osapa channel report 2010( nbc-cng367-r004rev0)
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2011
BY
HYDRAULIC ANALYSIS OF OSAPA CHANNEL(NICON TOWN REACH)
No 3 Asenuga Str. Off Opebi Link road Ikeja Lagos 08023356112, 07093189196
NOBLE BC LTD.
CLIENTS
NICON TOWN MANAGEMENT CO. PLC
Lekki Epe Express Lagos
REF: NBC-CNG367-R004REV0
REF: NBC-CNG367-R004REV0 Page 2 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
TABLE OF CONTENTS
TABLE OF CONTENTS ..................................................................................... 2
1. SCOPE AND METHOD OF WORK ............................................................ 7
1.1. GENERAL ............................................................................................ 7
1.2. TERMS OF REFERENCE ................................................................... 7
1.3. PROJECT AREA AND ITS BOUNDARIES .......................................... 8
1.4. METHOD OF WORK ........................................................................... 9
1.4.1. Data collection ................................................................................ 9
1.4.2. Preliminary design .......................................................................... 9
2. THE PROJECT SETTING ......................................................................... 10
2.1. EXTENT OF THE OSAPA CHANNEL CATCHMENT....................................... 10
2.2. IMPORTANCE OF STORM WATER DRAINAGE SYSTEM ............................... 10
2.3. CLIMATE .............................................................................................. 10
2.4. RAINFALL .......................................................................................... 11
3. OSAPA CHANNEL DOWN STREAM REACH ASSESSMENT AND
DESIGN ................................................................................................................ 14
3.1. DESIGN CRITERIA. .......................................................................... 14
3.2. RUNOFF CALCULATION .................................................................. 14
3.2.1. Rational Method ............................................................................ 14
3.3. CHANNEL DESIGN ............................................................................ 21
3.3.1. Open Channel Velocity. ................................................................ 21
3.3.2. Energy....................................................................................... 22
3.3.3. Flow Classification .................................................................... 23
3.3.4. Design Parameters ................................................................... 24
3.4. DESIGN PROCEDURES. .................................................................. 26
3.4.1. Special Features. .......................................................................... 26
4. DRAINAGE DESIGN .................................................................................. 27
4.1. TERRAIN HYDROLOGY .................................................................... 27
4.1.1. Catchment Areas ........................................................................... 27
4.1.2. Run-off calculations ...................................................................... 28
4.1. TOTAL FLOW HYDROGRAPH FOR SUB-BASINS......................................... 30
4.2. TOTAL FLOW HYDROGRAPH FOR NODES ................................................ 33
4.2.1. Osapa upstream with relevant nodes J-31, J-18, J-17 ............... 33
4.2.2. Osapa Downstream with relevant nodes J-29, J-23, J-26, J-
8 35
4.2.3. Igbokushu channel with relevant nodes J-1, J-21, J-22. ........ 37
4.2.4. ‘Primary A’ channel with relevant nodes J-23, J-24, J-25. .. 37
4.3. UNSTEADY FLOW HYDRAULIC ANALYSIS .................................................. 38
4.3.1. Hydraulic sections ......................................................................... 38
4.3.2. UNSTEADY FLOW ANALYSIS RESULTS (BYPASS
CHANNEL) 41
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.3.3. UNSTEADY FLOW ANALYSIS RESULTS (STRAIGHT
CHANNEL) 45
4.4. SEDIMENT TRANSPORT HYDRAULIC ANALYSIS ........................................ 49
4.4.1. Soil sample data ........................................................................... 49
4.4.2. Quasi-Unsteady flow data........................................................ 50
4.4.3. SEDIMENT TRANSPORT ANALYSIS RESULTS ................... 52
4.4.4. SEDIMENT TRANSPORT ANALYSIS RESULTS (BYPASS
CHANNEL WITH SEDIMENT BASIN) ...................................................... 54
4.4.5. CROSSECTION VIEW OF HIGH DEPOSITE AREA. ............. 55
5. CONCLUSION AND RECOMMENDATION .............................................. 56
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
EXECUTIVE
SUMMANRY
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
EXECUTIVE SUMMARY
The objective of this report is to provide the details of the
assessment and design of the Osapa channel downstream reach. A
brief was given by Nicon town management Company concerning
the Osapa channel and its discharge reach whose downstream
alignment is designed by the Lagos state ministry of environment
(drainage department) to pass through the premises of Nicon town
Lekki.
The Osapa channel is a about 2.3km in length from midstream at
the Lekki Epe express way to Its discharge point inside the Lagos
Lagoon behind Nicon town Lekki. As at the point of this report, the
naturally formed earth channel is beign lined to chainage 0+850
from the Express way. The Osapa channel is with top width of 12m
upstream before the Lekki-Epe express way and 15m top width just
after the bridge(in its midstream) on Lekki-Epe expressway to its
present point of construction. Average depth of the channel is 1.5m.
This report considers the hydraulic and hydrological parameters for
the Osapa channel catchment and these were used to assess and
design the downstream reach of the channel which is to discharge
into the lagoon. The challenge is to study the flow behavior of the
channel alignment, if straight and also if diverted via a Bypass
channel which will not pass throught the Nicon town premises.
Two downstream reach were considered for the Osapa channel.
• Straight reach through the premises of Nicon Town.
• Bypass reach circumventing the Nicon town premises on the
east and northern boundary wall.
Two hydraulic assessment procedures were employed
• Gradually varied or unsteady flow simulation for 12hrs.
• Quasi-unsteady flow for 3yrs for sediments transport
simulation
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
Our assessment results shows:
• unsteady flow simulation for 12hrs shows both bypass and
straight options having water profile elevation for upstream
channels on or below 4m elevation which is fit for purpose
• sediment transport simulations shows; about 0.35m
sediment deposition (in 3yrs) in the downstream reach if the
channel is made straight while sediment deposition reached
about 1.2m depth (in 3yrs) of the total 1.5m depth of the
channel if bypass channel is considered.
• Upstream regions becomes prone to flooding due to gross
increase in water profile elevation if downstream reach is
made into a bypass channel.
Our design results shows
• Introduction of a sediment basin just before the high
sediment deposition zone in the bypass channel will help
reduce drastically, the rate of depletion of available hydraulic
section required for storm water discharge in the sediment
prone area.
• In three years, with a sediment basin in place, about 0.5m
sediment deposition height is achieved above ambient invert
elevation of the channel.
• Water elevation profile is grossly decreased below flood
limits.
• Section for bypass channel or straight channel 12m bottom
width trapezoidal channel wit1.5m average depth and 1:2
walls
• Discharge channel into the lagoon is 15m bottom width
trapezoidal channel wit1.5m average depth and 1:2 walls
• The channel total length from the its junction with Igbokushu
channel to the Lekki lagoon is 1.54km for Bypass channel
• The channel total length from the its junction with Igbokushu
channel to the Lekki lagoon is 1.087km for Straight channel
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
1. SCOPE AND METHOD OF WORK
1.1. GENERAL
Noble B.C. Ltd. was commissioned by the Nicon Town
Development Company to perform hydrological and
hydraulic design for the reach of the proposed channel to
discharge storm water from the Osapa and Igbokushu
channels into the Lagos lagoon
1.2. TERMS OF REFERENCE
This project involves the assessment of two possible
routes for the Osapa channel downstream reach. The first
route is to pass straight on through the Nicon town.
Second route is to adjoin the Nicon town boundary wall
on the west and north and eventually discharge into the
lagoon. This assessment is to help ascertain a feasible
and resolving alignment for the osapa channel
downstream reach with focus on the possible re-route of
the alignment to follow the second route stated above.
NOBLE B.C is to provide a report in this regards and plan
profile drawings of the eventual route alignment. The
Lagos state government is to use this report as a guide in
assessing the Osapa channel and deliberate on actions
considering routing the Osapa channel downstream
reach. Survey data was provided by the Nicon town
development company via an independent surveyor. This
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
data was however updated by Noble B.C. Ltd to make it
fit for purpose.
1.3. PROJECT AREA AND ITS BOUNDARIES
The Osapa channel alignment starts with its upstream
coordinate at around (710371N and 556583E) it
crosses the Lekki Epe express way at approximately
0.2km east of the Jakande estate Lekki. It traverses
north westward towards the Lekki lagoon in its natural
course and eventually directed downstream into the lekki
lagoon amidst present developments. Other channel
contributing to the Osapa channel is the Igbokushu
channel under construction discharging midstream and
the ‘primary A’ channel discharging downstream.
Detail of the region around the Osapa channel reveals
the boundaries for its catchment area, see figure 1.
• Bounded in the North by the Lekki lagoon
• The South by Atlantic Ocean,
Figure 1: schematic representation of drainage network
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
1.4. METHOD OF WORK
1.4.1. Data collection
Survey data for alignment assessment and design
was supplied by Nicon town development
company. The survey features a plan layout of the
Osapa channel and its catchment and all
contributing channels into the alignment. It also
features the nicon town estate and other estates
within the environ of the alignment. We provided
inverts and bank elevation along the Osapa
alignment.
Soil samples were collected along the osapa
channel for sieve analysis and soil data
evaluation. Hydrological data for storm water
estimation was derived from the Nigerian
meteorological service company.
1.4.2. Preliminary design
Storm water runoff calculation was estimated for
the Osapa catchment for the assessment of the
channel from its upstream point at the Lekki Epe
expressway. Rainfall precipitation were
determined for various storm return period which
was used in the assessment and design
procedure. These form an integral part of this
report as described in Section 2.4.
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
2. THE PROJECT SETTING
2.1. Extent of the Osapa channel catchment
The entire catchment measures a land mass of about
900 hectares. The environment features appreciable
development with roughly about 68% of its land mass
being developed. Storm water infiltration is relatively high
and surface run off is also considerable and would
increase vis-à-vis increased development of the
catchment. The catchment Area is bounded in the north
by the Lekki lagoon and in the south by the Atlantic
Ocean. Fig. 11 shows the location of the Project Area as
defined for this study.
2.2. Importance of storm water drainage system
The need for an effective and efficient drainage system
for the Lekki catchment cannot be over emphasized.
Lekki lies between two immense water bodies with a land
mass that is almost flat in terrain.
2.3. Climate
The Project Area has a littoral type of climate with an
average daily temperature varying between 30°C in the
hottest month (March) to 24°cC in the coldest month
(August. The relative humidity varies in the region of
100% to 70
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Route Assessment and design Osapa channel
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2.4. RAINFALL
Rainfall generally occurs during the wet season (April to
October) and occasionally during the early part of Dry
Season (November to March). The rainfall is frequently
accompanied by thunderstorms and can be sudden,
heavy, and of long duration. The average
in the recent years is about 1648mm but a maximum
annual rainfall of about 2020mm has been recorded
during the period 1983-2007. The maximum monthly
rainfall of 567mm was recorded in the month of July
1996. Rainfall precipitation data for
was obtained from the Nigerian Meteorological agency
(NIMET). A 25years duration data was made available to
us for design purpose.
February 2010
Rainfall generally occurs during the wet season (April to
October) and occasionally during the early part of Dry
Season (November to March). The rainfall is frequently
accompanied by thunderstorms and can be sudden,
heavy, and of long duration. The average annual rainfall
in the recent years is about 1648mm but a maximum
annual rainfall of about 2020mm has been recorded
2007. The maximum monthly
rainfall of 567mm was recorded in the month of July
1996. Rainfall precipitation data for the catchment region
was obtained from the Nigerian Meteorological agency
(NIMET). A 25years duration data was made available to
Figure 2: sample rainfall precipitation data for year 2007 Obtained from NIMETsample rainfall precipitation data for year 2007 Obtained from NIMET
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Route Assessment and design Osapa channel
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The figures below features the Intensity duration
frequency curves generated from the rainfall precipitation
data for the catchment area obtained from the Nigerian
Meteorological agency (NIMET)
Figure 4: Intensity Duration Frequency (IDF) curve for 1year return storm event
February 2010
The figures below features the Intensity duration
the rainfall precipitation
data for the catchment area obtained from the Nigerian
Figure 3 Intensity Duration Frequency (IDF) curve for 2years return storm event
Figure 5 Intensity Duration Frequency (IDF) curve for 5 years return storm event
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Route Assessment and design Osapa channel
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Figure
February 2010
Figure 7 Intensity Duration Frequency (IDF) curve for 25 years return storm event
Figure 6 Intensity Duration Frequency (IDF)curve for 10 yeareturn storm event
ation (IDF)
years return storm event
Figure 8 Intensity Duration Frequency (IDF) curve for 50 years return storm event
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Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
3. OSAPA CHANNEL DOWN STREAM REACH ASSESSMENT
AND DESIGN
3.1. DESIGN CRITERIA.
The design criteria are developed on the basis of Land
use, proposed terrain, maintenance and safety of
residence. In the computation of runoff, the Rational
method is used which takes into consideration the runoff
coefficient for the land, contributing catchment areas and
the time of concentration.
For quantity of runoff, all calculations relating to runoff
analysis is based upon proposed land use and takes into
consideration any contributing runoff from areas adjacent
sub-basins.
Average land slopes along the terrain are used for the
estimation process of runoff rates.
3.2. RUNOFF CALCULATION
3.2.1. Rational Method
Design storms
The design storm within the jurisdiction of this
project shall be a 24-hour duration storm. Type ll
rainfall distribution shall be used in conjunction
with the 24-hour rainfall
depth.
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Route Assessment and design Osapa channel
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Rational equation
A hydrologic equation based on the premise that
the maximum flow rate occurs when the time of
concentration of the catchment is equal to the
duration of the storm. The maximum flow rate is
proportional to the product of the catchment area
and the rainfall intensity corresponding to the
storm duration.
Eq. 3.1
February 2010
A hydrologic equation based on the premise that
the maximum flow rate occurs when the time of
concentration of the catchment is equal to the
duration of the storm. The maximum flow rate is
proportional to the product of the catchment area
intensity corresponding to the
Where:
Q = Flow, m3/s (ft3/s)
C = dimensionless runoff coefficient
I = rainfall intensity, mm/hr (in/hr)
A = drainage area, hectares, ha (acres)
Ku = units conversion factor equal
English Units)
Time of concentration, tc
o The runoff travel time from the most
remote point of the catchment to the outlet
o It comprises the travel time from roof
gutters, open ground, kerb gutter, pipes
and channels
3.1
Q = Flow, m3/s (ft3/s)
C = dimensionless runoff coefficient
I = rainfall intensity, mm/hr (in/hr)
A = drainage area, hectares, ha (acres)
Ku = units conversion factor equal to 360 (1.0 in
English Units)
Time of concentration, tc
The runoff travel time from the most
remote point of the catchment to the outlet
It comprises the travel time from roof
gutters, open ground, kerb gutter, pipes
and channels
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Route Assessment and design Osapa channel
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Components of surface and
o Overland/allotment travel time from
kinematic wave equation
o Gutter travel time from Izzard's equation
Average Recurrence Interval (ARI), Y
o The average period between years in
which a value (rainfall or r
exceeded
o It is not the time between exceedances of
a given value
o Periods between exceedances are random
Rainfall intensity, I, is dependent on:
February 2010
Components of surface and gutter travel times
Overland/allotment travel time from
kinematic wave equation
Gutter travel time from Izzard's equation
Average Recurrence Interval (ARI), Y
The average period between years in
which a value (rainfall or runoff) is
It is not the time between exceedances of
Periods between exceedances are random
Rainfall intensity, I, is dependent on:
o Locality of the catchment
o Recurrence interval used In the design
o Time of concentration or duration of storm
Runoff Coefficient
The runoff coefficient, C, in equation 3
function of the ground cover and a host of other
hydrologic abstractions. It relates the estimated
peak discharge to a
percent runoff. Typical values for C are given in
table 3-1. If the basin contains varying amounts of
different land cover or other abstractions, a
composite coefficient can be calculated through
areal weighing as follows:
Locality of the catchment
Recurrence interval used In the design
Time of concentration or duration of storm
Runoff Coefficient
The runoff coefficient, C, in equation 3-1 is a
function of the ground cover and a host of other
hydrologic abstractions. It relates the estimated
peak discharge to a theoretical maximum of 100
percent runoff. Typical values for C are given in
1. If the basin contains varying amounts of
different land cover or other abstractions, a
composite coefficient can be calculated through
areal weighing as follows:
Eq. 3.2
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Route Assessment and design Osapa channel
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Table 3.1 Runoff Coefficients for Rational Formula
February 2010
Rainfall Intensity
Rainfall intensity, duration, and frequency curves
are necessary to use the
Refer to section 2.4 for the Intensity duration
frequency curves for the
As gotten from the Nigerian
Rainfall intensity, I, is dependent on:
o Locality of the catchment
o Recurrence interval used In the design
o Time of concentration or duration of storm
Time of Concentration
There are a number of methods that can be used
to estimate time of concentration (tc), some of
Rainfall Intensity
Rainfall intensity, duration, and frequency curves
are necessary to use the rational method.
Refer to section 2.4 for the Intensity duration
frequency curves for the Lekki, Epe region.
As gotten from the Nigerian Metrological Centre.
Rainfall intensity, I, is dependent on:
Locality of the catchment
Recurrence interval used In the design
Time of concentration or duration of storm
Time of Concentration
a number of methods that can be used
to estimate time of concentration (tc), some of
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Route Assessment and design Osapa channel
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which are intended to calculate the flow velocity
within individual segments of the flow path (e.g.,
shallow concentrated flow, open channel flow,
etc.). The time of concentration is calculated as
the sum of the travel times within the various
consecutive flow segments.
Sheet Flow Travel Time:
Sheet flow is the shallow mass of runoff on a
planar surface with a uniform depth across the
sloping surface. This usually occur
headwater of streams over relatively short
distances, rarely more than about 130 m, and
possibly less than 25 m. Sheet flow is estimated
with a version of the kinematic wave equation, a
derivative of Manning's equation, as follows:
February 2010
which are intended to calculate the flow velocity
within individual segments of the flow path (e.g.,
shallow concentrated flow, open channel flow,
ntration is calculated as
the sum of the travel times within the various
Sheet flow is the shallow mass of runoff on a
planar surface with a uniform depth across the
sloping surface. This usually occurs at the
headwater of streams over relatively short
distances, rarely more than about 130 m, and
possibly less than 25 m. Sheet flow is estimated
with a version of the kinematic wave equation, a
derivative of Manning's equation, as follows:
where:
Tti = sheet flow travel time, min
n = roughness coefficient. (see table
L = flow length, m (ft)
I = rainfall intensity, mm/hr (in/hr)
S = surface slope, m/m (ft/ft)
Ku = empirical coefficient equal to 6.92 (0.933
in English units)
Since I depend
the computation of tc is an iterative process.
An initial estimate of tc is assumed and used to
obtain I from the IDF curve. The
computed from equation
the initial value o
= sheet flow travel time, min
n = roughness coefficient. (see table 4-2)
L = flow length, m (ft)
I = rainfall intensity, mm/hr (in/hr)
S = surface slope, m/m (ft/ft)
Ku = empirical coefficient equal to 6.92 (0.933
in English units)
depend on tc and tc is not initially known,
the computation of tc is an iterative process.
An initial estimate of tc is assumed and used to
obtain I from the IDF curve. The tc is then
computed from equation 4.2 and used to check
the initial value of tc. If they are not the same, the
Eq. 3.3
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Route Assessment and design Osapa channel
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process is repeated until two successive tc
estimates are the same.
Table 3.2 Manning's Roughness Coefficient (n) for Overland Sheet Flow
February 2010
process is repeated until two successive tc Shallow Concentrated Flow Velocity.
distances of at most 130 m (400 ft), sheet flow
tends to concentrate in rills and then gullies of
increasing proportions. Such flow is usually
referred to as shallow concentrated flow. The
velocity of such flow can be estimated using a
relationship betwee
where:
Ku = 1.0 (3.28 in English units)
V = velocity, m/s (ft/s)
k = intercept coefficient (table 3
Sp = slope, percent
Manning's Roughness Coefficient (n) for Overland Sheet
Shallow Concentrated Flow Velocity. After short
distances of at most 130 m (400 ft), sheet flow
tends to concentrate in rills and then gullies of
increasing proportions. Such flow is usually
to as shallow concentrated flow. The
velocity of such flow can be estimated using a
between velocity and slope as follows:
= 1.0 (3.28 in English units)
V = velocity, m/s (ft/s)
k = intercept coefficient (table 3-3)
Sp = slope, percent
Eq. 3.3
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Route Assessment and design Osapa channel
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Table 3.3 Intercept Coefficients for Velocity vs. Slope
Equation 4.3
February 2010
Intercept Coefficients for Velocity vs. Slope Relationship of
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Route Assessment and design Osapa channel
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3.3. CHANNEL DESIGN
3.3.1. Open Channel Velocity.
Manning's equation can be used to estimate
average flow velocities in pipes and open
channels as follows:
where:
n = roughness coefficient (see table
V = velocity, m/s (ft/s)
February 2010
Manning's equation can be used to estimate
average flow velocities in pipes and open
n = roughness coefficient (see table 4.4)
R = hydraulic radius (defined as the flow area
divided by the wetted perimeter),
m (ft)
S = slope, m/m (ft/ft)
Ku = units conversion factor equal to 1 (1.49 in
English units)
Eqn3.4
Table 3.4 Values of Manning's Coefficient (n) for Channels
R = hydraulic radius (defined as the flow area
divided by the wetted perimeter),
S = slope, m/m (ft/ft)
Ku = units conversion factor equal to 1 (1.49 in
Values of Manning's Coefficient (n) for Channels
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Route Assessment and design Osapa channel
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3.3.2. Energy
Conservation of energy is a basic principal in
open channel flow. As shown in figure 5
total energy at a given location in an open
channel is expressed as the sum of the potential
energy
head (elevation), pressure head, and kinetic
energy head (velocity head). The total energy at
given channel cross section can be represented
as
where:
Et = total energy, m (ft)
February 2010
Conservation of energy is a basic principal in
open channel flow. As shown in figure 5-1, the
total energy at a given location in an open
channel is expressed as the sum of the potential
head (elevation), pressure head, and kinetic
energy head (velocity head). The total energy at
given channel cross section can be represented
Z = elevation above a given datum, m (ft)
y = flow depth, m (ft)
V = mean velocity, m/s (ft/s)
g = gravitational acceleration, 9.81 m/s2 (32.2
ft/s2)
Written between an upstream cross section
designated 1 and a downstream cross section
designated 2, the energy equation becomes
where:
hL = head or energy loss between section 1 and
2, m (ft)
Eq. 3.5
Z = elevation above a given datum, m (ft)
depth, m (ft)
V = mean velocity, m/s (ft/s)
g = gravitational acceleration, 9.81 m/s2 (32.2
Written between an upstream cross section
designated 1 and a downstream cross section
designated 2, the energy equation becomes
= head or energy loss between section 1 and
Eq. 3.6
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3.3.3. Flow Classification
All channels and drains is classified using the
following characteristics:
Figure 9: Total energy in open channels.
February 2010
is classified using the
o Subcritical
Subcritical Flow
is less than one (Fr < 1). In this state
depths greater than critical depth occur (refer to
figure 5-2), small water surface disturbances
travel both upstream and downstream, and the
control for the Flow depth is always
Total energy in open channels.
Figure 10: Specific energy diagram.
Subcritical, supercritical or critical
Subcritical Flow occurs when the Froude number
is less than one (Fr < 1). In this state
greater than critical depth occur (refer to
2), small water surface disturbances
travel both upstream and downstream, and the
control for the Flow depth is always
Specific energy diagram.
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downstream. The control is a structure or
obstruction in the channel which affects
Flow. Subcritical Flow can be characterized by
slower velocities, deeper depths and
while supercritical Flow is represented by faster
velocities, shallower depths and steeper
Supercritical Flow occurs when the Froude
number is greater than one (Fr > 1).
of Flow, depths less than critical depth occur
(refer to figure 5-2), small water
disturbances are always swept downstream, and
the location of the Flow control
upstream. Most natural open channel flows are
subcritical or near critical in nature.
supercritical flows are not uncommon for smooth-
lined ditches on steep grades.
3.3.4. Design Parameters
Parameters required for the design of channels in
free zone include discharge frequency, channel
geometry, channel slope, vegetation type and
freeboard. This section provides criteria relative to
the selection or computation of these design
elements.
Discharge Frequency
The Primary channels will be designed to
discharge 10-year design flows while the
secondary channels will be designed for 5 years
rainfall return period
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Channel Geometry
For Primary channels, they will be designed as
trapezoidal in shape. The secondary channels will
be designed as rectangular channels which will
form a network and feed the primary channels.
Channel side slopes
The trapezoidal channels to be designed for will
have side slopes not exceeding the angle of
repose of the soil in its environ.
Channel Slope
In this design, Channel slopes are generally
dictated by the proposed terrain vis-a-vis the flat
nature of the existing terrian. However, if channel
stability conditions warrant, it may be feasible to
adjust the channel gradient slightly
Freeboard
The freeboard of a channel is the vertical distance
from the water surface to the top of the channel.
The importance of this factor depends on the
consequence of overflow of the channel
bank. At a minimum the freeboard is made
sufficient to prevent waves, super elevation
changes, or fluctuations in water surface from
overflowing the sides.
REF: NBC-CNG367-R004REV0 Page 26 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
3.4. DESIGN PROCEDURES.
On establishment of design concept and design criteria,
design procedures were followed to establish the new
improved layout for the drains network, design runoff,
preparation of channel profiles and preliminary sizing of
drain. A computerized data input from field survey and
calculations were employed for sizing and evaluating the
capacities of proposed channels and pipes.
3.4.1. Special Features.
These involve the relationship of the Major drain
system with respect to other minor drains that it
discharged. A key challenge is the gate house
which presently has a ground floor level of about
3.5m and as such low in elevation as compared
to the proposed terrain elevations.
REF: NBC-CNG367-R004REV0 Page 27 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4. DRAINAGE DESIGN
4.1. TERRAIN HYDROLOGY
4.1.1. Catchment Areas
Note in the figure 11 that the catchment area for
the Osapa channel is the entire area shown in
green stripes. However, the catchment is divided
into sub-basins named sub-1, sub-2...etc.
This area is defined by the catchment for the
Osapa channel and its contributing secondary
channels.
Figure 11: catchment zones for runoff estimation
REF: NBC-CNG367-R004REV0 Page 28 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.1.2. Run-off calculations
The rational method is used and land use was
based on Residential but since Lagos residence
are known to have almost all land space paved,
run-off coefficient selected is 0.72. This forms the
basis of the design rational coefficient. Rainfall
return period is 10 years and Sheet flows were
estimated based on 2 years 24hrs rainfall. Note
that the contributing nodes as shown in red and
labelled appropriately.
REF: NBC-CNG367-R004REV0 Page 29 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
SN Element Area Weighted Average Equivalent Time Accumulated Total Peak Rainfall
ID Runoff Slope Width of Precipitation Runoff Runoff Intensity
Coefficient Concentration
(ha) (%) (m) (days hh:mm:ss) (mm) (mm) (cms) (mm/hr)
1 sub-1 23.59 0.7200 0.0015 442.44 0 02:56:18 82.14 59.14 1.31 27.950
2 sub-10 8.70 0.7200 0.0015 258.64 0 02:03:41 71.97 51.82 0.60 34.917
3 sub-11 11.94 0.7200 0.0015 272.49 0 02:31:35 77.60 55.87 0.73 30.731
4 sub-12 28.39 0.7200 0.0015 444.57 0 03:22:35 86.45 62.24 1.44 25.614
5 sub-13 32.27 0.7200 0.0015 471.84 0 03:33:32 88.18 63.49 1.59 24.781
6 sub-14 9.48 0.7200 0.0015 182.28 0 02:52:55 81.50 58.68 0.53 28.292
7 sub-15 10.37 0.7200 0.0015 197.81 0 02:54:02 81.72 58.84 0.58 28.178
8 sub-16 6.84 0.7200 0.0015 124.54 0 03:00:25 82.80 59.62 0.37 27.549
9 sub-17 14.28 0.7200 0.0015 235.98 0 03:14:19 85.16 61.32 0.75 26.294
10 sub-18 17.10 0.7200 0.0015 275.66 0 03:18:08 85.79 61.77 0.88 25.973
11 sub-19 13.14 0.7200 0.0015 50.00 0 10:02:15 129.72 93.40 0.34 12.921
12 sub-20 9.52 0.7200 0.0005 191.46 0 04:15:00 94.21 67.83 0.42 22.168
13 sub-21 73.69 0.3400 0.0002 491.03 0 15:49:05 153.59 52.22 0.67 9.711
14 sub-22 96.26 0.7200 0.0015 644.25 0 06:29:49 110.33 79.44 3.24 16.981
15 sub-23 103.93 0.7200 0.0002 691.16 0 15:50:33 153.69 110.65 2.00 9.701
16 sub-24 48.20 0.7200 0.0002 308.50 0 16:19:05 155.38 111.88 0.91 9.523
17 sub-25 139.54 0.7200 0.0002 1464.07 0 11:09:07 134.89 97.12 3.35 12.095
18 sub-26 27.01 0.7200 0.0002 50.00 1 18:24:43 221.68 159.61 0.28 5.227
19 sub-3 9.69 0.7200 0.0015 293.99 0 02:01:43 71.53 51.50 0.68 35.272
20 sub-4 7.75 0.7200 0.0015 256.64 0 01:53:49 69.80 50.26 0.57 36.790
21 sub-5 3.17 0.7200 0.0015 120.63 0 01:42:16 67.11 48.32 0.25 39.349
22 sub-6 8.08 0.7200 0.0015 279.12 0 01:50:10 68.95 49.64 0.60 37.551
23 sub-7 6.16 0.7200 0.0015 115.09 0 02:56:50 82.22 59.20 0.34 27.896
24 sub-8 7.07 0.7200 0.0015 196.58 0 02:10:12 73.35 52.81 0.47 33.811
Figure 12: Sub basin Hydrology data
REF: NBC-CNG367-R004REV0 Page 30 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.1. Total Flow hydrograph for sub-basins
REF: NBC-CNG367-R004REV0 Page 31 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
REF: NBC-CNG367-R004REV0 Page 32 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
REF: NBC-CNG367-R004REV0 Page 33 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.2. Total Flow hydrograph for nodes
4.2.1. Osapa upstream with relevant nodes J-31, J-18,
J-17
REF: NBC-CNG367-R004REV0 Page 34 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
REF: NBC-CNG367-R004REV0 Page 35 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.2.2. Osapa Downstream with relevant nodes J-29, J-
23, J-26, J-8
REF: NBC-CNG367-R004REV0 Page 36 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
REF: NBC-CNG367-R004REV0 Page 37 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.2.3. Igbokushu channel with relevant nodes J-1, J-
21, J-22.
4.2.4. ‘Primary A’ channel with relevant nodes J-23, J-
24, J-25.
REF: NBC-CNG367-R004REV0 Page 38 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.3. Unsteady flow hydraulic analysis
4.3.1. Hydraulic sections
Osapa Upstream
Before the Lekki Epe express way
Trapezoidal channel (Bottom Width (BW) 6m Side Slope
1:2
Existing bridge details at Lekki Epe express way
Existing culvert detail at chainage 0+500m along Osapa
channel
REF: NBC-CNG367-R004REV0 Page 39 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
Existing culvert detail at chainage 1+191m along Osapa
channel by Nicon town estate and Igbokushu channel
junction.
After the Lekki Epe express way
Trapezoidal channel Bottom Width (BW) 9m Side Slope
1:2
Bypass Channel (To Pass Through Or Bypass Nicon
Town Premises
Trapezoidal channel (Bottom Width (BW) 12m Side
Slope 1:2
Proposed culvert to cross proposed dual carriage road
behind Nicon town
REF: NBC-CNG367-R004REV0 Page 40 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
Igbokushu Channel
Upstream
Rectangular channel (Bottom Width (BW) 3m depth
1.5m
Midstream
Rectangular channel (Bottom Width (BW) 3m, depth
1.5m
Downstream
Rectangular channel (Bottom Width (BW) 5m, depth
1.5m
Downstream
Rectangular channel (Bottom Width (BW) 8m, depth
1.5m
Discharge channel
Trapezoidal channel (Bottom Width (BW) 15m Side
Slope 1:2
REF: NBC-CNG367-R004REV0 Page 41 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.3.2. UNSTEADY FLOW ANALYSIS RESULTS
(BYPASS CHANNEL)
In this scenario, the Osapa channel is directed in the
alignment indicated as the Bypass channel in the figure
14.
Also find below a schematic representation of the
hydraulic model developed for this analysis. Note that
simulation time is 12 hours based on input data
represented by flow hydrographs shown earlier in this
chapter. These hydrographs were built into the model and
representing total flow and lateral flow hydrographs at
relevant nodes within the reaches of the channel
Figure 14: Osapa channel with Bypass route.
Figure 13: Hydraulic
model
REF: NBC-CNG367-R004REV0 Page 42 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
WATER SURFACE PROFILES (OSAPA WITH
BYPASS CHANNEL)
Shows water surface elevation at 0hr, 12hrs and the
maximum water surface elevation attained during the
unsteady flow.
EG= energy grade WS= water surface
Crit= critical Max= maximum
Chnnl= channel Vel= velocity
REF: NBC-CNG367-R004REV0 Page 43 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
REF: NBC-CNG367-R004REV0 Page 44 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
PROFILE OUTPUT TABLE FOR OSAPA WITH
BYPASS CHANNEL
Figure 15: Summary output table for bypass channel route
REF: NBC-CNG367-R004REV0 Page 45 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.3.3. UNSTEADY FLOW ANALYSIS RESULTS
(STRAIGHT CHANNEL)
In this scenario, the mid reach of the Osapa
channel is straight and its alignment passes
through the Nicon town estate.
Figure 16: osapa channel with Straight reach
Figure 17: Hdraulic Model
REF: NBC-CNG367-R004REV0 Page 46 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
WATER SURFACE PROFILES FOR STRAIGHT
REACH
The profiles feature water surface elevation at 0hr, 12hrs
and the maximum water surface elevation attained during
the unsteady flow.
EG= energy grade
WS= water surface
Crit= critical
Max= maximum
Vel= velocity
Chnnl= channel
Figure 18: Unsteady flow analysis, Discharge channel, Bypass and osapa upstream(1) Figure 19: flow velocity plot (1)
REF: NBC-CNG367-R004REV0 Page 47 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
Figure 20:Unsteady flow analysis, Discharge channel, Bypass and Igbokushu(2) Figure 22: Flow velocity plot (2)
Figure 21:Unsteady flow analysis, Discharge channel, Primay A channel (3) Figure 23: Flow velocity plot (3)
REF: NBC-CNG367-R004REV0 Page 48 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
PROFILE OUTPUT TABLE FOR OSAPA WITH
STRAIGHT CHANNEL
REF: NBC-CNG367-R004REV0 Page 49 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.4. Sediment Transport hydraulic analysis
For this purpose we developed quasi-unsteady flow data
generated from rainfall data for the year 2007 and
simulated it for one, two years and three years sediments
deposit in the channel. Also, soil samples were taken
within the Osapa channel and its environ. Soil
characteristics tests and sieve analysis were conducted to
obtain data for design purpose.
4.4.1. Soil sample data
Figure 24: soil sample 1
Figure 25: soil sample 2
REF: NBC-CNG367-R004REV0 Page 50 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.4.2. Quasi-Unsteady flow data
Figure 26: Quasi- Unsteady flow graph for Bypass channel
Figure 27: Quasi-Unsteady flow graph for Osapa channel upstream
Figure 28:Quasi-Unsteady flow graph for Igbokushu channel upstream
Figure 29: Quasi-Unsteady flow graph for Primary A channel upstream
REF: NBC-CNG367-R004REV0 Page 51 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
Figure 30; Stage hydrograph for discharge point at the lagoon
REF: NBC-CNG367-R004REV0 Page 52 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.4.3. SEDIMENT TRANSPORT ANALYSIS RESULTS
Here also, two scenarios where considered at this point:
• Osapa with Bypass channel
• Osapa with Straight channel
WATER SURFACE, INVERT CHANGE PROFILES
This profile features the change in invert elevation of the
proposed channel reach over three years(2007-2010)
of sediment deposit after the three seasons of rainfall
event.
OSAPA WITH BYPASS CHANNEL
Maximum water surface profile is seen above 4.8m
upstreram which is above mean ground elevation. This
shows evidence of flooding upstream of the channel.
Sediments deposition is pronounced at the points shown
in figure 31. Rate of sediments deposit is about 0.65m
high per annum at these points and reaches up to 4.1m
elevation in the third year thus taking up about 90% of
the cross-sectional area of the channel section. Refer to
the legend in figure 31 to understand the water surface,
invert behavioural pattern of these reach of Osapa
Figure 31: Sediment deposit for Osapa with Bypass route
REF: NBC-CNG367-R004REV0 Page 53 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
channel when its alignment follows the Bypass route.
Figure32 also shows these points in the site layout.
OSAPA WITH STRAIGHT CHANNEL
Maximum water surface profile is seen below 4.5m which
is mean ground elevation. Sediments deposit is about
300mm in the third year of simulation.
Figure 32: layout showing areas of high sediment deposits (with Bypass channel) Figure 33: Sediment profile for Osapa channel with straight alignment
REF: NBC-CNG367-R004REV0 Page 54 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
BYPASS ROUTE ASSESSMENT
The hydraulic simulation shows that the bypass route will
cause accumulation of debris and sediments at the bends
shown in figure 32. This causes water surface elevation
to back up upstream and causing flooding. A straight
alignment shows debris accumulation to about 0.3m
height which is fair enough. However, the situation in the
bypass channel can be remedied. An option is the
introduction of sediment basins at major sediment deposit
areas. The next section shows the result of the
assessment of the bypass option but this time with a
sediment basin.
4.4.4. SEDIMENT TRANSPORT ANALYSIS RESULTS
(BYPASS CHANNEL WITH SEDIMENT BASIN)
This third option is same as the Bypass route but with a
sediment basin introduced at the corner where deposition
is high. Depth of sediment basin is 0.6m, 10m width and
about 40m long within the bed of the channel.
Note that the basin gets filled up in the third year of
rainfall event and sediment deposition only rises above
the channel invert to about 0.4m height. Maximum Water
elevation is about 4.1m.
Figure 34: Sediment deposit for Osapa Bypass option with Sediment basin
REF: NBC-CNG367-R004REV0 Page 55 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
4.4.5. CROSSECTION VIEW OF HIGH DEPOSITE
AREA.
Figure 37: cross section of high deposit area for bypass channel without sediment basin Figure 36: cross section at midpoint of straight reach passing through Nicon town
Figure 35: Cross section of High deposit area but with sediment basin
REF: NBC-CNG367-R004REV0 Page 56 of 55 February 2010
Route Assessment and design Osapa channel
Via Nicon town – Lagos - Nigeria
5. CONCLUSION AND RECOMMENDATION
Two major options were considered. The first is the
option of a bypass channel to align with western and
northern boundary walls of Nicon town and eventually
discharge into the lagoon. The second is a straight reach
to pass through Nicon town itself and then discharge into
the lagoon.
Option one shows high sediment deposition at the north
east corner of the Nicon town wall. However, a sediment
basin introduced at this corner will help tackle the effects
of deposition but with strict adherence to cleaning out the
sediment basin at least once every year.
The option two is to have a straight reach passing
through Nicon town. This option requires no sediment
basin but because of the unavailability of an appropriate
slope, velocity of flow is below 0.6m/s for most part of
the channel and sediment deposition is also imminent but
mild and not concentrated. However, routine maintenance
is also required to ensure good ambience and hygiene
conditions.
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