Sand, sand and yet more sand. Like, how much per year?

Seagullz love sand. New Brighton and indeed the whole of Pegasus Bay is largely sand. It comes from the mountains, down the Waimakariri and Ashley rivers, out to sea and down the coast. The prevailing north-easterly swells work this sand down the coast, and it ends up as more land, shallower shoreline, and higher features (dune systems) close to the shoreline.

Just how much sand is added to New Brighton, the Southshore spit, the Avon-Heathcote Estuary, and the river mouth off Sumner, is not a commonly quoted amount. We fly over it every day, and there's a lot more of it than there used to be. You, dear reader, will search in vain for a figure for this volume in any Environment Canterbury or Christchurch City Council Web resource. You will also, and less surprisingly, also be unable to find any strategy for managing it. Perhaps this is a state of denial, but one can never neglect good old fashioned ignorance as a possible explanation.

So how much sand does get added to the southern part of Pegasus Bay each year? It's not difficult to get a rough estimate. Using a Lands and Information map (M35, and conventional eastings-northings notation as set out on the map itself to give references) and the fact (disclosed in this pdf fact sheet) that the beach has been moving eastwards for around 450 years from a point we'll take to be near Travis Swamp, we'll do some basic arithmetic on a single A4 sheet. Calculators, map, pencil and paper ready! Here we go.

Looking at our map, we see that Brooklands Lagoon looks quite like an old, cut-off beachline. First simplifying assumption (seagulls have quite small brains, best not to clutter them up trying to be too clever): we'll take a point at the bottom of this lagoon, near a kink on Heyden's Road, and assume that all the beach movement has 'pivoted' around this single point. Grid ref is M35-864531. Think of the later beach growth as a tall triangle (or, tis the season, a Christmas Tree), point facing north. This is that point. And you have to use your imagination, because dere's no pictures on this blog. Seagullz claws can't hold a pencil that well, you see.

Second assumption: the beach 450 years ago was a straight line going down past Travis Swamp (which is an old oxbow loop of the Avon river), to our second reference point, at the western tip of the sewerage farm. Grid ref is M35-857425. The distance down this line is 10.5 km.
Third assumption: this was all foreshore and seabed 450 years ago, and has been raised by the addition of sand, by an average of 2 metres. This is probably an under-estimate, but what the hell. There are few trig points (formal survey marks) quoted on our M35 map, and we have no notion of average elevation. 2 metres it is.

Fourth assumption: The Southshore Spit (from the third point on our triangle, M35-890423) has taken only 30 or 35 years to build to its current height. This may surprise some readers, but tides used to wash over it, and terra firma essentially ended at the south end of Estuary Road. This gives us an opportunity to check the 'how much sand' figure we are about to calculate.

A small aside: Cross checking is always a Good Thing. I can't tell you how many times when flying fast and low, I've almost mistaken those splatters around pub carparks for just another infliction of abstract art - pavement graffiti, if you will. Whereas they are, upon Cross Checking, actually a warm and nourishing meal....life is just so full of confirming experiences.

Lets do the math.

Our triangle (right-angled at the sewerage ponds corner to Keep Things Simple) is 10,500 m high and 3,250 m wide at the base. Area is half base x height, thats, ummmm, 17,062,500 square metres. Using our third assumption of 2 metres accretion over this, that's 34,125,000 cubic metres of sand.

But wait, there's more. Our triangle needs a little extension, to take on the Spit and the lumpy bit just above the Spit, and below our line between grid refs M35 857425 and M35 890423. This area is sort of trapezoid but seagullz only do squares. so it's 750m wide and 1500 m deep. 2 metres uplift again, so we have 750x1500x2 = 2,250,000 cubic metres. Not enough to stop Pine Ave flooding during the odd storm slop, I'm told by an older and wiser bird.

And don't forget the Spit itself. It's 2800m long and say, oh, 250m wide (it wriggles about a lot, and 250 m is probably on the low side). Lets also say it has only 1.5m elevation too: our nanny Council is terribly, terribly worried about its lowness. I say, hey, they're mostly yachties live down there. They know how to handle things at sea. It's the damn landlubbers they can't stand. Oh, the calculation. 2800x250x1.5 = 1,050,000 cubic metres.

So we have three chunks of land, created from sand carried down the rivers, over a known period. Let's add them up: 34,125,000 + 2,250,000 + 1,050,000 = 37,425,000 cubic metres. Put that all on a 100 acre block (all right, a 40.47 ha block) and you have a 92 metre high stack of sand.

We're getting near a figure, aren't we. Divide that by 450, and we have a yearly sand volume. 37,425,000 / 450 = (tada!) 83,167 cubic metres per year. That's 23 10 metre truckloads every day of every year.

Let's say our fact sheet got it wrong. Stranger things have happened at sea. Let's say 600 years and do the division by 600, not 450. That's 62,375 cubic metres a year - 17 of those truckloads per day. That's still a shipload of sand, ain't it.

Now let's Cross Check. We know our Spit (the 1,050,000 cubic metre bit) grew over 30-35 years. divide one by t'other and we get between 30,000 and 35,000 cubic metres per year. Less than the original figure, eh? Where else could sand be going, to explain the difference between (say) 62,375 and (say) 35,000 cubes?

Well, dune height increase, for one thing. Over 30-35 years, lets assume (that's Number 5 if I'm still counting straight ) that the dunes grew from an average height of 4 to 7 metres. Let's say over 6,000 metres (a lot less than the total length of the beach, but let's Keep Things Simple). Our dune cross section is just like that darned hole near the end of the QEII mini-golf course - a 45 degree slope up, a flat bit with the hole in the middle (but oh so small), and a 45 degree slope down again. Our dune at a 4 metre height is thus 4 metres slope along, let's say 4 metres wide at the top, and another 4 metres down. Area is 4x4x2 = 32 square metres.

At a 7 m high dune, still 4 m wide on the top, we have 7 metres along the base of slope up, 7 metres height, 7 metres along the base of slope down again. That's 7x7=49 for the slopes plus 4x7=28 for the top width, for a grand total of, ummm, 77 square metres. So increasing from 4 to 7 m high makes the cross-section of our Real Simple Mini-Golf Dune go from 32 to 77 - a 45 sq m increase. Times 6000 metres of beach, that's 270,000 cubes. Divided by 30, that's 9,000 cubic metres per year. Or by 35, it's still 7,714 cubes.

So we have a sort of Cross Check: our Spit (35-30,000) and Dunes (9000-7714) together have taken around 37,714 oh lets just say 38,000 cubic metres per year (at the longer time period) or 44,000 cubic metres per year (at the shorter time period). Allowing say 1/3 of our original lower figure (62,375) to be Lost at Sea - filling in the Estuary (ask any member of any of the yacht clubs what's happened to average depths around the spit-ward marks over that time) - or filling in seawards of Sumner (ask Tug Lyttleton who got stranded on a bar that wasn't marked on the charts a few years back) would seem to fill the bill. There's our Cross Check if we allow around 20,000 metres of sea-filling.

So, young gulls, there we have it. Sand is being deposited on existing land (more freaking dunes), into the Estuary (Southshore Spit extension and Estuary depth decreases) and out to sea (obviously, if land rises, seabed must too). Around 62,000 to 83,000 cubic metres per year. Wasn't hard to figure, was it?

Hold that figure. Ask your Council whether it's about right. And particularly, ask what they plan to do with it. The Sand Conveyor doesn't respond well to bureaucratic suggestion. It needs actual management. Strategy. Real Planning. Engineering Works. The hard options, you see. Not that soft conservy stuff. Reclamation (accretion is the better geological term) is the de-facto strategy now. But it has reached it's practical maximum in terms of Length of Spit and Height of Freaking Dunes. Ask, where you gonna put that sand? Because, it just keeps coming.