Abstract

The paper draws on Colombo, Sri Lanka as the case study for developing Water Sensitive Planning and Design (WSPD) to deliver a flood resilient urban environment. Digital data and documentary data were evaluated to analyse water sensitive characteristics, potentials and constraints within the catchment. Spatial analysis tools in ArcGIS were used when analysing issues in the landscape mosaic. Evaluation of the landscape mosaic clearly identifies urban form and its significant issues in creating ecological links and patches, such as, marshes, streams, roads and shrubs that can create potential opportunities during stormwater management.

Geographically, the low-lying area plays an important role as the natural detention/retention basin for stormwater of the urban catchment during intensive rainfall. In addition, increased impervious surfaces created by high-density urban development and the limited availability of space have created challenges for retrofitting additional stormwater infrastructure. The study identifies the important role that urban planning can play in safeguarding strategies to deal with urban water related issues in more compromising and accommodating ways when situating stormwater infrastructure to optimise the connectivity and corridors. The study demonstrated the importance of street layouts in the urban landscape to support the development of WSPD. This approach provides sustainable ecological protection and outcomes to achieve a flood resilient environment in the catchment.

INTRODUCTION

Urban catchments become vulnerable due to climate change and increased intensity of rainfall. Increased modifications in the urban landscape, in turn, have created profound challenges, especially for maintaining the quality and quantity of surface water runoff (Goonetilleke et al. 2005). Therefore, proactive capacities to enhance urban adaptation, mainly focussed on issues related to urban stormwater and flooding are timely and important, especially for cities in the tropics. Water sensitive planning coupled with the concept of water sensitive cities can respond to these challenges (Siekmann et al. 2008; Howe & Mitchell 2011) and make cities resilient to these impacts on climate change.

To enable functions of water sensitive characteristics that improve the quality and the quantity of urban water, this paper examines approaches to integrating water considerations into urban planning through catchment ecosystem-based land use planning. This is because in many societies, the existing planning mechanisms that consider the catchment scale, as the basis of development are missing. In doing so, the research demonstrates how the selection of an appropriate spatial scale can help provide planning and design of land use that supports ecosystem services, including the reduction of stormwater generation and runoff.

As was pointed out in the introduction, this study is a component of current doctoral research by the lead author. The following sections of the paper present methods, empirical findings and capability of delivering WSPD by selecting two sub-catchments within the Colombo Urban Catchment (CUC) to influence control and management of stormwater. The paper ends giving recommendations for planners and policy makers to incorporate catchment scale planning into urban development.

CUC in Sri Lanka: the context of study and some lessons learned

Sri Lanka is an island country with a land area of 65,525 Sq·km, and located in the northern Indian Ocean off the southern sub-continent in ‘South Asia’ (Figure 1). The total population in 2012 was 20,359,439 (Department of Census & Statistics 2012). The country has been divided into 25 districts in 9 provinces. Colombo district is one of the three districts of Western Province, and can be recognised as the most highly urbanised district of Sri Lanka. Based on the rainfall, the country has been demarcated mainly into a three distinct climatic regions – the wet zone, dry zone, and the intermediate zone.

Figure 1

Spatial location of the Colombo District in Sri Lanka. Source: Compiled by author using Arc GIS data and Google Maps.

Figure 1

Spatial location of the Colombo District in Sri Lanka. Source: Compiled by author using Arc GIS data and Google Maps.

The rainfall pattern is based mainly on two seasonal monsoons (South-west and North-east) in Colombo. Figure 2 indicates that, the highest rainfall in Colombo occurs between the periods of inter-monsoon. For the last 10 years, the highest rainfall (393.7 mm–412.8 mm) has been received during the south-west inter-monsoonal months of October – November. The second highest rainfall (330.4 mm–320.5 mm) has been received between the North-east inter-monsoonal months of April–May.

Figure 2

Monthly Average Rainfall/mm in Colombo from 2001–2014. Source: Compiled by author using data obtained by the Department of Metrology, Colombo.

Figure 2

Monthly Average Rainfall/mm in Colombo from 2001–2014. Source: Compiled by author using data obtained by the Department of Metrology, Colombo.

Colombo is located in the wet zone and many parts of Colombo have been, and still are, subject to frequent flooding. Due to colonial influences, the natural landscape of the Colombo environment has been deliberately planned, designed and modified using artificial or man-made elements. Review of past planning reports supports the argument that appreciation of and incorporation into the urban form of existing natural factors such as geology, landform, river, natural drainage and streams, soils and natural vegetation have been less well considered. This has resulted in damage to the biodiversity and healthy ecosystems over time. In addition, negative human actions have resulted in pollution across the urban area.

Ultimately, these issues have contributed to increased flood risks of the urban area. Planning institutions and stormwater management authorities have implemented various projects/methods and strategies across the Colombo urban area to deal with flooding and related issues over several decades. But the intensity of the problem still persists, when there is heavy rainfall in the city. Therefore, authors in this study recognised that long term sustainability requires the planning of water resources to be done within the catchment scale.

METHODS

The research begins with a definition and justification of the concept of ‘catchmentscape’. This was developed as a result of an extensive literature analysis as reported below. The remainder of the research is presented in a case study format. The method section consists of discussing, mainly the three evaluation processes undertaken for the selection of a suitable set of sub-catchments to proceed with the study.

Firstly; the initial screening was undertaken using the CUC (that the sub-catchments sit within) to identify and select sub-catchments, that include natural water sensitive elements within the landscape. After this initial screening, secondly, the set of selected sub-catchments were re-evaluated, using a set of seven criteria and parameters to identify the most critical sub-catchments that require attention. This analysis of each selected sub-catchment considered its spatial and temporal issues, to find out the relative importance of each catchment for undertaking WSPD. Having selected a set of sub-catchments, thirdly, a validity check was carried out for each catchment, considering the theoretical framework and practical approach to exploring WSPD suitability. Finally, the researchers selected two sub-catchments to proceed with the study and to demonstrate potential WSPD considerations. The detailed process of methods for sub-catchment selection has been explained below.

Selection of sub-catchments within CUC

The case study began with defining a set of criteria for the selection of sub-catchment(s) within the CUC. The development of criteria was important, because it helps to identify the most critical sub-catchments that need integration of WSPD. The study was carried out using the CUC area map obtained from the Sri Lanka Land Reclamation and Development Corporation (SLLRDC). As demarcated by the SLLRDC, the CUC consists of 51 sub-catchments (Figure 3).

Figure 3

Colombo urban water catchment and boundaries of sub-catchments. Source: Author prepared using maps obtained from SLLRDC and Google Earth using ArcGIS 10.4.

Figure 3

Colombo urban water catchment and boundaries of sub-catchments. Source: Author prepared using maps obtained from SLLRDC and Google Earth using ArcGIS 10.4.

ArcGIS spatial query was run using two spatial data layers (1) Sub-catchments and (2) Streams network to acquire sub-catchments that are facing vital challenges among the total 51 sub-catchments. Following this methodological approach, the study initially screened and identified 25 sub-catchments ‘that completely contain water features within sub-catchment boundaries'. A list of seven main criteria (07) followed by several subsequent parameters under each criterion were developed to evaluate the initially identified 25 sub-catchments (see Table 1). The purpose of this evaluation was to consider fundamental aspects of the spatial and hydrological characteristics of Colombo catchment in regards to landscape and planning. Moreover, under each criterion a spatial map was prepared by using two data formats (vector and raster). Taking all these maps together, an evaluation was undertaken for each sub-catchment based on the given criteria and parameters (Table 1). From this evaluation, six sub-catchments were selected that approximately fulfilled the seven main criteria.

Table 1

Selection criteria and parameters for the selection of sub-catchments (SCs) using spatial layers

Criteria Parameters Description of SCs selection 
Existing land and environmental uses Built-up area SCs that contain more than 70% built-up lands, highest percentage of wetlands/Shrub lands including agricultural areas 
Recreational area 
Agricultural area 
Wetland/Shrub/Boggy 
Other 
Population density (Per sq·km) 2,872–5,919 SCs that has a population density between first two level of parameters (2,872–8,966) 
5,920–8,966 
8,967–12-12 
12,013–15,059 
15,060–18,106 
Elevation (metres) (Mean Sea Level) 0–10 SCs that contained a part of highest 20–30 elevation level 
10–20 
20–30 
Hydrological coverage >5% SCs that has highest percentage (16%) of hydrological features 
6%–15% 
16% < 
Imperviousness Yes SCs that are pervious (this was not a very accurate measure for imperviousness. But only available information in SLLRDC) 
No 
Zoning criteria Residential SCs that contained the highest coverage for residential-related uses. 
Mixed use 
Recreational 
Industrial 
Vulnerability to flood inundation Absolutely SCs that were absolutely or moderately inundated during previous flood events 
Moderately 
No 
Criteria Parameters Description of SCs selection 
Existing land and environmental uses Built-up area SCs that contain more than 70% built-up lands, highest percentage of wetlands/Shrub lands including agricultural areas 
Recreational area 
Agricultural area 
Wetland/Shrub/Boggy 
Other 
Population density (Per sq·km) 2,872–5,919 SCs that has a population density between first two level of parameters (2,872–8,966) 
5,920–8,966 
8,967–12-12 
12,013–15,059 
15,060–18,106 
Elevation (metres) (Mean Sea Level) 0–10 SCs that contained a part of highest 20–30 elevation level 
10–20 
20–30 
Hydrological coverage >5% SCs that has highest percentage (16%) of hydrological features 
6%–15% 
16% < 
Imperviousness Yes SCs that are pervious (this was not a very accurate measure for imperviousness. But only available information in SLLRDC) 
No 
Zoning criteria Residential SCs that contained the highest coverage for residential-related uses. 
Mixed use 
Recreational 
Industrial 
Vulnerability to flood inundation Absolutely SCs that were absolutely or moderately inundated during previous flood events 
Moderately 
No 

Source: Author developed considering the spatial and hydrological perspectives of the urban catchment.

Validity checks

Based on these selected six sub-catchments, the lead author undertook a validity check. This needed to be done before they could be considered for undertaking WSPD. Selected sub-catchments were compared across the theoretical framework and other circumstances. For example, several sub-catchments that were selected are isolated from the other sub-catchments. Therefore, the researchers had to aggregate selected sub-catchments considering the emphasis of ‘connectivity’ and other spatial elements such as patch and corridor. These elements were considered imperative for the design and development of WSPD components. Accordingly, there were six sub-catchments that were identified (Figure 4(b)). However, a detailed analysis was undertaken in only two sub-catchments of No 05 and No 03. The Colombo Flood Detention Area (CFDA) has been located on the left bank of the lower valley of the Kelani River (Figure 4(a)). A catchment provides the connectivity between physical and hydrological landscape for characterising negative effects on water resources and flooding of the study area.

Figure 4

(a) Colombo flood detention area (CFDA). Source: SLLRDC, Colombo. (b): selected sub-catchments within the city. Source: Author prepared using ArcGIS 10.4.

Figure 4

(a) Colombo flood detention area (CFDA). Source: SLLRDC, Colombo. (b): selected sub-catchments within the city. Source: Author prepared using ArcGIS 10.4.

Evaluation of the catchmentscape

As per the above Figure 4(b) indicates, the final selected six sub-catchments including No 05 and No 03 that were evaluated considering structural and functional characteristics using spatial analysis tools in ArcGIS 10.4. Undertaking this evaluation process helped create an understanding of the nature and extent to which WSPD could be applied in the selected two sub-catchment(s). To do this, spatial elements using the three (03) criteria of patch, corridor and connectivity were identified within the existing landscape matrix of the sub-catchment(s).

Finally, the two sub-catchment(s) were demonstrated using WSPD considerations as an example. This design process entailed identifying conflicts of existing street layout (roads) and natural ecological environments of the sub-catchments. Because the area of road is extensive, and it contributes a considerable volume of stormwater runoff within catchments. Therefore, integration of road designing and water sensitive planning for flood control and stormwater quality improvement in Colombo were considered important under the WSPD development.

DEFINING THE CATCHMENTSCAPE AND WSPD

To demonstrate the catchment scale as an appropriate spatial unit, the authors of this study recognise and define a new concept called ‘catchmentscape’. The term catchmentscape has evolved from reading a wealth of landscape planning, design and ecology related literature to define the planning scale for integrating WSPD. However, the more comprehensive meaning of catchmentscape has further evolved as an integration of the ecological, hydrological and topographical terms of catchment + landscape. Accordingly, in the present study, the ‘catchmentscape’ is used to refer to:

‘A spatial unit recognised within hydrological (catchment) constraints, characterised by networks of ecological structures and human-induced forces, and formed by interactions of patterns, functions and connectivity across the landscape’.

The term catchmentscape is recognised in this paper as the spatial unit of planning and design. This was because, the authors recognised, that at the preliminary stage of urban design and development, planning has a direct influence on understanding and managing stormwater and drainage services within an urban catchment. If planning does not consider fulfilment of this requirement before beginning development, impacts of planning can result in stormwater management and related impacts becoming long term unresolved problems within urban catchments.

Accordingly, development of WSPD provides strategies to understand spatial characters, values and effects of urban development on the built environment and to implement practices to maintain and protect water resources. The following section describes the importance and the evolution of the theory behind the new concept of catchmentscape. This is followed by a discussion of nested spatial scales of urban land within catchments and justification of the whole catchment area as the preferred planning unit for WSPD.

The evolving concept of ‘catchmentscape’ and WSPD

From a land mosaic perspective, there are three fundamental elements that can aggregate to form the variety of landscape structures or mosaic, namely, (a) patches (b) corridors and (c) matrix on land (Forman 1995, 2014). These elements can be natural or manmade at the landscape scale. But understanding the spatial relationships of each of those elements can be considered to be important for planners to arrange the urban landscape. Further to understand this relationship, Forman et al. (1987) have developed a basic principle called a patch-corridor-matrix model. This model provides the opportunity for land planners to handle land use planning decisions by understanding spatial pattern and the control function between natural systems and the human environment.

In addition to the above three elements, another aspect of measuring landscape structure namely ‘landscape connectivity’ has been introduced by Taylor et al. (1993). One major drawback of this approach is that the landscape connectivity has been mainly introduced to maintain animals’ movement and ability to access and supplement resource requirements between landscape patches. In the present study, together, the structural, functional and connectivity relationships need to be a central element of defining the catchmentscape, thus the development of planning scale to WSPD.

Johnson & Hill (2001) suggest an application of hierarchy theory as a useful method for understanding the urban landscape and its ecological complexities. They further state that hierarchy can be distinguished as structural or control hierarchies. Structural hierarchies are always nested, or each subsystem can be contained within systems that are part of components at the next higher level of the structural hierarchy, for example, watershed and sub-basins or catchments and sub-catchments. van Roon & van Roon (2009) suggest, the catchment as an appropriate design unit for delivering spatial planning frameworks. They explored the uptake of Low Impact Urban Design and Development (LIUDD) using a catchment basis, because, LIUDD focussed on an alternative approach to stormwater management that integrates land and water use design within a range of scales from neighbourhood-to-catchment scales. Moreover, the catchment scale has been applied on the basis of studying, measuring and managing water characteristics (Fohrer et al. 2001; Davis et al. 2016), landscape studies (Jain et al. 2016) floods (Baborowski & Einax 2016) and low impact approaches to stormwater management (van Roon 2011).

Investigating potential water sensitive elements that can be applied within a tropical urban environment, and identifying how to incorporate them into developing planning outcomes are major challenges. However, integration of elements of catchmentscape, water sensitive planing into developing water systems and urban land uses provides opportunities for responding to urban environmental problems. Therefore, the study defined:

‘WSPD is a holistic planning process that uses catchmentscape as the structural and functional spatial scale to design and develop local planning. It focusses on spatial patterns of water-land-human uses and their relationships as the key to effective land use planning in an urban region. WSPD operates within a nested scales approach considering interactions within and between catchmentscapes to design and develop spatial structures, and deliver them through a multi-level hierarchy of governance.’

In contrast to conventional approaches to water management in cities, concepts that are described above use innovation to integrate ecological elements into urban development. In order to achieve a meaningful analysis, the study was contextualised within this relevant theory. The following Figure 5 depicts the conceptual theoretical framework of the design and development of WSPD.

Figure 5

Conceptual theoretical framework of the design and development of WSPD as developed in the study.

Figure 5

Conceptual theoretical framework of the design and development of WSPD as developed in the study.

RESULTS AND DISCUSSIONS

Spatial patterns in the CUC versus planning

Planning practices in Colombo have neglected to understand and work with patch-corridor and connectivity relationships in the urban landscape. Such a missing relationship in urban planning practices has created key problems when managing natural resources in the CUC. The main problem found here relates to the boundary overlaps and cross over between administrative and natural environments. This overlapping relationship has created many negative implications, particularly, around development, management and uptake of the urban ecological environment, for example, management of Parliament Lake and the surrounding catchment. Accordingly, the lake and the catchment are crossed over by the three Local Authorities (LAs), namely, Sri Jayewardenepura-Kotte, Kaduwela and Maharagama (Appendix 1).

This has consequently given rise to several problems when establishing, maintaining and managing strategies for stormwater runoff within the planning process in this urban catchment. This is because each LA has developed its own planning and development regulations to operate. No attention has been given to the overall management of runoff within the catchment, or to understanding the nature and distribution of the patch-corridor network within authorities. The inclusive ecological connectivity between land uses and the catchment base requires better coordination and collaborations between LAs when they're developing planning outcomes to protect natural environments.

Scenario development

It is possible to reduce the runoff and improve the quality of stormwater if a primary goal for integrating human purposes and natural processes is established within the catchmentscape. In achieving this, a goal for the development of WSPD would be established.

The goal entails ‘uptake of capacities for self-regulation of stormwater within the existing catchmentscape’. As described before, stormwater is not a static resource, and it may be driven by several inputs and outcomes of planning. Therefore, tasks need to be performed within selected (sub) catchments to achieve expected outcomes. There were two WSPD scenarios identified that can potentially be helpful to achieve the goal.

  • (1)

    Self-regulation through natural elements

  • (2)

    Self-regulation through human-induced elements.

Since runoff generation varies within the land uses in the urban catchment, design and development of WSPD requires various elements. For example, within natural ecological structures of the catchmentscape as well as human-induced residential development. In order to be applicable, WSPD is required to be implemented at all spatial scales from lot, to neighbourhood, city and region. The combination of the two integrated scenarios addresses the enhancement of self-regulation of stormwater runoff within the catchmentscape and the urgent issue of urban flooding. However, the present study intends to continue only emphasising the development of scenario 1 of self-regulation of stormwater through natural elements.

Scenario 1: self-regulation through natural elements

To develop the first scenario, three landscape elements were incorporated as components to enhance natural capacity: (1) Patch (2) Corridor and (3) Connectivity. Within the patch component, wetlands (low lying areas) were considered ‘core areas (ecological nodes)’ that enable a particular mosaic of environmental conditions to conserve important ecosystems. Ecological connectivity within the patch component was enhanced using water sensitive corridors developed within the existing catchmentscape. This also was supported by providing buffer zones, considering the dispersed nature of wetlands and flood inundation areas. There are natural capacities that could enhance the water attenuation opportunities in the catchmentscape. But an evaluation of the spatial pattern and its composition revealed conflicts between uses of natural resources within the catchmentscape. After reaching an understanding of the spatial pattern process relationship the focus for scenario 1 shifted to the minimisation of conflicts and enhancement of the quality of runoff.

By considering broader constraints of structural and functional relationships within the catchmentscape, the scenario can potentially bring positive outcomes from the implementation of WSPD in the sub-catchments (no 05 and no 03). Figure 6(a) indicates the figure-ground map that helps visualise the relationship between built and unbuilt space of the two sub-catchments. The Figure 6(b) indicates, from the total catchment coverage, 72.4% has been already covered by built-up lands. This area is dominated by impermeable surfaces. Only 27.6% of the area is unbuilt and that includes wetlands, streams, recreational and agricultural lands. Therefore, incorporation of WSPD practices to the catchmentscape is important. These open areas provide multiple opportunities for the implementation of water sensitive practices.

Figure 6

Figure-ground map of the selected sub-catchments.

Figure 6

Figure-ground map of the selected sub-catchments.

Evaluation of the spatial pattern for WSPD in sub-catchments No 5 and No 3

The following Table 2 indicates a few examples for each of these landscape components that were recognised and analysed to incorporate WSPD in the sub-catchments. In the catchmentscape based planning approach, the objective of enhancing and detaining the stormwater through natural elements is addressed by taking the opportunities to implement discharge control at the landscape level. This is because, the landscape consists of several types of structural elements such as patches, corridors and potential connectivity among each of those structures within a given catchmentscape. So patchiness or heterogeneity of landscape structures denotes the opportunity to enhance self-regulated stormwater control potentials in the catchmentscape.

Table 2

Selection of spatial structures for the evaluation of catchmentscape

Area of sub-catchments No 05 and No 03
 
Scenario 1 – Natural elements
 
Patch (area) Corridor Connectivity 
Kotte marsh (95 ha) Canals and streams Blue-green infrastructure network corridors (extended detention using wet ponds) 
Shrublands Roads Main roads
 Local roads
 Footpaths/walkways 
Well connected stream and riparian systems linking flood plains to support stormwater runoff 
Working paddy
Abandoned paddy 
Riparian corridors Street buffers using native vegetation and greenways 
Diyawanna lake
Wetlands 
Walkways Connectivity to shrublands (between working paddy and marshlands) as stormwater treatment train to function in contaminant removal 
Area of sub-catchments No 05 and No 03
 
Scenario 1 – Natural elements
 
Patch (area) Corridor Connectivity 
Kotte marsh (95 ha) Canals and streams Blue-green infrastructure network corridors (extended detention using wet ponds) 
Shrublands Roads Main roads
 Local roads
 Footpaths/walkways 
Well connected stream and riparian systems linking flood plains to support stormwater runoff 
Working paddy
Abandoned paddy 
Riparian corridors Street buffers using native vegetation and greenways 
Diyawanna lake
Wetlands 
Walkways Connectivity to shrublands (between working paddy and marshlands) as stormwater treatment train to function in contaminant removal 

A capability assessment for the three structural landscape elements (Figure 7) was undertaken by preparing separate spatial layers for each element of the landscape composition (Appendix 2). Once each of the spatial layers was created, it was evaluated using the three geographical factors, namely, contour, soil structure and groundwater table within the catchment to find out their suitability for implementation of WSPD within the local environment.

Figure 7

Selection of core landscape metrics for WSPD (Suitability for the self-regulation of stormwater).

Figure 7

Selection of core landscape metrics for WSPD (Suitability for the self-regulation of stormwater).

Design consideration of connectivity/corridor for WSPD

Street level vs. streams and marshlands

Table 3 highlights the WSPD design evaluation, developed to maximise passive stormwater management opportunities into road designs within the catchmentscape. A road (re)development provides the opportunity to design roads around the natural topography and incorporate natural features of the catchmentscape into the design and implementation of stormwater friendly road layouts.

Table 3

Selection criteria of roads for WSPD consideration to restore natural hydrology and increase impervious control within catchmentscape

Criteria for selection for WSPD of the catchmentscape
 
Design Objective and Outcome(s) 
Roads vs. marshlands/streams Design elements Water quantity/ quality 
Local/main roads
  • i. Select roads that are within a distance of 50 m to existing marshlands

  • ii. Select roads that are within marshlands

  • iii. Select roads that are within a distance of 20 m of streams

(see Figure 8 for outcomes) 
Design of roads recognising hydrologic impacts and effective impervious area
(Streets with development one side and (or) both sides or no development either sides) 
  • ▪ Improve the quality of stormwater that discharges to marshlands/ streams and adjacent catchment environment

  • ▪ To increase the perviousness within development

  • ▪ Attenuate runoff

  • ▪ Minimise intensity and (or) slowing down velocity of stormwater generation – detaining high volume water flow through increased infiltration capacities along roads

  • ▪ Reducing the percentage of impermeable surfaces development

  • ▪ More space for species to migrate living in the wetland ecosystems

 
Design vegetated buffer strips along (adjacent) roads
Use of vegetated swales and bio-retention swales that function as treatment train elements and convey runoff
Disconnect impervious covers along road strips and design pedestrian paths using permeable pavement to continue infiltration
Retain existing natural areas along roads strips/stream networks to reduce effective impervious covers
Design street drainage using percolation pits to increased detention capacities to manage minor flows 
Limitation (s) 
  • ▪ No existing data/map provided on the distribution of impermeable surface within the catchmentscape. (Due to no data availability)

  • ▪ No proper stormwater drainage network exists within the catchment.

 
Challenge(s) 
  • ▪ Appendix 2(d) shows the topography and slope of the catchmentscape as more flat. Therefore, the movement of stormwater on land is slow making the slow draining onto the ground. But this may create infiltration is high.

 
Criteria for selection for WSPD of the catchmentscape
 
Design Objective and Outcome(s) 
Roads vs. marshlands/streams Design elements Water quantity/ quality 
Local/main roads
  • i. Select roads that are within a distance of 50 m to existing marshlands

  • ii. Select roads that are within marshlands

  • iii. Select roads that are within a distance of 20 m of streams

(see Figure 8 for outcomes) 
Design of roads recognising hydrologic impacts and effective impervious area
(Streets with development one side and (or) both sides or no development either sides) 
  • ▪ Improve the quality of stormwater that discharges to marshlands/ streams and adjacent catchment environment

  • ▪ To increase the perviousness within development

  • ▪ Attenuate runoff

  • ▪ Minimise intensity and (or) slowing down velocity of stormwater generation – detaining high volume water flow through increased infiltration capacities along roads

  • ▪ Reducing the percentage of impermeable surfaces development

  • ▪ More space for species to migrate living in the wetland ecosystems

 
Design vegetated buffer strips along (adjacent) roads
Use of vegetated swales and bio-retention swales that function as treatment train elements and convey runoff
Disconnect impervious covers along road strips and design pedestrian paths using permeable pavement to continue infiltration
Retain existing natural areas along roads strips/stream networks to reduce effective impervious covers
Design street drainage using percolation pits to increased detention capacities to manage minor flows 
Limitation (s) 
  • ▪ No existing data/map provided on the distribution of impermeable surface within the catchmentscape. (Due to no data availability)

  • ▪ No proper stormwater drainage network exists within the catchment.

 
Challenge(s) 
  • ▪ Appendix 2(d) shows the topography and slope of the catchmentscape as more flat. Therefore, the movement of stormwater on land is slow making the slow draining onto the ground. But this may create infiltration is high.

 

Different street levels (local roads/main roads) can be integrated as linear systems to connect stormwater circulation within the catchmentscape but depending on the local conditions (as mentioned before, such as soil conditions, contours and ground water table). So this integration has enabled many opportunities to minimise stormwater runoff and maximise recharge within the similar landscape structures.

There are a few factors that the lead author has identified during his field observations and analysis of planning documents at the initial stage of research development. For example, when considering the existing characteristics of the catchmentscape, integration of stormwater management practices into the planning of Colombo landscape and designing roads has been disregarded. Most of these findings have been documented in his draft PhD thesis. Road designing can significantly contribute to stormwater quantity and quality control and management. Incorporation of structural elements for connectivity and corridor in the catchmentscape can contribute to achieving multiple opportunities and benefits, mainly in stormwater management.

As shown in appendix 2, roads have been developed closely interconnected to the natural water systems and other environmental elements in the catchmentscape. Due to lack of drainage infrastructure in the area, the protection for water quality is poor. Moreover, lack of local detention and retention capacities have created problems, such as increased stormwater runoff and peak volumes during intense rainfalls. The following are elements that this research has identified as important to immediately integrate with planning WSPD in the Colombo catchmentscape.

Imperviousness control. Implementation of WSPD into road design helps detain rainwater, and minimise connected impervious surfaces. This informs the overall understanding of connectivity and corridor within the catchmentscape. The degree of human interaction and the lack of ecologically adopted design solutions in the sub-catchment has created negative stormwater consequences. Due to the urbanised nature of the catchmentscape, these consequences enhance the opportunity to incorporate design features, such as permeable paving to increase stormwater infiltration, development of vegetated swales to store or convey surface water downstream, rain gardens to attenuate and filter runoff, recreational amenities, and increase the connectivity between open stormwater canals/drains to enhance the flow of water through the catchmentscape.

Missing or lack of connectivity between natural stormwater conveyance elements. Through evaluation of the catchmentscape, it was recognised that functionality and the connectivity between existing streets and natural streams (including other sensitive environmental uses) are very high. For example, roads are either closely or directly located adjacent or boundary to the existing streams and marshlands in the catchmentscape. Due to decentralised stormwater conveyance systems, street runoff has been degrading most environmental functions of the catchmentscape such as wetland and stream habitats. Therefore, increased connectivity using WSPD considerations can provide opportunities to achieve sustainable outcomes when designing road corridors integrated with stormwater infrastructure.

Table 3 indicates criteria used to identify main and local roads to incorporate WSPD considerations within the selected catchmentscape. Despite the conventional approaches to stormwater drainage infrastructure design, it was also important to incorporate solutions that are sensitive to water which ultimately bring aesthetically pleasing, functional, usable stormwater management elements to road layouts that are also compatible with local conditions, such as topography.

According to the evaluation using the criteria in Table 3, Figure 8 shows the selected main and local roads that have potential for WSPD consideration during road design. The road layers were analysed comparing the distance factor to the structural elements of marshlands and streams. This was because the selected study sub-catchments area contained within the natural flood detention basin of Colombo. The available wetland network and streams of the catchments are providing an invaluable service to attenuate stormwater during rainfall events.

Figure 8

Roads which should be considered in connection with WSPD design considerations within the selected two sub-catchments.

Figure 8

Roads which should be considered in connection with WSPD design considerations within the selected two sub-catchments.

However, the function and value of the natural wetlands and the stream habitats, including the lake have been underestimated by current planning practices. For example, rapid degradation of wetlands water quality and stream quality due to first flush effects of street runoff in the catchmentscape. Negative consequences of this effect (that conveys high initial pollutant loads to the waterways and wetlands) have continuously damaged receiving environments due to seasonal rainfalls.

The lead author's field observations and analysis of land use patterns of CUC (within the PhD research study) has highlighted several issues. For example, re-vegetation is taking place in the wetland environments by converting marshlands into shrublands, resulting in the gradual reduction of the natural flood detention basin area compared to the available area in the past.

The roads that were selected in the catchmentscape are connected to natural environments such as marshlands, streams or shrubs. Therefore, WSPD objectives can apply to road designing at various scales to increase permeability, minimise peak volumes, to protect ecosystem health by protecting receiving environments at the broad, local and fine scale. The implementation of the WSPD within the catchment scale helps minimise discharges and preserves the hydrological regimes of catchment ecosystems.

In addition, development of WSPD will help respond to minimising problems that arise due to LA boundaries that have crossed over catchment boundaries. Because, when design begins at the catchment scale planning, LAs have to keep complying with WSPD targets on a sub-catchment basis to achieve outcomes. This may help assist in maintaining and/or improving ecosystem services and ecological health benefits from one catchment to each subsequent sub-catchment to the eco-regional scale. On one hand, it might not be possible to transfer WSPD practices directly across one catchment to another and to the regional level. But on the other hand, it may help the uptake of WSPD targets at the local level which ultimately can assist in managing urban water-related impacts at LA levels to regional and national level due to rapid urban development.

Therefore, the inclusion of WSPD that considers the connectivity and corridor stages of design within the catchmentscape has significant effects in planning. As such planners need to meet relevant authorities and to set design criteria at the initial stage of the planning process. This involves consideration of all the requirements that are unique to the catchmentscape, such as flood impacts, water quality related issues, problems related to catchment habitats and ecosystems to improve using WSPD.

CONCLUSIONS

Over the last three decades, CUC has been vulnerable and suffering from an increasing impact of flood and stormwater management issues. The frequency and the intensity of occurrence of these issues have now increased due to several factors that include both natural and human induced. Analysis of existing development strategies/solutions to stormwater management and flood controls in Colombo reveal that planning practices to date are mainly searching for treatment methods to fix ongoing symptoms rather than trying to resolve the actual causes and effects. Therefore, problems remain constant while making cities vulnerable to impacts.

Therefore, CUC requires to recognise alternative approaches that can enhance adaptation in addition to existing conventional approaches. The paper tried to focus on discussing significant characteristics of Colombo, the underlying need and importance of integrating WSPD as a means of initiating ecologically inspired adaptive capacities to optimise a flood and stormwater resilient urban environment.

However, in this paper, WSPD addresses the need for a combined approach that offers ‘landscape structure’ and ‘connectivity’ as two explicit components in land use planning at the catchment scale. Considering connectivity within water sensitive structures between patch and corridors, catchment scale based land use planning provides opportunities for the application of WSPD objectives to the area of urban development. By adopting the design objectives, an appropriate way could be found to combine urban land use effects and all catchment activities for urban stormwater health and runoff control to improve receiving waters and flood control in CUC.

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