CHAPTER 1
INTRODUCTION
1.1 Preamble
Mairuwa dam was constructed for water supply to Funtua town and its environs and it is the major source of water supply to Funtua town for domestic and irrigation purposes. However the water supply has been grossly inadequate. The amount of water pumped is insufficient to meet the increasing water demand. The residents of Funtua town rely mostly on hand dug wells, boreholes and rain harvesting. Yields from irrigated farms are often poor as a result of inadequate water supply.
Mairuwa dam which was constructed in 1970 had at inception a storage capacity of 5.5 million cubic metres. It has a catchment area of 120 km2, crest length of 457 m and maximum height of 12 m. Since the construction of the dam in 1970, no study has been done aimed at appraising its design storage capacity in view of the widespread poor land use practices over the impounding reservoir’s drainage basin which accelerates soil erosion. It is therefore possible that the initial storage capacity of the impounding reservoir may no longer be obtainable at present due to siltation.
The study focuses on the measurement of the remaining storage in Mairuwa impounding reservoir, rate of siltation in the reservoir and rate of erosion in its drainage basin
1.3 Location and Accessibility
The geographical coordinate of the studied area are latitude 70 13’ to 70 21’E and longitude 110 30’ to 110 38’N (Figure 1). The drainage basin lies on part of Funtua Sheet 78SW and SE. of the first edition of 1:50,000 topographic maps of 1967. The study area is accessible through a Trunk A road from Zaria through Funtua to Gusau. The dam is about 10 km northwest of Funtua.
1.4 Statement Of Problem
Slope stability is important in the design and construction of earth dam because exposed to dangerous conditions for the end of construction that mean no water level in reservoir (dry condition for upstream side slope)and rapid drawdown condition when the removal upstream water pressure that supported the slope for earth dam, it causes a danger to the upstream slope .
There are many methods for slope stability analysis to assessment factor of safety such as (limit equilibrium and Finite element) methods by computer software, the limit equilibrium including different methods (Ordinary, Bishop, Janbu ,Morgenstern-price and Spencer) these methods applying the computer program SLOPE/W is applied to define the potential slip surface and calculate the factor of safety of homogenous earth dam under change water level condition and rapid draw down for the reservoir with time. The minimum required of factor of safety for earth dams equal (1.3) for upstream slope (1).
Calculated the values of factor of safety of downstream and upstream slope for (NIAN) dam in IRAN for end construction. And the values of factor of safety of upstream slope for sudden drop in water level (rapid drawdown). And steady state seepage for downstream slope. by used (Ordinary, Bishop, Janbu, Spencer) methods, by using (Geo-studio)software (SLOPE/W and SEEP/W)programs.
The study showed, the value of factor of safety for the upstream slope in the end of construction greater than the value of factor of safety in the sudden drop in water level(rapid drawdown)and the value of factor of safety for the downstream slope in the end of construction greater than the value of factor of safety in the steady state seepage, while the earth dam still stable)2). Calculated the values of factor of safety for the dams in (Queensland- 106 Australia), for drawdown rates of the reservoir with time by using slope/w and seep/w programs. The study showed the values of factor of safety for upstream slope increase for a low rate of reservoir drawdown (3).
Recently, many researchers in Nigeria, such as (4, 5), interested in their research into the subject of the influence of several parameters on the factor safety for slope stability. In this study, the minimum values of the factor of safety are calculated using Limit equilibrium methods and Finite element method, when change values of (cohesion, angle of internal friction and unit weight of soil) and change water level in the reservoir, and rapid draw down of the water level.
1.5 Vegetation and Climate
The study area belongs to the Sudan savannah which is characterized by short grasses and scattered, drought resistant trees such as the Baobab, Tamarind and Acacia. There are two seasons; the rainy season and the dry season. The rainy season starts from April to October The annual rainfall is averagely 1000 mm.
Dry season is from October to April. Its first half till end of January is called “harmattan period”. During the day it is hot and cool at nights, both characterized often by poor visibility owing to dust haze of harmattan, north- easterly.easterly, and northerly wind. Rainfall is almost nil and the sky is cloudless during harmattan season. Relative humidity is low, about 40%, often as low as 25%. The second half of the dry season, “hot period” season, from early Februaly till end of April and is characterized by hot days and night and a good visibility (Schoeneich and Garba, 2010).
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