GIS-Based Hydrologic Analysis Using Remote Sensing Data

1. Introduction

Flooding is among the most destructive recurring hazards across Nigeria and sub-Saharan Africa. Improperly sized culverts, undersized drainage channels, and inadequately designed bridges are among the leading causes of infrastructure failure during extreme rainfall events. Yet, despite the frequency of these failures, many drainage structures in the region are still designed without any formal hydrologic analysis — relying instead on rules of thumb, precedent, or guesswork.

The traditional excuse has been data scarcity. Hydrological analysis requires topographic data, rainfall records, and land use information — none of which were historically easy or cheap to obtain in developing regions. That barrier has largely collapsed. Today, a practitioner with a laptop, an internet connection, and basic GIS literacy can perform a defensible peak-runoff estimation for any catchment in Nigeria using entirely free data and open-source software.

This article walks through that process end-to-end: from identifying a problem site, to acquiring and processing the necessary spatial data, to applying the Rational Formula to estimate design peak discharge. The approach is appropriate for culverts, minor bridges, small drainage channels, and other hydraulic structures draining catchments up to approximately 80 hectares. For larger catchments, we discuss the transition to more advanced methods.

2. Required Tools and Data

One of the most important insights of this approach is that no expensive proprietary software is required. The full workflow can be executed with tools that are either open-source or freely available:

Software

• QGIS (free, open-source) — catchment delineation, DEM processing, land use mapping. The SAGA provider and WhiteboxTools plugin handle all hydrological operations.
• Global Mapper — alternative commercial tool with an intuitive interface for DEM analysis and watershed delineation. A free trial is available.
• ArcGIS / ArcGIS Pro — industry standard; full hydrology toolbox available. Requires a licence.
• Google Earth (Desktop or Web) — land use interpretation for small catchments.
• Microsoft Excel or LibreOffice Calc — for the Rational Formula computations.

Data

3. Site Identification and Problem Definition

The starting point for any hydrologic study is identifying the design point — the location where a hydraulic structure exists or is planned — and understanding the nature of the problem. In the case study, the site is a culvert located near Galadimawa Roundabout in Abuja's Federal Capital City. The culvert receives runoff from a sizeable upstream catchment and has a known history of flooding during moderate to intense rainfall events, causing road overtopping and property damage in the surrounding area.

Site identification in modern practice involves a combination of field reconnaissance and remote sensing review. Before ever visiting a site, a practitioner should examine the following in Google Earth or a satellite imagery viewer:

• The visible drainage network and its relationship to the road/structure
• Upstream land use and impervious cover
• Evidence of erosion channels, gullies, or overbank flow scars
• Any recent land use change (new developments, clearing) that may have altered runoff characteristics

This desk-based review defines the analysis boundary and informs the selection of a pour point for catchment delineation — the specific outlet location on the DEM from which the upstream drainage area will be traced.

4. Catchment Delineation Using the DEM

Catchment delineation is the process of identifying all land area that drains to the design point. In a GIS environment, this is performed entirely from a Digital Elevation Model using the following sequence:

1. Download and clip the DEM Download the SRTM 1-arc-second (30m) DEM tiles covering the study area from USGS EarthExplorer (earthexplorer.usgs.gov) or the Copernicus DEM from OpenTopography. Clip to the approximate area of interest to reduce processing time.
2. Fill sinks Depressions in the DEM (caused by data artefacts or real terrain features) interrupt flow accumulation. The "Fill Sinks" tool in QGIS (via SAGA: Fill Sinks — Wang & Liu) removes these before routing flow.
3. Compute flow direction The D8 (eight-direction pour point) algorithm assigns each DEM cell a flow direction based on the steepest downslope neighbour. Run "Flow Direction" (SAGA or Spatial Analyst) on the filled DEM.
4. Compute flow accumulation Flow accumulation counts the number of upstream cells draining through each cell. Cells with high accumulation values form the stream network. Apply a threshold (e.g., >500 cells) to extract the channel network.
5. Snap pour point and delineate watershed Place the pour point at the culvert inlet location. Snap it to the nearest high-accumulation cell. Run "Watershed" (QGIS: Upslope Area from SAGA, or ArcGIS Watershed tool) to delineate the contributing catchment.
6. Extract morphometric parameters From the delineated catchment polygon, extract: drainage area (A), main channel length (L), and mean catchment slope. These drive the time-of-concentration and Rational Formula calculations.

5. Land Use Mapping and Runoff Coefficients

The runoff coefficient (C) captures how much of incident rainfall becomes surface runoff — reflecting land cover, soil permeability, slope, and antecedent moisture. It is the most significant parameter in the Rational Formula for urbanised or mixed catchments.

Mapping land use

For catchments up to approximately 10 km², Google Earth imagery provides adequate resolution for manual land use delineation. The practitioner digitises polygons corresponding to distinct land cover classes directly over the satellite imagery, then assigns standard C values from reference tables.

For larger catchments, or where a more systematic approach is preferred, the ESA WorldCover 2021 dataset (10m resolution, free download) provides classified land cover for the entire globe and can be used to compute area-weighted C values directly in GIS.

Standard runoff coefficients

Weighted runoff coefficient

Where a catchment contains multiple land use types, a weighted composite coefficient is computed:

For the Galadimawa catchment, the mixed land use — comprising paved roads, built-up residential areas, open spaces, and vegetated slopes — yields a composite C value that accounts for the progressive urbanisation of Abuja's periurban fringe.

6. Rainfall Intensity — IDF Curves

The Rational Formula requires rainfall intensity (i) in mm/hr for the design storm return period and duration equal to the time of concentration. In Nigeria, Intensity-Duration-Frequency (IDF) curves are the standard tool for this purpose.

Sources of IDF data for Nigeria

• Nigeria Highway Drainage Design Manual — the primary reference for hydraulic design; contains regional IDF curves for Nigerian cities and towns. The original data covers 1948–1978; practitioners should note this limitation and supplement with more recent records where available.
• NIHSA (Nigeria Hydrological Services Agency) — publishes updated rainfall frequency data and should be consulted for current design standards.
• State Ministry of Works — some state governments have developed updated IDF analyses for their capital cities.
• CHIRPS / ERA5 satellite records — can supplement sparse gauge data; require frequency analysis before use as IDF inputs.

Reading IDF values

For a given design return period (typically 10-year for minor drainage, 50-year for roads, 100-year for major structures), the design intensity (i) is read from the IDF curve at a duration equal to the catchment's time of concentration (Tc). This duration represents the critical storm — the one most likely to simultaneously fill the entire catchment and produce peak discharge at the outlet.

7. Time of Concentration — The Kirpich Formula

The time of concentration (Tc) is the time required for runoff to travel from the hydraulically most distant point in the catchment to the outlet. It determines the critical storm duration for IDF lookup and is a key input to the Rational Formula.

The Kirpich formula is widely used for small agricultural and urban catchments and is appropriate for the problem scale described here:

Both L and H are extracted directly from the GIS environment: L from the delineated stream network, and H from the DEM by comparing elevations at the catchment divide and the pour point. No field survey is required.

Alternative Tc methods

The Kirpich formula was developed for small Tennessee agricultural watersheds and may underestimate Tc in flat, poorly drained, or heavily vegetated terrain. Alternative methods should be applied as a check:

• NRCS (SCS) Lag Method — incorporates CN number and slope; suitable for mixed land use
• Bransby-Williams Formula — commonly used in South Africa and applicable to similar West African conditions
• FAA Formula — calibrated for airport drainage but applicable to urban catchments

Where multiple Tc estimates diverge significantly, engineering judgement is required. For design purposes, using the average of two methods and adding a safety margin is common practice.

8. The Rational Formula — Peak Discharge Estimation

With catchment area (A), weighted runoff coefficient (C), and design rainfall intensity (i) established, the Rational Formula provides the peak discharge estimate:

The formula embodies a simple mass balance concept: peak discharge equals the fraction of rainfall that becomes runoff (C) multiplied by the rate of rainfall input (i × A). Its elegance lies in its parsimony — only three parameters are required — but this simplicity also encodes its limitations.

Case Study Result — Galadimawa, Abuja

Applying the workflow described above to the Galadimawa culvert catchment, with design parameters extracted from the SRTM DEM, Google Earth land use interpretation, and IDF values from the Nigeria Highway Drainage Design Manual, the computed design peak discharge was 922.52 m³/s.

This figure reflects the full contributing area upstream of the culvert inlet, accounting for the mixed land use of the periurban Galadimawa basin, the relatively steep slopes of the Abuja terrain, and the high-intensity rainfall characteristic of the FCT during the wet season. The scale of this estimate underscores the importance of proper hydraulic sizing — and highlights why the existing culvert structure, dimensioned without formal analysis, is consistently overwhelmed during peak rainfall events.

9. Limitations of the Rational Formula

The Rational Formula is a powerful and practical tool, but its assumptions must be understood before applying results to design:

10. When to Use More Advanced Methods

The Rational Formula is an appropriate starting point for preliminary sizing of minor drainage structures. However, for the following scenarios, more rigorous hydrological modelling is required:

• Catchments > 80 ha — use the SCS-CN method, unit hydrograph analysis, or event-based modelling in HEC-HMS
• Storage routing — retention ponds, detention basins, or reservoirs require a full inflow hydrograph, not just a peak value
• Flood plain mapping — requires coupled hydrologic-hydraulic modelling (HEC-HMS + HEC-RAS or MIKE FLOOD)
• Dam safety analysis — Probable Maximum Flood (PMF) estimation demands full event simulation with Monte Carlo uncertainty analysis
• Multi-structure systems — interconnected drainage networks require dynamic routing (SWMM, InfoWorks ICM)
• Climate change sensitivity — future-scenario analysis requires climate model downscaling and stochastic rainfall generation

11. Updated Tools for Modern Practice (2024)

Since the original 2020 article, the toolset available to practitioners has expanded substantially. The following additions are worth incorporating into any GIS-based hydrologic workflow:

Improved DEMs

The Copernicus DEM (GLO-30), released by ESA and freely available via AWS, now outperforms SRTM in most accuracy benchmarks and should be the default DEM choice for new studies. For areas with dense vegetation, the FABDEM (Buildings and Forest Removed DEM) provides bare-earth elevations more suitable for hydraulic routing.

Google Earth Engine (GEE)

Google Earth Engine allows cloud-based processing of satellite imagery at continental scale without downloading data. It is particularly useful for land use classification over large catchments — Sentinel-2 time series can be classified using supervised machine learning directly in the GEE JavaScript or Python API.

QGIS Hydrology Plugins

The QGIS SAGA provider and the WhiteboxTools for QGIS plugin together provide a comprehensive free GIS-based hydrology toolkit, including DEM conditioning, watershed delineation, stream network extraction, basin morphometric analysis, and Strahler stream ordering — all without requiring ArcGIS.

HEC-HMS for larger catchments

The US Army Corps of Engineers' HEC-HMS (free download) is the international standard for event-based rainfall-runoff modelling. It accepts GIS-derived catchment parameters and outputs full storm hydrographs suitable for hydraulic structure design. For catchments above 80 ha where the Rational Formula is inadequate, HEC-HMS is the recommended next step.

12. Conclusion

GIS-based hydrologic analysis using freely available remote sensing data has transformed what a practitioner can accomplish with limited resources. The workflow described here — DEM-based catchment delineation, satellite-derived land use mapping, and Rational Formula peak discharge estimation — is reproducible by any engineer with basic GIS skills and a laptop. It removes the excuse of data scarcity from hydrologic design in Nigeria and across sub-Saharan Africa.

The Galadimawa case study demonstrates both the power and the responsibility of this approach. A computed peak discharge of over 900 m³/s from a peri-urban Abuja catchment is not a number to be filed and forgotten — it is an engineering mandate for properly sized, properly designed infrastructure. Where communities continue to flood and roads continue to wash out, the tools to do better exist and are free. The imperative is to use them.

For catchments above 80 hectares, or where storage, routing, or full hydrograph analysis is required, the step up to event-based modelling tools such as HEC-HMS is straightforward from the GIS foundation described here. The Rational Formula is a beginning, not an end.