Title: Evaluation of the On-Going Construction of Narok Town Storm Water Drainage System

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.

ABSTRACT

Narok 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.

ACKNOWLEDGEMENT

I 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.

Table of Contents

ABSTRACT

ACKNOWLEDGEMENT.

CHAPTER ONE: INTRODUCTION

1.1 BACKGROUND.

1.2 PROBLEM STATEMENT

1.3 GOALS AND OBJECTIVES.

1.4 SUMMARY OF APPROACH, TOOLS AND METHODOLOGY.

CHAPTER TWO: LITRATURE REVIEW.

2.1 HYDROLOGICAL CYCLE.

2.2 URBAN HYDROLOGY.

2.3 EFFECTS OF URBANISATION ON STROM RUNOFF.

2.4 URBAN STROMWATER MANAGEMENT.

2.5 URBAN STORMWATER DRAINAGE DESIGN.

2.5.1 INTRODUCTION.

2.5.2 PLANNING.

2.5.3DESIGN CRITERIA

2.5.4 BASIC DESIGN CRITERIA

2.5.5FLOOD ESTIMATION.

2.5.6CHANNEL DESIGN

2.5.7DRAINAGE DESIGN FOR URBAN STREETS.

2.5.8DESIGN OF CULVERTS

CHAPTER THREE: RESEARCH AND METHODOLOGY

3.1 RESEARCH APPROACH

3.2 METHODOLOGY

3.3TOOLS

CHAPTER FOUR: RESULTS, ANALYSIS AND DISCUSSION.

4.1 INTRODUCTION

4.2 PHOTOGRAPHY.

4.3 RAINFALL DATA ANALYSIS AND DESIGN OF DRAINAGE SYSTEMS.

4.3.1 RETURN PERIOD

4.3.2 RUN-OFF CO-EFFICIENT.

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

DISCUSSION

CONCLUSIONS AND RECOMMENDATIONS

REFERENCES

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 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 STATEMENT

Narok 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.

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

CHAPTER TWO: LITRATURE REVIEW.

2.1 HYDROLOGICAL CYCLE.

Hydrologic cycle is the water transfer cycle, which occurs continuously in nature; thethree 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 condensationnuclei 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 addstructures onto the surface, such as roads, houses, parking lots, sidewalks, and driveways. All of these are impervious surfaces, 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), runoffis 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 thannatural, and the flow rate increases much more rapidly and drops off more rapidlyafter the storm.Second, due to the reduced infiltrationvolumes, there is less water availableto be released slowly into the stream over time, resulting in lower waterlevels between rainfall events. In effect, much of the water that under naturalconditions infiltrated into the groundand slowly

Made its way into nearby creeks now enters the streamall 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)

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 thattraps litter and sediment to improve water quality in receiving waters. Communityinvolvement in cleanup programs and source controls, re-vegetation programs of disturbed land, and minimal bare soil in urban gardens (especially those on sloping land) are some of the non-structural measures (

The stormwater drainage network represents a large capital investment and hence dueconsideration 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.

2.5.1INTRODUCTION.

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 waterdrainage 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
• Water conservation in areas of low rainfall
• Integration of large-scale drainage works into overall town planning schemes

2.5.1.2Principles 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)

2.5.3DESIGN 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: