Seagullz, by flying around and sticking their big red beaks in to interesting spots, actually survey quite a lot. Humans make surveying quite hard work, it seems, by comparison. I mean, every young bird knows not to fly too close to certain humans on the beach: the lurkers in that wonderfully concealing native vegemation for starters - they're very annoyed at any squawking which gives away their position. And young humans who leave behind those nice shiny cans - they can be quite deadly with stones and suchlike. And the big noisy shiny boxes with wheels that chase us up and down the beach - best avoided. But somehow I don't think that these particular beach users will get interviewed by those earnest young humans - get Surveyed about their Preferences for Beach Values. You see, they don't have the Right Beach Values. Any old bird knows that you get the survey result you pay for, and who's running this one? That's right, young gullz, the Greenspace Unit. The Green Spacey guys who want native plants everywhere. Nature at her full, rampant, undisturbed best. Preferably, without those pesky humans.
But if you have to pick the humans least offensive to your Green Spacey self, best stack your sample with the Right People. So you can tell straight away (just like looking down at the back of a Fish and Chip Shop) what's not going to be there. At the chippie, don't go looking for leftover lattes. At the Beach Values Survey, don't go looking for motorcycle or drift car races on the beach, or car parking on the beach. Not the Right Beach Values, you see. Much too diverse. Even though, if you fly over at just the right time of day or night, you can see all these activities and more, if you have beady little seagullz eyes. But I can guarantee you that these won't ever, ever be noted, quoted or voted in a paid-for Survey.
You see, humans want many different things of their Beach, but Green Spaceys seems to want less diversity, not more. We seagullz have acquired quite a taste for the more exotic plants - but that's not the right Green, you see. Greenspace's motto for the beach (they don't have one but seagullz are clevver birds and can invent the odd motto) could well be "Ngaio and Pingao Up the Wazoo", to judge by the plantings up and down the beach. Talk about monoculture!
So, with this Survey, those Green Spaceys really want to confirm their own "Nature (as interpreted by and through Us) Knows Best" meme. Which when you think about it, is really strange, as it means Government by an Elite, rather than by We, the People. I seem to recall a very old gull in my distant youth, saying something like 'it will be a great day when the last emperor is strangled with the entrails of the last priest'. And look what those dumb humans have wound up with. A freaking Green Priesthood. Ain't life funny.
The really funny thing about Green Spacey plantings on the beach is that actual humans hate it. When we go human-watching (and what better than on a sunny clear day or early evening, when one or two of those shiny cans have been consumed, as then humans seem to get a little crazy, but I digress), we head for the sandy bits which is where the humans like to congregate. You don't see too many children happily sliding down the ice-plants, waving and shouting to Mummy and Daddy. And you don't see humans paired off and enjoying a quiet moment on a nice pingao mattress. I mean, have you ever tried to even sit on a pingao plant? Take it from a wise old bird - don't. But we certainly do see little humans rolling down the few sandy slides left, families nestled into sandy pockets of the lower dunes, and those slightly crazy ones well tucked away from the wind and other humans in their little sandy nests. And we get good pickings from them all.
You just have to laugh, don't you. Those stupid humans vote for councillors and pay their rates, and get staff that those same councillors cannot influence, surveys that are used to then confirm staff predilections, and plants they can't even bloody well sit on! And meanwhile the sand just keeps rolling on down to the Spit, untrammelled by Plans, Strategies and Beach Values. Truly, people come and go, but the Sand (and the Gullz) endure.
Thursday, December 16, 2004
Sunday, December 05, 2004
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.
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.
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