Role of Sediments in the Lower Zambezi River System

Informative Paper

What is the role of sediments in the Lower Zambezi River system?

JA! (JUSTIÇA AMBIENTAL) G.Manez & L.Scodanibbio

1

Q.1: What are the general impacts that arise from the loss of sediments due to impoundments on a river course? Do we see any such effects on the Zambezi, arising from Kariba and/or Cahora Bassa dams?

Based on impacts attributed to sediment loss in “Sediment Mobilization” by the Global Programme of Action for the Protection of the Marine Environment from Land­Based Activities (http://www.fao.org/gpa/sediments/coaster1.htm)

Reduction in fertility: Downstream lands that were once replenished by fertile silts from upstream lose fertility, and organic matter in topsoil can be reduced, increasing erosion and sedimentation into the river.

In Zambezi: Downstream of Cahora Bassa, there has been a decrease in the makande soils (dark nutrient­enriched soils), both due to decrease in amount of sediments, but also due to decline in flooding. The problem of erosion downstream of Cahora Bassa is documented by Davies et al. (2000), who compare the river below Cahora Bassa to a canal, with most sediment along riverbanks and islands washed away.

Channel siltation: Due to reduced volume and rate of flow, sediments are not carried as far into the coastal zone as before. Instead they settle in slack areas of waterways and deltas, increasing the need for dredging.

In Zambezi: The evidence from the Zambezi is varied, as local people along the river complain that the river has become shallower and wider, thus exacerbating the effects of floods, and also contributing to the fact that the batelão de Caia is often stuck.

Other evidence, such as in the delta from Chupanga to Luabo, shows that the Zambezi is down-cutting into a single channel or few deep channels (Richard Beilfuss, pers. comm., 2/3/04). Also, many secondary channels have become isolated from the main channel through silting of entrance points (Davies et al., 2000) Riverbank erosion: Studies have shown that following dam construction there is 2 ­ 16 times more erosion than was originally estimated and immediately downstream sediment yields increase by 50% per decade, due to the decreasing stability of soils through the lack of replenishment of fertile soils.

In Zambezi: Following Cahora Bassa, morphological responses to the reductions in sediment loads and flows have varied according to the different river–floodplain zones. For example, due to enhanced flow capacities in the gorges, the majority of the river channel bars have eroded. The loss of these temporary sediment storage areas resulted in a ‘canal’ like system with marked reduction of in­channel habitat. (Davies et al., 2000). Also see comments under channel siltation above.

2

Repercussions on marine environment: Silts from upstream provide nutrients for marine plant growth, which, together with organic matter and detritus from land are the basis of many food webs, providing food for fish and shrimps, particularly in relatively nutrient poor seas. Dam construction has reduced this source of nutrition and has had deleterious effects on coastal fisheries as their stocks have undergone dramatic reduction.

In Zambezi: Shrimp fisheries have undergone a dramatic decline – from 90kg/hr in 1978 to 30kg/hr in 1990s – due to lack of flooding, but also likely to be due to reduction in nutrients (Hoguane, in press). Local people who fish inland complain that fish size has decreased because less nutrients are available to the fish, also associated to the lack of flooding (Beilfuss et al., 2002). The patterns in sea fisheries are difficult to establish, given the shortage of research and long­term data on catch per unit effort.

We could however expect a decline in fisheries dependent on fluvial nutrients and freshwater input.

Coastal erosion: Lack of sediment replenishment increases the vulnerability of the foreshore areas of delta to wave erosion, e.g. 250 metres p.a. in areas of the Nile Delta. As the front erodes it threatens areas of human settlement and constructions.

In Zambezi: There is contrasting evidence regarding the extent to which coastal erosion and mangrove die­off is occurring in the Zambezi delta, and how much of it can be attributed to Cahora Bassa and Kariba. Loss in mangrove forests ranges from 5% in some studies (Doddema, 2000) to 40% according to aerial surveys by P. Dutton, a natural environment consultant, as cited by Davies et al. (2000). There are also many areas of regeneration that should be considered (Richard Beilfuss, pers. comm., 2/3/04). Coastal erosion, with subsequent landward progression of coastal delta is also discussed by Davies et al. (2000), but the role of tropical storms should not be underestimated, and neither should the fact that coastal erosion was a problem even before impoundments on the Zambezi (Richard Beilfuss, pers. comm., 2/3/04).

Salinisation: Encroachment of seawater into coastal agricultural land, previously buffered by fresh water, increases soil salinity and decreases fertility. Losses to coastal agriculture of around 10% p.a. have been quoted.

In Zambezi: Physico­chemical changes in the Lower Zambezi, including increased salinity, as well as changes in the composition and productivity of algal and plankton communities are cited as having taken place by Beilfuss and Davies (1999).

Reservoir siltation: Sediment accumulation over time dramatically reduces the volume of water that can be stored in the reservoir, thereby reducing the dam's effectiveness 3

In Zambezi: Cahora Bassa’s shape is unusual, with many inlets, valleys and bays, thus the problem of sedimentation does not seem to affect its reservoir.

Solutions: 'Flushing' of sediments from the dam, but can result in a sudden flush of anoxic water and sediments with detrimental results as they smother remaining fertile soils downstream.

In Zambezi: Unlikely to be feasible in the case of Cahora Bassa, due to the reservoir shape, the expense involved, and the lack of a sediment gate

Q.2: How do these losses come about and what potential is there for Mphanda Nkuwa to exacerbate these and Cahora Bassa’s effects?

Based on Grant, E., Schmidt, J.C. & Lewis, S.L. 2003. A Geological Framework for Interpreting Downstream Effects of Dams on Rivers. (A Unique River Water Science and Application 7, by the American Geophysical Union)

Geology’s role: The geology, including both the physical properties of the underlying rocks and their structural features and tectonic deformation, interacts with climate to produce topography, including relief and the drainage network pattern. The geological properties of rocks, such as their composition, degree of weathering, and relative hardness interact with climate to determine both the grain size distribution and rate of supply of sediment to the stream system.

In Zambezi: The area is mountainous. The geology of the reservoir area between Mepanda Uncua and Cahora Bassa is dominated by Precambrian granites and gneisses. Generally soils on the steep slopes are shallow, susceptible to erosion and unsuitable for crop production. However small pockets of fertile alluvium occur along the river. (Chapter 2: Joint Venture­UTIP, 2001) Precambrian Basement Complex (> 570 million years Before Present (My B.P.)) Precambrian formations, mainly of the Upper Precambrian, occur extensively in the project area. They comprise mainly highly metamorphosed and folded sediments with which ancient­eruptive rocks are associated. They are quite displaced, either due to major intrusions of basic and ultrabasic rocks or to granitic intrusions of several ages, most of them emplaced during the Upper Precambrian. Four divisions are recognised, the Luia Group, Chidue Formation, Barue Group and Tete Complex.

Luia Group (B,Lg) 4

The Luia Group, comprising granites, gneisses and granulitic gneisses, occupies much of the drainage basin of the Luia River. Rocks of this group also occur in the Songo region of Cahora Bassa and in a narrow, discontinuous band from South of Songo to Mepanda Uncua. Outcrops also occur on the left bank of the river near Mepanda Uncua.

Medium textured red soils (VM) and medium textured brown soils (KM) predominate over the granites and gneisses in the upper Luia valley. The topography is hilly. The soils are moderately deep (> 100 cm) and moderately to marginally suitable for agriculture. These soils will not be inundated by the reservoir. (Annex 2: Joint Venture­UTIP, 2001) How erodible are the gneisses and granites of the Luia catchment? How much sediment does the Luia contribute? Is it significant for downstream users and for the delta?

Role of tributaries downstream: Downstream impacts due to truncated sediment supply from upstream are most directly influenced by the rate at which sediment is re­supplied to the channel from tributaries, hill slopes, and channel erosion. A consideration is however that sediment re­supplied to the channel may not be of the same grain­size distribution as that which the dam removes In Zambezi: Tributaries downstream of Cahora Bassa appear at times to contribute large amounts of sediment to the main river stem but no data are available. (Chapter 2: Joint Venture­UTIP, 2001)

Dam’s operation: Downstream changes in sedimentation will be more common when dams cause a change in the frequency and magnitude of flows that are capable of mobilizing and transporting sediment.

In Zambezi: Severe riverbank erosion in the mid­Zambezi valley has been reported downstream of Kariba dam (Guy, 1981). It was attributed to out­of­season flooding due to sudden changes in discharge from the dam, silt­free water and erodible soils.

Similar conditions would apply under a regime of intermittent turbine operations at Mepanda Uncua. (Chapter 7: Joint Venture­UTIP, 2001) Sediment­transport regime: The hydrology and sediment delivery processes interact to determine the sediment­transport regime, including the frequency, volume, timing, and grain­size distribution of sediment transport.

In Zambezi: No recent sediment concentration data under flood conditions are available for the Zambezi River downstream of Cahora Bassa. Lake Cahora Bassa and its discharge are characterised by a relatively high turbidity, thought to be due to

5

high concentration of algae and a relatively high concentration of suspended clay particles. (Chapter 2: Joint Venture­UTIP, 2001) Cahora Bassa and Kariba as sediment traps: Dams alter two critical elements of the geomorphic system: the ability of the river to transport sediment and the amount of sediment available for transport. Most large dams trap virtually all of the sediment delivered from upstream into the reservoir, although trap efficiencies for smaller dams can range from 10­90% or higher In Zambezi: Sediment discharge on the main stem of the Zambezi River is trapped by dams at Kariba and Cahora Bassa. Bolton (1983, 1984) estimated that the mean input of sediment into Kariba and Cahora Bassa is c. 7­70 x106 and 20­200 x 106 t/y respectively. This effectively means that in the last 42 years (1959­2000) Kariba has deprived the downstream Zambezi river of between 294 x 106 and 2 940 x 106 tons of sediment, and in the last 26 years (1975 to 2000) Cahora Bassa has deprived the lower Zambezi of between 520 x 106 tons and 5 200 x 106 tons of sediment. This has implications for the present river morphology and ecology downstream. (Chapter 2: Joint Venture­UTIP, 2001). According to the Feasibility Study, Mphanda Nkuwa is expected to trap 300 x 106 m3 of sediment over a 50 year period (EIA working document n. 2: Joint Venture­UTIP, 2001).

Figure 1: Conceptual model of hierarchical linkages influencing channel and valley floor morphology. Dams can directly modify the hydrologic and sediment transport regimes. (Grant et al., 2003)

6

Q.3: What do the project proponents say about Mphanda Nkuwa’s impacts on sediment transport in the Zambezi?

Much of the suspended sediment load in Luia river may be trapped in the reservoir resulting in greater clearwater erosion downstream and less deposition on local in­ bank fields. The impact cannot be mitigated and it is suggested that local farmers should be compensated through the mechanism of good corporate­community relations (see Table 1 at bottom of document) (Chapter 5: Joint Venture­UTIP, 2001) As part of the Environmental Management Plan a hydro geological survey is expected to take place. (Chapter 9: Joint Venture­UTIP, 2001)

Q.4: What does JA say about Mphanda Nkuwa’s impacts on sediment transport in the Zambezi?

It is essential that more detailed and holistic studies be undertaken on the role of sediments and the potential Mphanda Nkuwa has for culling them. This is crucial at this point considering their potential in maintaining the delta’s ecology, which now is a Ramsar site. The feasibility study failed to take the issue seriously, especially as far as mitigation goes, with the main mitigation measure proposed for sediment loss, the provision of grants for social development of project­affected communities. This forgets the ecological impacts, losses to fishery and shrimp industry, losses to mangrove stands, etc.

7

RECEPTORS ACTIVITIES ISSUES AREA DIR MAG DUR EXT SEV REV CUM MAN ES MITIGATION AND COMPENSATION MEASURES Water resource Impoundment Change in sediment transport 3 ­1 3 3 2 ­18 3 3 6 ­108 Compensate through good community relations

Table 1: (Chapter 5: Joint Venture­UTIP, 2001).

References: Beilfuss, R.D. & B.R Davies. Prescribed flooding and wetland rehabilitation in the Zambezi Delta, Mozambique. In Streever, W. (ed). 1999.

An international perspective on wetland rehabilitation, Kluwer Ac. Publ. pp143­158.

Beilfuss, R.D., Chilundo, A., Isaacman, A. & W. Mulwafu. 2002. The impact of hydrological changes on subsistence production systems and socio­cultural values in the Lower Zambezi Valley. Working Paper #6 Zambezi Wetlands Conservation Programme Davies, B.R., Beilfuss, R.D. & M.C. Thoms. 2000. Cahora Bassa retrospective, 1974 – 1997: effects of flow regulation on Lower Zambezi.

Verh. Internat. Verein. Limnol., 27: 1­9.

Doddema, M. 2000. Environmental profile and proposed management plan for selected mangrove areas in the coastal districts of Dondo, Marromeu and northern part of Beira City, Province of Sofala. Volume II, Technical Report N° 7 of the Mangrove Resources Management Pilot Project in the Northern Part of Sofala Province, Mozambique (Project MZ011501). Maputo, DNFFB. Grant, E., Schmidt, J.C. & S.L. Lewis. 2003. A Geological Framework for Interpreting Downstream Effects of Dams on Rivers. (A Unique River Water Science and Application 7, by the American Geophysical Union) Hoguane, A.M. In press. The role of Zambezi runoff in the shrimp abundance in Sofala Bank.

Joint Venture­UTIP. 2001. Mepanda Uncua and Cahora Bassa North Project – Feasibility Study including Environmental Impact Assessment.

8