As it isLiverpool airport current runway is a modest 2,285m, while this is a reasonable length it prevents the biggest aircraft from taking off at their maximum takeoff weight, which limits either the payload(passengers) or the fuel load(range).
What is needed
In the images below the lengths of other airport's runways are marked. The pink is the current runway at Liverpool, while the red extension is the approach lighting system(ALS). The green line in the middle is Manchester airport's 3,050 m runway, the light bit again the ALS. Yellow is Heathrow's 3902-metre strip.
What is needed
|Madeira Airport Runway by Richard Bartz|
What it would look likeTo get to the length of Manchester's runway, with a taxiway would need a strip 375m wide and would require reclaiming a triangle 11.5 hectares
|Manchester Length Runway|
|Heathrow Length Runway|
How it could be done.There are several methods of reclamation that could be used the simplest is building a cofferdam around the perimeter of the area and then filling it with dredgings from the river.While this would damage the river bed and remove some of the existing shore, the total length of shore would be greater when it was finished. The river would be made slightly narrower which would increase the flow rate and probably result in a change in the deep channel position. This happens anyway and requires the river to be regularly surveyed.
To build on columns there are numerous methods but one to consider would be building floating the extension sections, the float them into position before sinking them on to a prepared river bed, this would interfere with the flow of the river less. The columns would soon be covered in various forms of life but would quite likely soon silt up. Unless this could be done considerably cheaper than the perimeter dam, I doubt it would be worth considering.
One system for doing this is below.
and another here
Why extend it at all?The world's largest aircraft in passenger service is the Airbus A380 at its Maximum Take of Weight it requires 2,950m of runway, meaning it can just about operate from Manchester's runway, while the 777-200ER takes 3,380 m and several 747 variants cannot take off from Manchester at full load . So any growth in that aircraft which requires a long run, will not be capable of using Manchester and obviously it cannot use Liverpool now. So if an extension is built then it would seem sensible to at least reclaim sufficient land to build a Heathrow length runway.
Changes in aircraft propulsion system to say electric may require longer runways as the installed power would be less, leading to lower acceleration and a need for a longer run to get up to speed.
The Northern Space Consortium investigated using Liverpool as a Spaceport that would need a longer runway to operate something like Skylon, the increase runway speed and weight would require a very strong runway. The current runway is very strong due to the nature of the ground, any extension would need to match that strength. Even for Suborbital Point to Point hops like the proposed Reaction Engine A2 or SpaceShip 3 the longer runway would be useful.
It is IMHO a project worth doing, I'm not sure of the cost though a lot less that Liverpool possibly in the region of £75,000,000 for a Manchester length & £200,000,000 for a Heathrow length but that is simply a guess, assuming land reclamation at about £5,000,000 per hectare ,runway at £7000 per linear metre of runway and £6000 per linear meter of taxiway see here.