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TITLE: EVALUATION OF THE ON-GOING CONSTRUCTION OF

NAROK TOWN STORM WATER DRAINAGE SYSTEM

BY

SOPIA .S. SHILLAH

F16/2289/2009

SUPERVISOR: MR. KIPKOROS KANDIE

This project is submitted in partial fulfillment for the award Bachelor of Science Civil and

Construction Engineering, University of Nairobi.

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ABSTRACTNarok town has experienced flooding for over several decades. The consequences being people

and cars drowning, destruction of property and indirectly has a link to slow development; this is

because the risks of investing in the town is increased by the frequent floods during rainy

seasons; hence investors shy away or avoid investing all together. Road pavement destruction is

a concern as water ingress into the sub-grade. Health and environmental impacts after the

floods are also a major concern.

The study involves the evaluation of the existing major drainage system in Narok town i.e. from

Kobil petrol station to Narok River, estimation of runoff, hydraulic design of open channels,

culvert and comparing the with existing ones.

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ACKNOWLEDGEMENTI hereby place on record my sincere gratitude to the following:

Eng: Kipkoros Kandie, my supervisor, who at every stage in my project buildup, played a

principle role by offering technical advice, relevant information, alterations, corrections,

additions and omissions to my work.

Mr. Lucas Wesonga, resident engineer for the project, who provided me with material support.

Employees of meteorological department, Dagoretti, who provided me with rainfall data,

analyzing the data and correction where necessary.

I also acknowledge the efforts of my parents, brothers and friends, for their support in my

education, financially and morally.

SOPIA .S. SHILLAH

APRIL 2015.

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Table of ContentsABSTRACT....................................................................................................................................................2

ACKNOWLEDGEMENT.................................................................................................................................3

CHAPTER ONE: INTRODUCTION...................................................................................................................6

1.1 BACKGROUND...................................................................................................................................6

1.2 PROBLEM STATEMENT......................................................................................................................7

1.3 GOALS AND OBJECTIVES....................................................................................................................7

1.4 SUMMARY OF APPROACH, TOOLS AND METHODOLOGY..................................................................8

CHAPTER TWO: LITRATURE REVIEW............................................................................................................9

2.1 HYDROLOGICAL CYCLE...................................................................................................................9

2.2 URBAN HYDROLOGY..........................................................................................................................9

2.3 EFFECTS OF URBANISATION ON STROM RUNOFF............................................................................10

2.4 URBAN STROMWATER MANAGEMENT...........................................................................................12

2.5 URBAN STORMWATER DRAINAGE DESIGN......................................................................................13

2.5.1 INTRODUCTION.........................................................................................................................13

2.5.2 PLANNING.................................................................................................................................14

2.5.3 DESIGN CRITERIA...............................................................................................................14

2.5.4 BASIC DESIGN CRITERIA............................................................................................................21

2.5.5 FLOOD ESTIMATION...........................................................................................................24

2.5.6 CHANNEL DESIGN..............................................................................................................26

2.5.7 DRAINAGE DESIGN FOR URBAN STREETS...........................................................................28

2.5.8 DESIGN OF CULVERTS........................................................................................................29

CHAPTER THREE: RESEARCH AND METHODOLOGY...................................................................................33

3.1 RESEARCH APPROACH.....................................................................................................................33

3.2 METHODOLOGY...............................................................................................................................33

3.3TOOLS...............................................................................................................................................35

CHAPTER FOUR: RESULTS, ANALYSIS AND DISCUSSION............................................................................36

4.1 INTRODUCTION...............................................................................................................................36

4.2 PHOTOGRAPHY................................................................................................................................36

4.3 RAINFALL DATA ANALYSIS AND DESIGN OF DRAINAGE SYSTEMS....................................................40

4.3.1 RETURN PERIOD........................................................................................................................40

4.3.2 RUN-OFF CO-EFFICIENT............................................................................................................41

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4.3.3 DESIGN OF THE OPEN DRAINS LFROM KOBIL PETROP STATION THROUGH THE SAMPURMPUR VALLEY TO NAROK RIVER...................................................................................................................43

DISCUSSION...............................................................................................................................................47

CONCLUSIONS AND RECOMMENDATIONS................................................................................................48

REFERENCES..............................................................................................................................................49

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CHAPTER ONE: INTRODUCTION

1.1 BACKGROUND.Narok town is located on the southern side of the Rift valley. It borders Tanzania to the south,

Transmara to the west, Kajiado to the east, Bomet and Nakuru to the north. Its latitude 00 50’

and 20 05’ south; longitudes 350 58’ and 360 05’ east. Looking at a topographical map of Narok

County, you note that the Narok town lies in lowland and is bordered on both sides by high

lands.

Narok County has ownership of the Konyo catchment, which is about 9km. Narok’s drainage

proceeds south from Mau Escarpment, through two seasonal tributaries known as River Siyapei

and River Narok. They flow south into the much larger Ewaso Ng’iro River which flows

southwards towards Tanzania. The two main tributaries pass through the Narok town centre, in

the low land valley causing flood havoc during the rainy season. (UNISDR Report: city resilience)

Narok receives two rainy seasons, with an average rainfall of 500mm to 1800mm per year

(metrological dept). Over the years the town has been experiencing flash floods during these

rainy seasons. The result is displacement, destruction of livelihoods and property, deaths and

injuries. Some residents are yet to recover from the floods of 1993 that killed over 50 people,

displaced thousands and destroyed properties worth millions.(standard paper 11/10/2012). To

date flash floods continues to haunt the inhabitants of the town, killing two children last year.

In the period of this research the metrological department has issued a warning of looming

flash floods with the expected El Nino rains.

The main cause of flooding in Narok town is loss of forest cover in outlaying water catchment

areas coupled with the closing up natural water ways in towns, due to structures done in total

disregard of the law (Daniel ole Sapit climate change advocate). All along the water way from

Kobil to Narok River, numerous structures have been constructed along the Sampurumpur and

Kakiya valley narrowing and in some instances blocking the valleys, resulting in increased

flooding. An efficient drainage system is the most important part of any developing urban area.

With well-designed drainage systems, loss of lives, damage and loss of property by floods is

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prevented. “The basic underlying purpose of any drainage system is to keep people from water,

to keep water from the people and to protect and enhance the environment while doing so.”

(Thomas and Dedo, 2002).

1.2 PROBLEM STATEMENTNarok town sits in a valley, thus run-off from higher grounds are drained into the town. Due to

increased urbanization, the runoff is increased creating the need for properly designed drainage

systems to avoid flooding. This however is not the case, Narok town experiences frequent

floods as existing drainage systems are not designed to cope with the increasing run-off. The

drainage that runs from Kobil petrol station to Narok River, forms the major part of the town’s

drainage system, if properly designed, it would ease the flooding in Narok town.

1.3 GOALS AND OBJECTIVES.The main goal of this study is to assess the existing drainage system that runs from Kobil to

Narok River so as to explain its adequacy, performance and give recommendations. It will

involve the determination of run-off quantities of the sub-catchment areas comprising the

drainage district and then comparing them to the existing drainage capacity.

The study will lead to identifying any failures in the drainage system and its causes with a view

of making recommendations on how to improve it and obtain a higher efficiency. Thus the

study will involve the collection and analysis of rainfall data for the area under study. Actual

sizes of existing structures will be measured at the field.

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1.4 SUMMARY OF APPROACH, TOOLS AND METHODOLOGY.

APPROACH.

Approach refers to the design on which the research data is to be obtained. In this research,

Primary and secondary data collection methods was used.

METHODOLOGY

Primary data was obtained by:

-field observation

-key informant interview

-focus group discussion

-applying questionnaires

Secondary data was obtained by:

- Literature review

TOOLS

-Observation schedule

-key informant schedule

-questionnaire

-focus group discussion notes

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CHAPTER TWO: LITRATURE REVIEW.

2.1 HYDROLOGICAL CYCLE.Hydrologic cycle is the water transfer cycle, which occurs continuously in nature; the three

important phases of the hydrologic cycle are: Evaporation and evapotranspiration, precipitation

and runoff. Evaporation from the surfaces of ponds, lakes, reservoirs, Ocean surfaces, and

transpiration from surface vegetation i.e., from plant leaves of cropped land and forests, take

place. These vapours rise to the sky and are condensed at higher altitudes by condensation

nuclei and form clouds, resulting in droplet growth. The clouds melt and sometimes burst

resulting in precipitation of different forms like rain, snow, hail, sleet, mist, dew and frost. A

part of this precipitation flows over the land called runoff and part infiltrates into the soil which

builds up the ground water table. The surface runoff joins the streams and the water is stored

in reservoirs. A portion of surface runoff and ground water flows back to ocean, again

evaporation starts from the surfaces of lakes, reservoirs and ocean, and the cycle repeats. Of

these three phases of the hydrologic cycle, namely, evaporation, precipitation and runoff, it is

the ‘runoff phase’, which is important to a civil engineer since he is concerned with the storage

of surface runoff in tanks and reservoirs for the purposes of irrigation, municipal water supply,

hydroelectric power supply e.tc. Urbanization affects the natural hydrological cycle hence the

need for urban hydrology. (Raghunath 2006)

2.2 URBAN HYDROLOGY.Urban hydrology is defined as the interdisciplinary science of water and its interrelationships

with urban people (Jones, 1971). A simpler definition of urban hydrology would be the study of

hydrological processes occurring within the urban environment.

Development on the land changes how water naturally travels through the watershed. With a

natural ground cover, about 50% of rainfall infiltrates into the ground, 40% evaporates or is

transpired through plants (these together are called evapotranspiration), and only about 10%

actually runs off the surface. As we develop the land, we add structures onto the surface, such

as roads, houses, parking lots, sidewalks, and driveways. All of these are impervious surfaces,

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which water cannot pass through them as it can through soil, and so instead of the water

infiltrating, it is forced to either evaporate or run off. (Dulo 2013)

On Evapotranspiration , this option is largely removed, and runoff increases as we construct

non plant-friendly structures. The amount of impervious surface within a watershed determines

how great the change in runoff will be. At 10 to 20% impervious (similar to medium-density

residential areas), runoff is doubled, and the amount of water infiltrating is reduced. At 30 to

50% impervious (such as in high-density residential developments), runoff is tripled. At 75 to

100% impervious (as is common in commercial areas), the majority of rainfall becomes runoff,

and infiltration is less than 1/3 of what it was prior to development (Dulo 2013). The result of

creation of large impervious areas, is significant problems such as regular flooding, inadequate

drainage facilities, erosion, sedimentation and deterioration of water quality in receiving water

bodies. Urbanisation in most developing countries exacerbates drainage problems; runoff is

increased by impermeable urban surfaces and, due to inadequate development control

mechanisms and their incompetent enforcement, settlements are constructed with little

consideration for storm water drainage. This therefore necessitates the need for proper storm

water drainage systems that are well planned for, designed and managed. (Parkinson 2003)

2.3 EFFECTS OF URBANISATION ON STROM RUNOFF.First, the large amount of extra runoff causes the streams to have much higher flows than

natural, and the flow rate increases much more rapidly and drops off more rapidly after the

storm. Second, due to the reduced infiltration volumes, there is less water available to be

released slowly into the stream over time, resulting in lower water levels between rainfall

events. In effect, much of the water that under natural conditions infiltrated into the ground

and slowly

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Made its way into nearby creeks now enters the stream all at once. (Dulo 2013)

Effects of urbanization on runoff (figure 1)

Source: S.O. Dulo, 2013

The amount of waterborne waste increases in response to the growth in population and

building density. The quality of storm water runoff deteriorates as contaminants are washed

from streets, roofs and paved areas. The disposal of both solid and waterborne wastes may also

have an adverse effect on groundwater quality. The degradation of the quality of flows in both

the drainage networks serving the urban area and the underlying aquifers, gives rise to major

hydrological problems. (Sunil 2000)

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2.4 URBAN STROMWATER MANAGEMENT.In early days, stormwater was considered as a nuisance and the main objective of stormwater

management was to dispose stormwater as quickly as possible to receiving water bodies. This

meant that no matter how large the rainfall or its duration, the drainage system was expected

to remove runoff as quickly as possible, in an attempt to restore maximum convenience to the

community in the shortest possible period of time. No consideration was given to stormwater

as a valuable resource. Furthermore, the receiving water bodies were adversely affected due to

poor quality stormwater. (Sunil 2000)

In recent times, stormwater has been considered as a resource due to scarcity of water

resources. Stormwater is a significant component of the urban water cycle, and its improved

management offers potentially significant environmental, economic and social benefits. Urban

stormwater management objectives now pursue the goal of ecological sustainable

development and better environmental outcomes. This objective results in vastly improved

stormwater quality. One of these technologies is the infiltration technology incorporating

soaking wells, pervious tanks and biologically engineered soil filter medium. Infiltration

techniques may provide an effective solution to overcoming stormwater contamination. One

other technique is the reuse of stormwater. Reuse of treated stormwater can be considered as

a substitute for other sources of water supply for non-potable uses. (Sunil 2000)

Structural and non-structural stormwater management measures often need to be combined to

manage the hydrology of urban runoff and to remove stormwater pollutants. One group of

stormwater management measures that has proved effective in removing stormwater

pollutants associated with fine particulates (such as suspended solids, nutrients and toxicants)

is constructed wetlands and ponds. Constructed wetlands also satisfy urban design objectives,

providing passive recreational and landscape value, wildlife habitat and flood control. A gross

pollutant trap is another structural pollution control measure that traps litter and sediment to

improve water quality in receiving waters. Community involvement in cleanup programs and

source controls, re-vegetation programs of disturbed land, and minimal bare soil in urban

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gardens (especially those on sloping land) are some of the non-structural measures

(http://eau.sagepub.com/content/15/2/115)

The stormwater drainage network represents a large capital investment and hence due

consideration should be given to its design, management and maintenance. To achieve the best

practice in the design and whole-life cycle management of stormwater infrastructure requires

the adoption of appropriate design standards dealing with major and minor storms, and the

encouragement of practices to extend the retention time in the storm water systems. (Sunil

2000)

2.5 URBAN STORMWATER DRAINAGE DESIGN.

2.5.1 INTRODUCTION.Urban storm drainage can be defined as the control of floods in urban environments. The main

purpose of storm water drainage is to collect and convey stormwater to receiving waters with

safety and minimal damage. However, recently the focus is slowly shifting from disposal of

storm water towards the total management of storm water considering it as a water resource.

The main components of urban drainage systems are:

Roof and property drainage

Street drainage (including both piped and surface flows)

Trunk drainage

Receiving water bodies (including ground water storage). (UNCHS report 1991)

2.5.1.1 Aims of storm water drainage systems.

The main aim of stormwater drainage systems is to collect and convey stormwater to receiving

water bodies with safety and minimal damage. Other objectives include;

Limit the adverse impact of urbanization like pollution, erosion and sedimentation

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Water conservation in areas of low rainfall

Integration of large-scale drainage works into overall town planning schemes

2.5.1.2 Principles of storm water drainage systems.

Analyzing of storm water based on measured/observed real time system behavior.

Planning of drainage systems in relation to total urban systems i.e. integrating with all

other elements if urban infrastructure.

Designing and operation of drainage systems to maximum benefit to the community at

large.

Designers should be influenced by professional considerations like ethics,

standardization and innovation. (UNCHS report 1991)

2.5.2 PLANNING.In planning and development of stormwater drainage systems, the existing natural drainage

system should be preserved in its natural state as far as possible. Other factors to consider

during planning include:

Drainage facilities should be coordinated with open space and transport amenities.

Natural water courses should be used for storm runoff waterways as this will minimize

peak flood flows in downstream, this is mainly because natural waterways have slower

flows.

Planning and design of stormwater drainage should not be based on the premise that

problems can be transferred from one area to another. Channel modifications which

simply transfer problems downstream should be avoided unless they are part of a

comprehensive upgrading of a particular drainage system.

Stormwater runoff can be stored in detention ponds or retarding basins which reduce

the downstream drainage capacity required, the amount of land acquisition and the

expenditure on downstream works.(E.M. Wilson 1990)

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2.5.3 DESIGN CRITERIA Design methods determine arrangement of drainage systems, characteristics and size of its

components. The many methods available are based on mathematical models of the physical

processes involved. These are expressed as a set of calculations performed manually, by

programmable calculator or computer programs. They include:

Hydrological models, which calculate peak flow rates, storage volumes or flow

hydrographs for system components.

Hydraulic models, which define size and other characteristics of components, or analyze

system behavior to define possible failures.

Other models and calculations concerning water quality, structural adequacy of

components, maintenance requirements and economics.

The rainfall/ runoff relationship is a complex one, especially in urban areas. Statistically derived

models are applied to the infinite variations in rainfall patterns to land catchment with an

almost chaotic distribution of features which sometimes impede or sometimes promote the

passage of water. (E.M. Wilson 1990)

2.5.3.1 Hydrological models.

Urban area models are the same in principles as those applied to rural catchments, the models

employed depends on the availability of data. In most counties in Kenya, there are insufficient

data to test and calibrate urban hydrological models on a wide scale. Thus rainfall-runoff

models employing statistical design rainfall data are mostly used.

The rational method is best known and has been the model closely associated with urban

drainage design, its formula is:

Q=C.I.A/360

Where Q~ is the design flow rate (m3/s)

C~ is a dimensionless runoff coefficient

I~ is rainfall intensity (mm/h), corresponding to a particular storm duration and average

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recurrence interval

A~ is the catchment area (ha)

Other hydrological models in use are shown in the table below:

TABLE 1. Hydrological models for urban drainage design and analysis.

Model Model type Appropriate

uses

Degree of

complexity

Major

variations

Examples of

models

Empirical

equations

Peak flow Estimation

of design

flows for

very small

catchment in

property and

road

drainage

Very

simple

Rules for roof

drainage design,

Talbot’s formula.

Rational

method

Peak flow Design of

small and

medium

street

drainage

systems, and

large

property

drainage

systems.

Only

applicable to

limited

forms of

analysis.

Simple to

medium( us

e of

computers

optional)

-

accumulation

of, flowrates,

equivalent

impervious

areas, areas

of different

land use

types.

-treatment of

partial area

effect

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Unit

hydrographs

hydrograph Design and

analysis on

large urban

catchments,

where data

are available.

Medium

( computers

optional)

Synthetic unit

hydrograph

methods such as

Snyder, Clark-

Johnstone.

Time

area(isochronal)

Hydrograph Design or

analysis of

all sizes of

systems,

including

tose

involving

storages.

Medium

(computers

required)

Linear or non

linear

storages.

TRRL,ILLUDAS

, ILSAX

storage routing hydrograph Too complex

for routine

design at

present.

Used for

detailed

analysis of

larger

systems and

scientific

studies.

Medium to

complex

(computers

required)

WASSPRORB,

RAFTS

Kinematic

wave

hydrograph Medium to

complex( c

omputers

required)

SWMM KWIM

Physical

process or soil

Continuous

hydrograph

Complex

(computers

SWMM, HSPF

17

moisture

accounting

models

required)

SOURCE: UNCHS 1991

2.5.3.2 DATA

Rainfall data Design of drainage systems is based upon the availability of rainfall data. It is therefore

imperative that the data is searched diligently in the central and regional offices of the agencies

responsible for metrological records. Rainfall data for use in storm designs is usually presented

in rainfall- intensity- duration and frequency curves (IDF/IDD). This is based on historical records

and give an average rate if rainfall corresponding to a given storm duration and specified return

period. Also required is information of magnitudes of rains of various frequencies and specified

duration. The relationship between rain intensity and frequency can be determined at the

recording stations. This relationship provides important information about run-offs.

FREQUENCY- INTENSITY- DURATION CURVESPrecipitation depths and intensities are useless by themselves unless they can be related to a

frequency of occurrence. The frequency of occurrence establishes the risk of failure. An

example is a storm drainage system designed for volume that occurs once in 5 years,this is

sufficient for all storms up to that volume. However, if a 50 year storm volume occurs, the

drainage system will be undersize and as a result fail.

It is frequently convenient to reduce intensities and volumes to more usable forms.

Transmission systems are designed to transport a particular rate of flow resulting from a storm

of a particular magnitude. Magnitude is specified by the intensity and duration of rainfall and

frequency of occurrence of the storm. Using data from many individual records, intensity-

duration- frequency curves are derived. (Surface water sewerage, 1981)

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Runoff data Little information is available on the runoff of stormwater in most counties. Its however

possible to obtain local information in small catchments by field measurements and gauging of

principle components in the drainage systems during storm events provided the designers has

sufficient time for the development of the systems design parameters (UNCHS1991).

Hydrology is not under any circumstances the most precise of sciences. The movement of water

can be at best only approximated, and a flood or run-off event must be calculated on the basis

of a recurrence frequency chosen largely by judgment, and it represents on a statistically-based

frequency over a long time base. (Waste water technology, 1975)

The hydrological phase of urban design is concerned with determining the magnitude,

distribution and timing of various run-off events. Maximum events are of utmost importance

since they are the basis for design of drainage structures. Run-off, which occurs in any drainage

area, is a function of both climate of the locality and the physical characteristics of the area.

Factors which may influence rainfall- runoff relationship include: rainfall type, rainfall intensity,

duration and distribution, soil type, evaporation, slope and land use characteristics of the

drainage area.

A brief study of these items should be sufficient proof that the hydrologic problem is complex. If

urban drainage works are to efficiently and economically serve the areas for which they are

designed for. Considerable emphasis must be used in determination of accurate and reliable

estimates of flow. In most cases the greatest number of hydraulic failures has been caused by

faulty determination of runoff magnitude and not structural inadequacies. (Design of sewerage

treatment works, 1970)

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2.5.3.3 STANDARD OF PERFORMANCE

Drainage standards are influenced by many factors, including:

The level of hydraulic performance required

Construction and operating costs

Maintenance requirement

Safety

Aesthetics

Regional planning goals

Legal and statutory requirements.

They are usually expressed by an Average recurrence interval (ARI). These measures determine

the magnitude of design rainfall or runoff event with which the system can cope. It is usually

appropriate to design for several performance levels, which may include:

Maintenance requirement (frequent event), related to a short design ARI, perhaps less

than one year.

A convenience or nuisance-reduction requirement (infrequent event), possibly a 1-2- or

–year ARI.

A flood damage prevention requirement, (severe or rare event), or about a 50- to 100-

year ARI.

A disaster management requirement (extreme event), related to extreme events such as

probable maximum floods..

The selection of appropriate design ARI must be made by the designer in the light of economic

and financial consideration of local conditions and requirements. It may be appropriate for

designers to vary the standards applied to different points in a drainage system, depending on

the perceived risks of failure. (UNCHS 1991)

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2.5.4 BASIC DESIGN CRITERIA2.5.4.1 Basis of design.

Drainage design is considered that two separate and distinct drainage systems exist. These are

the minor and major drainage systems.

a) Minor drainage systems~ this is that part of the total drainage a system which caters for

the maximum runoff from the initial storm. This system includes, street gutters,

roadside drainage channels and ditches, culverts, storm water pipes, open channels, and

any other features designed to handle runoff from the initial storm. All elements of the

minor drainage systems should be design for the least initial storm which may have a

design ARI of 2 or 5 years depending on the adjacent land use.

b) Major drainage systems ~ they include natural streams, rivers, flood plains, lined

channels, major pipes, major roads. Provision is made to provide capacity for the safe

discharge of a 20-year return period flood in rivers and main canals with catchment

areas in excesses of 100ha. Provision should also be made to minimize major property

damage and loss of life from the runoff expected from flood of a 100-year return period.

(Thomas Telford 2003)

2.5.4.2 Calculation of storm water runoff

Runoff calculation for areas up to 500 hectares should be based on the rational formula as

shown above. Where the catchment exceeds 500hectres, results determined by the rational

method should be compared with results obtained from the Snyder synthetic unit hydrograph

method.

2.5.4.3 Design recurrence intervals

In some circumstances, a cost-benefit analysis which takes into account the socio-economic

consequences of design flood exceedance may be appropriate in determining the design storm

21

recurrence interval. Runoff calculation for both initial and major storms are commonly based on

design recurrence intervals shown in the table below

Table 2 design storm recurrence intervals

Drainage systems Return period (years)

Minor Residential land use

Commercial land use

Industrial land use

2

5

5

Major All main water carriers with

>100 ha catchment.

All uses checked and assessed

for adequacy.

20

100

(UNCHS 1991)

2.5.4.4 Calculation of capacity of storm water drains

Manning’s formulae for open channel flow should be used in calculating the capacity of existing

and new storm water drainage channels. The Colebrook-white equations should be used for the

calculations of the flow in pipes.

Manning’s formulae c= R1 /6

n

Where C~ is the Chezy’s c

R~ hydraulic radius

N ~Manning’s n

Colebrook-whites equations 1√ f

=−4 log10 { K s

14.8R+ 1.26

ℜ√ f }Where KS~ Effective roughness

R~ hydraulic radius

22

Re~ Reynolds’s number

f~ friction co-efficient

(S. Ngare 2013)

2.5.4.5 Drainage reserves

Drainage reserves are necessary to prevent development encroaching on flood prone land and

to give access for plant and machinery for maintenance and repair operations.

Table 3 reserve widths for catchments less than 50ha

Drain Reserve width (m) for

drainage areas of:

0.50 ha 5-50 ha

Between buildings lots

Alongside roads

Top width + 1m

Top width

Top width +3m

Top width

Where downstream development is proposed in an undeveloped catchment, drainage reserve

to accommodate the ultimate 1 in 100 year flood with its natural flood path reserved or

alternatively used reserve widths in the table 4

Table 4 reserve widths for catchments more than 50ha

100 year discharge (m3/s) Reserve width (m)

30

30-100

100-200

30

40

75

23

200-300

300

90

Analyze separately

2.5.5 FLOOD ESTIMATION. For flood estimation for urban drainage system design the modified rational method is used. It

improves the accuracy of the standard rational method to take into account the variable runoff

coefficients in the catchment and the losses in rainfall and storage in the system. The modified

rational method:

Q ¿ FCC S IA

Where: Q~ the peak discharge in cubic meters per second of return period T years

I~ the average intensity of rainfall in mm per hour for a duration equal to the time of

concentration (tc) and return period T years.

A~ the catchment area in hectares

C~ coefficient of runoff

CS~ storage coefficient

F~ factor of proportionality (= 0.00278 when A in hectares)

(Dulo 2013)

The formula is based on the following assumptions:

i. The intensity of rainfall is constant throughout the storm and that same intensity of

rainfall is felt at all places.

ii. The impermeability of the catchment area remains constant and is evenly distributed at

full flow throughout the time of concentration.

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iii. The maximum rate of flow at any point in a surface water sewer occurs when the storm

has lasted the same length of time as it takes water to flow throughout the system to

that point. Therefore it means that maximum runoff will occur if the storm lasts

sufficiently long for every part of the area to be contributing to its quota to the run-off

at the point of discharge.

iv. The maximum run-off will occur during a storm which takes place after a previous wet

period so that the impervious and pervious surfaces alike will then have developed their

maximum run-off co-efficient

In applying the rational formula to a main out fall drainage system, a plan is prepared showing

the individual sub-catchment areas contributing to the main drainage. The formula is then

applied to each point of junction of the branch drainage with the main drainage to calculate its

size at that point. As the calculation proceeds downstream, the contributory area will be

constituted by different sub-catchment, the impermeability factor is obtained by calculating the

weighted mean of the co-efficient of various sub-catchment areas.(Water supply and pollution

control, 1975)

Time of concentration tc ; for urban stormwater drains the time of concentration consists of the

time required for runoff to flow over the ground surface to the nearest drain t0 and the time of

flow in the drain to the point under consideration td.

t c=t 0+t d

Rainfall intensity I ~ this is the average rate in millimeters per hour of precipitation from a

storm having a duration equal to the time of concentration tc. Rainfall intensity, duration and

frequency relationship must be derived for the locality.

Runoff coefficient C ~ this is difficult to determine precisely and can be interpreted in different

ways. A weighted coefficient (CW) should be used where land uses vary or surface

characteristics exist in catchment areas (see chart for co-efficient)

25

Storage coefficient CS~ as the catchment area gets larger the effect of channel storage on the

attenuation of the flood wave becomes more pronounced. To allow for channel storage effect

the peak discharge calculated should be multiplied by a storage coefficient. (Dulo 2013)

C s=2 t c /(2 tc+ td)

26

2.5.6 CHANNEL DESIGN The ideal channel is a natural one because velocities are usually low, resulting in longer times of

concentration and, hence, lower downstream peaks. Channel storage is also available to reduce

peak flows and maintenance costs are generally lower because of the natural stability of the

channel.

Flow computations~ manning’s formula for uniform flow is used in calculation

Q=A R

23 S2

n

Where Q~ flow, m3/sec

A~ area of flow section m2

R~ hydraulic radius, m

S~ slope of channel, m/m

n~ manning’s roughness coefficient which varies with the condition of the interior

surface of the drain.

Table2: manning’s co-efficient ‘n’

surface Condition

best good fair Bad

Concrete lined

channels

0.012 0.014 0.015 0.018

Cement rubble

surface

0.017 0.025 0.025 0.030

Dry rubble

surface

0.025 0.030 0.033 0.035

Cement motor

surface

0.011 0.012 0.013 0.015

Brick sewers 0.012 0.013 0.015 0.017

27

(Central public health and environmental engineering organization, 1990)

Lined channels ~ channel linings usually consist of concrete, stone pitching or a combination of

the two. The decision on lining materials used principally relies on local custom, availability and

cost of the materials. The sides’ slopes will generally range from vertical to one and should be

designed as retaining walls. The desirable minimum velocity is 0.75-1.0 meters per second for

self-cleansing and the maximum is 3.0meters per second for safety reasons.

Vegetated channels~ where there are no restrictions on right –of- way width and slope is

compatible, vegetated channels can normally be constructed at less cost. The invert of the

channel should be lined for low flows designed with capacity of 3-5 per cent of the design flow.

Natural channels~ natural channels in steep areas tend to have erodible banks and increased

erosion can be expected with urbanization. Increased erosion may also occur in flatter sections

as a result of increased urban discharge. Some modification may be necessary to create more

stable conditions and to ensure that the maximum velocity is kept below 2 meters per second

wherever possible.

Unkerbed streets~ to ensure satisfactory drainage to adjacent open channels, formation and

sealing should be taken to the end of main drains, and shoulders should be graded to the edge

of minor drains. If the road shoulder does not allow direct runoff to the drain, spaced inlets

should be provided. (S. Ngare 2012)

28

2.5.7 DRAINAGE DESIGN FOR URBAN STREETS. Design criteria for collection and transport of runoff water on public streets are based on a

reasonable frequency of interference to traffic when kerbs and gutters are constructed on each

side of the street. The making use of street as part of the urban drainage system should not

conflict with its primary function of traffic movement.

Gutter capacity and design~ for streets with formed kerbs, the theoretical gutter capacity Qg

(m3/s) can be calculated using a modified manning’s formula for flow in a shallow triangular

channel:

Q g¿0.375 z /n s1/2d8 /3

Where z~ the reciprocal of the crown shape, m/m

n~ manning’s coefficient of roughness

s~ gutter slope, m/m

d~ depth of flow in gutter, m

The gutter should be designed so that the width of flow on the road pavement does not exceed

2m in width during the initial storm.

Drainage inlets~ storm water drainage inlet is an opening into a drainage system for the

entrance of storm water runoff. It is important that inlets be properly designed, constructed

and maintained so that the drainage system operates at its full capacity. Sufficient openings

shall be provided to limit the gutter flow width to 2m.

Inlets maybe installed either on a continuous grade or in sag points, i.e. a low point in the road

grade. The inlets can be gratings or kerb opening inlets. (UNCHS 1991)

29

2.5.8 DESIGN OF CULVERTS Function of culverts, is to convey or transport storm runoff (or other discharge) from one side

of the roadway to the other. Culverts are usually designed to operate with the inlet submerged

if conditions permit. This allows for a hydraulic advantage by increased discharge capacity.

Bridges are usually designed for non-submergence during the design flood event, and often

incorporate some freeboard. (Hydrology and Water Quality Control 1990)

Culvert maintenance requirements include efforts to assure clear and open conduits, protection

against corrosion and abrasion, repair and protection against local and general scour, and

structural distress repair.

2.5.8.1 Hydraulic Design Considerations

1. Design Flood Discharge

• Watershed characteristics

• Design flood frequency or return interval

• All designs should be evaluated for flood discharges greater than the design flood

2. Headwater Elevation - check upstream water surface elevation

3. Tailwater - check that outlet will not be submerged

4. Outlet Velocity - usually controlled by barrel slope and roughness

2.5.8.2 Terminology

Headwater (HW) – Depth from the culvert inlet invert to the energy grade line (EGL). If the

approach velocity head is small, then HW is approximately the same as the upstream water

depth above the invert.

30

Tail water (TW) – Depth of water on the downstream side of the culvert. The TW depends on

the flow rate and hydraulic conditions downstream of the culvert.

31

2.5.8.3 Culvert Design Approaches

1. Design based on design flood discharge and allowable headwater elevation. Check tail water

conditions to verify design.

2. Flood routing through the culvert. Data inputs include:

• An inflow hydrograph

• An elevation versus storage relationship

• An elevation versus discharge relationship (rating curve)

2.5.8.4 Types of Flow Control

1. Inlet Control - flow capacity is controlled at the entrance by the depth of headwater and

entrance geometry, including the barrel shape, cross sectional area and the inlet edge.

2. Outlet Control - hydraulic performance controlled by all factors included with Inlet Control,

and additionally includes culvert length, roughness and tail water depth.

2.5.8.4 Culvert Design

• Most culverts operate under downstream control. This means that the hydraulic

computations proceed from the downstream in the upstream direction.

• The design discharge and allowable headwater elevation are initially established. Other

constraints such as culvert shape, material, aesthetics, etc., are specified.

• Assume a culvert size and check performance assuming both inlet and outlet control.

Whichever gives the highest HW elevation controls the hydraulic performance.

• Compare culvert performance with design constraints, and select the smallest least

expensive) size that meets the criteria.

32

2.5.8.4.1 Culvert (Hand) Design Procedure

1. Establish design data – Q, L, So, HWmax, Vmax, culvert material, cross-section, and

entrance type.

2. Determine first trial size culvert (arbitrary, A = Q/10, etc.).

3. Assuming INLET CONTROL, determine headwater depth HW. For unsubmerged and

submerged conditions, use Eqs. Q=CwB ¿ And

Q=C d A√Sg (HW−b2) respectively.

4. Assuming OUTLET CONTROL, determine headwater depth HW using the energy

equation.

i) Determine tail water depth TW for downstream control (for uniform flow use

Manning’s equation).

ii) If TW depth > b (height of culvert) set h0 = TW.

iii) If TW < b, set, h0=b+ yc

2 or TW, Whichever is greater

iv) Calculate the energy loss through the culvert, H = he + hf + hv.

a) For full culvert flow use H=(K e+29 n2L

Rh

43

+1) V2

2 g

b) For partially-full culvert use the direct step method to determine the

water level and energy losses along the culvert length:

∆ X=E2−E1

S0−¿Sr¿ where E= y+ Q2

2 gA2 =[ y+ q2

2g y2 for box culverts ] if y becomes > b, use the full culvert formula for the remaining

distance, but neglect the velocity head term. Calculate the headwater

HW from HW=E+H−S0∆ L where E is the specific energy

33

at the location where the culvert becomes full and ∆L is the length of full-flowing

culvert.

5. Compare HW values from steps 3 and 4. The higher HW governs and indicates the flow

control existing under the specified conditions for the trial calculation.

6. Try alternate sizes and characteristics and repeat steps 3 and 4 until design

specifications are met.

7. Compute outlet velocity assuming area based on TW, yc, or yn, as appropriate.

34

CHAPTER THREE: RESEARCH AND METHODOLOGY

3.1 RESEARCH APPROACHResearch design is the visualization of data and problems associated with the embodiment of

the data in the entire research project (Leedy, 1996). It is the arrangement of conditions for the

collection and analysis of data in a manner that aims to combine relevance to the research

process with economy in procedure and it constitutes the blue print for the collection,

measurement and analysis of data (Kothari, 2004).

This research undertook a research survey. Research survey refers to the collection of

information from a sample of individuals through their responses to questions (Weiss et al.,

2001). Research survey was employed in order to obtain information that would describe the

current state of drainage infrastructure in Narok Town road and how poor drainage system has

affected them during the rainy season. The survey involved; government institutions

responsible for construction and maintenance of drainage systems, engineering consultants

who took part in the design of the storm water drainage system, the contractor who is building

the drainage system, and residents of Narok Town. For the purposes of achieving the objectives

of the study therefore, a case study design was adopted where survey research was used.

3.2 METHODOLOGYVarious techniques of data collection such as applying of questionnaires, taking of photographs,

observation, key informant interview and focus group discussion for primary data and literature

review for secondary data were employed in the study to obtain the information required to

meet the objectives.

A questionnaire is a research instrument consisting of a series of questions and other prompts

for the purpose of gathering information from respondents (Gillham, 2008).

Whereas, photography is the art, science and practice of creating durable images by recording

light or other electromagnetic radiation, either chemically by means of a light-sensitive material

such as photographic film, or electronically by means of an image sensor (Schewe, 2012). It

35

provides a less biased recording than observations; in addition, they can then be analyzed by

others in their original formats (Flick, 2002).

Observation refers to the systematic examination of real-time processes or operations with the

goal of identifying needs/challenges or improving processes and practices that is, what can be

seen. Observations typically incorporate a prescribed protocol containing specific measures of

observable behavior and the narrative recording of the program activities and their context

(Lofland and Lofland, 1995).

The key informant interview seeks to describe the meanings of central themes in the life world

of the subjects. The main task in interviewing is to understand the meaning of what the

interviewees say (Kvale, 1996). It seeks to cover both a factual and a meaning level, though it is

usually more difficult to interview on a meaning level (Kvale). Interviews are particularly useful

for getting the story behind a participant’s experiences. The interviewer can pursue in-depth

information around the topic. Interviews may be useful as follow up to certain respondents to

questionnaires, e.g., to further investigate their responses (McNamara, 1999). The key

informant for this research was Eng timothy Ntimama, who is currently the Narok County

Engineer. His involvement in the project as a whole helped to give insight on overall project

appraisal.

Focus group discussion seeks to get the opinions and views of those involved in the actual

construction of the storm water drainage the group will include: the assistant resident

engineer, representative of the contractor, materials supervising officer and a procurement

representative from the contractor’s office. Their first hand experience will help give primary

data for the technical appraisal of the project.

A literature review is data collection method that involves reading and comparison of readily

available materials like, journals, publications, reports, design drawings, metrological data and

eBooks. It serves as a basis for comparison with data obtained from the field. Literature review

was done on the hydrological processes, effects of urbanization on storm water drainage and

design requirements for storm water drainage.

36

3.3TOOLSObservation schedule - this is a physical check list of items that were filled after an observation

in the field.

Key informant schedule- this contains the interview questions for the key informants. The

objective is to seek answers on the objective of the project, the expected benefits from

completion of project, data used to design peak flows, design standards used for construction,

role of county engineer in the project, environmental impact, social and economic impact of the

project.

Focus group discussion schedule – contains discussion topics that seek to explore the level of

technology used in construction, materials standards used, how the materials were acquired

and where from, involvement of the community in the construction phase and how they

benefit.

Questionnaire – this was structured to be filled by residents in general and those living along

the construction site. This was to help find out how the construction has affected them, if there

were any demolitions and if so, was there compensation from the county government. Also it

sought to find the environmental impact along the construction site and their expectations of

the drainage system solving their flood problems.

37

CHAPTER FOUR: RESULTS, ANALYSIS AND DISCUSSION.

4.1 INTRODUCTIONThe main focus of this chapter, is the presentation and analysis of data/results obtained from

research, observation and questionnaires. This is done to enable the achievement of the

objectives set out for this project.

4.2 PHOTOGRAPHY.PLATE 4.1

A box culvert at tropical house that is filled up of boulders and silt, as a result the capacity is

reduced and storm water over flows, flooding nearby shops and roads.

38

PLATE 4.2

Shallow and narrow open drain channel between mwalimu house and tropical, this increases

the velocity of the stormwater run-off posing a danger to pedestrians.

39

PLATE 4.3

Brick lined open channel, the bricks are eroded retrospectively due to high velocity of storm

runoff. Sprouting of vegetation upstream reduces the capacity of the conduits and hinders

smooth flow of storm water.

40

PLATE 4.4

Road section that is covered up in silt deposit due to flooding, this is caused by poor storm

water drainage system in the town.

41

4.3 RAINFALL DATA ANALYSIS AND DESIGN OF DRAINAGE SYSTEMS.Rainfall information was obtained from the metrological department in Dagoretti, Narok station

No.14, meteorological department No. 91.35.01. (See appendix 1) The type of rain gauge used

is the float type, to record parameters which can be used to calculate rainfall intensities. The

float type rain gauge provides a continuous strip chart record of accumulated precipitation. The

record shows the accumulation of rain which falls with time in the shape of a mass curve.

A record of monthly rainfall data, for the year 1990 to 2013 was obtained. This was used to

give a rough idea on how the rainfall is distributed over the years. (See appendix 2) Rainfall

intensity data was not obtained, due to the high cost of such data i.e. ksh500, 000, rather an

already analyzed intensity duration curve was obtained. This was used to generate rainfall

intensity for a given time of concentration tc.

4.3.1 RETURN PERIODThe size or scale of an urban drainage system can be expressed in terms of the return period of

the design flows, which can be carried by the system. For design purposes, return periods of 25,

50 and 100 years were done for the proposed stormwater drainage. (Appendix 3).

42

4.3.2 RUN-OFF CO-EFFICIENT.The study area was subdivided into several small sub-catchment areas. This is because the

smaller the catchment area, the easier the comparison due to the homogeneity of the area

Table5

Zone Area in km2 Descriptions Value of C

A 0.42 Rural steep slope 0.55

C 0.82 Residential

Low density

0.35

D 0.69 Residential

Low density

0.35

E 0.40 Residential

Medium density

0.45

F 0.25 Business

neighborhood

0.50

G 0.13 Residential

Medium density

0.40

H 0.03 Business

neighborhood

0.50

43

Weighted mean, of run-off co-efficient, at different points.

¿¿¿¿

At point 1

Contributing areas, C, D, A

= (0.82∗0.35 )+ (0.69∗0.35 )+(0.42∗0.55)

( 0.82+0.69+0.42 )

= 0.39

At point 2

Contributing areas, C, D, A, E, F, G

= (0.39∗1.93 )+ (0.40∗0.45 )+ (0.25∗0.50 )+(0.13∗0.40)

(1.93+.4+0.25+0.13 )

= 0.41

At point 3

Contributing areas: contributing areas at point 2 and H.

= (0.41∗2.71 )+ (0.03∗0.50 )

(2.71+0.03 )

= 0.41

At point 4

Contributing areas: areas contributing at point 3 and I, J

= (0.41∗2.74 )+(0.48∗0.35 )+(0.02∗0.55)

(2.74+0.48+0.02 )

=0.40

44

4.3.3 DESIGN OF THE OPEN DRAINS LFROM KOBIL PETROP STATION THROUGH THE SAMPURMPUR VALLEY TO NAROK RIVER.In the redesign, the following assumptions were made:

1. Minimum velocity = 0.75m/s

Maximum velocity = 3.5m/s

2. Manning’s roughness of co-efficient n= 0.015

3. Free board:

i) For channels, an allowance of 100mm has been provided for.

ii) For culverts, an allowance of 150mm has been provided for.

4. A constant time of entry of 20mins was used

5. Time of travel was obtained by dividing the length of the open channel by maximum

allowable velocity i.e. 3.5m/s.

45

TABLE 6 ESTIMATED RUN-OFF AT VARIOUS POINTS IN M 3 /S

Point

s

Tributary

areas

Are

a

Flow time Rainfall intensity

(return period.)

C Runoff, Q

In m3/s

Inle

t

Chan

nel

Tota

l

25

yrs

50 yrs 100y

rs

25

yrs

50 yrs 100

yrs

1 A,C, D 1.93 0.33 _ 0.33 102.5 103.5 105 0.39 21.45 21.65 21.95

2 A,C,D,E,F,G 2.71 0.33 0.067 0.40 101 102.5 104 0.41 31.20 31.66 32.12

3 A,C,D,E,F,G,H 2.74 0.33 0.1 0.43 100.9 102 103.5 0.41 31.51 31.86 32.32

4 A,

C,D,E,F,G,H,I,

J

3.24 0.33 0.1 0.43 100.9 102 103.5 0.40 36.35 36.75 37.28

47

TABLE 7 PROPOSED CHANNEL SECTIONS COMPARED TO CURRENT SIZES.

Channel

section

Channel

bed slope

Side

slop

e

Computed sizes Capacity M3/S Present

sizes

25 YRS 50 YRS 100 YRS 25 YRS 50YRS 100 YRS

Kobil petrol

station

culvert to

Mwalimu

house

culvert

0.00181 0.10 D =2.3m

B = 3.9m

D =2.3m

B = 3.9m

D=2.3m

B=3.9m

23.5 28.04 28.43

Mwalimu

house to

Tropical

house

culvert

0.00479 0.10 D = 2.3m

B = 3.7m

D=2.3m

B=3.7m

D=2.3m

B=3.8m

39.6 40 40.7

Tropical

house to

road

junction

0.00479 0.10 D=2.2m

B=3.7m

D=2.2m

B=3.8m

D=2.2m

B=3.8m

40.07 40.07 40.07

Road

junction to

Narok river

0.00479 0.10 D=2.3m

B=4.0m

D=2.3m

B=4.0m

D=2.4m

B=4.0m

43.4 43.4 43.4

48

TABLE 8PROPOSED CULVERT SIZES COMPARED TO CURRENT SIZES.

LOCATIO

N OF THE

CULVERT

SHAPE OF

CULVERT

COMPUTED SIZE COMPUTED CAPACITY

M3/S

PRESENT

SIZE

25 YRS 50 YRS 100YRS 25 YRS 50 YRS 100 YRS

Kobil

culvert

Rectangular

culvert

D = 2.2m

B = 3.7m

D=2.2m

B=3.7m

D= 2.2m

B = 3.7m

22.69 22.70 23.75

Mwalimu

house

culvert

Rectangular

culvert

D = 2.2m

B = 3.7m

D=2.2m

B=3.7m

D= 2.2m

B = 3.7m

32.93 32.93 33.95

Tropical

house

culvert

Rectangular

culvert

D = 2.2m

B = 3.7m

D= 2.2m

B= 3.7m

D= 2.2m

B = 3.7m

32.4 32.4 32.4

Road

junction

culvert

Rectangular

box culvert

D = 2.2m

B = 3.7m

D= 2.2m

B= 3.7m

D= 2.2m

B = 3.7m

36.8 36.8 36.8

50

DISCUSSIONThe dimensions of the open drain channels were designed on the basis of the quantity of run-off compute from the rational formula. However as stated earlier, the rational formula tends to overestimate the run-off due to its assumptions.

A physical survey in the study area revealed that measurements of dimensions of existing drains was rather difficult, this is because most of the sides of the channels have crumbled, in some sections there was narrowing due to encroachment, other sections were widened due to erosion of the channel’s sides.

According to the county engineer, the existing drainage system that was in place was never designed for and was built through the ‘juakali’ methods; this would explain the frequent flooding in the town

The proposed channels computed, a return period for 25, 50 and 100 years was used; this gives large cross-sections for the drainage systems hence lager capacity to accommodate the stormwater run-off and solve the frequent flooding problems in the town. For economical reasons, the 25 year return period would be more plausible, but given the rate at which the town is growing, a 50 year return period would be safe. This should also be applicable to the culverts.

A comparison between the proposed and existing channel clearly shows that the cause of flooding in the town is due to poor or lack of properly designed drainage systems.

51

CONCLUSIONS AND RECOMMENDATIONSThe drainage system of Narok town has failed majorly due lack of properly designed drainage systems. Solid waste dumping into the existing open drain channels, silting due to low velocities and erosion of the exposed earth on the side of the channel are also contributing factors.

RECOMMENDATIONS

1. Redesign and replacement of the entire town’s drainage system with bigger open drain channels and culverts so as to pass the runoff as quickly as possible to the Narok River. The culvert at the stage junction needs an urgent replacement as it creates a bottleneck effect during the rainy season.

2. Addition of other major drainage system all around the town so as to not over load one drainage system

3. Maintenance practices should be given priority; this includes removal and cleaning up of the drains to rid of debris and growing vegetation in channels and culverts. This can be achieved by increasing budgetary allocation for maintenances.

4. Given that Narok lies in a valley, efforts should be made to reduce runoff from the high-lying lands, this can be done by increasing vegetation cover through tree plantation and discouraging deforestation. This increase rain water infiltration into the ground.

5. Use of seepage trenches and soak-ways should also be incorporated into the drainage system. These are basically ground water recharge facilities which are located throughout the catchment areas.

6. Building of flood mitigation dams upstream, that provides enough storage capacity to retain water until there is sufficient drainage capacity hence reducing peak flows drastically.

7. Discourage encroachment of reserve areas for flood prone lands thus leaving enough room for plant and machinery for maintenance and repair.

52

REFERENCES1. Jones, D.E., Jr. “Where is Urban Hydrology Practice Today”, Proceeding of American

Society of Civil Engineering, Hydro. Div. 97 (HY 2), pp 257-264. , (1971),2. Sunil, T.D. , “ Modeling of urban storm water drainage systems using ILSAX” thesis,

(2000)3. Environment and urbanization-2003- Parkinson-115-26

(http://eau.sagepub.com/content/15/2/115) 4. United Nations Center for Human Settlement ,habitat, storm drainage and land

reclamation report (1991)5. Engineering Hydrology 9th Edition Macmillan, by E.M. Wilson, (1990).6. Rural and Urban Hydrology, MG Mansell publishers, by Thomas Telford, (2003).7. Hydrology and Water Quantity Control: by Martin Wiley and Sons, Inc. (1975)8. Lecture notes Fluid Mechanics by S. Ngare (2012)9. Lecture Notes hydrology by Dulo (2013)10. Lofland, J. & Lofland, L. H. (1995). Analyzing Social Settings: A Guide to Qualitative

Observation and Analysis. Belmont, CA: Wadsworth.11. Schewe, Jeff (2012). The Digital Negative: Raw Image Processing in Light room, Camera

Raw, and Photoshop. Berkeley, CA: Peach pit Press, pg. 72.12. Applied Hydrology, Mc Graw Hill series, by: Van Te Chow, David R. Maidment, and Larry

W. May.13. Hydrology: Principles, Analysis, Design 2nd Edition by: H.M. Raghunath (2006)14. Water Resource Engineering by IIT Kharagpur, (2008)15. Introduction to Environmental Engineering: By David A. Cornwell Mackenzie L. Davis

MC-Graw Hill International Editions, chemical Engineering series (1985)16. Solving problems in fluid mechanics: By J.F. Douglas. Longman Singapore publishers

(1986)17. Water and waste water technology: By Hammer M.J. John Wiley and sons inc. (1975)18. Urban Storm Water Drainage, proceedings of international conference held at

University of Southampton, April 1978. Edited by P. Helliwel. Department of civil Engineering University of Southampton.

19. Open-channel Hydraulics: By Richard H. French MC-Graw – Hill Book Company (1986)20. Surface water sewerage: Barlett R.E. Applied science publisher Ltd. London (1981)21. Central Public Health and Environmental Engineering organization @ Manual on

sewerage and sewerage treatment, ministry of works and housing New Delhi. (1980)22. Water and waste water technology: By Hammer M.J John Wiley and Sons Inc. (1975)

54

CALCULATION SHEETBEST SECTIONS FOR OPEN CHANNELS SHEET NO. 1

REF CALCULATIONS OUTPUT

TRAPEZOIDOL SECTIONS

W

D 1

ƶ

B

W =B+ 2ƵD

Area

A =(B+ƶD)D

Therefore: AD-1=B+ƵD

B=AD-1-ƵD

Wetted perimeter

P=B+2D√(ƶ2+1)

Substituting for B

P= AD-1-ƵD +2D√(ƶ2+1)

dPdD

=−AD2 −ƶ+2√(ƶ2+1)

55



56

For maximum discharge dPdD

=0

¿ AD2 +ƶ=2√(ƶ2+1)

Substituting for A

=Ƶ+DD2 (B+2ƵD )=2√(ƶ2+1)

Therefore:

(B+2ƵD )=2 D√(ƶ2+1)

Condition for

maximum discharge.



57

RECTANGULAR SECTIONS

B

D

Area

A= BD

Perimeter

P= B+2D

B=AD-1

Substituting B

P=AD-1+2D

dPdD

=−AD2 +2

For maximum discharge conditions dPdD

=0

Therefore A=2D2

CALCULATION SHEETOPEN DRAIN FROM KOBIL TO MWALIMU HOUSE SHEET NO. 4


58

For maximum discharge:

(B+2ƵD )=2 D√(ƶ2+1)

Substituting Ƶ=0.10

(B+2(0.10)D)=2 D√(0.102+1)

B=1.809D

Wetted perimeter, P =B+2D√(ƶ2+1)

=1.80D +2.099D

=3.819D

Area of flow, A=(B+2 D)D

=1.909D2

Hydraulic mean depth M1,AP=1.909 D2

3.819 D

= 0.5D

Using manning’s formula

Q=A∗M

23∗S

12

n

n=0.015

Q=1.909 D2∗(0.5D)

23∗S

12

0.015

D=[ Q

63.6∗S12 ]

38



59

25 YEAR RETURN PERIOD

Q=21.445m3/s

D=[ 21.445

63.6∗0.0018112 ]

38=2.17

Allow for a free board of 0.1m

B = 1.809 * 2.17 = 3.9

Hydraulic mean depth M1= 0.5D

= 0.5*2.17=1.085

By manning’s formula: V=(M )

23∗S

12

n

V=(1.085)

23∗0.00181

12

0.015

= 2.99m/s < 3.0m/s O.K

Area of flow, A= 1.909D2

=10.1m2

Capacity of channel QC = 23.5m3/s >21.445 m3/s

D=2.3m

B=4m

V=2.99m/s

A=10.1m2

QC=23.5m3/s



60

50YEAR RETURN PERIOD

Q=21.65m3/s

D=[ 21.65

63.6∗0.0018112 ]

38=2.18


B = 1.809 * 2.18 = 3.9


= 0.5*2.18=1.085


23∗S

12

n

V=(1.09)

23∗0.00181

12

0.015

= 3.0m/s = 3.0m/s O.K


=10.1m2


D=2.3m

B=4m

V=3.0m/s

A=10.1m2

QC=28.04m3/s



61


Q=21.95m3/s

D=[ 21.65

63.6∗0.0018112 ]

38=2.19


B = 1.809 * 2.19 = 3.9


= 0.5*2.1=1.095


23∗S

12

n

V=(1.09)

23∗0.00181

12

0.015

= 3.00m/s = 3.0m/s O.K


=10.3m2


D=2.4m

B=4m

V=3.0m/s

A=10.3m2

QC=28.43m3/s

CALCULATION SHEETOPEN DRAIN FROM MWALIMU HOUSE TO TROPICAL FASHION HOUSE SHEET NO. 8


62


Q=31.20m3/s

D=[ 31.20

63.6∗0.0047912 ]

38=2.08


B = 1.809 * 2.08 = 3.7


= 0.5*2.08=1.04


23∗S

12

n

V=(1.04)

23∗0.00479

12

0.015

= 4.7m/s < 3.0m/s NOT O.K


=8.26m2


D=2.1m

B=4m

V=4.7m/s

A=8.26m2

QC=39.6m3/s



63


Q=31.66m3/s

D=[ 31.66

63.6∗0.0047912 ]

38=2.09


B = 1.809 * 2.09 = 3.7


= 0.5*2.09=1.045


23∗S

12

n

V=(1.045)

23∗0.004791

12

0.015

= 4.7m/s > 3.0m/s NOT O.K


=8.34m2


D=2.2m

B=3.7m

V=4.7m/s

A=8.34m2

QC=40.04m3/s



64


Q=32.12m3/s

D=[ 32.12

63.6∗0.0047912 ]

38=2.10


B = 1.809 * 2.10 = 3.7


= 0.5*2.10=1.05


23∗S

12

n

V=(1.05)

23∗0.00181

12

0.015

= 4.7m/s > 3.0m/s NOT O.K


=8.4m2


D=2.2m

B=3.7m

V=4.7m/s

A=8.4m2

QC=40.7m3/s

CALCULATION SHEETOPEN DRAIN FROM TROPICAL TO STAGE JUNCTION SHEET NO. 11


65


Q=31.50m3/s

D=[ 31.50

63.6∗0.0047912 ]

38=2.1


B = 1.809 * 2.18 = 3.7


= 0.5*2.1=1.05


23∗S

12

n

V=(1.05)

23∗0.00479

12

0.015

= 4.7m/s > 3.0m/s NOT O.K


=8.4m2


D=2.2m

B=4m

V=4.7m/s

A=8.4m2

QC=39.56m3/s



66


Q=31.86m3/s

D=[ 31.86

63.6∗0.0047912 ]

38=2.1


B = 1.809 * 2.1 = 3.8


= 0.5*2.1=1.05


23∗S

12

n

V=(1.05)

23∗0.00479

12

0.015

= 4.76m/s 3.0 >m/s NOT O.K


=8.04m2


D=2.2m

B=4m

V=4.76m/s

A=8.04m2

QC=4.07m3/s



67


Q=32.32m3/s

D=[ 32.32

63.6∗0.0047912 ]

38=2.1


B = 1.809 * 2.1 = 3.8


= 0.5*2.1=1.05


23∗S

12

n

V=(1.05)

23∗0.00479

12

0.015

= 4.76m/s >3.0m/s NOT O.K


=8.4m2


D=2.2m

B=4m

V=4.76m/s

A=8.4m2

QC=40.07m3/s

CALCULATION SHEETKOBIL CULVERT SHEET NO. 14


68

Rectangular Section

For maximum efficiency

A=2D2 and M1= 0.5D

By manning’s formula

Q=A∗M

23∗S

12

n

For S=0.00181

n=0.015

Therefore;

D=[ Q3.3501 ]

38



69


Q=21.445m3/s

D=[ 21.4453.3501 ]

38 =2m

Allow for free board of 0.1m

B = 2D =4m

Area A = BD

= 2 * 4= 8m2

Hydraulic depth M = 0.5D

= 0.5 * 2 = 1

Velocity

V=(M )

23∗S

12

n V=(1)

23∗0.00181

12

0.015 =2.83m /s<3.0m /s

O.K

Capacity Qc = 22.69m3/s> 21.445 m3/s

D =2.1m

B =4m

A=8m2

V= 2.83m/s

Qc= 22.69 m3/s



70


Q=21.65m3/s

D=[ 21.653.3501 ]

38=2.01m


B = 2D =4.02m

Area A = BD

= 2 * 4.02= 8.04m2


= 0.5 * 2.01 = 1.005

Velocity

V=(M )

23∗S

12

n V=(1.005)

23∗0.00181

12

0.015 =2.83m /s<3.0m /s

O.K

Capacity Qc = 22.75m3/s> 21.55 m3/s

D =2.01m

B =4.02m

A=8.04m2

V= 2.83m/s

Qc= 22.75 m3/s



71


Q=21.65m3/s

D=[ 21.653.3501 ]

38=2.01m


B = 2D =4.02m

Area A = BD

= 2 * 4.02= 8.04m2


= 0.5 * 2.01 = 1.005

Velocity

V=(M )

23∗S

12

n V=(1.005)

23∗0.00181

12

0.015 =2.83m /s<3.0m /s

O.K


D =2.01m

B =4.02m

A=8.04m2

V= 2.83m/s

Qc= 22.75 m3/s

CALCULATION SHEETMWALIMU HOUSE CULVERT SHEET NO. 17


72


Q=31.20m3/s

D=[ 31.203.3501 ]

38=2.3m


B = 2D =4.6m

Area A = BD

= 2.3 * 4.6= 10.58m2


= 0.5 * 2.3 = 1.15

Velocity

V=(M )

23∗S

12

n V=(1.15)

23∗0.00181

12

0.015 =3.1m /s>3.0m /s

O.K

Capacity Qc = 32.93m3/s> 31.20m3/s

D =2.3m

B =4.6m

A=10.58m2

V= 3.1m/s

Qc= 32.93 m3/s



73


Q=31.66m3/s

D=[ 31.663.3501 ]

38=2.3m


B = 2D =4.6m

Area A = BD

= 2.3 * 4.6= 10.58m2


= 0.5 * 2.3 = 1.15

Velocity

V=(M )

23∗S

12

n V=(1.15)

23∗0.00181

12

0.015 =3.1m /s>3.0m /s

O.K


D =2.01m

B =4.6m

A=10.58m2

V=3.1m/s

Qc= 32.93m3/s



74


Q=32.12m3/s

D=[ 32.123.3501 ]

38=2.3m


B = 2D =4.6m

Area A = BD

= 2.3 * 4.7= 10.60m2


= 0.5 * 2.3 = 1.15

Velocity

V=(M )

23∗S

12

n V=(1.15)

23∗0.00181

12

0.015 =3.14 m /s>3.0m / s

NOT O.K

Capacity Qc = 33.95m3/s> 32.12 m3/s , O.K

D =2.01m

B =4.6m

A=1.60m2

V= 3.14m/s

Qc= 33.95m3/s

CALCULATION SHEETTROPICAL HOUSE CULVERT SHEET NO. 18


75


Q=31.51m3/s

D=[ 31.513.3501 ]

38=1.9m


B = 2D =3.8m

Area A = BD

= 2.3 * 4.7= 7.2m2


= 0.5 * 1.9 = 0.95

Velocity

V=(M )

23∗S

12

n V=(0.95)

23∗0.004791

12

0.015 =3.14m /s>3.0m/ s

NOT O.K


D =2.05m

B =3.8m

A=7.2m2

V= 4.5m/s

Qc= 32.4m3/s

CALCULATION SHEETTROPICAL HOUSE CULVERT SHEET NO. 19


76


Q=31.86m3/s

D=[ 31.865.758 ]

38 =1.9m


B = 2D =3.8m

Area A = BD

= 1.9*3.8= 7.2m2


= 0.5 * 1.9= 0.95

Velocity

V=(M )

23∗S

12

n V=(.95)

23∗0.00479

12

0.015 =4.5m /s>3.0m /s

NOT O.K


D =2.05m

B =3.8m

A=7.2m2

V= 4.5m/s

Qc= 32.4m3/s

CALCULATION SHEETTROPICALHOUSE CULVERT SHEET NO. 20


77


Q=32.32m3/s

D=[ 32.325.758 ]

38 =1.9m


B = 2D =3.8m

Area A = BD

= 1.9*3.8= 7.2m2


= 0.5 * 1.9 = .95

Velocity

V=(M )

23∗S

12

n V=(0.95)

23∗0.004791

12

0.015 =4.5m /s>3.0m /s

NOT O.K


D =2.01m

B =4.6m

A=7.2m2

V= 3.14m/s

Qc= 32.4m3/s

CALCULATION SHEETSTAGE JUNCTION CULVERT SHEET NO. 21


78


Q=36.33m3/s

D=[ 36.335.758 ]

38 =2.0m


B = 2D =4.0m

Area A = BD

= 2.0*4.=8.0m2


= 0.5 * 2.0 = 1.0

Velocity

V=(M )

23∗S

12

n V=(1.0)

23∗0.00479

12

0.015 =4.6m /s>3.0m /s

NOT O.K

Capacity Qc = 36.8m3/s> 36.33m3/s , O.K

D =2.15m

B =4.0m

A=8.0m2

V= 4.6m/s

Qc= 36.33m3/s

CALCULATION SHEETSTAGE JUNCTION CULVERT SHEET NO. 22


79


Q=36.75m3/s

D=[ 36.755.758 ]

38 =2.0m


B = 2D =4.0m

Area A = BD

= 2.0*4.0=8.0m2


= 0.5 * 2.0 = 1.0

Velocity

V=(M )

23∗S

12

n V=(1.0)

23∗0.00479

12

0.015 =4.6m /s>3.0m /s

NOT O.K


D =2.15m

B =4.0m

A=8.0m2

V= 4.6m/s

Qc= 36.8m3/s

CALCULATION SHEETSTAGE JUNCTIONCULVERT SHEET NO. 23


80


Q=37.28m3/s

D=[37.285.78 ]

38 =2.0m


B = 2D =4.0m

Area A = BD

= 2.3 * 4.7= 8.0m2


= 0.5 * 2.0 = 1.0

Velocity

V=(M )

23∗S

12

n V=(1.0)

23∗0.00181

12

0.015 =4.7m /s>3.0m /s

NOT O.K

Capacity Qc = 37.6m3/s> 37.28m3/s , O.K

D =2.15m

B =4.0m

A=8.0m2

V= 4.7m/s

Qc= 37.6m3/s

81

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