Split Submarine Outfall Impact Assessment and Pollutant Transport Modelling

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.

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.

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

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

"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 the area circumscribed by the reef and were not influencing hydrodynamics of underneath layers, where favourable (in terms of transport of discharged wastewater) off-shore turn of currents was recorded. Extremely small values of vertical components of velocity (of the order of magnitude of 10-~ds) recorded in such situations confm the fact that fluid is flowing around rather than passing above the obstacle. Hence, no serious threat of bottom denser water uprising and transport of wastewater plume towards surface is to be expected in such conditions. By increasing the E-W mean flow intensity to 5 cds and introducing density stratification into the model, vortices disappear and flow becomes 3. uniform, as shown in Figure

Figure 3: Perspective view on simulated currents vectors in sub-surface (-7.76 m) and bottom (-3 1 m) layers in Brac channel (prescribed velocity at the eastern model boundary v = 5 cmls)

By analysing plotted contours of density isolines in Figure 3a, one can observe certain concentration of slightly denser water (Ap=0.0005 kglm3) in front of the reef, as a result of denser bottom water uprising towards sub-surface layer. However, as no change in surface density has been recorded during experiments, it can be concluded that vertical transport of denser water towards sea surface (caused by increased vertical velocities) is stopped by density stratification dampening effect. Considering that maximum recorded magnitude of vertical velocity component has been of the order of w=10-~mJs, it can be deduced that hydrodynamic mechanisms of vertical transport in described circumstances would be too weak to cause the plume of discharged wastewater mixture to reach the sea surface. Further increase of horizontal velocities to higher values (v=10 cds) has lead to gradual increase of vertical velocity components in the region around the reef (Figure 4a). Tilting of thermocline layer and resulting change of initial density stratification profile (see Figure 4b and 4c, showing the E-W cross- section through the model) was observed too. However, even in such situations stratification dampening effect has prevailed, leading to gradual decrease of vertical velocity components and causing the bulk of denser bottom water to by- pass horizontally the reef in off-shore direction, instead of proceeding upwards toward surface layers. (a>

Figure 4: (a) vertical velocity components adjacent to the reef; (b) initial density profile; (c) density profile after 30 hours of simulation

Water Pollzctiorz 17 255

5 Hydraulic model

5.1 General

In addition to 3-D mathematical model analysis, which have enabled to study simultaneously both horizontal and vertical hydrodynamic processes in the channel, hydraulic model analysis have been focussed primarily on sealwastewater mixture plume shape and transport direction modelling in typical hydrodynamic situations in the Brac channel (Figure 5). Principal scope of the study consisted in checking hydrodynamic dispersion and dillution of the effluent on the way of transport towards coastal recreational zones and to review validity of the choice of wastewater discharge location in view of prescribed bathing water quality standards.

Figure 5: Tracer release via outfall difhser section in E-W channel currents

(currents magnitude v = 10 cm/s)

Generally, modelling of submarine discharge of wastewater by means of sea outfalls is complicated by fact that the effluent may disperse as a result of different mechanisms; although one may be predominant, several may be

influential in many cases. For the derivation of modelling laws the various mechanism have to be identified. The relevant criteria for jet modeling are Froude law and a large enough jet Reynolds number to avoid viscous effects. The mass transport of the effluent by ambient currents is governed by the laws valid for open channel flow models with the model operated according to the

Froude law, the ambient Reynolds number exceeding the limiting condition and a correct friction head loss reproduction [9][10]. In accordance with the above considerations, the Brac channel hydraulic model horizontal scale of 1: 1500 (distorsion 8) and artificially increased bottom roughness have enabled to meet the open channel Reynolds similarity law.

5.2 Performed simulations

Different conservative tracers (solution of hypermanganese, a dye used in cosmetic industry and sodium chloride solution) with densities approximately equal to ambiental one have been used in simulations. Horizontal transport of the dye tracer has been camcorded while concentrations of sodium chloride have been measured at several locations by conductivity probes. The depth of water in the model has enabled to analyse non-stratified flow only. As shown in previous chapter, that situation (although characteristic for winter period, when coastal water quality issues become less important) is to be considered critical from submarine discharge and vorticity generation viewpoints. Most frequently observed fields of E-W currents, with magnitudes varying between 0-20 cds, have been generated by means of closed pump-driven hydraulic circuit. Although several wind-driven currents field were modelled for sensitivity analysis purposes, difficulties in generating two-layer flow and impact of boundary conditions have directed experiments towards analysis in no-wind conditions (with currents magnitudes corresponding to wind driven flow situations). One representative example of results obtained by hydraulic modelling of wastewater plume transport in the Brac channel is shown in Figure 6.

Figure 6: Tracer concentration isolines after 24 hours of continuous release of 860 Us of wastewater into NW currents field (v = 10 cds)

It can be observed that one of the worst-case discharge scenarios, corresponding to constant 72-hour lasting discharge of maximum wastewater design flowrate of 860 Vs into stationary NW-oriented flow field with currents velocity of 10 cds, results in significant tracer concentration reduction on the way of transport towards coastal recreational zones, where 0.05 % of the initial effluent concentration values have been found.

Converted in bacteriological pollution figures, the above dillution rate

(resulting from hydrodynamics mechanisms only) would imply reduction from concentrations of 106 co1/100 ml, expected at the end of initial mixing phase, to the allowed value of 500 coll100 ml, required by the national bathing standards.

By taking into account bacterial decay rate (typical TgOvalues are between 2-3 hours), total dillution grows up exponentially with time, while coliform concentrations decrease accordingly. Considering the average plume transport duration of 18 h (between the source and recreational areas), it can be concluded that significantly better results than presented herein are expected in real situations.

6 Conclusions

Performed numerical experiments have shown that the probability of occurrence of oceanographic phenomena announced in [2] is rather low during typical hydrodynamic conditions in the Brac channel. Hence, the implementation of longer outfall would not result in much better ambiental conditions for submarine discharge of treated wastewater. On the other side, experiments on hydraulic model have shown that even hypothetical continuous wastewater discharge (at a maximum design flowrate) via shorter outfall would result in sufficiently high hydrodynamic dispersion on the way of transport towards coastal recreational zones. Considering that an intermittent outfall operation regime is expected in reality and that conservative tracer has been used in simulations of hydrodynamic dispersion, it can be concluded that sufficient protection of coastal strip can be guaranteed with the proposed outfall, providing that submarine discharge takes place via appropriately shaped diffuser section at a depths below the protective pycnocline layer.

7 References

[l] Institute for Oceanography and Fishery. Oceanographic Research of Brac and Split Channel - Integral Ecological Project Split-Solin-Kastela, Institute for Oceanography and Fishery: Split, 1990

[2] Institute for Oceanography and Fishery, State Hydrographic Institute. Results of the Research Works for Split-Stobrec' Submarine Oufall Final Design Preparation, Institute for Oceanography and Fishery: Split, pp. 113-136, 1998. [3] Civil Engineering Institute of Croatia - Regional Unit in Rijeka, SNC-

Lavalin, Faculty of Civil Engineering in Split. Optimisation Study for the 1'' Phase of SpWSolin Wastewater Disposal Project. Civil Engineering Institute of Croatia, 1999. [4] Ravlic, N., Optimisation of SplitISolin sewerage system. Proc. of the 3rd Int. Con$ On Environmental Coastal Regions III, eds. G.R. Rodriguez, C.A.

Brebbia, E.Perez-Martell, WIT Press: Southampton, Boston, pp. 343-352, 2000.

[5] Ravlic, N., Optimization of SplitBolin submarine outfall length. Proc. of the Int. Conf On Marine Waste Water Discharges 2000. eds. C. Avanzini, N. Bazzurro, AMGA, Genova, pp. 187-196,2000.

[6] Danish Hydraulic Institute. System 3 - A 3-0 Hydrodynamic Model. DHI, 1990. [7] Rasmussen, E.B., Vested, H.J., Justesen, P., Ekebjaerg, L.C. System 3 - A Three Dimensional Hydrodynamic Model, DHI, 1990. [g] Danish Hydraulic Institute. MIKEZI, DHI: Horsholm, 1994.

[9] Novak, P., Cabelka, J., Models in Hydraulic Engineering, Physical Principles and Design Applications, Pitman Advanced Publishing Program, Boston, 1981. [10]Kobus, H., Hydraulic Modeling, German Association for Water Resources

and Land Improvement, 1980.