Tag Archives: #drainagechannels

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.

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.

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).