# Stormwater harvesting and management/Surface water/Tyrolean weir

A Tyrolean weir is a water inlet structure in which water is abstracted from the main flow through a screen over a gutter. The gutter is usually made of concrete and built into the river bed. 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.
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.

### Suitable 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 weir or intake should be carefully sited.
The weir itself does not clean or purify the water.
| Advantages | Disadvantages |
| - More reliable and cheaper compared to dams and water inlet structures in river embankments - They do not affect water flow to communities downstream | - None known |

### Resilience to changes in the environment
#### Drought effects on cement

Effects of drought: Badly made concrete or cracked linings (e.g. in tanks, dams, waterways, wells, and other structures).
Underlying causes of effects: Less water used for curing; Impure water used for mixing.
To increase resiliency of WASH system: Ensure adequate mixing, ratios, purity of ingredients; Minimize water content in mixture; Ensure adequate curing.
More information on managing drought: Resilient WASH systems in drought-prone areas.

### Construction, operations and maintenance

A Tyrolean weir can either consist of parallel rods or a perforated plate, installed in the flow direction over the width of the stream with a 15-30 degree downward slope. Large stones, branches and large leaves cannot pass between the rods, and are prevented from entering the gutter. Because the rods / plate slopes downward, the material in the stream is pushed downstream, until it drops over the end of the weir.
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 m$^3$ of deposits and allows water to filter for 10 to 30 minutes at very low speed. It is cleaned by washing it out.

#### Maintenance

Several visits per year to the site are necessary for inspection, cleaning and minor repairs. Overall, maintenance is easy to carry out due to low-tech structure and the use of local labour and materials.
Regular inspection and cleaning of the grit / rack and possibly the gutter and sedimentation tank is required during and after storm periods.

### Costs

* Material (excluding the pipe and sedimentation tank): US$ 300 - 600.
* Labour (if site is easily accessible): 30 - 50 man days.

### Manuals, videos and links

* Surface water intake and small dams. Chapter 11. Revised by Nhamo Masanganise.

### Acknowledgements

* CARE Nederland, Desk Study: Resilient WASH systems in drought-prone areas. October 2010.
* Orignal Akvopedia.org article: akvopedia.org/wiki/Tyrolean_weir