Section B1
Mechanics of urban creek erosion
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CONTENTS
PAGE
B1.1
Introduction
101
B1.2
Introduction to creek morphology
101
B1.3
Impact of urbanisation on creek erosion
105
B1.4
Expected impact of stormwater detention systems on urban creek erosion
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B1.1 INTRODUCTION
The following methodology has been adopted to demonstrate the mechanics of urban creek erosion:
x
x
x
x
understand how creeks form
define the influencing factors
discuss how urbanisation affects these factors
draw conclusions about how creeks would be expected to react to urbanisation.
B1.2 INTRODUCTION TO CREEK MORPHOLOGY
In steep catchments, creeks are often established on a rock base so vegetation is not the primary means
of creek stability. In low gradient or moderately steep catchments, vegetation cover is often the main
means of bed and bank stabilisation in the upper reaches of the creek. For the purposes of this
discussion, the following comments are based on the formation of earth-lined creek systems in typical
low-gradient or moderately steep (hilly) catchments.
Overland flow paths
Minor creeks in Brisbane usually start as overland flow paths with continuous vegetative cover and no
defined banks. These overland flow paths can survive with a complete grass cover because flow velocities
resulting from frequent storm events are less than the erosive velocity for the given vegetative cover.
During these frequent storm events, damage to the vegetation is unlikely, but soil erosion around the
base of the vegetation (grass) would be expected on a regular basis. In undisturbed catchments, this
regular down-slope transportation of soil grains is closely balanced by the constant formation of soils
from weathering of soils and rocks.
However, during extreme events, grass can be damaged and scour holes may appear down- slope of
trees, rocks and clumped grasses. These scour holes slowly fill with sediment after years of light to
moderate storm events. Vegetation also establishes within these damaged areas, progressively
stabilising the sediment-filled scour holes.
Thus, equilibrium is achieved on overland flow paths through a constant cycle of damage and repair.
Formation of a watercourse with defined banks
Further down the catchment, some point is reached where a uniform vegetative cover cannot survive. At
this point, a defined low-flow channel or watercourse forms. This low-flow channel eventually forms into a
creek. In dry weather, a trickle flow or base flow may exist in small urban creeks because of groundwater
inflows and runoff from garden watering. This runoff can enter the creek directly through stormwater
pipes or indirectly through a groundwater spring.
The theoretical potential for erosion in creeks has traditionally been based on a relationship between the
average in-bank velocity (average velocity of the flow contained within the creek banks) and the erosive
nature of the creek's surface material (ie. soil, vegetation cover, or exposed rock). The average velocity of
the in-bank flow generally increases with the severity of the flood event. Therefore, erosion within a creek
is also expected to increase with the severity of the flood.
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Establishment of low-flow channels
Formation of a low-flow channel can be initiated by:
(i)
(ii)
(iii)
(iv)
the water table reaches ground level causing damage to the root system of the grass. Typically
this may occur in open grassed areas where trees cannot control groundwater levels.
a spring or continuous (or near-continuous) base flow saturating the ground causing failure of the
grass cover
frequent damage to the grass caused by overland flows exceeding the natural rate of repair of
the grass
erosion from a downstream open channel migrating up the watercourse during extreme flood
events and forming a ‘head cut’ or progressive upstream bed degradation.
In most creeks, significant damage is expected during extreme floods such as the 1-in-100 year event
(Q100). However, when it is considered that most creeks have experienced tens or even hundreds of
Q100, Q500 and Q1000 flood events, why is the in-bank area of most creeks so small and why hasn’t years
of constant erosion resulted in a wide, open creek bed?
To answer to these questions, we need to realise that creeks operate under the same damage-repair
cycle previously described for grassed overland flow paths. That is, damage that occurs during extreme
events is slowly repaired during the low-flow periods (ie. erosion holes are filled and damaged vegetation
is repaired or replaced). The creek establishes equilibrium based not only on the in-bank flow velocity,
but also the catchment’s general hydrology, soil type, riparian vegetation and the type of bed material.
Damage mechanisms
Soils: Flow velocity and associated turbulence draws soil particles into the flow resulting in the
transportation of these sediment particles.
Influencing factors include:
x soil cover (vegetation);
x flow velocity or shear stress;
x chemical reactions (ie. dispersive soils).
Vegetation:
For vegetation, a time component is introduced. In grassed channels, the allowable flow
velocity (uniform flow velocity that will not initiate failure) varies depending how long they are exposed to
that velocity. Grassed areas may withstand 5 m/s flow velocity for a few minutes, 3 to 4 m/s for a few
hours and 1 to 2 m/s over a period of days.
As floodwaters pass, soil particles are slowly removed from the base of the vegetation. If this flow stops
before the root system is exposed, the vegetation survives. The only repairs needed will be the
replacement of soil around the base of the plant. This soil replacement is achieved by normal
sedimentation during periods of low flow.
If soil erosion exposes the root system during the flood, then the plant could be undermined (this is the
main failure mechanism in grass-lined channels). Once failure occurs in isolated locations, damage
quickly spreads. Natural repair of these areas needs replacement of soil through sedimentation and
regrowth of vegetation (as previously described for grassed overland flow paths).
The above discussion refers mainly to bed vegetation. For bank vegetation, failure of the plant structure
can also result from high-velocity flows undermining the plant. This erosion eventually leads to partial
exposure of the root system, followed by the complete undermining and collapse of the plant, or long-
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term exposure of the plant’s roots. For most plants, any exposed roots will die from long-term exposure
to the sun or through damage caused by flood debris.
Failure or removal of the root system eventually exposes the soil under the plant, followed by bank failure
or simply the death of the plant. However, there are some plants that can withstand long-term exposure
of their root system. These plants are often best suited as bank vegetation.
It can be concluded that vertical or near vertical banks will not reach long-term stability because soil loss
cannot be replaced by normal sedimentation. However, steep banks can be stabilised by exposed rock,
erosion-resistant soils or a dense cover of mat-forming shrub and tree roots that can survive long-term
exposure.
Thus, the long-term stability of a creek channel incorporates:
x
frequent minor damage (bed and bank erosion, and minor vegetation damage)
x
varying degrees of bed and bank damage depending on the flood flow (ie. flood frequency and
flow velocity and duration)
x
significant damage and possibly the re-direction or re-location of creeks during extreme flood
events
x
continuous but slow sedimentation replacing eroded soils and
x
a continual damage-repair cycle for in-bank vegetation.
So why does a creek form with well-defined bed and banks?
In circumstances where a grassed swale or overland flow path cannot withstand the velocity, frequency or
duration of regular storm runoff, it is necessary to find another way to contain these flows. The problem is
that eroded open-earth channels typically cannot withstand the same flow velocities as grassed channels.
Adding to this problem is that the creation of an open channel increases flow depths and velocities.
To combat these problems, nature must either:
x
increase the channel roughness to decrease the average in-bank flow velocity
x
increase the length of the channel (ie. channel meandering) to decrease the channel slope and
flow velocity
x
increase the erosion resistance of the channel lining (ie. establish bed and bank vegetation or the
existence of natural rock protection) or
x
increase flow resistance by introducing bends and meanders, decreasing in-bank flow velocities.
In most cases, the formed creek will have a combination of these features.
Why do creeks typically form an in-bank channel capable of carrying between the 1-in-1 year (Q1) to the
1-in-2 year (Q2) flood flow?
If we accept that long-term creek stability is based on a damage-repair cycle and that significant groundcover vegetation typically has an establishment period (or repair time) of 1 to 2 years (including the
replacement of soil and regrowth of vegetation), a link between the two may exist.
In areas of a watercourse (including the channel and over-bank floodplain), where the frequency of
damaging flood flows is less than 1 to 2 years, then ground cover vegetation cannot stabilise, so an
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eroded open channel is formed (ie. a creek). In areas where the frequency exceeds 1 to 2 years (ie. within
the floodplain), vegetation can act as the primary erosion control mechanism.
In-bank area of a creek
The in-bank area of a creek can be governed by:
x
x
x
x
x
x
Q1 to Q2 flows
rock-lined bed and banks
trees providing exposed mat-like root protection of the banks
channelisation (constructed creek modifications)
the bed level, so the in-bank area of the channel is directly related to elevation of the bed relative
to the height of the over-bank topography or
local hydraulic factors, such as thick vegetation on the over-bank areas or major snags causing
increased stress on the creek channel.
This does not mean that in-bank vegetation should not occur, or that it has no impact on creek stability.
Rather, it means that the creek width is likely to be strongly influenced by the damage-repair cycle of the
bank vegetation, while the creek depth is likely to be strongly influenced by the damage-repair cycle of
the bed vegetation.
Therefore, it may be possible to study the in-bank size of creeks and correlate this with the damagerepair rate of the native bank vegetation (noting that riparian vegetation can change significantly over
short distances relative to the top of the creek bank).
The role of trees in the control of creek erosion
Trees have a much longer repair period than most ground cover plants. When exposed to the erosive
forces of in-bank flows, tree roots must be protected by ground cover vegetation or in-bank rocks if root
exposure is to be avoided.
Trees do not prevent creek erosion by binding the soil particles together. In fact, tree roots are easily
exposed if subjected to medium to high velocity flows.
Trees typically only stabilise a creek by providing structural strength to the banks. When masses of
weather-resistant tree roots are exposed (such as in the case of mangroves), these roots can prevent
high-velocity flows from reaching the underlying earth bank. In these cases, the trees and shrubs do
protect creek banks from erosion.
Successful bank-stabilising tree species have root systems that can withstand exposure without drying
out. The roots must also be long enough to pass below the active bank erosion (ie. the full bank height if
near vertical banks exist).
Finally, trees greatly influence the stability of static creeks (creeks not subject to meandering or changing
catchment conditions), but they have only limited long-term influence on mobile or dynamic creeks (ie.
creeks responding to changing catchment conditions caused by factors such as urbanisation).
B1.3 IMPACT OF URBANISATION ON CREEK EROSION
We know that urbanisation causes stormwater runoff during even the smallest storm. In undisturbed
catchments, surface runoff generally does not result from small storms because the rain is either used to
wet the surface area of the trees, leaves and surface mulch or allowed to infiltrate the soil. Runoff from
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undisturbed catchments only occurs in large or long duration storms that are sufficient to saturate the
catchment.
Urbanisation is reported to increase base flows in areas where regular lawn watering is common. Streams
that rely on base flows from ground water supplies can be affected by the increased impervious surface
of urbanisation (although natural catchments can also have natural impervious surface areas resulting
from certain soil conditions).
As a result of urbanisation, the velocity of surface runoff is usually increased and the delay time before
runoff starts to occur is reduced. Therefore, the response time of the catchment is reduced.
The hydraulic response time of a catchment is important because the flow in a creek at a given location,
and at any given time, will generally be governed by the volume of rain that has fallen over the preceding
period of time equal to the hydraulic response time of the creek at that location. Therefore, if we
consider a very long creek, at the top of this catchment, the response time may be just 15 minutes and
thus the flow in the creek at any point in time is dependent on only the previous 15 minutes of rainfall.
Half way down the catchment the response time may be 1 hour and therefore the flow in the creek at this
location is dependent on the previous 1 hour of rainfall.
At the end of the catchment, the response time may be 3 hours, so it will be the previous 3 hours of
rainfall that determines the flow in a creek. Thus during a given storm event, one part of a creek may
experience severe flooding while another part of the creek only experiences moderate flooding.
The response time of a creek is important because the peak flood flow in a creek is related to the average
rainfall intensity over a period equal to the catchment’s response time. For a given storm frequency, the
smaller the response time, the higher the average rainfall intensity.
If urbanisation decreases the response time of a catchment, then the average rainfall intensity over the
critical period of a storm will increase, thus increasing the creek’s peak discharge during that storm.
Impacts of urbanisation on creek flows
The following is a summary of the expected impacts of urbanisation on a drainage catchment.
Low-intensity, short duration storms:
x volume of runoff increases
x peak discharge increases
x frequency of runoff increases.
These storms are either short-duration summer thunder storms or slightly longer duration rainfall
depressions that generally cause only minor flooding of small creek systems, but are the source of most
in-bank flows in terms of total runoff volume.
Low-intensity, long-duration storms:
x volume of runoff increases
x peak discharge slightly increases
x frequency of runoff probably unchanged.
These are storms caused by rainfall depressions that may last several days and often have several
moderately intense bursts of rain throughout the storm. Such storms can cause minor to moderate
flooding in creeks and rivers. These storms can also be responsible for a significant proportion of in-bank
flows for medium to large creeks, and significant base flows in minor creeks.
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High-intensity, short-duration storms:
• volume of runoff increases
• peak discharge increases
• little change in the frequency of runoff.
These are concentrated rainfall depressions, possibly major thunder storms, that cause significant
flooding in minor creek systems, but only moderate flooding in major creeks and generally insignificant
flooding in rivers.
High-intensity, long-duration storms:
• volume of runoff, little change
• peak discharge, little change
• frequency of runoff, no change.
These storms are caused by cyclonic depressions, such as the 1974 Brisbane River flood. They can cause
significant flooding in rivers and major creek systems, but may cause only moderate flooding in minor
creeks.
Impacts of urbanisation on creek erosion
It can be concluded that urbanisation has the greatest impact on short-duration, low-intensity storms thunder storms of less than, say, a 1-in-10 year (Q10) intensity.
The greatest impact will be on events less than the 1-in-2 year (Q2) event. The storms that have the
greatest influence on the in-bank area of creeks are also the storms most affected by urbanisation. This
impact decreases with increasing size of the creek, ie. as the response time of the catchment increases.
If the Q1-Q2 in-bank area of a creek’s low-flow channel is controlled by the damage-repair cycle, and if
urbanisation increases the frequency of in-bank flows, it can be predicted that the in-bank area of an
urban creek would increase from, say, the natural Q1 peak discharge to the urban Q2 peak discharge.
So, if we look at a large creek system such as Brisbane’s Cabbage Tree Creek or Bulimba Creek, we
would expect:
•
the point where a grassed overland flow path fails and the open earth-lined channel starts, will
migrate upstream as a result of urbanisation
•
the region of a creek where the Q1 to Q2 peak flows are controlled by thunder storms (ie. the upper
catchment area), should experience the greatest in-bank erosion and channel expansion
•
further down the creek catchment, where the Q1 to Q2 flows are governed by long- duration cyclonic
events, we would expect little change in the Q1-Q2 flows, little change in the frequency of these
events, and therefore little change in the in-bank area of the creek.
B1.4 EXPECTED IMPACT OF STORMWATER DETENTION SYSTEMS ON URBAN
CREEK EROSION
Stormwater detention systems are installed within a drainage catchment to compensate for reductions in
stormwater infiltration and catchment response time resulting from urbanisation of the catchment. These
detention systems can vary in size from large regional detention basins (usually in parkland) to small
above or below ground single-dwelling systems better know as on-site detention (OSD).
Most regional stormwater detention systems are designed to mitigate the effects of urbanisation on
major flood events such as the once in 50 year or the once in 100 year events. However, some regional
systems are specifically designed to limit urban runoff to the maximum capacity of the constructed
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downstream drainage network. In either case, the basins are usually designed to have only a minor
impact on the frequent flood events such as the 1 in 1 year or 1 in 2 year storms.
In cases where it is necessary to design a regional detention system to have mitigating effects on the
frequent flood flows, the size of the basin is usually significantly larger than that of a simple flood control
basin. For environmental and serviceability reasons, these basins are better designed with a permanent
or near permanent water body incorporated into the basin such as a wetland or lake.
Such basins are known as retention basins as opposed to dry-bed detention basins. Both detention
systems may be referred to as retardation basins.
On-site detention systems on the other hand usually provide some degree of runoff control for a wide
range of storm events. However, these systems do not necessarily limit stormwater runoff to predevelopment conditions for all storms. In many cases on-site detention systems are sized to avoid
surcharging the downstream drainage network rather than to avoid damage to a downstream creek.
Most detention systems are designed to reduce the peak discharge of major storm events but not to
reduce the total volume of stormwater runoff. Thus these systems effectively reduce the volume of
overland (floodplain) flood flows and increase the volume and duration of in-bank flows. When it is
considered that creek erosion only occurs as a result of in-bank flows, and that the degree of creek
erosion depends on the duration of these in-bank flows, then it can be concluded that stormwater
detention systems have the potential to increase creek erosion rather than decrease creek erosion.
Due to the increased volume and duration of stormwater runoff from urbanised catchments, it is currently
believed that to limit the impacts of urbanisation on creek systems, it will be necessary to design
stormwater detention systems to reduce the 1 in 1 year to 1 in 2 year peak flood flows to levels
significantly less than the pre-development conditions. The degree to which a reduction in peak flood
flows is necessary will depend on the damage-repair cycle of the watercourse vegetation.
In conclusion, it would appear that large regional detention basins have the potential to increase in-bank
creek erosion downstream of the basin. However, the long-term effects of on-site detention systems on
urban creek erosion is still unknown. In either case, it is expected that stormwater detention systems
need to be designed to restrict regular flood flows to less than pre-development conditions if unnatural
creek erosion is to be avoided.
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