Permanent Structures for Gully Control: 4 Types | Soil Management

The following points highlight the four main types of permanent structures for gully control. The types are: 1. Spillway 2. Rubble Masonry Dam 3. Concrete Dam 4. Gabion Structure.

The gully control structures are laid in the gully section, throughout the length, according to the profile of gully bed.

Amongst different types of spillways, the chute spillways are located at the gully head for safe disposal of runoff water from head to the gully bed. The drop spillways are placed along the gully bed in series to act as control points, so that the gully bed cannot be eroded below the crest level of the structure. Drop inlet spillways are constructed at the places of high depression in the gully bed for storing the water.

Type # 1. Spillway:

It is a flow path constructed either in a dam section or in the drop structures for effective and safe disposal of water from upstream to downstream end. As per term meaning, the spillway is a path to cross the flow from one end to other end. The basic components of spillway, used as permanent gully control structures (hydraulic structure) are the inlet, conduit and the outlet.

Water enters the structure through the inlet and is conveyed through the conduit. The water leaves the structure through the outlet. The outlet is designed for dissipating the maximum kinetic energy of flowing water so that the water leaving the structure cannot cause erosion towards down-stream side of the structure. 

The various types of inlet, conduit and outlet that are commonly used are:

a. Drop Inlet Spillway:

This structure consists of a pipe (conduit) passing through an earth fill dam, is connected with a suitable inlet and outlet. The inlet is constructed with some drop which causes to flow the water from inlet to conduit. Sometimes, a hood type inlet is also used in drop inlet spillway. The outlet usually consists of a propped pipe or a chute. The size of outlet should be sufficient to allow an adequate flow to the downstream channel.

The drop inlet structure is versatile in use. It is a very efficient structure for controlling relatively high gully heads, usually more than 3m. It also serves as most suitable gully control structure, where large drops of gully profile are needed to eliminate. These are also used to raise the grade of deep gullies.

Also, for flood control the drop inlet spillways are ideally used, as they are constructed with an earth fill dam which forms sufficient storage capacity above the structure to hold the runoff. The drop inlet spillways have the dual purpose, i.e., gully control and flood control. However, various uses of drop-inlet spillways are given below.

Uses of Drop inlet Spillway:

Various uses of this structure are given as under:

i. It is used as a principal spillway in farm ponds or reservoirs for letting out the stored water.

ii. For stabilisation of gully grade.

iii. It can be used in combination of check dams for safe disposal of water stored in the reservoir.

iv. In forest roads it can be used as culvert.

v. In debris basins it is used as principal spillway.

vi. For controlling the flood, it is suitably used.

vii. It can also be used as a channel water inlet in drainage or irrigation structures.

Advantages:

It has following main advantages:

i. It requires comparatively less construction materials than the straight drop spillway for the same drop.

ii. The capacity of this spillway can be reduced, particularly where an appreciable amount of temporary storage is available in the gully length.

iii. Construction cost is less.

iv. For stabilisation of gully grade and prevention of flood, it is one of the most efficient structures.

Apart from above advantages, there are some disadvantages or limitations of drop-inlet spillway, which are as follows:

i. Small capacity drop-inlet spillways are more susceptible to get chocked by debris present in the flow water.

ii. At the place, where greater earthwork is required for construction, it is not suitable for use.

b. Chute Spillway:

It is an open channel like structure, which is constructed on steep slope of the gully face or head with a suitable inlet and outlet. Such spillways are used at the locations where head drop is fairly large ranging from 5 to 6 m. They are normally constructed to handle full flow at the gully head. Since they are used at larger drop i.e., at the gully head, so their main function is to control the gully head from getting enlargement by waterfall erosion. Chute spillway handles the flow with super critical velocity.

Chute spillways usually require less concrete work than the drop-inlet structure for the same capacity and drop. This type of spillway involves more danger of undermining by rodents and threatening of foundation due to seepage in poorly drained locations. The chute structures do not create water storage towards upstream side.

The other details of chute spillway are given as under:

Adaptability:

The chute spillways are suitable for following conditions:

i. For high over falls where a full flow structure is required.

ii. Where site conditions are not suitable for constructing the check dams.

iii. This spillway can be suitably used in combination of check dams and other detention-type structures.

Uses:

Its main uses are as follows:

i. For control of gully head advancement.

ii. For conveying the runoff from u/s areas into the gully, very smoothly without erosion.

iii. It can be used as a structure for water conservation and collection of sediments.

iv. For controlling the gradient of natural or artificial channels.

Limitations:

Various limitations of chute spillway are as under:

i. There is considerable danger of undermining due to rodents. For which additional precautions are required to follow.

ii. In poorly drained areas there is the problem of seepage. Such areas are not suitable for chute spillways, as seepage tends to weaken the foundation. In such areas if construction of chute spillway is very essential and no other substitutes are available, then provisions to control the seepage are essentially made.

iii. For safety point of view, the construction site should be compacted very well or when earth filling is done that should also be compacted thoroughly. It is an additional work, which involves time and money, both.

c. Straight Drop Spillway:

It is one of the most suitable and common permanent gully control structures which is mainly used at the gully bed to create a control point. Several such drop structures are constructed across the gully width throughout the length at fixed intervals.

The series of such structures develop a continuous break to the flow of water, causing deposition of sediments; and thus to fill the gully section. Sometimes, the drop structures are also used at the gully head to pass the flow safely and control­ling the gully head.

Different components of the drop structures are given as below:

i. Head wall

ii. Head wall extension

iii. Side walls

iv. Wing walls

v. Apron

vi. Longitudinal sills

vii. End sill; and

viii. Cut-off walls.

i. Head Wall:

It acts as a front wall against runoff flow in the drop spillway. It is constructed across the gully width. A notch of suitable size is also constructed at the top in the head wall for making easy water conveyance. Rectangular notch is most commonly used. The size of notch should be sufficient to allow the water very safely.

ii. Head Wall Extension:

It is the extended portion of head wall into the gully sides. Its main function is to provide structural strength against sliding of the structure; and also to check the flow of water from the sides of the structure.

iii. Side Walls:

They are constructed in the side along the gully walls. The two side walls form the apron section. The function of side walls is to prevent splashing of water flow over the gully banks; and also to confine the water flow within the apron.

iv. Wing Walls:

They are constructed at rear end of the structure with some inclination; usually at 45° from the vertical. These walls are extended up to the gully sides, and perform the function of preventing the flow backward into the space left between gully wall and side wall of the structure.

v. Apron:

It is one of the main downstream component of the straight drop spillway. It receives the gully flow with high velocity and makes the flow in such a condition as to cause negligible soil erosion from the channel, down-stream side. It includes several blocks which are elevated by some height and make the apron surface rough.

This feature of apron is responsible to dissipate the maximum kinetic energy of falling water by creating hydraulic jump, as result the velocity of outgoing water gets significantly reduced. The reduction in flow velocity is not only beneficial to reduce the soil erosion, but also very helpful to deposit the silt particles present in the flow water over the gully bed, causing control of the gully development.

vi. Longitudinal Sills:

They are constructed in the apron section. They are constructed in length wise parallel to the side wall. These sills are useful to make the apron, stable.

vii. End Sill:

It is the elevated portion of rear end of the apron. Its main function is to obstruct the water, going directly into the channel, below.

viii. Cutoff Walls:

They are constructed to provide structural strength against sliding of the structure. They increase frictional resistance of the structure, which opposes the force causing to slide. In other words, cutoff walls act as key for the structure.

The inflow capacity of straight drop structure is controlled by the size of the inlet, i.e. notch, used. The notch is in the form of rectangular weir, in which flow is directly proportional to the length of the weir. The drop structures are used up to the drop height of about 3 m.

There are following three major purposes of the use of drop structure:

i. To provide a transition between a broad or flat waterway and ditch or gully section.

ii. To raise the flow line, i.e., to form sufficient soil depth for vegetative growth where bottom of the gully is on risk.

iii. To raise the flow line of the waterway so as to provide drainage in the case of wet waterways.

In addition, the drop spillway can also be used as an outlet for disposing surface water from large areas, especially where drainage ditches exist. In some cases drop spillways are also used to reduce the channel grade.

However, the other details are given as under:

Uses:

Various uses of this spillway are given as under:

i. It is used for grade stabilisation in lower reaches of the waterways and outlets.

ii. Used for erosion control, to protect the roads, building etc.

iii. Straight drop spillway can be used as an outlet in tile drainage system and also for releasing the irrigation water into the field.

iv. In reservoir it is used for letting out the water, provided that total drop is low, i.e., not more than 3m.

v. It is also used for controlling the tail water at the outlet section of the conduit or spillway.

vi. For controlling irrigation the drop spillways are also used in the water distribution system.

Advantages:

These are as follows:

i. In this structure the danger of undermining by rodents is not possible.

ii. Straight drop spillways are less susceptible to get structural damage, as compared to the other structures.

iii. As in other spillways (especially in drop-inlet spillway) the conduit is likely to be clogged by debris, but in this spillway there is no such problem.

iv. Its construction is very easy; a village mansion can easily construct it.

Limitations:

The location of straight drop spillway has following limitations. They must be counted.

i. In the gullies where discharge is less than 3 m³/s and total head or drop exceeds 3 m, the construction of straight drop spillway proves to be a costly affair, should not be preferred.

ii. It requires a stable gully grade for construction.

iii. Construction of this structure is not technically justified, particularly where temporary storage is required.

Type # 2. Rubble Masonry Dam:

These dams are used in gullies or stream channels with high rates of runoff or where vegetations cannot be established. The construction of this dam is recommended only where rocks or stones are readily available in nearby areas. Minimum thickness of walls is kept 30 cm. The down-stream slope of the dam below the spillway is kept at least 1:2. The thickness of the base should not be less than 3/4 of the height of the dam.

The other details of rubble masonry dams are given as under:

i. The minimum thickness of side walls, cut off walls and apron should be about 30 cm.

ii. The thickness of main wall from the crest of spillway to the top of dam should not be less than 35 cm.

iii. To ensure proper settling, the upstream side of dam should be maintained at an angle of about 10° with the vertical.

iv. The length of apron should not be less than 1.5 limes the height of dam, measured from apron floor to the spillway’s crest.

v. For drainage, the provision of drains or weep holes should also be made. They should be located near the base of dam.

Type # 3. Concrete Dam:

Concrete dams are recommended, when inadequate materials are available for constructing the masonry check dams. The specifications used for construction of masonry dams, can be suitably used for concrete dams. The disadvantage of this structure is that it is very flexible. Once it is damaged, not repaired easily.

The main points about this dam are given as under:

i. Good grade of cement, reinforce and steel bars should be used.

ii. The buttresses are made to brace the head wall, especially when length of spillway is greater than 3 m.

iii. The cutoff walls should be placed at fairly greater depth both in the gully bed and sides, to make the structure safe against sliding.

Type # 4. Gabion Structure:

Gabion is also used as an erosion control tool that is originated in Italy. It is basically a rectangular wire mesh box, filled with stones. The size of stones filled is always greater than the mesh openings. These structures are flexible, permeable and economical, are constructed in those places where stones are abundant. The boxes are made of galvanized wires to ensure longer service life, as these wires are less susceptible to get affected by rust formation.

Advantages of Gabions:

Usually, it is assumed that a soil erosion control structure becomes ideal, when it is:

(i) Efficient

(ii) Permanent

(iii) Easy to construct; and

(iv) Economical

For a gabion structure is concerned, it fulfills all above requirements. In spite, it also has several special features, which prove its advantage.

These are given as:

i. It is a Flexible Structure:

Flexible structure is that which can be easily bend in any direction as per requirement of the site situation. For erosion control a flexible structure has great importance, because soil erosion that is either from the land surface, from the gully section or from stream bank portions, which never take place from a fixed point and within a limited extent, but it always varies depending on the characteristics of the soil, climate, stream flow etc. In this situation, a structure which is permanently fixed at a certain point cannot accommodate such fluctuations, and is not ideal for such specific purposes.

A gabion structure is well suitable for all such conditions, because it can be easily bend without breakage; and can also be shifted to any point as per site requirement. The gabion structures do not get collapse like the rigid or semi-rigid structures.

ii. It is a Permeable Structure:

A structure in permeable nature has good effect on soil erosion control, especially in gully section. Gabion structures are highly permeable, as these are constructed by filling the stone pieces in the box.

The permeable nature of gabion structure provides following main advantages:

a. It provides drainage space through the voids left between stones; by this effect the u/s water is easily drained.

b. It facilitates to ‘blead-off’ the ground water and relieves the hydrostatic pressure, thus prevents to collapse the structure.

c. Gabion structure does not require to get install a separate drainage system.

iii. It is a Stable Structure:

It is well known that, the gabion structure is a kind of heavy gravity unit; by this property it can easily absorb the earth’s thrust. In addition, the spaces left between stone pieces are also filled with the silt particles in due course of time, which makes the gabion solid. Furthermore, the soil filled spaces are also covered with vegetations, later on, which create a greater stability to the gabion. The root systems of vegetation also hold the stone pieces very tightly at their respective positions; and thus, further make the structure more stable.

iv. It is an Economical Structure:

The gabions are more economical as compared to the masonry and concrete structures, might be due to following reasons:

i. Cheaper construction.

ii. For construction of this structure, the operations like excavation below the water, driving of piles, establishment of underground drainage system, transportation of the materials at construction site etc. are not required. These operations are very costly. In this way the investment of money on above operations is completely saved in case of gabion structures.

iii. It does not involve complicated design and constructional features. An unskilled labour can easily construct it.

iv. For construction, mostly the boulders and stones are used, which are often available in the nearby areas, at low cost.

Construction of Gabion Structures:

Materials – The gabion boxes are constructed with the help of galvanized wire, hexagonal triple twisted mesh in the size ranging from 7.5 to 15 cm and stones or boulders of suitable size. The galvanized wire of No-8 gauge is commonly used. Less than No. 10 gauge wire is not used.

Construction Procedure:

Gabion boxes are constructed in any convenient size as per requirement. However, in order to have uniformity and also to meet all the necessary requirements, the specifications given in Table 4.4 can be adopted for construction purposes.

Construction of gabion box includes following guidelines for consideration:

i. The gabion box should preferably be fabricated in flat shape, as it provides easy handling for transportation.

ii. The edges of wire mesh forming the gabion box should be of 5 to 6 mm mild steel rod or two ply No. 10 wire.

iii. The diaphragm should be at the interval of about 1 m to form small segments, which is essential to control the internal movement of rock filling, and also for ensuring additional strength to the gabion structure.

iv. Most preferably the diaphragm should be tied from the base and laid flat, to make it convenient for transportation.

Construction Steps:

The construction of gabions is accomplished under following steps:

Step (1):

Assemble each gabion unit by binding their vertical edges with the help of about 12 to 15 cm long wire ties. It is being good to make 5 ties per metre height.

Step (2):

As per plan, place few empty gabion units in position, and tic them with the adjacent boxes, using at least 5 wire ties per metre.

Step (3):

With the help of standard fence stretcher, stretch the box before filling the stones to provide effective alignment after they are tied together.

Step (4):

Fill the rocks into stretched boxes and also insert connecting wires in each cell. When the box is filled by its 1/3 portion, then four wires, two in each direction in each cell are placed. Repeat the same procedure until the box is filled by its 2/3rd portion.

During rock filling, it is always kept in view that, if rectangular stones of large size are available, then they should be preferred to fill, first. However, if stones are available in different sizes, then larger stone pieces should be placed on the sides and smaller pieces in the centre of the box.

Step (5):

When filling operation of stones to construct the gabion boxes has been completed, then tie the lid from sides and end portions. For this purpose, a half metre long ‘pinch bar’ can be used.

Computation of Construction Cost of Gabions:

For computation of construction cost of gabion structures, the following parameters are taken into consi­deration:

(a) Size of gabion box (i.e. length, width and depth).

(b) Capacity of the box (m³).

(c) Area of wire mesh in each gabion (m²).

(d) Density of wire used (kg/m³).

(e) Cost of mesh construction (Rs/kg).

(f) Labour charge per box including construction cost of wire mesh.

(g) Cost of rock filling (Rs/100 m³).

Computation Procedure:

The following steps are followed:

Step (1). Find out the area of mesh involved in each gabion box.

Step (2). Calculate the weight of wire needed per box. It is given as –

Weight = (Density of wire) x (Area of mesh in each box).

Step (3). Calculate cost per box. It is obtained as –

(Weight of one box) x (Cost)

Step (4). Estimate labour charge per box including construction cost of wire mesh, which is computed as –

(Labour charge) x (Area of wire mesh of one gabion box).

Step (5). Compute the cost of rock filling per 100 m³. It is given as –

(Cost of rock filling) x (Capacity of gabion box).

Step (6). Find the total cost of construction, which is the sum of step (3), step (4) and step (5).