Tail EscapeTail Escape

Waterway

Road

Water Body

• Tail escape is a structure constructed Tail escape is a structure constructed at the end of a waterway to evacuate at the end of a waterway to evacuate the water to a water body.the water to a water body.

• Tail escape consists of a well where Tail escape consists of a well where its crest level at the high water level. its crest level at the high water level. It is also equipped with an orifice at It is also equipped with an orifice at its bottom to evacuate all the water its bottom to evacuate all the water in 24 hrs if necessary.in 24 hrs if necessary.

• A pipe evacuates the water to the A pipe evacuates the water to the water body under the road.water body under the road.

D

do

d

Road

Tail Escape

Hydraulic DesignHydraulic Design• Hydraulic design of the tail escape Hydraulic design of the tail escape

consists of three parts:consists of three parts: 1. Automatic evacuation (Weir)1. Automatic evacuation (Weir) 2. Controlled evacuation (Orifice)2. Controlled evacuation (Orifice) 3. Discharging to Sea (Pipe)3. Discharging to Sea (Pipe)

1.1. Automatic Evacuation:Automatic Evacuation:The excess water is evacuated over The excess water is evacuated over the weir's crest.the weir's crest.The maximum acceptable rise in The maximum acceptable rise in water level is 25 cm.water level is 25 cm.the discharge of water over the weir the discharge of water over the weir can be calculated as follows:can be calculated as follows:

QQww = T = Tss . h . V . h . Vss

Where:Where:QQww is the weir’s discharge in m is the weir’s discharge in m33/sec,/sec,TTss is the top width of the water is the top width of the water surface in m,surface in m,h is the rise in water level =0.25 m,h is the rise in water level =0.25 m,VVss is the surface water velocity in is the surface water velocity in m/sec.m/sec.

VVss = 1.17 v = 1.17 vww

Where vWhere vww is the mean water velocity of is the mean water velocity of the water way.the water way.

25 cm

Ts

H.W.L.

To design the size of the well, the weir To design the size of the well, the weir equation is used,equation is used,

Where Where Cd is the discharge coefficient = Cd is the discharge coefficient =

0.55,0.55,B is the crest length, andB is the crest length, and

5.1232

hgBCQ dw

DB 43

2. Controlled Evacuation:2. Controlled Evacuation: Orifice is designed to evacuate the Orifice is designed to evacuate the

waterway in 24 hrs. The following waterway in 24 hrs. The following equation is used to determine the equation is used to determine the size of the orifice,size of the orifice,

Where:Where:T is the time to empty the waterway T is the time to empty the waterway

= 24 . 60 .60 sec= 24 . 60 .60 sec

gyaCyLb

Td

e

2)(2

L is the waterway length in m,L is the waterway length in m, bbee is the waterway average width in is the waterway average width in

m,m, y is the depth of the waterway,y is the depth of the waterway, Cd is the discharge coefficient = Cd is the discharge coefficient =

0.60.6Ts

H.W.L.

be

b

y

a is the area of the orifice in ma is the area of the orifice in m22 and, and,

Notice: The orifice is equipped with a Notice: The orifice is equipped with a gate so the water in the waterway gate so the water in the waterway can be used for any purpose. The can be used for any purpose. The gate is open to evacuate the water if gate is open to evacuate the water if needed.needed.

2

4 oda

3. Pipe:3. Pipe: The pipe is designed to the The pipe is designed to the

maximum discharge for emergency maximum discharge for emergency as:as:

Where:Where:QQpp is the pipe discharge, is the pipe discharge,QQoo is the orifice discharge is the orifice discharge

owp QQQ

Where:Where:

And,And,

Where VWhere Vpp is the pipe velocity (2.0 is the pipe velocity (2.0 m/sec) and D is the pipe diameter.m/sec) and D is the pipe diameter.

02ghaCQ do

25.0yho

2

4DVQ pp

• Example:Example:A tail escape is required to be A tail escape is required to be constructed at the end of a waterway constructed at the end of a waterway according to the following data;according to the following data;

Q = 4.6 mQ = 4.6 m33/sec, Waterway /sec, Waterway length = 3000 m, Vlength = 3000 m, Vcc=0.45 m/sec=0.45 m/sec

7.6 m

5.8 m

4 m

1.8 m

• Sol.Sol.a. Automatic evacuationa. Automatic evacuation

QQww= 7.6 * 0.15 * (1.17*0.45)= 7.6 * 0.15 * (1.17*0.45) = 0.6 m= 0.6 m33/sec/sec0.6 = 0.6 =

(2/3)*0.55*B*(√2*9.81)*(0.15)(2/3)*0.55*B*(√2*9.81)*(0.15)1.51.5

B = 6.36 mB = 6.36 mD = 2.7 mD = 2.7 m

b. Controlled Evacuationb. Controlled EvacuationT = 24*60*60T = 24*60*60

a = 0.2 ma = 0.2 m22

ddoo= 0.5 m= 0.5 m

c. Pipec. PipeQQpp=Q=Qww+Q+Qoo

QQoo=0.6*0.2*(√2*9.81*2.05)=0.6*0.2*(√2*9.81*2.05) = 0.76 m= 0.76 m33/sec/secQQpp= 1.36 m= 1.36 m33/sec/secAssume VAssume Vpp= 2 m/sec= 2 m/sec

1.36 = 2 * A1.36 = 2 * App

D = 0.93 m take D = 1.0 mD = 0.93 m take D = 1.0 m

VVpp= 1.73 m/sec= 1.73 m/sec