Coastal managers and ecologists are confronted with tasks to estimate wave heights in the Kiashahr sea area, since hydrodynamic factors can profoundly impact this environment. In this study, wind data and an empirical wind wave predicting model were used to estimate deepwater wave characteristics in the Kiashahr sea area; while a numerical model was used to determine wave characteristics in shallow waters. Wave heights were also determined and compared at different stations before and after construction of 2 jetties at the lagoon mouth to evaluate their efficiency in wave height reduction. Results revealed that wave height is in the range of 0.8 to 1.2 m at Sefid Rud river mouth. Significant wave height reduction in the range of 0.15 to 1.2 m occurred at the entrance of the lagoon due to construction of the jetties. This attenuation of wave energy may result in some impacts on the ecosystem health of Kiashahr lagoon such as increased sedimentation, reduction in flushing rates, dumping of garbage and consequently, more pollution.
Sustainable management of aquatic ecosystems demands a deep understanding and ability to predict the behavior of the marine environment. Kiashahr Lagoon lies immediately east of the mouth of the Sefid Rud (river) in the southwest of the Caspian Sea at geographical coordinates on the 39° 57′ E and 37° 26′ N (Figure 1). The lagoon has evolved as a result of wide variations in water levels. In the early 1970s it was a shallow, brackish coastal lagoon with fringing marshes in an area of coastal sand-dunes and grassland; it was fed by local run-off, and drained north-east through a narrow channel into the Caspian Sea, having formed in 1960 as a result of the falling level of the Caspian Sea and development of coastal sand spits. The 1.8 m rise in the level of the Caspian Sea since 1978 has converted the wetland into a bay with broad entrance to the sea and exposed it to more energetic waves, similar to the situation in the 1950s. The marshy grassland and sand-dune areas at the mouth of the Sefid Rud have, however, remained more or less unchanged mainly because of a huge amount of sedimentation in the river mouth which has reduced the water depth and has kept that area from higher and more energetic waves.
Waves can profoundly impact the environment in coastal areas (Keddy, 1982; Fonseca and Bell, 1998; Fonseca et al., 1998a; Ashworth, 2001). Cheong and Okada (2001) studied the effects of wave and tidal actions on the penetration of spilled oil stranded on tidal flats, and thus on the tidal flat ecosystem. Waves are responsible for thermal mixing, coastal erosion and accretion. The re-suspending of bed materials by waves in shallow waters has large effects upon material balance and water quality, as well. Two jetties were constructed at the Kiashahr lagoon mouth to fix the lagoon entrance and prevent it from migration over time. Construction of jetties, moreover, has resulted in reduction of wave penetration into the lagoon. The objective of this paper is to determine the wave characteristics in the Kiashahr sea area and to study the efficiency of these jetties in reducing wave heights.
Material and methods
Wind data were collected from Anzali station and analyzed to determine wind roses in the study area. Deepwater wave characteristics have been determined by using wind statistics and Sverdrup-Munk-Bretschneider, SMB, method (U.S. Army Corps of Engineers, 2002; Azarmsa, 2003). MIKE 21 near-shore wave, NSW, module (Danish Hydraulic Institute, 2003) has been used to simulate the wave propagation, growth and decay of short period waves in near-shore areas and to estimate wave heights before and after construction of jetties on the mouth of Kiashahr lagoon. The model is a stationary, directionally decoupled, parametric model (Holthuijsen et al., 1989) and includes the effects of refraction and shoaling due to varying depth, wave generation due to wind (Johnson, 1998), and energy dissipation due to bottom friction and wave breaking (Battjes and Janssen, 1978) as well. The effects of current on these phenomena are also included. The model covers the whole Kiashahr lagoon and its vicinity with an approximate area of 72 km2 (Figure 2, Figure 3). The grid includes 170 × 170 nodal points distributed every 50 m in x and y directions. The bathymetry of the study area is modeled with 1:5000 scale.
Considering yearly deepwater wave characteristics in the region, 50 regional models (Table 1) were set up to model wave conditions in the Kiashahr sea area before and after construction of the jetties (100 models in total). The probability of occurrence of each specific wind duration and speed class and thus, resulted deepwater wave field, is given as P% in Table 1 for each direction. The model results include: significant wave height, average wave period, and average wave direction at all nodal points of model mesh in shallow waters for each model set up.
Results and discussions
Winds are calm about 63% of the time. The northwesterly (10.72%) and northerly (10.32%) winds are more dominant (Table 1). Wind speed ranges from 2 to 20 m s−1 in these directions. Local wind duration ranges from 3 to 9 hours. Northeasterly (8.73%) and easterly (7.31%) winds with maximum speed of 14 m s−1 and 11 m s−1, respectively, are in the second and third order of importance. Increase in wind duration and speed results in increase of wave height and period, but decrease of the probability of occurrence. The waves with 0.2 m height and 2.2 s period are more frequent (19.49%), although the northerly and northwesterly waves with 3.0 m height and 7.4 s period and yearly probability of occurrence of 0.02% and 0.01%, respectively, are rare, but the most crucial waves in the study area (Table 1).
Model results for four representative cases before and after construction of the jetties are considered and discussed here (eight cases in total). Specifications of their deepwater waves inputted into the model and propagated toward coastal waters are presented in Table 2.
In each case, wave characteristics calculated at each nodal point (in total 170 × 170 = 28900 points covering the whole model area). For the purpose of comparison, here, wave heights are shown and discussed only at 11 stations (nodal points) in the model area (Figure 4).
Results reveal that wave height is in the range of 0.8 to 1.2 m at Sefid Rud river mouth (Station P4) and is usually less than wave height in nearby locations (Station P3 and P5), mainly because of river flow and wave interactions (Figure 5, Figure 6, Figure 7, Figure 8). Even after construction of the jetties, wave height has not considerably changed in deepwater stations P1, P10 and P11 and near the Sefid Rud river mouth (stations P2-P5). However, significant wave height reduction in the range of 0.15 to 1.2 m has occurred at stations P6-P8. At lagoon mouth (station P8), only the easterly waves (Figure 8) are more significant following the jetties construction and still may have considerable influence on other hydrodynamic factors and thus, aquatic ecosystems in the Kiashahr lagoon.
Construction of two jetties at Kiashahr lagoon has resulted in improving wave conditions in that area. The influences of northerly, northwesterly, and northeasterly waves have been considerably reduced after the construction of the western jetty. As a result, wave energy and related impacts on the lagoon environment have decreased; harbor entrance has become a calm place and the wave penetration into the harbor has reduced. Moreover, small vessels can now have an easier and safer access to the lagoon and thus, monitoring and management of the ecosystem has become more feasible. However, increased sedimentation, reduction in flushing rates, dumping of garbage and more pollution are possible potential impacts on the ecosystem health of Kiashahr lagoon that would arise as a consequence of this attenuation of wave energy.
The authorities of Iran meteorological organization are thanked for providing necessary information.