Practical Steps for Urban Flood Risk Mitigation Using Nature-Based Solutions—A Case Study in New Cairo, Egypt

Abstract

This study investigated the effectiveness of nature-based solutions (NBSs) as a resilient strategy for mitigating urban flood risks in a developing hot arid country. The research method included the following steps: (a) performing a flood hazard risk assessment for the Fifth Settlement district in New Cairo, Egypt, (b) selecting best-fit NBSs, and (c) performance assessment. The process started with flood hazard analysis using hydrological data, topographical maps, urban planning, and land use maps, in addition to the history of storm events. This step defined the urban areas located in flood depth zones and categorized their flood hazard level. Exposure assessment considered the number and characteristics of population and buildings exposed to flood hazards. Vulnerability assessment determined the vulnerable characteristics of exposed populations and buildings to flood risk. The result of this assessment step indicated that there were 2000 buildings distributed in almost twenty neighborhood areas facing high flood risk. One of these urban areas with 72 building units, including residential, public, and services buildings, was selected to test the potential of integrating NBSs for flood-resilient land use planning and disaster preparedness. The selection of best-fit NBSs was based on a weighted-average sum matrix considering their climatic and contextual suitability and applicability. As a final step, numerical simulation models helped assess the efficiency of the selected NBSs for stormwater runoff reduction and the percentage of the volume capture goal. Five simulation models tested the efficiency of each NBS individually. Rain gardens achieved the highest stormwater capture percentage, while green roofs performed the least effectively, with capture rates of 43.6% and 9.9%, respectively. Two more simulation models were developed to evaluate the efficiency of NBSs when implemented in combination compared to the base case of using no NBSs. Permeable paving demonstrated the highest effectiveness in volume capture. The result indicated that applying combined measures of NBSs over 54.1% of the total site area was able to capture 8% more than the required volume capture goal. Consequently, this study underscores the necessity of adopting tailored solutions and integrated approaches using NBSs for flood risk mitigation. This necessitates testing their performance under site-specific conditions and future climate projections.

1. Introduction

The World Meteorological Organization reported that over the last 50 years, weather, climate, and water hazard-related catastrophes have accounted for 50% of all disasters, 45% of reported deaths, and 74% of economic losses [1]. Storms and floods, part of the top 10 disasters, led to 577,232 and 58,700 human losses, respectively. The UN ‘World Economic Situation and Prospects Report’ found that more than 91% of these deaths occurred in developing countries, which are greatly vulnerable to environmental problems [2]. Regarding the economic damage, storms and floods make it to the top 10 with a loss of USD 521 billion and USD 115 billion, respectively [2]. It is noted that flood risks are not only caused by heavy precipitation but also by human and environmental factors. These risks are escalated due to poor measures for flood risk assessment and management, as well as insufficient flood-resisting land use planning and infrastructure [3].

Urban flooding occurs within urban areas due to heavy rainfall, inadequate drainage systems, and rapid urbanization. Areas with a high proportion of impermeable surfaces are particularly vulnerable to these impacts [4]. Flood events can be slow onset, gradually inundating areas over time; however, they can also occur rapidly, and they are termed ‘Flash Floods’ [5]. Flood-prone areas are regions with a chance of experiencing flood events due to various factors such as geographical location, topography, climate, and human activities [6]. These are typically located within an X-year floodplain, referring to the probability of those events occurring [7]. Understanding these risks and analyzing the risk factors is crucial to understanding urban flood vulnerability for effective flood risk mitigation [8]. Thus, urban flood risk assessment is determined by hazards, exposure, and vulnerability [9]. Hazard assessment focuses on identifying and characterizing potential flood sources and their associated magnitudes and frequencies. Key factors include rainfall patterns, hydrological modeling, and coastal processes. Exposure assessment aims to identify and map people at risk. Vulnerability assessment examines the capacity of the urban system to withstand and recover from flood events. Thus, advanced data analytics and numerical modeling are needed to map flood-prone areas and asses their level of exposure and vulnerability [10,11].

NBSs are increasingly recommended for flood risk mitigation because they not only reduce surface runoff but also enhance biodiversity and improve urban livability [5]. Nevertheless, the selection among different types of NBSs is challenging considering their varying applications and suitability to different contextual and climatic conditions, as well as their capacity to withstand annual and extreme rainfall events. These challenges are escalated for vulnerable hot arid climates and under climate projection concerns [8].

2. Research Problem, Aim, and Objectives

Despite the growing interest in the effect of climate change on flood risk, developing countries remain underrepresented in the literature, even though they are greatly vulnerable to climate change impacts [12]. This is principally the case for the Middle East and North Africa (MENA) region with its arid and semi-arid climates, limited water resources, and high vulnerability to flash floods. By addressing urban flood risks through context-specific NBSs, this research advances scientific understanding by providing practical steps for flood risk assessment and management in a developing hot arid country. Egypt suffers from rapid urbanization and insufficient infrastructure. It has a history of storm events but has insufficient flood-resisting land use planning and infrastructure. The first step describes flood hazard risk assessment, analyzing hazards, exposure, and vulnerability for a flood-prone area. The analysis was carried out by scaling from a city, district, and neighborhood level to identify vulnerable locations, and it eventually adopted a microscale approach to investigate a small-scale neighborhood area in New Cairo. The following step selected the best-fit NBSs and tested their efficiency to mitigate flood risk in the study area. The results indicated their performance efficiency when applied individually and when combined. Thus, this study pinpoints the necessity of adopting an integrated flood risk assessment and mitigation strategy. This necessitates testing the performance of NBSs under site-specific conditions and future climate projections. This ensures that urban areas are better equipped to handle the challenges posed by climate change and rapid urbanization, ultimately contributing to more sustainable and resilient communities.

3. Literature Review

A systemic literature review was conducted in the last 10 years (2014–2024). Data collection included peer-reviewed published sources using Scopus, Web of Science, and Google Scholar databases. A research trend analysis was performed to scrutinize the scholarly publication output pattern and pinpoint the top keyword analysis for the topic of NBSs. The analysis categorized the share of publications in developed versus developing countries and in flood-prone versus non-prone countries. The results summarized in Figure 1 show rising attention across time (a total of 578 documents). They also indicate less scholarly input from developing countries (24%). They are prone to flood risk not only due to their environmental vulnerability but also because of their economic and social status. Moreover, less scholarly input from flood-prone countries (43%) stresses the need for more research concerning flood mitigation and management strategies. Relevant keywords pointed to the significance of adopting NBSs for flood control and risk management. They also pointed to its multifunctionality, including environmental, economic, and social benefits.

Effective flood risk management requires a combination of structural, non-structural, and integrated approaches.

Structural or traditional measures, also termed ‘Grey Infrastructure’, involve physical interventions designed to control or mitigate stormwater [13]. They include flood barriers and levees, stormwater drainage systems, and retention and detention basins. These systems are characterized by immediate action and high performance [14]. Nevertheless, their construction and maintenance can be prohibitively expensive and result in significant environmental impact. Also, they are often designed for specific flood scenarios and may not be adaptable to changing climate conditions or unexpected extreme events [15].

Non-structural measures, also termed ‘Green Infrastructure’, include the use of NBSs. These are usually ecosystem centric, multifunctional, sustainable, and socially inclusive [16]. They focus on reducing vulnerability and exposure to floods by mimicking natural hydrological processes to manage flood risk. NBSs primarily emphasize natural or ecological retention, detention, and infiltration processes, thus helping manage stormwater runoff volumes and peak flows [17]. They are increasingly recognized as critical tools for disaster risk reduction and flood risk management. These natural systems do not only mitigate urban flood risk but also enhance biodiversity, improve water quality, and support ecosystem services [13]. Additionally, they can be cost effective when considering the long-term maintenance and the multiple benefits they provide. Another advantage for NBSs is that they can evolve and adjust over time, so they are considered more adaptable to climate change projections [18]. Nevertheless, when compared to traditional infrastructure, they often require more space, which can be a limitation in densely populated urban areas, and the benefits as natural systems need time to establish and mature. Also, their performance can be less predictable, particularly in extreme storm events [19].

Scholarly publication output in the last 10 years listed different types of NBSs for stormwater management, as shown in Figure 2. Wetlands recorded the greatest publication share. They are usually applied for their multifunctional ecosystem services for flood management; nevertheless, concerns were raised about the availability of natural or constructed land plots that would serve the function [19]. The number of research papers on green roofs is also high. These systems have been used for urban flood management and urban heat island mitigation. The limitations of implementing green roofs were associated with installation costs, structural requirements, and maintenance needs [20]. The number of publications in rain gardens showed steady growth. This may have been driven by their effectiveness in stormwater management and aesthetic appeal in urban landscapes. However, their effectiveness for flood risk management would vary, depending on site conditions, design parameters, and maintenance practices [21]. Permeable paving demonstrated increasing attention. These systems have a dual role in flood mitigation and urban infrastructure development, but they require regular maintenance [22]. It is noteworthy that bioswales had the least number of publications. Nevertheless, they were gaining traction as cost effective and ecologically beneficial types of NBSs [23]. For bioswales, it is crucial to consider specific site conditions and design parameters for successful implementation, e.g., soil type, slope, and vegetation selection [4]. Implementing urban forests of native vegetation has received rising scientific attention. However, their effectiveness depends on proper design, planning, and management for tree species, forest density, soil conditions, and the extent of forest cover [24]. Infiltration trenches and rain barrels indicated steady but slower growth. For an optimized performance of infiltration trenches, factors such as soil permeability, groundwater levels, and potential for groundwater contamination must be carefully considered [25]. However, the effectiveness of rain barrels depends on rainfall patterns, catchment area, barrel size, and the presence of other stormwater management measures [4]. Hence, each type of NBS has its advantages and its drawbacks, which necessitates careful selection considering different factoring parameters [18].

For a full image of these discussed types of NBSs,

Table 1. Comparing the application of NBSs in hot arid urban zones.

Feature	Rain Garden	Rain Barrel	Permeable Pavement	Green Roof	Infiltration Trench	Bioswale
Infiltration	Moderate 1—limited by dry soil conditions and low rainfall [26]	None—water is stored, not infiltrated [27]	Moderate 1—effective if designed for local soil and climate [28]	Low—limited by shallow soil depth and high evaporation rates [29]	Moderate 1—depends on soil permeability and water availability [30]	Low 2—limited by low rainfall and high evaporation rate [31]
Retention	Low 2 to moderate—limited by high evaporation and infrequent rainfall [32]	High—ideal for storing scarce rainwater for reuse, e.g., for irrigation [27]	Low [28]	Moderate—can retain some water but is limited by evaporation [29]	Low [30]	Low—primarily for conveyance [33]
Detention	Low 2 to moderate—limited effectiveness due to infrequent storms [32]	None—does not detain water but stores it [27]	Moderate [28]	Low 2—limited by shallow soil and high evaporation rate [29]	Moderate1 [30]	Moderate 1 [33]
Primary Purpose	Manage Stormwater runoff, groundwater recharge, and pollutant removal [26]	Water storage for reuse [34]	Stormwater infiltration and reducing runoff [35]	Stormwater retention, insulation, and urban heat island mitigation [36]	Stormwater infiltration and groundwater recharge [30]	Stormwater conveyance, filtration, slowing runoff, and groundwater recharge [33]
Best Use Case	Rooftops, parking areas, driveways, and sidewalks [34]	Rooftops and parking lots [34]	Driveways, parking lots, and walkways [35]	Rooftops [36]	Areas with occasional heavy rainfall and permeable soils [30]	Roadside drainage in areas with occasional storms [33]
Maintenance	Moderate—requires irrigation during dry periods and soil maintenance [37]	Low—requires occasional cleaning and winterization [38]	Moderate—needs periodic cleaning to prevent clogging [35]	High—requires irrigation, weeding, and structural maintenance [37]	Moderate—needs sediment removal and inspection [37]	Moderate—requires vegetation management and sediment removal [33]
Cost	Low to moderate—depends on size and design [37]	Low [38]	Moderate to high—higher initial cost for materials and installation [35]	High—expensive due to structural and planting requirements [37]	Moderate—costs depend on depth and materials used [37]	Moderate—costs depend on size, vegetation, and engineering specifications [33]
Suitability in Hot Arid Regions	Moderate—limited by low rainfall and high evaporation, but useful for occasional storms [39]	Moderate1—useful for capturing and storing scarce rainwater but limited by low rainfall and high evaporation rate [39]	Moderate—useful for reducing runoff during rare storms but limited by high temperatures [35]	Low to moderate due to high maintenance and water requirements [30]	Low 2—useful for infiltrating occasional heavy rainfall but limited by dry soil conditions [30]	Low—limited by low rainfall and high evaporation but useful for managing occasional runoff [5,39]
Responding to Climate Change Projections	Moderate [39]	Low [39]	Moderate [35]	Moderate [40]	Moderate [41]	Moderate [41]

¹ It is generally high in other climatic zones, but it is moderate in hot arid climates. ² It is generally moderate in other climatic zones, but it is low in hot arid climates.

compares their suitability and applicability in hot arid urban zones. The table shows that infiltration is best achieved by rain gardens, permeable pavements, and infiltration trenches. Retention is best achieved by rain barrels and green roofs. Detention is best achieved by bioswales and infiltration trenches. Regarding the suitability of applying these strategies in hot arid regions, it is noted that permeable pavements are moderately suitable for reducing runoff during storms, but maintenance and high temperatures can be challenging. Rain gardens are also moderately suitable for managing occasional heavy rainfalls, but they are limited by dry soil conditions and high evaporation rates. Green roofs are moderately suitable, but they are limited due to high water requirements and maintenance costs in arid climates. Infiltration trenches are useful for capturing and storing scarce rainwater, but they are limited by low rainfall and high evaporation rates. Bioswales are limitedly suitable due to low rainfall and high evaporation rates, but they can be useful for managing occasional runoff. Thus, each method has its unique applications and trade-offs in terms of cost, maintenance, and effectiveness for flood management. Additional challenges should be noted when applying them in hot arid zones and when considering climate change projections.

4. The State of the Art of Flood Risk Management in the MENA Region: The Case of Egypt

The MENA region is amongst the most susceptible to floods, particularly in the last thirty years, causing casualties and loss of infrastructure. The region is characterized by arid and semi-arid climates, with limited water resources and high vulnerability to extreme weather events, including flash floods [42]. Its rapid urbanization rate led to unplanned urban expansion, which often encroached on floodplains, disrupting natural water flow and increasing runoff. Accordingly, flood risk management in the MENA region is a complex and multifaceted challenge. This is due to the region’s unique climatic, geographic, and socio-economic conditions. Furthermore, climate models predict increased variability in precipitation patterns, with more frequent and intense rainfall events. This is particularly concerning for urban areas, where impermeable surfaces, inadequate drainage systems, and insufficient flood-resisting urban planning and infrastructure prevail. Thus, flood risk management should be integrated with flood risk assessment for disaster risk reduction. Further, it should consider broader water conservation measures and climate adaptation strategies to achieve global and national sustainable development goals [15,43].

Egypt is considered highly vulnerable to climate change, particularly water scarcity, sea level rise, and extreme weather events [5]. The fact that it is a hot arid country with limited sources of freshwater urges scientists, engineers, and policymakers to develop water conservation measures in combination with flood-risk management strategies [39]. The overall average amount of rainfall that occurs annually throughout all weather stations is 52.5 millimeters, with a range of 9–172 millimeters [44]. Hence, it is not typically classified as a highly flood-prone country; however, certain regions in Egypt are vulnerable to flood events due to specific geographical, climatic, and human factors. This includes the Eastern Desert and Sinai Peninsula, coastal areas, the Nile Delta, and some main cities. Cairo is one of the metropolitan cities suffering rapid urbanization and deficient flood-resisting land use planning and infrastructure. Hence, research efforts are needed in flood control, adaptation, and resilience in urban areas. This is particularly needed in newly developed cities located in an X-year floodplain zone, e.g., New Cairo, Helwan, and Maadi, which were identified as flood-prone areas by the Egyptian Ministry of Irrigation [45].

5. Research Method

The research method is summarized in the following steps shown in Figure 3:

(1)

Hazard risk assessment provided background and justification for the selected study area (South Teseen Street).

(2)

Select the best-fit NBSs based on a weighted-average sum matrix.

(3)

Performance assessment was performed using an online stormwater calculator tool. This step tested and compared the performance of individual NBSs in the study area. Thus, five simulation models were developed, one for each type of selected NBS. Two more simulation models were carried out to compare the performance of the base case scenario (without implementing NBSs) to a proposed scenario (with all selected NBSs combined).

Figure 3. A schematic flow diagram summarizing the research method used.

Figure 3. A schematic flow diagram summarizing the research method used.

5.1. Hazard Risk Assessment

The flood risk assessment process for the Fifth Settlement area evaluated hazards, exposure, and vulnerability, scaling from a city, district, and neighborhood level, as shown in Figure 4.

The Fifth Settlement covers an area of approximately 70 Km². It includes residential neighborhoods, commercial zones, educational institutions, and green spaces. The area’s total population is estimated to be around 300,000 people (as of recent estimates [46]), with a population density of ≈3570–4285 people per Km². The study area includes 20,000–30,000 residential buildings and 200–400 service buildings (schools, hospitals, mosques, churches, shopping malls, and government offices) with a total sum of 25,000 buildings.

The flood hazard analysis identified and characterized potential flood sources and their associated magnitudes and frequencies in the study area. Additionally, it pinpointed the urban areas lying in flood depth zones and facing flood hazards. The analysis used hydrological and hydraulic models and considered a 100-year flood event scenario for the Fifth Settlement district in New Cairo. The hydrological data of annual and extreme rainfall records, in addition to the history of storm events and their impacts, determined the flood, frequency, extent, and depth. The average annual precipitation in New Cairo is about 23 mm, and there are 14 rainy days per year, with the least and the greatest captured rainfall volume occurring in May and January, respectively. However, the area is susceptible to unexpected occurrences of heavy precipitation, which may result in flash floods [47]. The extreme storm event ‘Dragon Storm’, hitting the country in 2020, led to flash floods. The water level exceeded one meter and resulted in casualties and major destruction to infrastructures and private properties. A topographic map showing digital elevation models indicated the lowest altitude level where stormwater runoff was most likely to occur. The lowest zone was towards the northwest with an altitude of 251 m, and the highest zone was towards the southeast with an altitude of 334 m. This step acquired a clearer knowledge of the present circumstances to be able to identify and map flood-prone areas. It was found that 65%, 25%, and 10% of the neighborhood area lay in a high (>1 m), medium (0.5–1 m), and low (<0.5 m) flood depth zone, respectively.

The exposure assessment process considered the number and characteristics of the population and buildings exposed to flood hazards. Hence, land use maps and infrastructure data defined the location of buildings, main roads, and critical facilities. The study area was selected in a densely populated flood-prone zone, missing flood-resisting land use planning and infrastructure. It was estimated that 40%, 40%, and 20% of the buildings were exposed to high, moderate, and low flood risk, respectively.

The vulnerability assessment process evaluated the vulnerable characteristics of the exposed population and buildings to flood risk. Additionally, it examined the capacity of the urban system to withstand and recover from flood events. The area included residential buildings and critical infrastructure, including hospitals, health care centers, religious buildings, and administrative and commercial buildings. It is proximate to a main road (South Teseen), which connects the city center to newly developed areas. The area included the lowest altitude level where stormwater runoff was most likely to occur, in addition to its history of recent flash floods in 2020. This analysis was further confirmed by field observations, which identified the existing conditions, and urban design features, underscoring the poor preparedness for flood risk. The impermeable surfaces included streets, sidewalks, driveways, parking surfaces, and flat roofs, whereas the permeable surfaces were minimal and included undeveloped lands, lawns, shrubs, and bushes. These factors contributed to identifying this study area as particularly vulnerable to flash floods, with almost 40%, 50%, and 10% of the buildings with high, moderate, and low levels of vulnerability, respectively.

The flood hazard risk assessment process identified and categorized urban areas into high, medium, and low flood risk zones. The result indicated that almost 2000 buildings (distributed in twenty neighborhood areas) face high flood risk. One of these neighborhoods was selected to test the potential of integrating NBSs within its urban design features for flood-resilient planning and disaster preparedness. The area of this neighborhood is 141,000 m², with 72 building units, including 67 residential buildings, each composed of three floors and seven apartments. Thus, the population was estimated to be ~2500 residents. This is in addition to five public and service buildings, including a hospital (Tabarak New Cairo Hospital), a mosque (Farouk Mosque), a trading company, a gym, and a training center, with almost 1000 visitors. The total population for the urban area was estimated to be ~3500. This area is vulnerable due to the capacity of public, service, and residential buildings. It was noted that when flash floods occurred in 2020, they not only affected the study area but they also extended to flood the main neighboring street (South Teseen), which led to high traffic congestion and delayed rescue arrival.

The flood hazard risk assessment process is shown in

Table 2. The flood hazard risk assessment process for the Fifth Settlement area, New Cairo, Egypt.

	High flood depth zones (>1 m)	Medium flood depth zones (0.5–1 m)	Low flood depth zones (<0.5 m)
Flood Hazard Analysis	65% of the buildings face high flood risk	25% of the buildings face medium flood risk	10% of the buildings face low flood risk
Exposure Assessment	40% of the buildings are highly exposed to flood risk	40% of the buildings are moderately exposed to flood risk	20% of the buildings are slightly exposed to flood risk
Vulnerability Assessment	40% of buildings are highly vulnerable	50% of buildings are moderately vulnerable	10% of buildings are slightly vulnerable
Flood Risk Calculation: Risk = Hazard × Vulnerability × Exposure [9]	2080 buildings	1000 buildings	40 buildings
	Total buildings at risk = 2080 (high risk) +1000 (medium risk) +40 (low risk) = 3120 buildings
Interpretations	▪ 3120 buildings are at risk during a 100-year flood event. ▪ High-risk areas are identified for the prioritization of mitigation measures using NBSs. This includes almost 2000 buildings distributed in 20 urban neighborhood areas.

.

5.2. Selection of NBSs

A weighted average sum matrix was performed following the work of previous studies [48] to select the best-fit NBSs. For a matrix A and a weight matrix W, the weighted average sum matrix S can be computed, as shown in Equation (1). It is noted that A is the input matrix, W is the weight matrix, and S is the resulting weighted average sum matrix.

Equation (1) shows the calculation of the weighted average sum for each type of NBS.

$$S_{i,j}=\sum _{k=1}^n \sum _{l=1}^m W_{k,l}·A_{i+k-1,j+l-1}$$

(1)

The parameters selected for the weighted average sum matrix were pinpointed in past studies [8,10,42] as being the most critical for selecting NBSs. These included suitability in hot arid zones, applicability in dense urban areas, their infiltration/retention/detention capacity, cost effectiveness, ease of implementation, and maintenance requirements, as well as their response to climate change projections. An input value of 0, white color (missing), 1, red color (least), 2 yellow color (moderate), and 3, green color (greatest) were assigned for every NBS for each individual parameter to reflect their evaluation score. Their weighted average sum was computed (noting equal weight for all parameters), as shown in

Table 3. A weighted-average sum matrix to select the best-fit NBSs in hot arid urban regions.

	Suitability in Hot Arid Zones	Applicability in Dense Urban Areas	Infiltration Capacity	Retention Capacity	Detention Capacity	Cost Effectiveness	Ease of Implementation	Maintenance Requirements	Responding to Climate Change Projections	Average
Green roofs	2	3	1	2	1	1	1	1	2	2
Rain gardens	2	2	2	1	1	3	2	2	2	2
Permeable paving	2	2	2	1	2	2	2	2	1	2
Bioswales	1	1	1	1	2	2	2	2	2	2
Native vegetation	3	2	2	1	1	3	3	3	2	2
Infiltration trenches	1	1	2	1	2	2	1	2	2	2
Rain barrels	1	1	0	3	0	2	1	3	2	1
Wetlands	1	1	2	1	1	2	1	2	2	1

0—white color (missing), 1—red color (least), 2—yellow color (moderate), 3—green color (greatest).

. The selected NBSs received a weighted average score of greater than or equal to 2 and were excluded from this study if less than 2. Thus, the selected types of NBSs were native vegetation, green roofs, rain gardens, permeable paving, and bioswales, whilst rain barrels and wetlands were excluded.

5.3. Performance Assessment

Site contextual and climatic data were input into the Green Values Stormwater Management Calculator, available on https://cnt.org/tools/green-values-calculator (accessed on 20 February 2025), to generate the typical yearly rainfall distribution [49]. This tool is a free online tool developed by the Center for Neighborhood Technology in the United States to guide practitioners to deliver innovative analysis for equitable, sustainable, and resilient solutions. The data were used to compute effective runoff throughout a year (23 mm water runoff) and during a storm event. Thus, a volume capture goal of 5911.6 m³ was determined based on the extreme scenario of the most recent storm in Cairo, ‘The Dragon Storm’ (42 mm water runoff). The volume reduction capture goal for each NBS was computed and compared to the defined volume capture goal. Five simulation models were run, testing the performance of the selected type of NBS individually. The comparison helped determine the most effective type of NBS considering the local contextual and climatic conditions. Two additional simulation models were performed, comparing the base case scenario (X) with no NBSs implemented and the proposed scenario (Y) with the implementation of the five selected NBSs combined. This step implied replacing some of the impermeable surfaces with alternative NBSs. Thus, some urban design features were modified for the proposed scenario, assuming the implementation of rain gardens on sidewalks and bioswales on roadsides, dedicating half the area of flat roofs for green roofs, using native vegetation in undeveloped lands as urban forests, and using permeable paving for all street surfaces. Figure 5 describes the input data for each NBS and the output stormwater runoff for the average annual rainfall during extreme storm events.

6. Results

The research provided practical steps for flood risk assessment and management using NBSs. The research method included flood hazard risk assessment, selecting the best-fit types of NBSs, and comparing the performance assessment of the selected NBSs for stormwater runoff reduction and volume capture goals.

This study adopted a microscale analysis focusing on a small-sized, densely populated neighborhood area with poor flood-resisting urban design and infrastructure. Hazard risk assessment considered the physical and social aspects of the study area. This process evaluated the expected hazards, exposure, and vulnerability based on the local conditions, topography, prevailing climate, storm events, and current urban design features.

A weighted average sum matrix was performed to shortlist suitable types of NBSs to the local context and climate conditions. The selected types of NBSs were native vegetation, green roofs, rain gardens, permeable paving, and bioswales. These systems are designed to enhance water infiltration, storage, and evapotranspiration, thereby reducing surface runoff and flood risk. Nevertheless, the selected types of NBSs vary in their mitigating nature and capacity for flood risk, including infiltration, retention, and detention. They also vary in their applicability in urban hot arid zones and their preparedness for climate change projections. Thus, performance efficiency simulation models were carried out to evaluate and compare the capacity of the selected NBSs for stormwater runoff reduction and achieve the volume capture goal.

Five simulation models shown in Figure 6a assessed the efficiency of applying each type of NBS individually. Thus, rain gardens placed on sidewalks achieved the highest percentage of 43.6%. Bioswales and permeable paving achieved a volume capture goal of 11% and 10.7%, respectively.

Another model tested the efficiency of applying the selected NBS in combination, as shown in Figure 6b. It was noted that permeable paving was the most effective type in volume capturing. Implementing native vegetation in urban forests in undeveloped lands was the third most effective technique. The result of the simulation model indicates that some NBSs perform better when combined than when implemented separately, and this requires a comprehensive understanding of the contextual and climatic conditions for proper NBS selection.

A comparison was carried out between Scenario X, the base case scenario with no NBS implemented, and the proposed Scenario Y, which included a combination of the selected NBSs, replacing some of the existing impermeable surface areas, as shown in Figure 7a. This significantly reduced the percentage of impermeable surface areas in the proposed scenario from 80% to 32%. The average annual rainfall decreased from 3.75 in Scenario X to 0.79 mm in Scenario Y, as shown in Figure 7b. In the case of a storm event, the runoff decreased from 22.86 mm in Scenario X to 7.34 mm in Scenario Y. Thus, compared to the stormwater volume capture goal, Scenario Y was able to capture 8% more than the required goal using a combination of the selected NBSs over 54.1% of the total site area.

The land use change for the base case and proposed scenarios and their volume capture capacity are described in

Table 4. The land use changes and volume capture capacity of the base case and proposed scenarios.

	Base Case Scenario (X): No NBSs		Proposed Scenario (Y): Combinations of NBSs Implemented		Volume Captured by NBSs
Land use area	m2	%	m2	%	m3
Total lot area	141,055	100
Impermeable areas:
Flat roofs	25,354	18	12,677	9
Sidewalks	24,088	17	16,380	12
Parking lots	1902	1	1902	1
Driveways	14,248	10	14,248	10
Streets	41,451	29	0	0
Total impermeable surface areas	107,043	76	45,207	32
Pervious areas:
Lawn/turf	1290	1	1290	1
Shrubs and bushes	18,391	13	18,391	13
Undeveloped land	14,331	10	0	0
Proposed NBSs:
Native vegetation	0	0	14,331	10	1019
Green roof	0	0	12,677	9	741
Rain garden	0	0	7708	6	1984
Permeable paving	0	0	41,244	29	2619
Bioswales	0	0	207	0.15	27
Total permeable surface area	34,012	24	95,848	68
Total NBS area	0	0	76,167	54	6390

. It is noted that for this study area, the majority of impermeable surfaces are streets (29%), rooftops (18%), sidewalks (17%), and driveways (10%). Thus, the proposed scenario intended to include urban design modifications to reduce the percentage of impermeable surfaces.

Table 5. The average annual rainfall and extreme storm events for the base case and proposed scenarios.

	Base Case Scenario (X) (mm)	Proposed Scenario (Y) (mm)	Difference
Average annual rainfall	23 mm
Runoff (mm)	3.75 mm	0.79 mm	79%
Runoff volume (m3)	530.65	111.62	419.04
Storm event rainfall	42 mm
Runoff (mm)	22.86	7.34	68%
Runoff volume (m3)	3224.26	1034.17	2190.09

calculates the stormwater runoff for the base case scenario (X) and proposed scenario (Y) for the average annual rainfall and extreme storm events. The difference in stormwater reduction for the average annual rainfall was 79%. On the other hand, the difference between both scenarios during extreme storm events was less, reaching 68%. Thus, it is noted that while NBSs perform well under moderate rainfall, their effectiveness can be limited during extreme storm events. In general, their efficacy under climate change projections depends on proper design, maintenance, and integration with other flood management strategies.

7. Discussion

This study explored the effectiveness of NBSs as a resilient strategy for mitigating urban flood risks in a developing hot arid country. The research method could be replicated to guide urban planning efforts in other regions facing similar challenges. This requires detailed analyses of local conditions, including topography, land use, and vulnerability, to ensure that selected NBSs are both effective and sustainable. The outcome results match with and build upon several key insights from previous studies on the benefits of using NBSs for flood risk mitigation [15,50]. Researchers have noted that NBSs should be selected based on their ability to address the most pressing risks in a given context [12,42]. Additionally, the integration of NBSs with existing land use planning plays a critical role in their success, and their performance is highly dependent on design specifications, such as storage capacity and vegetation type [16].

The efficiency of the selected types of NBSs should be empirically assessed under average and extreme events while considering climate change projections. Their performance should be tested under varying conditions, individually and in combination, to stand on their context-specific performance dynamics. This explains the superior performance of certain NBSs in this study, owing to their alignment with site-specific conditions and characteristics, as stated in previous studies [8]. For example, while rain gardens generally perform well in reducing flash floods, permeable paving emerged as the most effective solution when tested for this study area. The result underscores the importance of tailoring the selection of NBSs to local conditions. Urban areas with a high percentage of hardscape surfaces benefit most from permeable paving, whereas rain gardens and native vegetation are optimal for undeveloped land. Green roofs are best suited for regions with a high proportion of flat roofs but are less effective during prolonged heavy rainfall due to saturation [51], and bioswales are ideal for areas with extensive sidewalk surfaces [52]. This resonates with the work of [50], who highlighted the importance of tailoring NBSs to local conditions to maximize their effectiveness.

7.1. Linking SDGs and National Development Strategies

The research aligns closely with the National Development Goals (Egypt 2030) [53], several United Nations strategies, and the 2030 Sustainable Development Goals (SDGs) [54]. It contributes to SDG 11: Sustainable Cities and Communities by proposing NBSs to mitigate flood risks. This study supports SDG 13: Climate Action by contributing to climate adaptation strategies using NBSs. The focus on managing stormwater and reducing runoff through NBSs aligns with SDG 6: Clean Water and Sanitation. The integration of native vegetation and green infrastructure supports SDG 15: Life on Land by promoting the sustainable use of terrestrial ecosystems, biodiversity, and ecosystem services. The research also supports the Sendai Framework’s priorities by enhancing the understanding of disaster risks and promoting eco-centric measures, like NBSs, to reduce vulnerability and build resilience [8].

Additionally, the findings align with the growing recognition of the underrepresentation of developing countries in NBS research, as noted by [42]. These countries, despite being highly vulnerable to climate change, often lack the resources and data necessary to implement and monitor NBSs effectively. By focusing on a case study in a vulnerable zone, this research contributes to filling this gap [55]. While this research aligns with previous studies in many respects, it also advances the field by providing a more structured and quantifiable approach to evaluating the efficiency of NBSs in developing countries.

7.2. Study Limitations

The research focus was oriented towards assessing the efficiency of NBSs in hot arid regions; hence, the contextual and climatic conditions affect the selection of different types of NBSs. Also, the results of the performance assessment tests reflect their efficiency under site-specific conditions.

A weighted-average sum matrix with equally defined parameters was used for this purpose. Altering this list of parameters or changing their assigned weights can affect the selection process. For example, the environmental co-benefits of NBSs, such as enhancing biodiversity, improving urban cooling, and contributing to broader ecological goals, could be added to the matrix for a comprehensive understanding of their environmental impact.

The unreliability of climate change projections is also noted. This is due to data limitations for flood hazard maps and the limited availability of accurate hydrological and meteorological data. The stormwater volume capture goal was based on the recent storm event in 2020; nevertheless, identifying other target values shall affect the efficiency of the NBSs tested, and, accordingly, the reliability of the flood risk assessment process.

Despite being free and simple to use, the online stormwater calculator tool offers little options for NBS design. The tool deals with areas and volumes; thus, it is useful in preliminary decision phases when data are scarce and financial resources are limited. Nevertheless, a modeled simulation with urban design details and specifications is demanded during the design, construction, and monitoring phases. The model lacks integrating automated flood dynamics data and climate change projections; however, this calculation can be performed manually based on the latest flood event data and recent environmental studies.

To enhance the depth and breadth of this research, urban flood risk assessments should be integrated into broader multi-hazard risk frameworks. The model should address interconnected risks and climate projections, as stated by [12,56]. It may also be combined with a wider scope of environmental assessment, including Strategic Environmental Assessment and Value Engineering [57,58].

8. Conclusions

Developing countries remain underrepresented in the literature concerning flood risk assessment and management, even though they are greatly vulnerable to climate change impacts. This is of utmost importance in flood-prone urban areas characterized by impermeable surfaces and poor flood-resisting urban design and infrastructure. Thus, the research provides practical steps for urban flood risk mitigation using NBSs in a developing hot arid country, the case of South Teseen Street in the Fifth Settlement, New Cairo, Egypt. It highlights that the selection of NBSs should consider site-specific conditions, including the site area, topography, vulnerability and exposure level, climate, urbanization level, and land use and urban design features. This should also consider the percentage and type of impermeable surfaces.

The research method carried out hazard risk assessment, including hazard, exposure, and vulnerability valuation, to identify vulnerable locations in a small-scale urban neighborhood. A weighted average sum matrix was used to select the best-fit NBSs based on their suitability in hot arid zones, applicability in dense urban areas, infiltration/retention/detention capacity, cost effectiveness, ease of implementation, and maintenance requirements, as well as their response to climate change projections. Thus, the selected NBSs were native vegetation, green roofs, rain gardens, permeable paving, and bioswales.

Simulation models tested the efficiency of the selected NBSs to mitigate flood risk using an online tool for stormwater calculator. The numerical model obtained helped assess the efficiency of the selected NBSs for stormwater runoff volume reduction and the percentage of volume capture goal. Five simulation models were run to test the efficiency of each NBS individually. Thus, rain gardens placed on sidewalks achieved the highest percentage of 43.6%. Bioswales and permeable paving were two replacements for impermeable street surfaces with a volume capture of 11% and 10.7%, respectively. Another model was carried out to evaluate the performance efficiency of all NBSs when applied in combination. In this case, permeable paving demonstrated the highest effectiveness in volume capture. This is because the infiltration capacity of rain gardens and permeable pavements directly addresses the challenges posed by highly impervious dense urban areas in reducing stormwater runoff.

The result indicated that applying combined measures of NBSs over 54.1% of the total site area was able to capture 8% more than the required volume capture goal. This shows that there is an inherent dynamic for NBSs, which may cause them to perform better in stormwater management when combined rather than when implemented individually for the same study area. Also, the difference in stormwater reduction in the base case and proposed scenarios for the average annual rainfall was 79%, while the difference between both scenarios during extreme storm events was 68%. Thus, it is noted that while NBSs perform well under moderate rainfall, their effectiveness can be limited during extreme storm events.

The findings have important policy implications for urban planning, emphasizing the need for tailored solutions and integrated approaches using NBSs for flood risk mitigation. In this regard, land use planning should regulate development in flood-prone areas and promote sustainable land management practices. This study also pinpointed the necessity of testing the performance of NBSs under site-specific conditions and future climate projections. This is because their efficiency is not absolute but contingent on implementation, management, and maintenance scenarios.

Future research should explore the potential of developing a hybrid approach to flood management. Such an approach could combine the strengths of structural and non-structural measures, offering a more robust and adaptable solution under climate change projections. Also, an economic evaluation of the capital and operational costs of NBSs is essential to provide a balanced perspective on their financial viability. Addressing potential obstacles to implementation, such as ongoing maintenance costs, land use conflicts, and engineering specifications, is crucial for the successful adoption of these solutions. To conclude, the findings advocate for increased investment in NBS research and knowledge exchange, particularly in developing countries.