What makes a great wave great? Besides good swell that is. What is it that happens under the surface, down there among the bottom contours that can transform a line of swell into something surfers will devote their whole lives to?

At reefs and rock breaks, where the bottom is somewhat permanent, the answers are evident; bottom topography can be mapped and the secrets of a surf break exposed. Yet at sand bottom waves, where the system is in a constant state of flux, the secrets are less tangible and therefore harder to know.

Last year the entrance to Narrabeen Lagoon, which flows out in front of the famous North Narrabeen left, was blocked and a joint council project initiated to clear the channel. The project began in September 2011 and was finished early 2012. Interestingly, while the project was underway the waves at North Narrabeen were the worst they'd been in many years. It became apparent that Narrabeen - the most famous wave in Sydney, and the home of Australia's most successful and fiercest boardriders club - was dependent on continuous sand flow. It was one of the secrets to its wave quality.

The dredging project at Narrabeen Lagoon finished in February and the channel now flows freely again, yet local surfers have found that the wave quality hasn't been fully restored. They, along with local environmentalists, are now pointing the finger at the sand dune at the very tip of the Narrabeen Peninsula. The dune - known as Birdwood Dune as it sits at the northern end of Birdwood Park - is adjacent to the lagoon entrance and directly in front of Northy.

Upon being told of this news my first response was to dismiss the local reaction as misguided. So the waves aren't as great as they were, what role did a vegetated sand dune, seemingly in its natural state, have to do with it? Were the locals guilty of sporting rose tinted glasses and looking to apportion blame?

According to Brendan Donohoe, local surfer and Surfrider representative, Birdwood Dune has a lot to do with the current state of the waves at Narrabeen. The whole area has been mismanaged, and to explain how Donohoe gives me a short history lesson on the region.

Although the Birdwood Dune looks to me like a healthy, natural sand dune I'm surprised to learn it is an artificial construction. Birdwood Dune is actually man-made. The dune was built in 1974 and 1975 following a period of intense swell activity. It was during this time, when Sydney was ravaged by huge storms, that the dune was built as a buffer to protect the coast and the lagoon entrance. Vegetation was planted to stabilise it and access restricted to set paths.

Once it was built Birdwood Dune did its intended job of sand gathering well - too well, in fact. Originally designed at three metres above sea level the effect of a constant sand supply from both the lagoon and longshore drift combined with surface vegetation that trapped the sand meant its size grew far beyond the original design. Indeed, sand that would naturally find it's way out to sea, forming the banks under Alley Rights, Narrabeen proper and Carpark Rights, now gets trapped in the ever-growing Birdwood Dune. Such is the height the dune has reached that it now blocks the line of sight between North Narrabeen Surf Club and the waves. A sure sign of its ever-growing bulk and how it is impeding natural sand flow.

A commonly cited fact about Collaroy - Narrabeen is that it ranks as the beach third most at risk from coastal processes in Australia, though a source for this information isn't easy to find. Perhaps it's one of those factoids that gets forever repeated without anyone stopping to confirm it. What can be confirmed is that the Collaroy-Narrabeen stretch is an extremely dynamic coastal system with many smaller interdependent systems within it. If one component is tampered with the effect is felt throughout the whole system.

What can also be confirmed is that the Collaroy-Narrabeen stretch, including the beach and the lagoon, is one of the most intensely capitalised shorelines in Australia with Warringah Council estimating the real estate to be worth in excess of $500 million. The council have good reason to want to protect its coastal properties. What is being questioned by the likes of Brendan Donohoe, Tom Kirsop and others are the methods used to achieve this.

Recently Surfrider Foundation listed North Narrabeen as an endangered wave due to the sand-starving impact Birdwood Dune is having on the waves. Not a title given lightly, Northy is now keeping dubious company with Bastion Point in Victoria, Kirra in Queensland, and Tam O'Shanter in Tasmania.

It's with this background knowledge that Warringah Council has proposed a master plan to improve public amenity at North Narrabeen, including Birdwood Park and Birdwood Dune. "The development of a master plan is to create an overall vision for the area to stop ad hoc development and to ensure the area is conserved and maintained," said Warringah Mayor, Michael Regan last December. In the same statement Mayor Regan also said, "North Narrabeen is a National Surfing Reserve with iconic surf breaks and a rich surfing history." A statement aimed squarely at Narrabeen's large surfing constituency.

In May a draft version of the master plan was released by landscape architects Clouston Associates. To the dismay of local surfers it does nothing to address the state of the 'iconic surf breaks' of Narrabeen. To the contrary, this version further increases the sand trapping capability of Birdwood Dune. On the western side will be a new asphalt carpark, on the eastern side a new managed dune area replanted with Acacia, and on top of the dune a permanent boardwalk leading from the surf club carpark to the northern tip of the dune.

It's obvious what Warringah Council is trying to achieve at Narrabeen. Rather than overseeing an ever-changing coastline, and dealing with the ongoing costs and risks to property that may occur, they can simply erect unimposing yet permanent structures on the dune. Reclaim it as an area for recreation and lock it down for good.

The issue, however, is that sand bottomed waves such as North Narrabeen rely on the surrounding ecosystems to be in a constant state of flux. Sand needs to be available for storage and nourishment as the perpetual cycle of storms and swells change the profile of the beach and the underlying shape of the banks. What is becoming apparent to Narrabeen surfers is that the state of the Birdwood Dune is being reflected in the local bottom topography. An abundance of sand on shore heralds a shortage of sand underwater. It's a situation they don't want to become permanent.