Tag Archives: #floodwatercontrol

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.