duration of flow. These changes in the flow rCgime, particularly that in stage, should be expected to affect fluvial processes on the valleyside slopes, in addition to their effects on channel morphology which have been discussed by Mrowka (1974) and Schumm (1977). This response occurs because the level of flow in the main channel constitutes the local base level for fluvial processes on the valleyside slopes. Thus a decrease in stream stage downstream of dams should be expected to reactivate or increase the rate of soil erosion on such slopes. Piest et al. (1975: 65-66), for example, contend that a ‘change in base grade’ is one of the factors that induce ‘the initiation of . valley-bottom gullies’. The investigations described in this paper were based on the premise that the level of flow in a savanna channel is the local base level for fluvial processes on the valleyside slopes. That this premise is valid in the study area is not in doubt. A 0143~h228/X4/040321-12 $03.00 0
19X4 Butterworth
& Co (Publishers) Ltd
study of one gully mouth in the River Chalawa basin before the construction of major dams within the basin illustrates the close relationship between flow in the main channel and tributary alluviation. The first local rains in May of the year in question destroyed the alluviated gully mouth which originally lay above the bed of the Chalawa channel. The presence of this higher mouth, termed ‘hanging gully mouth’ by Olofin (1980b), is itself evidence of the height of flow in the Chalawa channel at the time of the last gully discharge during the previous wet season. The first rains were abie to remove the hanging element because they generated gully discharge at a time when the flow in the channel was low and diverted away from the gully mouth was alluviated to the gully mouth. Subsequently, however, different heights throughout the wet season of that year in response to changing heights of flow in the Chalawa channel. Thus the stage in the main channel acts as a regulator for the processes of overland flow which link the rainfall cascade with the channel flow cascade. Since the level of channel flow is likely to decrease downstream of a dam, it may be expected that the construction of the Tiga Dam would encourage entrenchment of valleyside tributaries. For this reason, a study was made of valleyside processes in areas immediately downstream of the dam (Fig. I). Valley-bottom (low terrace) gullies are used in this paper to serve as an index of valleyside erosion. The relevant investigations were undertaken between 1976 and 1979 as part of a wider project (Olofin 1980b) aimed at assessing: 1, 2. 3.
the magnitude of changes in flow stage and discharge; the effects of the changes in stage on valleyside processes, and the effects of the changes in discharge and valleyside processes morphology, debris storage and vegetation system.
Aspects addition
of the third line of investigations to the main work itself.
are contained
in Olofin
on channel
(1980a)
in
The study area Locution The study area is shown in Fig. 1, which also indicates the location of the sample sites for both the study of gullies and that of channel changes. The study area consists of the lower courses of the Kano and Chalawa rivers shortly before they join to form River Hadejia. The stretch of the Kano River within the study area is about 60 km but only about 15 km of the Chalawa is included. Both the river basins lie entirely in the Sudan savanna zone, although the River Kano rises in a more humid region at the footslopes of the Jos Plateau. Ctimute The climate may be classified as a tropical dry-and-wei type. The annual mean rainfall is between 800 mm and 900 mm. Variations about the annual mean value are up to f 30 per cent. More than 300 mm of the rainfall is received in August alone, while the truly wet season lasts from June to September. However, it is usual to regard mid-May to mid-October as the wet season. Rainfall intensity is in the range of 4%60 mm hr-‘, but it is particularly high at the beginning and end of the wet season when rainfall is characterised by heavy storms whose average intensity is about X0 mm hr- ’ (Leow and Oioge 1981).
E. A. Oiofin NIGER
A
I
‘Kura
Kadawa p Irrigation
Explanation +
+
-0
-
@
Boundary of study area Boundary of control area
Settlement
% Site of channel cross profile
z=zz -------
Major
road
A!q
Minor
road
-++t++
e .b
Dams
and
Site of low Railway
terrace
gully
line EAOlSOT
Reservairs Figure 1. The location
study
of the study area.
323
324
Effects of the Tiga Dam on valleyside erosion
There are three main temperature seasons. A cool and dry season lasts from November to February, during which the mean monthly temperature is between 21 and 23°C with a diurnal range of 12-14°C. The Harmattan winds prevail at this time. This period is usually followed by a hot and dry season which lasts from March to mid-May. The mean monthly temperature during this period is in excess of 30°C and the daily range is up to 20°C. This is followed by the wet season which is warm, with mean monthly temperatures about 26°C and a diurnal range of about lo”C, rising to 13°C in September. Evapotranspiration is generally very high throughout the year since potential evapotranspiration is not less than 120 mm for any month. Rainfall may exceed potential evapotranspiration only in four months (June to September), but in dry years a positive water balance may occur only in July and August. The mean annual potential evapotranspiration is in excess of 1800 mm. Geology
and landforms
The study area is underlain by the rocks of the Basement Complex which consist of Older Granites and old metamorphic rocks of various descriptions, The ancient rocks have been overlain in many areas by moderately thick regolith derived from them and by a top layer of windblown material (wind drift) up to 2 m thick. Outcrops of the ancient rocks are common, but are irregularly distributed. Apart from these rock outcrops, a typical valleyside slope profile shows the following landform units: an upland plain, a higher terrace, a low terrace and the present flat-bedded river channel. Each of these units is separated from the next by a scarp about 2-3 m high, densely gullied and steeper than 60 degrees in slope angle. The main units themselves are gently inclined at about l-2 degrees, except at eroded zones where slope angles in the range of 34 degrees are encountered. It is the occurrence of the scarps that encourages rapid rainfall runoff and increases rates of soil erosion on the valleyside slopes. Gully erosion is particularly active on these scarps. Attention here is focused on gully erosion on the low terrace (valley bottom), for this part of the slope is most responsive to changes of flow in the main channel. It should be noted also that gully erosion provides a more measurable index of erosion than other forms of fluvial erosion. Dam construction
in the study area
The construction of dams in Kano State started in 1969 and reached the study area in 1970, when the Bagauda dam was initiated on a tributary of the River Kano. Between 1970 and 1974 three dams were constructed in the study area: two in the Kano basin (Bagauda, 197(&71 and Tiga, 1971-74) and one in the Chalawa basin (Karaye, 1972). The dam at Karaye, on a tributary of the Chalawa, controls a stream with a mean annual discharge of only 0.45 m’s_’ and accounts for only 2.1 per cent of the normal flow of the River Chalawa. The Bagauda, on a tributary of the River Kano, controls a stream with a mean annual discharge of 1.25 m3 ss’ which is about 3.5 per cent of the normal flow of the River Kano. The Tiga dam was the only major dam operating in the study area at the time of this study. Its construction was completed early in 1974 and impoundment started in that year’s wet season. The dam controls a mean annual discharge of approximately 3358 m3 for about 91 per cent of the mean annual flow of the river. It is s-‘, accounting located directly on the river and has an optimum storage capacity of 1.97 x 10” m3
E. A. Olofin
325
and a reservoir surface area, at this capacity, of 178.1 km2 (Olofin 1980b). Statistical analyses of stage and discharge have shown that both the Bagauda and Karaye dams have had negligible effects on the hydrology of their relevant basins, but the effects of the Tiga dam on the hydrology of the Kano river basin have been great, as is shown below. The effects of the Tip Dam on basin hydrology
The analyses of stage and discharge of the River Kano, downstream of the Tiga dam, have shown that the dam and its reservoir have caused a lower peak discharge, a lower peak stage and a longer duration of flow in these reaches. For example, the mean annual discharge in the pre-Tiga period stood at 37 m3 s-l in the Chiromawa gauging station (KS in Fig. l), but this decreased to 9-6 m3 s-’ in the post-Tiga period. The net increase in evapotranspiration as a direct result of the Tiga Reservoir has been computed (Olofin 1980b) to be about 7.6 m3 s-‘. This net loss to evapotranspiration should be seen as a part of the total effect of the dam and its reservoir on the environment of the study area. Thus in terms of total volume per water-year, an annual discharge of 1.17 x 10” m3 at the pre-Tiga time decreased to only 0.30 x lo9 m3 in the post-Tiga period. The relationship can be stated thus:
Qm+I = 0.26 Qm,
(1)
where t = period before the Tiga dam t+l = period after the Tiga dam Qm = mean discharge in cumecs The wet season mean discharge also decreased from 86 m3 s-l at time t to 13 m3 s -r at time t+l, but the dry season mean discharge over the same time span increased from about O-5 m3 s-’ to approximately 7.0 m3 s-‘. Thus the flow regime was changed from a seasonal to a perennial type. The peak stage in the wet season has also decreased very significantly by more than one metre, particularly in July and August, while the dry season stage increased by up to 60 cm. In contrast to these changes in the flow regime and quantity of discharge of the Kano River, the analyses of the discharge and stage of the Chalawa River, over the same period of study (1965-70 and 1974-79), have shown that the discharge and stage of the Chalawa River did not change significantly from the normal. Methodology
The aim of the research was to compare the pre-Tiga magnitudes of the investigated variables with those of the post-Tiga period. For this purpose, a combination of field, laboratory and statistical techniques was used to study twelve low terrace gullies along the Kano River channel (A, B, C, . . . L in Fig. 1) and four similar gullies along the Chalawa River channel (A, B, C, D). Existing records, literature and air photographs were utilized to estimate the magnitudes of the selected variables, in addition to actual field measurements, observations and interviews. The main focus was on the depth, shape, gully-mouth condition and gully-floor material which were believed to provide clear indices of erosion or deposition in various combinations. Field measurements and observations were made twice during each dry season,
326
Effects
of the Tiga Dam
on valleyside
erosion
E.A. Olofin
327
at the beginning and at the end of the season. A study visit was paid to each accessible site during each wet season from 1976 to 1979. In addition, pre-Tiga gully characteristics were traced back 12 years, principally through a set of air photographs taken in December 1963. Another set flown in December 1978 provided adequate post-Tiga reference. The 1963 set was enlarged to a scale of 15000 while the 1978 set was actually taken at that scale. The results of the various analyses undertaken on the data generated through the investigations are presented below. The effects of the Tiga Dam on valley-bottom gullies Typical savanna valley-bottom gullies
An analysis of the available data on the characteristics of valley-bottom (low terrace) gullies along the Kano River channel in pre-Tiga time shows that the pre-Tiga valley-bottom gullies along the river channel were typical savanna ones. The characteristics of a typical savanna valley-bottom gully are illustrated in Fig. 2 and Table 1. These characteristics are believed to represent a process-response system in which the inputs of water and debris from upper slopes and gully sides are approximately the same as the outputs of water and debris into the main channel. Such a condition, where the valley-bottom gully acts merely as a conduit for the cascading system without measurable modifications to its characteristics, represents an equilibrium state within a savanna environment. However, the equilibrium state is limited to the valley-bottom section of a valleyside gully. Table 1. Mean
GLllly site A
B C D Mean s.d.
depths of typical savanna v~llley-bottom River basin, 1979
Distance from confluence (km) 14.00 13.00 8#0 4*OO
gullies. Chaiawa
Mean depth of whole gully (metres)
Mean depth of gully mouth (metres)
Height of gully mouth above river bed (metres)
1.06 0.70 1.06 1.16
1.19 1.05 0.94 1.45
l-00 1.35 1.05 0.95
1.00 0.18
1.16 o-19
1.09 O-16
Note: ~ulIy-in-gully morphology is absent; there is thick sandy alluvium on the gully mouth and gully floor. Source: Olofin (1980h)
From these illustrations (and the data on the first-phase gullies in Table 2), it is clear that before the Tiga dam and reservoir came into operation the valley-bottom gullies along both channels were first-phase, single-cycle types which were about 3-4 m wide and l-2-1-3 m deep. The gully floors were flat and consisted of an accumulation of sandy alluvium 40-60 cm thick, while the gully mouths were higher than the beds of the river channels into which they discharged by up to one metre. The entire gully unit (floor and wall) was without vegetation cover, but this bare surface did not result in measurabIe changes in the mean dimensions of valley-bottom gullies in the study area, as shown by studies of the Chalawa gullies.
During the pre-Tiga period, it is believed that the peak height of flow (HF) in the River Kano channel was slightly higher than the level of gully floor at the time of gully-mouth alluviation. Gully discharge, baseflow from soil moisture and seepage from groundwater must have responded, at least in part, to the height of Row in the main channel. Field evidence has shown that the valley-bottom gullies along the Chalawa channel maintained these typical characteristics of a savanna valley-bottom gully throughout the time of study. Some of these characteristics are shown in Table 1. The valley-bottom gullies along the Chalawa channel maintained these characteristics because no major dam existed in the Chalawa basin at the time. It is interesting to note that since then these gullies have undergone significant changes in depth and shape because of the numerous dams that have been constructed in the basin, including a major one, scheduled to be completed in 1984, on the Chalawa River itself. By May 1983 the valley-bottom gullies along the Chalawa channel had reached the phase of maximum incision shown in Fig. 3 and Table 2 (whole gully). When the major dam (the Chalawa Gorge) is completed and operated as the Tiga, these gullies along the Chaiawa channel will become similar to the post-T&a ones along the Kano channel described below.
The mean dimensions and other characteristics of the valley-bottom gullies along the Kano channel in post-Tiga period are illustrated in Fig. 3 and Table 2. These characteristics are believed to represent a control system at the point of optimum response to a negative change in the height of flow in the main channel. In this study they represent the point, roughly January 1978, shortly before the establishment of vegetation at the gully mouths. Field observations have shown that initial and rapid incision followed the lowering of the stage in the Kano River channel, increasing the depth of gully mouth from l-4 m to 2.02 m and the shape from single-cycle (1) to dual-cycle (2), while the top width (W) remained unchanged at 3.5 m. Gully water and debris discharge changed temporarily from the equilibrium state (it), described earlier. to a definite increase (+) condition due to additional inputs through activated erosion and release of part of the water which pre-existed as, or would have replenished, the groundwater storage at a higher water table. As these data show, the post-Tiga valley-bottom gullies have generally lost most of their pre-Tiga, typical savanna characteristics. They have become two-phased in shape with terraces standing at the levet of the former gully floor; their mouths have been incised to the level of the now aggraded bed of the main channel, increasing their mean depth from about 1.3 to 2 m, and the thick sandy alluvium on the gully floor has been removed. The mean incision of ail the gullies in the reactivated reaches is about 54 cm while the mean gully mouth incision is about 64 cm deep. In some cases the gully mouths have been extended over the aggraded bed of the Kano channel to reach the new effective channel. Such extensions are cut into the bed to depths ranging from 20 to 30 cm. During the 1977-78 dry season, however, vegetati~~n started to invade the gully mouths from sections of the former bed no longer utilized by the Kane River discharge. Lateral erosion removed most of the gully terraces in the following wet season, and by May 1979 the valley-bottom gullies along the Kano channel had achieved a kind of new dynamic equilibrium with thick, stabilizing, covers of grasses and small shrubs, as well as creepers and reeds.
E.A. Olofin Table 2. Mean depths
Gully site’
of post-Tiga
Distance from confluence’ (in km)
gullies along the Kano River
valley-bottom channel, 1979
Mean depth of whole gully (metres)
Mean depth of lst-phase gully’ (metres)
Mean depth of 2nd-phase gullyd (metres)
0.82 2.00
0.33 0.49
The entire reactivated section of the gullies A’ 59.97 1.15 B 58.40 2.47 C D
46.00 42.00
2.45 2.87
1.90 1.68
0.50 1.00
E F
38.50 38.00
1.74 1.77
1.30 1.19
0.49 0.58
G H
24.00 15.00
2.20 1.50
1.65 1.05
0.49 0.46
I J
13.00 7.00
1.18 1.53
0.61 1.00
0.57 0.51
K L
4.00 3.00
1.54 1.50
1.03 1.03
0.52 0.48
Mean s.d.
1.83 0.49
1.27 0.42
0.54 0.15
The gully mouth A’ 59.97 B 58.40
1.37 2.53
1.02 2.00
0.35 0.53
C D
46.00 42.00
2.45 2.76
1.80 1.71
0.65 1.05
E F
38.50 38.00
2.08 2.20
1.40 1.55
0.68 0.65
G H
24.00 15.00
2.21 1.77
1.65 1.15
0.56 0.62
I J
13.00 7.00
1.71 1.75
1.11 1.18
0.60 0.57
K L
4.00 3.00
1.67 1.70
1.02 1.15
0.59 0.55
2.02 0.40
1.40 0.32
0.64 0.12
Mean s.d.
329
“The first of each pair is from the right bank, the other is from the left *The Tiga dam is 60 km from the Kano-Chalawa confluence ‘M-phase = first-phase section, or pre-Tiga mean depth “2nd-phase = second-phase section, or post-Tiga incision ‘There is a strong structural control here, being rocky Source: Olofin (1980b)
The depths of gullies at time t (pre-Tiga) and at time t-t1 (post-Tiga) were statistically examined for significant differences, using the Student t-test. First, the first-phase gullies along the Kano channel were compared with the gullies along the Chalawa channel. Next the depth of the whole (first- plus second-phase) gullies along the Kano was compared with that of the gullies along the Chalawa.
0
HR q Average W = Average
of gully of gully
Figure 3. The structure
depth width
CASCADE COMPONENTS -_D Q=Discharge &Bed load Qo=Run off M= Suspended Load REGULATINg OR CONTROLLING >A = Exceedrng angle of repose? >F*= Exceeding Hg = Height of Gully mouth above channel flow HF q Height of channel flow GE = Gullv erosion MORPHOLOGICAL ’ 0 $I ; ;;&;ob;turrly
UPPER SLOPE GULLY SYSTEM
+_ = Balance
of a post-Tip
I
of
I
Debris Water
E:
gully along the Kano River channel.
change
I
SYSTEM
Condition
GULLY
valley-bottom
+, - = Direction
I
rYES
LOW TERRACE
STREAM
CHANNEL
SYSTEM
E. A. Olofin
331
Comparisons were made initially using all the 12 gullies investigated along the Kano channel, and then repeated using only the five Kano sites nearest to the Chalawa samples. The results of these comparisons are shown in Table 3. These results show that the first-cycle (pre-Tiga) gullies along the Kano channel were identical in depth with the gullies along the Chalawa channel at the time of study. None of the Student t-values shows a significant difference, even at as low a probability as 0.7. However, the mean depth of the whole Kano gullies (both the five selected ones and all the 12 studied) are shown to be significantly different from the Chalawa gullies at probabilities of 0.01 and 0401 respectively (see D2 and t2 in Table 3). Table 3. Statistical
analyses
of the changes
Mean gully depth along the Chalawa (metres) s.d. x
in valley-bottom
Mean depth of lst-phase gully along the Kano (metres) ic s.d.
gullies in the study area (1979)
Mean whole gully depth along the Kano (metres) x s.d.
Differences Student D,’ DZh t (mean) values (m) (m) tI t2
Five selected Kano gullies and four Chalawa gullies’ Entire gully I.00 0.18 0.95 0.16 Gully mouth 1.16 0.19 1.12 0.06
1.45 1.72
0.14 0.04
0.05 0.04
0.45 0.56
0.39 0.36
All lwelve Kano gullies and four Chalawa gullies’ Entire gully 1.00 0.18 1.27 0.42 Gully mouth 1.00 0.19 1.40 0.32
1.83 2.02
0.49 0.41
0.27 0.24
0.83 0.84
1.65 164
“Difference ‘Difference ‘Degrees of “Differences ‘Degrees of ‘Differences
between mean depth of Chalawa between mean depth of Chalawa freedom = 7 significant at 0.01 level freedom = 14 significant at 0.001 level
gully and mean depth gully and mean depth
360d 5.05d 4.59f 5.15’
of Kano lst-phase gully (=t,) of whole Kano gully ( =t2)
The cause of the significant changes in the valley-bottom gullies along the Kano River has been traced to the hydrological changes in the basin consequent on the construction of the Tiga dam and reservoir. Thus the changes in the characteristics of the gullies reflect some of the effects of the Tiga dam and its reservoir on valleyside fluvial processes in the study area. As stated earlier, the changes occurred very rapidly. Active erosion lasted for about three wet seasons after the Tiga dam was established, the first wet season being the most active. This type of reaction means that the lowering of the base level (control input) almost immediately escalated the erosion of the valleyside slopes (output), achieving what may be classed as episodic erosion. The rapidity of adjustments in erosional processes to management control is not peculiar to this study. For example, Dvorak and Heinemann (1967) have reported that more than 98 per cent of the sediments eroded from a gully in Nebraska, USA, between April 1951 and April 1956 occurred during the first year. One implication of such rapid adjustments is that such activities could be missed entirely if studies were not undertaken within the first or second year of their initiation. Conclusion One conclusion to be drawn from this study is that major dams and reservoirs built on seasonal streams in a savanna environment will generate changes in the
332
Effects of the Tiga Dam on valleyside erosion
components of the process-response systems in downstream reaches. One of the process-response systems that would be affected is the valleyside gully which responds to the consequent lowering of the height of flow in the river channel. The erosional phase is, however, very short and intense. It is relevant to state here that apart from the reactivation of valley-bottom gullies, other forms of fluvial processes must have responded in similar fashion. Indeed, the responses would have been transmitted up through the system because gullies exert a major control over many other processes operating on the valleyside slopes. Finally, where a perennial flow is maintained in the river channel, a dynamic equilibrium state is soon achieved with a substantial vegetation cover, which helps to stabilize the gullies and other valleyside fluvial processes. Nonetheless, this dynamic equilibrium state is quite different from the normal one which could exist under an undisturbed seasonal regime. Acknowledgements I wish to thank Mr S. 0. Taiwo of the Department University, Kano, for copying the figures of this paper Olofin (1980b).
of Geography, from the original
Bayer0 ones in
References Chorley, R. J. and Kennedy, B. A. (1971) Physicalgeography: a systems approach. London: Prentice-Hall. Dvorak, V. 1. and Heinemann, H. C. (1967) Cooperative runoff and sediment investigations on Medicine Creek Watershed in Nebraska. US Department of Agriculture. Report, 41-130. Leow, K. S. and Ologe, K. 0. (1981) Rates of soil wash under a Savanna climate, Zaria, Northern Nigeria. In Summaries of papers (Kano, 24th Annual Conference of the Nigerian Geographical Association), 18-22. Mrowka, J. P. (1974) Man’s impact on stream regimen quality. In Perspectives on environment (I. R. Manners and M. W. Mikesell, eds), pp. 79-104. Association of American Geographers, Commission on College Geography, Publication No. 13. Olofin, E. A. (1980a) The effects of the Tiga Dam and reservoir on the flow and channel characteristics of River Kano in downstream areas, Kano State Nigeria. Paper presented at the Pre-Congress Symposium of the IHP-IGU, Tsukuba, Japan. Summary contained in Abstracts for the Symposium, 6-7. Olofin, E. A. (1980b) Some effects of the T&a Dam on the environment downstream in the Kuno River basin. Unpublished PhD thesis, Ahmadu Bello University, Zaria. Piest, R. F., Bradford, J. M. and Wyatt, G. (1975) Soil erosion and sediment transport from gullies. Journal of Hydraulics Division, American Society of Civil Engineers 101. 65-80. Schumm, S. A. (1977) The fluvial system. New York: Wiley. (Revised manuscript received 16 January iY84)