Uzokwe's Searchlight

Wednesday, August 9, 2017

Alfred Obiora Uzokwe, P.E



Continued from part 1

Immediately Clear all Debris from Drainage Channels:

s the floods lasted, I saw a picture of a flooded street in Surulere Lagos with plastic bottles floating everywhere! It looked like something from the movies yet it was real. There were also pictures of debris floating in some areas of Lekki. Authorities in Lagos complained that the flooding was exercabated by the fact that residents were dumping thrash in drainage paths and channels.

It is true that people have no qualms with dumping thrash wherever they can in Nigeria, but authorities should not be creating enabling environment for them to do it. They say they are developing a trash collection and disposal system and my question is: How come a place like Lekki does not already have an efficient and lasting thrash collection and disposal program?

Also, I have often railed about the fact that many localities in Nigeria construct concrete gutters but then leave them open. This creates the temptation for unruly people to dump thrash into the gutters. To ameliorate this problem, in addition to implementing a robust thrash collection and disposal program, all gutters should be covered with concrete slabs and iron grates. The grates will allow water into the gutters but keep out thrash. The slabs and grates should be removable to access the gutters for periodic cleanout.

Discourage the Construction of New Buildings on Flood Plains and Drainage Paths and Consider Removing Buildings Already on Flood Plains

Authorities in Lagos state complained that flooding is made worse because people built on flood plains and drainage paths. Again, my question is: Who issued the building permits for these illegal structures? If they were built without permit, why did authorities allow them to stand for so long but are suddenly complaining?

Flood plain is the land area adjacent to a body of water. Flood plains have a purpose in nature. They absorb or soak up excess water that results from hightides or just swelling of the river as a result of rain. When people build on flood plains, the advantage of soaking up excess water is lost and the area becomes susceptible to flooding.

Because of its proximity and adjacency to both the Atlantic Ocean and Lagos Lagoon, Lekki is already in a vulnerable location. So, it is surprising that people were allowed to build in flood plains thereby worsening an already bad situation.

This problem needs immediate action to remediate. It should be clear by now that frequent rains will become the order of the day. The first step the Lagos state government must take to remediate this issue is to conduct a comprehensive assessment of Lekki using flood maps. Properly delineate the flood plains and determine which buildings are residing on the flood plains. Work with owners in a cooperative way to consider removing the buildings and compensating them for the fair market value of their buildings. This may sound draconian but the good of the majority must preponderate. Owners who agree with removal of their buildings from the flood plains should also be given enough time to find new properties for resettlement.

Authorities should also ensure that no new buildings are allowed to go on the flood plains anymore.

Mandate Construction of underground Catchment Basins in compounds with concrete pavements:

When you enter most compounds these days in Nigeria, you will notice that the compound grounds are paved with concrete or interlock pavers. This makes the whole compound impervious. When it rains, storm water will not infiltrate into the soil within the compound instead, it flows into adjacent streets through openings created on the perimeter walls. Storm water from multiple compounds flowing into the streets inexorably lead to excessive runoff and flooding. The problem becomes worse when 100-year storms occur or when it rains in a back to back fashion as it did in places like Lagos. The planners of Lekki development seemed to have saturated the landscape with buildings, roadways and interlock-paved compounds. As a corollary, a large swath of the landscape is impervious to storm water infiltration, leading to excessive runoff and flooding.

If excessive runoff in the area must be controlled, then storm water generated in all compounds in Lekki that are paved with concrete or interlock pavers must be infiltrated into the ground within each compound. Each home owner should be mandated to construct a covered underground “catchment basin” or ditch, within the compound, into which all storm water generated in the compound will flow. The water will then slowly dissipate into the ground within the compound without flowing into the adjacent street. This will reduce runoff and the stress placed on storm conveyance structures during rain fall. Engineers can calculate the size of needed underground basin that will do the job based on the size of the compound.

Implement Pervious/Porous Pavement in Parking lots:

In most urban developments, and Lekki is not an exception, paved parking lots make up the largest area of imperviousness in the landscape apart from building foot prints. Of course, storm water cannot seep into the ground in these paved areas. This increases the volume of runoff that stresses drainage infrastructures and create flooding.

Runoff generated by parking lots can be reduced to manageable levels through use of porous concrete or porous asphalt pavements for parking lots and walkways. Porous pavement is characterized by voids that allows storm water to seep into the ground during storms. The only caveat is that porous pavements require periodic vacuuming with specialized equipment to assure that the voids do not get plugged up by debris.

Design and Construct Retention and Detention Ponds in Strategic Areas of the Development:

I noted earlier that the landscape in Lekki, as with most other places in Nigeria, is saturated with buildings so when it rains, storm water has very little area left to infiltrate into the ground. It flows on the surface and exercabates the issue of flooding.

To minimize this problem, authorities need to again do a review of the topographical layout of Lekki. The topographic map will show the low points in Lekki. Low points are the areas that storm water gravitates to when it rains and if the storm water has nowhere else to go, it starts ponding or pooling on the surface leading to flooding. When these low points are identified throughout the development, authorities should construct retention/detention ponds in those areas to “catch” the storm water. A detention basin or pond is basically a big ditch dug in the ground in the lowest point of an area. The ditch is sometimes lined with stone or grass. During a rainstorm, rain water flows into the pond and slowly infiltrates into the ground. Inotherwords, it stores rainwater that would otherwise become runoff for a while and then allows it to slowly infiltrate into the ground.

Detention ponds require maintenance like removal of mud build up to ensure that rain water can always infiltrate into the ground. Also, it must be maintained to ensure that it does not keep stagnant water that becomes a mosquito den.

Continued in part 3…