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5.1. HYDROLOGY
Report No 678-F-001
METSI CONSULTANTS: SUMMARY OF MAIN FINDINGS FOR PHASE 1 DEVELOPMENT
SECTION 5. MINIMUM DEGRADATION SCENARIO
The Minimum Degradation IFRs are designed to minimise future degradation of the downstream river reaches or,
for the reach immediately downstream of the existing Katse Dam, to indicate what minimum degradation could
have been like. They represent the hypothetical “best case” situation with dams in place as it is assumed that
dams do not limit the IFRs. For IFR Site 2 - downstream of Katse Dam - this scenario used the pre-Katse
hydrological data.
For all parts of the affected rivers, the modified flow regimes encompassed two kinds of reduction - in the range of
low flows and in the number of floods. For instance, in the wet season, the range of low flows at IFR Site 1 could
be reduced from 0.02-6.75 m 3 s -1 to 0.02-6.00 m 3 s -1 , and at IFR Site 6 from 1.69-434.00 m 3 s -1 to
1.69-224.00 m 3 s -1 . Similarly the dry season low flow ranges at IFR Site 1 could be reduced from 0.00-4.03 m 3 s -1
to 0.00-4.00 m 3 s -1 , and at IFR Site 6 from 0.90-120.00 m 3 s -1 to 0.90-70.00 m 3 s -1 . Thus, it is indicated that
proportionally more low flow could be lost from the downstream sites than from the upstream sites without serious
impacts to river ecosystems.
With respect to floods, the main losses allowed are a reduction in the number of the smallest (Class 1) within-year
floods and, at some sites, the largest (Class 4) within-year floods, since these are deemed to have very similar
functioning to the Class 2 and 3 floods, respectively. Thus, the number of Class 1 floods at IFR Site 3 is reduced
from seven to five per annum, and at IFR Site 5 from four to two per annum. The number of Class 4 within-year
floods is reduced from two to one per annum at IFR Site 1, but no reduction of the natural flood situation is
imposed at IFR Site 4. The remaining within-year floods at each site are distributed proportionally according to
their natural occurrence. The 1:5 year flood is omitted for all sites on the assumption that the other major floods
(1:2, 1:10, 1:20 year) would maintain most of the same flood functioning.
For each site, the volumes encompassed in the new flow regime are estimated and compared to the present
Mean Annual Runoff (MAR). In total, the modified flow regimes comprised 55-67% of the present-day MAR
(Table 5.1). Summary values in Table 5.1 are provided for comparison purposes only. If actually applied, the
releases from the dams would have to be based on the capping levels for low flows and the flood volumes
provided in the detailed biophysical scenario report (No 648-F-04).
5.2. BIOPHYSICAL CONSEQUENCES
The reduced flow regimes would be expected to result in subtle shifts in river condition. A negligible (0-10%) or
low (10-20%) increase in the proportion of mud in the rivers, and in its deposition at river margins and in
backwaters, is predicted. A concomitant increase in the growth of slippery biofilms on underwater surfaces is also
expected. These changes are anticipated to be most obvious (20-40%) in the reaches downstream of Katse Dam
(IFR Reach 2). The lower Senqu River (IFR Reach 6) and lower Senqunyane River (IFR Reach 8) would be least
affected (0-5% change).
At the same levels of change, there would be a decrease in the movement of sand, gravel, cobbles and boulders
along the rivers. Over short periods more sand might accumulate in flow-sensitive cobble riffles and in pools and
thereby reduce their depths but, providing catchment erosion does not increase, larger floods would periodically
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