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.

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].

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 instead the vegetation is tall and rigid, typically that of trees, the effect on the height of the water, on the flow velocity and on the boundary roughness coefficient depends on the density and distribution of the plants. The practical application of the concepts outlined previously leads to various types of formulae. Among the simplest of these, one may mention that of Cowan [4], proposed both for the channel and for the flood plains (with differentiated coefficients), in which the final Manning’s coefficient is a function

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) River Basin Management III 447 of the sum of partial values, different in relation to various factors, corrected with a multiplicative coefficient for meandering channels. One relation to which a physical meaning could be attributed was furnished by Petryks and Bosmajian [8] for Strickler’s roughness coefficient in flood plains, which can also be extended to riverine vegetation. The estimate regards an equivalent roughness value (ks-veg) due to partially submerged rigid vegetation.

k so k −vegs = 1 A 1+ v 2 Rk 3 g2 so h

A pj = CA Rv aa yx where: kso= Strickler’s roughness coefficient; Rh= hydraulic radius; Av= vegetation density, which can be expressed by the following relation: in 2 which, CR= resistance coefficient, 1.0÷1.5; Apj= frontal area of vegetation [m ]; ax e ay= distance between plants in the flow direction and in the orthogonal direction [m]. The second extremely important aspect, correlated to the state of the vegetation on the river’s banks, is that regarding the transporting by floating of plants that have fallen into the channel and are carried away by the flow. The phenomenon may result from forms of erosion of the banks, which undermine the stability of plants otherwise in good condition, or from the falling of individual trees in poor condition or that are diseased or senescent. Nonetheless, in both situations the phenomenon is generally set off by events typical of flood conditions. The rapid increase in the flow brings about velocity values and distributions such as to favor and induce the subsequent transporting of vegetation, sometime having trunks and foliage of considerable size. This material then tends to lodge in areas of the channel that are already considered critical, such as particularly narrow sections or those near structures such as little overflow dam and bridges, even at a distance of many kilometers. In the latter case in particular, and also where there are piers set in the river, the vegetation finds favorable conditions for getting caught and jammed. The possible forming of actual barriers that occupy a large part of the surface may lead to various consequences. From a hydraulic perspective, this results in a possible rising of the water surface upstream from the obstruction, which may cause an increase in flood risk critical state conditions. In extreme cases, the accumulation of detritus, consisting mainly of vegetation, could cause the flow to be diverted abruptly toward the opposite bank, setting off concentrated erosion phenomena. Another hydraulic aspect that must be taken into consideration regards the possible sudden giving way of the barrier from the dynamic thrust exerted by the flood overflow. This situation could trigger an even greater flow peak and, at the same time, the impact of a flood wavefront made up of a large

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 448 River Basin Management III amount of detritus. Lastly, the dynamic thrust exerted by the flow on a bridge structure, amplified by the partial or total obstruction of the section due to detritus and/or trees carried by the same flow, could also cause serious damage to the bridge structure, Ballio et al [3]. Consequently it seems realistic to state that the presence of fallen plants in the channel must be avoided, also because this situation makes necessary urgent operations by specialized removal squads or, in the case of the obstruction of bridge spans, by the Fire Department, with a considerably negative impact from an overall (and particularly an economic) perspective. Some studies, conducted in laboratories or in the field, have attempted to delve further into the behaviour of tree detritus in open channel flow, Ginanni et al [7]; however, the laboratory experiments mostly refer to specific situations in the presence of certain works, whereas field experiments, though approximate, point out the extreme variability of the situations, also as concerns the characteristics of the river in terms of width, slope and flow velocity, Francia [6].

3 Results

The “control” or “reference” type management (Type 1) provides for: 1) the thinning out of trees on the internal slope, particular as concerns Robinia pseudoacacia L. and the plants in precarious health conditions or about to fall, located near the water surface in low flow conditions; 2) the identification and subsequent cutting of trees that show signs of dangerousness for public and private safety; 3) the thinning out of infesting shrubs and trees, including thorn bushes and trees with trunks up to 20 cm in diameter. After the “control” management, the density of the vegetation on the banks was 5.2 plants/100m² of ground area. In this type of management it has already been observed that the excessive thickness of vegetation found, along with the presence of large trees last standing has caused rotting in the trunks and the subsequent falling of the trees into the channel. The “propositional” management (Type 2) originated from a specific monitoring already carried out in a neighboring reach of river, and in particular it provided for: 1) the elimination of branches and suckers of plants that had a trunk leaning away from the vertical; 2) the elimination of senescent and large- sized plants, more subject to rotting, uprooting and leaning; 3) the elimination of trunks and branches excessively close to each other; 4) the elimination of exceedingly “infesting” shrub species. Following the thinning out, the density on the right bank was measured at 3.9 plants/100m² of bank area, and on the left bank 4.1 plants/100 m², fig. 3. In the “propositional” management priority was given to cutting large sized plants, avoiding having trunks excessively close to each other. The “experimental” management (Type 3) originated from a specific monitoring carried out according to a specially prepared chart. This monitoring showed the presence of invasive vegetation, with the recurring presence of medium and large trees that had fallen or were leaning sharply toward the river. In addition, many specimens were rotting at the foot, and others showed no signs

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) River Basin Management III 449 of vitality. According to this conclusion, although the density of trees measured previous to the intervention on the right bank was greater than that on the left bank, after the intervention this parameter was nearly the same, with 3.9 plants/100 m² on the left bank, compared to a density of 4.1 plants/100 m² on the left bank, fig. 4. It can be observed that during both the summer period as well as the autumn/winter period following the intervention, there was an “ideal natural maintaining” of the vegetation density established, with the exception of the rapid growth of suckers from the stumps of the Salix spp., and Robinia pseudoacacia L. tree species that had been cut down.

Prunus spp. Populus nigra L. var. Italica Plants remaining Morus alba L. Total plants before cutting Juglans regia L. Cornus sanguinea L. Fraxinus ornus L. Sambucus nigra L. Acer spp. Robinia pseudoacacia L. Populus nigra L. Populus alba L. Salix alba L. Alnus glutinosa L.

Cornus sanguinea L. Fraxinus ornus L.

Sambucus nigra L.

Robinia pseudoacacia L.

Populus nigra L.

Populus alba L.

Salix alba L. Alnus glutinosa L.

0 10203040506070 Plants (%)

Figure 3: Percent incidence of tree species in the reach of left bank (bottom) and right bank (top) involved in intervention type 2 – propositional.

The “image” management (Type 4) called exclusively for the cutting of plants obviously in poor condition, due either to age or unhealthiness. With this intervention the vegetational and hydraulic characteristics of the channels and banks were left practically unaltered, so as to have, over time, a reach that could be used as a parameter for comparison, in the hope of obtaining further indications on the optimal time frequency of the interventions. In this reach, after the cutting operations described above, the density of the vegetation on both banks remained virtually unchanged: 4.9 trees on 100 m² of bank area. During the summer considerable development of shrub growth was seen both on the bank, with shrubs of the Robinia Pseudoacacia L. and Salix spp. species, and near the river channel (Salix spp. shrubs), as well as at the top of the bank (Rubus fruticosus L. and Robinia pseudoacacia L. shrubs), Regni [10]. The hydraulic aspects connected with the types of managements on the vegetation were investigated more in depth using the HEC-RAS calculation

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 450 River Basin Management III procedures, which simulate the hydraulic behavior of a channel with certain topographic and roughness characteristics in one-dimensional flow conditions. The choice of this simulation procedure is entirely congruent with that done by the Tiber River Basin Authority as part of the Hydrogeological Conformation Plan (PAI), particularly in that part of the plan which regards hydraulic risk in the main hydrographic network, which also includes the reach of the Tiber River examined in this study. For this reason, the same hydraulic sections used in the PAI were used in this study, keeping the calculation parameters unchanged with the sole exception of hydraulic roughness induced by riverine vegetation and the outlining of the bridges, in which it was attempted to introduce the effect of possible obstructions at the bridge spans caused by the accumulation of material transported by the flow.

Fraxinus ornus L. Juglans regia L. Plants remaining Total plants before cutting Populus nigra L. var. Italica Sambucus nigra L.

Robinia pseudoacacia L.

Populus nigra L. Populus alba L. Salix alba L. Alnus glutinosa L.

Ulmus carpinifolia L. Populus nigra L. var. Italica Cornus sanguinea L. Sambucus nigra L. Acer spp. Robinia pseudoacacia L. Populus nigra L. Populus alba L. Salix alba L. Alnus glutinosa L. 0 10203040506070 Plants (%) Figure 4: Percent incidence of tree species in the reach of left bank (bottom) and right bank (top) involved in intervention type 3 – experimental.

On the basis of these hypotheses and the preceding considerations, various scenarios were simulated and were then summarized in the two situations considered to be most significant: a) comparison between the situation preceding the intervention, in which a maximum roughness induced by vegetation on the bank was assumed (n=0.200), and the situation after the maintenance operations, with a lower roughness value (n=0.050); b) possible partial obstruction of the Pistrino-Selci bridge from material transported by the flow. The results obtained do not show any considerable variations in the flood levels, with a return time period TR=50-100-200-500 years in simulation a). Indeed, the greatest effects are observed in the sections further downstream, with maximum level excursions 9 cm (Tr=50) to 26 cm (TR=500). In simulation b)

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) River Basin Management III 451 various situations were hypothesized: partial lateral obstruction; central obstruction, in the presence of piers; partial lateral and central obstruction; previous cases with different levels reached by the material as a function of the TR. In short, this second series of simulations pointed out the most important effects, with increases in the level ranging from a minimum of 13 cm to a maximum of 98 cm, showing particular sensitivity to the positioning of the hypothetical obstruction in relation to the main flow .

4 Discussion and conclusions

Approximately one year after the intervention, all reaches of the river subject to the experiment showed high spontaneous regrowth of suckers of willow, locust and poplar, fig. 5. The attentive periodic controlling of the vegetation balance that is established following the various interventions placed in comparison also assumes a preventive purpose, aimed at the safeguarding of the areas bordering the reach of river involved and at the proposing of effective indications for the management of riverbank areas. In order to rationalize the monitoring of the riverine vegetation and of its evolution in the reach of the Tiber River involved in the experiment, a post-management survey chart was prepared. This makes it possible to follow the evolution of the trees remaining after cutting and regards plants having a trunk diameter greater or equal to 10 cm. In particular, the following are defined: position on the bank, diameter growth of the trunk, the health of the plant, the inclination of the trunk, the number of fallen trees. A chart was also prepared for analyzing the spontaneously regrowth of shrubs.

Figure 5: Example of the situation after the intervention, in May (L) and in August (R).

At the end of the growing season following the interventions, the above chart enabled us to collect the earliest information on the most representative species of trees and shrubs. The “propositional” and “experimental” managements the cutting of larger plants as priority and that of younger plants as selective,

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 452 River Basin Management III avoiding also the presence of excessively close trunks and branches and trying to obtain, in both cases, a suitable vegetation density (3.9–4.1 plants/100 m2). Compared to the “control” management (5.2 plants/100 m2), these managements showed a lower incidence of plants in risk conditions resulting from the effects of winter atmospheric agents, whereas in reference to shrubby vegetation, there was spontaneous regrowth of suckers on the willow, locust, elder and poplar trees that had been cut, noting in particular the presence of brambles, which were particularly invasive. After the intervention, Salix alba L. (63.4% before and 47.7% after), followed by Populus spp. (15% and 15.1%), were the most numerous species. This species are hydric and typical of river and riverine environments, in addition, they are able to withstand prolonged periods of submersion. The poplar and the willow are not only the most common but also the largest species (the average diameter of the poplar trunks analyzed is 33.96 cm, and that of willows is 23.01 cm), compared to the other tree species present such as Alnus glutinosa L. (17.32 cm), Robinia pseudoacacia L. (15.60 cm), Sambucus nigra L. (13.93 cm). Furthermore, the poplar and willow were found to have a higher degree of trunk inclination and a higher degree of senescence, and consequently a higher degree of dangerousness compared to the other species, all of which confirms how important it is to give priority to the removal of the oldest and largest plants. The early results of vegetation monitoring were further confirmed by the hydraulic considerations carried out; in fact, the danger of floating vegetation that can accumulate near bridges, a phenomenon in which trees play a fundamental role due to their size and structural conformation, appears at the moment to be more significant. In this regard, annual monitoring may confirm a reduction in the phenomenon as a result of the application of the most appropriate types of vegetation maintenance. The incidence of river vegetation maintenance operations on the roughness of the channel during floods does not appear significant at the moment. However, this consideration must be confirmed by further tests conducted on longer reaches of river and in consideration of the speed at which vegetation grows back, with the latter compared to the feasible plurennial frequency of maintenance operations done on the same reach. In conclusion, it is emphasized once again that the experiment described must be considered entirely preliminary, and the identification of the type of management to be considered optimal, in terms of effectiveness and economy, will require an adequate period of monitoring. The experiment described is intended to be a starting point, which by means of guidelines may lead to the uniting of views from the perspective of environmental landscape and that strictly concerned with management and hydraulic protection.

References [1] Arcement G.J., Schneider V.R., Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Flood Plains, United States Geological Survey Water-supply, Paper 2339, 1989.

[2] Armanini A., Righetti M., Flow resistance in open channel flows with sparsely distributed bushes, Journal of Hydrology, 296, 2002. [3] Ballio F., Bianchi A., Pranzetti S., De Falco F., Mancini M., Vulnerabilità idraulica dei ponti fluviali, Catania, 1998. [4] Cowan W.L., Estimating hydraulic roughness coefficients. Agricultural Engineering, v. 37, n° 7, 1956. [5] Ferro V., Giordano G., Esperienze sulle resistenze al moto in alvei di tipo montano: riesami critici e nuove acquisizioni. Ingegneria Agraria, Anno XXI, 2. Edagricole, 1992. [6] Francia C., Il fenomeno della vegetazione flottante mossa negli eventi di piena, Master in Ingegneria del Suolo e delle Acque, Politecnico di Milano, 2004. [7] Ginanni F., Becchi I., Castelli F., Cinematica dei detriti arborei nelle correnti a pelo libero. Atti del XXVII Convegno di Idraulica e Costruzioni Idrauliche, Genova, 2000. [8] Petryks S. and Bosmajian G., Analysis of flow through vegetation: Proceedings. American Society of Civil Engineering, Journal of Hydraulics Division, 1975. [9] Prezedwojski B., Blazejewski R., Pilarczyk K.W., River Training Techniques: Fundamentals, Design and Application, A.A. Balkema, Rotterdam, Brookfield ed., 1995. [10] Regni M., La riqualificazione fluviale: stato dell’arte ed applicazioni in Umbria, Tesi di Laurea, Facoltà di Agraria, Università di Perugia, 2004.