Planning of Water and Hydropower Intake Structures (GTZ, 1989, 122 p.) 2. Planning of the intake structure 2.1 Requirements to be met by an intake structure 2.2 Principles for the arrangement of the intake structure on the river 2.3 Types of intake structures and their elements

Planning of Water and Hydropower Intake Structures (GTZ, 1989, 122 p.)

2. Planning of the intake structure

2.1 Requirements to be met by an intake structure

It is the task of an intake structure to divert from the channel at the tapping point the amounts of water necessary for whatever purpose with or without water being stored. For this purpose an intake structure for evacuating these amounts of water and possibly a structure for damming up the river are necessary. The bottom intake (Tyrolean intake) described in section 2.3.3 which combines intake and damming up in one structure is particularly important in this context.

The individual elements of the intake structure should always be so arranged on the channel that the following basic requirements are met:

1. The arrangement or the construction of a weir and intake structure must be chosen or carried out in such a way that the evacuation of the necessary amounts of diverted water is ensured at any regime of the channel.
2. The peak discharges must be safely evacuated from the weir and from the intake structure without damage being caused. To achieve this, hydrological data must be collected and evaluated in sufficient quantity in order to enable the dimensions to be planned in accordance with safety aspects (cf. sections 1 and 2.4).
3. A simple and moderately priced construction should be aimed at which allows maintenance-free operation and simple repairs to be carried out (cf. section 23).
4. If possible, the diverted water should be free from solid matter in order to prevent the diversion canal from being loaded with large amounts of bed load and/or suspended matter. To achieve this, the site of the intake structures should be selected in accordance with the river training rules explained in section 2.2.
5. It should be possible for the bed load and suspended matter, which is possibly deposited upstream behind the weir, to be evacuated by the water remaining in the river or by intermittent flushing. For this purpose, additional constructional measures should be taken (cf. section 2.2).

From this it is clear that the choice of the tapping point on or in the channel is just as important as the choice of intake structure. The decisions are mutually dependent. A simple construction should be the main objective. Observance of natural physical laws (cf. section 2.2) is an important prerequisite for the correct choice of site for the intake structure on the river bank, since the intake of bed load can be reduced by making use of these laws or by force, i.e. massive structures. Preference should always be given to the first solution.

Whether an intake is chosen with or without a river dam depends not only upon the cost of the weir. The following aspects should also be taken into consideration:

- The topographical conditions upstream of the structure. Damming up results in a backflow in the channel leading to a rise in the water level, which in turn may lead to flooding of the bank areas far upstream of the structure.
- The geotechnical conditions of the bank zones (talus material or rock).
- Height of the bank above the river bottom.
- The ratio of the quantity of diverted water to the residual quantity of water in the river at low discharge, with regard to existing rights of use of the downstream users.
- The channel width in the tapping point (dependence of the water level at times of low discharge in the river; meandering at low discharge in wide rivers, etc.; cost of damming structure, etc.).
- The routing of the diversion canal.
- The intake structure must not narrow the cross-section of flow of the channel; otherwise, at peak discharges, the bottom erosion in the area of the intake structure in the river bed would be increased, which in turn results in a change of the water level. A safe diversion of water at low discharges is therefore no longer ensured.

2.2 Principles for the arrangement of the intake structure on the river

As has already been mentioned, with the discharge, each river entrains solid matter in the form of suspended matter or as bed load (cf. also section 1.3).
The location of an intake structure must be so chosen that the largest possible portion of the bed load remains in the river and is not taken in in the diversion canal with the diverted water. A satisfactory arrangement of the intake structure does not remove the suspended matter; this is the task of a sand trap arranged downstream.

To hold off the bed load the natural hydraulic behaviour of the river can be profited from or technical measures taken:

1. Use of physical laws
In straight sections of river or stream, the water flows approximately in the cross-section of the channel, parallel to the banks. When the bed load transport begins, the bed load is transported accordingly on the bottom of the river.

In bends the direction of the bottom flow changes compared with the surface flow (Fig. 17a). A spiral flow forms which transports the bed load to the inner side of the river. On all streams and rivers it can be observed that gravel and sand banks form at the inside bend, i.e. the bed load is diverted from the deflecting bank. It could be concluded from this that the most favourable site for the construction of an intake structure is the deflecting bank. Fig. 17b shows for several examples the percentage of the bed load feed into a branch (intake) according to the arrangement of the intake structure or branch in the river section, a quantity of water to be diverted amounting to 50% being taken as a basis. Further results of the investigations are given in [6].

2. Technical measures
As technical measures bed load-deflecting structures in the form of ground sills, flushing canals, etc., in the flow area of the branch are a possibility.

A detailed discussion follows:

The following principles can be derived from the physical relationships:

(a) If at all possible, intake structures should be arranged on the outside bend.
(b) If it is necessary to construct the intake structure on a straight river section, a bent flow can be forced in order to be able to profit from natural physical laws.
(c) According to the rules of river training, special measures for keeping off the bed load are always necessary whenever more than 50% of the water is diverted from the river.
(d) In addition to the use of natural physical laws, technical measures are always necessary

- for intake structures where the water is not dammed up (case (c)),
- for intake structures where the water is dammed up, as the capacity of the silting space in front of the fixed weir is limited and the entrance of bed load into the intake structure cannot be prevented in the long term (cf. also Fig. 18).

Fig. 17a: Deposits in a river bend

The following guidelines for the construction of intake structures on various river sections are derived from these principles. They also serve to illustrate the examples previously discussed in a summarized form.

Intake structure on a straight river section
If the intake structure is arranged on a straight river section, the deflection of the flow by the power canal results in the bed load being transported to the inside bend, i.e. in the direction of the power canal. In order to prevent this, the flow of the river in front of the intake structure must be deflected so that the bed load remains in the river. For this purpose, groins (cf. Fig. 18) are arranged on the side of the river opposite the intake structure. This forces such a bend of the flow that the intake structure is now situated on the outside bend and the bed load is largely prevented from entering the power canal.

Fig. 17b: Entry of bed load in lateral intakes without additional structures according to [6]

	Distribution of bed load in the main stream and branch under the condition of a diversion of 50% QA = QH = 0.5 QO
	remaining of the bed load in the main stream, in %	entry of the bed load in the branch, in %
a	0	100
b	50	50
c	89	11
d	0	100
e	100	0

Intake structure on a bent river section
If intake structures are arranged on bends, the intake must always be situated on the outside bend, as the bed load is transported to the inside and the arrangement of the intake structure outside allows the bed load to be largely diverted from the intake.

The most favourable site for the intake structure is somewhat downstream of the apex of the bend. The spiral flow is strongest here, causing most of the bed load to be transported towards the inner bank.

If the bend in the river section is only slight (Fig. 19), the bending effect can be increased by a groin as described above (cf. Fig. 18). A bend is slight when the angle of the bend a<30° (Fig. 19).

Fig. 18a: lateral intake without damming and repelling of bed load from intake by technical measures

Fig. 18b: Lateral intake with damming and repelling of bed load from intake by technical measures

Fig. 19: Angle of bend a
Slight bend at a < 30º

2.3 Types of intake structures and their elements

Types of intake structures are chiefly distinguished by the method used to divert water from the river:

- lateral intake,
- bottom intake,
- overhead intake (intake of the water via inlets arranged in piers),

A lateral intake with water damming normally consists of two structures, the weir and the intake. The individual elements of the structures can be seen in Fig. 20. The individual structures and their elements have the following functions:

Weir

The weir is situated in the river and its function is to dam up the water level in order to ensure a constant minimum depth of water upstream of the weir and to allow the quantity of water for operational purposes (amount of service water) Q_A to be diverted from the river irrespective of the regime.

Fig. 21 shows the elements of the fixed weir. It consists of

- the actual weir body or the weir sill,
- the structure for energy dissipation: race floor, stilling basin with positive end sill, if necessary,
- the scour protection in the tail water in the area of transition between the structure and the natural river bottom.

Fig. 20: Elements of intake structure with damming. 1 retaining weir, 2 lateral intake, a forebay, b side weir and flood relief canal, c intake sluice/weir, d sand trap or direct connection to diversion canal

Fig. 21: Elements of a fixed weir

For reasons of economy, only fixed weirs are suitable for intake structures on small and medium-sized rivers for the tapping of relatively small amounts of water, as then maintenance is simple, local construction materials can possibly be used, and repairs be carried out by local staff.
In Fig. 22 different types of weirs are shown which will be described in detail in the following:

- Wooden weirs

The timber dam in Fig. 22 is suitable for construction heights up to 1.80 m. The dam wall must be sealed by staggered boards or by foil arranged inside. If the stream or river transports large quantities of bed load and suspended matter, a gradual sealing by the solids deposited can be expected. The race floor must be covered with thick planks or stones in order to prevent scours.

- Crib weirs

Crib weirs (Fig. 22) are suitable for greater impounding heads (up to 3 m) and have also proved their worth for rivers and streams which transport large quantities of bed load and suspended matter. They consist of wooden beams stacked at right angles and bolted, and filled with stones or bed material from the river. The front dam wall must be as tight as possible so that the weir is not destroyed by water flowing through it. To achieve this, staggered boards with foil between them can be used. According to the type of subsoil, the weir should be anchored with stone bolts (rock) or piles (subsoil into which piles can be driven). When the piles are driven close together, the front row may serve as sheet piling to reduce under see page. Where piles cannot be driven into the soil, the weir must be very long so as to obtain a long seepage line in order to reduce the buoyant forces due to the uplift (cf. also section 2.5).

Fig. 22: Different weir types of wood, masonry and concrete

- Stone or concrete weirs
Smaller impounding heads (< 1.50 m) can be reached with weirs of riprap and gravel core. The surface must consist of closely set press stones which offer sufficient resistance to the current. If the weir body is made of concrete, the weir surface should be paved with heavy granite blocks in order to avoid damage to the weir. For streams and rivers with coarse bed load, it is recommended that the downstream face of the weir be paved with hard wooden blocks. This lining of the weir surface has proved its worth, as in the case of too strong abrasion or damage, individual wooden blocks are relatively easy to replace. The blocks are placed in the mortar bed with the grain of the wood at right angles to the current. As wood placed in a dry state expands, this surface lining is very strong.

Intake structure

Fig. 23: Schematic potential arrangement of elements of an intake structure

According to the factors influencing the river and the amount of water to be diverted, the intake structure consists of the following elements:

- For intakes of less than 50% of the discharge of a river, the subdivision is shown in Fig. 23:

(1) Trash board to keep off floating matter.
(2) Coarse screen with a distance between the tears of 10 to 30 cm.
(3) Sill to keep off the bed load, construction height about half the height of the arithmetic mean water level when the water is dammed by a weir.
(4) Emergency gate at the inlet. Simple stop valves are arranged as emergency gates to allow the structures and elements situated in the tail water to be cleaned and repaired. Stop logs are suitable for this purpose which are guided in a stop log groove (recess on the left and right-hand side of the canal wall) and pressed by the water pressure against a seal in the groove. Fig. 24 shows such a stop log gate. The underside of each beam should also be provided with a seal so that the stop valve is absolutely watertight.
(5) Control structure. The intake structure is designed for a certain amount of water Q_A for a specific water level in the river. At higher water levels (periods of flood, etc.), owing to hydraulic phenomena, amounts of water larger than the design amount flow into the diversion canal. This is why a control structure for limiting the amount of water is to be provided enabling the excess water to be directly fed back into the river. A long side weir is a simple solution. However, a side weir allows only a certain portion of the discharge to be diverted. To achieve an exact limitation of the discharge in the canal to Q_A, several side weirs of different length and sill height would have to be arranged one behind another, resulting in a very long structure. By means of a control sluice in the canal downstream of the side weir, it is possible to achieve a greater excess head at the side weir and ensure a higher discharge capacity. A simple sliding sluice is suitable as control sluice (Fig. 25). Such a sluice is operated manually by a crank or a spindle. The latter is then arranged on the loadbearing system above the sliding sluice. When the sluices are designed and constructed, it should be borne in mind that the sluice must be pressed down by the higher sliding friction and the buoyancy during the lowering operation and possibly cannot be lowered by its dead weight alone. For the transmission of force for the lifting and lowering operations, the sluice is therefore equipped with racks which ensure the adhesion. The sluice is brought into the desired position with the crank and the spindle.

Fig. 24: Emergency gate

Fig. 25: Sliding sluice

Fig. 26: Example of an intake structure and flushing canal for bed load removal (QA > 0.5 · Q0) - basic sketch

- If more than 50% of the water is diverted from the stream or river, a bed load transport towards the intake structure must be expected due to the stronger deflection of the current in the direction of the intake structure. To prevent bed load from entering the diversion canal, an arrangement of flushing canals as proposed in Figs. 26 and 98 should be considered. These flushing canals must always have a bottom slope of at least 5%. As shown in Fig. 25, in order to minimize or prevent the entry of bed load in the forebay, it can be kept off by a first sill in front of the intake structure. If bed load still passes over this sill into the forebay, these deposits are led to the downstream side by intermittent flushing after a sluice in the flushing canal has been opened.

Fig. 27 shows the arrangement of weir, flushing canal and sand trap with the direct connection of a pressure pipe. This intake is suitable for the diversion of water without a power canal. Before entering the sand trap the bed load is kept off by a sill and led off to the downstream side by intermittent flushing after a sluice has been opened in the flushing canal. The sand trap is flushed by opening a flushing sluice at the end of the sand trap. Here, a spillway overflow in the form of a side weir can be constructed in order to feed excess amounts of water (closing of the turbines, flood) back into the river bed.

Fig. 27: Intake structure for a small hydroelectric power plant with sand trap and bed load removal (flushing canal) - basic sketch

In Fig. 28 the bed load is kept off by a first sill in front of the flushing canal. Solid matter that has been deposited in the flushing canal can be led off to the downstream side by intermittent flushing after a sluice has been opened. The intake structure in the form of a side weir prevents bed load from entering the power canal.

Fig. 28: Intake structure with bed load removal (flushing canal) and spillway (side weir) - basic sketch

If an excess amount of water enters the canal via the intake structure during a flood event, this is fed back into the river via a spillway (side weir, possibly with sluice in the canal to achieve a higher excess head) before it can enter the power canal.

2.3.3 Lateral intake without damming

In most cases lateral intake without damming is suitable only for the diversion of small amounts of water.

The inflow into the intake structure which is arranged laterally (Fig. 18) is directly dependent upon the water level in the river. According to the minimum regime of the river, the inflow is thus limited in quantity. Another limiting factor is that in the channel line the river bottom is normally situated at a lower level than the inlet bottom on the bank, with the result that in the inlet area, the excess head is smaller than the actual water depth of the river.

The limit up to which such intake structures are suitable is formed by an amount of water to be diverted of 1 to 2 m³/s << Q.

These few remarks already show that this type of intake without damming is suitable only in a few cases. In many cases it is nevertheless advantageous to dispense with damming in order

- to avoid an encroachment on the discharge in the case of insufficient knowledge being available of the hydrological phenomena,
- to avoid generating backwater to the upstream side and having to construct expensive jetties,
- to avoid aggradation in front of the dam in the case of rivers transporting a great quantity of bed load (often connected with a breakage of the weir body).

Fig. 29: Simple intake structure without damming with repelling groin

A typical intake structure with repelling groin is shown in Fig. 29. The water is diverted from a stream or river into the canal by a groin consisting of stones piled up in the river (repelling groin).

At times of medium and lowest discharge, when the river does not transport bed load or only small amounts of it, the diversion canal is not loaded by bed material. At times of the highest discharge when the bed load transport increases, the piled up stones of the repelling groin are entrained by the water, i.e. the groin is torn down so that the bed load can be freely transported by the river without hindrance. As the amount of diverted water is small in proportion to the amount of river water in the case of flood (Q_A << HQ₀), scarcely any bed load is transported into the inlet area. After the flood has subsided at the end of the rainy season, the repelling groin should be restored in order to ensure that water to be diverted is again introduced into the power canal at times of low discharge.

Fig. 30: Lateral intake without damming - basic sketch

Another method which has proved to be of practical value is to protect the intake structure by rock escarpments. Fig. 30 shows a typical arrangement. Owing to the small amount of diverted water in proportion to the discharge during the rainy season, bed load is scarcely introduced into the area of the intake structure, the more so as the intake structure is protected from the direct approach of water by the outcropping rock nose. When the discharge is at its lowest, the river carries almost no bed load at all, and the canal is therefore not loaded by bed material.

In periods of extreme drought, a repelling groin can be set up to ensure that the desired amount of water can be diverted.

2.3.4 Bottom intake (Tyrolean intake)

The Tyrolean intake occupies a special position. The water to be diverted is taken in through a collection canal built into the river bottom and covered with a screen (Fig. 31). The bars of the screen are laid in the direction of the current and inclined in the direction of the tail water so that coarse bed load is kept out of the collection canal and transported further downstream. Particles which are smaller than the inside width between the screen bars are introduced into the collection canal together with the water and these must later on be separated from the water for power generation by suitable flushing devices. The bottom intake can be constructed at the same level as the river bottom or in the form of a sill.

For the construction of the bottom intake attention must be paid to the following points:

- massive formation of the concrete body as it is subject to strong abrasion forces,
- recommended angle of inclination b of the screen between 5° and 35°,
- stable formation of the screen bars,
- sufficient freeboard between water surface in the collection canal and upper edge of the screen (at least 0.25 t = maximum water depth in the collection canal),
- sufficient slope in the collection canal to evacuate the solid matter which has entered through the screen, presorting of this matter by the inside width between the screen bars. In planning the dimensions of a Tyrolean intake it must be borne in mind that the whole inflow is taken from the river until the capacity limit of the screen is reached. If this maximum possible draw-off amount is greater than the lowest discharge, the tail water is drained. If the inflow exceeds the screen's capacity limit (e.g. during flood events), the amounts which are not diverted flow through the screen into the tail water. This is why the maximum amount of water for power generation to be evacuated can be more safely limited with a bottom intake than with a lateral intake with fixed weirs.

In Fig. 31 the elements of the intake structure with a Tyrolean weir are shown.

Fig. 31: Tyrolean weir / bottom intake

2.3.5 Selection criteria

In Table 4 (p. 53), the most important criteria for the selection of the lateral and bottom intake are given. The decision for one of the two intake types with a different arrangement of individual components should be taken bearing in mind the local conditions which are particularly influenced by the river's morphology and topography.

Selection criteria	Lateral intake	Bottom intake (Tyrolean weir)
Intake for water power utilization	Quite possible in connection with a sand trap	Quite possible in connection with a sand trap
Amount of water to be taken in	A favourable selection of the intake place will be a necessary prerequisite(outside bend, forcing of an artificial bend by groins) if the amount of diverted water is greater than 50% of the amount of water supplied.	The bottom screen draws off the river water up to the capacity limit of the screen.
Gradient of river:
- very great (I > 10%) to great (10% > I > 1%) gradient	Favourable; an as maintenance-free operation of the intake structure as possible should be ensured.	Very favourable; if the intake structure is well designed, the Tyrolean Weir can prove its worth owing to maintenance-free operation.
- mean gradient (1% > I > 0.01%)	Favourable in connection with a hydraulically very efficient sand trap.	Unfavourable; fine bed load falls into the collection canal and can result in strong alluvial deposits; difficult arrangement of the flushing installation.
- low gradient (0.01% > I > 0.001%)	Favourable in connection with a hydraulically very efficient sand trap.	Unfavourable.
Ground-plan of river:
- straight	Possible in connection with additional structures (groins for forcing a bent flow).	Very favourable, as bottom screen is uniformly loaded.
- winding	Very favourable when arranged on the outside bend.	Unfavourable, as bottom screen is not uniformly loaded.
- branched	Unfavourable; damming of the river recommended.	Unfavourable.
Solid matter transport of the river:
- Suspended matter concentration:
high	Suitable in connection with a hydraulically very efficient sand trap.	Less suitable.
low	Well suited.	Well suited.
- Bed load transport:
strong	Suitable as long as a sufficient amount of water remains in the river for flushing and transport purposes,	Well suited in the case of coarse bed load; expensive removal in the case of fine bed load with flushing devices.
weak	Well suited.	Well suited.

Table 4: Selection criteria for lateral and bottom intake