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Current characteristics of salinity stratification of two coastal lagoons in southern area of Sri Lanka after different human interventions
E. Furusato 1, G.L. Perera 1, N. Tanaka 1,2 , G. P. Amarasekara 3 and T. Priyadarshana 3 1Graduate School of Science and Engineering, Saitama University 255 Shimo Okubo, Sakura ku, Saitama 338 8570 JAPAN 2 Institute for Environmental Science & Technology, Saitama University 255 Shimo Okubo, Sakura ku, Saitama 338 8570 JAPAN 3 Faculty of Fisheries and Marine Sciences & Technology, University of Ruhuna Wellamadama, Matara (81000), Sri Lanka E mail: [email protected] u.ac.jp
Abstract: Field investigations were conducted to estimate the present state of density stratification of Koggala Lagoon and Rekawa Lagoon, Sri Lanka. The variation of salinity, water temperature and dissolved oxygen were measured vertically at different tidal conditions and rainy seasons. For both lagoons, salinity stratification occurred in not only lagoon but also inflow stream and lagoon mouth. Furthermore, the stratification was measured in mainly rainy season for both lagoons. On the other hand several temporal and spatial characteristics, such as salinity level, area of strong stratification and the reason of the seasonal difference of stratification, were contrastive for each lagoon. These depend on the inherent difference of hydrological characteristics of each lagoon, recently intensified by human intervention conducted in mouth area for both lagoons. Conceptual model has been constructed for the hydraulic characteristics of each lagoon related to the salinity stratification as the basis for the future quantitative analysis.
Keywords: seasonal difference, tidal effects, inflow stream, conceptual model.
1. INTRODUCTION
Water environment in Sri Lanka is recently under pressure with the rapid development activities and more concern is needed with respect to environmental resources preservation. Coastal lagoons play an important role not only by offering various “natural resources” for regional community but also by providing “natural infrastructure” for hazard protection and/or mitigation, which is commonly called “bio shield (Tanaka 2009). Appropriate preservation of natural systems and mitigation of such impacts will be strongly required in future due to development activities by various functions. Thus, it is important to know sufficiently about the various processes occurred in and around such systems.
Coastal lagoon system consists of various processes, such as physical, chemical, biological and ecological phenomena and is established on finely balanced various relationships among these factors (Hume et al. 2007). On the other hand, anthropogenic interventions effects lead diverse responses of the system. Salinity stratification and vertical mixing is such an important process that plays a central role. Further, salinity stratification is an interesting area considered under density current hydraulics. Many studies have been conducted on estuary hydraulics in the past (Uncles 2002). Coastal lagoon is one type of water body considered under various types of estuaries such as river mouth, fjord and bay (Kierfve and Magill 1989). In particular, narrow channel at downstream area of estuary water body is the inherent nature of coastal lagoons. There are little information about the relationship between lagoon mouth characteristics and salinity stratification in the past research.
The effects of mouth characteristics should be clarified for understanding the salinity stratification processes of coastal lagoons. Furthermore, regional or local endemism is important for such complex system of coastal lagoon. During past decade, Koggala Lagoon and Rekawa Lagoon suffered the effects
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of different human intervention around the mouth area. These lead some environmental social problems. Base on the backgrounds, field surveys were conducted about the present states of salinity stratification of both lagoons from 2011 as a preliminarily study. There are some other information on water temperature, water quality and biological/ecological situation of those two lagoons (Priyadarshana et al. 2007, IMMI 2006). However, salinity stratification behavior is not clear. Firstly, current situation of salinity stratification has been estimated. This paper shows the comparison of the salinity stratification characteristics of these two lagoons and discuss about the effects of human interventions on salinity stratification qualitatively.
2. MATERIALS AND METHODS
2.1 Site description
Koggala Lagoon and Rekawa Lagoon are located in the southern area of Sri Lanka (Fig. 1). These two lagoons have converse characteristics of not only lagoon mouth topography but also human interventions conducted last decade (Priyadarshana et al. 2007, Gunaratne et al. 2010a). Koggala Lagoon is situated in the southern coast of Sri Lanka (Fig.1) about 130 km south of the capital Colombo between 5º 59' 6º 02' N and 80º 18' 80º 21' E (Priyadarshana et al. 2007). The water body at present is a saline coastal lagoon, with a surface area of 574 ha and a catchment area of 55km 2. The water depth ranges from 1.0 to 3.2 m. The natural seasonally opening protective sand bar at the Koggala Lagoon mouth was removed for development activities and a groyne system has been constructed as a remedial action to minimize erosion. So the mouth is opened to sea throughout the year creating a high saline aquatic environment. That has caused various unexpected impacts on the lagoon (Priyadarshana et al. 2007, Gunaratne et al. 2010b). Rekawa Lagoon is a choked shallow lagoon located in the Southern Province of Sri Lanka. The average depth and the widest point of the lagoon are around 1.4 m and 2.5km, respectively (Gunaratne et al. 2010a). The water body at present is relatively low salinity brackish lagoon, with a surface area of 2.4 km 2 and a catchment area of 225 km 2. The main water body of the lagoon is connected with the sea by a narrow, meandering 3km long channel. A causeway constructed across this channel approximately 0.7km from the lagoon mouth to the inland has narrowed the width of the water flow of the channel to about 6m. Kirama Oya River, the main fresh water inflow connects to the lagoon outlet channel at the sea ward end 0.2km from the lagoon mouth to inland. The lagoon mouth keeps closed for most of the year and is intermittently opened naturally in rainy season or manually by local community to prevent flooding when high rainfall is received.
Sri Lanka N N
2km 2km
Koggala Rekawa
Figure 8 Map of Koggala and Rekawa Lagoon
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2.2 Field observation Table 1 Factors focused on in the preliminary survey for Koggala and Rekawa Lagoon Field observation for two lagoons have Time scale Koggala Lagoon Rekawa been conducted during about one half Lagoon year for several stations in each lagoons Several Rainy or Dry Opened or including mouth area and typical inflow months season Closed mouth streams (Fig. 1). A water quality Weeks Spring or Neap tide measuring equipment (multi probe) YSI Semi diurnal Ebb and Flood Model 55 was used to measure temperature, salinity, and dissolved Galle Colombo oxygen (DO) (approx. 0.5 m intervals). 1 Vertical profiles of these parameters were 0.8 measured longitudinally from Inflow 0.6
[m] 0.4 streams to the lagoon mouth through the 0.2 lagoon water body (Fig. 1). From Nov. elevation Tidal 0 0.2 2011 to Feb. 2012 several times survey Koggala 0.4 Surveys were conducted for each lagoons. The 600 Galle days of field survey are shown in Fig. 2 500 400 as circles with the temporal variation in 300 200 tidal change and rainfall related areas. Rainfall For selecting the survey timing, the some [mm/month] 100 0
different factor effecting on the mixing Jul Jul Oct Oct Apr Apr Jan Jun Jan Jun Jan Feb Mar Feb Mar Feb Sep Sep Aug Aug Nov Dec Nov Dec May May and stratification were focused. 2011 2012 2013 Generally, several factors, such as Rekawa Surveys Bata Ata block2 rainfall seasonal changes and tidal 400 350 conditions, should be considered for 300 250 discussing the salinity stratification in 200 150 coastal lagoons (Uncles 2002). For the Rainfall 100 [mm/month] 50 representativeness of field survey, some 0 Jul Jul Oct Oct Apr Apr Jan Jun Jan Jun Jan Feb Mar Feb Mar Feb Sep Dec Sep Dec Aug Aug Nov Nov factors were classified for selecting the May May analysis of field survey results. Table 1 2011 2012 2013 shows the classified factors related to the time scale of it. Qualitatively, longer time Figure 9 Rainfall and tidal data along with timing of scale of the factor means large effects of conducted surveys. White circles, once survey per day; the balance between saline water black circles, two times survey on one day for semi intrusion from the sea and freshwater (a) diurnal tidal effects survey. inflow from catchments.
30 3. RESULTS Surface layer Bottom layer M1 20 M2 M3 M4 L1 L3 L2 L4 10 3.1. Longitudinal salinity W1 Salinity [ppt] Salinity distribution for both 0 lagoons 0 1 2 3 4 5 6 (b) Distance from mouth end [km] Fig. 3 shows the longitudinal variation of surface and bottom salinity of both 18 lagoons. For understanding the Surface layer Bottom layer P1 P2 comprehensive states, temporal averaged 12 P3 P4 data for each station were used. Although L1 L3 L4 L2 almost of all station for both lagoons 6 G1 exhibit different salinity level between I1 Salinity [ppt] surface and bottom, different 0 characteristics of longitudinal salinity 0 1 2 3 4 5 6 distribution can be confirmed between the Distance from mouth end [km] two lagoons. Firstly, the salinity level of Figure 10 Longitudinal variation of salinity for both Koggala Lagoon was higher than that of lagoons (a, Koggala Lagoon; b, Rekawa Lagoon; Arrows Rekawa Lagoon. Secondly, the significant represent inflow stream point) difference in salinity level between surface
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and bottom occurred in converse Inflow(W1) Lagoon (L1) Mouth (M2) area. In Koggala Lagoon, upstream Salinity [ppt] Salinity [ppt] Salinity [ppt] inflow region exhibit the remarkable 0 10 20 30 0 10 20 30 0 10 20 30 vertical difference in salinity 0 0 0 compared to Rekawa Lagoon in 1 1 1 which down stream region exhibit it. . Depth These characteristics are related to Salinity 2 2 2 Dry Rainy not only the mouth conditions but 3 3 3 also the position of freshwater inflow DO [%] DO [%] DO [%] streams. Whereas in Koggala 0 50 100 150 0 50 100 150 0 50 100 150 Lagoon, upstream end of lagoon 0 0 0 water body is main freshwater inflow 1 1 1 region, freshwater inflow from down DO stream end of lagoon body (3.2km) Depth 2 2 2 and mouth area (0.3km) in Rekawa 3 3 3 Lagoon. Figure 11 Comparison of vertical profile of salinity and DO between rainy (22 Nov. 2011) and Dry (16 Mar. 2012) season 3.2. Koggala Lagoon for Koggala Lagoon in neap and ebb condition.
3.2.1.The effects of seasonality Inflow (W1) Lagoon (L1) Mouth (M2) Salinity [ppt] Salinity [ppt] Salinity [ppt] related rainfall 0 10 20 30 0 10 20 30 0 10 20 30 0 0 0 Fig. 4 shows the comparison between 1 1 1 each measured parameters of selected Depth Salinity 2 2 2 Neap stations of Koggala Lagoon during the Spring 3 3 3 monsoon rainy and dry season. For DO [%] DO [%] DO [%] avoiding the tidal effects, neap and ebb 0 50 100 150 0 50 100 150 0 50 100 150 condition data are selected. The 0 0 0 parameters exhibit differences 1 1 1 depending on the season. In the rainy DO Depth 2 2 2 season, partially mixing of the mouth area and strong density stratification of 3 3 3 both the lagoon and inflow stream were Figure 12 Vertical Salinity and DO profiles of selected measured. On the other hand, the stations of Koggala Lagoon in spring (30 Nov. 2012) and results of survey in the dry season neap (22 Nov. 2011) conditions indicated that such mixing states of Koggala Lagoon differ remarkably depending on the season.
3.2.2.The effects of weekly tide (spring and neap)
Fig 5 shows the comparison of vertical salinity and DO profiles of Koggala Lagoon in spring and neap conditions. These surveys were conducted in rainy season. Because of large quantity of freshwater inflow, salinity stratification in both tidal conditions can be found. Although surface salinity level exhibits relatively small difference, the effects of the tidal condition such as spring and neap, is smaller than that of seasonal rainy conditions. In both tidal conditions, bottom layer of lagoon (L 1) exhibits the decrease in DO. Regardless of the weekly tidal conditions, there exist high salinity and low DO in bottom layer because of the salinity stratification.
3.2.3. The effects of semi diurnal tide (ebb and flood)
Fig. 6 shows the comparison of salinity between ebb and flood condition in same day (30, Nov. 2012, rainy and spring tidal condition). Basically, only mouth region exhibits the difference by the semi diurnal tide. For estimating the lagoon water body, the effects of short period factor, that is to say semi diurnal tide, can be neglected except for discussing the hydraulic phenomenon in mouth region.
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Lagoon (L1) Mouth (M2) 3.3. Rekawa Lagoon Salinity [ppt] Salinity [ppt] For Rekawa Lagoon, as shown in Table 1, 0 10 20 30 0 10 20 30 the mouth opened conditions regulated by 0 0 ] the existence of natural sand bar at the end m1 1 [ of lagoon mouth is focused. The sand bar of h . pt2 2 Salinity Flood Rekawa Lagoon tends to be formed in e3 3 mainly dry season. D Ebb 4 4
3.3.1. The effects of mouth open DO [%] DO [%] condition 0 50 100 150 0 50 100 150 0 0 ] Fig. 7 shows the comparison of salinity and m1 1 [ DO vertical profiles between different mouth th2 . 2 open condition. Salinity stratification in DO p e3 3 mouth area is prominent compared to the D lagoon water body. It formed in both mouth 4 4 opened and closed conditions. On the other hand, in lagoon area, stratification can be measured in only mouth opened and flood Figure 13 Vertical Salinity and DO profiles of condition occurred in rainy season as small vertical difference in salinity. This seasonal selected stations of the Koggala Lagoon in ebb and tendency is similar to the tendency of flood tide conditions on same day (30 Nov. 2012, Koggala Lagoon. However, the salinity level Rainy season and spring condition). of mixed condition in dry season is Mouth (P2) Lagoon (L3) significantly lower than the level in Koggala Lagoon. One of the reasons of the mixed Salinity [ppt] Salinity [ppt] state occurring despite of continuous 0 10 20 30 0 10 20 30 freshwater supply even in dry season is the 0 0 mixing caused by external factors such as wind. Actually, we got an information from 1 1 th fishermen living at Rekawa village that the p .
Salinity e transparency of lagoon was low during D2 2 R17Mar12DN-C R21Oct12RNE-O relatively strong wind season. This R21Oct12RNF-O phenomenon suggests the effects of wind in 3 3 dry season on the mixing of lagoon water body as re suspension of bottom mud. DO [%] DO [%] 0 50 100 150 0 50 100 150 0 0 3.3.2. The effects of semi diurnal tide (ebb and flood) h1 1 DO pt e . D Fig. 7 also shows the comparison between 2 2 semi diurnal tide on 21 Oct. 2012, rainy and 3 neap tide condition. The effect of semi 3 diurnal tide was not significant in mouth and lagoon water bodies. This agrees with the Figure 14 Vertical profile of salinity and DO level in the mouth topographic condition of Rekawa Lagoon that is narrow, meadow and long main water body of Rekawa Lagoon and inflow in rainy characteristics preventing the sea water season with different mouth conditions. (See Fig. 1; ○○○, intrusion into the mouth channel. Mouth closed, 17 Mar. 2012; ▲▲▲, Mouth opened and ebb, 21 Oct. 2012; ▲▲▲, Mouth opened and flood, 21 Oct. 2012) 4. DISCUSSION
Based on the preliminary field surveys, the current states of salinity stratification with temporal and spatial
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[Rainy season]
[Dry season] KoggalaLagoon
[Rainy season]
Ocean Mouth Lagoon
Degree of mixing Impede mixing Mixing Assist mixing Impedence on free water flow Wind Tidal oscilation
Causeway
20-30 ppt <5 <5ppt ppt
5-10 ppt 5-10 ppt Mouth Opened Degree of mouth opened is important
[Dry season]
Ocean Mouth Lagoon
RekawaLagoon Stratification tendancy Impedence on free is higher water flow Mixing
Wind Causeway No tidal oscilation
<5ppt brackish water layer (inflow)
5-10 ppt
10-20ppt
20-30ppt Mouth closed Deep layer (high salinity, low DO)
Figure 15 Conceptual model of the effects of rainy season for Koggala and Rekawa Lagoon variation and related factors of both lagoons where different human interventions were done during last 236
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decade were understood. Fig. 8 shows the conceptual figures of salinity stratification for both lagoons. Based on the prominent factors for determining the stratification, two seasonal conditions are presented. Firstly, common temporal and spatial characteristics of salinity stratification for both lagoons exist. As spatially, salinity stratification occurred in not only lagoon water body but also inflow stream and lagoon mouth is common property. Generally, in Rekawa Lagoon mouth is opened in rainy season. As seasonal characteristics for both lagoons, salinity stratification is formed in mainly rainy season. Temporal characteristics for the stratification were also common. On the other hand, several temporal and spatial differences between two lagoons, such as salinity level, area of strong stratification and the reason of the seasonal difference of stratification, were contrastive as described below. The inherent difference of hydrological characteristics of each lagoon, recently intensified by human intervention conducted in mouth area, is considered to be main reason of the difference. Koggala Lagoon exhibits an entirely different mixing state depending on the season. In the rainy and monsoon seasons, the lagoon was brackish and a salinity stratified water body. On the other hand, in the dry season, the lagoon was high saline concentration and strongly mixed. In this season, only in the inflow stream, brackish and salinity stratification could be confirmed. The reason of this seasonal difference is to be the difference of inflow stream discharge caused by the seasonal rainfall variations. As Gunaratne et al. (2010) reported the balance of inflow quantity from the catchment and lagoon differs from the monsoon season. The hydraulic characteristics affect not only salinity level but also mixing state of the Koggala Lagoon. Similar to Koggala Lagoon, salinity stratification can be found in almost all survey stations in Rekawa Lagoon. In particular, the field survey results indicate that vertical salinity stratification tendency is greater at lagoon outlet channel compared to the main water body of the lagoon. The hydrological characteristics of Rekawa Lagoon, such as depth of mouth being greater than lagoon and the small amount of saline water inflow into the lagoon, lead the tendency. As temporal characteristics, salinity stratification of lagoon water body occur in opened mouth conditions. In such situation, not only increase in freshwater inflow from streams but also high salinity water inflow from ocean through opened mouth will act as the cause of it. The key factor determining the degree of vertical mixing in Rekawa Lagoon is mouth opened conditions as well as inflow of freshwater stream.
5. ACKNOWLEDGMENTS
This study was partly funded by JSPS Asia Africa Science Platform Program, Japan and partly by Grant in Aid for Exploratory Research, Japan (No. 25550007).
6. REFERENCES
Gunaratne, G. L., Tanaka, N., Amarasekara, P., Priyadarshana, T. And Manatunge, J. (2010b), Restoration of Koggala lagoon: Modelling approach in evaluating lagoon water budget and flow characteristics, J. Environmental Sciences, 22(6) 813–819. Gunaratne, G. L., Priyadarshana, T., Manatunge, J., Tanaka, N., Yasuda, S. (2010a), Water balance and renewal time of Rekawa lagoon, Sri Lanka; A restorative approach, International Conference on Sustainable Built Environment (ICSBE 2010), 37 44. IWMI (International Water Management Institute) (2006), Sri Lanka Wetlands Database. http://dw.iwmi.org/wetland/wetlandsinfooptions.aspx?wetlandname=Koggala%20Lagoon&wetland / (accessed February 10, 2011). Kjerfve, B. and Magill, K.E. (1989), Geographic and hydrodynamics characteristics of shallow coastal lagoons, Marine Geology, 88, 187 199. Priyadarshana, T., Manatunge, J. and Wijeratne, N. (2007), Report, Impacts and Consequences of Removal of the Sand Bar at the Koggala Lagoon Mouth & Rehabilitation of the Lagoon Mouth to Restore Natural Formation of the Sand Bar, Practical Action, Sri Lanka. Tanaka, N. (2009), Vegetation bioshields for tsunami mitigation: review of effectiveness, limitations, construction, and sustainable management. Landscape and Ecological Engineering, 5. 71–79,. Uncles, R. J. (2002), Estuarine Physical Processes Research: Some Recent Studies and Progress, Estuarine, Coastal and Shelf Science, 55, 829–856.
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