Emisarios (Sea Outfall)

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622 COASTAL ENGINEERING

15.7 Sea outfalls

15.7.1 General

The oceans have a great capacity to assimilate wastes and to make themharmless and they are being exploited as receivers of wastes from outfalls.A typical outfall consists of a pipe discharging effluents from the land tothe sea and laid on or buried in the sea bed (or housed in a tunnel). It isextremely important that the design of outfall should take account notonly of its hydraulic function but also of the environmental impact. Some

of the short outfalls constructed have failed from the environmental stand-point since effluents have found their way back polluting the beaches.Even long outfalls may sometimes be unsatisfactory since effluents canreturn to the beach depending upon the direction of currents that are pro-duced by the tidal flow, waves and wind.

There are essentially two outfall types. One consists of a pipe with adiffuser at its seaward end, both above the sea bed, and the wall ports,through which the effluent discharges, are just holes through the wall of the pipe; the second type, buried under the sea bed, has a number of risersfrom the diffuser. The outfall pipe and the risers must be protected fromship anchors and fishing lines.

In large outfall schemes and in schemes involving confined waters,the concentrations of the pollutants discharged should be checked periodi-cally by sampling the quality of water which is usually characterized by thebiological oxygen demand (BOD), bacterial content, suspended matter,turbidity, pH, temperature, toxic chemicals, minerals and organic andinorganic matter. The impact of these various constituents on the environ-ment is often complex and not necessarily immediate.

When the effluent is discharged into the sea, it is diluted rapidly byinitial dilution and subsequently by the secondary dispersion. The BODand the suspended solids are reduced quickly by these processes. It is,however, necessary that the ambient currents are favourable and that thesite is at a sufficient distance from the shore to reduce the bacterialcontent still further through dilution, mortality and sedimentation beforethe diluted sewage may reach the shore.

Apart from the above environmental aspects, the design of theoutfall system will depend on the characteristics of the sewage system,storm overflows, the outfall site, the headworks and the outfall itself. Thestorm overflows on combined systems (collecting both sewage and rainwa-ter run-off) are designed to reduce the flow in the sewers to about 6 timesthe dry weather flow and may also be a cause of pollution whose impact onthe environment has to be assessed.

The preliminary treatment of the wastewater on land and the sub-sequent marine treatment should complement each other in order to



SEA OUTFALLS 623

achieve environmentally acceptable standards. Before the effluent is dis-charged into the sea, the grit is removed to avoid its deposition in theoutfall pipe and the suspended matter is reduced to small sizes (thescreens should pass a maximum particle size of 5–6 mm). If the suspendedload in the effluent is such that the marine treatment is not very effectiveenvironmentally, then primary sedimentation may be incorporated beforedischarging the effluent into the sea. If the effectiveness of the outfall fallsshort of the acceptable bacterial content, then disinfection may be recom-mended. It is worth noting that the sludge produced in primary sedimenta-tion may be difficult to dispose of and that disinfection of the effluent canbe detrimental to the aquatic life.

In this concise text many aspects of sea outfalls could be touched ononly briefly; for a fuller treatment of the subject, the reader is referred toCharlton (1985) and Neville-Jones and Dorling (1986).

15.7.2 Site surveys

For effective design of outfalls, site surveys should be undertaken to assessthe quality of receiving water, tides and currents, and environmental con-ditions (e.g. seaweed, animal life, bacteria, etc.). Charlton (1985) describesthe methods used to obtain a form of tidal atlas in which the current vari-

ation throughout the tidal cycle is presented on an hourly basis. Theoverall performance of the outfall is predicted by tracer tests. These testsare carried out usually in normal conditions of the sea by introducingtracers at the site and the dispersion and dilution of the tracer are continu-ously monitored mainly at a depth less than 1 m.

15.7.3 Initial dilution and secondary dispersion

The effluent passing through a port is lighter than the surrounding seawater; the density of sea water is typically 1.026kgm3 and depends on theambient temperature and salt content. Thus a density gradient across

the depth may exist due to variations of temperature and salinity with thedensity increasing in depth. The density stratification is likely to be morepronounced in summer than in winter.

Consider that the effluent from the port exhausts in the verticaldirection into the sea of uniform density without ambient currents. Theresulting buoyant jet spreads owing to its initial momentum rate and thebuoyancy effects. Without the momentum rate, the effluent from the portwill spread as pure plume driven only by the buoyancy. As the jet rises tothe surface, entrainment of the surrounding water takes place with the




maximum velocity and concentration along the centreline of the jet decay-ing in the streamwise direction. Instead of concentration, the term dilutionis commonly used. If, say, 99ml of pure salt water mixes with 1ml of theeffluent, then the dilution is 100:1 or simply 100. On reaching the surfaceof the sea the effluent tends to spread horizontally.

The density of the effluent increases through dilution. If the receiv-ing water is density stratified, then the effluent may not even reach thesurface as, when it reaches a value at some depth equal to the density of the surrounding water, the plume is arrested and spreads horizontally.

The next phase of the effluent transport is the advection by oceancurrents and turbulent diffusion. The phase, called the secondary disper-

sion, depends on the current and the turbulence structure of the ocean.Ports generally discharge effluents horizontally to achieve the best

dilution (Fig.15.11). Figure 15.11 also shows the spread of the jet to be dis-cussed further in Section 15.7.6. For horizontally discharged effluent,about 20% to 50% greater dilutions are obtained at the water surface thanfor vertical discharge.

Fig.15.11 The spread of horizontal jets in still water (after Charlton,1985)



SEA OUTFALLS 625

15.7.4 Still ambient fluid

The initial dilution of the effluent Sm along the centreline of the jet issuingfrom the port may be expressed in still water conditions in a functionalform as

Sm f 0 , (15.34)

where V p is the velocity at the exit of the port, y0 is the depth of the portexit below the surface and Dp is the diameter of the port. g g( a 0)/ 0where a is the density of the ambient fluid and 0 is the density of theeffluent. The first term in the brackets on the right-hand side of equation(15.34) is the densimetric Froude number denoted as FD (equation(8.20)).

The spread of the buoyant jet horizontally discharged in still sur-rounding fluid has been studied extensively and the results are presentedin the form of equation (15.34) as charts or equations. Figure 15.12shows the graphical expression of the functional form due to Fan andBrooks (1966). Cederwall (1968) expresses the dilution in the form of equations as

Sm

0.54FD

7/16

for 0.5FD (15.35) y0

Dp

y0

DpFD

y0

Dp

V p

( gDp)1/2

Fig.15.12 Centreline dilution as a function of y 0/D p and FD




and

Sm 0.54FD 0.665/3

for 0.5FD. (15.36)

Sm is the minimum dilution occurring along the centreline of the plume.For vertical port discharges, the average plume dilutions Sa in the initialmixing region can be determined by the equations due to Muellnoff et al.

(1985) with Qp being the port discharge:

Sa 0.13 g1/3Qp2/3 y0

5/3

which can be written in non-dimensional form as

Sa 0.13π42/3

5/3

. (15.37)

The average plume dilution Sa is approximately 1.8 times the minimumdilution Sm at the centreline.

Figure 15.12 shows that the same dilution can be obtained for eitherlow or high exit densimetric Froude numbers. Low velocities mean lowdriving heads while large velocities need large heads. The choice of lowvelocities can lead to malfunctions of the outfall if deposits of sediments on

the bed occur or if the densimetric Froude number is below the critical limitof unity when salt water intrusion reducing the effluent flow will develop.

15.7.5 Moving receiving water

When the effluent is discharged into receiving water moving with velocityV a the dilution is significantly modified from that in calm water. Fieldstudies (Agg and Wakeford, 1972) give an equation for an additional dilu-tion factor which is a ratio of the dilution in moving water to the dilutionin still water. The empirical equation for applicable in the range0.1 V a/V p 2.0 is

log 1.107 0.938log(V a/V p). (15.38)

(The dilution in moving water is determined by multiplying the dilution instill water of Fig.15.12 by the factor .) Lee and Neville-Jones (1987)present the following equations for dilution in moving water:

Sm for y0 , (15.39)5B

V 3a

0.31B1/3 y05/3

Qp

y0

Dp

1

FD2/3

y0

Dp

0.38 y0

DpFD



SEA OUTFALLS 627

Sm for y0 (15.40)

and

B Qp g .

For small values of y0/Dp, the results of Agg and Wakeford are comparablewith equations (15.39) and (15.40) but, for y0/Dp between 50 and 100, theyunderpredict the initial dilution (Neville Jones and Dorling, 1986).

For vertical single ports discharging into moving receiving water,Muellenhoff et al. (1985) present the following equation for average dilu-tion Sa:

Sa . (15.41)

15.7.6 Spacing of ports

The spacing of the ports depends on the geometry of the plume as itspreads. Under still water conditions the buoyant rising plumes should

not overlap till they reach the surface. The geometry of the buoyantplume in still water according to Brooks (1970) is shown in Fig.15.11.The dimensionless radial extent of the plume W /Dp at the free surface ispresented in Fig. 15.11 as a function of the densimetric Froude numberFD and y0/Dp. The spacing of the ports should be greater than W . Inmoving receiving water, the size of the plume is increased with theincreased degree of dilution. Neville-Jones and Dorling (1986) suggestthat for V a/V p 0.2, W 0.5 y0 for y0 2B/V 3a. The width W 0.9 y0 forV a/V p 0.2.

15.7.7 Diffuser design

The flow through the outfall may be due to gravity depending on the headavailable between the headworks and the highest water level at the dis-charge location in the sea. If enough head is not available, then the waste-water is pumped through the outfall.

The frictional loss in the straight pipe of the outfall leading to the dif-fuser is calculated using the Darcy–Weisbach equation (equation (8.4))with friction factor computed by equation (8.5) or the Colebrook–Whiteequation (Worked example 13.1). During the operation of the outfall,

0.49V a y20

Qp

a 0

0

5B

V 3a

0.32V a y20

Qp




slime builds up on the wall of the pipe increasing the roughness height andreducing the effective diameter of the pipe; this should be taken intoaccount in headloss calculations.

Figure 15.13 shows typically a diffuser with three ports with verticalrisers. An expression for the discharge through a port is obtained byapplying the energy equation between the exit of the port and the centre-line of the diffuser at the junction. Consider for example, port 2:

l p2 (ke kst kb) (15.42)

where p2 and V 2 are the pressure and velocity respectively just before the junction at 2. l p2 is the vertical distance between the port exit and thecentreline of the diffuser and pa2 is the ambient pressure at the port exit.

ke, kst and kb are respectively the coefficients for the headlosses at entry tothe riser, along the straight riser and in the bend (Miller, 1994). The dis-charge through the second port Qp2 is then Qp2 ap2V p2.

Wall ports are simply holes through the wall of the diffuser and mayhave a rounded or sharp-edged entrance. Equation (15.42) applies withl p2 kst kb 0. Therefore for V p2

V p2 1

1

ke

2 gE21/2

(15.43)

V 2p2

2 g

V 2p2

2 g

pa2

g

V 22

2 g

p2

g

Fig.15.13 Diffuser with risers



SEA OUTFALLS 629

where

E2

p

g

2

p

a

g

2 (15.44)

and the flow rate is

Qp2 cap2V p2 cdap2(2 gE2)1/2 (15.45)

with

cd c 1/2

.

An empirical equation (Koh and Brooks, 1975) based on laboratoryexperiments for port diameters less than one-tenth of the diameter of thediffuser for a bell-mouth entrance is

cd 0.9751 3/8

(15.46)

and for a sharp-edged entrance is

cd 0.63 0.58 . (15.47)

E j is as given by equation (15.44). (V j is the velocity in the diffuser justbefore the junction j and V pj is the velocity through the port at junction j .)For the riser ports the experimental discharge coefficients vary from 0.648to 0.848 (Grace, 1978). These coefficients may be used for the preliminarydesign; for detailed calculations equation (15.42) taking account of thelosses in the individual components of the riser assembly has to be fol-lowed.

(a) Analysis of flow in the diffuser

Referring to Fig.15.13, the energy equation for the segment of the diffuserbetween ports 2 and 3 is

∆Z 23 hf23 hc. (15.48)

p3 and V 3 are respectively the pressure and velocity in the diffuser at thesection immediately downstream of junction 3 (Fig.15.13). p2 and V 2 referrespectively to pressure and velocity occurring upstream of junction 2

V 22

2 g

p2

g

V 32

2 g

p3

g

V 2 j

2 gE j

V 2 j

2 gE j

11 ke

V 22

2 g




(Fig.15.13). ∆Z 23 is the change in elevation . hf23 is the frictional head lossin the straight pipe and hc is the head loss in the contraction which isusually negligible. Z 2 and Z 3 are the elevations of junctions 1 and 2 respec-tively (Fig.15.13).

The pressure across the junction (Fig.15.13) is determined using theenergy equation which, as an example, for junction 2 is

k2 . (15.49)

Note that

Q2 Q2 Qp2 (15.50)

where Q2, Q2 and Qp2 are respectively the discharges upstream and down-stream of the junction and through the port.

The analysis of flow in the diffuser is a step-by-step procedure start-ing from the extreme seaward end. The calculation procedure for the wallports (or ports with risers) is as follows.

1. Estimate E1 relevant to port 1 in the diffuser at the junction with thefirst port for the discharge Qp1 through the first port, using equation(15.44); determine p1.

2. Determine the pressure p2 just downstream of junction 2 from equa-tion (15.48).3. Solve equations (15.49) and (15.50) or (15.45) to find the pressure p2

just upstream of the junction and the flow Qp2 through the secondport.

4. Continue the procedure to port 3 and so on.5. Check whether the sum of the discharges through all the ports is the

same as the design discharge.

The calculation procedure can be carried out using computer programs.It is difficult to avoid at all times deposition of sediments in the

outfall pipe and the diffuser (at low flows) as a small pipe diameterincreasing the flow velocity would result in large energy losses and greater

pump power requirements. Usually some sediments are allowed to depositat low flows to be flushed out at high flows. Generally velocities in excessof about 0.9ms1 will be required at high flows. For further information onsediment transport in sewers and outfall see Novak and Nalluri (1987) andAckers (1991). In order that the velocities along the diffuser are suffi-ciently large to dislodge the sediments from the bed and transport them,the diffuser diameter is reduced progressively as shown in Fig. 15.13.

An outline of a preliminary outfall design is given in Workedexample 15.4.

V 22

2 g

V 22

g

p2

g

V 22

2 g

p2

g

Emisarios (Sea Outfall)

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