Modelling of Nature-Based Solutions (NBS) for Urban Water Management—Investment and Outscaling Implications at Basin and Regional Levels

This manuscript is an attempt to demonstrate effectiveness of nature-based solutions (NBS) and measures to reduce risk of flooding and environmental impact in urban settings. The nature-based solutions (NBS) were assessed as scenarios from experience of urban storm drainage and sewerage systems based on practices that improve urban water management through modelling using urban stormwater management model (SWMM). The model has been applied in a typical urban environment in the second city in Botswana, the City of Francistown, which has a population of more than one hundred thousand. By considering the 2-yr and 10-year storm events in a calibrated SWMM, NBS scenarios from a mix of low impact and drainage measures were considered. The considered NBS scenarios were used to determine their effectiveness in terms of reducing and controlling peak runoff, flood volumes, infiltration and evapotranspiration in the study area, which are vital in assessing the opportunity and challenge for sustainable management of water resources and associated tradeoff of investments in the urban contexts. The study demonstrates the usefulness of implementing effective measures for achieving NBS in urban context and possibility of outscaling at basin and regional levels.


Introduction
Improving urban water management, both its quantity and quality, require a bute to widespread LID technique was introduced in April 2006 by Singapore's water agency to carry out stormwater management in a more sustainable manner. This ABC Program is primarily used to managing urban stormwater and controlling flood which is also becoming popular in countries of the northern hemisphere [18].
The urban storm runoff treatment concept was introduced which synonymously called as Sustainable Urban Drainage System (SUDS) ( [10] [11] [19]. The concept focuses on making use of and strengthening natural process to simulate hydrologic development in early time, which subverts principals of rapid transit [20]. Besides, runoff quantity, SUDS is also applied as an intervention to improve urban flood and water contamination and urban beautification, and to provide a more livable environment for both human and wildlife [21] [22].
The high degree of urbanization put pressures on ecosystems of both local and regional lifesupport where ecological principles could guarantee sustainability where a practical tool for eco-sustainable planning and management is required [17] [23]. The idea of Sponge city plays a pivotal part in the construction of ecological landscape, provides a livable and sustainable environment, also becomes the basis of evaluating the ecological environment of urban residents [24].
In Botswana where the study case site is considered and the southern Africa in general, there is a growing focus on sustainable development planning in rural Journal of Water Resource and Protection and urban areas, improved urban development standards and improved drainage manuals to respond to infrastructural challenges and changed climate conditions which resulted in increase of frequency and intensity of heavy storms [25]- [30]. The main objectives of this study were therefore: To model and assess various plausible NBS scenarios in urban setting.
To analyze the implications for implementing NBS at urban scale.
To discuss the challenges and limitations of NBS implementation.
To elucidate the enabler conditions for uptake of NBS across different scales.
Furthermore, through the studied case study site and different cases in Africa, recommendations for improving implementation of NBS at a scale of urban settings, basin and regional levels is also provided.

Study Area, Sub-Catchments and Design Storms
The study area is focused on the watershed area encompassing the drainage and sewerage layout of the City of Francistown in north-eastern Botswana. The area drains the Confluence Rivers of Ntshe and Tati Rivers before joining Shashe River towards the Limpopo Drainage Basin. Data from [31] was used to define the drainage areas and drainage characteristics, such as slopes, lengths, besides the network map of the sewerage system. The considered subareas and the location are presented in Table 1 and Figure 1.
Storm Intensity-Duration-Frequency (IDF) Curves were used to determine the design storms which are the basis of the design of the drainage system. In addition to the average storm intensity within certain duration, the time-distributed form, i.e. the rainfall pattern, is also an important factor. For rainfall patterns in many generalized studies, the Chicago model is widely used as suggested by [32]. This model, in the rainfall intensity mode represents the maximum average intensity rule of the same frequency, introduces the average shape and intensity peak position, and can obtain the average rainfall intensity, time-interval rainfall intensity and instantaneous rainfall intensity. The model described by the  instantaneous intensity summarizes the special rainfall patterns, forming a rainfall pattern that more fully reflects the characteristics of storms.
In the absence of long-recorded data of short storm events/storm depths of short durations, it is always difficult to develop an IDF curve for a given region.
However, based on a parsimonious robust frequency modelling approach it was possible to model 24-hour maximum rainfall frequency and construct the IDF curves from existing best practices. For this purpose, the IDF curves were constructed based on [33] and rainfall incremental depths for hourly durations were based on the Botswana Roads Design Manual, Volume 3, Hydrology and Road Drainage [28].

Flooding and Flows
In the main study area of an urban drainage challenge in City of Francistown, there have been frequent incidences of flooding in the City as summarized in Furthermore, effect of wet weather flow increases in the existing sewerage system and the existing wastewater treatment plant cannot be undermined.
These flow increases, will require upgrades in capacity of the urban drainage and sewerage system, with pronounced impact on investments to be made. Journal of Water Resource and Protection Unless, sustainable management of excess runoff and treated wastewater are utilized to meet co-benefits, cost-effectiveness of public investments in water supply and sanitation infrastructure will remain questionable and practically unjustifiable.

Review of the Existing NBS Tools
Using prior concepts prior to the advent of NBS, similar approaches are elaborated by different authors. Notable studies were undertaken since 1970s that aimed at developing an urban system which is a combination of several components of urban runoff management and water treatment, synonymously known as Integrated Urban Drainage System (IUDS) [35]. The concept focusses mainly in the integration of sewer, wastewater treatment plant, water receiving systems, and sustainable stormwater management, with economic factors taken into consideration [36]. The concept of Water Sensitivity Urban Design (WSUD) is also applied for planning and design, which first popularized in Australia in the mid of 1990s. The WSUD primarily targets at minimize the adverse effects of urban development on surrounding hydrological environment [37].
The best solution to the rainwater problem in cities should be drainage which involves efficiently collecting and discharging stormwater to receiving waters. This tendency which was popular up to the 1990s has transformed to developing water sensitive city infrastructure and ecosystem services as illustrated in Figure 3.
Review of the existing NBS tools for sustainable water management in urban contexts are elaborated in [2]. Prudent urban water management and promotion of practices of best NBS, included five categories as noted in [2], namely: 1) stormwater management, 2) flood protection and risk management, 3) implementation of blue-green infrastructures, 4) urban water in the field of food, water and energy ecosystem and 5) urban water pollution control and constructed wetlands. Journal of Water Resource and Protection

Modelling Approach
The modelling approach allows for considering development pattern in urban or watershed level based on source control and decentralized processes for assessing and managing generated runoff at selected outlets.
The modelling facility within EPA's Storm Water Management Model (SWMM) has provisions that allow the conceptualization of LIDs attributable to each target subareas. LID practices are modeled based on the conceptualization of the vertical layers between which SWMM tracks of how much water moves and is stored ( Figure 4). Common LIDs are summarized in Figure 5.
Sustainable Urban Drainage Systems (SUDS) technologies consider environmental, social and economic pillars in the design process. SUDS should integrate stakeholders in the decision making and ultimately, could achieve multiple benefits along with flood and inundation mitigation. There are several SUDS technologies available. Within the scope of this study, four of the most popular SUDS technologies were considered: 1) Rainwater harvesting-which can be a supplement for water supply sources; reduce extra direct discharge to the drainage system and prevent urban flooding [40] [41] [42].
3) Urban green space provides improved resiliency in runoff management and Journal of Water Resource and Protection

4) Pervious pavement-a technology that both enhances infiltration and im-
proves surface runoff quality [57] [58] [59]. There is some concern about clog- A combination of layers used to simulate any LID or a combination of LIDs such as pavement layer, bioretention, vegetative swale, infiltration basins, green roof, and the flow pathways between the layers to allow the surface layer to receive direct rainfall or runoff from adjacent land areas, stores excess inflow in depression storage, and generates surface outflow that either enters the drainage system or flows into adjacent land areas. Vertical layer structure of the general arrangement of bioretention in SWMM is shown in Figure 5.
The pavement layer is the layer of porous materials with permeable system.
The soil layer in SWMM is the defined as natural soil mixture or engineered soil mixture used in LIDs to support vegetative growth or provide bedding and filtration. The storage layer consists of crushed rocks or gravels for water storage.
The drain system allows water effluent from the storage layer into a common outlet channel or pipe. The drainage mat layer used in green roof is a plate or mat between the soil and the roof to convey water off of the roof.

Consideration of NBS for the Study Area
NBS through LID practices that provide stronger control and reduction of runoff volume are biofiltration, rain garden, green and vegetative swales. These measures have some advantage of reducing runoff pollution, landscape enhancement and some economic benefits in terms of tourism and recreation. Equivalently, permeable pavements and infiltration trenches also have some similar advantage with some degree of challenges of economy and landscape due to non-vegetative character of change of surface of the landscape.
The risks associated with flooding faced in the study area urban floods and waterlogging which are currently the most prominent problems which affect not only urban lifestyle but also flooding of the sewerage system at major outfalls and junctions. In most parts of the study area, rainwater pipes in many residential areas directly discharge roof rainwater to the roads, and serious missing of vegetation layer has occurred in landscape flower beds, and unrepaired pavement exits in almost each residential areas. Using the conventional methods, a lot of time and money need to be invested in rebuilding or renovating these facilities.
The urban development standards [29] and the revised hydrology and road drainage design manual [28] calls for better management of urban runoff and harvesting excess runoff and by connecting roof downspouts to urban drainage systems, replanting vegetation layers and paving new roads, etc. With the introduction of LID concept, according to the characteristics of different LIDs, it will be more effective to apply them or the introduction in urban areas, such as add- ing Rain Barrels connected with roof downspouts to directly collect rainwater from the roof for household water utilization, landscape flower beds into Rain Garden, and transforming damaged pavements into Permeable Pavements, as Journal of Water Resource and Protection illustrated in Figure 5.
Therefore, based on the characteristics of the various LIDs and the major problems faced by the study area, taking the total runoff and runoff peak control as objectives and meanwhile considering economy and landscape, in this study, vegetative swales, Infiltration trench and permeable pavements or open pavement systems are applied as basic LIDs. The DCD measures were included apart from the basic LID practices to cater for development control [29] and revised road drainage manual [28] drainage Lot Grading and Drainage requirement to a minimum lot grading around houses and buildings of up to 2% and the minimum grades for side lot swales and rear lot swales be 2%. Also the hydrology and road drainage manual recommends that all grading design shall be completed in accordance with the governing guide-

Results
Rainfall characteristics and system response for a 2-year and a 10-year return period rainfall with existing drainage network is shown in Figure 6. Whereas  Table 2 and Table 3 show the summary of the effect of 2-year storms for the various NBS Scenarios on runoff volume and runoff peak, and Table 4 and Ta

Discussion of Results
Under scenario 1, runoff volume and peaks reduction of the range of 6.1% to 9.4% are achieved among the six subareas considered in Francistown. For scenario 2, 3 and 4, more reductions in runoff in the order of 20% to 40% are prevalent. It is within the expected range as more measures to decrease in surface runoff coefficient and imperviousness would reduce runoff substantially.
In the entire drainage area, runoff reductions in between 5.3% and 46.0% can be evident for the four NBS scenarios. It is evident that as more measures to decrease infiltration and increase runoff coefficient are introduced, more urban runoff will be generated with implications to creating surface storage sites, and also for controlling increases in wet weather flow into the sewerage systems.
Generally, the changes in runoff storages and peaks as well evaporation and infiltration for the 2-year and 10-year recurrence interval storms are different slightly. This is due to changes of rainfall intensities used for the two cases. This

Implications for Implementing NBS at Urban Scale
Urban sewerage and storm management investments are considered as part of the water supply and grey water/sanitation, and urban road infrastructure sector, respectively. In the drinking water supply and sanitation sector as a whole, NBS appear to be severely underfunded in comparison with grey infrastructure.
In the case of the City of Francistown, a great opportunity exists to harness the excess runoff generated at different localities/watershed outlets as described in the modelling study. Furthermore, the effluent from wastewater treatment plant can be stored in wetlands and be used to create more ecological and agricultural reuse opportunities.

Context of Implementation
In the context of water and sanitation, constructed wetlands for wastewater treatment can be a cost-effective NBS that provides effluent of adequate quality for several non-potable uses, including irrigation, as well as offering additional benefits, including energy production [1]. With over 80% of all wastewater released to the environment without any prior treatment globally, and over 95% in some developing countries [1], constructed wetlands can provide great opportunities for communities of all sizes and regions in Africa and beyond.
Effectively collected urban runoff in drainage systems in combined sewerage systems or wet weather flows in sewerage networks can be used in the nature based solutions in the following manner: • reuse of wastewater for urban and peri-urban agriculture.
• reuse of wastewater for urban landscaping and gardening.
• constructed wetlands as cooling of urban runoff.
• constructed wetlands, ecological benefits and reuse in aquaculture.
• artificial groundwater recharge as wastewater reuse.
Moreover efficiently built wastewater treatment facilities can have advantages in downstream nature based solutions and co-benefits of: • nutrient recycling and sludge reuse.
• phosphorus recovery and reuse from wastewater.
• sustainable energy generation as reuse.

Context of Implementation
The biological and geophysical characteristics of a river basin directly affect the quantity and quality of water flowing downstream over time and space. Any significant changes in the characteristics of landuse/landcover (LULC) and climate change can alter these hydrological features. Improved land management can therefore be seen to include an ensemble of NBS that can collectively enhance water security. There are examples of such practices across different regions.

Regional and National Frameworks of NBS
NBS can merely get focus as a standalone solution in the water supply and sanitation sector unless it is employed as add-on concept and practice to enhance water management, environmental quality and multi-purpose water management in urban and rural settings. Although most often driven by local stakeholders, such as large water users and municipalities, to achieve specific water management outcomes, broader frameworks and partnerships at national and regional levels play a critical role in fostering implementation of NBS. No separate national policies do usually exist to facilitate and oversee implementation of NBS, which is particularly critical, unless it is driven as part of the traditional water-related infrastructure.
Large-scale national-level implementation of NBS as part of a broader policy framework is required for achieving a specific water management objective-in this case flood management-with complementary objectives such as spatial planning and environmental protection. Figure 10 shows the framework for evolving approaches to the water-ecosystem nexus where emphasis has shifted from looking at impacts on ecosystems to managing ecosystems to achieve water management objectives.
NBS provide a mechanism for realizing participatory approaches to water and land use management, facilitating the exchange of information and in some cases drawing upon traditional knowledge and historically tested natural resource management approaches, such as the Integrated water resources management (IWRM) approach. They can assist in formalizing and activating part-  Although many relevant frameworks either mandate or enable NBS to be considered, the final decisions will often depend on a more detailed consideration of the costs and benefits of various options. A notable feature of recent legal/regulatory/framework development is their emphasis (whether legally mandated or not) that all benefits, and not just a narrow set of hydrological outcomes, need to be factored into assessment of investment options. This requires a detailed systematic approach to evaluating costs and benefits, which is possible and will lead to improved decision making and overall system performance [1]. the Urbanizing World is provided in [66]. A more specific regional review of wastewater treatment performance efficiency of constructed wetlands in African countries is provided in [67]. In a natural setting, [68]  • Understanding of regional design storm and flood modelling with risk implications in ungauged catchments [33] [69].

Implementing NBS at National Levels
• Maintenance and management of surface water and groundwater recharge in data scarce arid catchments [70].
• Management of water supply reservoirs and dams through technical and engineering tools under uncertainties in arid and urbanized environments [71].

Implementing NBS at Southern African Level
The Southern African Development Community (SADC) region has developed a regional water policy and regional water strategy to promote regional integration and poverty reduction within SADC, which particularly requires and promotes two objectives, namely: 1) Cooperative management of shared watercourses within the region, primarily through the Protocol on Shared Watercourses, and 2) Harmonisation of national water sector management between SADC Member States to facilitate integration and the achievement of endorsed targets.
A regional water policy is developed based on principles and objectives from the Millennium Development Goals, World Summit on Sustainable Development, NEPAD (goals of AMCOW on water) and multi-lateral agreements between Watercourse States. The policy was synthesized to underpin the following policy principles [73].
• Water as an instrument for peace, cooperation and regional integration • Development of SADC water resources through the joint planning and construction of storage, in order to rectify historical imbalances and promote water supply for irrigation and poor communities • Efficient use of water through demand management, conservation and reuse/ recycling, and the efficient use of water in agriculture • Recognition of the environment as a resource base and a legitimate user of water • The protection of the environment through appropriate user charges and the enforcement of "the polluter pays" and "waster pays" principles, taking into account equity and social justice • Integration of water supply, sanitation and hygiene education programs • Capacity building to ensure that managers of water, waste and sanitation have the requisite knowledge and tools • Ensuring that waste is safely managed at or as close as possible to the point of generation • Preventing the import (and export) of harmful waste across the national and regional boundaries The conceptual framework for the SADC regional water policy presented in Figure 11, which was implemented during the policy formulation to illustrate the linkage to the SADC goals of regional integration and poverty reduction, water at the center domain of developmental in the region. The policy indirectly can be used to embrace on the implementation and use of NBS tools to improve sustainable development and management of the region's water resources.
Source: [73]. Figure 11. Conceptual framework of the SADC regional water policy. Journal of Water Resource and Protection The key water related objectives that can be considered as part of the NBS implementation are linked to industrial development (including agri-businesses), food security, access to water and sanitation, water for peace, energy security and safety from disasters. Underlying these is the objective of sustainable development, or development that does not compromise the environment.
With possible opportunity that can be garnered to promote NBS in the region, the tenets of Integrated water resources management (IWRM) is the fundamental approach that has been adopted in the SADC water policy, which is enabled through the development of tools related to institutional development, capacity building, stakeholder participation, information management, integrated planning, conflict resolution and environmental management. Each of these objectives and tools is addressed in the policy, with IWRM being the common thread that links them all together.
In the context of southern African region, there are a number of experiences which show the benefits and opportunities of nature based solutions in the management of water resources. These efforts found in the region are also highlighted in related publications that include: • Improved understanding of agricultural water management such as climate change impacts and adaptation in rainfed farming systems through improved modeling frameworks for scaling-out climate smart agriculture in Sub-Saharan Africa as noted in [74].
• Improved understanding and evaluation of drought Severity, drought regimes and impacts at a basin scale in the Limpopo basin as described in [75] [76]. Africa through application of a large scale hydrological model [78].
• Evaluation of evapotranspiration at regional scale based on the FAO Penman-Montheith, Priestly-Taylor and Hargreaves models for estimating reference evapotranspiration at a regional scale in southern Malawi [79].

Challenges and Limitations
Implementation and provision of NBS often require cooperation among mul- • There is a lack of technical guidance, tools and approaches to determine the right mix of NBS and grey-infrastructure options.
• The hydrological functions of natural ecosystems, like wetlands and floodplains, are much less understood than those provided by grey infrastructure.
• NBS are even more neglected in policy appraisal and in natural resource and development planning and management.
• Insufficient research and development in NBS and lack of robust assessments of current NBS experience, especially in terms of their hydrological performance, and cost-benefit analyses in comparison or conjunction with grey solutions, especially in the developing world.
• There are limits to what ecosystems can achieve and these need much better identification. For example, "tipping points", beyond which negative ecosystem change becomes irreversible, are well theorized but rarely quantified.
• The high degree of variation in the impacts of ecosystems on hydrology (depending on ecosystem type or subtype, location and condition, climate and management) cautions to avoid generalized assumptions about NBS. For example, trees can increase or decrease groundwater recharge according to their type, density, location, size and age.
• Natural systems are dynamic and their roles and impacts change over time.
• Understanding of cost-effectiveness of NBS and including consideration of co-benefits. While some small-scale NBS applications can be low-or no-cost, some applications, particularly at scale, can require large investments.
• Ecosystem restoration costs, for example, can vary widely from a few hundred to several millions of US dollars per hectare. Site-specific knowledge on the field deployment of NBS is essential yet often inadequate. Journal of Water Resource and Protection With more attention given to NBS and more knowledge and practices are understood, NBS practitioners need to greatly increase knowledge to support decision making in considering NBS in projects that are directly or indirectly associated with NBS.

Enabler Conditions for uptake of NBS
A number of enabling conditions to accelerate uptake of NBS can be considered equitably alongside other options for water resources management. According to the compendium of experience of [1], the required responses to these challenges essentially involve creating enabling conditions for NBS to be considered include the following: Leveraging financing-NBS do not necessarily require additional financial resources but usually involve redirecting and making more effective use of existing financing. Investments in green infrastructure are being mobilized thanks to the increasing recognition of the potential of ecosystem services to provide system-wide solutions that make investments more sustainable and cost-effective over time.
Creating an enabling regulatory and legal environment-The vast majority of current regulatory and legal environments for water management were developed largely with grey-infrastructure approaches in mind. Consequently, it can often be challenging to retrofit NBS into this framework. However, rather than expecting drastic changes in regulatory regimes, much can be achieved by promoting NBS more effectively through existing frameworks.
Improving cross-sectoral collaboration-NBS can require much greater levels of cross-sectoral and institutional collaboration than grey-infrastructure approaches, particularly when applied at landscape scale. However, this can also open opportunities to bring those groups together under a common approach or agenda.
Improving the knowledge base-Improving the knowledge base on NBS, including in some cases through more rigorous science, is an essential overarching requirement. Established evidence helps convince decision makers of the viability of NBS. For example, a frequently raised concern is that NBS take a long time to achieve their impact, implying that grey infrastructure is quicker. However, the evidence shows that this is not necessarily the case and timescales to deliver benefits can compare favorably to those of grey-infrastructure solutions.
Using the 2030 Agenda for Sustainable Development as entry point-NBS offer high potential to contribute to the achievement of most of the targets of SDGs (Sustainable Development Goals), SDG 6 (on water). Areas in which this contribution translates into particularly striking positive direct impacts on other SDGs are with regards to water security for underpinning sustainable agriculture (SDG 2), healthy lives (SDG 3), building resilient (water-related) infrastructure (SDG 9), sustainable urban settlements (SDG 11) and disaster risk reduction (SDG 11 and, as related to climate change, SDG 13).

Conclusions
Reducing the impervious percentage could contribute to alleviating the flood situation of the City of Francistown urban development area. Future development should try to maintain per cent imperviousness to reduce in the order of 30% to 40% or less, assuming no other interventions (e.g. SUDS) is considered.
The findings of this study also concur with similar efforts made in investment of urban green infrastructure, from the revegetation of impermeable surfaces to green roofs and constructed wetlands, which can yield positive results in terms of water availability, water quality and flood reduction, as exemplified by China's experience of the "sponge city" project [24]. water security, energy security, ecosystems functions & services, sustainable human settlement, sustainable landuse, climate resilience and disaster risk reduction, and pollution & waste management [81].
The social investment and development in these areas will help achieve the global development agendas constituted in agenda 11 of the sustainable development goals (SDGs), "Make cities and human settlements inclusive, safe, resilient and sustainable" (United Nations, 2015). Consideration of urban water management through efficient collection and treatment systems will help to sustain some of the green infrastructure requirements. This will further help nations to achieving the national development agenda and the Sustainable Development Goals (SDGs) through investment in ecological infrastructure [27].
The drinking water supply and sanitation sector as a whole, NBS appear to be severely underfunded in comparison with grey infrastructure since NBS as components of sewerage and storm management investments are considered as mere aspects of environmental management in water supply and grey water/sanitation, and urban road infrastructure investments. This challenge remains at large in the developing world and with dividends envisioned in co-benefit appraisal of NBS investments, the future trajectory of adoption and implementation in urban