Distribución angular de los fragmentos

Urban land cover is formed of a complex mosaic of different surface types, some of which are of a solely anthropogenic origin (e.g. road) whilst others may be of a disturbed natural form e.g. park. Detailed assessments of surface type, condition and connectivity are needed to characterise the design and layout of the study sub- catchments.

A number of field and desk based investigations are performed to define surface cover within Arley Close and Winsley Close. First Ordnance Survey Master Map Topography Layer (OSMM) data is downloaded via the Digimap online service. OSMM is the most detailed, accurate and widespread dataset on the locations of buildings, roads and pavements available for the whole of the United Kingdom. The data is available free of charge for the academic and public sectors, subject to licence agreements. With its countrywide coverage, detail of the urban environment and wide availability, OSMM data is an appropriate dataset with which to begin to characterise the surface cover of the study catchments. The data is downloaded in OSGB GML file format which is then processed through the ESRI Productivity Suite to convert the GML file into an ESRI Geodatabase; a file format that is used within ArcGIS (a commonly used GIS software). OSMM is produced, managed and updated by the Ordnance Survey and is based on a structure of five interrelated layers: Topography, Address, Integrated Transport Network, Aerial Imagery, and River Network. The Topography layer is formed of features that appear in the landscape, such as buildings, land, water-bodies and roads. These features are represented either by points, lines or polygons. Each feature has a number of attributes that provide detail of physical characteristics, such as area for a polygon. The Topography layer is organised into “Themes” that allow the user to group features together based on the value of their attributes; for example, a polygon representing a road would be listed as “Manmade” under the theme “Make”. This allows users to query, select and measure the urban form with a readily available and consistent dataset.

On site, Individual Parcel Assessments (IPAs) are used to first verify the data contained within the OSMM and to expand surface type classifications and detail. IPAs are defined as the practice of collecting detailed, site-specific data suitable for estimating imperviousness of individual (residential) land parcels (Keeley, 2007).

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IPAs are used for collecting detailed data on the urban environment for use in surface water modelling and management (Krebs et al., 2014). The methods previously used within the literature involve the modification of existing land cover data such as maps (Lee and Heaney 2008), resident questionnaires (Keeley, 2007) and the use of high resolution remote sensing (Beighley et al., 2009). Here, maps of the study areas are annotated to demark any deviations in the land surface from the OSMM. Surface types within domestic parcels and at road sides are defined along with information on surface condition which is identified by noting defects in surface integrity (e.g. potholes or cracks). Features that could affect runoff generation such as garden walls, fences and kerbing are also recorded. The data from the IPAs are digitised onto the OSMM using ArcGIS, tracing surface boundaries with aerial photography and site notes as a guide.

3.3.1 Surface cover estimation in inaccessible locations

Both Arley Close and Winsley Close contain residential buildings that have garden areas to the front and rear of properties. It is not possible to enter rear gardens to perform site based IPAs (due to restricted access) and so aerial photography (supplied by getmapping under the PGA agreement 2003, 25 cm resolution) is digitised through visual inspection within ArcGIS into two land surface types; vegetated and semi- impervious. The semi-impervious category is chosen as it is possible to view some surfaces in Winsley Close that are block paving, rather than sealed surfaces e.g. concrete or tarmac. However, this approach is uncertain and may underestimate imperviousness, especially as it is also possible to view garden sheds and other likely impervious surfaces within rear gardens at both sites. The uncertainty of defining land cover to the rear of properties is discussed in other studies where imperviousness is defined through the interpretation and digitisation of aerial photography in urban residential settings of the UK (Perry and Nawaz, 2008; Pauleit et al., 2005). In these studies, either a conservative approach to estimating imperviousness is applied (e.g. surfaces are defined as impervious only if other surface types are discounted) or assumptions are applied e.g. non-vegetated rear garden surfaces are considered impervious. Estimates of imperviousness are likely sensitive to the methodological assumptions applied to define surface cover within residential land parcels. To determine the sensitivity of estimates of total imperviousness to the methodological

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assumptions applied, the total imperviousness of each catchment is estimated under three methodological assumptions:

(i) All rear-garden non-vegetated surfaces are semi-impervious (and not counted towards total imperviousness). This is termed the “Low” estimation method.

(ii) All rear-garden non-vegetated surfaces are impervious (and counted towards measurements of total imperviousness). This is termed the “High” estimation method.

(iii) 50% of all rear-garden non-vegetated surfaces are impervious. This is termed the “Medium” estimation method.

In Arley Close, the definition of front and rear garden areas is relatively straightforward as boundary features (e.g. fences) clearly demark the two areas. In Winsley Close, where there are fewer boundary features, front gardens are defined as those areas within domestic parcels that are viewable from the public highway and footpaths, whilst rear-gardens are defined as those surfaces which require access onto private land to verify surface classifications.

Digitising aerial photography to define surface cover within rear gardens is a challenging process as, depending on the time of day and the position of the aerial platform in relation to the study location at the time of image capture, shadows form across the landscape, especially in close proximity to buildings and other elevated features (such as trees) obscuring the view of the land surface. Additionally, because the photographic angle is not directly above buildings, roofs block the surfaces immediately to the periphery of building perimeters due to parallax. Surfaces are therefore defined into the vegetated and semi-impervious categories by judging the colour (green indicates vegetated, whilst grey/black indicates semi-impervious materials) and location i.e. surfaces under tree cover are assumed to be the same as surrounding surfaces within individual gardens.

It is not possible to ascertain the number or type of hydraulic drainage features that are present within rear gardens from aerial photography, other than roof drainage which is assumed to mirror the arrangement of downpipes to the front of properties. For some properties it is possible to visually verify roof drainage positions at the rear

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of houses from publicly accessible land, confirming that roof drainage to the rear of properties mirrors that to the front.

3.3.2 Land ownership

Though land ownership is not hydrologically important, the division between private and public land and their constituent surface types offers a descriptive structure for defining differences in residential surface layout and design, thus aiding in the characterisation of the two study sub-catchments (Perry and Nawaz, 2008). Land ownership is classified into one of two categories, defined as:

Public: land that is owned or under the regulatory authority of a public body, e.g. road or public open space.

Private: Land that is privately owned by one land owner or shared between more than one private land owner e.g. a garden serving a property or driveway.