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 (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 bodies with safety and minimal damage. Other objectives include;
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 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( use of computers optional) of, flowrates, equivalent impervious areas, areas of different land use types. -treatment of partial area effect - accumulation
Whereas, photography is the art, science and practice of creating durable images by recording 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 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 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
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 Weighted mean, of run-off co-efficient, at different points. At point 1 Contributing areas, C, D, A = = 0.39 At 2 Contributing areas, C, D, A, E, F, G
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, 3.24 0.33 0.1 0.43 100.9 102 103.5 0.40 36.35 36.75 37.28 TABLE 7 PROPOSED CHANNEL SECTIONS COMPARED TO CURRENT SIZES. Channel section Channel bed Side slope Computed Capacity M3/S Present sizes 25 YRS 50 YRS 100 YRS 25 YRS 50YRS 100 YRS Kobil petrol station culvert to Mwalimu house culvert 0.10 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 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
house culvert 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 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 TABLE 8PROPOSED CULVERT SIZES COMPARED TO CURRENT SIZES. LOCATION 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
50YEAR RETURN PERIOD Q=31.66m3/s Allow for a free board of 0.1m B = 1.809 * 2.09 = 3.7 Hydraulic mean depth M1= 0.5D = 0.5*2.09=1.045 By manning’s formula: = 4.7m/s > 3.0m/s NOT O.K Area of flow, A= 1.909D2 =8.34m2 Capacity of channel QC = 40.04m3/s >31.66 m3/s D=2.2m B=3.7m V=4.7m/s A=8.34m2 QC=40.04m3/s CALCULATION SHEET OPEN FROM MWALIMU HOUSE TO TROPICAL FASHION HOUSE NO. 10 REF CALCULATIONS OUTPUT
100YEAR RETURN PERIOD Q=32.32m3/s for a free board of 0.1m B = 1.809 * 2.1 = 3.8 Hydraulic mean depth M1= 0.5D = 0.5*2.1=1.05 By manning’s formula: = 4.76m/s >3.0m/s NOT O.K Area of flow, A= 1.909D2 =8.4m2 Capacity of channel QC = 4.07m3/s >32.32 m3/s D=2.2m B=4m V=4.76m/s A=8.4m2 QC=40.07m3/s CALCULATION SHEET KOBIL CULVERT SHEET NO. 14 REF CALCULATIONS OUTPUT
50 YEAR RETURN PERIOD Q=31.86m3/s Allow for free board of 0.15m B = 2D =3.8m Area A = BD = 1.9*3.8= 7.2m2 Hydraulic depth M = 0.5D = 0.5 * 1.9= 0.95 Velocity NOT O.K Capacity Qc = 32.4m3/s> 31.86 m3/s , O.K D =2.05m B =3.8m A=7.2m2 V= 4.5m/s Qc= 32.4m3/s CALCULATION SHEET TROPICALHOUSE CULVERT SHEET NO. 20 REF CALCULATIONS OUTPUT