By Engineer John Cee Onwualu (FNSE)

Storm Water Project in Delta State, Nigeria.

In my last post, I took a deep dive into the details of the findings and observations for Storm Water Management and Control Measures for Asaba, Warri, Effurun, and the environs.

It is clear that the topography of Asaba and its environs and that of Warri, Effurun, and its environs have similarities in their flood management and control approaches.

There were recommendations for optimal and efficient Pre-cast Open and Closed (underground sewers) concrete systems put forward for consideration and implementation for these project areas.

This article enumerates the merits to be derived from the use of these recommended pre-cast concrete structural elements of rectangular, square, and circular channels.

1) The use of precast primary receivers for the construction of the downstream channels to receive storm water discharges from the streets’ right-of-way will reduce the construction time required for evacuation of the current flooding menace that is being encountered in the environment, since non-functioning drains will now have discharge points.

Storm Water Construction at Effurun, Delta State.

2) There are savings in project delivery in terms of time, handling, and cost optimization during fabrication and execution of the project, since all these various types of channels with different operations will run simultaneously.

Construction of Drains at Okpanam, Delta State.

3) There is reduced construction risk to workers and passers-by during deep execution works since earth-moving equipment will be deployed for the placement of heavy precast concrete elements in deep excavations. This will improve the overall safety rating of the project.

Okpanam Drainage Construction, Delta State, Nigeria

4) There will be reduced exposure time to the excavated sections with the use of precast structural elements over cast-in-place concrete types, as the works are in built-up areas. This will improve the overall safety rating of the project.

Construction Work at Effurun, Delta State, Nigeria.

5) There is no need for long-exposed excavated trenches, as is the case with cast-in-place concrete, since smaller segments of work excavation are needed to place the precast elements.

6) The use of precast concrete structural elements will reduce in situ construction errors, which would create greater quality control on the project.

Storm Water Construction at Effurun, Delta State.

7) Finally, with adequate funding, the State government would complete the project within a short time, which would save her citizens the agony of economic and human losses occasioned with the hazardous and devastating flood water.

I hope that the state governments of different nations can take a keen look at the merits of these recommendations, and execute them where necessary.

Recommendations for Stormwater Management and Control

By Engineer John Cee Onwualu (FNSE)

Recommendations for Stormwater Management and ControlRecommendations for Stormwater Management and ControlRecommendations for Stormwater Management and Control

The details from the findings and observations of this study and design by Jefcon & Associates Ltd (Consultant) for the Stormwater Management and Control Measures for Asaba, Warri, Effurun and environs, showed that the topography of Asaba & its environs and that of Warri, Effurun & environs have similarities in its flood management and control approaches. 

Due to the nature of the terrains, the study on these cities and their environs have recommended optimal and efficient Pre-cast Open and Closed (underground sewers) concrete systems.  Therefore, the recommendations put forward for consideration and implementation for both project areas are that: 

1. The natural watercourses and downstream primary drainage channels should be considered as Priority No.1.  This involves clearing of the natural watercourses of weeds, waste, silt deposits and demolition of encroached properties along their paths; and construction of the downstream primary drainage channels that would receive flood water from the streets’ drains to these natural watercourses.  This will bring immediate improvements to flood water discharge on existing systems that do not have discharge points.

Recommendations for Stormwater Management and Control

2. Canalization of some of the watercourses with concrete mattresses, (See the figure above) is recommended as Priority No.2.  This involves the construction of 6m – 10m wide trapezoidal channels made with concrete mattresses. This will stop encroachment by property developers and create a definite demarcation of the natural watercourses. This will bring improvement on the existing systems while the construction of new infrastructures for more efficient and effective evacuation of flood water from the environment would be in progress.

Recommendations for Stormwater Management and Control

3. The need for updated Survey Maps for Asaba, Warri, Effurun, and environs by the Delta State Ministry of Lands and Survey is recommended as Priority No.3.  The presence of these maps will aid future planning and construction of infrastructural developments, as the lack of these maps has caused delays and increased project cost.

Recommendations for Stormwater Management and Control

4. With the implementation of items. 1 &2 above, the need to construct drainage channels in flood-prone areas is recommended as Priority No. 4. The construction of these street drainage channels (tertiary and secondary channels) in the identified flood-prone areas will create greater improvement in flood management and control of the environment.

Recommendations for Stormwater Management and Control

5. Government Ministries and Agencies in charge of urban development must stop approval of property developments on the natural watercourses and streets’ rights-of-way while identifying and demolishing those properties built on these mapped-out routes for quick evacuation of the flood water generated within the environment.

Recommendations for Stormwater Management and Control

6. Government Ministries and Agencies in charge of drainages must have a holistic maintenance and repair response plan with a motivated team.  This team should carry out a quick repair to damages on drainage channels as well as carry out periodic manual de-silting of the drainage channels.

Recommendations for Stormwater Management and Control

7. The State Orientation Agency and similar Agencies in charge of public enlightenment should do more at educating the populace on the hazards associated with the discharge of waste materials and blocking of drains and natural watercourses, as studies have shown that some see it as a normal way of living.

Recommendations for Stormwater Management and Control

8. The government should make it mandatory for property developers to create more areas for grassing and vegetation than stone or concrete interlocking pavements, as the use of the latter increases runoff in the streets.

In the next article, we take a look at the merits of these recommended options.

Considerations in the design of stormwater channels

By Engineer John Cee Onwualu (FNSE)

There are many important considerations in the design of durable stormwater channels, and the next series of posts considers some of them.

Time of Concentration (Tc)

The time of concentration (Tc), is defined as the longest time it takes the rain falling at the most distant point within a watershed to reach the point under reference. It is one of the most important variables in the estimation of design discharges.

It should be noted that while dealing in watersheds with flat terrain or low topographic slopes, the calculation of Tc, using commonly accepted equations often results in unreasonably large values.

That is, as the slope approaches zero, the travel time approaches infinity. Through research work, it is recommended that an adjustment of 0.005 be made to the slope in both the Kerby and Kirpich methods to allow for more realistic results.

The adjusted slope becomes Slow Slope = So + 0.0005 ….…………… Eq. 3

Therefore, if So ≤ 0.002 m/m (0.2%), a low slope condition exists, and adjustments should be made.

There is also another method of computing Tc, using the application of Kerby formula, which is expressed as:

tc = 1.44(Ln)0.467S-0.235 …………………………………………………………. Eq. 4

Flow Length, L = {[tc/1.44]1/0.467}/ns-0.467 ……………………………. Eq. 4.1

Retardance roughness coefficient,

n = {(tc/0.1.44)1/0.467}/Ls-0.5 ……………… Eq. 4.2

One of the common methods for estimating the Time of Concentration (Tc), is the application of Kirpich formula.

This is expressed as:

tc = 0.0195{L0.770/S0.385} ……………….…………………….………………. Eq. 5

Travel Length, L = {tcS0.385/0.0195}1/0.770 …………….……………. Eq. 5.1

Slope, So = {0.0195L0.770/tc}1/0.385 ……………………………………. Eq. 5.2


tc = is in minutes.

L = maximum length of travel in meters.

S = slope of the catchment in mm-1 over the total length, L.

Kirpich method yields very conservative or short times of concentration that result in high peak runoff rates, especially from the Rational method.

Kerby – Kirpich Method

The Kerby – Kirpich method of computing time of concentration (Tc), is one of the best approaches, which produces Tc estimates that are consistent with watershed time values independently derived from real world storms and runoff hydrographs.

This is expressed as: Tc = Inlet time + Channel flow time Tc = tov + tch …………………………………………………………………… Eq. 6

Tc = tov + (0.0195L0.770/(√so)0.770) ……………………………… Eq. 6.1


tov = overland flow time tch = channel flow time

The Kerby – Kirpich method for estimating Tc is applicable to watersheds ranging from 0.70Km2 to 389 Km2, main channel lengths between 1600m and 80000m, and main channel slopes of between 0.002 and 0.02 (m/m).

In the next post in the series, we look at OVERLAND FLOW and CHANNEL FLOW.

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

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