CHAPTER 2.The Lake and the Secondary Dam

The Secondary Dam of Sella Zerbino L'Energia Elettrica Fasc. XII Vol.2 1925)

Why the name “Main dam”?
The decision to increase dam height of 13 mt. in respect of primary project was the cause of a big problem: in a point of the perimeter of the new coming lake, particularly where the depression of two ridges produced a saddle (Sella Zerbino), the natural bank would be placed below the max storage capacity level. From here, water could “spill”, flowing into the meander of the Orba torrent, just downstream the main dam.
Therefore, a secondary barrage was built, made up of a wall, 110 mt. overlength and 14 mt. height. This barrage was designed and built very quickly, without any geologic research, since Sella Zerbino had to be ”...solid rock made...”. This second barrage (called Secondary Dam of Sella Zerbino) looked as a large high wall , where the way to the Main Dam ran along.

Cross section secondary dam L'Energia Elettrica Fasc. XII Vol.2 1925)

On December 1 st 1923, at 7.15 a .m., some hundreds kilometres north-east far from Ortiglieto: the Scalve Valley (Alta Bresciana - Orobiche Alps) was hurt by a terrible accident. Owing to a rainstorm, the Gleno dam, on the homonymous river, almost completed, collapsed letting flow into the narrow Alpine creeks more than 6 million water and detritus. The flood swept everything away, till the Iseo Lake and 400 persons about died. The following inquiries of the competent Authorities will ascertain numerous defects both in designing and in constructing.

Designs of the secondary dam (L'Energia Elettrica Fasc. XII Vol.2 1925)

When in 1924, the so named “Commissione di Controllo del Gleno” visited the erection site of Ortiglieto in Molare, some water leakages and losses under the Secondary Dam of Sella Zerbino were put in evidence. Actually, the involved ground showed some high permeability areas, that soon, when the lake was full, were the cause of seeping through. In order to solve this problem, the engineers of O.E.G., urged by the Control Committee, made some concrete injections, but no results were obtained. If O.E.G. had carried out some geologic researches, they could find out that Sella Zerbino was laying on an old fault and made up of easily broken rocks, in some cases even “soapy” type. What could happen once 18 million cubic meters of water filled the reservoir?

Construction of the secondary dam

The Molare plant was put in operation January 1925.

The Ortiglieto lake, generated by the two barrages placed around Bric Zerbino, extended, running unevenly, for 5 km about upwards and had a 400 mt. max width (click here to examine the lake topographical map). Maximum storage capacity (322 mt. max) was 18 million cubic meters. It was necessary to expropriate many cultivated lands and farms. Two affluents of the Orba torrent near the lake, had a share in its provisioning: the Meri Rio on the left side and the Rio delle Brigne on the right, a few hundreds meters far from the Main Dam. The mountains of the Orba Valley have medium height slopes and are nearly covered with woods.

The old postcard shows the lake, the main dam with the house of dam-keeper

As to geological composition, this area of the Ligurian Alps is featured by metamorphic rocks forming the “Gruppo of Voltri”. Actually, these matters are coming from an oceanic crust (and corresponding sedimentary settling) existing between Africa and Europe before the alpine orogenesis. The converging movement of the two Continents (Cretacious-Low Eocene ages) caused sinking down of this crust for some kilometers. The following continental collision (Medium Eocene) had as a consequence that these materials, deep modified by the very high pressures, were pushed up to the surface and then displaced in alpine orogenetic evolving strata (Eocene-Miocene). These rocks have therefore a very complex structure and their deformations are mainly brittle with a very high quantity of different faults, as to importance and positions.

CHAPTER 3. August 13th : The Rainstorm