Ask an Engineer

Ask an Engineer

Netflix's 'Tiger King' captivated audiences during lockdown. A question that sprung to our minds was, "Why can't you release captive tigers into the wild"?

Take a look at Ecologist Kat Longstaff's answer below:

There’s a number of reasons you unfortunately cannot release captive-bred tigers back into the wild:

  • Many captive breeding facilities (like that demonstrated in Tiger King) are not accredited wildlife/conservation organisations and subspecies have been interbred, which creates hybrids. This messes with the genetic diversity of subspecies of tigers and loses traits that have evolved to adapt to their unique habitats, e.g. a tiger from Indonesia wouldn’t do so well in frozen valleys of Russia and vice versa. Inbreeding is another issue that impacts the health of tigers.
  • Captive-bred tigers have not had to hunt for their own food so are lacking skills that are gained from the mother as cubs, locating, stalking, capturing and dispatching prey. Without this exposure they are not competent predators in the wild.
  • Captive tigers associate humans with food. This is a twofold problem - the tigers lack the innate fear of humans that wild tigers have making them susceptible to poaching, while their boldness can bring them closer to human populations that can cause conflict resulting in the culling of tigers.
  • Wild tigers have vast home ranges and are territorial – captive tigers have been confined to hideously small enclosures and have not carved out their own territories from reaching maturity.

The conservation of tigers depends on protecting their threatened habitats in the wild and reducing human conflict. Make ethical choices in products you buy, support registered wildlife charities, and if you really want to see a tiger only visit accredited registered zoos that are part of genuine conservation programs.


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How do foils work on America's Cup yachts? How do foils make such a large yacht with many people on board sit and fly above the water? 

Take a look at Civil Engineer Keith Dickson's answer below:


Almost exactly the same way that the metal wings on a 747 can lift 440 tonnes off the ground at a speed that’s the same as the self-imposed speed limit of many top-end German cars (i.e. at around 240 km/hr).

In simple terms, the difference between the boats and a plane is that water is far more dense than air – so you can develop a lot of lift at low speed with smaller wings. America's Cup boats take off at around 8 km/hr.

America's Cup boat wings are made largely with carbon fibre and fibreglass resin (they need massive strength to take the loads applied on such small wings).  It is highly likely that they have kevlar in the leading edges as it’s a certainty that they will strike weed, bits of driftwood etc and at relatively high speeds. Carbon fibre has excellent stiffness and strength, however, it also shatters pretty well if you hit it hard. Kevlar has lower strength but massive toughness and therefore excellent impact resistance through absorption and dispersion of the energy of the impact (which is why it’s the key component of bulletproof vests and woven into bulletproof suits (yes these do exist!).  


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How do you make sure ground isn't contaminated - and what happens if it is?

Take a look at Contaminated Land Consultant Natalie O'Rourke's answer below:

A: The first step is to engage a contaminated land specialist, preferably someone who loves acronyms (like SQEP). Contaminated land specialists often come from a range of formal training, and can comprise environmental scientists, environmental engineers, geologists, hydrogeologists, chemical engineers, and many more. They’re a friendly bunch.

In short form, they will:

-          Identify previous uses or activities that may be sources of contamination

-          Design a sampling plan for a site investigation

-          Collect samples of potentially contaminated media (this could be of soil, groundwater, surface water, soil gas, ambient air or building materials, depending on the issues)

-          Send samples to a laboratory for testing

-          Interpret the laboratory data and assess the risk with respect to potential receptors (human health and the environment) and the regulatory setting

If contamination (generally taken to be chemical concentrations above background levels) is found, do not panic. Contamination can exist without posing an unacceptable risk to human health or the environment. So what happens next depends on the risk assessment as well as overarching regulatory requirements. Depending on the specifics of the site, it’s possible that no further action would be required. Or, further stages of site investigation may need to be undertaken to delineate contamination and to help understand whether remediation is required. If remediation is required, there are lots of fun options to consider depending on what is being remediated, but that’s a story for another time.


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Although Easter has just passed, we thought we'd ask our Structural Engineers Geoff Radley and Alex Vink to put their egg-ceptional heads together to give this question a crack:

Q: Why are eggs stronger when standing vertically than when they are lying down horizontally? 

A: Eggs are ovoids where the sharper the curve, the more rigid or stronger it is. Hence why the tips of eggs are harder to crush than their sides. Their curved shape when squeezed distributes pressure all around the shell evenly. In engineering terms, this pressure experienced by the shell are in plane shear or tangential forces and stress in the normal direction is negligible.  It has been recognised in structural engineering that curved membranes or shells are more efficient than flat or straight members in resisting external pressure or gravitational loading. Hence arch bridges and domed roofs were in abundance historically. 


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With working from home arrangements well underway, it's not uncommon for parents to be interrupted by the shouting, hollering and the general hullabaloo of kids with pent up energy. But how loud are kids? We passed that question on to Acoustics/Noise Specialist Lindsay Leitch. Here's her response:


In this new era of working from home with kids in tow, it may already be painfully obvious that children can be pretty noisy. But how loud are they?

Firstly, we need to consider what constitutes a sound. A sound is created by something that vibrates, such as an engine, or a musical instrument, these vibrations travel through the air as a pressure wave - like ripples in a pond. When your ears detect these pressure variations, they are converted into electrical impulses that are transmitted to your brain and interpreted as sounds. An amazing amount of information can be transmitted through variations in frequency (how high or low a sound is), loudness, duration and a lot of more subtle factors such as for speech and whether it is pleasurable or annoying.

So how do you measure sounds? A microphone works roughly in the same way as your ears. The diaphragm of the microphone picks up the pressure variations and converts them to electrical signals. If the microphone is connected to a sound level meter, these sounds can be measured in units of pressure (Pascals - Pa) then converted to decibels (dB, a logarithmic scale).

The acoustics team in Christchurch and Auckland took a few sound level measurements in and around their homes offices of typical sounds using a phone app.


Sound pressure level, dB

Family eating lunch and conversation (small table so at 1m)

57 dB, 2 min

Mother telling off child at 3m

61 dB, 6 seconds

Standing while watching the kettle boil

66 dB

Son playing Xbox with headphones on

51 dB

Outside feeding the chicken

56 dB

On the patio

45 dB

In my office


34 dB with window closed

41 dB with window open

45 dB with music on

In the lounge with the window open where I’m working and my partner is doing crochet

39 dB

Partner talking at 2m

41 dB

Boiling the jug (highly reverberant room)

65 dB

At desk working

42 dB

Outside house, port in distance, no traffic

45 dB

Kids playing swingball

66 dB

Children yelling at each other (1m)

80 dB

And finally, to answer the original question – how loud are my children? I gave my two boys the challenge of seeing how high they could get the sound level to measure on my phone app. The result? 95 dB. Ouch. If you had that level of sound all day you would quickly go deaf!


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Ever wondered how you know where to drill a well? We asked hydrogeologist Tara Forstner, "How do you find well water"? 

Take a look at her response below!


Well water, also called groundwater, is found everywhere.

Groundwater is simply subsurface water that fully saturates pores/cracks in soils, unconsolidated and bedrock formations.

Therefore, a very simple answer to 'how do you find well water' could be countered with, ‘how deep are you willing to go?!’ It is not a matter of what location has groundwater, but rather a question of how DEEP do you have to go to find bore water.

A number of factors affect the depth to groundwater, such as climate, topography, geology, and seasonality. Groundwater/surface water can be thought of as a singular system in many cases.

Streams, lakes, and the sea level, are surface representations of the water table intersecting the topography. Therefore, within close proximity to these features and at lower relative topographies (such as floodplains), we often have a higher confidence in determining depth to groundwater.

However, there are too many exceptions to go into, such as perched areas (where groundwater may seasonally exists above the water table), or where streams are disconnected from the water table. Surface water – groundwater hydraulic relationships are often not straightforward and a good conceptual model is required.

In addition, just because we know where the water is doesn’t mean we can always effectively use it. Well water abstracted for the purpose of household use or irrigation also has to consider the aquifer yield - this is how much water you can actually get out of the aquifer. This is where geology becomes very important, as even though you may be in saturated ground, a well screened in clay is not going to be nearly as productive as a well screened in sand.

a)       Simple description of “typical” groundwater system.


b)       Example of a more complex system, such as karstic (limestone geology).



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Q. Although there is no definitive answer to the question ‘How long is a piece of string’, there has to be one with ‘How long is a can of spaghetti’?

You could lay the spaghetti out end-to-end and measure it out that way. But is there a way you could calculate the length while keeping the pasta within the tin?

We asked physicist and acoustic engineer, Darran Humpheson to answer this perplexing pasta problem.

A. I was given an 820g tin of spaghetti. The data on the can says it is 52% spaghetti (48% sauce). I measured the height and diameter of the can and estimated the diameter of the spaghetti at 6mm.

I then calculated the volume of the spaghetti in the can. Dividing this by the cross-sectional area gave me the total length of spaghetti.

The working figures:

Can of Spaghetti


Height of can in cm


Diameter of can in cm


Diameter of spaghetti / mm


Volume of can in cm3


Quantity of spaghetti per can


Volume of spaghetti per can in cm3

Once I had worked out these numbers, I put them into the following formula to figure out the length of spaghetti:

The final calculation was that there should be 16.61 metres of spaghetti in the can.

We verified the answer by literally laying out the spaghetti end to end. See the video below of how we did it!

The total length was 16.24 metres. That makes the expected and actual answers only 37cm apart – pretty good calculations! Although the final answer means you get slightly less spaghetti than you would expect for what you pay.


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Located in Wyoming, USA, Devils Tower is a peculiar lone formation that rises tall above the surrounding grassland and pine forests.

But how was it formed?

Senior Engineering Geologist Kevin Hind has an idea.

A: Devils Tower was formed from magma that has risen up through weaker sedimentary rocks. It may or may not have been part of an established volcano or it was a new volcano erupting through the surface – no one really knows. Once the magma stopped rising, it cooled and solidified. As it cooled it shrank in size, with cracks opening up in the characteristic columns - like what happens in mud when it dries. Over time, the weaker sedimentary rocks were eroded away around the tower leaving the stronger volcanic rock behind.

Cooling columns or columnar jointing of this type can be seen in the lava at Auckland Grammar School although they are not as high quality or as strong and therefore not as large or prominent. This is likely because they formed within lava on the surface rather than a slowly cooling subsurface mass.


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The buzz around EVs (electric vehicles), is, well, electric.

EV ownership in Australia and New Zealand has seen exponential growth over the past decade with savings on petrol and a reduced carbon footprint among the leading purchasing reasons for consumers.

But for those particularly concerned about reducing their carbon emissions, it isn’t just the vehicle itself that needs to be factored into the equation – the source of energy must be accounted for as well.

In New Zealand, 80% of energy comes from renewable sources. In Australia, 80% of energy comes from coal.

The question on our minds is:

Is it still considered “green” to use an EV in a country reliant on non-renewable energy sources?

We asked T+T Environmental Engineer Rob van de Munckhof and below is his detailed answer:


The question is not quite as simple as it seems at first, but assuming you mean “green” to be carbon emissions two aspects need to be considered. Firstly, the carbon footprint associated with the manufacture of the vehicles and secondly the carbon footprint associated with running the vehicle.

The estimated carbon emissions during the manufacture of an electric vehicle have been calculated at 8.8 tonnes CO2e which can be compared to a standard petrol vehicle at 5.6 tonnes CO2e and a hybrid vehicle at 6.5 tonnes of CO2e. This shows that the manufacture of electric vehicles emits nearly 60% more carbon than the manufacture of normal petrol vehicles. This is commonly reported as stating that electric vehicles are worse for the environment than petrol vehicles but ignores the carbon contribution in use.

During use, many electric vehicle sellers highlight the CO2 emissions as 0, where in reality the emissions depend on the source of electricity used. As the question highlights this varies significantly, with NZ sourcing approximately 80 % of our electricity from renewable sources which have zero carbon emissions where certain states in Australia rely heavily on coal and on average only has 15% renewable sources.

Like petrol vehicles, the fuel (or in the case for electric vehicles energy efficiency) varies depending on the size of the car. The efficiency is commonly reported as kWh/ 100 km with efficiencies ranging from 15.7 kWh/ 100 km for the Hyundai Ioniq to 24 kWh/ 100 km for bigger vehicles such as the Tesla Model X.

In New Zealand this works out to be approximately 2 to 3 kg of CO2e per 100 km, while in Australia this works out to be approximately 14 to 22 kg of CO2e per 100 km on average. It is worth noting that the carbon emission factors vary throughout Australia with the emission factors for Tasmania similar to NZ while the emission factor for Victoria is significantly higher (0.15 kgCO2e per kWh for Tasmania compared to 0.9 overall and 1.02 for Victoria).  This can be compared to the emissions from normal petrol vehicles such as a Toyota Corolla of 14 kg of CO2e per 100km, or 9 kg of CO2e per 100km, for a hybrid Prius or up to 23 kg of CO2e per 100km for a large SUV such as Toyota Highlander.

Based on a travel distance of 100,000 km, the total carbon emissions can be calculated as follows:


Manufacturing emissions (tonnes CO2e)

Running emissions  (tonnes CO2e)

Total  (tonnes CO2e)

New Zealand

Standard petrol vehicle




Large petrol vehicle




Small electric vehicle




Large electric vehicle





Standard petrol vehicle




Large petrol vehicle




Small electric vehicles




Large electric vehicle




Electric vehicles in Victoria


18 to 28

27 to 37

Therefore, in New Zealand, owning an electric vehicle can significantly reduce your carbon emissions. In Australia, if you rely on the grid, owning an electric vehicle will increase your carbon emissions comparing similar sized vehicles and is worse if you live in Victoria.

The other factor associated with electric vehicles is the impact on urban air pollution. Studies have determined that over 1700 people die prematurely in Australia and nearly 400 people in New Zealand due to exposure to emissions from vehicles in particular emissions of nitrogen oxides and fine particulate. Electric vehicle emissions have zero tail pipe emissions and therefore reduce the potential emissions to the communities where the vehicles are used. Further, emissions of nitrogen oxides and fine particulate from power stations are tightly controlled to ensure these are not causing any significant effects to surrounding communities.

In summary, driving an electric vehicle in New Zealand reduces both your carbon emissions and the health impacts from tailpipe emissions. In Australia, generally unless you use your own solar panels to charge your EV, driving an electric vehicle will increase your carbon emissions but will reduce the potential health impacts from tailpipe emissions.

In both scenarios, if you really want to reduce your carbon emissions, ditching the car and walking or cycling will make the most difference with both walking and cycling generating no carbon emissions and being better for your overall health.  With one third of vehicle trips in New Zealand less than two kilometres, and two thirds less than 6km leaving your car at home is easy.


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When it comes to water on Earth, you could say that we’re only scratching the surface.

Hydrogeology (not to be confused with Hydrology), is the study of groundwater and is part of the service offerings we provide here at T+T. But what does a Hydrogeologist do? What sort of work do they deal with?

We passed that question on to Hydrogeologist Kevin Ledwith:

Q: What does a Hydrogeologist do?

A: Earth is known as the blue planet because of its abundance of water, however only a small percentage (about 0.3%) is usable by humans. It’s estimated that 98% of all this useable freshwater exists as groundwater, however the vast majority of it is hidden away from view. Occasionally groundwater moves fast and makes a spectacular display in a geyser, an underground cave, or a large spring, but for the most part, it moves very slowly and exists in small pore spaces underground.

Depending on where you are in the world, groundwater can be either the solution or the problem. Areas of the planet with low rainfall or polluted surface waters depend on groundwater as a source of drinking water, for irrigation, and to maintain healthy ecosystems. Whereas, construction projects that require earthworks below the water table (e.g. basements or tunnels) need to temporarily pump groundwater away from areas in order to build foundations in suitably dry conditions.

Regardless of where you are in the world, groundwater requires careful monitoring and management to ensure that it remains available in sufficient quantities, is safe to consume, and will not create land stability or engineering related problems. Enter the role of the Hydrogeologist!

A Hydrogeologist is a person who studies the flow of water underground, compared with Hydrologists who are primarily concerned with surface water. They tend to be either office-based or field-based, or they may work in a combination of these environments. Hydrogeologists are responsible for identifying and assessing the location, quantity and quality of valuable groundwater resources so that future development works do not negatively affect humans or natural and built environments.

The science of Hydrogeology deals with how water gets into the ground (recharge), how it flows in the subsurface (through aquifers) and how groundwater interacts with the environment and surrounding soil, rock and surface waters (rivers, lakes, and the ocean). To access groundwater, it is necessary to drill into aquifers and to install wells, these wells are then either used for water supply or to monitor how groundwater levels change with time.

Hydrogeologists are trained in mathematics and sciences including physics, chemistry, biology, geology, geography, and environmental science. They draw on knowledge and skills in these areas to collect and interpret data, apply scientific and engineering principles, and to present their work in reports. Some of their day-to-day tasks include:

  • Visit sites to supervise drilling or to collect groundwater level or quality data from wells
  • Compilation of groundwater information from previous reports and datasets
  • Data analysis. Depending on the project, this may involve relatively simple calculations or may require more sophisticated statistical analyses
  • Use of specialist software to predict how and where groundwater will occur in the future
  • Preparing drawings, maps, graphs, and tables to support technical reports
  • Preparation of interpretative reports to provide advice for non-technical audiences

Governments and councils depend on professional Hydrogeologists to ensure that human health and environmental regulations are appropriate and are adhered to. To achieve this, sometimes it is required for a Hydrogeologist to attend local Hearings and Environment Courts as an expert-witness relating to enforced laws, policies or legislation.

Groundwater plays an important role in a large diversity of projects at Tonkin +Taylor. Our Hydrogeologists are awarded the opportunity to solve a wide array of complex problems involving multiple specialist areas. Their efforts positively contribute to society, directly impact urban and rural communities, and help protect natural and built environments. In return, they are given the surety of a challenging but rewarding career. A selection of projects that Tonkin +Taylor’s Hydrogeologists have worked on recently include:

  • Protecting and securing drinking water from contamination
  • Planning for future water supply challenges associated with population growth
  • Building resilience against climate change and natural hazards
  • Assessing potential engineering impacts of construction dewatering projects, including assessments of tunnels, basements, and other excavations
  • Assessing potential effects on the environment, ensuring that proposed projects do not: impact existing groundwater users, induce the depletion of rivers or intrusion of saltwater into aquifers, cause land stability or engineering problems, create water contamination issues 

For more, check out the hydrogeology page and stay tuned for some upcoming infographics!


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It's been a fantastic three months for our interns, and, with it being their final week here at T+T, we decided to switch up our Ask An Engineer slot and hand it over to our interns! 

We've fired through a novelty question:

Why do socks disappear and what can we do to mitigate this? 

A: It’s 7am and you’re getting ready for work. You shower, get dressed and reach for your socks… and realise you can’t find two that match. Amid the scramble, you wonder to yourself, just where do these socks go?

This question has haunted humanity for years. While it’s not one we can answer, we felt sure that we might be able to contribute to the debate “why it is that socks disappear?” and “what we could do to prevent or mitigate their loss?” Rather than ask just one engineer, we called upon the brightest minds of tomorrow – the 2019 T+T interns (engineers, scientists, and business services professionals) – to answer this most pressing question, before waving them off to another year of study.

The interns quickly identified five major variables that influence sock loss:

  • Care
  • Storage method
  • Washing and drying technique
  • Sock characteristics
  • Household residents

Variable one: care

‘Care’ includes factors such as relative income compared to the price of socks, and personality traits. If you’re as laid back as Bob Marley was, you probably don’t worry much about the odd misplaced sock or whether you have a matching pair. Similarly, if you have a high enough income that the price of socks doesn’t concern you, you may be more prone to sock loss. It’s debatable whether the Queen of England has ever even realised that sock loss was a thing. Meghan and Harry, however, might be a bit more conscious of the phenomenon in future.

Variable two: storage method

This is actually a multi-factor issue. Factor one: do you pair your socks together when you put them away? Our merry band of interns hypothesised that those who lack motivation for the proper pairing of socks when storing would be more prone to loss of individual socks. Factor two: the bigger the space you launder and store them in, the more room you have to lose your socks. Combine these two factors together, and you’ve got a veritable equation for losing socks.

Variable three: washing and drying technique

Just how often do you do the washing? Do you dry your clothes using a machine, or on the line? Our interns thought that those that wash more frequently, lose socks more frequently. The interns could have digressed at this point onto whether ‘tis nobler to lose a sock occasionally or have smelly socks, but fortunately for us they moved on to address drying technique. The group thought those drying socks on lines in windy areas may be more prone to losing. Here’s hoping that one of them goes back to university and suggests scientific study comparing average rate of sock loss between Wellington and everywhere else in New Zealand. It wouldn’t be the oddest study out there, after all.  

Variable four: sock characteristics

Factors include what size your socks are and what colour they are. Firstly, the group thought that those with smaller feet would be more likely to lose socks, as smaller items are generally easier to lose (keys, anyone?). Unfortunately, they were unable to quickly check this hypothesis amongst themselves as it turns out that they all have awfully average-sized feet. Secondly, if your socks are similarly-coloured to the background in which you remove them, you may be more likely to lose them. Particularly if, for example, you take off your photo-realistic grass socks while walking on a field.

Variable five: household residents

If you have small children, teenagers or dogs, let’s face it: you’re just going to be more likely to lose socks. If it’s small children or teenagers, just write those socks off now. If you have a canine friend in residence, there’s many, many articles online of what to do if your dog swallows a sock (aka very expensive sock retrieval), along with plenty of advice on how to prevent socks from going missing via this route.  The latter, our interns hypothesised, may have been helpful to the owners of the Great Dane who swallowed 43.5 socks.

Sock loss prevention and mitigation

Key variables identified, we asked our interns to discuss solutions that could prevent or mitigate sock loss. At this point, they definitely demonstrated just how creative they could be, with suggestions including:

  • Chameleon socks that change design so they match whatever other sock you are wearing.
  • Adding GPS trackers to socks.
  • Attaching socks together with a magnet – the implications for your washing machine are unknown.
  • Let humans evolve into one-footed beings, so that you never need a pair of socks. We may just have to wait a couple of tens of millions of years for evolution on that one, though – and even then, evolution might not prioritise sock loss as necessitating one foot.

One of their more cunning solutions, perhaps demonstrating some understanding of psychological engineering, was to not in fact attempt to control sock loss in anyway – rather to change the narrative, so that odd socks are perceived as “lucky”. After all, if it worked for bird poop, surely it can work for socks?

Their last solution – which we’re a massive fan of during these hot summer days – is for everyone to migrate to the beach, thus eliminating the need for socks altogether.

Many thanks to our 2019 T+T Summer Interns for their input into this article. We wish them all the best back for their studies this year.


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Q: If you were to fly to Fiji for the Christmas holidays, how many trees would you have to plant to offset your carbon emissions?

Sustainability Engineer Kate Boylan, with help from Environmental Consultants Roger MacGibbon and Kate Draper, has an answer:

Firstly, it feels like my duty to say, that offsetting your emissions is great, but reducing your emissions is the best! So, flying to Fiji (or anywhere) less regularly would be the best thing to do. However, your bags are packed and, being the good human you are, you would like to offset the negative impacts of your holiday flight. Carbon offset calculating websites are becoming popular, but we’ll use good old distances and emissions factors here.

To calculate the emissions, we use an ‘emissions factor’ which allows us to estimate emissions from a unit of activity data (e.g. litres of fuel used). In New Zealand, the Ministry for the Environment (MfE) provides emissions factors for most activities. Many human activities produce multiple Greenhouse Gases (GHGs) at once, however, to allow for meaningful comparisons, we commonly express GHG emissions as carbon dioxide equivalent (CO2-e).

Given you are just one person on the plane taking up just one seat, you are only responsible for a small portion of the flight’s overall emissions. If you are in an economy seat, you are responsible for fewer emissions than those in business or first class, purely because you take up less space.

The flight to Fiji, according to Google, is 2160 km. Assuming you can drag yourself away from the beautiful white sand beaches at the end of your stay, a return trip is therefore 4320 km. Because these flights are less than 3700 km (each way), it is considered short-haul. The emissions factor provided by MfE for an average passenger on a short-haul flight is 0.162 kg CO2-e/, including Radiative Forcing Factors (where Radiative forcing factors help account for the wider climate effects of aviation, including water vapour and indirect GHGs (MfE, 2019)).

Therefore the emissions are the total flight distance multiplied by our emissions factor; 4320 km x 0.162 kg CO2-e/ = 701 kg CO2-e.

MfE also sets out standard ‘sequestration’ rate (carbon sucking up ability) emission factors for some of our common forestry carbon sinks. Planted forests can remove 33,807 kg CO2-e per hectare, whereas naturally regenerating forest can remove 5,097 kg CO2-e per hectare.

So, to offset your Fiji holiday flight emissions of 701 kg CO2e, we would need to plant 207 m2 of planted forest, or regenerate 1376 m2 of natural forest. Let's assume you’re looking to plant some fast-growing trees (e.g. radiata pine), where standard forestry spacing is initially around 1000 stems per hectare.

You would, therefore, need to plant 21 trees to offset your return holiday flights to Fiji.

If you are interested in some further reading, check out the MfE measuring emissions resources online, or for more information on specific sequestration rates see the Carbon Look-up Tables for Forestry in the ETS.

Kate (with help from Roger MacGibbon & Kate Draper)


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What’s not to love about the great Kiwi outdoors? Cool, pristine bush, magical vistas and crystal clear waters await those of us who love to roam. But just how safe is it to drink straight from our streams and rivers? We asked one of T+T’s hydrogeologists, Dr Jeremy Bennett.

A: Water, water, everywhere – but which drops can we drink? It depends – water has a ‘memory’ of where it has been, or more specifically, what it has travelled through. Generally, the closer you are to where the water came from (i.e. the sky), the cleaner the water is likely to be. If you’re up the top of one of our many mountain ranges, then the water in the streams has not travelled that far and won’t have too many ‘bad memories’.  As you move down to lower areas, particularly where there are more people or animals, then it is highly likely that the water will be unsafe to drink due to the presence of microbiological contaminants like E. coli and Giardia. When you get sick from these nasties, the vistas are not magical.

The risks of waterborne disease are lower in conservation areas, and many trampers and hikers do drink directly from streams without too much trouble. However, you need to think carefully about where the water is coming from (its catchment). Are there farms or wild animals upstream of your location, or is the area often used by people? Even in the most pristine bush environment, there could be something just around the corner which will make the stream water unsafe to drink without treatment (dead possums are a common one). The Department of Conservation recommends that you boil water if you have any doubts about its quality and Wilderness Magazine has some good tips on collecting water during a tramp.


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With summer almost upon us and tens of thousands of people already heading for the beach, we asked T+T Coastal Geomorphologist Eddie Beetham:

Q: How do you spot a rip current and what causes them?

A: An area of low wave height and limited breaking located next to an area of larger breaking waves will likely be a rip. A danger of rips is that low energy sections of beach are often more inviting for bathers than a section with breaking waves. Always watch the beach for a few minutes to identify rips before entering the water and swim between the flags.

Rip currents form when waves break on irregular sandbars spaced along the beach. This forces an offshore current to develop between sandbars, as wave energy piling up at the coast is released. The process of wave breaking creates a local area of elevated water level called ‘wave setup’ landward of the breakpoint.

If smaller waves are breaking at a neighbouring section of the beach, a setup differential is established and this drives an alongshore current from areas of high setup (where waves break) to areas of low setup (small wave locations). Setup driven currents converge in lower energy locations between sections of wave breaking and form a seaward directed rip current. The velocity of a rip current typically increases with larger waves and high-speed rips can erode a channel between sandbars that enhances the development of a rip cell.


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Q: Can you hurry up the pedestrian crossing by pressing the button lots of times to tell the machine there are lots of people waiting?  

A: Unfortunately not. Pushing the button more than once doesn’t make any difference to the lights at a pedestrian crossing. This is because the lights are programmed in a specific sequence made up of a number of phases. When you push the button the pedestrian phase is called, meaning the pedestrian light will turn green next time it appears in a phase in the sequence. However, once a phase is called pushing the button again doesn’t do anything because the call has already been made.




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Q: Obviously it's more expensive to Uber than to Bus but if your only concern was sustainability, would it be better to Uber to travel to work in an electric car or ride in a fossil fuel-powered bus?

Our Sustainability Engineer, Kate Boylan has an answer:

We believe that despite the emissions that our diesel bus fleets emit, they are the more sustainable option. The more people we have on each bus, the less emissions there are per person when that is split evenly. Also, given a standard bus can take around 50 passengers, that’s around 50 cars that are not travelling on the same roads. Even if they are electric or hybrid cars; fewer cars means less congestion, which would make it easier for our buses and public transport vehicles to get around. Less congestion also means that the buses have to stop and start again less often, which saves on fuel, and therefore emissions. Less congestion also means that everyone travelling would save time, and we all know that time is precious, and time is money.

Fewer individual cars on the road saves space. If everyone used a bus, train, or active modes of transport to commute, we would need less road space overall. Just Google image ‘road space cars vs bus’ for some visual examples of this. We would also need less space to be used for car parking at workplaces, this would save you or your company money, as that space could be utilised for something far more useful.

However, vehicles on roads pollute more than just fuel emissions. Vehicles often pollute rubber and brake pad particles to the surrounding environment of roads. Stormwater run-off from our roads flows directly out to sea with these contaminant particles in tow. Buses also contribute these pollutants, however, as we’ve seen visually above, fewer buses are required to transport the same amount of people. Fewer individual cars on the road could also reduce the wear and tear on the road. Currently, high-traffic roads need resealing every seven years. If there was far less traffic on these roads, they could be replaced less often. This would require fewer materials and construction works, therefore a dramatic saving in construction emissions and embodied resource emissions.

As a fare-paying passenger of your local public transport system, you are supporting your local council or transport body. This revenue can help increase the investment in more efficient and better public transport vehicles and overall systems. After all, every dollar you spend is a vote for what you want to see in the world. Our councils are already starting to look into Electric and Hydrogen fuel buses, so they could be cleaner than your Uber in the near future.

Lastly, from a high-level sustainability perspective, buses and public transport represent changing people’s habits to a more sharing and circular economy. Instead of owning cars, electric or otherwise, could we hire them as we need them?

We hope that answers your question - see you on the bus!




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Q: How come the Japan vs Scotland game at the Rugby World Cup was able to go ahead at Nissan Stadium in Yokohama, despite the massive floods caused by Super Typhoon Hagibis? 

A: It comes down to some pretty nifty engineering. The stadium is built on massive pillars that lift it above the flood level of the river. You might remember seeing that the adjacent Shin-Yokohama Park was totally awash, even while the pool game was on. The Park acts as a large flood retarding, or storage basin for the Tsurumi River, and, during a flood, overflows into the Park as it did during Hagibis. After the flood a drainage gate feeds water from the Park back into the river. Result? One flood-beating stadium - so the players at Yokohama stadium get to keep their feet dry! 


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Q: Can wind seriously blow a building over?

A: Alex Vink, Structural Engineer has an answer!

In a word, yes wind can absolutely blow buildings over. What’s really interesting is all the ways wind can cause structures to fail. 

Wind is mainly caused by the rotation of the earth, which causes the air in the earth’s atmosphere to move. When the wind blows, structures like buildings and bridges obstruct the air flow. This causes the air to flow around the obstruction and generates a pressure or drag on the obstruction. Engineers need to design their buildings and bridges to make sure that they can withstand this pressure.

In a storm, the air particles can move very quickly. This can generate large pressures and drag on buildings. If these buildings and their foundations have not been designed to be strong enough to withstand these large forces that are generated, they can topple or collapse. This happens quite commonly in the Pacific Islands during cyclones.

It’s not just large wind pressures that can make buildings or bridges collapse. During the life of a bridge, it will experience a lot of changing wind with different pressures. It may only experience a couple of really big storms, but lots of small gusts of wind can cause the bridge to fail due to a phenomenon called fatigue. Some materials, like steel, get weaker when they are exposed to many cycles of loading (like changing gentle breezes) and when they experience enough cycles, they can break. This is a bit like bending a paper clip back and forward until it breaks.

Perhaps the most famous example of a bridge collapsing due to wind is the First Tacoma Narrows Bridge collapse. The wind caused the suspension bridge to vibrate excessively without the wind speed increasing because the bridge was not able to absorb all the energy from the vibrations. Eventually these vibrations became too big and the bridge became unstable and collapsed. This phenomenon is called aerodynamic flutter. Engineers need to consider these effects when designing bridges.


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Q: If The Wall from Game of Thrones was built in real life, how deep would the foundations need to be? Would it actually be possible to build it out of ice? NB: The Wall is meant to be 300 miles long, over 700 feet (213m) high, and made of solid ice.

- Grant, Tikitere

A: Mark Thomas, Senior Geotechnical Engineer has got the answer!

Mark: "How come I get the question about fictitious magic stuff?!

I know more about soil and rock than I do about ice. I know glaciers get way thicker than that, (~1500m thick) and the Greenland and Antarctic ice sheets are in the order of 2000 to 5000m thick! But they don’t have vertical cliff faces nearly that high.

In order to be strong enough to form a 213m high vertical face, the ice would need to have a theoretical mass strength of at least 2MPa UCS, which doesn’t even take into account defects within the ice. On top of that, Martin Truffer has pointed out that ice doesn’t hold its shape, and I guess that’s where you need a bit of Old Gods magic to keep the whole structure from falling apart.

Building a structural “foundation” for the wall wouldn’t really cut the mustard. You’d really want to dig down and build the wall on solid bedrock. Depending on the strength and fabric of the rock, you might have to dig tens of metres deep to support the steep sides of the wall. Given the size and weight of the wall, you’d probably also induce quite a few localised earthquakes during construction. That could be a problem for nearby masonry castles and forts due to their brittle construction; but less likely to be an issue for a typical wildling hut.

It’s a bit strange that the wall was built out of magic ice. If I was in charge of the wall’s construction, I would have opted for magic rock or magic concrete, which would be much more robust in both a structural sense, and in its resistance to dragons and long summers. Donald Trump, if you’re reading this, give me a call".

- Mark Thomas, Senior Geotechnical Engineer


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Q: How long will the eruption of the volcano on Kilauea in Hawaii last?

A: One of our resident Volcanologists Alec Wild has got an answer for you!

The short answer is we don’t know! It’s difficult to determine how much magma beneath the volcano is going to be ejected, and over what timeframe, as we cannot directly observe it.

Kīlauea Volcano on Hawaii’s Big Island has been erupting on and off since 1983. However, this most recent phase has formed long cracks in the ground called fissures, along the East Rift Zone to the east of the main vent. The rift zone is a thin strip of land that is being separated, allowing magma to travel up and down. Once the magma reaches the surface and forms fissures, it produces fire fountains and lava flows. Of particular note are the fissures that have formed in a residential subdivision, Leilani Estates, resulting in the evacuation of residents and damage to properties and infrastructure.

The lava level from Kīlauea main summit has dropped, which indicates that the lava could be moving along the rift zone. Volcanologists are monitoring the lava level, seismic data and ground deformation to try and predict where the magma is moving and where it might come up to the surface.

This eruption is considered, based on activity and affected area, similar to the 1955 event which occurred in the same rift area, and lasted 3 months. During the 1955 event with lava flowing both north east and south-west of what is now Leilani Estates. Although this is similar, we cannot definitively say that this eruption will be of the same duration.

- Alec Wild, Natural Hazards Specialist, MSc Quantitative Volcanic Hazard and Risk Modelling


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Q: I have been transfixed by that dramatic, jagged, golden cliff face that features in the British TV programme, Broadchurch. Would one of your engineers kindly explain (in layman's terms) how it was created.

- Shirley, Remuera.

A: Great question, Shirley. Our legendary engineering geologist Kevin Hind was dead keen to answer!

"The steep cliffs of West Bay in Dorset on England’s Southern coast are made of extremely weak and highly erodible sandstone. They originally formed from layers of sand, mud and shell fragments that settled onto the ocean floor sometime between 183 and 174 million years ago.

It took a whopping 860,000 years for the full 43 metre thickness of cliff to develop! The build-up of sedimentary sand layers happened extremely slowly, forming at an average rate of one metre per 20,000 years. All those horizontal layers of sediment are clearly visible in the cliff face - just like they are in the cliffs of Auckland, which formed in a very similar way.

Years of wave erosion has formed the cliff face itself, most of which occurred since about 6,000 years ago, at the end of the last ice age, when sea levels stabilised at their current levels.

The corrugated look of the cliffs is caused by two things. Firstly, the geology contains two sets of strong sub-vertical joints, along which cliff collapse generally occurs. This results in tall, flat-faced columns, which stand out from the overall cliff face with recesses between them.

This close-up image clearly shows the sub-vertical joints in the rock formation, this effect is called ‘buttress and groove’.

Secondly, extensive landslips that occur at the top of the cliff within the soil and weathered rock. These landslips occur on a regular basis along the length of the cliff, giving it that serrated and ‘bread knife’ appearance. This, combined with the rock buttresses, give the cliffs their remarkable corrugated appearance".

Unique geology and erosion combined to create the ‘bread knife’ appearance of the cliffs along the Jurassic Coast.

So what about that golden yellow colour?  

"That’s actually the result of an oxidation process enhanced by beautiful cinematography. The rock is naturally a grey-blue colour, but when exposed to oxygen, the mineral ‘pyrite’ (also known as ‘fool’s gold’) changes to yellow limonite – snap – there’s your yellow stone".

- Kevin Hind, Senior Engineering Geologist


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Q: How did the Devil’s Marbles get there? 

- Paul, Melbourne

A: Great question! We asked Senior Engineering Geologist Trevor Smith - and he's got the answer!

The Devil’s Marbles are located in the middle of the Northern Territory. Unlike Stonehenge, the marbles, consisting of granite rock, were actually formed exactly where you see them now – 1.7 billion years ago – as a result of molten magma squeezing between sandstone layers. As granite cools and ages, it shrinks and cracks, making horizontal and vertical fractures. Over a very long period of time, the surrounding softer rock layers have washed away – and the granite itself has been eroded by water travelling along those crack lines. With the clay washed away, the boulders have been left exposed. The rock is formed in layers, like an onion, and over time, the edges have then been exposed to a process of chemical weathering (from water and the substances dissolved in it), which peels away the top few layers of the rock to leave it softened and rounded – which is how we see them today.

There are many, many boulders, in an area of approximately 18 square kilometres. The boulders range in size from a mere 50cm across, up to around 6m.

The local Aboriginal name for the rock is Karlwekarlwe or Karlu Karlu – round rocks. Sacred to them as an important dreamtime site, they have many legends and traditions associated with the area.

The area that the rocks are found in is now part of a conservation reserve. It belongs to four local tribes, but is currently under the joint authority of the Aborigines and Parks and Wildlife Rangers. To find out more about it, or to plan a visit, check out the Northern Territory government website.

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