DRAINAGE PLAN
for
Proposed Cement
Factory,Clarendon.
Volume: 932 Folio:447
Prepared for:
The Owner
May 2010
1
Introduction .................................................................................................... 1
1.1
Background ............................................................................................ 1
1.2
Objectives of the report........................................................................... 1
1.3
Methodology ........................................................................................... 1
2 Description of Development ........................................................................... 1
2.1
The Catchments ..................................................................................... 3
2.2
Soils ........................................................................................................ 5
2.3
Meteorology ............................................................................................ 5
2.4
Anecdotal Evidence of Historical Flooding Events (Hurricane Ivan) ....... 5
2.5
Limitations of Hydrologic Analysis .......................................................... 7
3 Proposed Drainage Infrastructure .................................................................. 7
3.1
Drain Sizing Guidelines .......................................................................... 7
3.2
Hydrology ............................................................................................... 7
3.2.1
SCS Method .................................................................................... 7
3.2.2
Curve numbers ................................................................................ 7
3.3
Hydraulics ............................................................................................... 8
3.3.1
Methodology .................................................................................... 8
3.3.2
Hydraulic Analysis of the proposed site ........................................... 8
4 Conclusions and Recommendations ........................................................... 10
4.1
Conclusions .......................................................................................... 10
4.2
Recommendations ................................................................................ 10
2
1 Introduction
1.1 Background
The proposed site is located approximately 4 km east of Freetown and 6 Km south south east of Old Harbour, Clarendon.
It is the site for the proposed cement factory with a volume of 932 and a folio of 447. The site is bordered to the north
Highway 2000 and bordered to the west by the HIPRO FEEDS factory. Please see Figure 1.1 which shows the location of
the proposed site.
Figure 1.1. Map shows location of proposed site.
1
1.2 Objectives of the report
The Client is desirous of developing a drainage plan as requested by the
National Works Agency. This drainage plan examined the flow of stormwater
which will be conducted through the site.
The goals of this report are as follows:
 To outline the flows expected to be generated from the proposed site.

To design preliminary drainage works for the site.
1.3 Methodology
The methodology adopted to complete these goals are as follows:
 Field investigations to determine past flooding and rainfall runoff event
effects.
 Hydrological analysis of the associated catchments
 Hydraulic analysis of the existing and proposed drains and drainage
features
 Prepare engineering analysis, designs and drawings for preliminary
drainage infrastructure.
2 Description of Development
The site at present is has a gentle slope from north to south with the site
elevation ranging from 23.5 m to 14m. The site receives an earth drain at the
north end approximately 2m wide and 2.7 m deep which runs through the centre
of the proposed site. Investigations of the site topography as well as anecdotal
information revealed that the drain is unable to contain the stormwater which is
expected from even the 2 year return period and so ponding in several low spots
on the site occurs. Please see Figure 2.1 which shows an overview of the site,
the existing earth drain and proposed layout of the factory.
1
Figure 2.1. Overview of site showing existing earth drain and factory layout
2
2.1 The Catchments
It was crucial for us to delineate the catchments associated with the site so that
we could deduce the rainfall runoff flows expected to reach the site.
The area of concern has several catchments which are generally bounded by the
natural topography. From the site visit it was evident that in its existing form, the
land is drained via overland sheet flow to the earth drain travelling through the
site as well as the low spots. The catchments associated with the site have an
overall area of 223 HA. The catchments associated with subdivision one have
moderate slopes reaching a summit of 39 m above mean sea level north of the
site. Please see Figure 2.2A which shows the catchments associated with the
site.
3
Figure 2.2A. Map shows associated catchment area.
4
2.2 Soils
The soil types as described in the “GIS data sets obtained from the
Meteorological Service of Jamaica for the area contained by the main
catchments are clay loam and sandy loam. These soils are rapidly draining soils
having a high propensity for landslides.
2.3 Meteorology
The meteorological data for the catchment was obtained from the Intensity
Duration Frequency Curves data set analysed by the Meteorological Service of
Jamaica. Please see Table 2.
Return Periods
Predicted Rainfall Intensity
2
5
10
94
140
170
50
236 mm/24hrs
Table 2.1. Table shows predicted rainfall intensity for different return periods at the Bodles
Station
Bodles is approximately 2km, north of the proposed site. The rainfall extremal
data was obtained for the 50 year return period as instructed by the NWA
guidelines for designing internal drainage and was found to be 236 mm/24hrs.
2.4 Anecdotal Evidence
(Hurricane Ivan)
of
Historical
Flooding
Events
To supplement meteorological data, information regarding historical rainfall
events was obtained by conducting interviews with present residents in the area.
On speaking to 5 residents in the area, it was concluded that the proposed sites
had no historical flooding events even during severe weather systems such as
Hurricane Ivan in 2004 and Hurricane Dean in 2007. It was the general
consensus that all the rainfall runoff tended to travel over land and on the existing
roads to the existing earth drain. It should be noted however that there are no
residents directly adjacent to the site and so they could not speak for what
happens on the property.
A preliminary hydrologic analysis was conducted to conclude whether the
existing drain could handle the expected flows from catchment. The analysis
revealed that the existing drain could not handle the expected flows for even the
2 year event however the closest residents interviewed (less than 1km from site)
had no recollection of flooding in past storm events. Please see Table 2.1 which
shows the calculations.
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Input Parameters
Area
Mannings Coefficient for land
Lengths
Main stream length, L
Secondary length
Distance from outlet to centroid, Lc
Elevations
Lower elevation
Upper elevation
Upper elevation-catchment ridge
Slope
Main channel slope
Catchment slope
Ct
Cp
Runoff Coefficient, C
Curve Number, CN
Values
Box Channel
Length of main channel
Slope
Mannings Coefficient
Width
Depth
Depth + freeboard
R
P
A
Velocity
Flow
Tt
Hydrology
Time of concentration
Time entry
Overland/shallow flow, Tt (NCCS revised)
User switch (Box = 1, V = 2, Trapezoidal =3; Pipe = 4)
Time of travel in main channel
Values
1426
0.67%
0.015
2.000
2.500
3.13
0.7
7
5
4.3
21.74
0.091
Units
m
0.083
0.042
1
0.091
hours
hours
0.22
hours
Units
2,233,733 m
2
0.035
783.2
m
m
m
14.00
23.50
39.50
m
m
m
1426
904
0.7%
1.1%
1.50
0.17
70%
87
Tc
m
m
m
m
m
2
m
m/s
3
m /sec
hours
hours
Rainfall-24 hours ( 1 in ARI year return period)
94
mm/24hours
Rainfall Intensity
41
mm/hour
Rainfall Intensity for Tc
7.83
3
m /sec
25.34
Table 2.1. Calculations of the capacity of the existing drain and the expected flow from the
2 year rainfall event.
Effective Runoff Rate
6
2.5 Limitations of Hydrologic Analysis
It is important to note that there was insufficient topographic information with
respect to the earth drain upstream of the site and so the true elevation of the
invert of the gully was not known. Given this limitation, as well as limited site
topographic data, topographic data from the12 500 map series was used in the
analysis. A detailed site specific topographic survey as well as a survey of the
earth drain upstream is recommended so that an accurate hydrologic and flood
plain analysis can be conducted. It is possible that depressions upstream which
during rainfall events, act as ponds could significantly decrease the peak flow
downstream and so may explain why flooding is not recalled at the site.
3 Proposed Drainage Infrastructure
3.1 Drain Sizing Guidelines
The drainage for the project was designed according to specific guidelines set
out in the Ministry of Transport and Works Guidelines for Drainage review of
Subdivisions and Development Applications (May 2007). Some of these
guidelines are as follows:

The return period to be used for flood control is 1 in 50 year.

The internal diameter of sewer or drainage pipes must not be smaller than
450 mm

The freeboard in all drains must be at least 25% of the depth of channel
3.2 Hydrology
The hydrology was analyzed using the Natural Resources Conservation Service
(NRCS) Method for generating peak flow. This method incorporates selecting a
single curve number to describe the runoff land and soil conditions.
For the analysis a type III distribution curve was used.
3.2.1 SCS Method
SCS method is an empirical model for rainfall runoffs which is based on the
potential for the soil to absorb a certain amount of moisture. On the basis of
field observations, this potential storage S (millimeters or inches) was related to
a 'curve number' CN which is a characteristic of the soil type, land use and the
initial degree of saturation known as the antecedent moisture condition.
3.2.2 Curve numbers
The curve number used in the SCS method was extracted from the
recommended set of values given in the ‘Stormwater Collection Systems Design
Handbook’ – written by Larry W. Mays, PH.D., PE., P.H.
7
The selected curve number was 87 for the type of soil and the observed land use
corroborated by that stated in the “Soil and Land-Use Surveys” for Clarendon.
3.3 Hydraulics
3.3.1 Methodology
The hydraulics was approached using a steady state analysis. The hydraulics
was programmed using the Manning’s Equation for the design of an open
channel (v-drain), a box drain and drain pipes.
The Manning’s equation is one of the most commonly used equations governing
open channel flow. The Manning’s equation is an empirical equation that applies
to uniform flow in open channels and is a function of the channel velocity, flow
area and channel slope.
Where:
Q = Flow Rate, (ft3/s)
v = Velocity, (ft/s)
A = Flow Area, (ft2)
n = Manning’s Roughness Coefficient
R = Hydraulic Radius, (ft)
S = Channel Slope, (ft/ft)
3.3.2 Hydraulic Analysis of the proposed site
The proposed plan is to redirect the route of the existing earth drain to a
proposed open U-drain which will run along the northern boundary in a westerly
direction to the western boundary of the site. The drain will then continue south
along the western boundary to the southwest corner of the site before discharge
into an existing culvert which will have to be upgraded. The site will then be
graded to drain via overland sheet flow to internal drainage to be designed once
the construction of the plant begins, and ultimately discharged to the Open Udrain being proposed along the western boundary.
The proposed catchment is expected to generate a flow of 79.22 cubic metres
per second for the 1 in 50 year event. The proposed U-drain will therefore be 8.6
m wide and 1.9m deep with a slope of 0.67% in order to handle the expected
flow. Please see Table 3.1 which shows the calculations.
8
Table 3.1 Calculation of proposed drain size for the 50 year return period.
Input Parameters
Values
Units
2
Area
2,233,733 m
Mannings Coefficient for land
0.035
Lengths
Main stream length, L
1426
m
Secondary length
904
m
Distance from outlet to centroid, Lc
783.2
m
Elevations
Lower elevation
14.00
m
Upper elevation
23.50
m
Upper elevation-catchment ridge
39.50
m
Slope
Main channel slope
0.7%
Catchment slope
1.1%
Ct
1.50
Cp
0.17
Runoff Coefficient, C
70%
Curve Number, CN
87
Box Channel
Length of main channel
Slope
Mannings Coefficient
Width
Depth
Depth + freeboard
R
P
A
Velocity
Flow
Tt
Hydrology
Time of concentration
Time entry
Overland/shallow flow, Tt (NCCS revised)
User switch (Box = 1, V = 2, Trapezoidal =3; Pipe = 4)
Time of travel in main channel
Values
1426
0.67%
0.015
8.600
1.500
1.88
1.1
11.6
12.9
5.8
75.35
0.068
Units
m
0.083
0.042
1
0.068
hours
hours
Tc
0.19
hours
Rainfall-24 hours ( 1 in ARI year return period)
236
mm/24hours
Rainfall Intensity
128
mm/hour
Rainfall Intensity for Tc
20.32
Effective Runoff Rate
79.22
9
m
m
m
m
m
2
m
m/s
3
m /sec
hours
hours
3
m /sec
The proposed drain will discharge into an existing culvert in the wouthwest corner
of the site. The culvert will need to be upgraded to a box culvert of similar
dimensions (8.6 m wide by 1.9m deep) to handle the 50 year return event.
4 Conclusions and Recommendations
4.1 Conclusions
The following could be concluded from the analysis conducted to date:
 The Catchments associated with the site are primarily bounded by the
natural topography in the area.
 Presently the site is drained by overland sheet flow to the existing earth
drain which runs through the site.
 Anecdotal information retrieved on the site visit from 5 residents in the
surrounding areas showed that there are no historical flood events on the
site.
4.2 Recommendations
The following are our recommendations based on the analysis and design
conducted:
 A detailed topographic survey of the proposed site and the earth drain
upstream should be conducted to confirm the expected flows reaching the
site.
 A flood plain analysis should be conducted before internal drainage
infrastructure as well as design floor levels are concluded.
 Use the proposed drainage infrastructure to conduct the flow out of the
site
 Site should be graded to encourage runoff to proposed drainage
infrastructure.
Supplementary Paragraph
An earth drain presently runs in a southerly direction from as far north as Bodles
Agricultural Research Station, under Highway 2000 via a culvert before crossing
the proposed site. Please see drawing attached. The catchment associated with
this drain is approximately 223 HA and is denoted by the broken blue line on the
drawing.
During a rainfall event, water travels via over land sheet flow to the earth drain
which conducts the water through the proposed site before discharge into a
culvert located in the south west corner of the site. The expected runoff rate of
rainfall during a 50 year storm event is 79.22 m3/s as stated in the drainage
report. The direction of flow is denoted on the drawing by the magenta arrows.
10
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