A Single Solution for Rathnapura Floods and Water Shortage in Hambantota

1. Introduction

Rathnapura is a major city located on the middle reach of Kalu Ganga (Figure 1) which is subjected to frequent floods. The city floods are highly destructive due to heavy damages to public and private properties. The residents and the political leaders of the area continuously request for an effective solution to protect their lives and properties from flooding. Post Images

Figure 1: River Basin Map of the Kalu Ganga with existing hydrometric Stations

The Kalu Ganga catchment above Rathnapura covers an area about 600 km2 in the Wet Zone of the country which receives nearly 3200 mm of annual average rainfall. The basin elevation goes up to 2250 m towards the origin of the river in the central mountains. The catchment receives heavy intense rains causing severe floods during May, June months hampering economic development in the lower basin. This enormous water resource has not been used, so far, for any economic development even though there are high potentials for hydropower generation and trans-basin diversion.

2. Historical Floods

During the period of last 35 years, from water year 1984/85 to 2019/20, Rathnapura city has faced 7 major floods. Three of them in May 1989, May 2003 and May 2017 were devastating and classified as critical floods. The Irrigation Department has recorded all important details regarding those flood events.

3. Flood Frequency Analysis

Flood frequency analysis relates the magnitude of an extreme event with its frequency of occurrence or, more precisely, the probability of exceedance. Practically, the results of frequency analysis are used to design of hydraulic structures for water resources development, flood control works and various other purposes. Such analysis was conducted for the Kalu Ganga catchment with Gumbel’s Extreme Value Distribution using the annual maximum water levels at Rathnapura river gauging station. According to that the return period of 2003 flood (23.57 m) and 2017 flood (23.57 m) have return period approximately equal to 20-year. The return period of 1989 flood (24.75 m)is equal to the recurrence interval of 50-year as per the same analysis.

4. Design Flood

When considering the flood mitigation options, flood levels are important in designing flood protection levees. If the designer expects to provide protection from 50-year flood, he can take the design level of the flood bunds as 24.75 m above MSL with some suitable freeboard. However, the flood volume (more precisely the excess volume) is more important in the designing of detention reservoirs. The hydrograph in 1989 flood was more peaky but shorter in duration in comparison to the 2017 flood as illustrated in Figure 2. If the designer goes for a detention reservoir for flood mitigation it is more appropriate to select 2017 event as his design flood.

5. The Most Suitable Approach for Flood Mitigation

There are numerous methods successfully adopted for flood mitigation in the various parts of the world. Selection of an appropriate method for a particular case is a competency of the designer. Rathnapura catchment is relatively short, steep, and susceptible for flash floods. Hence ‘Flood Forecasting and Warning’ is not much effective since the response time is inadequate to evacuate people and valuables from the flood prone areas. Construction of Levees (flood protection bunds) also restricted by the high population density and the scarcity of lands. Pumping of storm water from the protected areas is also expensive since such volumes are quite large due to the heavy rains experienced in the area. Some consultants have proposed to construct dry dams at Malwala and Dela, on the tributaries of the Kalu Ganga, for temporary detention and controlled release of flood water at a low rate which is manageable within the downstream river channel. This is technically feasible, but the lands inundated may not be usable for any purpose since the Kalu Ganga floods are so frequent. The crops cannot be survived under the circumstances of frequent inundation. Further, it will be a wastage of this enormous water resource while the neighboring areas (Southeastern Dry Zone) are suffered critically from the scarcity of water.
Out of remaining options, construction of multipurpose reservoirs in the upper catchment seems to be most appropriate for the Rathnapura flood problem. This can be incorporated with trans-basin diversion of water to the Walawe Ganga catchment which is adjacent to the Kalu Ganga basin in the eastern direction (Figure 3) and suffers from severe scarcity of water. The elevations of two river basins are also favorable for a trans-basin diversion project. The project can be made economical by introducing the component of hydropower production using the potential head of diversion flow at several places.

5.1 Construction of Multi-Purpose Reservoirs in the Upper Catchment

The two upstream tributaries, Wey Ganga and Bambarabatu Oya, mainly contribute for the flooding at Rathnapura. Flood flows can be brought to a tolerable level (safe carrying capacity of the river) by damming these streams at suitable locations. The project can divert water directly to Udawalawa Reservoir if the reservoir sites are selected at the elevations above 100 m MSL.

The existing Udawalawa Reservoir has adequate storage capacity (268 MCM) to regulate the diversion flow. The Full Supply Level (FSL) of the reservoir (88 m above MSL) enables to receive water from the detention reservoirs and distribute over the water scarce areas of South Eastern Dry Zone (SEDZ).

5.2 Reservoir Sites and the Catchments Intercepted

Suitable sites should be selected for the reservoirs on tributaries considering the elevation requirements for diversion. Bambarabatu Oya meets the 100 m elevation at Bambarakotuwa (6.69 * 8.48), at the downstream of existing mini-hydropower plant. The location seems to be suitable for construction of a medium size reservoir. Upper portion of the Kalu Ganga meets 100 m elevation at (6.68 * 80.48) where existing KDU mini-hydropower plant is located. The site is suitable for the second reservoir. The third reservoir will be built on the Wey Ganga at (6.57 * 80.53), Yainna village, a few kilometers downstream of Kahawaththa township (Figure 3).
The catchment area of the Kalu Ganga above Rathnapura is 603 km2. Total catchment covered by three reservoirs will be about 2/3 of Rathnapura catchment and it is possible to reduce flood flows substantially by constructing three reservoirs.

5.3 Detention Capacity Required

Three reservoirs are designed to cope with 2017 flood. If volumetric analysis is carried out this flood will be equivalent to the flood frequency of 50 - year. The reservoir capacities should be designed to retain the excess volume of flood inflow.
Minor flood level of the river at Rathnapura has been defined as 19.44 m above MSL. This is the level begins inundation of agricultural areas and the infrastructure facilities of minor importance. Allowing 1 m for safety, the discharge at 18.44 m MSL (241.55 m3/sec) is taken as the safe carrying capacity of the river section at Rathnapura. Total discharge of 20 m3/sec is supposed to be diverted to the Walawe Ganga catchment continuously. Under these assumptions, the capacity needed to retain excess runoff is estimated at 65 MCM. The calculation procedure is graphically illustrated in Figure 4.