By Dr J Floor Anthoni (2000)
As populations increase, so does the number
of cars and roads. People also demand more comfort and speed, resulting
in road developments increasing more rapidly than population. People need
houses, shops, offices and factories. All this development requires frequent
cutting of the earth, resulting in increasingly severe erosion. Fortunately,
substantial gains can be made to reduce erosion. Understand how roading
and development have become major contributors to land loss and erosion,
pollution of waterways and coastal seas. Discover how to prevent or minimise
erosion from landscaping, urban development and roading. How to stabilise
banks and how to compensate for ecological changes.
Roading and land development contribute largely to the amount of mud
reaching the sea. As the world population increases, so do the number of
cars and houses and also the number of road works and development sites.
It is an accelerating process.
Road construction is a high level engineering job which includes the
building of bridges and tunnels. Some understanding of low level engineering
is necessary for this chapter: cut-and-fill, slope, strata, drains and
The killer kerb: a little-known fact
kills innocent people every day. It is time to stop this carnage.
Ever since humans have been roaming the Earth, tracks and roads have been
made and maintained to facilitate travelling. First there were foot tracks,
for humans only (humans are one of the very few species able to walk, swim
and climb a tree. Tigers do so too.). These tracks were later widened and
smoothed for pack animals, and later again for draught animals and their
carts. Then came the automobile which required better roads, but progress
did not stop here. The world is continuously upgrading its roads to accommodate
more vehicles, heavier vehicles and more connections. At the same time
roads are altered to accommodate higher speeds and improved safety. As
societies became more affluent, an increasing sum of money was spent on
roading. At the same time, gains in mechanisation have made the process
more incisive, resulting in roads becoming a major factor in soil erosion.
In this chapter we'll examine the effects that roading has on the loss
of land, and its continually increasing contribution of mud to our oceans.
Please note that no scientific publications could be found on this matter,
which makes this chapter rely heavily on my own observations over a period
of twenty years in New Zealand. The many photographs illustrate the issues.
Modern urban development demands the 'landscaping' of the
soil, requiring the cutting and shifting of large quantities of it. During
the entire time it takes to do the cutting and filling and spreading of
topsoil, the whole developed area is left bare and extremely sensitive
to erosion by rain storms. The shifted soils are easily saturated by rains,
resulting in high levels of runoff.
In an aerial view of a new motorway between Auckland and
Orewa in New Zealand, one can see how much of the land has been slashed
bare, causing massive flows of mud to the sea. The process never stops
or abates because society progresses and the expensive machines must be
This property access road was delayed because of heavy rains,
which left the soil bare for many months. Such 'Acts of God' cannot entirely
be foreseen, but it illustrates the dimension of the risks.
The same road as in the picture on left, took two years to
be sufficiently grassed to diminish soil erosion. For two years it
was a major contributor of mud to the local estuary. Even now its sides
are not completely covered.
Although this is not the place for a course in civil engineering, it is
nonetheless helpful to understand a few engineering issues. This diagram
shows three roads cut in a hill with sloping rock layers. Roads need a
horizontal cross-section, wide enough for its purpose. Engineers cut into
the hill and use the fill to level the down-hill side of the cut. Fill
is often transported several hundred metres to fill gullies. The
easiest places to cut roads is on the crests of hills. Here the soil is
nearly level, requiring very little cutting and filling. Besides, the soil
is dry, which hardens the road. Most connecting roads run over hill crests.
the left, a road is cut through the down-sloping rock strata. When rock
layers slope down, soil slumps are hard to contain because water (blue
arrow) seeps under the soil and over the rock, lubricating it. Gravity
makes it slip onto the road. To remedy this problem, requires draining
the soil, cutting terraces and planting trees. In a case like this, the
roadside bank is easy to revegetate because of the availability of moisture.
On the right-hand side of the hill another road has been cut. Because
the rock strata slope backward, its banks are stable. If the hill side
is capped with trees, enough moisture will be provided to revegetate the
bank. Whether a bank faces the sun or is turned away from it, matters enormously
to the amount of available moisture, and consequently to how to revegetate
it and with which species.
the diagram a profile of a cut road is shown. Wherever a cut is made, the
water table drops down, as shown in the profile. The uphill ditch serves
not only to drain water during rain storms, but often also to lower the
water table to keep the body of the road dry. If the water table is too
high, the road becomes soft, no longer able to carry heavy loads during
the wet season (top left picture).
The diagram also shows green vegetation willing to grow where it can
reach the water table, on top of the bank, around the ditch and below the
road. The upper part of the bank is usually too dry, resulting in bare
soil, exposed to raindrop impact erosion. It can now also be seen that
by rounding the cut (dotted line), the cut profile can be made to follow
the water table more closely, which makes revegetating the bank easier
to do. It is a simple and cheap measure to reduce bank erosion considerably.
An example of poor engineering and politics. The Goat Island
Road needed to be tar-sealed, but in order to attract a government subsidy,
it had to be made much wider than strictly needed, resulting in steep road
cuts intruding on neighbouring farmland. To make matters worse, the cuts
were not rounded and no form of erosion control was applied. All this soil
ended up in NZ's first marine reserve, damaging the marine environment
A bank bordering a motorway, shows how difficult it is to
control erosion by revegetation. When unattended, this bank will creep
backward like it has been doing for at least six years. Every year, this
scar alone delivers at least two ton of mud to the sea, about 200x the
rate of natural erosion or 50x that of the grassed soil above it. The problem
here arises not from scouring or gullying but from the deep volcanic soil
crumbling away faster than seedlings can grow. Also the water table is
out of reach.
Wire netting baskets filled with stones are called gabions.
Here they are stacked to protect an unstable bank. It is an expensive method,
and what will happen when the baskets rust through?.
These landscaping machines have not been bought to stand
idle. Modern machinery has enabled humans to shift more soil in a shorter
time. From this perspective, they are both a blessing and a curse.
This tall bank was easier cut than contained. Banks like
these should have been terraced and replanted in deep rooting trees. Now
it has become a problem too hard to tackle. Erosion denudes the rock face
and all soil ends up in the sea.
Drains and ditches are often not part of the engineering side of road
design, but they should. Inadequately designed drains and ditches cause
much erosion. One of the problems is that a permanent solution, such as
a concreted waterway, is often unaffordable, and it sits in the way of
further widening or straightening of the road. Usually the roading process
is one of gradual improvement, requiring many successive events of cutting
and grading of the road's cross-section. The road's shoulders and ditches
are frequently shifted over time.
Some of the requirements for road-side ditches are:
They should transport all the water from banks, road surface and often
adjoining property, without flooding the road.
Adequate drains are required to cross under the road.
Downhill farm land should not carry the burden of concentrated water spouts
running over its pastures or cropland.
Ditches should be paved with a natural substance which costs little, is
easy to shift, and does not impede water flow, while being easy and cheap
Grass can successfully be used to pave ditches, requiring specially designed
mowers, but a single application of herbicide may negate all the effort
of establishing it. Where water runs fast, ditches should be paved with
stones and check-dams to slow the water down.
An example of how a poorly designed ditch has eroded almost
beyond salvation. In the process, the road has also become more dangerous.
This ditch is now about 1.5m deep. How can it be repaired?
Another example of a poorly designed ditch that was scoured
out to 1m depth, and in the process took most of the bank with it as well.
Now an expensive bed of rocks was needed to repair a design fault that
could have been avoided.
The roadside ditch has been paved with coarse gravel, and
check dams made of sand bags have been placed to slow the flow. The sand
in these bags may have been mixed with a small amount of cement, to make
them last longer. It is a cost-effective solution.
This soil rests on a down-sloping bedrock, reason why it
is being drained. Without the drains, a slip would be very likely. Notice
how much water drains from this bank.
The whole idea of check dams is to change the direction of the rushing
water such that it works against itself. The most effective way is to send
the water up in the air like a spout such that its forward speed is lost.
Check dams must not bank up water, but always leave a small opening for
small flows. They are needed most for the rare occasions of torrential
rainfall and should be dimensioned for these events.
The killer kerb Sometimes little details matter. How often do you read in the newspaper
of pedestrians being hit by cars; of cars hitting lamp posts placed well
free of the road? This is caused by the unknown and invisible terror of
the killer kerb. It is everywhere and it kills innocent people, while blaming
or even jailing other innocent ones. I've driven some 2 million kilometre
and seen only four accidents happen before my eyes. Three of these were
caused by the killer kerb. Fortunately, both ended only with material damage.
one I just escaped by luck. Here is what happens.
killer kerb is the abrupt boundary between the raised side walk and the
road. Sidewalks are raised for the pedestrian's safety and also to keep
it high and dry, above the rainwater runoff from the road. The water runs
against the kerb until it encounters a storm water drain. When a rubber
tyre grazes this vertical kerb, it causes enough friction to slow the wheel
down, but the car pushes forward and turns the wheel slightly, resulting
in the wheel being pushed firmer against the kerb, causing more friction
and more slowing down. Very suddenly the wheel is almost stopped. The steering
wheel jerks out of the driver's hands with tremendous force and the car
jumps onto the kerb, into innocent children or a lamp post (figure A on
left). On motorways with median barriers or side barriers, the car turns
almost entirely around, facing the other direction, and is bounced off
the barrier into the opposing traffic, traversing two lanes and the median
strip, causing mayhem (figure B).
Killer kerb accidents happen in a moment of inattention, a misjudgement
of where the wheels on the other side of the car are, sometimes by slight
drunkenness and an unsteady course, sometime carelessness. So many innocent
drivers are sued for an accident that they could neither foresee nor avoid.
It is particularly acute on bridges. Drivers say that 'they suddenly lost
control' and 'the car changed direction suddenly'.
Figure C shows how a slightly turning wheel rubs increasingly more against
the sharp kerb. Kerbs can be made safe by following a profile somewhat
like the blue dotted line. In figure D some of the kerb profiles in use
are shown. The bottom one creates more depth for the water to flow, but
at the same time becomes the worst killer kerb in use today. The profiles
above it are progressively safer.
It is true that modern kerbs are more gentle, but the killer kerb is
still actively laid today, everywhere in the world. No research has been
conducted into how a kerb should be shaped to be safe, nudging a car safely
back onto the road. The killer kerb has a large effect on small wheels,
much less on buses and trucks. Some tyres (radials??) are more susceptible
than others. Civil engineers are apparently unaware of this problem, and
so are city councils and motorists. To help eradicate this problem, refer
engineers and councillors to this chapter on the Web. Likewise, all motorists
should be made aware of this hidden danger in their driver training.
This innocently looking edge between the road and the kerb
is the cause of many accidents. It causes drivers to suddenly lose control
of their vehicles, which jump onto the kerb, killing innocent pedestrians.
In new road constructions, associated with motorways, a more
benign kerb is laid, which has a sloping edge. However, even this kerb
can be dangerous for some tyres. Obviously, more research needs to be done.
Roads are the world's most lethal environments. Just imagine a landscape
entirely tar sealed over, for as far as the eye can see. What sort of environment
would that create?
heat: the heat from the sun would heat the tar-seal close to boiling
point, gradually also heating the air to similar temperatures. No earthly
creature could live on it, above it or under it. The soils underneath would
adiabatic wind: the hot air would rise unpredictably causing fast
uplifting winds that draw in fresh air from outside. The hot air would
rise to unmatched heights of over 1000m, interfering with the water cycle.
wind: with nothing to slow them down, winds would rage unimpededly
at their maximum speeds.
runoff: rain water would not be absorbed but run down-hill in ever
Clearly, the roads we build are environments from hell, and no matter how
small they seem in relation to the total landscape, they do affect the
environment and local climate in no uncertain ways. So what could engineers
do to compensate?
heat: what could be done to ameliorate the heat from roads?
by using light coloured rock (or broken concrete) in the tar seal matrix,
the road surface becomes lighter, thus absorbing less solar energy.
shading trees can be placed on the side facing the sun. It makes an enormous
cool marshes and ponds can be incorporated in the road's design, also serving
as water traps.
wind: the tar seal and road sides will tunnel the wind, so what
can be done?
sheltering trees of varying heights will slow the wind down.
rather than placing trees in continuous rows, they can be placed in alternating
rows left and right and in groups.
by mixing deciduous trees with non-deciduous ones, there will still be
trees with foliage in winter or in the dry season.
runoff: roads do not absorb any amount of water, so extra provisions
must be made to compensate for this.
by including ponds and marshes in road design, runoff can be slowed and
even stored for gradual evaporation.
deep ditches with check dams to guide the water to environmentally friendly
discharge points taking account of downhill effects.
frequent stormwater drains to guide the water under the road to discharge
points. Rather than having a few large discharge points, it is better to
have many small ones.
As stated before, the pressure on roading and housing developments comes
from increasing populations and their increasing welfare. So the amount
of soil cut and shifted is increasing exponentially, reason why erosion
control needs to keep up with this trend. From the chapter on soil erosion
follows that the main damage arises from raindrop impact and water scouring.
The most effective control of erosion is thus achieved firstly by covering
the soil and secondly by trapping running water.
Immediately after cutting, the soil must be covered with straw or better
still, wood peelings or wood bark, while at the same time re-seeding it.
Fertiliser must be applied when seeding and should be maintained by regular
dressings, even when the grass is fully grown.
Sedimentation ponds at strategic locations, help to trap coarse particles
like clumps, crumbs, peds and sand but the finer clay which needs over
two weeks to settle out, won't be trapped (see also erosion/rain).
Covering the soil and preventing the clay particles from being released,
is thus a far better solution.
The idea of a sediment trap is simple: create a point where the water
slows down, to allow soil particles to settle out. If the pond is large
enough, it could trap the entire runoff without flowing over. But a number
of facts render their effect rather disappointing:
There is never enough room for the pond. Developers want to use all of
their land, and a pond is a costly sacrifice and of a 'temporary' nature.
Besides, the soil everywhere around it, is being shifted. Ponds are usually
much smaller than 5% of the developed area. Even a small rain of 10mm results
in a 200-2000mm rise in the pond level, causing it to flow over.
Small particles like silt take hours to settle out; clay weeks. These particles,
often constituting the bulk of the soil, pass through the ponds.
The damaging effects of rain drops increases very rapidly with their size,
causing hardly any damage during light rains but massive damage during
rainstorms, when the sediment traps are least effective. The water flows
too fast and is more turbulent and the ponds are too small to steady the
water for long enough.
A sedimentation pond has been dug at the bottom of an industrial
development site. Its level is low at the moment, caused by underground
seepage, but during rainstorms it will flow over into the storm drain in
the foreground, from which it flows without further hindrance, into the
The area draining into it is about 100 times larger and
a 50mm rain storm would raise its level by 5 m!
This digger is cobbling a check dam together along the motorway.
Check dams slow the water down, to reduce scour. But these dams are placed
where water flows slowly already, perhaps intended to trap sediment. Erosion
control at the bottom of a hill is always ineffectual. Look at the amount
of bare soil on the slope! Here is where erosion control works.
A small sediment trap at the foot of a bank covered with
straw. Geotextile matting has been placed around the pond as an experiment
to see how effectively it traps loose soil. The fact that no soil is found
either inside or outside the pond, shows how ineffectual they are.
A system of cascading sediment ponds was installed as an
experiment to determine their effectiveness. Little benefit can be expected
because erosion happens only during extreme events, when runoff grossly
exceeds the capacity of these ponds.
To prevent erosion, road sides need to be stabilised. Natural vegetation
is cheapest and most pleasing, while blending in with the natural landscape.
In order to provide immediate cover, the bare soil is covered in straw,
hay, wood chips or bark while the seed mix is applied. Where banks are
steep, paper pulp is used.
Where banks slope gradually, such as near motorways, they can be grassed
over, even grazed for natural maintenance, and motorised grass mowers are
able to drive over these banks. The problem here is similar to that of
maintaining a pasture.
But steeper banks cause problems, both in their establishment and their
maintenance. Establishment problems are mainly caused by using the wrong
species of plant. If native pioneer plants were included in the seed mix,
they could resist drought better. Also the leguminous alfalfa should be
part of the seed mix. Much research needs to be done on the right mix of
plants. An important start is to acknowledge that not all roadsides need
to look like a golf course. Hardy native vegetation is to be preferred
because it is more permanent, resists drought better, blends in best and
grows slowest. My personal suggestion for the mix is:
some leguminous plants like clover and alfalfa.
some grasses for quick establishment, but it needs to be a mix, including
some fine-leaved, low and slow growing drought-resistant native species
capable of being mowed.
Ironically, the species most successful for revegetating bare, infertile
land, have been declared noxious for this very same reason (gorse, pampas
grass, ladder fern).
A regrassing machine in action, spraying a mix of water,
paper pulp and grass seeds onto the lower bank of a recently upgraded road.
Note the bank on right appears several times in the following photographs.
This low-key technology consists of a tank with a stirrer inside, and a
water pump outside. For each batch, used daily newspapers are shredded
into the water-filled tank, and grass seeds are added.
Just before being sprayed with drenched paper pulp and grass
seed, the bank on the picture at left, looked like this, a very deep, poorly
weathered soil, typical of New Zealand's native forests.
Only months after application, the grass formed this dense
cover, but not for long. Lacking maintenance applications of fertiliser
and not surviving the summer droughts, most of the grass died off and was
This is the situation two years after regrassing the bank
shown above. Lack of ground water and the use of the wrong seeds, caused
this regrassing attempt to almost fail completely. Grass did establish
lower down, within reach of the ground water.
Here the roadside was treated with a ryegrass-clover mix
(standard meadow seed), of which only the clover survived, due to its self-fertilisation
and resistance to drought. On the slipped patch top right, a new mix has
been applied and one can see the young ryegrass coming up.
Where rocks are hard enough, and particularly on shaded banks,
lichens and hardy mosses develop an attractive and effective cover, requiring
Banks underneath a cover of native forests, are much more
stable than those without, because the forest retains moisture during summer,
also allowing the bank to revegetate successfully. The evenness of moisture
over the year, prevents clay from expanding and shrinking, which would
break the soil, causing it to crumble and erode and slump.
Ladder ferns are stable, drought tolerant ground cover, ideal
for road verges but they have recently been declared a noxious weed.
The maintenance of road sides is very much a hit and miss affair, lacking
clear policy. It was usually done by the staff of the local council, but
has recently been contracted out (a freemarket concept). It worsened the
condition of the banks, and diminished accountability and responsibility.
By now, one can see that maintaining roadsides in a sustainable and environmentally
friendly way, is not easy. For this reason, the roadside vegetation must
be looked at as a fragile environment.
Once established, vegetation needs to be controlled for the following
being able to see the road's shoulder and ditch, the area for emergency
being able to see oncoming traffic around a bend.
to prevent roadside fires arising from discarded cigarette butts.
to control pest plants.
to clear waterways.
The going methods of vegetation control are:
scraping by diggers and bulldozers.
motorised mowing by cage mowers, flail mowers and slashers.
hand mowing by rucksack slashers and weed-eaters.
motorised spraying of herbicides.
hand spraying of herbicides.
The use of herbicides should be discouraged on banks simply because they
lay the ground bare, requiring the vegetation to start all over again each
time. It is preferred by councils and contractors because it is easy and
cheap and does not need to be repeated as often. However, there is a strong
case for outlawing it altogether, mainly because of the erosion it brings
The use of diggers for weed control on clay road verges is also to
be discouraged, but their use on road shoulders (and some ditches) is correct.
Mowing the road verges close to the ground can only be tolerated where
grassland adjoins and where the bank is robust and fertile. But close shaving
prevents hardy and lasting perennials from establishing on the more difficult
banks. If we had a nation-wide policy of not mowing closer than 40cm to
the ground, such shrubs and ferns could establish themselves. It would
require far less maintenance, produce less erosion, while offering a pretty,
bushy look of our road sides.
Many roads in poor countries are still unsealed, providing
a major source of mud. In this picture a grader is seen grading a gravel
road on a clay base. A grader works like a wood planer, cutting the surface
between the front and rear wheels, thus levelling any unevenness. It requires
skill. As long as the road remains covered in gravel, raindrops will have
This bank was once perfectly vegetated but tractor-mounted
slashers have damaged it severely in an attempt to shave closely. With
contractors in charge, and council staff not checking the work, the environment
is left at their mercy.
This bank is maintained by spraying with herbicides. It bares
the soil, which allows rain drops to carve it out. Herbicides are tolerable
on the gravel road shoulder but not on clay.
In the hands of poorly instructed workers, roadside weed
control can inflict serious damage to this fragile environment. Here a
worker uses a motorised weed-eater to trim the grass, but the soil is bared
and slashed in many places.
A growing number of people are taking stewardship of sections of roadside,
revegetating them in attractive ways, often with robust perennial flowering
plants. Passers-by are now enjoying their initiatives and efforts.
The median strip of this motorway has been sown with wildflowers.
Their gaudy colours provide a most welcome change of the monotony of the
motorway. Once sufficiently tall, they also shield the traffic from oncoming
A stretch of roadside is maintained by the local landowners,
who planted these attractive perennial flowers. County contractors are
often too bloody-minded in their grass mowing, which has caused some clashes.
What can we do?
It is not a matter of what we can do, but rather of what we must do. We
must contain roadside erosion to acceptable levels, which means close to
the naturally occurring erosion of about 1 ton/ha. The outlook that so
much can be improved, rests on the sobering fact that so little has been
done about it. But there is a role for everyone:
Improved engineering: including the ditches and revegetation in
the road design.
Shifting priorities: national roading agencies should shift funding
priorities toward the sealing of unsealed roads.
Taking responsibility: local councils should take full supervision
Protecting bare land: by covering it, with whatever means. No amount
of bare land is acceptable.
More research: into improved vegetation and seed mixes, improved
kerb profiles for sunlit and shaded margins.
Improved management: by not spraying on clay, mowing only to 40cm
off the ground.
Education: educating engineers, local councils, motorists, the public.
Blowing whistles: don't accept bare soil or shoddy practices anywhere.
Report unsatisfactory situations.
Form care groups: form a care group to take care of sections of