Osapa Channel Report 2010( Nbc-cng367-r004rev0)

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




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




EXECUTIVE

SUMMANRY




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




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




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




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




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.




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

REF: NBC-CNG367-R004REV0 Page 11 of 55



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




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




Figure

February 2010


Figure 6 Intensity Duration Frequency (IDF)curve for 10 yeareturn storm event

ation (IDF)

years return storm event





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.




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


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




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


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




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


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.


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




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)


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)


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




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)


k = intercept coefficient (table 3-3)

Sp = slope, percent

Eq. 3.3




Table 3.3 Intercept Coefficients for Velocity vs. Slope

Equation 4.3

February 2010

Intercept Coefficients for Velocity vs. Slope Relationship of




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


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)


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)


Values of Manning's Coefficient (n) for Channels




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




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.




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




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.




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.




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




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.




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




4.1. Total Flow hydrograph for sub-basins










4.2. Total Flow hydrograph for nodes

4.2.1. Osapa upstream with relevant nodes J-31, J-18,

J-17







4.2.2. Osapa Downstream with relevant nodes J-29, J-

23, J-26, J-8







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.




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




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




Igbokushu Channel

Upstream

Rectangular channel (Bottom Width (BW) 3m depth

1.5m

Midstream

Rectangular channel (Bottom Width (BW) 3m, depth

1.5m

Downstream


1.5m

Downstream


1.5m

Discharge channel

Trapezoidal channel (Bottom Width (BW) 15m Side

Slope 1:2




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




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







PROFILE OUTPUT TABLE FOR OSAPA WITH

BYPASS CHANNEL

Figure 15: Summary output table for bypass channel route




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




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)




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)




PROFILE OUTPUT TABLE FOR OSAPA WITH

STRAIGHT CHANNEL




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




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




Figure 30; Stage hydrograph for discharge point at the lagoon




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




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




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




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




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.

Documents

Osapa Channel Report 2010( Nbc-cng367-r004rev0)