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this document is a design report of outlet structure of a small dam...

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1.1.1

Intake Tower existing Dam

To control and regulate the entry of water from the reservoir to the proposed types of waterway a 10m high intake tower has been provided upstream of the dam body. The tower is located on the left abutment at 234353E, 743420N adjacent to the toe of the upstream dam body. To ensure the effective withdrawal of the reservoir water and fulfill all the functional requirement of the outlet work the following arrangement has been provided to the intake tower: 

A 3.3m X 3.3m rectangular stem and

10m high from its base at 2597masl up to

2606masl ; 

Two sets of emergency gate accompanied by vertically slide trash racks.



An rectangular control room superstructure building at the top of the Intake Tower, 2m long sided and 2m high, housing the lifting gear for the various hydro mechanical equipment.



A 40m long footbridge from the superstructure building to left abutment Dam crests level ground.

Access to the base of the tower will be by steel stairways in the Intake Tower stem, with steel platforms at 5.6 m intervals of height. Space is also allowed in the Intake Tower stem for an access shaft, for removal of the hydro mechanical equipment. 1.1.2 Outlet Pipes 1.1.2.1 Determination of Outlet Design Discharge Outlet works controls are designed to release water as specific rates, as indicated by downstream needs, and according to the the gewha water supply project the total water suplly demand and live stokGefersa water treatment facility is designed to produce 30,000 m³ per day or 0.347 m³/sec treated potable water. Thus to ensure this production target sufficient amount of raw water should be conveyed into the treatment plant. Accordingly with the absence of the actual raw water requirement of the scheme the outlet structure has been designed to convey 40,000 m³ per day or 0.463 m³/sec of raw water into the treatment plant.

1.1.2.2 Determination of Outlet Levels According to the as built drawing done by the COYNE ET BELLIER and Tractable Development Engineering Consultants the invert level of the existing outlet pipe is 2595.5 m.a.s.l And in order to ensure the supply of the required discharge the invert level of the inlet gate has been determined considering the 50 years

sediment storage level, the minimum

submergence and total head needed upstream of the outlet pipe to avoid air entrainment and overcome the various head losses in the pipe.

According to the hydrology study of the project the annual entry of sediment into Gefersa dam III reservoir is estimated to be around 0.330MCM

and the required reservoir level to

3

accommodate the 50 years 2597Mm sediment load is computed to be at 2599masl which is higher than the outlet pipes invert level. In the new provision of intake tower the minimum level is considered by fixing the required demand of wet and dry season. Thus, an invert level of 2598.5 m.a.s.l. is fixed for dry season as well as 2601 and 2603.75 m.a.s.l. for wet season of rainfall. Design Consideration The design of new intake tower which is to be build has considered the following key assumptions in order to be safe in function and economical. 

Short distance location of pedestrian bridge for gate operation and easily maintain work inside on tower.



Cost effective tower integrated with its accessory like provision of sliding gate, ladders, flow control valves, trash rack, and stop log hydro mechanical equipment.



Easily and cost effective system of construction stages and selection of materials.

Hydraulic computation of Intake structure The head loss calculations are shown in the following section in detail and the summery of the reservoir control points are shown below. 

Full Reservoir Level = 2604.5m



50 Years sediment storage level = 0.330MCM



Sill elevation of outlet pipe (Intake One) = 2603.75m



Sill elevation of outlet pipe (Intake Two) = 2601m



Sill elevation of outlet pipe (Intake Three) = 2597m

 

Size of steel pipe conduit = 500mm



Sill elevation of the lower intake tower inlet = 2598 m



Maximum Discharge for Intake 1 and 2= 1.98m3/s and Minimum= 0.53m3/s



Maximum Discharge for Intake 3= 1.94m3/s and Minimum= 0.39m3/s

As the quality of water decrease downward from the full reservoir level two additional inlet opening with trash rack and emergency gate has been provided at 2603.75 and 2601 so that the withdrawal of good quality of water can be ensured for most of the plant operation period. Calculation of required discharge is based on maximum and minimum reservoir level in addition to consider depletion time of empty reservoir with in short period. So the total head loss (K L) and iterate optimum size of pipe diameter in accordance with hydraulic efficiency of the pipe outlet system. Thus, the formula used from small dams , where to head and total head loss calculation.

a)

Head losses and loss coefficients

a1 is the required area of pipe size in addition ….….Eq. 1.1

The head losses in the proposed outlet work arrangement includes the head loss due to the trash rack interference, entrance shape, regulating gate valve installation, horizontal bend and pipe fittings and total head loss of the outlet system has been computed using following procedure:

b) Trash rack loss coefficient, kt To prevent the entry of floating & submerged debris as well as floating animals a fixed inclined trash rack of size 1.2 X 1.2 has been provided in front of the intake tower inlet gates. The trash rack is arranged to have 10cm wide rack of bars with 11cm center to center spacing and to facilitate manual cleaning the trash rack is designed with an installation angel of 750. The head loss due to the trash rack has been computed using the formula recommended by USBR, 1974 (Design of Small Dams, pp472) ht =K t (

V2) ………………………………………………………………………………… 2g

(1.2) Where, ht : head loss due to the trash rack interference Kt : Trash rack loss coefficient V : Entrance velocity through the net rack area The Trash rack loss coefficient has been computed using the empirical formula given by: kt = 1.45 - 0.45 × (an / ag) ×(an / ag) ………………………………………………(1.3) Where, an : net area through the trash rack bars ag = Gross area of the racks and supports,m2 Width of rack = 1.3m

Height racked above Invert level of pipe = 1.3m ag = 1.42 m2 an = 0.71 m2+ ag/ an = 0.42 kt = 1.08

c) Entrance shape and loss coefficient, ke Usually to reduce the loss at entry it is recommended to provide smooth entrance shapes, accordingly to facilitate the entry of the reservoir water into the intake tower inlet opening that has a slightly rounded shape has been provided. And as per USBR recommendation the entrance head loss coefficient for slightly rounded entrance, ke should be: 

Maximum = 0.18

square cornered entrance, small dams page 458 table

10.1/slightly rounded entrance. Similarly an entrance contraction head loss coefficient of 0.5 has been adopted for calculating the head loss at the pipe entry.

……………………….. Eq.1.4

d) Regulating - Gate valve loss coefficient, kg As it was mentioned before the flow through the outlet pipes has been designed to be regulated using the existing Flanged Gate Valve mounted downstream and for fully Open Flanged Gate Valve, the coefficient, kg is 0.7. (USBR, Design of Small Dams, losses in Gate table 10.1 small dams’ page 458). e) Fitting loss coefficient

The fitting loss coefficient value is highly dependent on factors such as bend radius, contraction and expansion ratios and for 0 .4m diameter line fitting an average coefficient, Kf of 1.0 and 0.4 has been adopted respectively. f) Pipe friction loss coefficient, hf Pipe friction loss coefficient is determined based on transition region of flow. The ColebrookWhite equation is recommended for computing the resistance coefficient, f since it is applicable to either smooth, transition, or rough flow conditions. ……………. ………Eq 1.5

a. Total head loss The total head loss, observed in the system includes, entry loss, regulating - Gate valve loss, loss in pipe and friction losses are calculated H L = 1.36 m for intake 1 & 2 and H L = 1.26 m for intake 3 respectively. Table 5-3: Calculated Head losses of Gefersa Dam III Intake 1 and 2

s.no

Element

Area

(a3/ax)2

Loss type

Loss symbol

Loss coefficient

4*7

1

2

3

4

5

6

7

8

1

Entrance

1.13

0.03

Kt

0.930

0.028

1.13

0.03

Trash rack loss Entrance loss

Ke

0.180

0.005

2

Transition

0.66 0.66

0.09 0.09

contraction friction loss

Kc kf

0.500 0.003

0.044 0.000

3

Pipe Loss

0.50

0.15

friction

Kf

1.607

0.248

0.50

0.15

V. bend 1

Kb1

1.000

0.154

0.50

0.15

V. bend 2

Kb2

0.000

0.000

0.20

1.00

Gate

Kg

0.700

0.700

0.20

1.00

Entrance

ke

0.180

0.180

4

Gate Loss

0.20

1.00

Friction

kf

0.003

0.003

KL

1.36

Table 5-1: Calculated Head losses of Gefersa Dam III Intake 3

s.no 1

Element 2

Area 3

(a3/ax)2 4

1

Entrance

1.13

0.03

1.13 0.66 0.66 0.50 0.50 0.50 0.20 0.20 0.20

0.03 0.09 0.09 0.15 0.15 0.15 1.00 1.00 1.00

2

Transition

3

Pipe Loss

4

Gate Loss

Loss type 5

trash rack loss entrance loss contraction friction loss friction V. bend 1 V. bend 2 Gate Entrance Friction

Loss symbol 6

Loss coefficient 7

4*7 8

Kt

0.930

0.028

Ke Kc Kf Kf Kb1 Kb2 Kg Ke Kf

0.180 0.500 0.003 1.949 0.000 0.000 0.700 0.180 0.003

0.005 0.044 0.000 0.301 0.000 0.000 0.700 0.180 0.003

KL

1.26...