Appendix A: Outfall Pipe Hydraulics - · PDF fileAppendix A: Outfall Pipe Hydraulics. Project...

Appendix A: Outfall Pipe Hydraulics

Project number: 14830.RDated: 2014/07/16Revised:

a. GENERAL

This study is concerned with dense brine plumes released into the ocean: the brine is transported from thereservoir via a pipe and released into the ambient waters through a diffuser structure (Appendix B:Drawing K1000 and K1001). The brine stream released into the ocean is denser and thus heavier than theambient receiving waters. As a result, the effluent brine will tend to sink to the bottom of the ocean undergravitational forces and accumulate at the sea bottom.

The brine discharge pipeline will be equipped with a diffuser system. The primary objective for the designof a diffuser for a sea outfall is to discharge the effluent in such a way that the environmental criteria (forthe specific area and the project) are adhered to during the weather (sea) conditions throughout the designlife of the outfall. The hydraulic design of the outfall (main pipe and diffuser) must be correct in order toachieve this objective.

Normally, ocean outfalls will attempt to dilute the discharge at the outfall site via the configuration of thediffuser before the natural ambient conditions (currents, tidal oscillations, etc.) occurring in the regionfurther disperses and advects the effluent.

Section (b) details the design parameters for the main discharge pipe up to the diffuser; Section (c)describes the design considerations for the offshore brine diffuser; while Section (d) describes the designconsiderations for the surf zone brine diffuser.

b. MAIN BRINE OUTFALL PIPE

At Volwaterbaai, the brine is discharged from a sump at the RO plant at +16.5m to MSL via the main brineoutfall pipe to a diffuser located in one of two locations:

In the surf zone with a maximum water level of +3.3m to MSL (max water depth of 4.9m at the outfalllocation); or

Off shore to a depth of -10m to MSL (max water depth of 13.3m).

Two brine discharge flows are considered: 316 l/s (6 million m3/year scenario) and 422 l/s (for the 8 millionm3/year scenario) .

The surf zone discharge has a pipe length of 880m and an off shore discharge a total pipe length of 730m.

Design assumptions:

i) HDPE pipe: PN 6.3 SDR 21: Pipe roughness = 0.8mm (for an old pipe)

ii) Gravity flow

iii) V = 1.7m/s

iv) Flow conditions: max = 422 l/s and min = 316 l/s

v) Minimum static head available = +16.5m – (+3.7m) = +12.8m

Pipe friction losses were calculated using Colebrook-White. Table 12 gives the calculated pipe frictionlosses for the proposed brine outfall pipes for the flows of 6Mm3/a and 8Mm3/a.Table 12: Colebrook friction factor & friction head loss

EffluentFlow

(m3/s)

Pipelength

(m)

Pipediameter

(m)

Effluentvelocity

(m/s)

Total friction loss(m)

6Mm3/a (Surf Zone) 0.316 880 0.51 1.57 5.138Mm3/a (Surf Zone) 0.422 880 0.57 1.65 4.936Mm3/a (Off Shore) 0.316 730 0.51 1.57 4.148Mm3/a (Off Shore) 0.422 730 0.57 1.65 4.14

c. OFFSHORE BRINE DIFFUSER

The off shore diffuser system will consists of a number of discharge ports, through which the brine will bedispersed over a relatively large area (Appendix B: Drawing K1000).

(i) Offshore Bathymetry

It can be seen in Appendix B: Drawing SKK019 that the offshore outfall pipe extends approximately 560minto the ocean up to the –10m to MSL depth. The sea floor slope (1:56) is relatively flat in the offshoredirection.

The dive survey (UCT, 2014) reports that exposed rocks / boulders are visible in water depths of -5m toMSL and shallower while a prominent sand layer is present in water depths of -5m to MSL and deeper.The depth of the sand layer was not determined during the dive.

(ii) Diffuser Friction Loss: Off ShoreAppendix A: Drawing K/1000 shows the conceptual designs for the dimensions of the offshore diffuser ascalculated by the CorHyd model (Bleninger & Jirka, 2005). The hydraulic analysis of the program is basedon the hydraulic energy balance for the complete system (multi-port diffuser and main pipeline) bycomparing the specific energy between any two points in the system, taking into account all friction andlocal fitting losses between two adjacent points.

Table 13 gives the hydraulic head losses calculated for the offshore outfall assuming full pipe flow.Table 13: Offshore diffuser friction losses for the 6Mm3/a and 8Mm3/a flow scenarios

Riserheight

(m)

PortDiameter

(m)

PipeFriction

Loss(m)

Port ExitVelocity

(m/s)

DiffuserHead Loss

(m)

Total Headloss(m)

6Mm3/a (OffShore) 0.20 0.13 4.14 7.95 6.57 10.71

8Mm3/a (OffShore) 0.20 0.13 4.14 7.95 6.62 10.76

From Table 13 it is clear that there is sufficient static hydraulic head available for gravity flow brinedischarge. Table 14 summarises the offshore outfall diffuser configurations.Table 14: Diffuser configuration for the 6Mm3/a and 8Mm3/a flow scenarios.

6Mm3/a Scenario(Flow 316 l/s)

8Mm3/a Discharge(Flow 422 l/s)

Diffuser depth 10 m

Diffuser length 7.5 m 10.5 m

Main pipe diameter (ID) 510 mm 570 mm

Nr of ports 3 4

Port direction 45 degrees from horizontal, discharging to alternate sides of the main diffuser pipeNumber of diffusersections 2 sections 2 sections

Tapers in main diffuserpipe (ID) Taper to 400 mm Taper to 400 mm

Main pipe and portdiameter per diffusersection

1st section: Main pipe dia = 510 mm ID;2 ports; Port dia = 130 mm ID

1st section: Main pipe dia = 570 mm ID;2 ports; Port dia = 130 mm ID

2nd section: Main pipe dia = 400 mm ID;1 port; Port dia = 130 mm ID

2nd section: Main pipe dia = 400 mm ID;2 ports; Port dia = 130 mm ID

Port spacing 3.5 m


d. SURF ZONE BRINE DIFFUSER

The surf zone diffuser system will consist of a singular discharge port, through which the brine will bedischarge at a high velocity (Appendix B: Drawing K1000).

(i) Near shore Bathymetry

The surf zone discharge pipe is located inside a gully (Appendix B: Drawing SKK019) and will discharge ata depth of -1.56m to MSL. The slope inside the gully can be regarded as flat i.e. no slope.

The dive survey (UCT, 2014) reports that mainly boulders are present around the entrance of the gullywhich varies in size. No significant big gullies were observed nor any long-shore gullies where brine mightget trapped.

(ii) Diffuser Friction Loss: Surf ZoneAppendix B: Drawing K/1001 shows the conceptual designs for the surf zone diffuser. Table 15 gives thehydraulic head losses calculated for the surf zone outfall:Table 15: Surf zone diffuser friction losses for the 6Mm3/a and 8Mm3/a flow scenarios

PortDiameter

(m)

DiffuserLength

(m)

Port ExitVelocity

(m/s)

PipeFriction

Loss(m)

DiffuserHeadLoss(m)

TotalHeadloss(m)

6Mm3/a (Surf Zone) 0.175 1.00 13.13 5.13 3.75 8.88

8Mm3/a (Surf Zone) 0.200 1.10 13.43 4.93 3.94 8.83

From Table 15 it is clear that there is sufficient static hydraulic head available for gravity flow brinedischarge. The angle of discharge for the beach outfall is horizontal.

Appendix B: Drawings and Figures

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BRINE OUTFALL,GULLY

PROPOSED BRINEOUTFALL, OFFSHORE

INTAKE BASIN

DEPTH= -10m to MSL

NOTE:

LEVELS TO MSL

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Effluent DischargeQuasi Steady State2008 Water Level Time Series

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6Mm3/a: 1.5ppt ABOVE AMBIENT8Mm3/a: 1.5ppt ABOVE AMBIENT

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Effluent DischargeQuasi Steady State1998 Tidal Level Time Series

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Figure B.4: Plot of dilutions achieved for the 2008 time series calculated at a fixed point 10m from the point of initial dilution for the 6Mm3/a discharge. Thered line indicates the required dilution of D=5.

Figure B.9: Plot of dilutions achieved for the 2008 time series calculated at a fixed point 20m from the point of initial dilution for the 8Mm3/a discharge. Thered line indicates the required dilution of D=5

WSP Group Africa (Pty) LtdStein HouseBrandwacht Office Park, Trumali StreetStellenbosch7600South AfricaTel: +27 21 883 9260Fax: +27 21 883 3212

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Appendix F: Marine Ecology Impact Assessment

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