Water Portal / Rainwater Harvesting / Surface water / Tyrolean weir

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Dams and water inlet structures in embankments are vulnerable and expensive elements in river-fed water systems. They are easily damaged by floods, underflow, seepage and suffer from build-up of sediment or rubbish in the water. The Tyrolean weir forms a more reliable and cheaper alternative. Water is abstracted through a screen (or inlet) over a gutter, usually made of concrete and built into the riverbed. The screen on the crest should slope downstream (15-30 degrees), to increase flow velocities and prevent sediment carried by the stream from blocking it. From the gutter, water enters a pipeline, which drains into a sedimentation tank and then flows by gravity into the rest of the system.

Tyrolean weir

Advantages	Disadvantages/limitations
- More reliable and cheaper compared to dams and water inlet structures in river embankments

Costs

• Material (excluding the pipe and sedimentation tank): US$ 300 - 600.
• Labour (if site is easily accessible): 30 - 50 man days.
• Operation and maintenance: several visits per year for inspection, cleaning and minor repairs. Overall easy to carry out due to low-tech structure and the use of local labour and materials.

Applying conditions

• Tyrolean intakes are used in small permanent rivers and streams where the sediment content and bed load transport are low, or on the crest of a dam spill.
• The threshold can be a concrete elevation above the rocky bed of a mountain stream, or a vertical low weir structure, anchored in the embankment.
• The capacity of the inlet pipe / drain (diameter and gradient) should be 30% more than the design flow and have an uniform gradient to prevent accumulation of sand.
• The sedimentation tank can accumulate 1.5-2 m3 of deposits and allows water to filter for 10 to 30 minutes at very low speed. It is cleaned by washing it out.

Maintenaince

• Regular inspection and cleaning of the grit / rack and possibly the gutter and sedimentation tank is required during and after storm periods.

Considerations

• The weir or intake should be carefully sited.
• The weir itself does not clean or purify the water.

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External links

• General Background www.aquaforall.nl