Technical Characteristics

Location in the estuary

The mill is located in the Bay of Biscay, where the high water of the most spring tides raises until about 4.75 meters.
The location of the mill is in a medium zone, approximately to five (5) kilometers of the coast line, place where it obtains to a very high advantage due to the great duration of the low tides and the express that the tide raises. Approximately it raises in three hours and the ebb tide in nine hours, which allows two daily cycles of seven useful hours.

Due to the variation of the cycle of the tides the water contribution of the Koba spring can be a negligible quantity, in the spring tides, or allow a minimum cycle of operation in the neap tides. The average water flow of Koba spring is 16.5 liters per second. When rains, very usual in the Basque Country, the contribution of the water is greater of the one than it is freed by the floodgates of the mill having to be evacuated by the overflow of the pond.

Machinery of the mill

Portu Errota is an horizontal water wheel mill. The transmission between the turn of the turbine and the wheel of the milling is direct.

The following graph has been extracted of the treaty of D. Pedro Bernardo Villarreal de Berriz " Hydraulic machines of mills and blacksmith shops and government of the trees and mounts of Biscay " written in 1736. A great similarity between this drawing exists and the distribution of the machinery of the mill.


The turbine of the mill is constructed in an only stone piece. The diameter is of one hundred forty centimeters (140 cm.). The thickness is of twenty centimeters (20 cm), and has thirteen (13) blades carved in the stone.

The turbine has an adhered an iron rim.

Channel of the the mill pond

The mill pond is an authentic jewel of hydraulic engineering. It was artificially constructed changing the natural channel of the spring and directing waters to the mill. Several sections of the pond, some superior ones to one hundred meters in length exist, in which the wall directly separates the channel of the pond to the arms of the estuary.

The channel, or aqueduct, runs of winding form from the Koba spring to the building of the mill. The resultant as far as the direction talks about, between spring and mill, is southwestern and the slant range is of about six hundred forty meters (640 m.s).

The total length of the channel is about thousand two hundred meters (1,200 m.s). Additionally it has two arms of approximately one hundred meters each one, that gives a total length of thousand four hundred meters (1,400 m.s).

The total surface of the channel of the pond is eighteen thousand square meters (18,000 m2).

The difference of the level of the water in the pond between the high water and the low tide is of two meters and a half (2.5 ms.) to the line of the building of the mill, and in about eighty centimeters (80 cm.) on the level of the Koba spring.

The water contribution considered of the Koba spring to the pond of the mill is of sixteen and average liters per second (16.5 m/s).

The system of fluvial water evacuation of adjacent lands to the pond is resolute by of hydraulic constructions with very singular characteristics. It is possible to emphasize the case of lands of the Argatxa that because it cannot drains to the pond, because the level of the water of the pond is superior to the level of the own land, has a floodgate that connected with wood pipes puts below the bottom of the channel to empty directly to the arm of the estuary.
The following one is a plane on scale of the pond of the mill.

Closing wall of the mill pond

The foundations of the mill serve as closing wall of the millpond, with a length of twenty meters (20 m.s). This wall extends in its right side in a length of fifty meters and a half (50.5 m.s), with which the total length is of seventy meters and a half (70.5 m.s).

It was constructed of stone, having a width of three meters (3 m.s).

At the end of the pond closing exists an overflow of a width of a meter and twenty centimeters. (120 cm.).

The altitude difference in low tide between the bottoms of the pond and estuary is more or less one meter.

The fall of the water to the turbine is of about eighty (80 cm.)