U-Shape Spillway and Hydraulic Considerations in Sonnateh Dam in Iran 1

U-shape spillway and hydraulic considerations in Sonnateh dam in Iran1

Shahin Koohbezan Manager of dam and hydropower department , Dezab consulting Engineers co

Elham Saki Malehi Hydraulic expert, Dezab consulting Engineers co

1. INTRODUCTION

Design of an appropriate layout of a dam and its components in a dam site considering topography, geotechnical aspects, access road, and safety is very significant. One of the most important components that require a major cost is the spillway.

2. LAYOUT OF SONATTEH STORAGE DAM SPILLWAY

Sonnateh storage dam is a rock-fill dam with clay core which is designed to regulate and control floods and provides agricultural demands on Khorkhoreh river. Electrical energy production is also considered as a minor objective in this project. Sonnateh dam is located in a U shape open nonsymmetrical gorge. The spillway is located in the right abutment in a saddle. The river bed

1 U évacuateur de crues et de l'hydraulique dans des considérations Sonnateh barrage en Iran elevation in the dam axis is 1514 m, the normal water level is 1567 m, the dam crest elevation is 1573 m, the total reservoir volume is 100 mcm and the design flood is 1133 m3/s. A saddle can be noticed at elevation of 1565 m in the right abutment. Geotechnical explorations indicate that the surface material of the saddle up to 15 m depth consists of overburden and high weathered fair rock. Therefore, these surface materials should be excavated and replaced by a massive and structural concrete of 10 m depth up to the spillway crest in order to get structural stability. It is obvious that this may increase the cost significantly. One of the alternatives which may require less excavation and eventually less costs is the side spillway. The flow passes the side spillway in a normal direction and enters the side channel which is designed parallel to the spillway crest. In general this channel has a significant depth and ends with a chute. Considering the topography of the site, a side U shape spillway is designed as depicted in Fig. 1. The flow passes the spillway crest and enters the side channel, then passes through a steep chute in which the flow regime changes to super-critical. Fig. 1 Plan and cross section of the traditional Side spillway

Fig. 2 Plan and cross section of the U shape Side spillway 3. SELECTION OF TYPE OF THE SIDE SPILLWAY

The plan of the topography and the geotechnical profile for the spillway implementation are shown in figures 3 and 4. The RQD's of various layers are depicted in this profile. Thickness of the overburden at the location of the side channel varies between 3 to 8 meters. A 2 to 7 m rock layer which may be classified as a weak to intermediate rock layer exists beneath the overburden layer. The rock beneath this layer has a good RQD and appropriate mass strength. In general, the depth that should be excavated from the natural ground surface varies between 5 to15 meters. Calculations indicate that design of a side spillway in this site requires a much less structural concrete compared to the usual chute spillway.

Fig. 3 Plan of the side spillway Fig.4 Geological longitudinal profile in the spillway direction

4. GOVERNING EQUATIONS OF FLOW IN A SIDE CHANNEL

The governing differential equation for the side channel which is considered as a spatially varied flow with lateral inflow is given as follows:

2Q dQ S  S  dy ° f gA2 dx  [1] dx Q 2 B 1 gA3

Where y : flow depth in the side channel x : distance from the beginning of the side channel

S° : bed slope

S f : energy grade line slope which can be calculated from The Manning formula A : cross sectional area Q : flow discharge g : gravitational acceleration B : top surface width

Using the finite difference method, equation (1) can be descretized as follows:

2 2  n Qm  2Qm Q2 Q1 S°  4 / 3 2   2 y2  y1  Rm Am  gAm x  2 [2] x Qm Bm 1  3 gAm

Where Q1  Q2 Qm  2 Q  (Q  ql)  1 1 2 ql  Q  1 2 R  R A  A B  B R  1 2 , A  1 2 , B  1 2 m 2 m 2 m 2

l in the above relations is the length of the spillway reach from which the flow passes over. It should be equal to 2 x because there are two sides of flow in reach of the U shape spillway. But for the curvature in the upstream reach l should be equal to the real curved length of the spillway and x is selected in the axis direction of the side channel.

Fig. 5 A typical water surface profile in a distance equal to ∆x in the side channel

4.1. METHOD OF SOLVING EQUATION (2)

Using the head in the reservoir above the spillway crest and having the total length of the U shape side spillway, the total discharge is calculated by the over flow spillway relationship Q = CLH 3/2. Since the flow is subcritical in the side channel the control point has to be located downstream which means that the solution should be started from a downstream point. The control point in this case is located at the end of the side channel just upstream of the chute. The depth at this point can be calculated using the overflow spillway relationship. If we recall the depth at this point as y2 then y1 should be calculated from equation (2) as an unknown. It should be pointed out that in the distance from this point to the end of the location at the end of the side spillway the third term in the nominator of the fraction should be set equal to zero because there is no lateral inflow, but further than that the mentioned term should be involved in the equation. The algorithm to calculate y1 in equation (2) is as follows:

1- Guess y1 ( It can be guessed as y1 = y2). 2- Substitute y1 in the right hand side of equation (2) and calculate y1 of the

left hand side. If the absolute value of the difference between the new y1 and the old one is less than the accepted tolerance, then the equation in

this stage is solved and the final y1 at this location is obtained other wise

substitute the new value of y1 in the right hand side of equation (2) to start a new iteration. Continue this procedure until convergence is achieved. 3- Repeat steps (1) and (2) for other cross sections upstream. q for every reach should be calculated using the over flow discharge relationship ( q CLH 3 / 2 ) in which L indicates the total length of spillway flow in the reach. It has to be mentioned that the depth in the side channel may become higher than the spillway crest. In this case, the submergence depth interacted 2 with the spillway flow should not be greater than H above the spillway crest. 3 °

5. RESULT OF DESIGNING

Length of spillway 40 meter Length of side channel 17.9 meter Breadth of side channel's end section 20 meter Total Length of the parallel sides of the ogee 25.8 meter Slop of the side channel's apron 0.001 Peak discharge of dam 1133 m3/s Height of side channel 11 meter Depth of water in side channel 12.5 meter Depth of water behind the dam as measured from ogee crest to maximum water level 5.9 meter Length of converged channel 20 meter Breadth of converged channel's end section 18 meter Slop of the converged channel's apron 0.01

6. CONCLUSION

Selection of a spillway type depends on topography, geo-mechanical and hydraulic conditions of the dam site. The alternative which has the least cost and can achieve all requirements despite its design difficulties should be selected. But it has to be born in mind that such alternatives may be approved by employing an appropriate hydraulic model in order to reduce uncertainties of the design. AKNOWLEGEMENT

The author would like to thank Dr. H. M. V. Samani for his valuable recommendations and also would like thank Mr. Liaghatjoo direct manager of Dezab consulting Engineers Co. for his supports.

REFRENCES

[1] "Design of small dams",water resources Technical Publications,U.S. Bureau of Reclamation,1965. .Hydraulic design of spillways", us army corps" [2]

SUMMARY AND KEY-WORDS

Engineers usually take into account the efficiency and cost in design of hydraulic structures. An uncommon geometry may sometimes be chosen to achieve an efficient hydraulic structure with the least cost. Sonnateh earthfill dam with a reservoir volume of 64 MCM and 61 meter height at the western part of Iran is planned to be constructed. The geometry of the dam site gorge and the depth of the appropriate bed rock in addition to the abutments’ geometry constraints have led to a U-shape spillway design. The flow passes the spillway crest and enters a basin which acts as a side channel. The flow will then pass through a chute that guides the flow to an energy dissipater. The hydraulic design of the U-shape spillway and the side channel is based on the spatially varied flow principles. The final design satisfies all constraints and economically is optimized. The design flood is 1132 m3/s. The spillway crest is 40 meters long and the design head is 5.9 meters. In this paper details of the layout and hydraulic calculations will be presented.

keywords : dam - spillway- U shape - unusual spillway-Hydraulic structures RESUME ET MOTS-CLES

Les ingénieurs prennent généralement en compte l'efficacité et le coût de conception d'ouvrages hydrauliques. Un peu de géométrie mai parfois être choisie de façon à atteindre une bonne structure hydraulique avec le moins de coût. Sonnateh barrage en terre avec un réservoir de 64 MCM volume et de 61 mètres de hauteur à la partie occidentale de l'Iran est prévu pour être construit. La géométrie de la gorge de barrage et de la profondeur de la roche-lit en plus les culées "la géométrie des contraintes ont conduit à une forme de U déversoir design. Le flux passe la crête de l'évacuateur de crues et de l'entrée d'un bassin qui sert de côté canal. La circulation sera alors passer par une goulotte qui guide le flux d'énergie à dissipater. La conception hydraulique de l'évacuateur de crues en forme de U et le côté canal est basée sur le débit varie spatialement principes. La conception finale répond à toutes les contraintes et économique est optimisée. La conception d'inondation est 1132 m3 / s. La crête de l'évacuateur de crues est de 40 mètres de long et la conception de la tête est de 5,9 mètres. Dans ce document, les détails de la mise en page et les calculs hydrauliques seront présentés. mots-clés: barrage - déversoir U-forme - de l'évacuateur de crues inhabituelles-structures hydrauliques