Representing connectivity within the UK variable runoff model

A sensitivity analysis is conducted to explore the uncertainty of representing the surface connectivity of Arley Close and Winsley Close within a design rainfall-runoff model, the UK Variable Runoff Model. This model is chosen as a result of its simplicity (it can be easily executed in MS Excel), sensitivity to imperviousness and connectivity and its prevalence as a method within surface water drainage design in the UK (Woods-Ballard et al., 2015).

Connectivity is typically considered as a binary process in current urban hydrology theory, i.e. an impervious surface is either connected or disconnected to a surface water drainage system (Kong et al., 2017). Section 3.4 expands this simplistic understanding of connectivity to include connection efficiency and defines surfaces with direct and indirect connections. Directly and indirectly connected surfaces are likely to contribute different amounts of runoff to the surface water drainage system given that joins and gaps between surfaces lead to losses from storage, evaporation and infiltration (Chapter 2, Section 2.5). The connectivity of surfaces is currently represented within the UK Variable Runoff Model as a proportion of the total imperviousness of a catchment:

𝐶𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 𝑆𝑢𝑟𝑓𝑎𝑐𝑒𝑠 = 𝐼𝐹 ∗ 𝑃𝐼𝑀𝑃 Equation 3.20 Where IF = effective impervious area factor, and PIMP = the percentage imperviousness of a catchment.

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A sensitivity analysis of the New UK Runoff Model is used to assess the sensitivity of modelled PRURB values to values of IF and PIMP. The PIMP value is then set to the MEDIUM value defined in Section 3.3 and a number of methods of defining connectivity (IF) are tested to determine the accuracy of PRURB predictions. The sensitivity analysis methodology is as follows:

(i) The model parameter values of NAPI and PF are set to 17 and 200 respectively at the start of each event.

(ii) Values of PIMP are increased between the LOW, MEDIUM and HIGH methods for each site, defined in Section 3.3.

(iii) The values of IF are increased from 0 (no connectivity) to 1 (full connectivity) by 0.1 and PRURB calculated.

(iv) A three dimensional scatter plot is used to visualise the sensitivity of PRURB to values of PIMP and IF.

(v) Values of PIMP are then set to the MEDIUM value and values of IF are then derived using the following methods:

Method 1: Standard values of IF are used from guidance documents based on surface condition: 0.75 for “good” condition surfaces, 0.6 Fair and 0.45 Poor, Woods-Ballard et al. (2007).

Method 2: IF is defined as the proportion of PIMP with a direct connection to the surface water drainage system (i.e. a surface with at least one hydraulic connection to the surface water drainage system, defined in Section 3.4).

Method 3: IF values are adjusted until the PRURB output from the model matches the mean PR value from the observed events.

The values of PRURB derived from the different values of IF defined under Methods 1 and 2 are compared to those derived under Method 3, and the resulting runoff volumes calculated through Equation 3.10.

The sensitivity of hydrological models to the uncertainty of understanding and defining surface cover, connectivity and rainfall-runoff processes within urban residential catchments is therefore quantified. This allows a discussion of how residential land covers can be better represented within commonly applied rainfall-

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runoff models. The succeeding chapters report the results of applying the methods described here.

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Chapter 4

DEFINING SOIL PROPERTIES, SURFACE COVER AND CONNECTIVITY WITHIN ARLEY CLOSE AND WINSLEY CLOSE

This chapter compares the soil properties, surface cover and surface connectivity of Arley Close and Winsley Close, reporting the results of new methodologies to compare the overall connection efficiency and those surfaces with direct and indirect connections to the surface water drainage system.

4.1 INTRODUCTION

Hydrologists need a consistent and hydrologically relevant methodology with which to define urban surface cover and surface connectivity, so that different types of urban development can be represented within hydrological models and surface water management planning. Quantifying and understanding the different ways in which urban surfaces connect to the surface water drainage system and defining different types of urban surface cover that exhibit a range of hydrological behaviours with detail at small-scales remains a scientific and engineering challenge (Yao et al., 2016a). There is a lack of studies examining the physical features and processes within urban areas that control the connection of the urban surface to the surface water drainage system, increasing the uncertainty of representing urban areas within hydrological models. For example, Kjeldsen (2009) relies on large scale estimates of connectivity (70% of surfaces are connected) to represent urban areas within a flood prediction model, whilst others demonstrate the complex and non-linear relationship between imperviousness and connectivity (Lee and Heaney, 2003).

The connectivity of surfaces is often estimated with a simple definition that relates connectivity to the presence or absence of a surface water drainage system (Sahoo and Sreeja, 2016), or via empirically derived equations relating imperviousness to connectivity (Sutherland, 2000). However the actual connectivity of surfaces is

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dependent on the presence of hydraulic entry points (such as road gullies) whose spatial distribution varies across the urban landscape (Prichard et al., 2009); in addition surface features such as kerbing affect the generation of runoff on urban surfaces (Ozdemir et al., 2013). It is likely that the efficiency by which runoff can enter the surface water drainage system is sensitive to the types of surface being drained, their condition and the physical arrangements of surfaces in relation to drainage connection points (Redfern et al., 2016). These are small-scale features and processes (for example road gullies are smaller than 0.5 m2_{) that are difficult to detect in aerial} photographs or satellite remote sensing and thus require detailed study to record their occurrence (Wiles and Sharp, 2008; Keeley, 2007). Whilst such intensive detailed study may be impractical across large catchment scales, the understanding gained from small-scale studies could be applied to estimation methods of surface connectivity at larger scales. In addition, in the United Kingdom at least, there are a number of legislative and policy drivers that may increase the availability and resolution of data on the locations and types of surface drainage features (e.g. Section 21 of the Flood and Water Management Act 2010). With greater understanding of the connection process, hydrologists and engineers will be better placed to quantify hydrological impacts in urbanised catchments and design the urban environment in a manner that reduces such impacts, warranting further study of the surface cover and connectivity of urban surfaces within Arley Close and Winsley Close.

This chapter reports the results of applying methods described in Sections 3.2-3.4, which define surface soil characteristics, surface cover and connectivity of surfaces within Arley Close and Winsley Close, building on readily available data with detailed site visits. The chapter examines how representative large-scale soil maps are of small- scale soil properties within urban areas. In addition an assessment of how the number and type of drainage connection points impacts overall connectivity of the urban surface is made by applying a novel methodology (Section 3.4.2).

Ordnance Survey Master Map data (OSMM), Light Detection and Ranging data (LiDAR) and aerial photography are combined in a GIS environment and site based Individual Parcel Assessments (IPA) are used to define surface cover and connectivity within Arley Close and Winsley Close (Section 3.3 – 3.4). The chapter culminates in the definition of not just the surface cover and connectivity, but explores the efficiency

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of connections, comparing surfaces with direct and indirect connections to the surface water drainage system.