This presentation notes that in economic terms flooding is the major natural disaster experience in Trinidad and discusses the lack of sustainability of the current approach to drainage and flood mitigation and proposes some solutions such as storm water management at source as a possible solution to flooding
1. APETT FREE PUBLIC
CONSULTATION
SERIES
Bridging Engineering and the Society
FLOODING AND DRAINAGE
“TOWARDS A SUSTAINABLE
DRAINAGE SOLUTION”
Presentation by:
Eng. Vaughn I. Lezama R Eng, FAPE.
2. INTRODUCTION
This discussion will address the following issues:
Current circumstances and approach to drainage
and flood mitigation and whether the current
approach is a sustainable solution.
Objectives and Elements of a Sustainable
Drainage System
Some recent approaches which need to be further
expanded and explored
Conclusions/Recommendations
3. CURRENT CIRCUMSTANCES AND
APPROACH TO DRAINAGE AND FLOOD
MITIGATION
Flooding in economic terms is by far the major
natural disaster experienced in Trinidad &
Tobago. This is so since we have little experience
of devastating hurricanes and earthquakes and
none of volcanoes or other such natural
phenomena.
Given the increasing frequency and magnitude of
flooding and the resulting social dislocation and
devastation, the economic losses over time could
have a serious negative impact on the country’s
development and economy.
4. CURRENT CIRCUMSTANCES AND
APPROACH TO DRAINAGE AND FLOOD
MITIGATION
To date the major attempt to eliminate or
mitigate flooding has been in trying to improve
the carrying capacity of existing watercourses by:
Construction of river embankment
Dredging, paving, lining and widening of water
courses
5. CURRENT CIRCUMSTANCES AND
APPROACH TO DRAINAGE AND FLOOD
MITIGATION
Notwithstanding these efforts the improved
capacities of many of the newly lined and paved
channels are soon exceeded and there is no
further opportunity for increasing channel
capacity
Over the last twenty years or more several
million dollars have been spent on dredging,
widening and embankment construction along
the Caroni River. Recently there was a bulletin
warning that this river was likely to burst its
banks.
6. CURRENT CIRCUMSTANCES AND
APPROACH TO DRAINAGE AND FLOOD
MITIGATION
Extensive erosion in the upstream catchment
areas and the resulting sediment and siltation of
watercourses has served to reduced channel
capacities.
15. OBJECTIVES AND ELEMENTS OF A
SUSTAINABLE DRAINAGE SYSTEM (SDS)
Surface water drainage systems developed in line
with the ideals of sustainable development are
collectively referred to as Sustainable Drainage
Systems
The objective of a SDS design would be to reduce
runoff by integrating storm water controls
throughout the drainage catchment area in
small, discrete units.
Through effective control of runoff at source, the
need for continuous drain capacity improvement
is minimized
16. OBJECTIVES AND ELEMENTS OF A
SUSTAINABLE DRAINAGE SYSTEM (SDS)
Because of our limited land space and to a lesser
extent lack of land use control we do not have
the advantages of a large land mass with
expansive and varied topographic and geological
which can easily facilitate the various
components of a SDS.
Some of the components of a SDS include:
large constructed wetland ponds with wetland
vegetation to which storm water can be directed for
storage
Filter strips or wide densely vegetated areas that can
treat runoff from adjacent impermeable areas
17. OBJECTIVES AND ELEMENTS OF A
SUSTAINABLE DRAINAGE SYSTEM (SDS)
SDS Components CONT’D
Wet ponds which are basins with a permanent pool of
water and which provide temporary storage for
additional runoff above the permanent water level.
Wet Ponds could provide amenities for recreational
and aesthetic benefit provided that water quality can
be enhanced.
Extended Detention Ponds. These are normally dry
and are designed to detain a certain volume of runoff
as well as provide water quality treatment.
18. OBJECTIVES AND ELEMENTS OF A
SUSTAINABLE DRAINAGE SYSTEM (SDS)
Detentions Ponds represent the one element of a
SDS which may be most suitable and applicable
to our local circumstance and to the extent that
this may be so these should become a mandatory
part of our drainage system design.
19. SOME RECENT APPROACHES TO FLOOD
MITIGATION
In recent years with the advent of the EMA that
agency has established a requirement that in any
new development for which a CEC is required the
drainage system must be so designed as to
ensure that the pre-development runoff from the
site must not be exceeded after the development
is completed.
20. SOME RECENT APPROACHES TO FLOOD
MITIGATION
Invariable this has resulted in the need for some
form of detention structure and as a consequence
a number of new land developments have
incorporated the use of detention ponds.
As of now there are no known locally established
design criteria with respect to the required
storage volume of these ponds so that design and
performance of detention ponds is currently a
work in progress
34. CONCLUSION
The traditional hydraulic engineering approach
to solving the drainage problem has provided
some relief in some instances but has not
resulted in any long term solution to the flooding
problem
The financial allocation budgeted for drainage
works on a yearly basis is essentially for
improving existing drainage channels and
cleaning of watercourses. Much has been talked
about but little else have been seriously
attempted over the years
35. CONCLUSION
The current drainage regime throughout the
country is in crisis and a reversal of the present
situation is the only feasible solution. As such all
new land development should be required to
produce a post-development runoff that is not
more than 80% of the pre-development runoff.
Managing storm water at source with more
emphasis on short term storage and attenuation
of discharge into downstream watercourses could
provide a more sustainable solution to flood
mitigation.
36. CONCLUSION
The use of Detention Ponds as a means of
managing storm water discharge should become
mandatory for all land developments
In any land development the provision of an
appropriate plot should be required for storm
water management in the same way that there is
a current requirement to set aside plots for
recreational and communal use as part of the
land use planning
37. CONCLUSION
Consideration should be given to offering an
incentive to urban townhouse developers to
provide a certain volume of storage for roof water
which can be used for purposes which do not
required potable water.
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