International workshop on Flash flood and Debris flow risk management in Mediterranean areasSalerno, 26th January 2012

Predicting the river morphology after River Restoration. The methodology VALURI

Nardini, A.(1)and Pavan, S.(2)

(1) CIRF - Centro Italiano per la Riqualificazione Fluviale, Technical Secretariat, Mestre, IT - e-mail: a.nardini@cirf.org(2) CIRF - Centro Italiano per la Riqualificazione Fluviale, Technical Secretariat, Mestre, IT - e-mail: s.pavan@cirf.org

super-lacualeCHIESE

sub-lacuale

OGLIOPO

VALURI is an innovative methodology developed ad hoc for the prediction of the morphologic and geometric configuration of a river, consequent to a substantial change in the defence works and in the exploitation system setting typically introduced by a large scale River Restoration project.

Case study. The methodology VALURI was developed and applied to the whole stretch of Chiese River downstream of Lake Idro, until the confluence with river Oglio (about 70 km). Most of the river runs in a semirural area, touching several small towns and rural settlements. Almost its entire course is highly artificialized with several big size weirs and semi-continuous longitudinal defences, and big, sometimes multiple, levees.

River history. It considers the likely causes of morpho-logical modifications: big floods, longitudinal defences and levees, and the likelihood of formative discharge, together with some state variables as the number of bars and islands

Considered Alternatives:ALT_0: the ‘quasi business-as-usual’ alternative, which implies high Operation, Maintenance and Replacement costs for keeping the current defence and exploitation works system.ALT_SdF: this represents the solution proposed by AdBPo, which basically espouses the criterion of putting the river corridor in safe conditions with respect to the 200 years recurrence time flood.ALT_Base: this is a first step of restoration which implements the criterion of eliminating as much as possible concrete works, while keeping the impact on the anthropogenic system as low as possible.

River interpretative theory: (1) Original situation: meandering stretch of length L0 with low slope S0; (2) right after rectification: thestretch is shorter, LC ≪ L0, and with higher slope SC ≫ S0; (3) new equilibrium, assuming that upstream control variables Q and QS did

state by comparison of aerial photographs, analysis of differential transport capacity of stretches and field observations; c) identify fixed points as weirs, rock formations, rigid longitudinal defence, for each alternative; d) infer, from the interpretative theory and the response of equilibrium assessment, how will the river stretch respond to alternatives, in qualitative terms; e) speculate, as a first trial, which will be the corresponding new equilibrium bankfull geometry, by applying mechanistic-engineering reasoning and qualitative relationships; f) for fully alluvial stretches, modify the guessed geometry until, by trial and error, the corresponding bankfull flow QB acceptably equals the efficient flow QE and check, with empirical relationships, whether the identified features are consistent with empirical evidence; g) for the whole river corridor, possibly iterate on the assumed geometry of all stretches until topographic and system consistency is found.

(progressively disappearing), width and length of stretch (narrowing and shortening) and river bottom elevation (incising)

The morphologic prediction in practice. For every geomorphologically homogeneous reach: a) By means of a deep historical analysis the river history is traced and subsequently an interpretative theory is developed; b) assess current equilibrium

not change: it has incised upstream, when down-stream a weir stays in place (built and re-built several times), in order to reduce the excess of transport capacity until a slope similar to the original is reached

References: (i) CIRF, 2006. La riqualificazione fluviale in Italia. Linee guida, strumenti ed esperienze per gestire i corsi d'acqua e il territorio. A. Nardini, G. Sansoni (curatori) e coll., Mazzanti editore-VE (www.cirf.org) . (ii) Nardini A. and Pavan S., 2012b. River restoration: not only for the sake of nature, but also for saving money while addressing flood risk. A decision making framework applied to the Chiese River (Po basin-Italy), Journal of Flood Risk Management, 5 (2012), 11-133. Blackwell Publishing (UK). (iii) Nardini A. and Pavan S., 2012a. What river morphology after restoration? The methodology VALURI. Journal of River Basin Management. Taylor & Francis (UK). Vol.10, n.1, pp.29-47