Trash rack (or screen) losses

By Ahmad Suhendra

A screen is always required at the entrance of a pressure pipe . The function of screen (trash rack) is :

• keep debris away from the entrance to the outlet works where the debris will not clog the critical portions of the structure;
  
• capture debris in such a way that relatively easy removal is possible;

The flow of water through the rack also gives rise to a head loss. Though usually small, it can be calculated by a formula due to Kirchmer .

ht = [Kt * (t / b)^(4 / 3) * (Vo ^2 * Sin α )] / (2 * g)

ht = Screen head loss (m)
Kt = Resistance coefficient
t = Bar thickness (mm)
b = Width between bars (mm)
Vo= Approach velocity (m/s)
g = Gravitational constant (9.8 m/s^2)
α = Angle of inclination from horizontal


Ref :
ESHA (European Small Hydropower Association),”Layman’s Handbook on How To Develop a Small Hydro Site,”2nd ed, 1998 http://www.scribd.com/doc/8885765/Layman-Handbook-for-hydro-electric-power-plants

http://www.knoxcounty.org/stormwater/pdfs/vol2/3-3-5%20Trash%20Racks%20and%20Safety%20Grates.pdf

Calculation of flood spillway

By Ahmad Suhendra

Flood spillway was designed based on the excess water and high water in the channel carrier (head race)

Example :

Flood Spillway will be made on a channel that has a water depth of 0.691 m, with a normal discharge (Q normal) of 0.6 m^3/sec. What is the length of Flood spillway?

Spillway length (L spillway) is obtained by using the equation of calculating Weir and must be able to pass through if the flow (Q spillway) in a state of excess but not based on the flood peak.

Dicharge Coefficient (Cd)
Profile of Crest Weir	μ- factor	Cd=2.953 μ
Board, sharp edge	0.49-0.51	1.44 - 1.51
Board, round edge	0.50-0.55	1.48 - 1.62
Round overfall	0.65-0.73	1.92 - 2.16
Sharp edge,	0.64	1.89
Rounded	0.75	2.21
Roof Shaped	0.79	2,33

Standard Weir equations:
Q = Cd * L weir * (h over the top )^1.5
In this case :

• L spillway = L weir
• Q spillway = Q minor flood - Q normal = Cd * L spillway * ( h over the top)^1.5

There are some values of  Discharge Coefficient  (Cd) and in this calculation the value of Cd was taken 1.6 (Board, the round edge)

Then :

• L spillway = (Q minor flood – Q normal) / (Cd * ( h over the top)^1.5 )

To increase the water on the head race taken a number of around 20% of normal conditions and is considered as a minor flood (small flood).

• Q minor flood = 1.2 * 0.6 = 0.72 m^3/sec
• h minor flood = 1.2 * h = 1.2 x 0.691 = 0.829 m
• h over the top = 0.829 - 0.691 = 0.138 m
• Q spillway = 0.72 - 0.6 = 0.12 m3/sec
• L spillway = ( 0.12) / (1.6 * 0.138 ^1.5) = 1.46 m

Examples of open-channel calculation

1. What are the types of open channel flow?
2. Water is flowing at a velocity of 12 ft/s and depth of 10 ft in a channel of rectangular section. Find the change in depth and absolute water level produced by (a) the smooth upward step of 0.5 ft, (b) the smooth downward step of 1 ft in the channel bed. Also (c) find the maximum allowable size of upward step for the upstream to be possible as specified.
3. etc

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