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Split Submarine Outfall Impact Assessment and Pollutant Transport Modelling Transactions on Ecology and the Environment vol 49, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 Split submarine outfall impact assessment and pollutant transport modelling N. ~avlic',~.~jetvaj~, v.~ndro~ec* I Civil Engineering Institute of Croatia Regional Unit in Rijeka, Croatia. 2Faculty of Civil Engineering, University of Zagreb, Croatia. Abstract Split (Southern Croatia) submarine outfall, whose construction will begin in September 2001, represents a key disposal facility of centralized Split/Solin Northern Basin sewerage system, which drains the area with some 60.000 inhabitants. Construction of this outfall will enable all preliminarly treated communal wastewater to be discharged into the Brac channel via 2,750 m long HDPE pipeline, ending up at a depth of 36 m. In order to check the consequences of various discharge scenarios, the outfall operation impact has been preliminarly tested by means of 3-D mathematical and hydraulic model. Performed tests confirm that the submerged morphological barrier, located westward to the planned outfall diffuser section location, contributes to the complexity of sea currents pattern in the wastewater discharge zone, while characteristic energy dampening effect due to seasonal seawater column stratification represents a favourable factor for submarine release of wastewater. This paper presents results of both physical and mathematical modelling experiments in typical hydrodynamic and oceanographic conditions in the Brac channel, where various driving forces act simultaneously. Results obtained by two models, both in terms of modelled currents fields and spatial extent of the pollution filed, has lead to a conclusion that reasonably high safety factors (in terms of meeting prescribed coastal water bacteriological quality standards) can be obtained with the proposed optimized outfall length. That conclusion is further strenghtened by the fact that expected outfall intermittent operation regime will enable flushing of the mixing zone between two consecutive discharge batches, so even better results than presented herein are expected in real situations. Transactions on Ecology and the Environment vol 49, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 1 Introduction Within the framework of Split MEIP (Municipal Environmental Infrastructure) project, the construction of one of the largest submarine outfalls ever built in the Republic of Croatia is scheduled for the September 2001. A 2,750 m long PEHD submarine pipeline with outer diameter 1000 mm will be laid down in Split channel (Figure la) in southern Croatia, with the scope to perform submarine discharge of wastewater originating from the towns of Split and Solin (60,000 people). Longitudinal profile of the designed outfall is shown in Figure lb. Figure 1: (a) Geographic location of the town of Split (b) Designed longitudinal profile of the outfall This paper presents results of mathematical and hydraulic modelling of outfall operation, performed with the scope to review validity of the choice of discharge location for the 1" stage of outfall implementation. 2 Oceanographic data Several oceanographic research campaigns [l][2] were undertaken in past 10 years in the region of BraE channel (Figure 2a). All of them confirmed high assimilative capacity of the channel with respect to discharge of appropriately treated communal waste water. Further, they have undoubtely proven the existence of favourable conditions for submarine release of wastewater in the channel, both in terms of predominant sea currents direction and summer stratification of the water column. Sea currents measurements [1][2], taken simultaneously at several stations in the channel, have determined prevailing E-W currents along the whole profile of the water column, with typical currents velocity magnitudes between 1-10 cmis in no-wind situations. Maximum sea currents intensity in superficial layer, recorded in wind driven flow conditions, can reach 60 cmis. Most frequent summer winds (mistral) can temporarily reverse ~upe~cialcurrents in opposite eastward direction, while the most frequent winter winds (scirocco and bora) accelerate the prevailing superficial currents in wind direction, aligned (scirocco) or perpendicular (bora) with the E-W channel axis. Transactions on Ecology and the Environment vol 49, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 Figure 2: (a) Brac channel bathymetry map (b) Bathymetry map in the planned wastewater discharge area However, following the latest oceanographic campaign in 199711998, focussed on oceanographic research in planned outfall difhser section area (Figure 2a), a hypothesis on less favourable ambiental conditions than determined in [l] were advanced by the oceanographers. As described in [2], two submarine reefs, located westward to the planned outfall route (Figure 2b), represent a physical barrier for water masses and may therefore influence regime of currents in the examined area. Although hypothetical, two following hydrodynamic scenarios have put a question mark on the choice of optimum location for submarine disposal [3]: first scenario, possibly occuring in SE wind driven flow conditions, can supposedly result in an onshore turn of predominant E-W currents, which can eventually be transformed in an anticyclonic vortex in the planned diffuser section zone. Second threatening scenario, expected presumably in summer two-layer flow conditions, consists in combined effect of the reef by-passing and bottom water uprising, resulting in formation of closed circulation patterns in the examined area. Although neither hypotesis was confied by direct current measurements, other oceanographic indicators (salinity distribution, phytoplankton density and composition of zooplankton population) were used to support this theory. 3 Outfall length optimization tools In order to achieve wastewater discharge into the area less hydrodynamically influenced by the reefs and to avoid concerns related to relatively poorly explored currents regime in the proposed submarine discharge area (see Figure 2a and 2b), a proposal has been advanced to increase the optimized outfall length [4] of 2,750 m to approx 4,500 m [2]. Considering both the importance of the outfall for overall efficiency of Split/Solin disposal scheme and respective increased investment and operation costs of longer outfall (with questionable environmental benefits), it was Transactions on Ecology and the Environment vol 49, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 deemed necessary to analyse channel hydrodnamics by means of two advanced optimization tools: 3-D mathematical and hydraulic model. Models were expected to help both in checking validity of the proposed outfall length and to determine the relevance of hypothesis advanced in [2]. 4 Mathematical model 4.1 Model description A three dimensional hydrodynamic flow modelling tool [6], based on the conservation of mass and momentum in three dimensions, has been applied. The flow is decomposed into mean quantities and turbulent fluctuations, while the closure problem is solved through the Boussinesq eddy viscosity concept, introduced by Smagorinsky subgrid model. To handle density variations, the equations for conservation of salinity and temperature are included. An equation of state constitutes the relation between the density and the variations in salinity and temperature. Hydrodynamic model equations are discretized in an implicit finite difference scheme on a staggered grid and solved by use of alternating directions technique. The transport of scalar quantities (salinity, temperature) is solved by using explicit finite difference technique based on quadratic upstream interpolation in three dimensions [7]. 4.2 Performed simulations In order to model local hydrodynamics around future Split outfall, several mathematical models were set up for different purposes: 2-D mathematical model [g], covering the area shown in Figure 2(a), was used to provide velocity boundary conditions for nested 3-D mathematical model, extending over the area shown in Fig 2(b). Resolution of the 3-D mathematical model grid (Ax = Ay = 100 m and Az = 7.76 m) enabled to model accurately both the sea bottom morphology changes and local hydrodynamics in the frontal part of submerged reefs. Simulations were performed for both winter homogeneous water column situation and summer stratified flow conditions, with initial salinity and temperature data taken from [2]. Evolution of the flow field was monitored during 24, 48 and 72 hour simulations, depending on the period required to establish stationary flow conditions. Principal scope of modelling work was to provide answers to two following questions: in which hydrodynamic conditions the probability of vortex occurence in the planned zone of wastewater discharge is at its highest ?; what is the magnitude of vertical velocity components in the zone of the submerged reef in typical and most frequent hydrodynamic conditions ? Performed initial tests with no stratification of water column have shown that the submerged morphological barrier located westward to the planned outfall contributes to the complexity of sea currents pattern in the area Transactions on Ecology and the Environment vol 49, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 "downstream" (in terms of predominant channel currents) to the outfall, especially in extremely slow flow conditions (0.5-1 cds). Although it has been determined that in such conditions several vorticies may emerge [5], observed eddies stayed in
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