For thousands of years, sand and gravel have been used globally in the construction of roads and buildings. Today, demand for sand and gravel continues to increase. Sand accumulation as layers of sand deposits in river courses is a dynamic phenomenon. Rivers all over the world are under immense pressure due to various kinds of anthropogenic activities due to rapid urbanization, among which indiscriminate extraction of sand and gravel is most disastrous, as the activity threatens the very existence of river ecosystems (Kondolf 1994; Rovira et al. 2005;).
Approximately 175,484 tonnes of sand are harvested yearly in the greater Makueni district with very little income earned being ploughed back to the local area for development. In recent years there have been increased concerns for maintaining high quality groundwater supplies as a result of the recognition of the potential negative impact of sand mining since the mining operations are often located in areas favourable for developing potable ground water.
Sand mining results in the destruction of aquatic and riparian habitat through large changes in the channel morphology. Impacts include bed degradation, bed coarsening, lowered water tables near the streambed, and channel instability. These physical impacts cause degradation of riparian and aquatic biota and may lead to the undermining of bridges and other structures.
In addition, continued extraction may also cause the entire streambed to degrade to the depth of excavation. Sand mining generates extra vehicle traffic, which negatively impairs the environment. Where access roads cross riparian areas, the local environment may be impacted.
Mining and dredging activities, poorly planned stockpiling and uncontrolled dumping of overburden, and fuel spills can cause reduced water quality for downstream users, increased cost for downstream water treatment plants and poisoning of aquatic life.
Rapid increase in population of 3% per annum, unprecedented urbanization estimated at 40% and rising economic growth has led to increased and continued demand for river sand as material for housing and general infrastructure construction in Makueni region and the main source is from river channels because they provide high quality material at low cost.
River sand is particularly desirable because weak materials are eliminated by abrasion and attrition leaving durable, rounded and well-sorted materials that require less processing than many other sources (Barksdale, 1991) and are commonly located near the market or transportation routes.
However, sand mining has been shown to cause severe negative environmental impacts that are not reversible (Kondolf, 1997; Rovira, et.al.,2005; Rinaldi, et.al.,2005; Nabegu, 2012). One of the most serious and subtle but ignored negative consequence of sand mining is on ground water recharge and quality as a result of the extraction process (Herlling, 1982), because for centuries, humans have been enjoying the natural benefits provided by rivers without understanding much on the river ecosystem (Naiman 1992; Naiman and Bilby 1998), particularly alluvial channels such as Kano River (Lu et.al., 2007). For instance, some of the characteristics that make sand a valuable resource also make it a very good aquifer and recharge materials.
Also, sand mining within an aquifer recharge area will increase the vulnerability of the aquifer to be contaminated because it decreases the distance between the ground water table and land surface. In some cases, the excavation actually penetrates the shallow aquifers, leading a direct access to ground water (Depreeze, 2000).
In recent years there have been increased concerns for maintaining high quality groundwater supplies as a result of the recognition of the potential negative impact of sand mining since the mining operations are often located in areas favorable for developing Potable groundwater.
Despite this, there have been no studies on the extent of sand mining operations or of the impacts in Makueni region. In addition, minimization of the negative effects of sand mining requires a detailed understanding of the nature and sources of the impacts on ground water resources (Kondolf, 1997,Rinaldiet.al.,2005, Rovira, et.al., 2005) . Sand mining has also caused social conflicts among mining groups in the entire county (Makueni County First Integrated Plan 2013-2017).
The conceptual frame work is based on the fact that sand mining has several impacts to the environment both negative and positive. The advantages of sand harvesting include giving a source of employment, economic development, development of infrastructure to mining sites and use as a raw material for domestic construction.
|During high flows, a nick point becomes a location of bed erosion that gradually moves upstream
Head cutting mobilizes substantial quantities of streambed sediments which are then transported downstream to deposit in the excavated area and locations further downstream. In gravel-rich streams, effects downstream of mining sites may be short-lived when mining ends, because the balance between sediment input and transport at a site can re-establish itself relatively quickly. Effects in gravel-poor streams may develop rapidly and persist for many years after mining has finished.
Regardless of downstream effects, head cutting in both gravel-rich and gravel-poor streams remains a major concern. Head cuts often move long distances upstream and into tributaries, in some watersheds moving as far as the headwaters or until halted by geologic controls or man-made structures. A second form of bed degradation occurs when mineral extraction increases the flow capacity of the channel. A pit excavation locally increases flow depth and a bar skimming operation increases flow width. Both conditions produce slower stream flow velocities and lower flow energies, causing sediments arriving from upstream to deposit at the mining site.
As stream flow moves beyond the site and flow energies increase in response to the “normal” channel form downstream, the amount of transported sediment leaving the site is now less than the sediment carrying capacity of the flow. This sediment-deficient flow or “hungry” water picks up more sediment from the stream reach below the mining site, furthering the bed degradation process. This condition continues until the equilibrium between input and output of sediments at the site is re-established.
A similar effect occurs below dams, which trap sediment and release “hungry” water downstream, where channel incision usually ensues. Instream mineral excavation downstream of dams compounds this problem. Although other factors such as levees, bank protection, and altered flow regimes also promote channel incision, mineral extraction rates in many streams are often orders-of-magnitude in excess of sediment supply from the watershed, suggesting that extraction is largely responsible for observed channel changes. Susceptibility to hungry-water effects would depend on the rate of extraction relative to the rate of replenishment. Gravel-poor streams would be most susceptible to disturbance.
Channel incision not only causes vertical instability in the channel bed, but also causes lateral instability in the form of accelerated stream bank erosion and channel widening. Incision increases stream bank heights, resulting in bank failure when the mechanical properties of the bank material cannot sustain the material weight. Channel widening causes swallowing of the streambed as deep pools fill with gravel and other sediments. Swallowing and widening of the channel also increases stream temperature extremes, and channel instability increases transport of sediments downstream.
Mining-induced bed degradation and other channel changes may not develop for several years until major channel-adjustment flows occur, and adjustments may continue long after extraction has ended.
Constructions and extraction has been widely recognized for its significant importance to the building industry and the State’s economy. However, without appropriate sitting and management, sand extraction can have impacts on the surrounding environment and water resources. It has also come out clear that the premises in chapter one that the river has several social economic benefits is true. It is also evident that there are inadequate measures to control the situation of sand mining.
The recommendations made as a result of this study are based on several principles.
- First, policy makers and the local people have the enormous task of protecting the environment. This requires inter-sectoral co-operation and community participation in the management of the river ecosystem.
- Second, a controlled sand harvesting system can only come about if there are serious planning effort and clear political thinking.
- Third, the provision of basic needs, poverty alleviation, the raising of economic well-being of the people, and improved technology will reduce the reliance on sand mining. Fourth, dealing with sand mining crises requires new scientific knowledge, new engineering and technology as well as new practices and methods.
Based on these principles the following recommendations are made:
Increasing employment opportunities
Agriculture and fishing should be made lucrative. This involves the introduction and extension of improved modern production techniques that are affordable to farmers. This can include introduction fish ponds. Furthermore, the promotion of small scale businesses such as poultry and carpentry can reduce the problem of over dependence on sand harvesting.
To achieve this, enabling environment should be created through the provision of basic facilities like better access roads, electricity and water supply.
Regulating sand mining
The county government should regulate the activities carried out by sand miners. Local citizens in the neighbouring areas should be allowed to utilize sand to build their own houses.
When sand mining is carried for commercial basis, licenses or permits should be given by the county government. These licenses should be awarded to contactors who are able to prepare Environmental Impact Assessment to demonstrate their competence, technological know-how to deal with environmental degradation. The activities of these contractors should be closely monitored and evaluated to ensure effective implementation of the regulatory strategy.
Increasing environmental awareness
A change in altitude is required for successful implementation of an environmental management plan. Therefore, application of environmental education is very important. This would change people’s values, altitude, skills and behaviour needed for sustainable development. The environmental education should be based on the integration between sand harvesting and social environmental factors.
Public education programs about sand harvesting in relation to water supply should be done from the national to county level.
Encouraging participation in community based activities
The communities near the river should adopt a participatory planning and decision making approach. When people are directly involved in planning and decision making process, they will be willing to contribute to community projects. The County government should conduct workshops; seminars and field work for their employees so that they can interact with communities better. Another aspect of encouraging community participation highlighted by Cruz (1996) is use of participatory management and negotiations to resolve conflicts.
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