Description of the Study Area
Location of study area
Guna-Tana Landscape is located in the South Gondar zone, which is part of the Amhara Regional state covering an area of 3528.16 km² (Fig.1). The coordinate systems of the basin extends from 37° 53' 13.555" E to 11° 53' 17.460" N. The elevation ranges from more than 4090m in the highland to around 1752m in the floodplain. An undulating and rugged topography is dominating the basin containing steep slopes in the mountainous region in the east and more gentle slopes towards Lake Tana.
Identification of sites and determination of flows at selected sites have been the requirements for assessment of hydropower potential. The methodology used to assess hydropower of Guna-Tana landscape is identified on Gumara and Ribb rivers. The hydraulic head and the availability of flow are the two major components of hydropower generation. The assessment of head, site selection and simulation of flow at each selected site was carried out using the ArcGIS. The ArcGIS tool generates stream network characteristics, the length of the river, and elevation difference for each stream within the watershed boundary. The model has provision for addition or deletion of outlets and inlets by user intervention, which affects the delineation and number of sub-watersheds created by the model, and this facility was utilized in assessing the head variation along the river, by placing sub-basin outlets at different locations.
Acquisition of Digital Elevation Model (DEM)
A DEM supplied from Alaska data were used to obtain Elevation difference, slope and other spatial information. The Alaska DEM with a spatial resolution of 12m was freely available and it was downloaded from website Alaska satellite facility’s vertex.daac.asf.alaska.edu. the steps followed to download this DEM was as follow from the website mentioned above downloading single band for study area, then mosaicking each band in ERDAS 2016 software and finally prepared the composed band which give the DEM of study area (figure 2a). DEM is used as the input in ArcGIS software to generate various maps based on different basin characteristics. It is a raster whose grid values signifies the height of the surface. The elevation data (figure 2b) for the study area is obtained from Digital Elevation Model (DEM) is used to find out the head available at different locations.
Slope Map of the study area
Slope calculates the maximum rate of change in value from that cell to its neighbours. The maximum change in elevation over the distance between the cell and its eight adjacent cells identifies the steepest downhill descent from the cell. The slope of the streams were generated from the slope map of the study area to identify the potential hydropower areas on the rivers.
Flow direction map of the study area
The flow direction map represents the direction of flow out of each cell. The input required is the DEM of the study area in the form of raster. The flow direction is based upon eight direction (D-8) flow model (Kinner, D.,. et al., 2005). In this model, there are eight valid output directions relating to the eight neighbouring cells into which flow could travel.
In the figure 4a, we can see a raster of elevation surface. Let us consider the 1st grid whose value is 78. Its adjoining cell have values 72, 67, 74. The minimum value among these is 67. Thus, the maximum slope is in the direction joining the cell 78 and 67 i.e. the southeast direction. Similarly, the value is assigned for every cell of the elevation surface. The resultant raster formed is the flow direction raster. The direction of flow is determined by the direction of steepest descent, or maximum drop, from each cell (figure 4b). The Flow Accumulation tool in ArcGIS software calculates accumulated flow as the accumulated weight of all cells flowing into each downslope cell in the output raster. In a simplified manner, we can say that the flow accumulation value of a cell represents the number of cells accompanying flow to that cell.
Assessment of hydraulic head
To assess potential hydraulic head along the river, computations were started at the main outlet of the watershed and then preceded in the upstream direction (Hundecha, Y. and Bárdossy, A., 2004). A location is identified as a potential hydropower site when a head of 20 m or more is available in a stream and the distance between the current location and the site immediately downstream exceeds 500 m (Kusre et al. 2010). A low head hydropower application uses river current or tidal flows falling through 20 m or less to produce energy. This is to ensure that the tailrace of the upstream site is not influenced by the reservoir of the downstream site. The assessment for the next potential site is carried out from the last selected site and the process continues to the upstream end of the river. In this study, only the potential ROR sites where power could be generated without constructing a reservoir were identified. Run-of-river schemes do not require a dam or storage facility to be constructed; instead, they divert water from the stream or river, channel it into a valley and then into a turbine via a pipeline called a penstock. Such ROR schemes can generate power for use in homes, hospitals, schools and workshops, providing an environmentally friendly way of harnessing energy from the falling water of steep mountain rivers such as Gumara and Ribb Rivers in Guna-Tana landscape. Small hydropower schemes such as these do not cause damage to the environment, can make a sustainable contribution to the electricity crisis and help the economic development of hilly areas. A hydropower plant without pondage has no storage, is, therefore, subject to seasonal river flows, and serves as a peaking power plant, whereas a plant with pondage can regulate water flow and serve as either a peaking plant or base-load power plant. Therefore, it is important to be able to estimate river flow.
Estimation of flow
Many times situation arises when the discharge observations are not available at all for streams and flow assessment has to be made for planning and the preparation of project report of a possible project site (Adhau, S.P.,. et al., 2012). Depending on the availability of data of the sites or basins there are various methods to estimate the discharge of the streams in order to identify potential hydropower sites on the stream. The discharge data was obtained from Amhara metrological station for both Ribb and Gumara rivers. The missing discharge data were filled for final analysis. To first locate the feasible locations based on head, we need to find streams with adequate flow. More is the order of the stream; more probability is there that adequate flow is available. Hence, to ensure the sufficiency of flows only streams with a minimum stream order of 3 is considered. The stream order is based on Stalher’s criteria. According to the Strahler method, the stream originated from the top most elevation is marked as first order stream. The uppermost stream are assigned stream order number 1. When two Nth order stream meets, the resultant stream becomes (N+1)th order stream. Similarly, when a higher order stream meets the lower order stream, the resultant stream becomes the higher order stream.
Criteria for identification of sites for selection of the potential sites for a hydropower project, the following criteria were adopted:
(i) Availability of flow
The availability of adequate flow is to be ensured:
By considering only streams that have flow accumulation of 12000 cells or more, as ascertained from the flow accumulation map; and as two second-order streams join to become a third-order stream, this will have sufficient runoff for installation of a powerhouse; thus the flow accumulation map was examined along with the digitized drainage map to make sure that only streams of third-order or more are considered.
(ii) Site spacing
(a) The minimum distance between two consecutive sites should not be less than 500 m
(Kusre et al. 2010). This will ensure that there is sufficient gap between the tailrace of one site and the diversion arrangement of the next, so that the river ecosystem will have sufficient opportunity to rejuvenate.
This will also ensure that the tailrace of the upstream site is not influenced by the reservoir/pondage of the downstream site.
(b) The maximum distance of river considered to find the head should not be more than 3000 m.
(iii) Head availability
(a) The head availability is assessed starting from the main outlet of the watershed.
(b) A project should have at least 20 m of head. (c) If the 500 m criterion is not satisfied, the
Hydraulic head is repeatedly raised until the interval constraint is meet.
Estimation of hydropower potential
The amount of power generated when a discharge is allowed to fall through a head difference of H is given by:
Where: P is power, ρ is the density of water (1000 kg/m3
g is the gravity acceleration (9.81 m/s2) and
η is the overall efficiency of the turbine or generator. The amount of power generated will increase with the increase in Q and H. In this study, the theoretical power is estimated by using the ArcGIS environment.
Units and Power Estimations
Power: watts [W] or Kilowatts [kW] 1 kW = 1000W
Flow: 1 m³/s = 1000 l/s
Gross heat: height difference the water "falls down"
Net head: a little smaller than gross head. Gross head deducted by energy loss due to friction in penstock.
Potential power ('electric)' is calculated as follows:
Power [W] = Net head [m] x Flow [ m³/s] x 9.81 [m/s²] (est. gravity constant) x (turbine/generator efficiency)
Potential power is estimated as follows:
More estimations that are accurate take into consideration:
- exact net head (intake to powerhouse)
- Exact flow (constant during the year?)
- combined efficiency of turbine and generator (depends on quality)
Criteria for identification of sites
- Order of stream: Only fifth and higher order streams are considered for selection of sites to ensure sufficient amount of water flow.
- Bottom gradient: Selected site should be such that average gradient along the bottom of the stream should be 1:50 (i.e. 2%) or more to ensure sufficient potential head.
iii. Minimum hydropower site interval: Distances between two consecutive hydropower sites should not be less than 500 m.
Identification of hydropower sites
To select suitable sites, the Digital Elevation Model (DEM) and stream network were used. The DEM that describes the terrain features of the study watershed was obtained from website as described earlier. The contour data at 20 m interval were extracted from the DEM. DEM, the stream network were overlaid to ascertain the elevation, and hence the available drop along the streambed. Search for the suitable locations initiated from the outlet of the watershed stretching upstream was continued until the final location. The decision of suitable hydropower sites was taken based on set criteria (figure 7).
Selection of potential hydropower sites
The stream network generated by the ArcGIS tool using a 12m DEM and threshold value of 12000 closely follows the stream network extracted from the DEM map. The smaller the threshold value, the more detailed is the stream network generated by the ArcGIS interface. The model generates the stream network giving details of stream length and elevation difference for every stream in the study area.