(a) (i) Design a trapezoidal concrete lined channel to carry a discharge of 300 m³/s at a slope of 1 in 4000. The side slope of…

(a)(i) Design of the concrete-lined trapezoidal channel

Given Q = 300 m³/s, S = 1/4000, n = 0.0125, side slope 1.5 : 1 (z = 1.5) and B/D = 5, so B = 5D.

Area A = BD + zD² = 5D² + 1.5D² = 6.5D². Wetted perimeter P = B + 2D√(1 + z²) = 5D + 2D√3.25 = 8.606D. Hydraulic radius R = A/P = 0.755D.

Manning: Q = (1/n) A R^(2/3) S^(1/2) = (1/0.0125) × 6.5D² × (0.755D)^(2/3) × (1/4000)^(1/2) = 6.82 D^(8/3).

So D^(8/3) = 300/6.82 = 44.0 and D = 4.13 m, B = 5D = 20.67 m.

Check: A = 111.0 m², V = 300/111.0 = 2.70 m/s, which is acceptable for a concrete lining. Top width = 20.67 + 3 × 4.13 = 33.06 m, hydraulic depth = 111.0/33.06 = 3.36 m, and Froude number = 2.70/√(9.81 × 3.36) = 0.47, so the flow is subcritical.

Adopt a freeboard of 0.75 m, so the lined depth is 4.13 + 0.75 = 4.88 m, with the top width at the top of the lining = 20.67 + 3 × 4.88 = 35.3 m.

(a)(ii) Spillways and energy dissipation

Types of spillway: the ogee (overflow) spillway, whose shaped crest follows the underside of a free nappe and is used on concrete dams; the chute (trough) spillway, an open inclined channel along an abutment; the side-channel spillway, where the flow passes over a crest and turns along a channel, suited to narrow valleys; the shaft (morning-glory) spillway, a funnel inlet with a vertical shaft and tunnel; the siphon spillway, for automatic regulation of small heads; and the labyrinth or stepped spillways, which give a longer crest or a rougher chute. Gated and ungated forms, and a separate emergency or fuse-plug spillway, are also used.

Energy dissipation: a hydraulic jump stilling basin with chute blocks, baffle blocks and an end sill (USBR types I to IV, chosen by Froude number); a roller or slotted bucket; a ski-jump (trajectory) bucket that throws the jet into a plunge pool; impact baffles for small discharges; and stepped chutes, with riprap and apron protection downstream.

(b) Air pollutants: sources and health effects

Lead: from leaded petrol, battery and smelter works, paints and pipes. It damages the nervous system, reduces the IQ of children and causes anaemia, kidney damage and hypertension.

Cadmium: from nickel–cadmium batteries, electroplating, smelting, pigments and cigarette smoke. It causes kidney tubular damage, bone disease (itai-itai), emphysema and lung cancer.

Nickel: from stainless steel and alloy works, electroplating, oil and coal burning and tobacco smoke. It causes dermatitis, asthma, bronchitis and cancer of the nasal passages and lung.

Mercury: from coal burning, chlor-alkali plants, gold extraction, smelters and incinerators. The vapour and methylmercury damage the nervous system (Minamata disease), kidney and the developing foetus.

Carbon monoxide: from incomplete combustion in vehicles, domestic fuel burning and industry, and from tobacco smoke. It binds haemoglobin as carboxyhaemoglobin, cutting oxygen delivery, and causes headache, dizziness, heart strain, unconsciousness and death at high levels.

(c) Sanitary landfill

(i) Factors in evaluating landfill sites: the capacity and life of the site for the expected waste volume; its distance from the waste sources and the quality of access roads; the geology and soil, with low permeability clay and a deep water table; the distance from surface water, wells and aquifers, and avoidance of flood plains; the topography and drainage; availability of cover soil; the climate and rainfall; the distance from housing, airports and protected areas; land use, cost, public acceptance and closure and post-closure care.

(ii) Leachate is formed when rain, the moisture in waste and any groundwater percolate through the waste and dissolve its soluble organic and inorganic matter. It has high BOD, COD, ammonia, chloride and metals, and can contaminate groundwater if it is not collected.

Gas is formed in stages. First, aerobic decomposition uses the trapped air: organic matter + O₂ → CO₂ + H₂O + heat. In the anaerobic phases, hydrolysis breaks down polymers: (C₆H₁₀O₅)ₙ + nH₂O → nC₆H₁₂O₆. Acid formation follows: C₆H₁₂O₆ + 2H₂O → 2CH₃COOH + 2CO₂ + 4H₂ (and volatile acids, which lower pH and dissolve metals into leachate). Methane formation then gives CH₃COOH → CH₄ + CO₂ and CO₂ + 4H₂ → CH₄ + 2H₂O. Mature landfill gas is about 50–60% CH₄ and 40–50% CO₂, with traces of H₂S and NH₃. Uncontrolled it migrates, causing odour, fire and explosion risks, so it is vented or collected.