The engineered alluvial plain, called the “colmata”
Until the end of the 18th century the marshlands were drained hydraulically using an engineered alluvial plain or “colmata”. An alluvial plain is a relatively flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which the alluvial forms.The hydraulic construction uses earthen embankments to fence in an area destined to collect sediment and floodwater channeled in by an alluvial canal. The alluvial canal draws water from the web of existing canals and highlands, intentionally flooding the “colmata” by way of a fillway. The fillway controls the rate at which water flows into the earthen embankments from the alluvial canal. It is built at an elevation lower than the perimeter embankment summit. When this happens, water that has entered the fenced in earthen embankments loses nearly all momentum and the sediment carried in by the flow settles. The spillway, located near the summit of the embankments, releases filtered water into the emitting canal. The emitting canal flows filtered water towards a major waterway, where it is distributed.
When the accumulated sediment reaches the desired elevation, the “colmata” is decommissioned and the alluvial canal feeding the “colmata” is deviated to move the water flow to another area. The “colmata” can be formed from one or more embanked areas.The Brolio Colmata is the only “colmata” still functioning in the Valdichiana. It covers about 30 hectares of surface area and was built in its current state in the early 20th century. This coincided with the construction of the Allacciante dei Rii Cortonesi Canal and the Esse and Mucchia streams, all of which flow into the turbid waters of the “colmata”.