Dissesto idrogeologico, milioni di italiani esposti a frane ed alluvioni

di Michele Costanzo 57

dissesto idrogeologico ISPRA 2015
La scorsa settimana l’Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA) ha presentato un nuovo rapporto sul dissesto idrogeologico in Italia con dati aggiornati al 2015. Questo documento costituisce un vero e proprio punto di riferimento per la valutazione del rischio idrogeologico e traccia un quadro abbastanza allarmante sulle condizioni del nostro territorio.

A rischio frane il 19,3% del territorio

La complessa storia geologica del territorio italiano ne fa uno dei paesi ha più elevato rischio frane in Europa. Il rapporto ISPRA spiega come in Italia siano state censite 528.903 frane che interessano complessivamente oltre 22 mila chilometri quadrati di superficie pari al 7,3% del territorio nazionale. La distribuzione dei fenomeni franosi attraversa tutto il Paese con le sole significative eccezioni nella grandi zone pianeggiati in Pianura Padana ed in Puglia. Viceversa la maggiore concentrazione di fenomeni franosi riguarda le aree alpine e la catena dell’Appennino. Circa un terzo delle frane censite è di tipo ‘a cinematismo rapido’ caratterizzate cioè da una elevata velocità di spostamento che può raggiungere anche i diversi metri al secondo. Rientrano in questa classificazione crolli e colate di fango e detriti, fenomeni che proprio per la rapidità con cui si manifestano sono anche tra i più distruttivi.

Per il costante monitoraggio dei fenomeni franosi ISPRA, regioni e provincie autonome hanno realizzato l’Inventario dei Fenomeni Franosi in Italia (IFFI), un documento importante per la corretta valutazione del dissesto idrogeologico. Proprio sulla base di questi dati avviene la valutazione della pericolosità da frana dei Piani di Assetto Idrogeologico (PAI). Da queste valutazioni risulta che il 19,3% del territorio italiano, pari a oltre 58 mila chilometri quadrati, è esposto al rischio frane. Al 2,9% del territorio in particolare è attribuita una pericolosità molto elevata (categoria P4) ed al 5% una pericolosità elevata (P3).

Considerando proprio le aree a maggiore rischio (P3+P4), il rapporto ISPRA stima in oltre 1,2 milioni gli italiani esposti a rischio frana un valore pari al 2,1% della popolazione. Considerando invece tutte le aree a rischio inserite nei PAI si arriva oltre i 5,6 milioni di persone. Le imprese esposte a rischio frana elevato o molto elevato sono calcolate in 79.530 pari all’1,7% del totale. Preoccupante è anche la situazione del bei culturali; si stima infatti che siano ben 34.651, pari al 18,1% del totale, i siti esposti ad un rischio elevato o molto elevato (P3+P4).

Dissesto idrogeologico, la pericolosità idraulica

Il secondo grande rischio che da sempre caratterizza il dissesto idrogeologico del territorio italiano sono le alluvioni. Anche in questo contesto il rapporto ISPRA per il 2015 mostra dati preoccupanti. In questo caso sono soprattutto le aree fluviali ad essere esposte a rischio con una particolare criticità per le aree attraversate dal fiume Po. Il modello di analisi della pericolosità idraulica individua tre diversi gradi:

  • P3, pericolosità elevata con tempo di ritorno fra 20 e 50 anni (alluvioni frequenti): riguarda oltre 12 mila chilometri quadrati pari al 4% del territorio.
  • P2, pericolosità media con tempo di ritorno fra 100 e 200 anni (alluvioni poco frequenti): riguarda oltre 24 mila chilometri quadrati pari all’8,1% del territorio.
  • P1, pericolosità bassa (scarsa probabilità di alluvioni o scenari di eventi estremi): riguarda oltre 32 mila chilometri quadrati pari al 10,6% del territorio.

Considerando lo scenario di pericolosità media (P2) è particolarmente delicata la situazione dell’Emilia Romagna dove il 45,7% del territorio regionale è considerato a rischio. Tra le regioni maggiormente esposte alle alluvioni troviamo quindi Toscana (11,1% del territorio), Veneto (9,6) Lombardia (8,5%), Piemonte (7,8%), Friuli Venezia Giulia (7,5%) e Valle d’Aosta (7,1%).

Tenendo conto di questa distribuzione geografica del rischio alluvioni, il rapporto stima in 1,9 milioni gli abitanti esposti ad un grado di pericolosità elevata (P3), in 5,9 milioni gli abitanti esposti ad un grado di pericolosità media (P2) ed in 9 milioni gli abitanti esposti ad un grado di pericolosità basso (P1). Quadro preoccupante anche per quanto riguarda l’esposizione alle alluvioni delle attività produttive: nello scenario P2 sono 576.535 le imprese a rischio pari al 12% del totale. Il rischio alluvioni si dimostra un grave pericolo anche per il patrimonio culturale italiano con 29.005 beni culturali esposti nello scenario di pericolosità idraulica media P2 (15,2% del totale) ed addirittura 40.454 (21,2%) nello scenario di pericolosità idraulica bassa P1.

Quasi 900 km di coste a rischio erosione

Se frane ed alluvioni costituiscono i due aspetti forse più visibili del dissesto idrogeologico, un capitolo a parte il rapporto ISPRA dedica anche all’erosione delle coste. Rispetto ad uno sviluppo totale di circa 8300 km, circa 7500 km di coste sono ancora naturali, vale a dire prive di strutture marittime o di strutture di protezione. Cira 4800 sono in particolare i chilometri di costa bassa e quindi maggiormente esposta al rischio di erosione.

Secondo dati raccolti tra il 2000 ed il 2007 il 37% di tutte le coste basse ha registrato variazioni superiori a 5 metri. In questo scenario i tratti di costa in avanzamento (849 km pari al 18,0%) sono state inferiori per estensione alle coste in erosione (895 km pari al 19,0%). Dati che ne complesso mostrano una vulnerabilità del territorio costiero e la necessità di interventi mirati.

Photo credits | Rapporto ISPRA 2015

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