Promoting Sustainable Drainage Systems in Islington
1. Introduction
1.1
Figure 2: Rivers and springs in Islington
Managing rainfall in the city
In the beginning
Once upon a time, rain would have fallen on a forest landscape where the borough of
Islington now stands. Rainfall filtered gently through a canopy of leaves before falling to the
woodland floor and soaking into the ground before finding its way through the surface and
soil pathways to marshy places, ponds and streams. The shallow winding streams led
eventually to a wide, marsh fringed river of many channels that would later be tamed and
called the River Thames.
The city grows
As agricultural settlements developed in Islington, this natural condition began to change.
Water was drained to ditches and then along straightened streams to a river confined by
banks. As London grew, development in Islington increased, much of it focusing around the
many streams, rivers and springs in the area (see figure 2). As London’s population grew
further and existing watercourses became polluted, the New River was developed, bringing
clean drinking water from Hertfordshire, via Islington, into London.
The Victorian period led to major changes in London’s water landscape as the demands of
London’s burgeoning population, escalating land values and the pollution caused by human
waste resulted in development of the London sewer system. This network of pipes collected
sewage and rainwater under the city for transport to treatment works and then to the sea.
Most of London’s watercourses were also undergrounded during this period and are no
longer visible; however, clues to the past importance of water on Islington’s development
remain in evidence in many of Islington’s street names (see figure 1).
Figure 1: Water and Islington place names
Early habitation in Islington grew up around springs and shallow wells and there remains evidence
of this in many of the borough’s place and street names:
•
Clerkenwell and Goswell both take their names from ancient springs in the area - Clerk’s
Well and Good Well or God’s Well.
•
Sadler’s Wells is also named after a spring, which became the home of ‘Islington Spa’. The
spa was famous for the quality of its water which was said to rival that at Tunbridge Wells
in its healing properties.
•
Turnmill Street follows the course of the River Fleet, which was also known as Turnmill
Brook owing to the number of mills along it.
•
A line of ponds once marked the edge of a gravel terrace carried by glacier meltwaters –
this boundary is now known as Ball’s Pond Road.
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Promoting Sustainable Drainage Systems in Islington
Unintended consequences
The problems of conventional drainage are inherent in the way it deals with water that flows
from hard surfaces in the city. Water is collected and conveyed away from where it falls as
quickly as possible, causing flooding when the drainage system cannot cope with the large
volumes of water entering it or if it is blocked. This can also cause sewer overflows
containing foul waste which create a range of additional problems.
A cocktail of oils, sediments, spillage, animal droppings, herbicides, pesticides, road
cleaning chemicals and debris is washed into the drainage system directly to streams or
treatment works. In times of low flow this causes severe pollution of streams, while in times
of high flow streams are damaged through erosion or silt build up. All the benefits that
derive from use of clean water locally, for example in creating attractive water features or
wildlife habitats and naturally irrigating landscapes, are foregone as it disappears
underground in the drainage system. Clean water cannot soak into soil naturally to
recharge groundwater.
Figure 3: SUDS Policy Drivers
There are a range of drivers for SUDS in Islington, ranging from national to local, including:
•
Pitt Review – This review took place following the floods in Summer 2007 and recommended
a range of changes needed to prevent similar problems in the future. It recommended that
Local Authorities and the Environment Agency should be given greater responsibility for
managing surface water flood risk.
•
Flood and Water Management Bill – The Bill includes measures to address flood risk,
including by clarifying responsibilities around flooding and encouraging sustainable drainage
systems. The Bill will end the automatic right to connect to sewers for surface water drainage
and require developers to put SUDS in place in new developments. Connection to the
drainage network for surface water will be conditional on meeting new national standards on
SUDS. Local Authorities will be responsible for approving SUDS schemes as well as their
adoption and maintenance.
•
Planning policy – a range of national, regional and local planning policies, including PPS 25
and the London Plan, promote the incorporation of SUDS within developments and more
widely across existing neighborhoods. Policy 4A.14 of the London Plan states that any
development should incorporate SUDS with the aim of achieving greenfield run-off rates (see
Appendix 1).
•
National Indicators 188 (climate change adaptation) and 189 (flooding) both promote the use
of SUDS to address flood risk. Islington has been identified as a priority borough for support
on NI189 following the Environment Agency’s initial assessment of surface water flood risk.
•
Islington’s Climate Change Adaptation Strategy and Biodiversity Action Plan – SUDS will
support delivery of both of these strategies, as well as wider objectives around physical
regeneration and improvement of the public realm.
The problem in Islington
Islington suffers from many of the problems described above. The borough is highly
urbanised with few permeable surfaces and a very dense population. As a result it is
deemed to have a high risk of surface water flooding, which is likely to be increased by
further growth and intensification of the built environment as well as increasing risk of heavy
rainfall due to climate change. As a result of its location and characteristics, the
Environment Agency has identified Islington as a one of a number of priority areas for
action on surface water flooding.
Constraints on the amount of water which filters into the soil also creates a number of
problems in Islington. Natural watering of trees and other vegetation is restricted, and we
may increasingly have to rely on artificial watering as our summers become hotter and
dryer. Drying of soils due to a lack of infiltration has led to significant subsidence problems
in some areas. We are also missing major opportunities to bring water back into Islington’s
environment and to enjoy the amenity benefits it can bring, both as part of our urban
landscape and for play and recreation. Water could also be harnessed to create a range of
habitats which would support and enhance biodiversity.
A new approach to managing rainfall
It has now been widely recognised that a new way of managing rainfall is needed to deal
with the problems created by the traditional pipe drainage system we have inherited from
the past. The inspiration for this new approach, known as Sustainable Drainage Systems,
or SUDS, can be found in the way nature intercepts rain through vegetation and soils,
allowing water to flow slowly into the ground or to wetlands, ponds and streams.
SUDS is a more environmentally friendly way of dealing with runoff from development that
uses landscape techniques that mimic nature to control flows, prevent pollution, and
provide attractive water features that enhance wildlife and provide benefits for the local
community. SUDS provide robust, easily managed and usually cheaper ways of dealing
with rainfall. The shift towards this new approach is reflected in a range of policy drivers
which promote SUDS; these are outlined in figure 3.
Benefits of SUDS
In Islington, SUDS offer a number of benefits:
• Reduces the risk of surface water flooding and related sewer surcharging by providing
on site storage and a reduction of rate of runoff into the combined sewer.
• Improves the water quality of runoff from a site – in Islington this is particularly important
where this water will be reused to provide local benefits.
• Improves the quality and attractiveness, and in many cases the value, of public realm
and private developments by creating attractive landscape features.
• Enhances biodiversity through the creation of habitats such as wetlands, ponds and
planted raingardens.
• Reduces the need for artificial watering of trees and landscaped areas by offering
techniques for naturally irrigating landscapes.
• Provides clean water for reuse by residents or businesses, either through an outdoor
rainwater butt or internal rainwater recycling, for example to flush toilets.
• Reduces capital and maintenance costs associated with conventional drainage systems.
• Provides education and play opportunities by making the water cycle visible.
• Provides a lower carbon drainage solution by reducing embodied energy in hard
infrastructure and the volume of water requiring energy intensive treatment downstream.
• Increases flexibility of the drainage system, for example to adapt to climate change.
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Promoting Sustainable Drainage Systems in Islington
1.2 SUDS characteristics and design criteria
SUDS in the city
SUDS use a variety of techniques in sequence to slow the flow of runoff and improve the
quality of water by natural cleaning processes. Where there is enough space in
development, this can be achieved in open vegetated features like swales (flat bottomed
grass channels), basins, ponds and wetlands. But in urban areas it is usually necessary to
use more engineered techniques to make the most of confined space. Every urban surface
must be considered as a rainfall collector, allowing water to pass through to a drainage
layer below or flow to a soakage area so that volumes do not build up to cause problems
downstream. Pollution is trapped and treated where it is generated. Important techniques
include ‘green’ roofs, permeable surfaces, ‘raingardens’ and bioswales, where water can
soak into planted areas, or any other underdrained surface that intercepts water ‘at source’
near to where it falls as rain.
SUDS manage runoff from development in an integrated way to reduce the quantity of
water entering drains, especially at peak periods, to improve the quality of runoff and
promote amenity and biodiversity benefits from using the water in the urban environment.
These objectives are detailed further below, along with the design criteria which SUDS
schemes in Islington will be expected to meet.
The principles of SUDS are the same in urban areas as less dense development, but
whereas runoff should infiltrate or flow to natural watercourses where possible outside the
city, it is usual for runoff to enter the combined or storm sewer in London. This condition
informs the collection and control of runoff and particularly the rate at which water leaves
development (see quantity section below). It also means that where water is available for
amenity or wildlife interest, ensuring the water is clean through the ‘management train’
treatment system is critical. These issues are described further below.
Figure 4: SUDS in urban environments: urban wetland, Western Harbour, Malmo and
combined accessible and green roof at Ropemaker, Islington
Design criteria
(a) Quantity - managing the flow
The rate that water flows from development together with the volume of water falling on a
site needs to be managed to reduce the impact of runoff on surrounding development, local
watercourses or the combined sewer. The measures used to achieve this protect both the
development itself, and the downstream environment from flooding and other effects of
uncontrolled runoff.
The role of SUDS in reducing flood risk is likely to become increasingly important in
Islington as climate change increases the frequency of heavy rainfall and as ongoing
development and intensification of the borough lead to additional water (both waste water
and surface water) being drained to the combined sewer.
Retrofitting SUDS in Islington would reduce local surface water flood risk in a variety of
ways. SUDS reduces the frequency with which runoff occurs by encouraging water to soak
into the ground or be lost through evaporation. Although much of Islington has clay soils,
some infiltration will still take place and will offer a range of benefits. SUDS also reduce the
rate of runoff entering the sewer by slowing the flow of water as it passes through planting,
soil or layers of crushed stone below permeable surfaces, and by providing additional
storage for runoff onsite. In this way SUDS can help prevent local ‘flash flooding’ caused
when runoff is channelled quickly off hard surfaces by conventional gratings and pipes,
causing sewers to fill with water quickly and resulting in unpredictable overflows of
contaminated water from the combined sewer when it surcharges. In addition, SUDS
features intercept rainfall ‘at source’ removing silt and debris that can block conventional
pipe systems and result in flooding.
In order to ensure SUDS are designed to control the flow of rainwater leaving any site, the
basic characteristics of rainfall events must be understood. Controlling the rate of runoff and
the volume of water falling on a site will depend on:
•
the storm probability or the chance of a storm of given magnitude happening in any
year, known as the storm return period
•
the storm duration or period of time over which it rains
•
the storm intensity or the depth of rainfall over a period of time
There are a number of storm durations and intensities for any given return period. For
example, a very short duration storm may have a very high intensity of rainfall and these
are more likely to occur in summer, washing pollution from hard surfaces and causing
blockage of pipe inlets. Long duration storms with lower intensity can also occur within the
same return period but are more typical of winter storms.
The critical duration event is the storm that causes the largest peak flow within a particular
return period. The nationally agreed return period which should be designed for is a 1 in
100 year with a 30% allowance for climate change. The largest peak flow is calculated
through computer simulation of a range of storms likely to occur within this period to see
which one creates the greatest storage volume requirement. This will determine the volume
of water which needs to be stored on a site.
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Promoting Sustainable Drainage Systems in Islington
The two key elements of quantity are:
(b) Quality - preventing pollution
1.
Whereas the quantity element of SUDS is assessed numerically, the quality of runoff
involves a qualitative assessment of how runoff is managed using a sequence of
techniques that control and clean flow as it passes from one stage to the next. This is called
the ‘management train’ and is shown in figure 6 below.
Runoff rate
The natural flow from an undeveloped site is called ‘greenfield rate’ of runoff and is usually
between 3-8 litres/second/hectare. In Islington the Environment Agency have agreed a
figure of 8 l/sec/ha. In urban areas most existing development generates much higher
uncontrolled flow rates, often between 200-350l/sec/ha, as gullies and pipes collect rainfall
almost instantly and carry the water away quickly in the sewer.
Figure 6: SUDS Management Train
This increase in runoff rate leads to a range of problems which SUDS aims to address by
reducing flows of water through the landscape to a ‘greenfield rate’ of runoff. The London
Plan (policy 4A.4) states that developers should aim to achieve greenfield run off from their
site. On sites where it can be demonstrated that this is not possible, an estimation of the
previous runoff rate can be undertaken and this reduced by at least 50% to a rate agreed
with the Local Authority and regulators, in line with the Mayor’s essential standard (see
Appendix 1 for full policy details).
2.
Storage volumes
SUDS should be designed to provide an agreed volume of storage on site. This storage
volume should be equivalent to the largest peak flow expected within a 1 in 100 year (plus
30% allowance for climate change) return period. This is calculated through modelling of
the storm which would create a flow, known as the critical duration event (see above). The
amount of storage required also depends on the rate that water can leave the site and can
be affected by the underlying geology. The amount of water which will need to be stored on
any site in Islington, with different runoff rates, is shown in the table below.
Runoff
rate
Figure 5: Storage volumes for Islington with different return periods and runoff rates (m3/m2)
Return period (yrs)
1
2
30
100
100 + 30%
8//sec/ha
0.010
0.014
0.032
0.045
0.062
50 l/sec/ha
0.004
0.006
0.016
0.024
0.035
100l/sec/ha
0.002
0.003
0.01
0.016
0.025
As the table shows, the depth of water that needs to be stored for any area of hard surface
in a worst case situation of a 1 in 100 year (+30%) return period varies from 62mm for
8l/sec/ha flow rate down to 25mm for 100l/sec/ha. The total storage volume requirement for
the site can then be calculated by multiplying the area of hard surface on the site by the
relevant depth of water to be stored (ie 25mm, 35mm or 62mm).
The table also demonstrates the benefits of a hierarchical approach to providing storage.
For example, day to day rainfall (such as a 1 in 1 year event, requiring only 2-10mm
storage) can be easily stored within smaller SUDS features such as swales or green roofs.
Storage for moderate rainfall events (such as a 1 in 30 year event) can be provided through
features which will be required periodically, for example detention basins. Larger volumes
that only occur very infrequently (for example in a 1 in 100 plus 30% return period) can be
accommodated in multifunctional spaces such as playing fields, play grounds, car parks or
amenity green space which can be designed to provide storage for short periods after
extreme rainfall.
The sequence begins with ‘prevention measures’ to reduce the risk of contamination at
‘source control’ stage, where features, such as green roofs or permeable pavement,
intercept and deal with runoff as close as possible to where it falls as rain. Source control is
critical to SUDS as it ensures that contamination and silt are dealt with before they enter the
drainage system.
Volumes that cannot be managed ‘at source’, flow slowly to storage or ‘site control’
features, such as detention basins, ponds or wetlands, usually found toward the edge of a
development. These volumes should be clean enough to provide amenity and biodiversity
interest for the SUDS scheme. Open conveyance like swales and vegetated low flow
channels provide additional cleaning of runoff by trapping and treating any pollution that has
passed through the ‘source control’ stage.
In some developments where public open space is available, then ‘regional control’ (or
‘community control’) can be incorporated to provide additional storage and ‘polishing’ of
runoff.
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Promoting Sustainable Drainage Systems in Islington
As part of this approach, an appropriate number of ‘treatment stages’ should be provided
within the management train to ensure runoff is of sufficient quality. The number of
treatment stages incorporated should be based on a risk assessment of pollution.
Roofs and housing areas require at least 1 treatment stage, roads and commercial sites
need 2 stages, and high risk areas like industrial sites, HGV parks and fuel storage areas 3
or more. At least one ‘treatment stage’ must be provided before runoff is used in open
landscape features, ponds or wetlands to protect wildlife and public amenity.
Treatment of the polluted ‘first flush’ should also be addressed – this is the volume of runoff
(10-15mm) that flows from hard surfaces during short rainfall events or the beginning of
storms, carrying with it accumulated silt and pollutants. It is important that this polluted
volume is intercepted for treatment via small sub-catchments or rain collection areas.
Figure 7: Planted filter strip at Exwick School, Exeter and multifunctional wetland courtyard
as part of SUDS at Western Harbour, Malmo
(c) Amenity and biodiversity – maximising ‘use’ and ‘pleasure’
Unlike traditional drainage that directs rainfall underground into a network of pipes, SUDS
provide a controlled flow of clean water at or near the surface to deliver benefits for the
community and wildlife. Even where water flows just below the ground in permeable
pavement or SUDS planters, the visual quality of the surface can be attractive and provide
biodiversity benefits. Keeping water at or near the surface is one of the features of SUDS
design as it makes the ‘water story’ legible, is safer, cheaper and offers a range of benefits,
as long as it has been cleaned through source control features and sufficient treatment
stages.
Amenity and biodiversity benefits include:
•
attractive, safe SUDS features which make use of clean water at the surface to enhance
landscape design and create a sense of place
•
provision of multi-functional spaces such as sport, recreational and wildlife areas
•
creation of ecological habitats such as ponds, wetlands and other planted areas
•
enhanced wellbeing and educational opportunities as the water cycle is made a part of
people’s everyday lives
•
water reuse opportunities, from naturally watered landscapes to rainwater recycling
•
well designed SUDS details including rills, channels, canals, spouts, cascades and
pools
•
visually acceptable and safe inlets, outlets and control structures (see figure 9).
Figure 8: Islington SUDS Design Standards
Schemes should clearly demonstrate, through design drawings and a SUDS design
statement, how the following standards have been met:
1. Quantity
Runoff rate - Schemes should be designed to reduce flows to ‘greenfield rate’ of runoff 8l/sec/ha for Islington. In cases where it can be demonstrated that this is not possible due
to site constraints, a higher runoff rate may be agreed with the Local Authority and
regulators, which should provide a minimum of a 50% reduction on the existing runoff rate.
Storage Volume - The volume of runoff to be stored on site should be based on the largest
peak flow of any storm expected within the nationally agreed return period of 1 in 100 years
plus a 30% allowance for climate change.
2. Quality
Schemes should demonstrate they follow the ‘management train’, maximising the use of
source control, providing the relevant number of ‘treatment stages’ and identifying how the
‘first flush’ will be dealt with.
3. Amenity and Biodiversity
The design must maximise amenity and biodiversity benefits such as those described
above, while ensuring flow and volumes of runoff entering public open space are
predictable and water at the surface is clean and safe.
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Promoting Sustainable Drainage Systems in Islington
1.3 The design process
It is important that SUDS proposals are developed based on an understanding of the SUDS
design process to ensure that key design elements have been demonstrated at the correct
stage in the process. The key stages in design are described below, while figure 10
demonstrates how these stages fit into the wider design and planning process.
Key stages in designing SUDS
This should begin with a consideration of the major components of the site, for example
roofs, pedestrian areas and carparking, and the drainage opportunities these provide.
Appropriate SUDS solutions should be considered for each surface, for example permeable
paving, green roofs and areas of soft landscaping. Once these major surfaces have been
assessed to perform a drainage function, then each component can be linked by surface
conveyance in the form of rills, channels, linear wetlands or other surface features. This
element of the design process is particularly important on very small sites, where it may not
be possible to distinguish flow routes and sub-catchments.
1. Identifying flow routes through the site
SUDS design begins with an assessment of the natural drainage pattern for the site and
how this will be modified by development. This will be determined largely by topography
and geology, together with a review of historical drainage measures that have modified the
original pattern including land drainage, culverts and the sewer network.
The flow route analysis should identify how original flow routes are modified by proposed
development and create a framework for appropriate SUDS techniques to collect, clean and
store runoff in a ‘management train’ before discharge to an outfall. The identified flow routes
should provide corridors for day to day low flows, overflows that can operate when
surcharge or blockages occur and exceedance pathways when exceptional rainfall
overwhelms the SUDS.
At convenient points along the path taken by the controlled flow of clean water, it can be
used to create biodiversity or other amenity features. Spouts, cascades and other devices
can be used to enhance the urban and landscape design and to make visible water’s
journey through the site. Reuse of water to provide watering of the landscape or to flush
toilets could also be built into the design.
Figure 9: Example SUDS design details within the urban landscape - T-piece control as
pond overflow at Springhill, Stroud and final slot weir to SUDS at Riverside Court, Stamford
The flow route analysis should also show the destination of runoff after collection, cleaning
and storage. This should be to infiltration wherever possible, to a watercourse where
present, and finally to the storm or combined sewer as a last resort. In Islington, due to
limitations on infiltration and lack of watercourses, the outflow is normally to the combined
sewer.
2. Identifying sub-catchments
Identification of flow routes will be likely to lead to designation of ‘sub-catchments’,
particularly on larger sites. These are small, discreet drainage areas, each with their own
drainage characteristics. A sub-catchment deals with its own rainfall using a ‘management
train’ with appropriate treatment stages, first flush volume and often a separate flow control.
Each sub-catchment should manage its own flows and volumes wherever possible.
The characteristics of the sub-catchment will influence the selection of SUDS techniques
and the amount of water which can be stored within its boundary. In many cases a certain
amount of water will be stored in each sub-catchment with a control point to hold back
runoff during heavy rain. Large volumes of clean water from bigger storms may bypass the
control to reach storage structures further down the management train.
3. Building up the design
Once the flow pattern is determined and any sub-catchments established, the SUDS
features that best suit the conditions they need to drain are selected. The SUDS scheme
should be built up around the flow routes and sub-catchments in a way which meets the
agreed design criteria for quantity, quality and amenity/biodiversity.
By following the above design process, the SUDS design should demonstrate how it will
meet quantity criteria, follow the ‘management train’ including the use of ‘source control’
techniques that characterise SUDS schemes, and should be integrated with the urban and
landscape design to maximise amenity and biodiversity benefits.
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Promoting Sustainable Drainage Systems in Islington
Design and planning process
1.4 Adoption and maintenance
The flowchart below explains how these design stages fit into a wider development design
and planning process for a particular site. This diagram has been tailored to the Islington
context to reflect the types of schemes generally seen in the borough.
Planning for who will adopt, or be responsible for, SUDS and who will maintain them to
ensure they continue to provide a drainage function and contribution to the landscape, are
important considerations. While many SUDS features, such as permeable pavement, have
been shown to still function with very little maintenance, it is important that management
considerations are addressed early on to get the best performance from SUDS.
Figure 10: Design process for SUDS schemes
CONCEPTUAL DRAINAGE DESIGN
A conceptual drainage design which identifies opportunities, constraints and basic
criteria for drainage design should be produced at pre-application stage. The
developer should enter into initial discussions with Islington Council to agree
basic SUDS design criteria and adoption requirements. A drawing showing flow
routes and sub-catchments (where feasible) with likely SUDS features and a
preliminary SUDS design statement will be appropriate at this stage.
OUTLINE DRAINAGE PROPOSALS
An outline drainage proposal drawing and SUDS design statement should be
provided at application stage. These should develop the conceptual design to
demonstrate how the design criteria for quantity, quality and amenity/biodiversity
(described on page 7) will be met and provide further detail on proposed
techniques. The proposals should also describe how the drainage system will be
integrated into the landscape design and the methods that will be used for linking
systems together and managing flows in excess of the design event.
SUDS are essentially landscape features even when they are part of pavement design.
Maintenance therefore comprises simple tasks like litter collection, grass cutting and
inspection of simple inlets and outlets at normal monthly visits for open space management.
Occasionally small amounts of silt and wetland vegetation may need to be removed as site
waste. Design of SUDS should minimise maintenance requirements. For example pipe
connectors should be shallow and short, allowing simple jetting to keep them clear. Inlets
and outlets and control structures should be at or near the surface to allow day to day care
by landscape contractors or site managers.
This simple maintenance of surface SUDS features can therefore be easily incorporated
into landscape management. This would be carried out by the landscape managers for any
site, who may range from a private management company for a new development, the local
authority parks contractor, school facilities managers or highways maintenance teams.
Figure 11: SUDS features managed as part of new development, Riverside Court, Stamford
DETAILED DRAINAGE DESIGN
Detailed drainage design proposals, including necessary calculations, detailed
drawings and a management plan, should be submitted to the Planning Authority
in order to discharge relevant conditions. The final design should meet the overall
design objectives described in this guide and the SUDS Manual. A full SUDS
design statement should be provided at this stage.
Designing the case study schemes
Each of the case study designs within this document has followed the above process as far
as possible to demonstrate good practice SUDS design. While they are only worked up to a
conceptual stage, and therefore would need further detailed calculations to demonstrate
how they have met all of the design criteria in detail, the solutions presented show how it is
possible to meet the design criteria in a simple, cost-effective way.
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