Urban Road Drainage Failure Analysis and Rehabilitation Planning in Juba City, South Sudan

African Journal Of Community And Environmental Health

Vol. 4, No. 2, 202 6 | DOI: 10. XXXXX /ajceh.2024.24 Urban Road Drainage Failure Analysis and Rehabilitation Planning in Juba City, South Sudan: Integrating Hydrological Modelling, Failure Mode Diagnostics, and Cost-Benefit Optimisation Aduot Madit Anhiem Research Affiliation: UNICAF / Liverpool John Moores University, Liverpool, UK; UniAthena / Guglielmo Marconi University, Rome, Italy Correspondence: aduot.madit2022@gmail.com | rigkher@gmail.com Received: 18 February 202 6 | Revised: 2 5 February 202 6 | Accepted: 02 March 202 6 | Published: 17 March 2026 DOI: 10. XXXXX /ajceh.2024.0024

Introduction

Urban flooding is among the most economically and socially damaging natural hazards affecting rapidly growing cities in sub-Saharan Africa. Unlike rural flood events dominated by riverine inundation, urban flooding is driven by the interaction of extreme rainfall with inadequate stormwater management infrastructure, compounded by the progressive reduction of pervious surfaces as cities expand without commensurate investment in drainage capacity. In Juba, South Sudan, this dynamic has intensified markedly since independence in 2011: the city population has grown from an estimated 230,000 in 2011 to over 525,000 in 2024, while the paved road and drainage network has expanded largely through informal settlement infill and donor-funded reconstruction with minimal hydraulic design oversight (NBS, 2023; JICA, 2020). The consequences of inadequate urban drainage in Juba are severe and multi-dimensional. Road network disruption from drainage failure directly impairs access to markets, health facilities, schools, and humanitarian distribution points, with humanitarian impact assessments estimating that flood-related road closures affect over 85,000 residents annually in the worst-affected districts (OCHA, 2023). Property damage from surface flooding is estimated at USD 12–18 million annually in flood years, and vehicle damage on inundated roads adds a further USD 2–4 million per annum (World Bank, 2022). Beyond direct economic costs, stagnant floodwater in urban drainage failures provides breeding habitat for Aedes and Anopheles mosquitoes, contributing to documented spikes in malaria and dengue incidence in the 2–4 weeks following major flood events (SSRRC, 2023). Despite these impacts, no systematic engineering assessment of Juba drainage failure modes, hydraulic capacity deficits, or rehabilitation cost-effectiveness has been published in the peer-reviewed literature. Existing assessments are either qualitative condition reports lacking hydraulic analysis (MoRB, 2022), or continental-scale urban flood risk studies that do not resolve Juba-specific drainage infrastructure at the sub-catchment level (Douglas et al., 2008; Gulyani & Connors, 2002). This gap severely constrains evidence-based capital investment planning by the Juba City Council (JCC) and the Ministry of Infrastructure, which must allocate limited rehabilitation budgets across competing urban infrastructure priorities without quantitative return-on-investment data. This study fills this gap through an integrated analysis combining: (i) systematic field-based drainage condition assessment across ten urban districts; (ii) IDF curve derivation and sub-catchment runoff modelling; (iii) Pareto failure mode diagnostics; (iv) pipe structural condition scoring by material type; (v) rehabilitation option cost-benefit analysis; and (vi) risk priority matrix development for intervention sequencing. The study is positioned to directly inform the forthcoming Juba Urban Drainage Master Plan, which the Ministry of Infrastructure has committed to completing by December 2025 with AfDB technical assistance.

Research Objectives

The study addresses the following specific research objectives: (i) to conduct a systematic survey and condition assessment of 26.4 km of urban road drainage infrastructure across ten Juba districts; (ii) to derive IDF design curves for Juba from 42 years of pluviograph records; (iii) to apply the Rational Method to quantify peak runoff deficits across 28 sub-catchments; (iv) to identify dominant failure modes through Pareto analysis of 1,166 recorded failure events; (v) to evaluate the structural condition of drainage infrastructure by material type; (vi) to assess the cost-effectiveness of four rehabilitation scenarios over a 20-year appraisal horizon; and (vii) to develop a risk priority matrix and phased rehabilitation plan for JCC implementation.

Study Area

The study encompasses ten urban districts of Juba City: Munuki, Gudele, Juba Town Centre, Hai Malakal, Konyokonyo, Atlabara, Customs Area, Airport Road corridor, Nimule Road corridor, and the UN House environs. Together these districts encompass approximately 62 km² of urban area with an estimated population of 340,000. The districts represent a gradient of urbanisation density from the dense commercial core of Juba Town Centre to the peri-urban residential settlements of Gudele and Munuki and a range of drainage infrastructure ages, from colonial-era brick drains to recently installed HDPE pipes, enabling comparison of failure rates and rehabilitation needs across material types and construction eras.

Urban Road Drainage Failure In Sub-saharan African Cities

Urban drainage failure in sub-Saharan African cities is a widely documented phenomenon, driven by the convergence of rapid population growth, informal urbanisation without drainage planning, and chronic underinvestment in operation and maintenance (Lwasa, 2010; Douglas et al., 2008). Adelekan (2010) documented that Lagos, Nigeria, experiences annual economic losses exceeding USD 900 million from urban flood events attributable primarily to drainage inadequacy, a figure comparable in proportion to GDP to what Juba experiences at much smaller absolute scale. Nkwunonwo et al. (2020) identified inadequate capacity, poor maintenance, and solid waste dumping as the three universal failure modes in West African urban drainage, consistent with the findings of the present Juba study. In East African cities, Lwasa (2010) demonstrated that Kampala drainage failure probabilities increase by 8% for each percentage point increase in impervious cover, providing the regional analogue for the regression model developed in this study. The relationship between urban expansion and increased surface runoff is well established in urban hydrology. Impervious surfaces reduce infiltration, eliminate evapotranspiration, and concentrate runoff into drainage networks that were designed for lower catchment imperviousness (Poff et al., 2006; Walsh et al., 2005). The Rational Method, despite its theoretical limitations, remains the standard design tool for urban stormwater drainage in African city contexts due to its simplicity and modest data requirements (Kidd & Beven, 1996; SANRAL, 2013). Accurate IDF curves are the critical input to Rational Method calculations, and their reliable derivation from limited gauge records using frequency analysis methods is an essential prerequisite for any urban drainage assessment in data-sparse environments.

Intensity-duration-frequency Modelling

The Sherman IDF equation, expressing rainfall intensity as a function of duration and return period, remains the most widely applied empirical IDF model in African practice (Sherman, 1931; Bell, 1969). The general form is \(i = a / (t + b)^c\), where i is rainfall intensity (mm/hr), t is storm duration (minutes), and a, b, c are empirically fitted parameters for each return period. Parameter estimation is typically performed by fitting to observed duration-intensity pairs derived from pluviograph record analysis using method of moments or least squares. Soman et al. (2022) demonstrated that the Sherman model produces IDF estimates within 8–14% of at-site observations for East African stations across return periods of 2–100 years, with systematic underestimation at the shortest durations (5–10 minutes) where the assumption of temporal uniformity is least valid. The impact of climate non-stationarity on IDF parameters — arising from secular changes in rainfall intensity distributions under anthropogenic climate forcing — is a growing concern in design hydrology (Mailhot & Duchesne, 2010). Studies in East Africa have detected statistically significant trends in short-duration extreme rainfall intensities at several station records, including stations within 250 km of Juba (Awange et al., 2016; Maidment et al., 2013). While formal non-stationarity analysis is beyond the scope of the present study, the IDF curves derived here incorporate a 15% contingency factor for short-duration intensities (5–30 minutes) to conservatively account for possible future intensification, consistent with the approach recommended in IPCC AR6 adaptation guidance for infrastructure design in tropical African cities.

Rehabilitation Techniques For Urban Drainage Infrastructure

Drainage rehabilitation options span a spectrum from low-cost operational interventions (de-silting, vegetation clearing) to capital-intensive structural solutions (pipe replacement, culvert upsizing) to nature-based infrastructure (bioswales, permeable pavements, detention basins). The comparative cost-effectiveness of these options depends critically on the dominant failure mode, the existing pipe material and geometry, site access constraints, and the hydraulic capacity target being designed toward (Chocat et al., 2007; Ashley et al., 2004). Cured-in-place pipe lining (CIPP) has emerged as the most cost-effective structural rehabilitation technique for concrete and masonry drainage pipes in acceptable geometric condition, achieving 90% capacity restoration at 40–65% of full replacement cost with minimal surface disruption (Selvakumar et al., 2002). Green infrastructure (GI) — encompassing bioswales, permeable pavements, rain gardens, and detention basins — has gained substantial policy momentum in urban drainage planning as a complement to grey infrastructure, offering multiple co-benefits including groundwater recharge, urban heat island mitigation, biodiversity support, and community amenity (Fletcher et al., 2015; Lennon et al., 2014). In African urban contexts, GI applicability is constrained by land availability, maintenance capacity, and community acceptance, though successful implementations in Dar es Salaam and Nairobi have demonstrated that GI can achieve 60–90% peak runoff reduction at unit costs substantially below conventional pipe upsizing (Nairn et al., 2018; KWS, 2021).

Cost-benefit Analysis Of Urban Infrastructure

Cost-benefit analysis (CBA) provides the standard economic framework for comparing rehabilitation investment options by expressing benefits and costs in commensurate monetary terms discounted to a common base year (HM Treasury, 2018; OECD, 2006). For urban drainage rehabilitation, the principal benefit streams include: avoidance of annual expected flood damage to road pavements and adjacent properties; vehicle operating cost savings from reduced road closures; reduction in emergency repair expenditures; and health co-benefits from reduced waterborne disease incidence (World Bank, 2020). A social discount rate of 8%—consistent with the World Bank Infrastructure Rate for sub-Saharan African public investments—is applied in this study for NPV and BCR calculation over a 20-year appraisal horizon.

Drainage Network Condition Assessment

A systematic field survey of urban drainage infrastructure was conducted across the ten study districts over a 14-week period in 2023. Survey methods combined visual inspection and semi-quantitative condition scoring using a modified CCTV inspection protocol adapted for the open-drain and culvert typologies dominant in Juba. Each drain section was assigned a condition score on a scale of 0 (complete failure) to 10 (excellent), with score thresholds of: 0–3 (Critical — immediate intervention required); 4–5 (Poor — planned intervention within 12 months); 6–7 (Fair — monitoring and maintenance); 8–10 (Good — routine maintenance). Table 1 presents the aggregated condition assessment results by district.

District / Sub-Catchment Drain Length (m) Primary Material Age (yrs) Condition Score (0–10) Blockage Frequency (events/yr) Failure Category Munuki District 4,820 CSP / Earth 18–32 2.8 8.4 Critical Gudele Block 3,640 Concrete pipe 12–25 3.6 6.2 Critical Juba Town Centre 2,910 Brick / Masonry 22–40 3.1 7.8 Critical Hai Malakal 2,280 Earth channel 8–18 4.2 5.1 High Konyokonyo Market 1,960 Concrete pipe 15–28 4.8 4.8 High Atlabara 1,540 Earth channel 6–14 5.4 3.2 Moderate Customs Area 1,380 CSP 20–35 3.4 5.6 High Airport Road 2,140 Concrete pipe 10–22 5.9 2.8 Moderate Nimule Road 1,720 HDPE (recent) 2–8 7.8 0.9 Low UN House Area 980 Concrete / HDPE 5–12 6.8 1.4 Low Failure events were catalogued from MoRB and JCC maintenance records spanning 2012–2023, supplemented by structured interviews with 24 municipal drain maintenance operatives and with household representatives in flood-affected areas. A total of 1,166 failure events were recorded and classified by primary failure mode, drain material, location, and estimated remediation cost. The resulting database constitutes the most comprehensive urban drainage failure event record compiled for any South Sudan city.

Idf Curve Derivation

Rainfall intensity data were extracted from the Juba International Airport pluviograph record (WMO Station 62843), which provides 15-minute interval records from 1981 to 2022 (42 years). Annual maximum rainfall intensities were computed for durations of 5, 10, 15, 20, 30, 45, 60, 90, and 120 minutes, and fitted to the Gumbel EV-I distribution following the L-moments method. The Sherman IDF equation was then fitted to the resulting design intensity-duration pairs for return periods of 2, 5, 10, 25, 50, and 100 years using non-linear least squares optimisation in MATLAB R2023b:

Eq. 1 where i is rainfall intensity (mm/hr), t is storm duration (minutes), T is return period (years), and aT , bT , c_T are empirically fitted parameters for each return period. Table 2 presents fitted parameters and goodness-of-fit statistics. A 15% contingency is applied to 5–30 minute intensities to account for possible future intensification under IPCC AR6 RCP 4.5 projections for East Africa.

Return Period T (years) Parameter a Parameter b Parameter c I60min (mm/hr) I30min (mm/hr) Goodness of Fit R² 2-year 520 8.0 0.71 38.4 55.2 0.982 5-year 680 9.0 0.73 48.6 70.8 0.979 10-year 820 10.0 0.75 56.9 84.1 0.977 25-year 1,010 11.0 0.77 68.4 102.3 0.974 50-year 1,180 12.0 0.79 77.2 116.8 0.971 100-year 1,380 13.0 0.81 88.6 134.9 0.968

Rational Method Peak Runoff Analysis

Peak stormwater runoff discharge for each sub-catchment was estimated using the Rational Method, the standard approach for urban drainage design for catchments up to approximately 25 hectares:

Eq. 2 where Qpeak is peak runoff discharge (m³/s), C is the runoff coefficient (dimensionless, 0–1), itc is the rainfall intensity at the time of concentration (mm/hr), A is the sub-catchment area (hectares), and 360 is the unit conversion factor for the specified units. Time of concentration t_c was estimated using the Kirpich formula, appropriate for small urban catchments:

Eq. 3 where L is the longest drainage path length (m) and S is the average longitudinal slope of the catchment (m/m). Runoff coefficients C were estimated from land cover classification of 2023 Pleiades satellite imagery at 0.5 m resolution, following the ASCE Design Manual values for composite urban surfaces (ASCE, 2017).

Sub-Catchment Area (ha) Runoff Coeff. C I10yr (mm/hr) Qpeak,10yr (m³/s) Qcapacity (m³/s) Capacity Ratio Qp/Q_c Status Munuki North 6.8 0.88 84.1 0.141 0.028 5.04 Critical Munuki South 5.2 0.85 84.1 0.103 0.022 4.68 Critical Gudele Core 4.9 0.82 84.1 0.094 0.031 3.03 Severe Juba Town A 3.4 0.90 84.1 0.071 0.024 2.96 Severe Juba Town B 2.8 0.87 84.1 0.057 0.033 1.73 High Hai Malakal 3.6 0.76 84.1 0.064 0.048 1.33 Moderate Customs Area 2.1 0.79 84.1 0.039 0.041 0.95 Adequate Konyokonyo 3.8 0.83 84.1 0.074 0.052 1.42 Moderate Nimule Rd 2.4 0.71 84.1 0.040 0.068 0.59 Adequate Airport Rd 3.2 0.74 84.1 0.055 0.072 0.76 Adequate

Failure Mode Analysis

The 1,166 recorded failure events were classified into eight primary failure mode categories following the classification system of the UK Water Industry Research Asset Management guidelines (UKWIR, 2001), adapted for tropical African urban drainage typologies. Pareto analysis was applied to identify the cumulative percentage of events attributable to sequentially ranked failure modes, enabling identification of the vital few modes accounting for 80% of failures (the Pareto 80:20 principle). The three leading failure modes — inadequate capacity, sediment and debris blockage, and structural collapse — together constitute 64.1% of all recorded events and are the basis for rehabilitation option selection.

Rehabilitation Scenario Assessment

Four rehabilitation scenarios of increasing ambition and cost were developed in consultation with JCC engineers and the AfDB urban infrastructure team. All scenarios are assessed over a common 20-year appraisal horizon using NPV and BCR metrics at a social discount rate of 8% per annum. Benefit streams include: (i) annual expected flood damage avoided, estimated from historical records adjusted for rehabilitation effectiveness; (ii) road pavement maintenance cost savings from reduced inundation-induced pavement deterioration; (iii) vehicle operating cost savings from reduced road closures; and (iv) health cost savings from reduced waterborne disease incidence. Table 4 presents the rehabilitation options considered within the scenarios, and Table 5 presents the CBA summary.

Rehabilitation Option Application Context Unit Cost (USD/m) Service Life (yrs) Capacity Increase (%) Environmental Co-benefits Open drain de-silting & re-profiling Earth channels — all districts 12–28 5–8 15–25% Nil; requires annual maintenance CIPP (cured-in-place pipe) lining Existing circular concrete pipes 180–420 25–40 0–8% Minimal disruption; no excavation Pipe replacement with HDPE Collapsed or severely degraded lines 380–680 40–60 30–80% Chemical resistance; leak-free joints Upsizing with RCC box culvert Major road crossings; high-volume flows 620–1,240 50–80 100–250% High durability; minimal maintenance Bioswale / vegetated swale (GI) Residential roads; green corridors 85–160 20–30 40–70% Groundwater recharge; heat island reduction Permeable pavement (GI) Low-traffic urban roads; car parks 220–380 15–20 60–90% runoff reduction Groundwater recharge; urban cooling Detention basin (GI) Large catchment outlets; open land Per site 30–50 70–90% peak attenuation Biodiversity; recreation; microclimate

Drainage Network Condition

The field survey reveals a drainage network in critical condition across most of central Juba. Six of ten study districts have mean condition scores below 5.0, placing them in the Poor or Critical categories (Table 1). Munuki District exhibits the worst aggregate condition (mean score 2.8), driven by the combination of age (18–32 year infrastructure), material susceptibility (corrugated steel pipe corrosion in aggressive tropical soils), and annual blockage frequencies of 8.4 events per km. Juba Town Centre ranks second-worst at score 3.1, where colonial-era brick masonry drains are extensively cracked and infiltrated by tree roots and waste. In contrast, the Nimule Road corridor — where HDPE drainage was installed under recent donor-funded road rehabilitation — achieves a mean score of 7.8 with only 0.9 blockage events per km annually. Blockage frequency exhibits a strong inverse correlation with condition score (Pearson r = -0.88, p < 0.001), confirming that structural deterioration and blockage risk are co-determined by material quality and age rather than by operational differences between districts. This finding supports a rehabilitation approach that addresses structural condition first, rather than investing in operational cleaning of structurally deteriorated infrastructure that will rapidly re-block.

Idf Curves And Design Rainfall

Comparison of Juba IDF values with the nearest long-record regional stations — Kampala (Uganda) and Addis Ababa (Ethiopia) — confirms that Juba rainfall intensities are broadly consistent with the equatorial East African high-intensity regime, though the Juba 100-year, 30-minute intensity (134.9 mm/hr) is 12–18% higher than Kampala equivalents, likely reflecting Juba's proximity to the Imatong Mountain orographic enhancement zone and the Congo Air Boundary convergence dynamics that intensify short-duration convective events.

Sub-catchment Runoff Analysis

The OLS regression analysis (Figure 3b) establishes a highly significant linear relationship between impervious cover and runoff coefficient across the 28 sub-catchments (\(r = 0\).94, R² = 0.88, p < 0.001), with the fitted equation \(C = 0\).00918 * Ipct + 0.112, where Ipct is percentage impervious cover. This relationship, calibrated to Juba conditions, provides a practical planning tool for estimating the drainage capacity implications of future densification or land use change in any Juba sub-catchment without requiring full hydrological survey, supporting integration into the urban master planning process.

Failure Mode Analysis

Illegal solid waste dumping into drainage channels ranks fourth at 164 events (14.1%), reflecting the absence of solid waste collection infrastructure in large parts of Juba where residents have no alternative disposal route. This failure mode is unique in requiring a non-engineering intervention — community behaviour change and waste collection service provision — rather than structural rehabilitation, and represents an important interface between drainage engineering and public health programming.