Download - Drain Design (Proag)
-
8/13/2019 Drain Design (Proag)
1/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
7
Abstract
Many areas in Mauritius get ooded regularly due
to
(a) sudden rain of unexpected magnitude
(b) building permits given on historically ood-
able land
(c) inadequate drains.
Rainfall frequency and intensity records can be
used to estimate the magnitude of rains and the en-
suing ood ows. There is a 26 % probability that
a 100 year rain will occur during the next 30 years
(a generation). Even if a higher return interval, e.g.
1,000 years is taken, it is found that there is 7.2 %
chance (not to be neglected as being small) that a1,000 year ood will occur during a 75 year span
a mans lifetime.
In a small country like Mauritius, it is difcult to
give ood warnings in advance that one can aban-
don house and move furniture. The ooding might
occur suddenly in the middle of the night, when
there has been a power cut.
The only solution to have dry feet would be to haveadequate drainage.
Is it acceptable for ones house to get ooded every
10 years? Or every 30 years? Or never at all during
ones lifetime? The three alternatives will require
drains of different sizes, with different costs.
Once the desired safety from oods has been ac-
cepted preferably through legislation it would
be easy todesign for the adequate drain capacity(1)
earmark the boundary of the reserved low lying(2)
areas reserved for extreme ood conditions.
This paper presents the relevant criteria to adopt for
drain design in Mauritius.
1. IntroductionThere are many places which receive heavy rains
without anybody being aware of the fact, simply
because no area gets ooded. In other places, how-
ever, there are many tell-tale signs, either during or
after the heavy rains. The signs noticed afterwards
indicate the levels to which the water levels rose
during the peak of the storm. If the existing drains
are unable to carry the ood peaks generated, then
people do notice the ooding of the surroundings,
sometimes with devastating results and loss of life
and material damage. A sudden heavy rainfall will
cause ooding if there is no drain to carry the water
away.
A low lying area will certainly be ooded because
all water will eventually accumulate there and it is
usually difcult to make drains which are at a still
lower level. The area nearby is also likely to be af-
fected in case the drain, if any, has an inadequate
carrying capacity.
Flooding seems to be a regular phenomenon which
occurs in many countries for several reasons,
namely: (a) building permits given on historically
oodable land (b) inadequate drains (c) sudden
rain of unexpected magnitude
It is therefore important that the catchment area of
the urban environment be studied for the low lying
areas and the natural draining channels. These areas
DRAIN DESIGN FOR DRY FEET
Virendra PROAG
University of Mauritius
-
8/13/2019 Drain Design (Proag)
2/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
8
should in the rst instance be completely avoided
for building purposes.
2. Outline Planning Schemes
An outline planning scheme aims to dene zoneswhere different activities are allowed, including
housing. The main parameters that are considered
are environmental, but rarely do we nd drainage
being taken into account because of possible impacts
of its non existence.
If there were no oods during some 50 years in liv-
ing memory (or sometimes even during the last 40
years average service time of a building permit
ofcer), it is reasonably felt that there is no dangerof any big ood occurring in the area. Very often,
there are historical records which can conrm that
the given area had been ooded so many years ago
sometimes, 300 years or 500 years. Unfortunately,
it is not always easy to go through these records or
to check them.
Thus, building permits are very often given on land
which, according to historical records, is prone to
ooding.
Building lots have often been earmarked by the land
promoter within drainage channels low lying con-
tours. While these should be a constraint against
giving the building permit, political pressure or an
unwary building permit ofcer may come in the
way. At other times, a whole set of houses have been
built in a low lying area which could very well form
a lake if a regular means of feeding it with water was
available. In this case, it is usually difcult to makedrains which are at a lower level.
Very often, ooding occurs because the drain, if any,
has an inadequate carrying capacity, or has a carry-
ing capacity which has not been designed to take
sudden heavy rainfalls Sometimes, the drains are
permanently inadequate as one road engineer ex-
plained, he designed road drains to cater only for the
rainfall coming from the road. The drains coming
from the nearby building lots were not supposed todischarge into his road drain !!!.
While the above explains how building permits
wrongly given or drains with inadequate capacity
allow ooding to occur, it is judicious to examine
sudden rainfall. If intense rainfall magnitudes can
be estimated, this could help in designing the appro-
priate drains. Thus, before designing drains to carry
ood ows, it is necessary to determine the magni-
tude of the ood ows.
3. Determination of Flood Flows
One approach to the problem is as follows:
Walk over survey of the area
Obtain local historical ooding levels from
the residents
Collect data
Analysis of collected data to estimate ood
ows
Estimate size of channel sections under the
bridge
4. Walk Over Survey
Site visits undertaken on the existing or nearby re-
gions will enable meeting people, sometimes old,who recollect what they (or their grandparents) saw
during ood conditions - the ood levels observed.
A backow analysis may help in crosschecking the
present ood estimates.
These are certainly of use, as a check, during design
work.
5. Data Collection
5.1 General Approach
How often have promoters accepted a consultants
offer to design drains capable of carrying ow with
a return period of 2 years?
Many codes of practice indicate a good guideline to
design drains, etc with a return period of 50 years.
This section refers to the catchment parameterswhich will enable determining some further factors
-
8/13/2019 Drain Design (Proag)
3/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
9
needed for the calculation of oods. In a rst stage, the catchment area, slope (= elevation difference/stream
length) are required to nd the time of concentration.
The peak ood ow is given by the relation Qp= CiA, adjusted (for the units given) to
Qp= 0.278 C i A
Where C = runoff coefcient
i = rainfall intensity (mm/hr)
A = drainage catchment area (km2)
Qp= Design Discharge (m3/s)
The runoff coefcient is a function of the vegetation, urbanisation and other factors of the catchment. The
rainfall intensity to be used depends on the time it takes the whole catchment to contribute to the ow in the
drainage channel.
These parameters are discussed below.
Figure 1 shows the process of rainfall, wherein rainfall (or precipitation when it includes hail, snow, etc) is
the sum of the ensuing evaporation, inltration and runoff.
The lands surface always has a slope, however small it might be, which determines the direction of ow
(here, the runoff).
Figure 2 indicates how the ridge at the top of a valley slope will divide rainfall, which will run along slopes
of either side of the ridge. The area enclosed by a given ridge determines a catchment area. Depending on
the point of interest, the catchment area will vary. Point X determines a smaller catchment area than point
Y, and it turn area at point Y is smaller than that governed by point Z. Eventually, the estuary governs aneven bigger area.
Figure 1 : How rainfall is shared among different components
-
8/13/2019 Drain Design (Proag)
4/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
10
So, this diagram illustrates how rain from the valley will run to a low point. Therefore, unless a drain
has been specically designed to take this rainwater, it will run into the drain besides the road, even if
the engineer wrongly believed that only water from his road would run into the road drained he designed
to take water, just from the road. And, if there are no road drains, the road itself will act as a well-
designed drain. The recent heavy rainfalls in Port Louis and in other places bear good testimony to this
phenomenon.
Figure 2: The catchment area gets bigger downstream of the valley
Figure 3: The estuary is the lowest and nal exit drainage point
-
8/13/2019 Drain Design (Proag)
5/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
11
Therefore, one rst rule to avoid ooding is to
make sure that the catchment area of the drain
being designed is not underestimated. Not rocket
science, but how often ignored by engineers and
planners!
Figure 3 gives an overall picture of a valley (with
smaller valleys inside) and indicates how everything
discharges into the lowest point which happens to be
the estuary.
In this connection, there is a parallel with trafc
ow. Unless the conveying capacity QOUT
is greater
than the incoming ow of trafc QIN
, there is going
to be a trafc jam. While this results in a halt or
lower speed in case of vehicles, unfortunately with
water, this higher inow leads to non-stopping ow
which results in overtopping the drain and ooding
the sides.
Therefore, another rule to avoid ooding would
be to make sure that the carrying capacityCOUT
of the channel drain exceeds the peak discharge
Qp.
COUT
Qp
Simple logic, but how often ignored!
5.2 Runoff Coefficient Value
The runoff coefcient C represents the ratio of a peak
ow and rainfall rate of selected duration determined
or the same average recurrence interval from
frequency analysis of ood peaks and rainfalls.
There are various factors affecting the runoff
coefcient which must be considered. In
consideration of these, the Institution of Engineers,
Australia (Abbey, 1999) recommends that the runoff
coefcient C be taken as
C = Fy(0.45 + 0.20 f
i)
Where
fi = impervious factor, taken as 1 as a worst
case.
Fy = frequency factor
= 1.20 for a 100-year return period.
The runoff coefcient thus works out (for this return
period) to beC = 1.2 (0.45 + 0.20 x 1) = 0.78
Different authors give other formulae or tabulated
values, depending on soil cover.
If Figure 1 is examined again, several observations
may be made:
The equation,
Rainfall = Evaporation + Inltration + Runoff
while holding true in all cases, does not indicate that
runoff or any of the other parameters are constant,
though they may be taken to be taken as an average
over the year and so on.
For example, during a hot sunny day, imagine that
some rain falls. As the rain drops touch the ground
(soil or road surface) water vapour can be seen torise in the air. Evaporation is actually occurring!
If it is a light rain, the ground surface will be seen
to dry up quickly. Either all rain water evaporates
on the road or some of the rain is absorbed into the
earth : inltration is taking place.
The end result is, however: there is NO runoff!
At the other end of the scale, even under the samesunny conditions, if there is a heavy rain, there will
a substantial runoff towards a low point (drain, river,
pond), because the soil has reached its inltration
capacity. The soil is momentarily saturated.
The ratio of this runoff to the measured rainfall is
the runoff coefcient.
Again, this runoff coefcient may be measured as an
average over a period of time, or at every instant orover very short intervals.
-
8/13/2019 Drain Design (Proag)
6/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
12
Typically, it is usual to give the average over a
long interval of time for the runoff coefcient.
However, for those people who have experienced
cyclonic conditions, the situation is different.
When there are heavy rains, in fact, rain might be
falling continuously/on and off, during several days.
The soil is now saturated over a longer period, and
this can be felt even outside cyclonic conditions.
Imagine now a sudden, heavy rainfall under these
conditions. This will just be runoff. There will be
NO inltration (saturated soil) and little evaporation
(the air is saturated with water vapour).
So now, the runoff coefcient C = 1, taken as 1 is a
worst case, that needs to be considered.
Although this might be difcult to swallow, it is
judicious to examine the situation in the light of
actual experience. Mauritius is a tropical island with
tropical heavy rains, not a desert where it rains 20
mm per year !!
If, on top of that (as in Port Louis), the ground
surface consists of clayey soil or is mostly paved,
again the runoff coefcient is going to be C = 1.
This is the third rule to consider: In tropical
countries, take C = 1
This factor will increase the design ow to be
considered, for sure. However, though the engineersjob is to do an economical design, he should not
underestimate the loading conditions (here, the
possibility that the rain will not inltrate at all, nor
evaporate, is real. It does happen.). Furthermore,
the drain is expected to be effective under extreme
conditions, not only when it rains slightly.
5.3 Intense Rainfalls
Normal rainfalls do not cause ooding to occur. So
a serious study of ooding needs to consider intense
rainfalls.
The worlds greatest recorded rainfalls, according to
the World Meteorological Organisation are approxi-
mated by the equation
P = 422475.0
dT
Where
P = the rainfall (precipitation) depth in
millimetres
Td= the duration in hours
The equation was obtained by tting data from
observed extreme rainfalls at many locations for
durations ranging from one minute to several
months. This equation is an estimate of the
precipitation depths that could occur under very
extreme circumstances.
If Td
is taken as 1 hour, the rainfall is 422 millimetres.
Something to think about!
Fortunately, the rainfall records in Mauritius do
not indicate such extremes in Mauritius, but heavy
rains with 100 mm/hr over an hour or so are not
uncommon (89 mm at Dubreuil on 22ndDecember
1979 over 1 hour , 310 mm at La Brasserie over 150
minutes on 6th February 1992 and more recently 91
mm at Line Barracks, Port Louis, between 2 and 3
p.m, on 30thMarch 2013. The rainfall collected over
the rst half hour was 50 mm which amounts to an
intensity of 100 mm/hour)
5.4 Rainfall Intensity and Frequency
To introduce the subject, a 100 m sprinter runs at
a speed of 36 km/h on a 10 second race, but theaverage speed is much lower (22 km/h) when another
Table 1: Typical Rainfall Intensities (mm/h)
Duration (mins)
T = 100 yearsSeychelles Mauritius
30 mins 150 120
60 120 100
Duration (mins)
T = 50 years
30 140 110
60 105 90
-
8/13/2019 Drain Design (Proag)
7/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
13
runner (or the same person) undertakes a 10,000 m
marathon.
Similarly, though a rainfall may last several hours
(long distance race), the critical condition to observe
in drain design is the highest rain intensity (highestspeed over a short distance race).
The rainfall intensity i is the average rate of
precipitation in mm/hr from a storm having a
duration equal to the time of concentration.
It is assumed that runoff due to a heavy rainfall will
reach a peak at the time of concentration when the
whole catchment is contributing to ow. Then, the
duration of the design storm is equal to the time ofconcentration.
The time of concentration tcis thus the time taken
for water to travel from the catchment boundary to
the point of interest (Points X, Y or Z or estuary) in
Figure 2).
For small, steep areas, (e.g. Mauritius, Seychelles),
the Kirpich formula has been found to give reasonable
estimates for tc. In this formula,
tc= 0.01947 L0.77S -0.385
where
tc = time of concentration in minutes
L = maximum length of travel of water (m)
and
S = slope of catchment = H/L in which
H = difference in elevation between the most
remote point on the catchment and the outlet.
Mays (2004), Reddy (2008) and Rmniras (1986)
give other similar formulae, applicable in different
conditions.
Rainfall intensity-frequency-duration curves are
usually derived by the countrys Meteorological
Services. The rainfall intensity (mm/ or mm/min)
gures are available for different periods of time,
such as 5, 10, 15, 20, 30, 60 and 120 minutes. Table 1
shows examples of rainfall intensities for Seychelles
and Mauritius.
The time of concentration rarely falls exactly on the
duration time for which gures have been provided
by the Meteorological Services. A judicious
interpolation helps.
As previously indicated, a 50 year return period is
a good guideline, but sometimes the designer might
feel that a 100 year return period might be better.
For example, a bridge (Bindra, 1975) is a structure
that is expected to be in operation during a very long
period. In this context, it is natural to consider events
with a return period of 50 years or even more. There
are so many bridges in the world which have been
standing for more than 50 years.
A 100-year rainfall has a 1% chance of occurring in
any single year. This issue will be discussed below.
6. Is a Return Period of 50 years Acceptable?
The results obtained from the above calculations
can prove to be very important in the design of hy-
drological structures such as bridge culverts and
channels to drain the area under consideration and
prevent ooding. Every structure is designed for a
certain design life and it must be ensured that this
structure serves for its purpose without endangering
any life and property.
The risk that failure of such a structure occurring
during its design life has been explained by Mays
(2004) as follows:
Let P be the probability of the occurrence of an
event,
1 P = probability that the event will not
occur
(1 P)(1 P) = probability that the event will notoccur in two successive years.
(1 P)(1 P)(1 P) = probability that the event will
not occur in three successive years.
(1 P) N = probability that the event will not
occur during a span of N successive years.
Hence, the risk, R or the probability that the event
will occur during a span of N years is given by,
R = 1 (1 - P)N
The probability P is given by P = 1/Tr. Table 2 shows,for return periods T
rand various spans of years N,
-
8/13/2019 Drain Design (Proag)
8/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
14
the risk Rthat a ood with certain return period will
be equalled or exceeded during periods of span N
years.
Rainfall frequency and intensity records can be used
to estimate the magnitude of rains and the ensuing
ood ows. In this respect, it is important to notethat there is a 26 % probability that a 100 year rain
will occur during the next 30 years (a generation).
In practical terms, this means that each generation
has a 1 in 4 chance of experiencing ooding, even
if an exceptional (?) rainfall intensity of 100 year
has been considered. Over a 75 year lifetime, the
likelihood rises to 0.53, i.e., the average person has
a 1 in 2 chance of experiencing ooding during
his lifetime.
Is the population ready to accept this?
Even if a higher return interval (e.g. 1,000 years) is
taken, it is found that there is 7.2 % chance (not to be
neglected as being small) that a 1,000 year ood will
occur during a 75 year span a mans lifetime.
It can be noticed that the risk that an event is reached
or exceeded for a certain span of time, decreaseswith an increase in return period. This result is of-
ten used in the design of huge structures. There is
also an increase in cost by considering the design of
a structure for a long return period. However, this
should be done to be safe from calamities causing
loss of life and property.
Many Codes of Practice indicate that one of the
reasons for choosing a return period of 50 years has
been that the average lifetime of most buildings and
structures is near 50 years.
This may have been true at one time. There
are, however, other factors which need to be
considered:
(1) the use of better materials has increased the
lifetime of the buildings and structures. Similarly,the corresponding drains or bridge culverts will
have a longer life.
(2) why should any owner, demolish his building
after 50 years, if it is still serviceable? The Eiffel
Tower was built in 1889, to be demolished just after
the Universal Paris Exhibition. It is still standing and
being regularly maintained. We have not yet seen
any drain being demolished to be enlarged, except
when they have really been shown to be inadequate.
Even if a local authority tried to do so, it very likelythat adjoining structures would prevent this.
Table 2: Risk R, that a ood of a given return period will be equalled or exceeded during
periods of various lengths.
Return
Period
Tr (years)
Risk R for various spans of N years
5 10 30 50 75 100 200 500
1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
5 0.67 0.89 1.0 1.0 1.0 1.0 1.0 1.0
10 0.41 0.65 0.96 0.995 1.0 1.0 1.0 1.0
50 0.10 0.18 0.45 0.64 0.78 0.87 0.98 1.0
100 0.05 0.10 0.26 0.40 0.53 0.63 0.87 0.99
500 0.01 0.020 0.058 0.095 0.14 0.18 0.33 0.63
1,000 0.005 0.010 0.03 0.049 0.072 0.095 0.18 0.39
5,000 0.001 0.002 0.006 0.010 0.015 0.020 0.039 0.095
10,000 0.0005 0.001 0.003 0.005 0.0075 0.0099 0.020 0.049
-
8/13/2019 Drain Design (Proag)
9/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
15
There so many cathedrals and nearby bridges built
in the eighteenth century in Mauritius (thirteenth
century in Europe) still standing today. Would any
present day designer still consider a 50 years lifetime
for such monuments?
(3) the cost of demolition becomes so high that the
owner is likely to push the time limit before he has
to really bring down the structure..
If these factors are considered, what return period
should be considered?
In dam hydrology, the notion of maximum possible
ood (return period of 10,000 to 50,000 years,
depending on authors) has made its appearance, for
exactly the same reasons the possible danger to
human life.
It might be argued that with only some 100 years
data or, in most cases, even less, it is difcult to
make predictions (or wild guesses) about 10,000
years recurrence intervals. But, if a bridge culvert or
drainage channel is needed now nobody will wait
to collect another 50 years of rainfall data.
7. Estimation of the Peak Design
DischargeAt this stage, the peak design discharge may be
calculated and hence used to design the drain or
bridge culvert as the case may be.
Once the design ow has been established, channel
hydraulics may be used to design the channel or
culvert. One typical carrying capacity formula is
that of Manning
where
COUT
= ow in channel (m3/s)
A = wetted area (m2)
R = hydraulic radius (m) = wetted area/wetted
perimeter (m)
S = channel slope
n = Mannings roughness coefcient
= 0.010 smooth, cement lining = 0.013 good brickwork
= 0.030 rivers in good condition
Design constraints are usually channel or river
width and slopes, but the designer should try to see
if other accompanying measures need to be taken.
The choice is likely to be governed by minimumheadway clearances under the bridge due (1) to the
possibility of branches and trees being carried away
and (2) other facilities passing under or by the side
of the bridge.
Some river training works might be necessary just
upstream or downstream of the bridge.
In this context, this formula is enlightening. The
same channel will have different carrying or
discharge capacities if any of the variables changes.
A bigger cross sectional area will increase the
channel capacity, but the effect will be attenuated
if the roughness changes from a smooth, cement
lining to a river in badcondition.
8. The Case for Port Louis
The motorway was ooded at Caudan between
Rogers House and the waterfront on 11thFebruary
2013, without much damage. There was a worse
incident on 30thMarch 2013, with loss of life.
The rainfall recorded, on 30thMarch 2013, at Line
Barracks (less than 100 mm in 1 hour) would
indicate, from Table 1, a return period of some 50-
100 years. However, the fact that there was a similar
ooding at Place dArmes/Caudan (apparently
without the underground pedestrian pathways
getting submerged) on 11thApril 2003 (Wright A.,
Moonien V., 2013) conrms the values of Table 2.
A 50 year ood does not occur every 50 years! It
will certainly occur during a period of 500 years, but
may also occur within the next 10 years !!
In the light of the above discussion, it is judicious
to ask whether ooding can occur again, and how
soon?
The motorway from Montebello towards Port Louis
is lined, practically on both sides with concrete
borders or walls, which are supposed to be very
-
8/13/2019 Drain Design (Proag)
10/12
-
8/13/2019 Drain Design (Proag)
11/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
17
effective against cars trying to rub into them. The
walls are also provided, at regular distances, with
weep holes, which are expected to evacuate water
into the side drains.
While these weep holes can be very effective inevacuating low ows, their small size (some 30 x 10
cm to 40 x 15 cm) becomes inadequate when heavy
rains and winds bring in their loads of gravel, leaves,
and mud. When these weep holes are blocked, the
bituminous motorway becomes a very well designed,
bitumen lined channel, which was well evidenced
during the heavy rains of 30th March 2013. The
motorway was conveying water which was supposed
to be evacuated into the side drains.
At end of July 2013, the weep holes are still of the
same size!
Between Edith Cavell street and the Government
House, the lowest points in Port Louis occur along
the La Poudrire street. Rightly so, the two channels
Le Pouce stream and La Butte Thonnier canal are
located on both sides of this road. The ground also
has a downstream slope towards the sea. This means
that any rainfall will be channelled towards these two
canals/channels and towards the sea.
The only problem is that at the level of the Harbour
Front and Place dArmes, there is an uprising obstacle
(Photos 1 and2) in the form of the motorway and the
Caudan Esplanade. This now implies, that should the
peak discharge ow from heavy rainfall exceed the
discharge capacity of the channels, the ood waters
will not go directly towards the sea, unless and until
they have overtopped the motorway and the Caudan
waterfront Esplanade. Of course, with a consequential
ponding of the area between the Port Louis museum
and the Place dArmes. Again, this is simple logic,
borne out by the events of 11thApril 2003 and 30th
March 2013.
Even assuming that the motorway constitutes a
roadblock in the evacuation of rainwater from Place
dArmes, historical records (Chelin 1989) show
that oods have occurred several times, prior to
the construction of the motorway. This implies that
the existing canals/streams are not enough or are
inadequate to evacuate the water reaching Place
dArmes in case of heavy rainfall.
So, knowing that a rainfall of intensity 100 mm/
hr is not uncommon (see examples and valuesMeteorological Services Table 1), have we proposed
any new canals to evacuate more water?
9. Conclusion
This study has proposed an approach to be adopted
prior to the approval of planning or zoning schemes
with respect to possible ooding.
Rule 1: Do not underestimate the catchment to be
drained, particularly when designing roads. The area,A, is much bigger than the road itself.
Rule 2: In a tropical country like Mauritius, take C
= 1, to cater for extreme conditions when the soil is
saturated.
Rule 3: Determine the rainfall intensity, i, using
the proper and adequate return period, which is
commensurate with what the population expects from
engineers for leading a comfortable life.
Rule 4: Determine the peak discharge Qp fromthe equation Q
p= 0.278 C i A. The size (width and
height) of the channel must consider the possibility of
avoiding blockage by shrubs, leaves and trees during
cyclones.
Rule 5: Design the drain carrying capacity
Rule 6: Check that COUT
Qp
It has been argued that a 50 year return period is prob-
ably too low and higher return periods should be tak-
en, given the relatively high probability of occurrence
during a mans lifetime.
Once the desired safety from oods has been accepted
preferably through legislation it would be easy to
earmark the boundary of the reserved low lying areas reserved for extreme ood conditions. This should
-
8/13/2019 Drain Design (Proag)
12/12
THE JOURNAL OF THE INSTITUTION OF ENGINEERS MAURITIUS
18
ensure that houses do not get ooded regularly.
It is essential that such guidelines and low lying
boundaries be properly adhered to, particularly when
establishing planning zones.
A discussion of the ooding occurrences in Port Lou-
is, before and after the construction of the motorway
in the 1970s, tends to highlight a possible inadequacy
of the existing drainage exits into the sea.
References
Abbey, P. 1999, Storm Water Drainage Design
Guidelines, report for Government of Seychelles,
Ministry of Environment and Transport.
Bindra S. P. 1975, Principles and Practice of Bridge
Engineering. Dhanpat Rai, New Delhi.
Chelin A. 1989, Maurice : Une le et son pass.
Editions du CRI, Ile de la Runion.
Mays L. W. 2004, Water Resources Engineering.
John Wiley, USA.
Moonien V. 2013, Inondations: Port Louis dj sous
leau 2003. LExpress, 11thAugust 2013, p. 20.
Parker D. J. 1998, Warnings for Torrential Rain and
Floodings in Mauritius. Recommendations for the
Government of Mauritius.
Proag V. 1995, The Geology and Water Resources of
Mauritius. Mahatma Gandhi Institute, Mauritius.
Reddy P. J. R. 2008, A Textbook of Hydrology.
University Science Press, New Delhi.
Rmniras G. 1986, Lhydrologie de lingnieur.
Eyrolles, Paris.Ven Te Chow., Maidment D.R., and Mays L. W. 1988,
Applied Hydrology. McGrawHill, New York.
Wilson E. M. 1990, Engineering Hydrology.
Macmillan, England.
Wright A . 2013, Rien na chang pour les autorits
. LExpress, 10thAugust 2013, p. 9.