Design of side intake with trashrack

Given: river bed RL 3315.00 m; weir crest RL 3317.50 m; 100-year flood level RL 3319.55 m; canal water level RL 3317.30 m; $Q=1.45$ m³/s; circular bars 10 mm thick at 100 mm clear spacing. Assumed: rack velocity 0.8 m/s, rack inclined at $75^\circ$, $C_d=0.6$ for the gate orifice.

Step 1: Level of the sill and opening

• Sill level: 1.0 m above the river bed, i.e. RL 3316.00 m, to keep bed load out.
• Opening height $h=1.3$ m, so the top of the opening is at RL 3317.30 m. This is the canal water level and 0.2 m below the weir crest, so the opening stays submerged at low flow.

Step 2: Rack area and width

$$A_{net}=\frac{Q}{V}=\frac{1.45}{0.8}=1.812\ \text{m}^2$$ $$A_{gross}=A_{net}\times\frac{s+b}{b}=1.812\times\frac{110}{100}=1.994\ \text{m}^2$$ $$\text{width }=\frac{A_{gross}}{h}=\frac{1.994}{1.3}=1.53\ \text{m}\ \Rightarrow\ \text{adopt }1.6\ \text{m}$$

Adopted gross opening $1.6\times1.3=2.08$ m²; net area $=2.08\times\frac{100}{110}=1.891$ m²; actual velocity $V=\dfrac{1.45}{1.891}=0.767$ m/s (< 1 m/s, so acceptable).

Step 3: Orifice check at normal pond level

Head difference between the pond (crest RL 3317.50 m) and the canal (RL 3317.30 m) $=0.2$ m.

$$Q_{cap}=C_dA\sqrt{2g\Delta h}=0.6\times2.08\times\sqrt{19.62\times0.2}=2.47\ \text{m}^3/\text{s}>1.45$$

Required head $\Delta h=\dfrac{1}{2g}\left(\dfrac{1.45}{0.6\times2.08}\right)^2=0.069$ m. The gate is throttled to keep the canal level at RL 3317.30 m.

Step 4: Trash rack loss (Kirschmer)

$$h_t=\beta\left(\frac{s}{b}\right)^{4/3}\frac{V^2}{2g}\sin\theta=1.79\,(0.1)^{4/3}\frac{0.767^2}{19.62}\sin75^\circ=0.0024\ \text{m}$$

which is negligible.

Step 5: Flood condition and levels

At the 100-year flood (RL 3319.55 m) the opening is submerged on both sides (canal WL 3317.30 m), so the head across it is $\Delta h=3319.55-3317.30=2.25$ m. Without control the intake would pass $0.6\times2.08\sqrt{2g\times2.25}=8.3$ m³/s, so the gate must be throttled or closed. To pass only 1.45 m³/s, the opening area is $\dfrac{1.45}{0.6\sqrt{2g\times2.25}}=0.36$ m², i.e. a gate opening of about $0.36/1.6=0.23$ m. Top of the intake wall and hoist deck: flood level + 0.6 m freeboard $=$ RL 3320.15 m.

SECTION (levels in m)
 3320.15 ____ deck
 3319.55 ~~~~~ HFL
 3317.50 ~~~~~~~~~~ weir crest (pond level)
          | rack  |gate|
 3317.30  |###\   |    |~~ canal WL 3317.30
          |###     |    |
 3316.00  |_____sill____|   (1.0 m above bed)
 3315.00 /////////////// river bed

Answer: sill RL 3316.00 m; opening 1.6 m wide x 1.3 m high (top RL 3317.30 m); rack velocity 0.767 m/s; rack loss 0.0024 m; gate opening about 0.23 m at the design flood; deck RL 3320.15 m.