Overland flow control via surface management techniques over the last century in the tuscan-emilian apennine range: the Rio Maggiore case study

The current study introduces a representative case study of a mountain tributary of the Reno river, where, in 30 years from the beginning of the 1900, has been led an intense erosion and flood control program concerning streams and hillsides. Although its limited extent, the study area (1.65 km2) is a perfect example of the history of the Apennine landscape: two conflicting and simultaneous phenomena occurred in the Apennines from the 50s until nowadays: population moving both to cities and valley bottoms and agricultural mechanization. Their consequences have been evident on land use: abandonment of unproductive fields and enlargement of the remaining plots. Until the fifties, agricultural practices, dealing mainly with soil surface and surface runoff control systems, were habitually maintained. However, the apparent sustainability of soil use seems to be incoherent with respect to historical documents, witnessing heavy hydrogeological instability in naturally fragile areas. This critical situation is confirmed also by the realization, from the beginning of the 1900, of important and spread hydraulic works in torrential streams and retaining works on hillsides, besides specific political initiatives. Current aerial photographs show how vegetation cover is mainly composed by meadows, shrubs, woods and crops, although some hillsides still suffer of mass movements, mainly due to the natural fragility of the land rather than to insufficient control works. Soil erosion is now limited to hillside foots in the lower Apennines, where agriculture is still economically valuable. Torrential streams are approximately at equilibrium, indicating the success of the works done, even if many of them need now urgent and wide maintenance. The populated areas nearby Porretta Terme (Provincia di Bologna) and its mountain basin used to experience a severe hydro-geological disruption status, whose evidences can be found since the beginning of the 20th Century: extremely frequent and flashy flood events together with high sediment yield, hill slopes disruption, high soil erosion rates even occurring in woods, which used to be deforested because of energetic or livestock requirements. Such a critical situation, which was quite common over the whole Tuscan-Emilian Apennine range, led to embark on a massive and widespread commitment in building runoff control works both on hillslopes and streams since the early 1900s. On one hand, about 162 weirs were built in the Rio Maggiore and its tributaries’ riverbeds, using both stone-mortar works or plane stone-works. On the other hand, relevant extensive works, mainly forestal-hydraulic arrangements and even some early bioengineering techniques, were applied on hillslopes (e.g. terraces, which allowed for reafforestation of steep and stony slopes, live fascines). Those control works were highly needed, as soil used to be extremely impoverished, especially on those slopes where pastures and cutting of trees had been practiced for long time spans. The terraces are easily recognisable even nowadays, although they are now well covered by fully-grown woods. Together with reafforestation, some historical “forestal roads” were built as, serving plants or nurseries, they were thought to be fundamental in order to achieve good management practices of woodlands. Thus, the mountain landscape of the Apennines (from 900 to 1600m a.m.s.l) changed in a time span of some decades, mainly because of the widespread use of conifers: black pine (Pinus nigra), spruce (Picea abies), silver fir (Abies alba), douglas fir (Pseudotsuga menziesii). Subsequently, more control works were built and extensive maintenance of the existing ones was carried out, concerning both hydraulic works and reafforestations. A survey of in-stream works, run in the mid 90s by the Reno Basin Authority and the Italian Forest Rangers Department, reported 223 weirs, 2/3 of them being probably built in the first half of the 20th Century. As a tangible con...