Side intake sketch; trash rack loss

Sketch of a side intake

PLAN
 river ==========================>
   weir |||||||  undersluice
   bank +---------------------+
        |trash rack  | gate   |--> gravel trap --> canal
        +---------------------+

SECTION
  pond ~~~~~~~~~~~~~~~~~~~~~~~~
   boom o  |  rack (75°)  | gate | hoist deck
        ###|##\           |      |
        ###|###\__________|______|======> canal
   sill (1.0-1.5 m above undersluice)
 /////////////////////////////////////////// bed

Components: trash boom, trash rack with rake, intake sill, gate with stop-log slot, hoist deck, flushing undersluice and the transition to the canal.

Head loss at the trash rack (Kirschmer's formula)

$$h_t=\beta\left(\frac{s}{b}\right)^{4/3}\frac{V^2}{2g}\sin\theta$$

where $h_t$ is the head loss (m), $\beta$ the bar shape factor (2.42 for rectangular bars, 1.83 for rectangular bars with a rounded upstream edge, 1.79 for circular bars, 0.76 for streamlined bars), $s$ the bar thickness, $b$ the clear spacing between bars, $V$ the approach velocity (m/s) and $\theta$ the angle of the rack from the horizontal.

A rack in service is partly clogged, so the clean-rack loss is increased by a clogging allowance of 25-50%, and this increased loss is used in the design of the intake.

Example: circular bars ($\beta=1.79$) 10 mm thick at 100 mm clear spacing, $V=0.8$ m/s, $\theta=75^\circ$:

$$h_t=1.79\,(0.1)^{4/3}\frac{0.8^2}{19.62}\sin75^\circ=1.79\times0.0464\times0.0326\times0.966=0.0026\ \text{m}$$

This is very small, so the loss at a clean rack is negligible.