Tuesday, 10 March 2015

Environmental Damage from Cyclone Yasi


Survey 54 - Yasi Recovery


Survey Name:Cyclone Yasi Damage Assessment
Survey Purpose:After Cyclone Yasi, a category 5 cyclone, hit the coast it was my job to assess the damage and calculate the repair costs. This photo essay is drawn from the 7000 photos I took to document the damage.  To manage these photos, I wrote a database application and the following information is from a database report.
StaffAndrew Mitchell
Begin Date3/05/2011
End Date



Environmental Impact of a Powerful Cyclone

   Records damage to coastal vegetation and some aspects of vegetation recovery (Photo Set: 54-249)
Unless otherwise stated, photos were taken approximately 8-10 weeks after the cyclone impacted the coast line.

Beach Retreat

1:   In many places, the beach retreated from 10-20 m. Coconut palms provide useful markers of where the crest of the beach once was. photo: 7054


2:   Remains of another coconut palm which was once on the crest of the beach.
photo: 7068

3:   Sometimes all that is left to show where the crest of the beach once was are palm tree roots.
photo: 7066

Tree Damage

4:   Even trees that only live on foreshores and which are well adapted to cyclones are thrown by strong cyclones. See how the roots of this tree are like strong flexible cables that run for many metres and anchor the tree against most cyclones.
photo: 7076

5:   Paperbarks growing around the edges of mangrove swamps cannot grow deep roots and are often also wind thrown.
photo: 7072

6:   This view shows how the paperbark has very shallow roots as roots cannot survive deep in the airless mud.
photo: 7074

7:   However paperbarks growing in sandy areas or on rocky coasts are almost indestructible. Can you see the watermark on these trees about 3 m above the ground.
photo: 7087


8:   Some trees stand up against the wind but are ringbarked by floating objects. This tree cannot survive for long even though it has leaves as the bark of a tree is needed to transport food down to the trees roots.
photo: 7071

9:   In some places entire forests were defoliated. These trees are white apples (Syzygium forte). Many trees took more than six months to regrow leaves. Many trees regrow leaves but were fatally stressed by the cyclone and died over the next few years. Climate change may be a factor. In 2003, we had a record drought which was followed a few years later by a record wet year where the ground was waterlogged for the entire year. Big trees have difficulty tracking such wide environmental swings and use their stored reserves to cope. If a cyclone follows a period of climatic stress, the trees cannot recover if their reserves are already depleted.
photo: 7077

10:   Looking closely at the defoliated canopies, every twig is still present but the trees were leafless. Other cyclones generally snapped the tree crowns off completely, however Cyclone Yasi passed over quite quickly which probably spared the tree crowns. It is not at all clear why cyclone Yasi was so deadly to trees when it caused so little visible structural damage to tree crowns. After previous cyclones, trees recovered much more quickly.
photo: 7126

11:   Saltwater may be responsible for defoliating the trees but the cause is not known for certain. In this view the tree closer to the beach remains defoliated, whereas the tree on the right which is growing on slightly higher ground which was just above storm surge level is recovering quickly.
photo: 7075

12:   Further inland vast swaths of eucalypt plantation were killed by the cyclone. Many of the trees were not broken by cyclone, something else killed the trees, perhaps being violently shaken in waterlogged soil. This can damage the roots enough to allow fungal pathogens in, which kill the roots. I have collar rots on such trees where the bark rots away at ground level, but it is hard to know if the collar rot killed the trees or infected an already dying trees.
photo: 7080

Storm Surge

13:   In Cardwell, the storm surge was about 2 m above normal high tide level. The highwater mark can be seen in this photo as a high mound of debris. In Edmund Kennedy National Park, the storm surge was a full 7 m, however this area is very difficult to get to.
photo: 7073

14:   Beach sand was swept into the mangroves in some places, however much more sand was sweep along the beach or out into the shallow waters just off the beach. Where sand had been swashed into the forest, there was a smooth and almost level surface and there were no washouts. The inland edge of the sand drift is steep and more or less parallel to the beach. It is important to note they type of feature created by major cyclones so that we can look for evidence of past cyclones. However, such geomorphological evidence is subtle and needs a trained eye to identify.
photo: 7097

15:   Before the cyclone, this was a shady natural foreshore. Hopefully, this area was just fenced off and allowed to recover. There is an enormous mound of cyclone debris in the centre which will regenerate very quickly if left alone. Cleaning the place up and planting trees would actually slow recovery down and introduce weeds.
photo: 7063

16:   Before the cyclone, the bank of this creek at Cowley Beach which runs parallel to the beach was fairly straight and now it has headlands of sand. Swash from the beach flowed along 4WD tracks that went from the beach to fishing spots on the creek and created several rounded sand headlands that now jut into the creek. Possibly similar features occur near the Environment Centre or at nearby Yorkeys Creek.
photo: 7096

17:   In areas near rivers, sand washed up by the cyclone was really coarse. See the normal beach sand on the left and the cyclone sand on the right. The cyclone sand is river sand which is too heavy to be washed right up the beach during normal weather. Compared to normal sand, the cyclone sand holds less water. Following the Cyclone Yasi, there was almost no rain for three months and most of the seedings that germinated in the cyclone debris died from drought stress.
photo: 7104

18:   On islands which have coral sand, things are a little different. This place was a sandy beach before the cyclone and cyclone has expose some previously covered granite boulders which are white. Normally boulders are black with endolithic algae that lives in the surface layer of the rock. In the foreground there is a sheet of beach rock which is a concretion formed from dissolved shell grit. The beach rock provides a layer of armour that helps to protect the beach. Holloways beach does not have buried stone but island beaches usually do and some mainland beaches such as Wangetti Beach also do. This is Garden Island, near Cardwell.
photo: 7085

19:   Waves crashing into the base of hills can also trigger landslides. Wet soils and trees being moved around in the wind like levers are also a factor. In this case I were told by a geotechnical engineer to leave the landslide debris in place to protect the toe of the cliff from the next cyclone. That meant we had to design a boardwalk that went over the top of the landslide debris as this landslide covered a famous coastal walking track.
photo: 7086

20:   This place at Mission Beach was a picnic area but the cyclone washed in coconut-sized stones and covered the entire area.
photo: 7088

21:   Waves from Cyclone Yas also broke up hundreds of metres of concrete path and most of the pieces were washed out to sea. The path had reinforcing steel in it and the concrete mix included glass fibres to make it strong, but the sea destroyed it. The first version of the Edmund Kennedy path was destroyed by Cyclone Larry, that was a bitumen path, so a stronger one was constructed from reinforced concrete and that was destroyed. So what can we use that is stronger than concrete? The answer is wood. Some of the wooden bridges survived when the adjacent concrete was destroyed. The secret is that wooden structures have gaps in them between the boards that let the power of the wave through rather than being a solid surface that receives the full force of the wave.
photo: 7089

22:   A Tully Heads, even the stone sea wall could not do much to stop the waves. Rocks from the seawall were scattered across parks and peoples lawns.
photo: 7092



Mangroves

23:   Vast areas of mangroves were killed by cyclone Yasi. The trees appear structurally intact but are dead. I do not know why. My personal theory is that the mechanical shaking of the trees by wind and waves is transferred to the ground and the oxyzone around the mangrove roots breaks down allowing poisonous hydrogen sulfide to contact the roots. Normally crab holes and mangrove roots can pipe enough air into the mud to maintain an oxyzone that extends out for 1-2 cm from each root.
photo: 7084

24:   Close up of mangroves in a swamp behind a low sandy beach.
photo: 7130


25:   In this stretch of coast, all that remains of the mangroves are places where their roots snapped. However only a very few places were like this.
photo: 7060

Previous cyclones

26:   Much of the broken timber from mangroves and trees carried out to sea by floods seems to accumulate at the foot of the beach.
photo: 7061

27:   The heavy watelogged timber remains at the toe of the beach. Interestingly, the timbers are aligned to point out to sea.
photo: 7067

28:   At the toe of the beach, some sand was swept away and revealed what looks like very old trees trunks pointing out sea. This may be evidence for previous, possibly prehistoric cyclone.
photo: 7062


Natural Regeneration

29:   Many trees regenerate rapidly. When trees need to grow leave quickly, they grow them from buds underneath their bark. This photo was taken ten weeks after the cyclone.
photo: 7069

30:   When trees have lost their leaves they grow a new set from epicormic buds under the bark. Later on, when the crown recovers, these shoot die off, however they leave knobs of the truck where they grew. Old trees are sometimes covered with knobs as they have seen many cyclones.
photo: 7125

31:   Some plant can grow back from broken roots. This is a Clerodendron, a common coastal species. Further inland, bushland species which regrow from the ground after bushfires are also regenerating vigorously.
photo: 7064

32:   The pulverized debris is packed full of native tree seeds that germinate as soon as the cyclone is over. The cyclone debris includes a lot of pumice, which is washed out of the beach sands.
photo: 7070

33:   A thicket of seedlings growing from the a strip of cyclone debris.
photo: 7082

Helping Regeneration

34:   This tree has had the broken branches removed for public safety and to reduce points of injury where insects and infections can get in. The cost of treating a tree like this is often more than $2500
photo: 7127

35:   Each tree has to climbed by a skilled tree surgeon using ropes and harnesses. The work is difficult and dangerous and the tool of trade is a chainsaw.
photo: 7128

36:   The council covered the tree roots with sand to try and keep the trees alive. This happened as soon as it was possible to get a machine on the beach.
photo: 7102

37:   The thick green grass is called Urochloa humidicola. It is a pasture grass but is listed as a pest. Following the cyclone, revegetation plantings were left alone and the humidicola thrived. However look at the trees, they are bushy and green. The humidicola does not interfere with tree growth in the way Singapore daisy does but nobody did any science and people spent years battling the grass which is actually useful. I now use humidicola in revegtation plantings as it keeps out the Singapore daisy and guinea grass, which are harmful weeds.
photo: 7098

Regeneration Failures

38:   Unfortunately some common weeds also have growth spurt after a cyclone. Singapore daisy is blanketing the ground and smothering all the native tree seedlings. Coastal rainforest are now critically endangered. Officially, the main threatening process is local councils converting forest into parkland, however fragmentation, weeds, cyclones and post-cyclone clean ups also do tremendous damage.
photo: 7079

39:   In some places, nearly all the foreshore vegetation is gone. A combination of rising sea levels, roads build too close to the sea, cyclones and post-cyclone clean-ups have removed all of the trees from a landscape that would naturally be crowded with trees.
photo: 7091

40:   Mangroves on fringes of a swamp often survived but in the centres of swamps, tree death is almost total. This photo was also taken 3 years after the event.
photo: 7056

41:   The mangrove swamps are still devastated more than 3 years after the cyclone. A patch of mangroves near Cairns that was damaged by cyclone Winifred in 1986 is only just beginning to look mature after almost 30 years of recovery. It may take much longer before the trees reach full size.
photo: 7057

42:   This boardwalk passed through some magnificent mangrove areas with trees larger than any street trees. Many of the dead trees were cut down to protect the public.
photo: 7058

43:   Coconut palm roots are like a natural seawall. Unfortunately other plants cannot grow well under coconut palms, not even lawn. Most plants cannot endure the palm root-choked, salty, shady, windy and dry conditions that occur under coconut palms. Coconut palms are not compatible with native coastal plants.
photo: 7094

44:   The cyclone also buried fences and allowed people to drive on the beach, which can damage recovering vegetation. It is quite common for people to clear views or make tracks just after a cyclone and most of them get caught and fined as clearing coastal vegetation is forbidden.
photo: 7095

45:   Three years later, it seems that some of the larger trees did not survive being toppled. Trees that fall into rivers just change direction and keep on growing but that may not apply to trees of the strand. People often demand that these dead trees be removed as they are reminders of the cyclone. I very strongly object and believe they should be retained where possible.
photo: 7055

Off-shore Impacts

46:   Even the bottom of the sea was impacted. This is a view of the normally famously muddy seabed off the end of the Cardwell jetty. The mud had been washed away and replaced with a clean sandy bottom, which is disaster for animals like turtles and dugong which require the higher productivity of muddy areas with sea grass.
photo: 7083

exported from PhotoSurvey: 11/03/2015 8:28:39 AM

1 comment:

  1. Really good article. Thanks for taking the time to explain things in such great detail in a way that is easy to understand.

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