CONSIDERATION ON HYDRAULIC RUNNING PARAMETERS OF HYDROTECHNICAL FREE FLOW CULVERTS

 

MIHAIL M. LUCA and VASILE HOBJILA

 

Technical University of Iassy, Departament of Hydraulic Structures, Romania

C.P. 2404, O.P. 11, 6600 Iassy, ROMANIA,

Phone: 0040-32-177553 / Fax: 0040-32-272022

 

 

ABSTRACT

The paper concerns the hydraulic study of hydrotechnical free flow culverts functioning with variable discharge and warp load. An analysis is made on the concrete case of culvert of a mountainous stream under a mining warehouse mass over its course. The hydrotechnical culvert made of steel concrete, with a circular shape and 1.80m diameter. The total length of the culvert is of 1368 m. After a free running of 22 years the hydrotechnical culvert has been seriously eroded in the apron area. The destructive process is the result of water dynamic action loaded with solid particles, in a free and fast flow and of cavitation phenomenon moulded on the damaged apron of the culvert in a nonuniform way. The formation of high speed can lead to the appearance and maintenance of the cavitation phenomenon. The erosion and cavitation phenomena operate on the flow section independently or together. The correlation of the two water erosion phenomena in their action destroys the safety of the hydrotechnical culvert running.

 

Keywords: hydrotehnical culverts, free flow, water dynamic action, erosion phenomenon, cavitation phenomenon,

 

INTRODUCTION

Designing and implementing projects within acceptable economic limits, projects that while functioning ensure an adequate safety and a minim risk of weakening, represents a main problem for the engineering activity. Any inadvertence in and between the phases of the designing, implementing and operating processes can lead to the weakening of the construction. Underground hydrotechnical constructions, designed to carry water by free or underpressure flow are subjected to a destructive process caused by erosion and cavitation phenomena. These processes lead to the changing of the design values, of the function parameters, the reduction of mechanic resistance, the endangering of stability and the decrease in the safety exploitation. To remedy the effects means expensive working which are done under very difficult condition.

 

CONSIDERATION ON THE EROSION PROCESS IN FREE FLOW CULVERTS

The running section of culverts where the water course has a free flow can undergo an erosion process due to the exceeding of the critical thresholds specific to the physic-chemical parameters of the material that constitutes the constructive structure as well as the parameters of the water-course. In the certain conditions the erosion process is amplified by the presence of sediment discharge, especially dragged load in the free flow. The origin of solid particles, their shape and size, the solid phase concentration and the traveling speed have a high influence on the intensity of the erosion process.

The erosion and cavitation phenomena operate on the flow section independently or together. This take place depending on the geometrical characteristics of the water bed, the fluid's physical state parameters, the water-course hydraulic parameters. In some functional situations of the culvert when the cavitation process has no conditions of developing on its own, the erosion process can create through its effects the primary stages for the cavitation phenomenon to take place and then further lead to its development. The presence of sediment discharge causes a continuous erosion action as opposed to the cavitation phenomenon, which has an intermittent development, due to the appearance and maintenance of the formation parameters [1, 2, 6]. The changing of the chemical characteristics of the water carried through the culvert can positively influence the erosion phenomenon.

 

THE DESIGN VALUES OF THE UNDERPASSING CULVERT SCALDATORI V.

The previously mentioned phenomena have been observed in a culvert underpassing a mine waste deposit (Scăldători Valley) located in the northern part of Romania. The hydrotechnical culvert made of steel concrete achieves the taking over and the deviation of the Scăldători stream waters under the heap of steril in process of formation. The heap of steril has, in time, occupied the natural riverbed and the afferent area of the water course (fig.1) [3].

Fig.1. The culvert underpassing the sterile deposit.

 

The hydrotechnical culvert was built in 1973 on a course parallel to the Scaldatori Valley stream riverbed. The interior profile of the culvert has a circular shape with a 1.80m diameter. The thickness of the steel concrete wall varies form 0.35 m to 0.70 m depending on the loads given by the height and composition of the steril heap. The steril heap has a height of 70 - 85 m at the moment [3]. The total length of the hydrotechnical culvert is of 1368 m out of which about 1250 m are located under the steril heap. The longitudinal gradient varies form 6.2 % to 0.8 % with a weighted mean of 4.65 %.

The hydrotechnical culvert takes over the water of the Scaldatori Valley stream without removing the solid deposits. Downhill the culvert discharges its load into a channel, which acts as a stepped energy-destroying spillway and as a socket to the initial waterbed. The culvert has been functioning continuously since 1975. However, phenomena of pollution of the Scaldatori Valley stream waters have been observed over the last period of time, pollution due to seepage form the steril heap through the crazes of the culvert's walls and through the severely damaged apron.

 

CHARACTERISATION OF EROSION PHENOMENA

After about 22 years of continuous activity the culvert shows important changes of the hydraulic and geometrical parameters specific to the running section in longitudinal and traverse profiles. The water current loaded with solid particles of mineral origin caused an increased erosion of the culvert's apron. The erosion phenomenon took place by digging-up a new channel for the minim and multiannual average outputs.

Water dynamic erosion was completed by cavitation erosion. This was caused by the formation of flow speeds with high values in section with high slopes. The microrelief of the culvert's apron modeled by abrasion also contributed to the erosion phenomenon (fig.2). Mechanic erosion was completed by chemical erosion caused by the seepage of water form the steril heap through the crazes and the construction joints of the culvert. Seepage water chemically activated in the sterol heap acted upon the concrete block, the gout of the joints and the transversal and longitudinal street reinforcements.

 

Fig.2. The effects of the dynamic action of water in the flowing (running) section of the culvert.

 

The erosion zones are continuous on the apron of the culvert and exhibit important fluctuations of the geometrical parameters. Their shape in a cross-section is semicircular with variable surface width (B = 0.40...0.90 m). The depth of the erosion zone also varies o the sections of the culvert (the measured values were h > 0.05...0.28 m). The concrete belonging to the flow area was eroded and carried downhill. The steel reinforcements (transversal bars f 18 mm and longitudinal bars f 21 mm) have been uncovered and differentially corroded until totally destroyed.

Measurements of flow rates and flow speeds inside the culverts have been taken as well as samples of alluvial material and water, in order to better understand the process of erosion. These measurements have been taken during the months of August in 1995 at minim flow and April in 1996 at high flow. The section to be measured have been chosen on transoms showing differentiate erosion phenomena. During the first stage the values recorded were: flow Q = 32.11 l/s...35.16 l/s, water depth h = 3.5...5.5 cm and speed v = 1.78...2.93 m/s. During the second stage (1996) the same transoms of the culvert have been measured and the new recorded values were: Q = 118.5...125.5 l/s; h = 7.5...9.0 cm; v = 2.94...3.84 m/s [3].

It is also worth mentioning that the running channel in the measured areas has a nonuniform microrelief due to the presence of concrete aggregates and traces of corroded and destroyed reinforcements. The microrelief of the riverbed had a high influence on the rugosity coefficient value. This value has increase in time, comparative of initial value and which led to an increase in the height of the water level in the culvert. The Manning coefficient of rugosity had an initial value of on n = 0.012...0.013, in accordance to the degree of concrete treating, but at the moment shows a higher value in the apron area where n = 0.020...0.022 (fig. 1) [3, 6].

The checking of the delivery value was done by comparing the flows likely to be passed by the culvert in a free flow with those formed in the hydrographic basin of the hydrotechnical construction's gathering area. The following values were obtained by processing the hydrologic parameters: Q_max,1%= 14.91 m³/s; Q_max,5%= 10.11 m³/s; Q_50%= 2.14 m³/s and Q_90%= 0.76 m³/s.

The initial condition and the condition of the contour used in computing the hydraulic parameters of the culvert were [3, 6]: the input flow is constant over the considered period of time; the running in the culvert is considered to be of the free flow type permanent and gradually varied; the calculus sections have variable slopes and rugosities in longitudinal and transversal sections; admissible velocities depend on the nature of the material (B 250/Bc 15).

The analysis done on the transoms with slopes I_min=0.8%, I_med=4.65%, I_max= 6.2% and rugosity coefficients n_min= 0.012, n_max= 0.013 allows to draw the following conclusions:

A. On the medium slope transoms the flow Q_med= 26.65 m³/s at a height of h = 1.60 m is higher than the considered maximal flows taken into calculus, therefore the culvert passes these flows at higher velocities than the ones maximally admissible. For h > 0.40, velocities v >v_max, (v_max@ 6.0 m/s) and v = 5.95...11.20 m/s respectively are obtained. The state of motion is supercritical (at the height h = 0.05...1.60 m and Fr = 3.77...10.46 is obtained).

B. On the maximum slope transoms, the culvert discharges flows with high calculus probability (Q_max= 33.80 m³/s). But velocities which are obtained even at small heights of the water surpass by much the maximum admissible values for concrete (at h > 0.30 m a v of 6.10...14.0 m/s and a Fr of 6.00...16.44 are obtained, a supercritical state of motion).

C. On the minim slope transoms, the culvert enter under-pressure for Q > Q_max,5%=10.11 m³/s. For h = 1.65 - 1.70 m, an Fr < 1 is obtained, and the state of motion within the culvert goes from "supercritical" to "under-critical". This has repercussions on the outflow from upstream through the formation of a hydraulic jump, the creation of a vacuum zone and so on.

The modeling of the culvert's apron through hydrodynamic erosion of the water current loaded with solid particles as well as certain deficiencies in the execution of the concrete section have determined the formation of individual irregularities or distributed in a nonuniform manner in the running section. Mainly these irregularities are represented by differences in the elevation rates of the pouring joints, concrete residuum and uncorroded reinforcements, stripped pouring joints, holes, cracks and crazes in the concrete mass and so on. These, especially the individual types, constitute centers for the appearance and development of cavitation processes where it will later on develop in all destructive aspect.

The intensity of the destruction of culvert through cavitation and erosion is highest where the flow regime employs bigger debits than the nominal debit. In the analysed case, these are represented by the high water discharge whose appearance is reduced with a relatively small propagation time but with unavoidable destruction. At the same time the high water discharge dislodges a drift debit of high value. Since the culvert functions in a supercritical state of motion (Fr > 1) velocity presents high values even at low debits.

In the area of the microrelief forms obtained on the apron, similar to those presented by the speciality literature [1, 6], the existence of a cavitation phenomenon of a fixed and turbionary type has been evaluated. The calculus of the cavitation coefficients has taken into account the parameters determined on the location "in situ" (water temperature, altitude, form of irregularities, velocities, characteristic heights and so on) as well as some approximations on the subject of speed and pressure distributions.

The following formulae have been used throughout the medium slope sections, for h = 0.20...1.00 m, water temperature 15°...20° C, p_v/rg = 0.18...0.24 m, altitude z = 820...850 m, speed v₀ @ v_m [1], [6]:

(1) (2) (3)

where s_g - s_ext is the cavitation coefficient of the water flow in the culvert for p_min and v₀; s_cr is the cavitation coefficient characteristic to the apron's irregularities for p_n and v_n; s_er is the erosional cavitation coefficient corresponding to the maximum intensity of the cavitation phenomenon. The resulted values are synthesised in Table 1.

 

 

Table 1. Cavitation coefficients (orientative values)

 

The analysis of the values shown in table 1 and their comparison with s_cr whose determined measures for various profile (s_cr = 3.00 ... 4.00 for individual reinforcement, s_cr = 2.00 ... 6.00 for concrete sill, s_er = 2.00 ... 3.50 - individual forms of microrelief and so on) are presented in [1] and [6] and emphasise the conclusion that the cavitation phenomenon starts and develops in the flow section of the culvert. Also, it should be taken into consideration that the cavitation phenomenon appears at small depths, for h = 0.20...0.30 meters, depending on the slope of the culvert sections and respectively on the current exploitation heights.

Since dynamic and cavitation erosion cannot be eliminated completely the functioning of the culvert, efforts should be concentrated on the causes which determine the phenomenon. In this respect the following should be taken into consideration:

-          the limitation of the solid flow transport through the culvert;

-          the decrease in the rugosity of the apron and the elimination of irregularities, which form the microrelief, shapes in the flow section;

-          the rebuilding of the culvert's apron by using materials more resistant to the phenomena of abrasion and cavitation.

 

CONCLUSIONS

The culvert which undercrosses the stream under the sterile heap in process of formation shows after 22 years of free-flow running, an important change in the flowing section caused by the an intense and complex process of hydrodynamic and cavitational erosion. The destructive process of the flowing section is caused by the dynamic action of the water loaded with solid particles with angular shapes, carried at high speed through a geometrical section not suited for a free flow.

The water current loaded with solid particles, favoured by the high gradient of the culvert intensified the hydrodynamic erosion, of the apron, a fact that led to the appearance and maintenance of the cavitation phenomenon in the areas where the microrelief were created in the flowing section. The two correlated types of water erosion (dynamic and cavitational), certain construction deficiencies the quality of the materials used and so on led to the significant degradation of the culvert's apron as well endangering safety in exploitation.

The continuos functioning of the culvert in its present technical state would have negative effects on the stability of the construction as well as on the stability of the sterile heap. This is why it is important that technological rehabilitation construction of the internal section of the underpassing culvert, especially in the apron area is done as soon as possible.

 

REFERENCES

1. Anton, I. - Cavitation - vol. I and II, The Romanian Academy Publishing House, 1985.

2. Luca, M. - The hydraulics of hydrotechnical constructions, The Iassy Technical University publishing House, 1994.

3. Luca, M., Hobjila, V. - Considerations on hydraulic parameters of free flow culverts exploitation, Hydrotechnica Review, vol. 42, no. 8, Bucharest, 1986.

4. Priscu, R. - Hydrotechnical constructions, vol. II, The Didactic and Pedagogical Publishing House, Bucharest, 1976.

5. Ratiu, M., Constantinescu, C. - The behaviour of constructions and hydrotechnical systems, The Technical Publishing House, Bucharest, 1989.

6. Sliskii, C., M. - Hydraulic calculus of the hydraulic structures of the high pressure, Energoatomizdat, Moskva, 1986.