PROTECTED STILLING BASINS DOWNSTREAM OF LOW-HEAD RIVER TRAINING STRUCTURES: ENERGY DISSIPATION

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1 Netherlands E-proceedings of the 36 th IAHR World Congress 28 June 3 July, 2015, The Hague, the PROTECTED STILLING BASINS DOWNSTREAM OF LOW-HEAD RIVER TRAINING STRUCTURES: ENERGY DISSIPATION STEFANO PAGLIARA (1), MICHELE PALERMO (2) (1) Department of Energy Engineering, Systems, Land and Construction University of Pisa, Pisa, Italy, s.pagliara@ing.unipi.it (2) Department of Energy Engineering, Systems, Land and Construction University of Pisa, Pisa, Italy, michele.palermo@ing.unipi.it ABSTRACT Low-environment impact structures have become very popular in the last decades. They are widely used in river restoration both to control sediment transport and, at the same time, to create optimal conditions for fish species. They generally improve water quality due to higher oxygenation. Nevertheless, these are complex structures which require a particular attention to the effects of their presence on the local eco-system. Among this structure typology, block ramps are probably the most common. They are characterized by a quite complex flow regime and by significant erosive processes occurring downstream of them. Therefore, the knowledge of hydraulic characteristics is a fundamental aspect for a correct functioning of the structure. In particular, the downstream stilling basin morphology strongly depends on both hydraulic and geometric parameters. Namely, the stilling basin geometry has to be carefully analyzed in order to prevent structural collapse and to create opportune pools where fishes can find an optimal natural habitat. This paper focuses on energy dissipation process occurring in correspondence with block ramps and downstream stilling basins. The analysis of the dissipative phenomenon is conducted in the presence of protection structures (sills) located in the stilling basin in order to minimize scour characteristics. Two stilling basin configurations were compared: prismatic configuration having the same width of the ramp and symmetrically enlarged configuration. The energy dissipation for both the tested configurations was analyzed and discussed emphasizing the effect of spatial positions of the sills on it. Keywords: energy dissipation; hydraulics; low-head structures; sills; 1. INTRODUCTION River restoration is one of the most important topic for hydraulic engineers. In particular, the location of hydraulic structures in river body has an impact on the in situ conditions and it has to be carefully analyzed in each aspect in order both to prevent excessive localized erosion and to assure a correct hydraulic behavior. Namely, hydraulic structures have to control upstream sediment transport and, at the same time, they have to be structurally stable, i.e. they should not be endangered by downstream localized scour holes. In this perspective, low-head structures have become very popular in the last decades. In particular, they are effective both in controlling river bed morphology and in dissipating a relatively large amount of the upstream flow energy. On the other side, this structure typology minimizes the anthropic impact on the in situ environment. The study of low-head structures was conducted by several authors who proposed both theoretical and experimental approaches to foresee the main scour characteristics. Veronese (1937) analyzed the hydraulic behavior and the scour mechanism downstream of smooth horizontal channels for different stilling basin configurations. Tests on scour evolution downstream of rigid apron were conducted by Mason and Arumugam (1985) and Hassan and Narayanan (1985) who proposed useful relationships to predict the main scour hole features. The scour mechanism downstream of low-head control structures exhibits substantial similitudes with that due to plunging jets. This last aspect was analyzed and highlighted by several researcher (among others Rajaratnam, 1981; Rajaratnam and Macdougall, 1983; Breusers and Raudkivi, 1991; Hoffman and Verheij, 1997; Hoffman, 1998; Pagliara and Palermo, 2008a; Pagliara et al., 2010; Pagliara et al. 2011a; Pagliara et al., 2012a). But, one of the most important study on the topic is that proposed by Bormann and Julien (1991), who analyzed the jet diffusion properties in the stilling basin. Namely, they stated that the scour phenomenon occurring downstream of grade control structures depends on several parameters among which the most important are the structure geometry, the jet diffusion length and the bed material characteristics. D Agostino and Ferro (2004) further developed the analysis and results proposed by Bormann and Julien (1991) providing useful semiempirical relationships in order to predict the scour hole geometry. In the last years the study of low-head control structures has received a particular attention due to their capability to be flexible and eco-friendly structures. In particular, some peculiar structures were studied and analyzed. Namely, Pegram et al. (1999), Peyras et al. (1992) and Pagliara and Palermo (2013) analyzed stepped gabion weirs highlighting both their hydraulic behavior and scour mechanism occurring downstream of them. In particular, Pagliara and Palermo (2013) focused on the scour mechanism analysis occurring downstream of stepped gabion weirs and rock grade control structures. In addition, they studied the onset conditions of the different flow regimes occurring on the structures. 1

2 E-proceedings of the 36 th IAHR World Congress, Nevertheless, among low-head hydraulic structures, block ramps can be considered one of the most suitable structure to achieve the mentioned goals, i.e., high energy dissipation and sediment transport control. They are characterized by a rough sloped bed on which energy dissipation takes place. In addition, the erosive process occurring downstream of them can be easily controlled. A correct design of these structures requires a deep and accurate knowledge of both hydraulic and morphological characteristics. Therefore, many studies were conducted to understand their peculiarities. In particular, several experimental researches were conducted to evaluate the critical conditions which determine a collapse of the structure. In other words, several studies were conducted to evaluate the critical discharge conditions for the incipient movement of stones located on the sloping bed (among others Parker et al. 1982; Whittaker and Jaggi, 1986; Robinson et al., 1997; Pagliara and Palermo, 2011a). Further researches focused on scour hole geometry prediction. Namely, the scour morphology of the movable stilling basin was analyzed under different hydraulic and geometric conditions and several useful practical relationships were provided for both protected and unprotected stilling basins (Pagliara and Palermo, 2008b; Pagliara and Palermo, 2008c; Pagliara et al., 2009a; Pagliara and Palermo, 2010a; Pagliara and Palermo, 2011b; Oertel et al., 2011). In addition, the scour mechanism was also analyzed in the case of live-bed conditions and different geometric configurations of the stilling basin (Pagliara et al., 2011b; Pagliara et al., 2012b). Nevertheless, one of the most important aspect, which still requires further investigations, is the dissipative mechanism occurring in correspondence with this structure typology. Namely, there are relevant similitudes among low-head structures dissipative mechanisms as highlighted by Pegram et al. (1999), Peyras et al. (1992) and Pagliara et al. (2013). But, block ramps are characterized by peculiar dissipative characteristics, due to formation of vortexes between the stones. In addition, the dissipative process cannot be limited only to the structure, but has also to take into account the energy dissipation occurring in the stilling basin, which is influenced by both the geometric characteristics of the basin itself and by the presence of protection sills. The global dissipative process occurring in correspondence with block ramps (i.e., from the block ramp entrance up to the hydraulic jump downstream section in the stilling basin) was carefully analyzed by Pagliara et al. (2008). The authors showed that the global dissipative process does not depend on the ramp slope and roughness in the case in which the block ramp is not submerged by the hydraulic jump. Furthermore, they proposed useful relationships to evaluate the relative energy dissipation in the case of prismatic stilling basin (same width of the ramp). The results of this analysis were further extended by Pagliara and Palermo (2010b) who studied the effect on the global energy dissipation of protection rock sills located in the stilling basin in different spatial positions concluding that, in this last configuration, the energy dissipation is slightly bigger than in the case of unprotected stilling basin. In addition, Pagliara et al. (2009b), Pagliara et al. (2009c), Pagliara et al. (2010b) and Pagliara and Palermo (2012) analyzed the global energy dissipation in the case in which the downstream stilling basin is unprotected and enlarged. They showed that the enlargement of the downstream stilling basin causes an increase of the global energy dissipation, as also proven by Bremen and Hager (1993). This behavior is mainly due to the fact that a 3D hydraulic jump takes place in the downstream stilling basin causing an increase of the scour hole depth. The present paper aims to analyze the effect on the global energy dissipation of the presence of rock sills in a symmetrically enlarged stilling basin located in different spatial positions. In addition, the analysis is extended to two different rock sill widths, i.e., equal to the stilling basin width and to the ramp width. The results of the experimental tests are compared with previous relationships relative to both enlarged and prismatic basins in the absence of any protection sills. Both similitudes and differences of hydraulic behavior are highlighted and discussed. 2. EXPERIMENTAL SET-UP An experimental model was built in a flume whose dimensions are: 6 m long, 0.5 m wide and 0.8 m deep. Water depth and scour morphology were accurately measured by using a point gauge (0.1 mm accuracy), fitted on a movable trolley located on the channel. The ramp was simulated using a metal sheet on which granular materials were glued. The mean diameter of the ramp bed material is D 50=14.8 mm, whereas the non-uniformity material coefficient is =1.18. Three block ramp slopes S were tested: 0.083, and The width of the block ramp was b=0.18 m. A uniform granular material, with d 50=5.75 mm and =1.2, was used in the downstream stilling basin. According to Pagliara et al. (2009a) and Pagliara et al. (2009b), two different downstream stilling basin enlargements were tested. Namely, the downstream stilling basin was symmetrically enlarged in such way that the tested ratios between stilling basin width and ramp width were B/b=1.8 and 2.8, where B is the stilling basin width. Therefore, the tested widths of the downstream channel were m and 0.5 m. Preliminary tests (reference tests), in the absence of any stilling basin protection sill, were carried on in order to extend the relationships proposed by Pagliara et al. (2009b) to wider block ramp slope range. In fact, the minimum ramp slope tested by Pagliara et al. (2009b) was S=0.125, whereas in the present experimental investigation the minimum tested slope was S= The analysis of the results allowed to extend the deductions proposed by Pagliara et al (2009b). Furthermore, preliminary tests allowed to obtain the reference scour hole characteristic values. Namely, once the equilibrium scour morphology had been reached, the reference values of the maximum scour depth z max and scour length l 0 was carefully measured (see Figure 1a). In addition, the water level measurements allowed to evaluate the water depth at the ramp toe h 1 and the water level h 2 downstream of the hydraulic jump in the stilling basin. For the tested slopes and configurations, critical depth k always occurred at the ramp entrance. Figure 1a illustrates a diagram sketch of the tested apparatus (adopted for the reference tests) along with the mentioned geometric and hydraulic parameters. Note that H is the ramp height measured from the original stilling basin level. In Figure 1c-d a plan view of the experimental apparatus is shown. Note that for reference tests, no sills were located in the stilling basin. Figure 2 shows a top view of a reference experimental test. The 3D hydraulic jump occurring in the stilling basin is evident. 2

3 E-proceedings of the 36 th IAHR World Congress According to Pagliara and Palermo (2008b) and Pagliara and Palermo (2008c), reference values of the maximum scour depth z max and length l 0, were used to locate the rock sill in the stilling basin. In the presence of a rock sill, tests were repeated in the same hydraulic and geometric conditions. Namely, a combination of four longitudinal positions of the rock sill (x s=0.25l 0, 0.5l 0, 0.75l 0 and l 0) and three vertical positions (z op=+0.5z max, 0z max and -0.5z max) were selected. In addition, two different rock sill widths were tested. Namely, rock sill width was equal either to the ramp width b or to the stilling basin width B (see Figure 1c-d). Therefore, for each reference hydraulic and geometric configuration, 24 tests involving rock sill were performed. Rock sills were made of crushed stones whose mean diameter was 46 mm. Note that x s is the longitudinal distance of the sill from the ramp toe and z op is the vertical distance of the upper edge of the rock sill from the original bed level (see Figure 2b). In Figure 2b, the main geometric and hydraulic parameters are also reported. Namely. z maxs and l s are the maximum scour hole depth and length in the presence of a protection structure. In the following elaborations, both the longitudinal and vertical non-dimensional positions of the sill were taken into consideration, i.e. Z op=z op/z max and s=x s/l 0. Figure 3 is showing a picture of an equilibrium scour morphology for a test performed with a rock sill whose width is B, whereas, Figure 4 is showing the same for a test in which the width of the sill is b. Both the pictures refer to tests performed with a symmetrically enlarged stilling basin (B/b=2.8) and for ramp slope S= Figure 1. Diagram sketch of a block ramp and downstream stilling basin: side view with (a) unprotected stilling basin and (b) protected stilling basin; plan view illustrating protected stilling basin with sill width equal to (c) ramp width and (d) stilling basin width. 3

4 E-proceedings of the 36th IAHR World Congress, Figure 2. Unprotected symmetrically enlarged stilling basin Figure 3. Symmetrically enlarged stilling basin with protection sill whose width is equal to B Figure 4. Symmetrically enlarged stilling basin with protection sill whose width is equal to b 4

5 E-proceedings of the 36 th IAHR World Congress 3. RESULTS AND DISCUSSION 3.1 Literature background The global dissipative phenomenon occurring in correspondence with block ramps and downstream stilling basin was carefully analyzed by Pagliara et al. (2008), in the case in which no protection sills are present in the stilling basin and for prismatic channels (B/b=1). The authors showed that the global relative energy dissipation E 2=(E 0-E 2)/E 0 mainly depends on the parameter k/h (relative critical depth) and the effect of both relative roughness and ramp slope is negligible in the tested range of parameters, i.e., 0.1<k/H<1.2, 1<k/D 50<42 (which includes small, intermediate and large roughness conditions according to the definition given by Pagliara et al., 2008), and 0.125<S<0.25. Note that previous deductions are valid for hydraulic jump entirely occurring in the stilling basin, i.e., F MB (Free jump in Mobile Bed) hydraulic jump type according to Pagliara and Palermo (2008b). In other words, Pagliara et al. (2008) showed that E 2 is slightly affected by ramp configuration, but mainly depends on the dissipative phenomenon occurring in the stilling basin. Note that E 0=H+1.5k is the total upstream energy head (section 0-0 of Figure 1a) and E 2=h 2+V 2 2 /(2g) is the energy head at the section 2-2 of Figure 1a (downstream section of the hydraulic jump). Based on experimental observations, they derived the following Eq. (1) to predict E 2: E0 E2 E2 A 1 E 0 C k / H A e where A and C are constants equal to 0.25 and -1.9, respectively. It can be also noted, that for prismatic channel the relative tailwater (h 2/h 1) effect on the variable E 2 is negligible. This is mostly due to the fact that the hydraulic jump typology was also the same (F MB) in the tested range of parameters, therefore the hydraulic behavior and the dissipative mechanism were not significantly affected by h 2/h 1. In addition, a substantial non-dimensional longitudinal profile similitude is occurring when B/b=1. These deductions were further extended by Pagliara et al. (2009a) and Pagliara et al. (2009b). Namely, the authors analyzed the global dissipative process occurring in correspondence with block ramps and unprotected stilling basins in the case of symmetrically enlarged channels. They conducted tests for B/b ratios up to 2.8. In this last case, the hydrodynamic behavior inside the stilling basin is significantly different from that occurring in prismatic channels. Four different 3D hydraulic jump types take place in the stilling basin according to the inflow conditions and h 2/h 1 ratios. A peculiar flow pattern takes place inside the symmetrically enlarged stilling basin contributing to deeply modify the equilibrium scour morphology. Based on the studies conducted by Bremen and Hager (1993), Pagliara et al. (2009a) distinguished four 3D hydraulic jump types which were termed as follows: Repelled Symmetric jump (R-S); Repelled Oscillatory jump (R-O); Toe Symmetric jump (T-S); and Toe Oscillatory jump (T-O). They showed that the relative energy dissipation increases with enlargement ratio. This last occurrence is mainly due to the fact that the approach flow concentrates to the basin center if the channel expansion increases, resulting in a deeper scour hole. In other words, two lateral re-circulating flow zones occur in the stilling basin which contributes to axially deflect the entering flow, resulting in a substantial increase of the discharge per unit width. Therefore, the axial velocity results to be higher and the local shear stresses increase. The role of the parameter h 2/h 1 is essential in enlarged channels. It causes the hydraulic jump to occur either at the ramp toe or in the stilling basin. In addition, the increase of the parameters h 2/h 1 and the densimetric Froude number F d90 at the ramp toe determines periodical oscillations of the flow inside the stilling basin, resulting in a less 3D scour hole morphology. Note that F d90=v 1/(d 90g) 0.5, where =( s-)/ with s sediment density and water density, d 90 the diameter of the stilling basin material for which 90% is finer, g acceleration due to gravity and V 1 approaching flow velocity at the ramp toe. Based on these observations, and on the fact that for symmetrically enlarged stilling basin, non-dimernsional transversal profile similitude does not occur, Pagliara et al. (2009b) showed that the stilling basin material non-uniformity is not significantly affecting the dissipative phenomenon and, at the same time, a larger energy dissipation occurs increasing h 2/h 1. Namely, the relative energy dissipation is linearly depending on h 2/h 1. Therefore, they modified Eq. (1) including both the effects of B/b and h 2/h 1, resulting in the following general expression of the coefficient A of Eq. (1): with, h2 / h1 A Ck / H 1 e [1] [2] 10 k / H e B / b [3] and, C k H k / H e e B / b / 1 [4] 5

6 E-proceedings of the 36 th IAHR World Congress, Note that also for symmetrically enlarged stilling basins C=-1.9 in Eq. (1). As it can be easily observed, for B/b=1, A=0.25, therefore, Eq. (1) in which A is assumed as per Eq. (2), can be considered the general expression for the global relative energy dissipation for unprotected stilling basin whose enlargement ratio varies up to 2.8. In addition, Pagliara et al. (2009b) analyzed the dissipative process occurring in the stilling basin. Namely, they compared the relative energy dissipation (E 1-E 2)/E 1 in the stilling basin with the values predicted by Eq. (5), proposed by Hager (1985) and valid for symmetrically enlarged horizontal fixed stilling basins. 2 E 1 E [5] 1 E1 B / b F1 They concluded that there is a general increase in the energy dissipation performance, especially for the higher enlargements because of more prominent scour hole formation. This evidence confirms the significant role of the hydraulic jump in the global dissipative process. Especially for enlarged channels, the peculiar flow features occurring in the stilling basin causes a prominent lateral vorticity, which contributes to further dissipate flow energy. Note that in Eq. (5), E 1= h 1+V 2 1 /(2g), where V 1 is the approach flow velocity at the ramp toe, and F 1= V 1/(gh 1) 0.5 is the Froude number at the section 1-1 (ramp toe). Nevertheless, based on the previous mentioned studies, Pagliara and Palermo (2010b) analyzed the effect of rock made sills located in the stilling basin on the global energy dissipation process. In particular, according to Pagliara et al (2008c), they conducted their analysis in prismatic basins (B/b=1) and tested the following non-dimensional longitudinal and vertical positions of the rock sill: 0.25< s<1 and -0.5<Z op<+0.5. A detailed data analysis allowed to establish that, being constant Z op, the effect of s on E 2 is negligible. In addition, it was experimentally proven that there is only a slight effect of Z op on E 2, meaning that, for practical purposes, Eq. (1) is well predicting also the relative energy dissipation occurring in a prismatic and protected stilling basin. However, it is worth mentioning that the presence of the sill slightly contributes to increase the energy dissipation. Furthermore, the authors showed that also the relative energy dissipation in the stilling basin is slightly affected by the sill presence. It means that the sill is contributing to slightly increase the energy dissipation due to the flow disturbance induced inside the hydraulic jump. 3.2 Energy dissipation analysis for protected and enlarged stilling basins The previous results and deductions allowed to compare the dissipative process occurring in symmetrically enlarged channels in the presence of protection rock sills. Namely, the data analysis was conducted by steps. In the first step, the global energy dissipation occurring for all the tested configurations was compared with that predicted by Eq. (1) in the case of prismatic stilling basin and in the absence of any protection structures (Figure 5). It was observed that the global energy dissipation increases with B/b, even if a clear trend cannot be detected. This last evidence is in agreement with that observed by Pagliara et al. (2009b) in the case of enlarged unprotected basins. In addition, experimental data relative to different ramp slopes do not exhibit different trends. It means that the effect of ramp slope can be considered negligible, thus confirming the previous findings for all the tested configurations and hydraulic conditions. Figure 5 reports the experimental data in a graph E 2(k/H). It is evident, that being H constant, the higher energy dissipation is occurring for lower k, i.e. lower inflow discharges, for which the flow disturbance on the ramp due to the protruding boulders is more prominent. In addition, a preliminary analysis allowed to establish that the effect of rock sill width on the dissipative process is not significant. This occurrence is mainly due to the fact that the flow exiting from the ramp toe is axially deflected by the lateral flow re-circulation. Therefore, the central part of the channel is the zone of the stilling basin in which the sill presence results to be more effective. It means that the part of the sill closer to the stilling basin walls is not significantly influencing neither the scour morphology nor the hydraulic jump characteristics. Figure 5. E 2 vs k/h for all the tested vertical and longitudinal positions of the sills and ramp slopes. The second step of data elaborations was performed to highlight the effect of both longitudinal and vertical nondimensional positions of the sill in the stilling basin. Namely, data relative to reference tests, were compared with those relative to tests conducted in the same hydraulic conditions and configurations but varying the spatial position of the sill. 6

7 E-proceedings of the 36 th IAHR World Congress The results are shown in Figures 6 and 7. In particular, Figure 6 reports experimental data for all the tested s, for S=0.083 and for B/b=1.8. Data were grouped according to the parameter Z op. It is evident that for B/b>1, the effect of different non-dimensional vertical positions on the dissipative process is practically negligible, thus confirming the findings of Pagliara and Palermo (2010b) for prismatic channels. This is due to the fact that, in the tested range of parameters, energy dissipation is mainly occurring during the scour process. But the presence of the sill determines two different opposite effects. On one side, if the sill is located in a lower vertical position, its influence on scour morphology reduction is less prominent, therefore the amount of flow energy dissipated in the stilling basin is very close to that occurring in unprotected channels. On the other side, if the sill vertical position is Z op=0 or +0.5, its effect on the erosive process is more prominent contributing to substantial reduce the scour hole depth, therefore a less prominent energy dissipation should be expected. But, at the same time, a higher vertical position of the sill determines a more significant flow disturbance, resulting in an increase of the flow turbulence and energy dissipation. These two opposite effects concur to slightly modify the global dissipative process in the presence of a rock sill, respect to the unprotected configuration. Figure 6. Effect of Z op on energy dissipation for S=0.083, B/b=1.8 and all the tested longitudinal positions of the sills Figure 7. Effect of Z op on energy dissipation for S=0.083, B/b=2.8 and all the tested longitudinal positions of the sills The third step of data elaboration regarded the evaluation of the energy dissipation in the stilling basin. Namely, the relative energy dissipation (E 1-E 2)/E 1 occurring in the stilling basin was compared with that predicted by Eq. (5) proposed by Hager (1985) and relative to horizontal unprotected and enlarged stilling basins. Experimental data relative to enlarged protected stilling basin are reported in Figures 8 and 9. In the mentioned figures, x-axis represents the measured values of the variable (E 1-E 2)/E 1, whereas y-axis represents the computed values of the same variable using Eq. (5). In particular data were grouped for different B/b tested. Namely, Figure 8 compares (E 1-E 2)/E 1meas with (E 1-E 2)/E 1calc for protected basin with B/b=1.8, whereas Figure 9 shows the same comparison for B/b=2.8. As mentioned above, Pagliara et al. (2009b) noted that there is a general increase in the energy dissipation performance, especially for the higher enlargements because of the more prominent scour hole formation. This evidence is confirmed also in the presence of protected enlarged channels. As it can be noted, Eq. (5) underestimates the relative energy dissipation in the stilling basin. In addition, it is evident, by comparing Figures 8 and 9, that increasing B/b, the predictive performance of Eq. (5) becomes worst, as to be expected according to Pagliara et al. (2009b). It means that, also in this case, the presence of the sill, due to the opposite mentioned effects, which it determines, does not cause a substantial modification in the dissipated amount of energy. Therefore, based on all the previous considerations, Eq. (1), in which A was calculated using Eq. (2), was adopted to estimate the global relative 7

8 E-proceedings of the 36 th IAHR World Congress, energy dissipation also in the presence of protections sills in a symmetrically enlarged stilling basin. Figure 10 shows the comparison between measured and calculated values of the variable E 2, confirming that, for practical purposes, Eq. (1) is well predicting the totality of experimental data and therefore it can be considered as a general predicting tool, valid for 0.083<S<0.25, 0.1<k/H<1.2, 1<B/b<2.8 and both in the absence and in the presence of rock sills located in the stilling basin. Figure 8. Energy dissipation in the stilling basin for B/b=1.8 and all the tested ramp slopes Figure 9. Energy dissipation in the stilling basin for B/b=2.8 and all the tested ramp slopes Figure 10. Comparison between measured and calculated (with Eq. 1) values of the variable (E 0-E 2)/E 0. 8

9 E-proceedings of the 36 th IAHR World Congress 4. CONCLUSIONS The present paper analyzed the effect on the global energy dissipation of the presence of rock sills in a symmetrically enlarged stilling basin located in different spatial positions. The analysis involved a comparison with previous tested configurations, i.e., prismatic channel with and without protection sills and unprotected symmetrically enlarged stilling basins. The experimental data showed a substantial similar behavior in terms of dissipated energy between protected and unprotected stilling basins for enlargement ratios up to 2.8. This occurrence is mainly due to the fact that the presence of the sill determines a reduction of the scour depth and at the same time increases the flow disturbance inside the stilling basin. These two aspects are strictly related to the amount of dissipated energy. In fact, a reduction of the scour hole dimension results in a decrease of the energy dissipation. Nevertheless, the flow disturbance induced by the sill determines an increase of flow turbulence in the stilling basin, resulting in an increase of the energy dissipation. This two opposite effects are almost equivalent in terms of modifications induced to the dissipative process. Therefore, it can be concluded that the presence of a protection sill in an enlarged stilling basin does not significantly modify the global dissipative mechanism. REFERENCES Bormann E., and Julien PY. (1991). Scour downstream of grade control structures. Journal of Hydraulic Engineering, 117(5), Bremen R., and Hager WH. (1993). T-jump in abruptly expanding channel. Journal of Hydraulic Research, 31(1), Breusers HNC., and Raudkivi AJ. (1991). Scouring. IAHR Hydraulic structures design manual 2. Balkema: Rotterdam, the Netherlands. D Agostino V., and Ferro V. (2004). Scour on alluvional bed downstream of grade-control structures. Journal of Hydraulic Engineering, 130(1), Hassan NMKN., and Narayanan R. (1985). 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