Some environmental aspects of marine disposal systems with particular reference to UK waters

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Pergamon

Wal. SeL Tech. Vol. 32, No.2, pp. 167-174, 1995.

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SOME ENVIRONMENTAL ASPECTS OF MARINE DISPOSAL SYSTEMS WITH PARTICULAR REFERENCE TO UK WATERS YusufKaya Principal Engineer. Babtie Shaw & Morton. Consulting Engineers. 95 Bothwell Street, Glasgow G2 7HX, UK

ABSTRACT Disposal to the marine environment is generally regarded as an effective means of disposing of sewage effluent. In recent years legislation has been introduced to enable marine disposal in a more controlled and environmentally acceptable manner. The discharger mUSl prove to the regulatory authorities that the proposed discharges comply with the relevant legislation. This paper concentrates on the environmental aspects of marine disposal systems and gives examples from the UK. Emphasis is given to mathematical modelling techniques used to assess compliance with relevant legislation. Data requirements for model set up and calibration are discussed, together with assessment of the level of calibration achieved for two specific coastal applications. An improved representation of wind stress effects on the water column and requirements for a special radiation condition at open boundaries to render the boundary transparent to outgoing transients are also discussed.

KEYWORDS Comprehensive studies; dispersion; effluent; ecosystem; legislation; predictive modelling; radiation boundary; wind stress. INTRODUCTION The marine environment is used as the final receptor of local and land based pollution sources. In recent years there has been a widespread realisation of the pressure that ever increasing discharges impose on the marine and coastal ecosystems. Increased public awareness has led to the introduction of "green" politics in many countries. In parallel with increasing public awareness and concern, the technical aspects of marine disposal systems have also received considerable attention from engineers, marine biologists and environmental scientists. Legislation has been introduced and guidelines proposed for minimising the adverse effects discharges could have on the marine environment. The degree and extent of the impact any marine discharge could have on the environment depend on various factors including the quality and quantity of the wastewater flows, the level of treatment provided before the 167

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effluent is discharged. the hydraulic perfonnance of outfall and diffuser system. and the mixing and dispersive characteristics of the receiving waters. Within the European Community (EC) countries discharges to the marine environment are controlled by various international and national legislation. In most cases secondary treatment is presumed sufficient. For less sensitive areas less stringent treatment may be sufficient providing that it is proved by comprehensive studies that the environment will not be adversely affected. Predictive mathematical modelling constitutes an integral part of such studies. ENVIRONMENTAL ASPECTS It is generally accepted that enhancement and protection of the aquatic and marine environment should be controlled by setting consents for discharges. The member states of the EC are required to control marine discharges within the framework of EC Directives which, if necessary. are supplemented by more strict locally imposed conditions based on the assimilative characteristics of the receiving water. EC Directives concerned with coastal sewerage schemes include the Bathing Water Directive. the Urban Waste Water Treatment Directive. the Dangerous Substances Directive. the Shellfish Waters Directive. the Bivalve Molluscs (Health Conditions) Directive and the EC Directive on Environmental Impact Assessment. The Bathing Water Directive requires that the designated bathing water areas must comply with bacteriological. physico-chemical and aesthetic standards. It prescribes the level of monitoring required at bathing waters and the level of compliance required. The Urban Waste Water Treatment Directive (UWWTD) requires prOViSion of treatment of coastal discharges serving a population equivalent greater than 10.000. The degree of treatment depends on the classification of the receiving waters. For receiving waters which are identified as "sensitive areas" secondary or more stringent treatment would nonnally be required. On the other hand for "less sensitive areas" (or high natural dispersion areas) at least primary treatment would be required providing that it is proved by means of comprehensive studies that the coastal ecosystem will not be adversely affected. Similar conditions apply for the other directives. for example, the Shellfish Waters Directive sets bacteriological and physico-chemical standards for the designated shellfish waters. Comprehensive studies The Urban Waste Water Treatment Directive (UWWTD) does not defme precisely the adverse effects nor does it prescribe the context of comprehensive studies required. The Marine Pollution Monitoring Management Group (MPMMG) published a report in early 1994 (MPMMG. 1994) in which an attempt was made to define adverse effects and the nature of necessary comprehensive studies. The MPMMG recommend that predictive modelling technologies should be used to evaluate the impact of a given outfall/discharge regime on the marine environment. Prediction of biochemical oxygen demand. nutrients (both dissolved available inorganic nitrogen and dissolved available inorganic phosphorus) and suspended solids would be required to assess the adverse effects. Furthennore. regular monitoring would be necessary in respect of eutrophication and the benthic impact of solids. Monitoring of dissolved oxygen levels is also required. TECHNICAL ASPECTS The technical aspects of planning and implementation of marine disposal systems include the collection and conveyance of the wastewater through treatment, dilution and dispersion investigations and final design. In

Environmental aspects of marine disposal in UK waters

169

this section emphasis is given to dispersion calculations using predictive mathematical modelling techniques. The initial dilution of wastewater is the reduction in sewage strength as it rises buoyantly from its discharge point near the sea bed to the sea surface. Initial dilution depends on a number of factors including diffuser geometry. water depth. the buoyancy of the effluent, the magnitude of the discharge and the strength of the ambient current. Methods for predicting initial dilution in still water and in moving water can be found in Lee and Neville-Jones (1987) and WRc (1990). Mter initial dilution. the effluent is further diluted through turbulence. eddies and shears induced by the water movement which mix the effluent with an increasing volume of the receiving water as it spreads away from the discharge area. The most common and accurate method of predicting dispersion is the use of mathematical models. Predictiye modemn!: The fIrst step in any modelling investigation is to identify the type of mathematical model which would be required to represent the conditions within the coastal waters likely to be affected by the proposed discharge.

In coastal applications where flow is predominantly two-dimensional a depth averaged mathematical model is generally used to predict the dispersion of pollutants. Such models are based on the basic assumption that the vertical acceleration is negligible (hydrostatic pressure) and the water is well mixed along its depth. Where the flow is stratifIed or has a three-dimensional structure. a two-dimensional layered model or a three-dimensional model should be used. In this paper examples from the application of a two-dimensional depth-averaged hydrodynamic model (FLOFIELD) and a two-dimensional depth-averaged dispersion model (DISPOL) to UK coastal waters are presented.

Hydrodynamic model

Basic equations: The hydrodynamic model is based on the solution of the two-dimensional depth integrated equations of motion and continuity. For a constant density turbulent flow described in a cartesian co• ordinate system these equations can be expressed as (Falconer. 1986): 2

oUH _ _ +

H P [OU --

oVH "8t

R

at

Or) +

at

ox

+ ..

[OUVH --a;c

oUH Ox

+ _oUVH] -

NH

+

gH -Or) ox

OV H] + fUH -;;y

+

gH Or) _ 'I' Oy Iy

Oy

2

+

+ ~ _

Oy

'1'..

+ 'I'1lx

+ 'I'

by

-

-

E

EH [Q2U 2 -;;:;r

ox

axr

H [Q2V

+

Q2V]

Q2U + _ _ + --.. oy'

2 ayt Q2V

+

OxOy

Q2U]

OxOy -

0

0

(1)

(2) (3)

0

Eqs (1) and (2) are the momentum equations in the x- and y- directions. respectively. and Eq.(3) is the continuity equation. In the above equations U.V depth mean velocity components in the x. y directions. respectively. H total depth of flow. t time. ~ correction factor for non-uniform vertical velocity profile. f Coriolis parameter. g gravitational acceleration. Tl water surface elevation with respect to datum. £ = depth mean eddy viscosity. t. x and t. y = water surface shear stress components in the x and y directions and tbx and tby = sea bed shear stress components given by

=

=

_ 'I' lX,y

P. C' WX,yW P

=

=

,

C:

gq",y q

tbx,y-~

H2

= =

=

(4)

170

Y.KAYA

=

=

=

where Pa air density, C· air-water interfacial resistance coefficient, Wx,y wind velocity components in x and y directions, w absolute wind speed, P water density, qx,y depth integrated discharge per unit width, q = absolute discharge per unit width and C z = Chezy roughness coefficient.

=

=

=

The above equations were solved by an alternating direction implicit finite difference scheme on a space staggered grid. All partial differential terms were approximated by fully centred difference equations in both space and time by iteration. The scheme is basically second order accurate with no stability constraints. Full details of the finite difference scheme are given in Falconer (1986).

Surface wind effects: In most depth averaged tidal numerical models the effects of surface wind have been included as an additional shear stress term at the free surface (see Falconer, 1986). Also an assumption is generally made for the shape of the vertical velocity proflle taken, in most cases, either as a logarithmic or a seventh power law velocity distribution (Falconer. 1986). Koutitas (1988) and Koutitas et al. (1986) stated that wind generated velocity proflles may vary considerably over the depth and suggested using a second order parabolic velocity distribution (Falconer and Chen, 1991). For the x-direction this can be expressed as

(5) where u

=velocity at elevation Z, and z =0 at the surface and z =-H at the bed.

c _ Hew I

..

P.

'I

Jw: P+ w:

(6)

in which Cc is the air-fluid resistance coefficient (Kaya, 1992). Once the depth averaged velocity field is determined the values of U and C x can be substituted directly into Equation (5) to give the vertical velocity proflle which permits the computation of the current pattern at any depth. If, for example, the free surface velocity components or the averaged velocity components for a predefmed top layer are required for subsequent use in a surface/near surface advective diffusive pollutant transport model they can be computed from Equation (5). Water Quality model The water quality model is based on the solution of the two-dimensional depth averaged advection-diffusion equation expressed as (see Falconer. 1986) aSH + aSUH + asVH a rHO as HO at --ax ayax[ Xlax+

'Y

as] Oy

a rHO as +ay[ Ylax+

HO

as]

W7ij

(7)

in which S is the depth mean solute concentration and 0u' Oxr 0 and 0 are the depth mean dispersion-diffusion coefficients in the x and y directions, respectively (F~oner, 19~; Kaya, 1992). The solution to the above equations can be accomplished similar to the hydrodynamic equations by conventional second-order fmite difference methods. However, to increase the accuracy of predictions, particularly in the regions of high concentration gradients, the advective transport terms are represented with a higher order spatial fmite difference scheme known as QUICK (Quadratic Upstream Interpolation for Convective Kinematics). The application of the method to the present model essentially follows that of Falconer and Uu (1987).

EnvironmenUll aspects of marine disposal in UK waters

171

The water quality model can simulate total Coliforms, faecal Coliforms, biochemical oxygen demand (BOD), total organic nitrogen, ammonical nitrogen, nitrate nitrogen, dissolved oxygen (DO), suspended solids, temperature and salinity. Equation (8) is adequate to describe the conservative pollutants. For non-conservative pollutants additional reaction equations must be solved simultaneously with this equation. For the coliforms a fIrst order reaction equation is used.

(8)

dS/dt= - KS

where S is the concentration and K is a decay rate (constant). A conservative pollutant such as salinity or dye can be modelled by setting the K coefficient to zero. North Deyon mathematical model As part of a comprehensive strategy for environmental protection, South West Water Services Limited required a mathematical modelling suite to simulate tidal flow, bacterial dispersion, and water quality in the North Devon coastal waters. The model area covers about 52 km of the North Devon coast in the Bristol Channel from Porlock in the east to a point about 6 km west of Baggy Point as shown in Fig. 1.

L

Figure 1. Boundaries of North Devon mathematical model.

The North Devon model was constructed on a space-staggered grid having a regular mesh size of 400 m. The hydrodynamic model was calibrated and verified using tidal data which was made available from a comprehensive environmental survey. This included measurements of current velocities at five locations and water surface elevation at four locations over a continuous 29 day period. Further details are given in Kaya (1992). The model incorporates three types of open boundaries. The eastern open boundary is driven by current velocities while the western and south-western open boundaries are driven by water surface elevations. The northern open boundary was assumed to have zero normal velocities (no flow is allowed across the boundary) and free slip tangential velocities. This assumes that the tidal flow is mainly in the east-west direction along the northern open boundary. This assumption was later justified by the field survey data which indicated a predominantly east-west flow direction. Some difficulties were encountered in trying to calibrate the hydrodynamic modd when a water elevation boundary was used along the western and south-western open boundaries as shown in Fig. 1. Such boundaries do not allow transients generated inside the region to be transmitted outwards. Model instability which appeared after several tidal cycles was investigated by numerical experiments which showed waves being reflected back and forth between the south side of the western boundary and the opposite coastline. The numerical oscillations were first generated in the south west region and propogated towards the north

172

Y.KAYA

and east directions. The amplitude of these oscillations gradually increased and the model became unstable after two consecutive tidal cycles. To overcome these numerical oscillations an appropriate fonn of a radiation boundary condition was introduced along the western open boundary. The method employed is fully described by Blumberg and Kantha (1985) and by Bills and Noye (1987) which uses a simplified radiation boundary condition given as

0" at

+

(gH)"

Urt __

on

(TJ-TJk)

t

(9)

T,

in which n is the direction nonnal to the planar boundary. The tenn on the right hand side represents damping which tends to force the value of TI at the boundary to some known value Tlk with a time scale of the order of Tf. T f 0 corresponds to the original level boundary conditions where no disturbances are allowed to pass out through the boundary: and T f -+ 00 represents the pure radiation condition (Blumberg and Kantha, 1985) and renders the boundary transparent to waves travelling in the positive n direction with phase speed (gH) 112. In this study T f 2r/(gH)112 was used where r is the average distance between the western boundary and the opposite coastline.

=

=

Numerical experiments carried out with the above boundary conditions showed no oscillations in both water levels and current velocities for simulations of several consecutive tide cycles each with approximately 12.4 hours period. The hydrodynamic model was calibrated against a neap tide and validated against a range of spring and neap tides. The model predicted water surface elevation and current velocities with good accuracy as can be seen in Figs 2 and 3.

C

4.0

NORTH DEVON MATHEMATICAL MODEL

3.0 2.0 .§. 1.0 c 0.0

~

~

-1.0 } ·2.0 W .3.0 -4.0 +---r---,.-----,,..----r---r---r-----, 8 8 o 2 10 12 14

TIme (hours)

Figure 2. Comparison of predicted (-) and measured ( • ) water surface elevation at a gauging location.

Ayr sewera~e scheme - mathematical modemo/: As part of Ayr sewerage scheme investigation undertaken for the Department of Sewerage of Strathclyde Regional Council a mathematical model of the entire Ayr Bay in the west coast of Scotland was set up. This model was used to ensure compliance with relevant legislation concerning coastal discharges. The investigation also involved modelling of the sewer system serving a population of about 50,000 and assessing frequency of spill into the coastal waters from a number of stonn water overflows. In assessing the impact of the proposed scheme on the marine environment intennittent discharges from the stonn water overflows were also taken into account

The mathematical model was calibrated and verified against data obtained from a comprehensive marine survey. These included measurements of current speeds and water surface elevation, drouge tracking and dye tracing.

Environmental aspects of marine disposal in UK waters

173

Field observations showed that current velocities in the Ayr Bay are relatively small, having an average velocity of about 0.2 mls during the survey period which extended over a month. In such low flowing waters the effects of wind on both the flow and dispersion of effluent plume becomes more pronounced.

In general. good correlation between the predicted and observed water surface elevations and current velocities was achieved (Fig. 4). However, spring tide predictions where the water surface displacement and current velocities were relatively higher showed better correlation than neap tides. NORTH DEVON MATHEMATICAL MODEL

!'~!L=v::: 8. en

0.5 0

o

2

•

II II TIme (hours)

10

Figure 3. Comparison of predicted (-) and measured ( • )velocities at a current meter location.

, 12

AYR BAY MATHEMATICAL MODEL

i"~ ! "0

0.2

0

o

2

•

II 8 Time (hours)

10

12

Figure 4. Comparison of predicted (-) and measured ( • ) velocities at a current meter location.

CONCLUSIONS

In order to enhance and protect the aquatic and marine environment discharges to coastal waters must be controlled by setting consents. Standards required for these consents should be determined on a case by case basis within the framework of international and national legislation. Predictive models play an important role in assessing compliance with relevant legislation. As a large number of such models are available it is extremely important that the selected model should represent local flow conditions with acceptable accuracy and be suited to the particular modelling application. Care must be taken in selecting proper input data and in interpretation of model results. Typical application of a two-dimensional depth averaged mathematical model to two locations in the UK has been presented. For one of the applications a special radiation boundary condition was introduced along a water elevation open boundary which successfully allowed outgoing transient waves through the boundary yet maintained the background mean tidal elevation. This illustrates the need for modelling software to be sufficiently flexible for it to be enhanced, if required, to suit particular local circumstances. Representation of surface wind effects has been improved by introducing a parabolic vertical velocity profile which permits advection of buoyant plumes with near surface velocities. This technique can be of considerable practical value in this type of marine investigation. ACKNOWLEDGEMENTS The author wishes to express his thanks to South West Water Services Ltd and Strathclyde Regional Council, Department of Sewerage for their permission to publish this work. The encouragement provided by colleagues and contribution from Professor R A Falconer is greatly valued by the author. The views expressed in this paper are those of the author, and should not be assumed to be those of either South West Water Services Ltd. Strathclyde Regional Council or his employers.

. . . .4 ..

Y.KAYA

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REFERENCES Bills, P. and Noye, J. (1987). An Investigation of Open Boundary Conditions for Tidal Models of Shallow Seas. In: Numerical Modelling: Applications to Marine Systems, J Noye (Ed), North Holland. Blumberg, F. A. and Kantha. L. H. (1985). Open Boundary Conditions for Circulation Models. Journal of Hydraulics Engineering, ASCE, Ill, (2), 237-255. Falconer, R. A. (1986). A Two-Dimensional Mathematical Model Study of the Nitrate Levels in an Inland Natural Basin, Proc. Int. Con,{. on Water Quality Modelling in the Inland Natural Environment, BHRA Fluid Engineering, Bournemouth, Paper n, pp. 325-344. Falconer, R. A. and Chen, Y. (1991). An Improved Representation of Flooding and Drying and Wind Stress Effects in a 2-D Tidal Numerical Model, Proc. Inst. Civil Engineers. Pan 2, Research and Theory, 91, 659-678. Falconer, R. A. and Liu S. (1987). Modelling Solute Transport Using QUICK Scheme. Journal of Environmental Engineering. ASCE, 114, (I), 3-20. Kaya. Y. (1992). Some Aspects of Tidal Flow and Water Quality Modelling of the North Devon Coastal Waters. In: Proc. 2nd Int. Con,{. Hydraulics and Environmental Modelling of Coasta~ Estuarine and River Waters, R. A. Falconer, S. N. Chandler• Wilde and S. Q. Liu (Eds), Vol. I, pp 187-199, Asbgate, UK. Koutitas. C. G. (1988). Mathematical Models in Coastal Engineering, Pentech Press Limited, London. Koutitas. C. and Gousidou-Koutita. M. (1986). A Comparative Study of Three Mathematical Models for Wind-Generated Circulation in Coastal Areas. Coastal Engineering, 10, 127-138. Lee. J. H. W. and Neville-Jones, P. J. D. (1987). Initial Dilution of Horizontal Jet in Crossflow. J. Hydr. Eng. ASCE. No.5. Lui, S. and Falconer. R. A. (1989). Application of the QUICK difference Scheme for Two-Dimensional Water Quality Modelling, Proc. Int. Con,{. Hydraulic and Environmental Modelling of Coasta~ Estuarine and River Waters, University of Bradford, pp. 360-370. Marine Pollution Monitoring Management Group (MPMMG) (1994). Comprehensive Studies for the Purposes of Article 6 of DIR 91rl71 EEC, The Urban Waste Water Treatment Directive. Forth River Purification Board, UK. WRc. (1990). Design GUidefor Marine Treatment Schemes, Volumes I and n, WRc. UK.