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Channel Selection for Stormwater Control

By Engineer John Cee Onwualu (FNSE)

In channel selection for control of stormwater in Asaba, Warri, Effurun, and environs, great care was taken in channel depth selection because of the high-water table, especially in Effurun and Warri Metropolis.

The shapes and types of hydraulic channel sections recommended for use were dependent on the magnitude of the designed peak flow discharge of the floodwater, topography, and geology of the project areas.  This contributed to the various shapes and types adopted such as cast-in-place Concrete or Precast Concrete Rectangular, Square, and Circular shapes.

In the design of stormwater drainage systems, manholes are the most common appurtenances because of their various uses.  Their primary functions include:

  1. Providing maintenance access.
  2. Serving as junctions when two or more channels merge.
  3. Providing flow transitions for changes in channel sizes, slope, and alignment.
  4. Providing ventilation.

Manholes are generally made of precast or cast-in-place reinforced concrete. They are typically 1.2m to 1.5m in internal dimension and are required at regular intervals, even in straight sections, for maintenance reasons.

Manholes provide gradual transitions from circular pipe flow to box ducts flow (open or closed) alignment to minimize energy losses.  Another primary function is to provide a transition that minimizes erosion in the receiving water body.

From the hydraulic design analyses carried out, which were based on the results of the hydrological analyses of the drainage basins, various types of drainage channels were recommended for use, such as Rectangular, Square, and Circular shapes. 

These recommended channel sections were for the Tertiary, Secondary, and Primary channels that would safely and efficiently convey the Stormwater to River Niger and Warri River. 

Hydraulic Design of Drainage Channels

By Engineer John Cee Onwualu (FNSE)

The hydraulic design of a drainage channel requires excellent knowledge of the topography and rainfall data of the project area.

The design of a drainage channel is affected by factors such as the topography of the project area, selection type, design frequency, expected volume of stormwater, and economy.

The design discharge is affected by channel section, channel roughness, channel slope, and run-off coefficient factors.

In hydraulic engineering, two types of flow are usually encountered, laminar and turbulent flow. Laminar flow is rarely encountered when dealing with flows in pipes and ducts.

Most flows in nature are turbulent, and this is encountered when dealing with floodwater.

The primary consideration for the final selection of any drainage channel is that its design should be based on appropriate hydrological, hydraulic, and geotechnical analyses, which will lead to economic and efficient hydraulic control systems.

These control systems are the designed Tertiary, Secondary and Primary channels that would safely control and convey stormwater to the natural receivers, without destroying lives and property.

In addition to sound structural design, good construction practices are necessary for a drainage channel to function efficiently.

Description of Core Hydrological Terminologies in Flood Control Analysis

By Engineer John Cee Onwualu (FNSE)

Rainfall Intensity

This term is used to define the amount of rain falling during a specific time within the most intense period of the rain.  This value is then converted into the amount of rain that will fall in one hour.  The rainfall intensity is expressed as:

Rainfall Intensity, I = Maximum Rainfall Depth (mm)/Duration (hr) ………………           Eq. 1

Rainfall Intensity has an important effect on runoff proportion, as it determines the rate at which rainfall runoff arrives at the soil surface and, consequently, when infiltration rate of the soil is low to allow absorption, flooding of the environment does occur. 

Return Period

Since this study involves the design of hydraulic structures for the management and control of high flood levels without failing, a knowledge of design frequencies (Return Periods) of these maximum rainfall intensities must be obtained.  This Return Period (T) is the average number of years during which a flood of a given magnitude will be expected to be equalled or exceeded once.  It is computed using Weibull’s method of ranking and expressed as:-

Return Period, T = (n+1)/m                          ……….………….…………………….                         Eq. 2


            n = number of years

            m = event ranking

            T = recurrence interval

The derived data were used to generate the rainfall Intensity-Duration-Frequency (IDF) Curves for Asaba and Warri for the various rainfall intensities used in the computation of peak flow discharge (Qp) at different durations or time of concentration (Tc). 

Engineering Design and Hydrological Analysis for Effective and Efficient Flood Control

By Engineer John Cee Onwualu (FNSE)

The engineering design of a stormwater drainage system requires a large data collection effort.  The data requirements in the stormwater management and control measures for Asaba, Warri, Effurun, and environs include knowledge of topography, drainage boundaries, imperviousness, soil types, and locations of existing drainage channels, manholes, culverts inlets, and outlets. 

In addition, identification of other types of utilities and their locations in the ground is critical.  These data collated enabled the study and design work for the new drainage network systems to be achieved.  The engineering analysis of the drainage system recommended for this project considered four major aspects:

  1. Hydrological analysis
  2.  Hydraulic analysis.
  3. Structural analysis of sections
  4. Bill of Engineering Measurement and Evaluation (BEME)

Hydrological Analysis (Stormwater Estimation)

The design works relied on information provided from rainfall data of over 30 years collated from three meteorological stations (Enugu, Benin, and Onitsha) that are climatically similar and near to Asaba.

The design works also obtained information from the Federal Ministry of Works, Highway Manual Part 1: Design, for the rainfall Intensity-Duration-Frequency (IDF) Curves for Warri. 

The hydrological analysis of the rainfall data for a drainage basin is the most essential in the hydraulic design of drainage facilities.

Intensity-Duration-Frequency (IDF) design curves are used to estimate peak rainfall intensity (I) with different design frequencies (5, 10, 25, 50, and 100) years for the determination of peak Discharge (Qp).

Engineering Survey for Effective and Efficient Flood Control By Engineer John Cee Onwualu (FNSE)

To develop efficient and effective flood water control measures, a good knowledge of the topography of the study area is important for the design.

As much as practicable, the engineering survey here followed the existing street layouts.

The delineation of the study areas into catchments and sub-catchment using road networks was made for ease of collation and analysis of ground data.

Survey control points were established within the project areas for ease of referencing during the study and construction periods.

The spot heights (ground surface data) obtained during the Engineering Survey work were used to produce the terrain modelling showing the surface flow directions, generate a contour map of the entire project area, as well as perform the hydrological analysis.

With the modelling of these rainfall-runoff flow patterns, an idea of the natural surface flow pattern was obtained for all the catchments.


Well-researched studies were conducted over the entire flood-prone areas, valleys, natural watercourses, swamps, and rivers in Delta State of Nigeria, including existing drains in Asaba, Warri, Effurun, and environs. Several observations were made. Eight of these observations were discussed in my last post. This post is a sequel to the former and a continuation of the observations from these studies:

ix.) The geographical location of Asaba makes it possible to receive all the floodwater generated in Okpanam, since Okpanam town is situated at about 175m above Asaba. This difference in height created the heavy flooding that gathers at the centre of Asaba;

x.) Tidal waves from Warri River create temporary stoppage of discharge of floodwater from the environment, as its occurrence is independent of rainfall;

xi.) Most of the property developers make their foundations very low to the ground, which now makes the roads and drains created in these low-lying terrains, become elevated above the environment, and make intercept of flow discharges difficult from the streets’ drains. These drains provided, only address the removal of floodwater generated on the roads as they now act as embankments, leading to increased flooding in the environments;

xii.) All the marshy swamps and lakes have been occupied with property developments having no regard for the natural watercourses. This is more noticeable in Effurun GRA, Warri GRA, Okumagba/Ugborikoko Marshy Lakes and Edjeba;

xiii.) Some of the drainage channels and culverts on the streets are under-sized, while some culverts across some of the waterways lie above the channel beds, making it difficult to speedily evacuate the floodwater. This is noticeable across Jakpa Road, Uti Road, Olumu Street/Okumagba Avenue;

xiv.) Some streets completely lack the drainage system to evacuate the floodwater generated in their environment. This is more noticeable in Baptist Mission Layout in Warri, Enerhen and;

xv.) Most constructed drains along the major roads are with concrete slabs over them, making it difficult for desilting and other maintenance works since their sections are small to allow human entrance.


Studies were conducted over the entire flood-prone areas, valleys, natural watercourses, swamps, and rivers, including existing drains in Asaba, Warri, Effurun, and environs. The studies showed that:

i. There were no previous records of survey data and maps for the cities;

ii. While Warri had an old Master plan, Asaba had none;

iii. A large part of the flooding noticed in Asaba, Warri, Effurun and environs could be attributed to blocked drainage channels with waste materials (decomposable and non-decomposable), silts, overgrown weeds etc. The result is that the flow discharge into the drainage channels is disturbed and hence could not be evacuated from the environment;

iv. Most of the existing drains do not have discharge points as they were constructed without design specifications;

v. The rapid urban development in Asaba, Warri, Effurun, and environs have caused the stripping of the vegetative covering, leaving the soil surface exposed to the damaging effect of erosion. This has created serious silt deposit inside the existing channels, thereby reducing the design flow volume;

vi. Serious flooding noticed in many parts of Asaba, Warri, Effurun, and environs is a result of non-functioning drains that have been filled with silt deposits over the years thereby reducing the original flow section as designed;

vii. Most developers have encroached into the streets’ right-of-way and in some cases completely blocked the natural waterways. These are more noticeable in Okpanam area of the Capital Territory, Effurun, Warri GRAs and Ugbogboro creek in Ugborikoko, Sokoh Estate, Bendel Estate, Lower and Upper Erejuwa;

viii. All the natural watercourses are overgrown with weeds and a large amount of silt deposits, and in some cases have become refuse dumps, especially in Tori Creek, Crawford Creek, and Ugbogboro Creek/River;

Read about more of the observations and findings in my article, FINDINGS AND OBSERVATIONS IN FLOOD-PRONE AREAS OF DELTA STATE -2

Hydrological Basins in Delta State, Nigeria by Engr. John Cee Onwualu (FNSE)

Studies were conducted to identify all the flood-prone areas in Asaba, Warri, Effurun, and environs, all in Delta State of Nigeria.

These studies were to help develop efficient and effective flood water control measures that would safely convey run-off from the streets to the natural watercourses and rivers without loss of lives and property.

The major objectives of the study were:

1. To identify the root cause(s) of flooding in Asaba, Warri, Effurun, and environs.
2. To provide primary drainage channels that would incorporate the existing and new secondary and tertiary drains for efficient and effective evacuation of the floodwater from the environment to the natural watercourses.
3. To make the natural watercourses efficient by desilting and removal of obstructions on flow paths.

Asaba, Warri, Effurun, and environs are viewed as hydrological basins that drain to the major Rivers (Niger and Warri) within their localities. These basins were broken into catchments using existing road networks for delineation as boundaries.

In the case of Asaba, the absence of water bodies limited the design approach to nature-based types, such as valleys, streams, and rivers that are tributaries to the River Niger.

But in the case of Warri and Effurun, the catchment areas contain lots of water bodies (swamps) and natural watercourses, which serve as the receiving basins for floodwater evacuation from the environment.

The delineation with existing road networks into catchment areas made the application of the Rational Formula and the Hydrological Analysis convenient.

Studies were conducted over the entire flood-prone areas, valleys, natural watercourses, swamps, and rivers, including existing drains in Asaba, Warri, Effurun, and environs.

The outcome of these studies is properly captured in my next post, Findings and Observations in Flood-prone Areas of Delta State, Nigeria.

Oil Hubs in Delta State, Nigeria by Engr. John Cee Onwualu

Warri and Effurun as shown in the figure, are located on Latitudes 5o27’ N and 5o36’ N and Longitudes 5o40’ E and 5o48’ E. They sit on the bank of Warri River which joined Forcados, and Escravos Rivers through Jones Creek in the lower Niger Delta Region to the Atlantic Ocean.

Studies have shown this region to have moderate rainfall and humidity from May to October. Experience has shown it to have a short dry season from December to March, making construction activities to be at their peak during these months. The natural vegetation predominant in this region is a rain forest with swamp forest in some areas.

Warri is one of the oldest cities found within the lower Niger of Nigeria. The city is described as a low-lying plain that consists mainly of unconsolidated sediments of Quaternary age. The sediments are partly of marine and fluvial origin.

It is one of the oil hubs in Delta State with Udu and Uvwie kingdoms having been integrated into the larger cosmopolitan Warri. It originally, comprised three ethnic groups of Urhobo, Itsekiri, and Ijaw people.

Tributaries to the River Niger Within Asaba, DELTA STATE, NIGERIA by engr. john cee onwualu

Asaba is situated at the western bank of River Niger, overlooking the point where the Anambra River flows into it.  It is sandwiched between Anwai in the North, Issele-Azagba in the West, Ibusa in the South, and River Niger in the East.  

The eastern axis of the territory is marked with low relief that falls to about 22 metres above mean sea level, while the western axis shows the features of high relief that rises to about 175 metres above mean sea level.

The 2006 census shows that Asaba had a population of 149,603.  Asaba has a large population of Igbo-speaking people, but her position as Delta State Capital has brought her a cosmopolitan population drawn from other ethnic nationalities such as Urhobo, Isoko, Ijaw, Itsekiri, Hausa and Yoruba people, etc. 

Its geographical location lies between latitude 060 15’ 17.84” N and 060 09’ 38.49” N and longitude 060 36’ 23.48” and 060 45’ 13.35” E as shown in Figure 1.  Asaba lies in a plain between 88m to 41m, and about sixty-five percent (65%) within this plain is sitting in a flat terrain, which is between 44m – 41m.

There are some nature-based drainage routes that are tributaries to River Niger within Asaba, such as Iyi-Abi that is in the South, and Anwai River in the North-East.  The project section covers only an area of about 8,521.24 hectares (85.212-sqkm).