This paper evaluates riparian buffer zone violations and examine its consequences on the building-level flood

vulnerability along River Ngadda in Maiduguri, northeastern Nigeria. Geographic Information Systems (GIS)

was used to integrate high-resolution satellite images and building footprint data in order to determine structures

buildings in the 30m buffer, of which 716 were in the most vulnerable 0-10m area. The VI values were 1.73-

1.90 which showed moderate to high flood risk, and central urban segments were most vulnerable.

The spatial analysis revealed mostly negative autocorrelation indicating fragmented and not clustered

vulnerability likely reflecting incoherent informal encroachment. Comparative analysis of distance-based

vulnerability and building density showed complementary data, whereby single-proxy measures could be a poor

fitting characterization of risk. The results indicate that the river could have been less effective in flood

attenuation capacity due to weak setback enforcement which was worsened by the excessive solid waste disposal

which diminished the conveyance of the channels. The management suggestions are based on spatially oriented

intervention: phased relocation of highest-risk structures in central segments, prohibition of the emergence of

new communities, clearing of channels and waste management plans, and institutional reinforcement. Preventive

significant growth in the frequency of flood disasters in the past several decades (Manandhar et al., 2023;

Nkwunonwo et al., 2016). It is estimated that urban centers in the area will increase three times by 2050, which

will only increase the burden on the environment and flood risks (UN-Habitat, 2020; Guneralp et al., 2017).

Nigeria, the most populous country in Africa, has experienced devastating floods that have claimed thousands

of lives, displaced thousands of people, and destroyed billions of nairas of infrastructure (Umar & Gray, 2023).

Maiduguri becomes one of the most flood-prone cities in northeastern Nigeria to floods, where the River Ngadda

serves as the main city drainage channel (Uba, 2024; Obroh & Sambo, 2022).

River Ngadda, a tributary of the Lake Chad Basin, traverses the middle of Maiduguri and has traditionally been

a source of important ecosystem services, such as supplying water, flood management, groundwater

replenishment, and nutrient cycling (Sanyaolu, et al., 2025). However, processes of systematic intrusion of its

exacerbated by extensive riparian buffer encroachment and reduced channel conveyance along River Ngadda.

Riparian buffer zones (setbacks) have various environmental and hydrological roles, which are important to

flood control: (a) storage of temporary floodwater; (b) attenuation of surface runoff; (c) filtration of sediment,

and (d) conservation of aquatic life (Graziano et al., 2022; Majumdar and Avishek, 2025). The typical effective

buffer widths in the world are 30 to 100 m in accordance with the order of streams, slope, and protection goals

(Graziano et al., 2022; Stoffyn-Egli and Duinker, 2013; Sweeney and Newbold, 2014). Buffer width

recommendations are generally narrower in African contexts, with South Africa suggesting 32 m in non-

perennial rivers, and 100 m in perennial rivers (DWAF, 2005), whereas Ghana suggests 10 to 90 m depending

on the order of the streams, the slope, and the danger of pollution (Ministry of Lands and Natural Resources,

2013).

flood exposure along riverine platforms is spatially clustered or dispersed- knowledge gaps that this study intends

to fill along the River Ngadda. Further, there is no research study that specifically evaluated the riparian corridor

of River Ngadda, even though Maiduguri was affected by catastrophic flooding on a frequent basis. This paper

utilizes graduated buffer of 10 m, 20 m, and 30 m to determine the level of encroachment and vulnerability of

the river to flood along River Ngadda. This range represents: (1) the minimum statutory setback of 10m by the

Nigerian Federal Water Resources Act and the minimum statutory riparian setback of about 10 to 16m in

Maiduguri (Borno State Geographic Information System) in forming the inner buffer limit; (2) empirical studies

of Nigerian cities showing that most flood damage is within 30m of the channel in wet seasons (Komolafe et al.,

2015).

The relationships between fast urbanization, riparian encroachment, and exposure to urban flooding are theorized

autocorrelation in order to understand whether encroachment is due to coordinated development or haphazard,

opportunistic expansion. As part of urban resilience models, the ability of cities to absorb, adapt, and recuperate

following flood shocks is critically determined by the integrity of riparian corridors as natural infrastructure

(UN‑Habitat, 2020). The current paper states that the governance of floods in Maiduguri can be characterized

by a compliance gap, which is promoted in the literature concerning risk governance, despite the existence of

statutory setback mechanisms, the institutional means of their enforcement is lacking (Majumdar and Avishek,

2025). It is thus necessary to Embed riparian management within the overall urban resilience measures such as

community-based monitoring and adapting land-use governance to generate a bridge linkage between this gap

and alleviating exposure to buildings to flood risks on the River Ngadda. The study aims to: (1) to quantify the

degree of building encroachment within 10m 20m and 30m riparian buffers along River Ngadda; (2) to compute

segment-specific Vulnerability Indices that can forecast high-risk areas; (3) to examine the patterns of

vulnerability spatial distribution with the aim to inform specific policy interventions.

unimodal rainfall regime, where the rainy season is usually between June and September and highest in August.

During wet years rain can start as early as in April and last until to October. The average annual temperatures

are between 30°C and 40°C with temperatures in the dry-season hitting up to 42°C, and Harmattan temperatures

can drop down to about 15°C. The major urban drainage system that drains Maiduguri is River Ngadda and its

tributary River Ngaddabul (Shettima et al 2025), which comprises the greatest urban drainage system throughout

the city. River Ngadda is the river that is significant in stormwater transportation and flood management. This

research paper concentrated on River Ngadda, which is the major flood-prone channel in Maiduguri.

To capture spatial variation on encroachment and flood exposure by the buffer zone, River Ngadda was further

broken down into 5 segments (A- E) using physical features especially major bridges (Figures. 2 and 3). The

Segment E: Custom Bridge to Moromoro Bridge

buffer zones on a segment-by-segment basis.

acquired by the Microsoft Global Building footprints dataset which offers polygon representations of building

outlines, as the result of machine-learning categorization of high-resolution satellite images. High spatial

coverage, vector format, and a consistent geometry characterize the dataset, and render it appropriate to the

datasets can fail to capture emerging trends or informal networks of various cities that are growing at rapid rates

(Braun, et al., 2023).

In order to overcome these constraints, the dataset was validated and revised by GeoEye-1 satellite imagery

having a spatial resolution of 3.96 cm (Fig. 3). GeoEye imagery has enough spatial resolution to discover discrete

buildings, roof profiles and new developments and is suitable to be manually digitized and refine accurateness

in data-averse urban settings. Visual comparison of the GeoEye imagery showed that 287 structures had been

removed, 143 polygons had been misdigitized, and 89 of the structures were obsolete; the anomalies were later

corrected by manually digitizing in ArcGIS 10.8.2. About 17 per cent of the total layer of buildings required

modification, mainly in informal settlements which were tightly packed together along Segments C and D. A

positional-accuracy test with 50 ground control locations had a root-mean-square error (RMSE) of 1.2m, which

is well within acceptable limits of building-scale analysis of flood vulnerability.Figure 4. depicts the

Methodological workflow.

Table 1: Summary of Datasets and Data Sources

digitization was performed at Full Tank Level (FTL) of the river, which is the farthest point along the river at

high water levels. FTL was determined based on visual analysis of channel morphological indicators such as:

(1) vegetation transition zones which indicate the upper limit of regular inundation, (2) alteration in soil moisture

deposits that could be seen. Even though the data of continuous hydrological gauges were not provided, this

method is equivalent to the existing knowledge on floodplain delineation in data-sparse settings (Flannery et al.,

2017; Wu et al., 2012; Bradbrook et al., 2004). While this visual interpretation method is appropriate for data-

In order to determine the amount of encroachment and flood exposure in the buffer zone, three concentric buffer

areas (10, 30 ,20 m) were created using the Buffer tool of ArcGIS 10.8.2 based on the digitised riverbank at FTL.

Where:

respectively: 0-10m, 10-20m and 20-30m). The performance of Spatial Autocorrelation (Global Moran I) was

done in ArcGIS 10.8.2 with the help of an inverse-distance spatial weights matrix. The inverse-distance

weighting scheme was chosen due to the fact that it is suitable in terms of the hypothesis according to which

spatially close buildings are more likely to have similar levels of vulnerability since they have similar

environmental background and development trends. A threshold of conceptualization distance of 100m was used,

as it was proposed by the average size of informal settlements localities in Maiduguri. The Moran's I values

Table 2: Spatial Distribution Segment-wise Buffer Zone Area, Building Counts, and Density across 10m, 20m,

and 30m Buffer Distances