URBAN DRAINAGE
Investigating urban creep
map and from the creep map. In addition to the properties there is a larger increase in the
A study to identify factors
areas of paved surfaces and buildings the area of impermeable area per year than for terraced
underlying the growth of urban
the sample was also measured along with a houses, which is as expected. Looking at the
count of the number of properties within the weighted averages of the five sample towns, the
creep in five English cities will
sample. This data was then exported into Excel increase for a detached property is four times that
help identify future patterns
to allow further analysis to be undertaken to of a terraced house. No clear relationship was
of urban spread. Martin Allitt identify any trends. The Land Cover Classification found between any individual variable and urban
of consultancy RAA and
Maps were not the only dataset used in this creep, so more complex analysis is being
Andrew Tewkesbury of
study. To help us with the sampling strategies the undertaken by Scottish Water.
Infoterra share their findings
respective sewerage undertakers were asked to
provide the following datasets: Hydraulic modelling
■ Address point data Another important part of this study was to
U
rban creep is the term used to describe ■ Sewer records develop a methodology for including allowances
the change of permeable areas within ■ Postcode boundaries for urban creep within hydraulic models which is
the urban environment to impermeable ■ Soil maps clearly auditable. While the methodology for this
areas. Typical types of urban creep are the ■ ACORN data (socio-demographic data) phase of the study has been finalised, the actual
creation of patios, paving the front gardens to ■ Property age data (where available) values to be used are still to be finalised
create hard standing parking areas or house following the ongoing statistical analysis.
extensions. UKWIR commissioned Richard Allitt Sampling and analysis The majority of hydraulic models are built
Associates (RAA) to undertake a study with the A complex sampling strategy was developed to using three or four of the 12 available runoff
objectives of providing data on the degree and sample this data according to drainage system surfaces within InfoWorks. The proposed
rate of urban creep and to develop a type, soil type, house type, house footprint area, methodology for including urban creep within
methodology for modelling the degree of urban depth of front gardens and perhaps most the hydraulic models is to use an extra one or
creep within hydraulic models so that future importantly, by socio-demographic factors. This two of the runoff surfaces for the area to be due
projections of the impact of urban creep on CSO resulted in a total of in excess of 8,500 actual to urban creep and also to add the design
spills, pumping costs, treatment costs and samples after the more extreme values had horizon to the network name.
flooding could be determined. been removed. The network file name will contain the date of
Five sample locations, each covering a Following thorough sampling and data the flow survey used to verify the model, then if,
100km
2
area, were chosen: Leicester, Maidstone, analysis, the urban creep in m
2
per house per for example, it was necessary for the hydraulic
Chester, Norwich and part of Newcastle-upon- year was calculated. The image shows the model to reflect the operation of the sewerage
Tyne. The availability of data to allow Infoterra to change in impermeable area in Leicester from network in 2020, the additional area due to
produce Land Cover Classification Maps helped 1999 to 2006. There is a difference between the urban creep over the 11 years would be added
decide the locations and such large samples five towns, for example Chester has the largest and the networked could be renamed
were used to allow for any extremes to be average change at 1.094m
2
/house/year, Network_2020. The proposed data structure in a
removed, avoiding any skewing of the results. whereas Newcastle-upon-Tyne has the lowest at model would be:
Remote sensing techniques allowed large 0.381m
2
/house/year. Taking an average across Column 1 Paved Areas (no change)
areas to be sampled and two snapshots in time the five towns, the average urban creep is Column 2 Roof Areas (no change)
were used to identify change. As this study was 0.749m
2
/house/year, which is a significant Column 3 Urban Creep
focused on urban creep it was important that increase and could have a greater impact on Column 4 Permeable Areas
growth within the sample areas was removed sewerage networks than climate change will
early on. For this study the definition of growth have. The average increase in impermeable area The study found that urban creep is a serious
was as follows: “Growth is either the addition of per house per year for each of the four property problem for sewerage undertakers and it is unlikely to
a new property where there was no previous types across the five sample towns was also go away for many years to come. It is also clear that
property, or the addition of paved areas identified. The results show that for detached there are many drivers for urban creep and there is a
associated with new properties. Major highway complex mixture of different variables acting as
improvement schemes, which may not be drivers. The use of remote sensing technology to
directly linked to a new development, are also to detect urban creep has enabled large and
be considered as ‘growth’”. statistically valid sample sets to be extracted and
The next stage was to generate the samples
Image cour
analysed. The methodology is repeatable in future
within each of the five sample areas which we years as more data becomes available. ■■■
were going to analyse to identify any links with
tesy of Infor
the degree of urban creep. Once the samples This article is an extract from a paper at
were generated the next step was to use
ter
WaPUG’s 2009 conference, the full paper can
MapInfo to calculate the area of each sample
ra
be found at www.raaltd.co.uk.
that is covered by paved surfaces or buildings
GeoPerspectives image of Leicester in 2006
The author’s would like to thank UKWIR for
both from the latest Land Cover Classification
identifying urban change since 1999
funding and support
8 Consultant & Contractor File February2010
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