Engineers’ Angle | Flooding and drainage control
Recent flooding events have again thrust this issue into the public discourse.
Following the massive flooding in Montego Bay St James, there is was flooding in May Pen, Clarendon and recently on the important Marcus Garvey Drive corridor in the Corporate Area.
Flooding can lead to loss of property, loss in economic activity and in the worst case, loss of life.
When there is a failure in the drainage system there is also the opportunity to learn from the flood event, define the problems that exist, and correct them, or mitigate against a recurrence.
Procedures can also be strengthened, or developed, for the identification of the circumstances that caused the problem, the preparation of remedies or mitigation measures and actual implementation of the remedial works or mitigation strategies to alleviate the problem.
The flooding last week along Marcus Garvey Drive, in the vicinity of the Kingston Wharves, must be of great concern. Not only did it inconvenience many people who use the roadway, it also highlighted a possible deficiency in our lifeline infrastructure there.
The country depends on export to and imports from the wharf along with the associated commercial activity there. Therefore the level of risk and protection needs definition to set the quality of the storm water management design required.
Not all infrastructure is designed to the same level of protection. The more persons affected, or the more important an area is, the more investment in protection is required. The 100 year storm recurrence is the level of risk usually applied to storm water design for lifeline infrastructure here.
Put differently that infrastructure should not be inundated more than once and possibly twice in a person's lifetime, as the 100 year flood can occur two years in a row.
The public needs to understand the difference between drainage and flood control and the level of risk being contemplated and what they may need to do to protect themselves.
Drainage is about convenience, while flood control is about protection of life and property. If a roadway serving a relatively small number of people or community is being inconvenienced once every five or 10 years for a few hours in a day, is it worth the cost to build drainage that will make the cost of homes more expensive?
There are always trade-off between risk and cost in engineering. Risk is something we all understand in Jamaica as with the risk of being burglarised.
If all one can afford is burglar bars one does that, but if one can afford a camera system, maintain and care for large dogs, high fence and security guards, then you do that. In some areas no burglar system is needed.
The susceptibility to the risk of flooding and the ability to pay for the infrastructure to mitigate the risk needs balance. The state agencies set the level of protection required for various developments, including roads.
So what causes drains to become flooded?
The likely issues include:
1. The rain event exceeds the design event.
2. There is a blockage that restricts the flow in the channel
3. The drain was undersized
4. The entry to the drain is undersized (inlet control)
5. The outlet, or receiving drain, or water body, is inundated causing backup.
6. Change in bed slope or obstacle that cause hydraulic jump.
7. Other head loss situations.
What needs to happen next?
The first step is to understand actually happened. Once there is flooding there is the usual speculation that the drains are undersized, drains were blocked, a lack of maintenance that redirected the flow to an unusual location, or the road grade was at an elevation that allowed for water to pond in areas too deep.
But speculation cannot solve the actual problem or problems. The state agencies have the capability to evaluate the rainfall event that occurred during the flooding on Marcus Garvey Drive in the vicinity of Tinson Pen last week and determine the likely recurrence interval, such as once in two, or five, or 10, or 100, or 200 years, whatever it is.
The next step is to have experts define the catchment that caused the flow of water to get to the flooded area and report on all observations without any preconceived idea of what caused the problem.
The sizes of the drains and the openings as well as the elevations of the road surface need to be determined. One of our engineering mentors always cautioned that a safety valve is always needed in engineering design to ensure that, for whatever reason the intended design fails, the effect is reduced.
In road design an overflow path is a usual approach.
In the case of Marcus Garvey Drive, it is close to the sea, which is its ultimate outlet. If such a flood way or overflow path is defined then concrete walls should not be allowed in those areas so that the surface water can flow through premises as an alternative outlet.
That may not be the best solution for this case, but it is an example of an approach to reduce the effects of flooding.
If the flooding that occurred is deemed to have a frequent recurrence interval, then an early warning system may be necessary to close that roadway and divert traffic to other routes.
An emergency traffic management plan will have to be developed by the Ministry of Transport and associated agencies to manage similar events.
Business owners can then assess the need to make their buildings watertight depending on how frequently the floor levels are likely to be inundated.
Going forward consideration have to be given to issues inclusive of:
1- Greater planning and zoning to implement storm water best management practises
2- Greater capital investment and data use
3- Application and adherence to national building codes.
4- Greater attention to waste management.
5- Proper communication and reporting to the public so that they can make wise decisions on how to operate when similar events occur.
- Ivan Foreman and Dwight Ricketts are members of the Jamaica Institute of Engineers. Send questions and comments to, firstname.lastname@example.org or email@example.com. You may also leave your comments for the JIE's Technical Committee at our Facebook page: Jamaica Institution of Engineers JIE