Seawater Marine Outfalls

[Ariel Reyes]

Hello Everyone,

I'm looking for some technical advise on the design of seawater marine outfalls, particularly, technical standards, codes which govern the sizing of outfall pipes, diffuser ports. For a 30,000 cubic meters per hour discharge rate, I've come up with these figures.

1. Pipe Material: HDPE
2. Pipe Roughness: 0.1 mm
3. Pipe Length: 107 meters
4. Seawater Density: 1022 kg/m3
5. Effluent Density: 1019 kg/m3
6. Average Velocity: 1.84 m/s
7. Frictional Head Loss: 0.084 meters (using Darcy Weisbach)

Given are the ff:

High Water Level of 51.74 meter
Elevation of Diffuser Port Tip: 40.90 meters

What would be the elevation or the static head needed to drive the effluent for proper dilution and the velocity of the diffuser ports? Attached is my sketch showing plan and profile.

Thanks and Best Regards,
Ariel

 

[mk3223]

IMO, it could be a real help to hire a Consultant specialized in the Mrine project to design your effluent outfall system. It's a complicate design and calculation, and involves the environmental and local authority review and approval.

 

[Ariel Reyes]

Hello mk3223,

We actually have a consultant who'll design the outfall port side and generate some thermal plume study. What we're after here is if the feeder/outfall pipe can generate a static head strong enough
to counteract the high water level of 51.74 meters.

Regards,

 

[mk3223]

I am not sure what is the static head on the pipe.
Is it a piece of "open ended" pipe to be filled with water and sunk under the water at EL. +35.9?

 

[bimr]

We actually have a consultant who'll design the outfall port side and generate some thermal plume study. What we're after here is if the feeder/outfall pipe can generate a static head strong enough to counteract the high water level of 51.74 meters.

The feeder/outfall pipe does not "generate a static head".

Your consultant should know the desired flow velocity in the diffusers in order to design the outfall.

Calculate the outfall head loss using the Darcy-Weisbach equation for the desired flow velocity in the outfall pipe. The headloss should be calculated for the worst case condition, the high water level condition. You should also calculate the headloss for the low water level conditions and confirm that the flow velocities will be acceptable for operating the outfall. The flow velocities will be higher for the low water level condition. Since your drawing shows the difference between high water level of 51.74 and MSL of 50.38 being 1.36 meters, the low water level conditions appear to be acceptable.

Where the plant discharge into the outfall, your drawing shows an invert elevation of 54.736 which means you have at least 54.736 - 51.74 = 2.996 of available static head. There is also additional static head available since the outfall pipe is 96-Inch diameter.

The minimum static head required will be the elevation of 51.74 plus the calculated outfall headloss (you have stated a Frictional Headloss = 0.084 meters).

It appears there is adequate elevation to gravity discharge into the outfall.

 

[Ariel Reyes]

Many thanks for your replies, this would give us much needed direction in our design!