Watercourse Maintenance: a Look at the Plants and Hydrology of a Case Study on the Tiber River

River Basin Management III 443 Watercourse maintenance: a look at the plants and hydrology of a case study on the Tiber River M. Bellezza1, L. Nasini2, S. Casadei1 & A. Standardi2 1Department of Civil and Environmental Engineering, University of Perugia, Italy 2Department of Agricultural and Environmental Science, University of Perugia, Italy Abstract In order to prevent damage caused by extreme hydrological events in a river basin, it is of the utmost importance that the surrounding territory and watercourses of the river are kept clear and well maintained. Although this is universally recognised as a valid concept which should be put into operation, numerous practical problems arise, mainly linked to areas to be found along many of these rivers, such as river parks and sites of community interest, which are often protected. This situation gives rise to a complex debate, in which river ecology is often in direct contrast with hydrology. For this reason a study was developed in conjunction with the Province of Perugia aimed at taking a detailed, integrated look at both watercourse plants and hydrology simultaneously. The results obtained on the reaches of the Tiber River examined have shown how plant data (the fruit of three years of monitoring existing vegetation) enable us to provide an extremely detailed picture of the state and nature of the riverine vegetation, so that ways to cut and thin it out can be proposed according to the botanical and agronomic knowledge of these species. The hydrological effects of this approach were simulated using HEC-RAS calculation procedures in order to outline the effects in flood conditions. Special attention was paid to the possible phenomenon of the partial obstruction of the stream connected with the bridges and their dimension, and with the amount of vegetation which can be expected to be uprooted by virtue of its state of conservation and its position on the river banks. These results are also useful for identifying an operational protocol, which can be helpful in contracting such maintenance work along rivers with similar characteristics to the one being studied. Keywords: riverine vegetation, plant management, roughness coefficient, flood condition, floating vegetation. WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 444 River Basin Management III 1 Introduction The reach of the Tiber River involved in the experiment (about 1 km) is located in Umbria, near the town of Città di Castello (PG) and is identified as a Site of Community Interest (SCI) as part of the Nature Network 2000 program. The soil in the area is rather plain, and the river follows a straight course with a regular bed and a generally uniform slope, the elevation ranges between 290 and 230 m a.s.l.. The riverbed consists mainly of gravel, cobbles and silt; rocks or significant protrusions are rare. The reach of river examined has a torrent-like flow regime, with a relatively limited hydraulic shape. This characteristic, together with the possibility of trees from the banks falling in the river, has created problems in terms of the obstruction of the hydraulic shape, especially near bridges, fig. 1. On-site investigations showed the riverine vegetation to be considerably degraded, with limited species variability. In particular, shrub species are prevalent at the top of the banks, while tree species are found near the low water level, in direct contact with the water and frequently leaning away from the vertical. This situation denotes a bad plants situation, a possibly consequence of earlier maintenance work that was improperly done, which in any event have serious problems, especially during floods, with a significant amount of floating vegetation being carried away by the flow. Figure 1: Trees fallen into the river and accumulating near bridges. 2 Materials and methods In order to define the possible solutions for this situation, it was decided to compare four types of plant management carried out (in 2004) at four different reaches of river, more or less homogeneous and about 100 meters in length, fig. 2. The reaches, and consequently the types, were divided into: 1) “control” or “reference”, set up according to the specifications adopted by the Province of Perugia; 2) “propositional”, set up on the basis of the indications ensuing from an investigation on a preceding reach of the Tiber River; 3) “experimental”, having come from specific monitoring; 4) “image” or “minimum management”, tending to reduce the impact on the landscape to a minimum, Regni [10]. WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) River Basin Management III 445 An accurate survey chart, consisting of a series of multiple-choice questions, was used to obtain the appropriate reference data. The main parameters examined took into consideration: the vegetational and territorial characteristics, the hydrogeological problems found, the presence of works in the channel, the presence of embankments, the presence of bridges, and civil works in general. Type 4 (Image) Type 3 (Experimental) Type 2 (Propositional) Type 1 (Control) Figure 2: Location of the four intervention types. The chart also took into consideration the characteristics of the ordinary river section and of flood plains. The analysis of the bed of the channel regarding mainly the naturalness of the wetted section, the level of meandering, the composition of the materials present, the structures found in the channel, irregularities in the river bed and, above all, the type of vegetation and the presence of the various species examined, distinguishing among aquatic herbaceous, consistent herbaceous and shrubby plants. As regards the channel banks and the riparian strip, first an analysis was done referring to the stability, erosion and irregularity of the banks, the presence of artificial coverings and the width of the riparian strip. The trees and shrubs present were also identified and quantified, evaluating their state of health and timeliness for thinning (on the basis of which various types of management were proposed). In the “propositional”, reach of management, the riverine vegetation surveyed before the intervention was composed of 23.8% shrubs and 76.2% trees. The total number of plants was 539, of which 317 on the right bank, with a clear prevalence of Salix alba L. (66.8 %); this prevalence was found also on the left bank, where the species Salix alba L. represented 56% of the 222 specimens WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 446 River Basin Management III counted. In the “experimental” reach, 18.5% of the plants contributing to the formation of the riverine vegetation were shrub species, and 81.5% were tree species. Before the intervention, a total of 459 trees were counted, 294 of which on the right bank and 165 on the left, with a prevalence of the species Salix alba L., 62.2% on the right and 68.6% on the left. The experimenting was focused on the “propositional” and “experimental” management types, as the remaining types (“control” and “image”) had management criteria already established by precise specifications, and they would not have allowed enough freedom of action to justify interest in experimentation. Both the “control” and the “image” types were in fact used for comparison with the other two management types for the analysis results. In the flood area, the presence of seasonal herbaceous crops, orchards, footpaths, bicycle paths, recreation and sports areas and developed areas. The problems in the interaction between riverine vegetation and hydraulic aspects are essentially connected with the concept of hydraulic roughness and the behavior of floating vegetation carried by floods. The problem of evaluating the effect of vegetation on the hydraulic roughness value is rather controversial, with examples of both theoretic, Armanini and Righetti [2], and experimental laboratory, Ferro and Giordano [5], approaches along with some cases of subsequent verification in the field, Arcement and Schneider [1]. Reading the results does not always lead to conclusions entirely in agreement, due to the considerable influence that the characteristics of the shrub and tree species have when examined on different occasions. Indeed, vegetation is the element that has the sharpest space-time variability, and it is also an element that can be modified by specific hydraulic maintenance operations, such as in the case of the reach of the Tiber River being tested. As concerns the resistance caused by vegetation, unlike that from the material found in the channel, what happens is that it changes over time depending on the vegetative period and the flow level. Due to its flexible nature, vegetation tends to bend spontaneously under the force of the flow’s drag. The degree of bending depends on the resistance of the stalk or trunk and on the hydrodynamic thrust exerted by the flow. In general, the degree of resistance to the flow depends therefore on the plant’s morphology, on the density of the growth and on the level of submersion. In attempting to summarize the problem of the resistance of vegetation to the flow, referring to this study, it could be hypothesized that grasses and shrubs are of limited importance, and their flexible behaviour could in fact make them uninfluential in terms roughness, with the increasing of the water level, Prezedwojski et al [9]. When

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