Environmental Issues & Construction Aspects of an Outfall

Contents
ENVIRONMENTAL ISSUES IN A RATIONAL DESIGN OF AN OUTFALL
CONSTRUCTION ASPECTS OF A SEWAGE OUTFALL

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ENVIRONMENTAL ISSUES IN A RATIONAL DESIGN OF AN OUTFALL

A summary of simple analytical expressions for the environmental design of medium size sewage outfalls in Greece is presented herein. The work is directed toward the designer of a small or medium size system who has no access to complex computer programs, or is limited because of budget constraints.

Rational approach
It is necessary for the designer to determine first the critical pollutant, defined from:

S = max (Cë/Cá)(1)

where S is the dilution, Cë is the concentration of the pollutant in the outfall pipe and Cá the maximum allowable concentration in the waters. The environmental design of the outfall need to satisfy Eq. (1). Typically, discharge limits are set for the initial dilution that is, immediately after the discharge, and at remote locations of special protection such bathing waters, shellfish farms, etc.

Initial dilution
In most cases the discharge behaves like a plume, that is, the buoyancy force exceeds the initial momentum flux and hence determines the height of rise and dilution. In homogeneous waters criterion for the behaviour of the discharge is the Richardson number

R = Q0 B01/2/ M01/4

which approaches 0 for a jet and 1 for a plume, while in stratified waters criterion is the non-dimensional number

N = M02 å /Â02

which exceeds 4 for jets is less than 4 for plumes. In the above expressions, M0 is the initial momentum flux, B0 is the initial buoyancy flux (divided by the density of the discharge, ñw), Q0 is the volume flux) defined from:

Â0= Q0 g(ñs-ñw)/ñs, M0= Q0 u, Q0=ðD2u/4

and å=g(Äñs/z)/ñscharacterizes the degree of stratification, D is the diameter of the round opening, u is the initial velocity of the discharge, ñs is the density of the water and ñw is the density of the discharge.
In the greek waters, and for discharges in depths less than about 20 m, it is safe to state that waters are homogeneous or weakly linearly stratified. For these two cases there are simple analytical expressions for the initial dilution of plumes:

Sm = 0.128 F-2/3 (x/D)5/3 when F < 15(2)

for a vertically discharged plume, and:

Sm = 0.54 F (0.38 H/(D F) + 0.66)5/3 when Ç/D > 0.5 F(3)

Sm = 0.54 F9/16 (H/D)7/16 when Ç/D < 0.5 F

for a horizontally discharged plume [1], where F = u/[gD(ñs-ñw)/ñw]1/2 is the densimetric Froude number and H is the discharge depth.
For a plume in a linearly stratified environment [7]:

Sm = 0.80 B03/4 å-5/8 Q0-1(4)

and the terminal height of rise zmax is:

zmax = 4.5 B01/4 å-3/8

To prevent adjacent plumes from merging, it is recommended to design the ports at distances greater than H/6 where H is the depth of the port.

FAR-FIELD DILUTION
Computer programs of transport and dispersion of pollutants are available [2,8] for the calculation of dilution far from the discharge location. For a first estimate, the horizontal dispersion can be calculated from [4]:

Sm = e-kx/u erf{ [1.5/((1+0.66 â x/b)3 - 1)]1/2 }(5)

and the width L of the plume at distance x:

L/b = (1 + (2/3) â x/b)3/2

where â = 12Kh / (u b).

SUMMARY
The simple expressions for pollutant dilutions presented here are adequate for small and medium size outfall systems. For large systems, or for discharges in environmentally sensitive areas, the design must be performed using available computer programs that account more thoroughly for the complex phenomena of trasport and dispersion in coastal waters.

CONSTRUCTION ASPECTS OF A SEWAGE OUTFALL

Design and construction guidelines are presented herein, which, if followed, will ensure the successful operation of a sewage outfall made from HPDE. This material offers a number of advantages over conventional material for outfall of sizes up to 1000 mm, and is widely used in Greece. The HDPE outfalls have been operating successfully all over the world for a number of years, and any failures are attributed to mishandling of the outfall during launch.

The main advantage of the HDPE outfall is the ease in handling. It can be launched with minimal equipment and within short periods of time, which is important when construction takes place under adverse weather conditions. However, the HDPE material can be damaged by sharp tools or filling material. For this reason, but also because it is lighter than water, the HDPE outfall requires extra protection, constructed usually in the form of a layer of reinforced concrete surrounding the outfall. The thickness of this layer is calculated so as the outfall weights about 20 kps/m in the water.
Launching of the outfall is performed in lengths that are several multiples of the 12-meter factory lengths. The outfall length is equipped with air-filled devices and is floated over its designed position. Air is then removed in a controlled manner from the floating devices so that the outfall is brought to rest on the ocean floor slowly and in a manner that does not allow the development of excess stresses on the outfall material. The outfall can be brought to the ocean floor empty in shallow water as long as its buckling strength is not exceeded. In deeper water, the outfall is gradually filled with water as it moves to the ocean floor.
When the ocean floor material is loose, the preferred method of trenching is the so-called post-trenching technique, whereby a special device proceeds along the outfall, buries it and refills the trench. In rocky areas, explosives are used to open a trench where the outfall is laid. The trench is then filled with graded material by divers so that the material of the layered filter is not mixed.
Protection from the waves is provided by artificial concrete blocks connected with a fibermesh and layered on geofibers in cases where the ocean floor profile undergoes seasonal changes (loose bottom). In rocky ocean floors, quarry stones can be used, are stones of adequate size that are often available in the rocky floor nearby.
Protection from boating and fishing equipment must also be provided in the area of the risers. IPB steel beams can be very successfully used to construct a protective structure spanning the risers.

References
1. Abraham, G, "Jet diffusion in stagnant ambient fluid", Delft Hydraulics Laboratory, 1963, Publ. 29.
2. Bogle, G,V, Valioulis, I.A., and Meiorin L., Estimation of far-field dilution of coastal discharges, ASCE J. of Waterway, Port, Coastal and Ocean Engineering, to appear in January/Febryary, 1993.
3. California Ocean Plan, Water Control Plan, Ocean Waters of California, State of California Water Resources Control Board, March 22, 1990, pp. 23.
4. Fisher, H.B., List, E.J., Koh, R.C.Y, Imberger, J., Brooks, N., 1979, Mixing in Inland and Coastal Waters, Academic Press, New York. 5. King, I.P., 1988, Program Documentation, RMA2 A TwoDimensional Finite Element Model for Flow in Estuaries and Streams, Version 4.2, RMA, Lafayette, California.
6. ÊïéíÞ ÕðïõñãéêÞ Áðüöáóç 46399/1352)89, Åöçìåñßò ôçò ÊõâåñíÞóåùò, Áñ. 438, 3 Éïõëßïõ 1986, óåë. 4316-4331.
7. Kotsovinos, N. "ÄéÜèåóç õãñþí áðïâëÞôùí óôçí èÜëáóóá", Ðñüãñáììá COMETT, Åñã. ÕäñáõëéêÞò Á.Ð.È., 21 Ìáñôßïõ 1989.
8. U.S. Environmental Protection Agency (ÅPA) Report PB86-137478, U.S. EPA, Newport, Oregon, 1985.