2.1 Project Location
Gubadida irrigation scheme is situated in Gubadida sub location, Kinna south location, Kinna Ward, Garbatulla sub-county, in Isiolo County. The scheme is about 404 km North East of Nairobi.
The scheme location in Google Map (Figure1).
Source: Consultant (Google Earth 2020)
Figure 2.1: Isiolo County Map showing where the irrigation scheme is based.
2.2 Project Layout Design
The siting, design concept and criteria was developed in accordance with the general guidelines and standards used in the design of similar projects in Kenya and are in line with international standards and for best practices. The design adopted best practices and experiences drawn from past. Flood based farming can be described as the use of often unpredictable and occasional destructive water from the impermanent/seasonal, semi-perennial and even semi-perennial river through spate irrigation (direct diversion of flashy floods) or flood inundation and recession (rivers overflow their embankments and flood adjacent areas and through flood spreading weirs and flood harvesting dams for multiple use such as crop, rangeland and agro-forestry production, domestic and livestock water supply and recharge of ground water.
The proposed flood-based irrigation will help utilize the flood amount that comes from Ethiopian highlands as maximum as possible in order to sustain the livelihoods of inhabitants of the project area rather than its devastating and wasteful nature.
2.2.1 Bush clearing
There will minimal selective bush clearing to create access road and the site of the farms. The main type of vegetation in project site is proposes Juliflora and other encroaching bushes.
2.2.2 Canal Alignment/Layout
The main canal of the system will be laid along the high edge of the irrigable area in order for the largest possible area to be commended for irrigation. In most cases, the canal will closely follow the land contours, losing only enough elevation to maintain the slope needed for suitable flow velocity. From the main canal, secondary canals will be laid out to each irrigation unit, following the line of highest elevation in each unit so as to maximize the area served by each secondary canal. Tertiary canals, or field channels, will then be laid out from the secondaries to deliver water throughout the unit.
2.2.3 Bed slope
On flat sloping, non-undulating lands, canals will generally have the same slope as the terrain. In steeply sloping lands, canals will be given a slope which is less than the terrain to avoid high flow velocities. In such cases, drop structures will have to be installed to connect the canal sections. When a canal crosses a depression or a gully, it cannot follow the terrain and should be constructed in fill, and if a ridge in the terrain has to be crossed the canal will have to be constructed in cut.
2.2.4 Canal Embankments
Canals in fill are constructed above ground level by building embankments with soil brought from other locations or scraped from the adjacent field. For small canals, sometimes only one large embankment will be constructed, then the canal cross-section is excavated in the middle. Before an embankment is to be constructed, its elevation and its width have to be calculated. The elevation of the top of the embankment (ETE) can be calculated as follows:
• For a canal with a bed elevation higher than the field ETE = h + bed elevation
• For a canal with a bed which is lower than the field ETE = h - bed elevation
ETE is the Elevation of the Top of the Embankment in metres; h is the Height of the canal cross-section (relative to bed elevation) in metres; and Bed elevation is the elevation of the canal bed in relation to the adjacent field, in metres.
2.2.5 Steps in construction of an embankments
The following steps can be used as guidance for the construction of an embankment.
Based on the canal layout, locate the alignment of the canal and plough a strip in the terrain where the embankment is planned. A strip in the field is ploughed to clear all vegetation and roots. The material removed can be used to dress the sides and top of the completed embankment before planting grass. The width of the strip should be larger than the total width of the future canal embankment.
Hammer pegs in the soil every 50 m in a line to mark the centre line of the embankment and its final level.
Every 10 m, mark the outer line of the body of the embankment to be constructed. The top level of the embankment has been marked in Step 2. The outside slope of the embankment
must be stable and depends on the material which is used. As a rule of thumb, a slope of 1:2 (1 vertical to 2 horizontals) may be taken.
Construct the embankment. The embankment is constructed by adding soil in 5 cm thick layers, with each layer compacted moist.
2.3 Baseline physical environmental conditions
The project site is located in Gubadida sub location, Kinna south Location, Kinna Ward, Garbatulla Sub County, Isiolo County. The site is situated at an approximately on the coordinates 37N 0410158 UTM 0033028.
2.3.1 Climate, Vegetation Cover and Soil Cover
The county is hot and dry in most months in the year with two rainy seasons. The short rain season occurs between October and December with the peak in November while the long rain occurs between March and May with the peak in April. The topography of the landscape influences the amount of rainfall received. The higher ground areas near Mount Kenya and Nyambene Hills (Bulla Pesa, Burat and Kinna wards) receive between 500-670 mm of rainfall per year. The drier eastern and northern part of the county receives less than 300mm (County Government of Isiolo, 2018).
High temperatures are recorded in the county throughout the year, with variations in some places due to differences in altitude. The mean annual temperature in the county is 290C.
The county records more than nine hours of sunshine per day and has a huge potential for harvesting and utilization of solar energy.
Monsoon winds blow across the county throughout the year and attain their peak during the months of July to August, sweeping away all the moisture. The strong winds provide a huge potential for wind generated energy (County Government of Isiolo, 2018).
Three main soil types are red sandy soils, grey sandy soils and alluvium. The red sandy soils are developed over most of the rocks of the Basement System. The grey sandy soils are derived from the Upper Pliocene sediments and are rather similar to black cotton soils with admixture of sandy material. The alluvium is best developed along Ewaso Ngiro (Matheson, 1971).
2.3.2 Existing water supplies and demand
The area is supplied by the same Galamila River that supplies the farms/ irrigation scheme the project area therefore requires a complete rehab of the current water supply to ensure the residents get hygienic and reliable water fit for human consumption to meet the daily demand of water for agricultural, livestock and domestic use. There is no bulk water supply in vicinity of the area to meet this demand.
2.3.3 Geology
The project area has sedimentary rocks of the Precambrian age which includes; gneiss, schist and quartzite which form a basement system that lie beneath a veneer of deposits of younger ages.
2.4 Flora and Fauna
The project area is not densely populated and there are some several areas covered by savannah grassland, thick shrubs and big trees. There are few small wild animals in the area
2.5 Surrounding Land Use
The area is occupied by small scale mixed farmers who practice both crop and livestock and residential plots. The crops mainly grown in the area are tomatoes, onions, maize and beans.
They also have cows, goats and sheep.
2.6 Hydrogeology
The hydrogeology of an area is determined by the nature of the parent rock, structural features, weathering processes and precipitation patterns. Within volcanic rocks, groundwater primarily occurs within fissure zones, fractures, sedimentary beds, lithological contacts and Old Land Surfaces (OLS). Lava flows rarely possess significant pore space; instead, their porosity is largely determined by secondary features, such as cracks. However, pyroclastic deposits and especially sediments do have a primary porosity: the cavities between the mineral grains or clasts are usually open and interconnected. Consequently, they can contain and transmit water.
2.7 Demography
Population of entire Kinna ward is put at 14, 618 persons based on the last Census carried out in the Country.