A Review of Hydrological Aspects of the Proposed Epupa Dam and Reservoir, Cunene River, Namibia
From a hydrologist’s point of view, the Feasibility Study of proposed
hydroelectric power dams on the Cunene River has some serious
deficiencies. In order of importance, they are: 1) The study is
organised so as to be virtually inaccessible to even a careful
reader. Separate pieces of the same subject matter are scattered
in illogical places throughout the voluminous corpus of the study.
2) Flow data, and estimations in the absence of data, are of low
reliability. The entire hydrological analysis is based on the
premise that a meagre 12 year streamflow record from a location
200 km upstream of the dam site can be linked with a longer record
from another river basin to synthesise a reliable theoretical
streamflow data base with which to assess the viability of a 1.9
billion dollar hydropower project. This proposition is shaky at
best. 3) The analysis of the effects of changes in the river’s
sediment regime as a result of building the dam are incomplete
and ignore the most important issues, such as increased degradation
downstream and upset of the sediment regime in the delta. 4) The
study lacks a definitive appraisal of the thermal and nutrient
regime in the reservoir and river. The potential of drastic changes
in the temperature, oxygen and nutrient regimes of the reservoir
and lower river to upset existing ecological and cultural relationships
have been naively understated. These points are elaborated below.
The conclusion is that the Feasibility Study is not a sound basis
for evaluating either the costs or the benefits of the project,
nor for proceeding with major financial or other irreversible
commitments to the project.
2. Basis of Review
This review is based on the electronic version of the Feasibility
Study, which was posted on the Burmeister Partners web site. This
version of the Feasibility Study is incomplete in several respects,
the most important of which is the missing figures from the hydrology
chapters (Baynes Main Report, Chapter 7; Epupa Main Report, Chapter
7) and the reservoir operation (Epupa Main Report, Chapter 13).
Appendices with supporting data and references are also missing.
The hydrology sections are necessarily quantitative, and much
of the analysis is best portrayed graphically. If the figures
are not available, this severely limits the scope of review that
can be carried out on the study.
I have concentrated my attention on the hydrology and sediment,
and have referred to the chapters on reservoir and dam engineering
design and construction. The chapters on the hydrology of the
Epupa dam site and the Baynes Mountain site appear to be virtually
identical with the exception of the names and physical characteristics
of the sites. I have not read the entire electronic version of
the Feasibility Study.
3. Organisation
The Feasibility Study was not written to assist the reader in
understanding the project and its impacts. The organisation of
the study is convoluted and redundant. The duplicate chapters
on Baynes and Epupa sites with nothing but the names changed is
an example. Material relating to hydrology and reservoir management
is not presented or even referred to in the appropriate sections,
but is referenced in general sections elsewhere. A reservoir stratification
model was considered (Chapter 2, Epupa Summary Report), but it
was not mentioned in the report section on reservoir operation
(Epupa Main Report, Chapter 13). Hydrologic simulations of downstream
effects on the river were included with the economic simulations,
but were not mentioned in the chapters on hydrology. This lack
of coherent organisation makes a technical appreciation of the
project nearly impossible.
4. Flow Data and Estimations (Chapter 7)
The central proposition of the hydrologic analysis is as follows:
There is no flow record at either proposed dam site (Epupa or
Baynes). There is only a short record at an upstream site (Ruacana),
that we hope will tell us something about the dam sites. There
is a long record from one site on another entirely different river
basin (Rundu). So we can synthesise an artificial hydrological
picture for Ruacana based on the twelve years for which Rundu
and Ruacana have an overlapping record, and then translate that
picture 200 km downstream to the proposed damsites. The correlation
between the two gauges on the Cunene and Okavango Rivers depends
on the assumption that the two watersheds are hydrologically similar.
There is no detailed description of those two watersheds that
would convince us of this similarity. The whole integrity of the
analysis depends on our confidence in the correlations among all
the sites. While the consultant should be commended for developing
a creative approach to the problem of inadequate data, confidence
in the synthetic streamflow record as a basis for evaluating the
Epupa hydropower project is not warranted. The limitations of
the data have been conveniently forgotten: Chapter 11 says, "In
this simulation, the last 49 years of stream flow for the Cunene
river are used." 49 years of stream flow for the Cunene river
do not exist. Only 12 years of stream flow for the Cunene exist,
and the rest are based on an assumed similarity to a distant location
on the Okavango. The simulation in question is the core of all
the predictions of economic behaviour of the Baynes and Epupa
schemes.
The analysis also assumes that future behaviour will be the same
as the past. There is an explicit assumption of "stationarity"
in the record; i.e. that there is no long-term trend in the data,
and that past variability of hydrologic behaviour is considered
a reliable guide to future variability. The data series being
used does not take account of recent extreme low runoff conditions,
nor does it take into account global warming possibilities. The
essence of global warming is a long-term upward trend in temperature
data, which would drive trends in other characteristics such as
evaporation, rainfall, and runoff. The assumption of a stationary
record, in the face of evidence to the contrary, is a risky basis
for a very long-lived and highly capital intensive investment
that depends on streamflow.
The feasibility study has a prolonged discussion (Section 7.5)
of speed gauging data. What this refers to is the point velocity
measurements that are supposedly taken on a regular basis, in
order to establish the relationship between depth of water, i.e.
of gauge height, and volume of discharge; which must be calculated.
This relationship is depicted graphically in a rating curve that
is unique to the gauging site and the watershed’s hydrologic response.
Various manipulations were carried out on the rating curves at
Rundu on the Okavango River and at Ruacana on the Cunene River.
These rating curves are not available for inspection, the explanation
is not clear, and the work is not shown. This makes it impossible
to evaluate the hydrologic conclusions. It appears that certain
river discharge measurements were rejected because they "were
clearly identified as outliers" and because of "documentation
discrepancies." The study does not say which measurements
were rejected, or how many, or how many were left as a basis for
the correlation. The shape of a rating curve is extremely sensitive
to the speed gaugings at both high and low flows. It is curious
that one would hand-pick one set of data that is considered reliable
and then say that the correlation with another set of data is
good. Would it get better if more data were thrown out? There
must have been a problem with low velocity discharge measurements,
where "current meters are at their limit of applicability"
(7.5.1.2). There is no reason why a standard current meter should
not function acceptably at velocities well below 0.1 m/sec., which
would give adequate discharge measurements in suitable river cross
sections.
The study says "all authors agree" on the good correlation
between Rundu and Ruacana, yet the seasonal lack of correlation
between the two is glossed over (Section 7.4.4). The study allows
for possible error in the rating curve by introducing two error
bands, the lower of which is 15% below the best fit rating. Yet
they use the best fit rating curve instead of the more conservative
"minimum" or "worst case" ratings as a basis
for the estimate of mean annual runoff. Given the weaknesses of
the data, the estimate based on best fit is not entitled to great
confidence. The difference could have severe impacts on the economic
performance of the hydropower schemes.
The analysis (7.6.1) treats the data as though there is no serial
correlation between annual runoff volumes, in other words the
study assumes dry years do not necessarily follow other dry years.
Elsewhere, the study says (7.9.3) that "dry periods are considerably
longer than one year." It says (11.6.5) that Baynes could
run dry at the end of the first of a series of dry years. A conclusion
of no annual serial correlation is also at odds with the findings
of other investigators (Kerr, 1985), and with emerging understandings
of cyclical El Niño - Southern Oscillation effects (Philander,
1989). It is a generous assumption that low flow years do not
occur consecutively. If in fact they do occur back to back, then
the estimates of the duration of a drought or the runoff produced
during a drought would be over-optimistic and the economic performance
of the hydroelectric scheme could be considerably worse than assumed.
Evaporation data for the Epupa site was used directly for the
Baynes site, which is about 35 km farther west and 180 m lower
in elevation. The evaporation at this site would be higher, but
the magnitude is not known. Evaporation rates higher than estimated
would degrade the economic performance of the reservoir scheme.
5. Sediment (Chapter 8)
Discussion in the Feasibility Study: The scope of the sediment
discussion appears to be confined to a very narrow set of questions
to be examined, enumerated in the beginning of Chapter 8: "the
long term effect on reservoir storage volume . . . the likely
deposition pattern . . . possible effects on the intake structure
to the powerhouse." These are important questions, but they
are hardly a complete treatment of the issue. The study could
reasonably be expected to offer a reach-specific discussion of
the characteristics of the bed load of the river above and below
the proposed reservoirs. Section 8.4.1 outlines an approach that
includes "landscape analysis and the application of sediment
yield figures based on empirical data from similar environments,"
yet the analysis does not make use of experiences with sediment
erosion and deposition at other reservoirs. While observing that
the "sediment data is insufficient for a conventional analysis,"
i.e. frequently sampled data over five years, the study ignores
river characteristics that would be very useful and would not
take much time or money to find out: reach gradient, channel confinement,
sediment storage volume estimates, sediment particle size distribution,
lithology and weathering characteristics -- in short, a geomorphologic
analysis of the river. While observing that the river has several
gradient and sediment regimes over its length, the generalised
description is never interrupted by an application of these differences
to the two proposed dam sites. The sediment analysis is based
on "weak data . . . no measurements" (Section 8.5).
No measurements of bed load have been made. Nonetheless, the study
says "the reach between Ruacana and Epupa shows clear evidence
of substantial bed load transport," and "large quantities
of sediment in channel storage, especially in the ephemeral tributary
streams." This sediment is now in an equilibrium with the
rest of the river; the study does not acknowledge that the equilibrium
will be upset by dam construction.
The sediment chapter has an unwarranted preoccupation with sediment
rating curves, which even the Feasibility Study admits are of
little use in the Cunene situation. As an illustration, Leopold
et al. (1964) show a sediment rating curve for the Rio Grande
River in New Mexico that exhibits two to three orders of magnitude
of variability in the sediment transport rate for the same flow.
Authority for the "normally used sediment discharge rating
curves" (section 8.5.1) is lacking. No details are offered
of the techniques used for the described sediment samples; sediment
sample results are highly sensitive to method (Leopold et al.,
1964).
The Ruacana reservoir sediment survey (section 8.7) is inconclusive.
The study refers to modelling effort applied to the case, but
the model and its inputs are not described. Apparently this modelling
effort was never finished, because the study points out the necessity
of completing it (Section 8.9).This section has several unsupported
assertions about the sediment at Ruacana: "the impact [of
extreme events] is generally balanced out within less than 10
years" and "it is found that . . . after 10 years the
accumulated volume provides a reliable measure of the long term
average load, irrespective of hydrological patterns during the
period."
The loss of capacity due to sedimentation at Baynes or Epupa is
probably, as the study concludes, not likely to be of major short-term
consequence in reservoirs with storage capacities that are a large
multiple of the likely sediment accretion rate. The sediment analysis
is not a strong one, however. A paraphrase of the analysis would
be "We don’t know what the sediment yield would be, all we
can do is make a couple of educated guesses. But even if the sediment
yield is twice as much as our biggest guess, the reservoir will
still hold it."
Sediment effects not considered in the Feasibility Study: The
bed load will be interrupted by the reservoir; there will be no
bed load moving beyond the still water at the head of the pool,
and none moving through the reservoir to the downstream reaches.
Energy now utilised in transporting upstream sediment will be
available to move other sediment, in unforeseen and possibly undesirable
ways. This difference in the dynamics of the river can produce
far-reaching effects downstream. The dams will impose a complete
change on every physical characteristic of the river. Each perturbation
of one characteristic will have a reaction on the other characteristics.
The likely direction and magnitude of these adaptations is an
essential piece of the analysis. No discussion is offered at all
on the downstream effects of sediment trapping. This can consist
of bed armouring, channel bed and bank scouring, destabilisation
of existing gravel bars, desiccation of riparian habitat, and
perturbation of the sediment budget of the estuary at the mouth
of the river. The riparian vegetation, the dynamics of the estuary,
the littoral drift to the north of the river mouth, the lagoon,
and the biological systems that depend on them are delicately
adjusted to the accretion of sediment in the river. This sediment
supply will be interrupted permanently. Some combination of bed
erosion and bank erosion downstream of the reservoir can be expected
to compensate for this loss of sediment supply. The stream reaches
in question are high gradient reaches with relatively high velocities.
At velocities of 4 to 5 km/h (Simmons et al., 1993), this clear
water will have considerable erosive power.
Sediment problems other than loss of reservoir storage capacity
are well known to the dam building industry (Hu, 1995). These
problems have been known to have drastic effects on the operation
and economics of reservoirs. For numerous projects, downstream
starvation of river sediment load threatens to exceed reservoir
siltation as a cost of dam construction (Kazmann, 1972). The Feasibility
Study would be strengthened by a discussion of relevant African
cases, and an explanation of why the Cunene River should be considered
exempt from these kinds of problems.
6. Temperature
It is virtually certain that either the Baynes or Epupa reservoirs
would develop a stable thermal stratification early in the life
of the reservoir. Although the information in the study is inconsistent,
it appears that Epupa is planned for a maximum depth of 157 M
and volume of 11.5 billion M3, and Baynes is planned for 200 M
depth and volume of 2.6 billion M3. The stratification phenomenon
does not draw comment in the reservoir operation chapter (13)
of the Feasibility Study, although references to it are scattered
in other sections. A stratification model is mentioned (2.5.1)
but neither inputs nor outputs to the model are supplied.
Stratification means that the water body segregates into two distinct
layers of water that rarely if ever mix. These layers are different
in temperature, oxygenation, nutrients, and density. In deep tropical
water bodies, a small temperature difference between surface and
bottom can be sufficient to maintain a stable stratification (Hutchinson,
1957). An initially weak thermal stratification can become stronger
because of differences in chemical constituents. It is known that
the temperatures at the mouth of the free-flowing Cunene River
are warm, ranging from about 19 C. to 26 C. (Simmons et al., 1993).
The dam design makes allowance for variable depth releases, so
that the chemistry and
temperature of release water can be managed (20.2.2). What the
Cunene reservoirs will be like is unknown; but typically, reservoir
releases from the epilimnion (upper layer) tend to be warm, partially
oxygenated, and relatively lean in nutrients. Releases from the
hypolimnion (lower layer) tend to be colder, anoxic, rich in nutrients,
and rich in anaerobic decomposition products.. Releasing cold
water that may be completely devoid of oxygen will have far-reaching
effects on the downstream ecosystem. On the other hand, releases
from the upper layer could isolate the hypolimnion and cause its
water quality to degenerate as it accumulates nutrients and organic
matter. Even with variable depth releases, it will be impossible
to mimic the characteristics of the free-flowing river.
The Feasibility Study deals with these issues by saying, in essence,
"We do not know what will happen as a result of impounding
the river. We do not know what kind of aquatic habitat is there
now, nor what will become of it, nor what the needs of the surviving
species will be. But we will monitor it, and adapt mitigation
after the fact, including operational constraints, to suit the
situation." It is a severe defect of the proposals that no
one knows what the possible effects are; much less what the mitigation
measures will cost directly, nor how they would affect the economic
benefits of the project.
7. Reservoir dynamics
Damming the river will bring about dramatic changes in its sediment,
temperature, and nutrient economy. These changes will have equally
dramatic, but unknown, consequences on stream morphology and ecology
in downstream reaches of the river. The Feasibility Report makes
no acknowledgement of the complete change of state of the river
basin attendant on construction of a reservoir. It does assume
unrestricted operation of the reservoir, presumably to meet hydropower
objectives at all times (Section 13.3). The Baynes reservoir pool
level will apparently be highly variable: the standard deviation
of the synthesised annual flows is 45%, and the active reservoir
storage is 1.5 times the mean annual runoff. This raises doubts
that the proposed 50 m drawdown in the simulation chapter will
be observed. The study acknowledges the possibility of running
the reservoir empty (11.6.5).
Wetzel and Likens (1991) offer a well rounded but general description
of reservoir dynamics:
- Reservoirs receive runoff water mainly via large streams, which have high energy for erosion, large sediment-load carrying capacities, and extensive penetration of dissolved and particulate loads into the recipient lake water. Because the inflows are primarily channelized (along the prior river course before it was inundated) and often not intercepted by energy-dispersive and biologically active wetlands and littoral interface region, runoff inputs to reservoirs are larger, more directly coupled to precipitation events and extend much farther into the lake per se than is the case in most natural lakes. All of these properties result in high, but irregularly pulsed, nutrient and sediment loading to the recipient reservoir . . .
- Large areas of sediments are alternately inundated and exposed. These manipulations usually prevent the establishment of productive, stabilising wetland and littoral flora. Erosion and resuspension of floodplain sediments augment loadings from elsewhere in the drainage basin. Sediments may be shifted between aerobic and anaerobic conditions, which enhances nutrient release . . .
- High nutrient and organic matter loadings occur during the early "trophic surge" period following the damming of a river . . . The environmental conditions of reservoir ecosystems tend to have large, rapid, and erratic fluctuations. Often insufficient time exists for complete population growth and reproduction to occur before a succeeding major disturbance occurs, e.g., a plume intrusion and disturbance of stratification patterns following a major rainfall. These instabilities result in biota that tend to be few and well adapted with broad physiological tolerances (low diversity, less specialisation, rapid growth). As in all restrictive, stressed environments, the productivity of adapted organisms can be high, as high or greater than in more homeostatic natural lakes.
8. Conclusion
There is no reason to think that dams on the Cunene River will
be different from others in their effects or operational characteristics.
A complete understanding of hydrology and river morphology are
fundamental building-blocks for predicting the economic performance
and side-effects of a river basin development scheme. The Epupa
and Baynes Feasibility Reports available for review in December
1997 do not contain such an understanding.
Author’s Biographical information
Education: B.A., University of Washington, Seattle, Washington. M.S., Ph.D., Cornell University, Ithaca, N.Y.
Professional Affiliations: American Water Resources Association; American Geophysical Union .
Dr. Willing is Principal in the Bellingham firm of Water Resources Consulting, L.L.C. Since founding the firm in 1989, he has carried out a wide variety of assignments for public and private clients seeking to solve water-related technical questions. Examples are: hydroelectric system design, flood frequency analysis on North American rivers, wellhead protection, surface water - ground water interactions, storm water management strategy, and hydrologic basis of water rights. He has participated in technical reviews of African river basin development plans. In public sector positions, he has served as general manager of a public water supply system. He also served as chief environmental officer of a large municipal electric utility. Dr. Willing holds Adjunct Faculty appointments in Geology and Huxley College at Western Washington University, Bellingham.
References
Hu Chunhong, 1995. Controlling reservoir sedimentation in China. Hydropower and Dams, March 1995.
Hutchinson, G.E., 1975. A Treatise on Limnology. Wiley & Sons. 2nd ed.
Kazmann, R. 1972. Modern Hydrology. New York: Harper & Row. 2nd ed.
Kerr, R.A. 1985. Fifteen Years of African Drought. Science 227: 1453-1454.
Leopold, L.B., M.G. Wolman, and N. Miller. 1964. Fluvial Processes in Geomorphology. W.H. Freeman.
Philander, G., 1989. El Niino and La Nina. American Scientist 77(5): 451-459.
Simmons, R.E., R. Braby, S.J. Braby. 1993. Ecological studies of the Cunene River mouth: avifauna, herpetofauna, water quality, flow rates, geomorphology and implications of the Epupa Dam. Madoqua 18(2): 163-180.
U.S. Bureau of Reclamation, 1960. Design of Small Dams.
Wetzel, R.G. and G.E. Likens, (1991). Limnological Analyses. 2nd ed. Springer Verlag. 391 pp.
Comments by
Peter Willing, Ph.D.
1903 Broadway
Bellingham, Washington 98225 U.S.A
Tel: 360-734-1445 Fax: 360-676-1040
email: pwilling@telcomplus.com
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