Show with a diagram the different components of a permanent soil conservation drop structure? What role is played by each component?


Drop Spillway

It is a weir structure, in which flow passes through the weir opening, fall or drops on an approximately level apron or stilling basic and then passes into the downstream channel. Its use is limited to a maximum drop of 3 m. It 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 flow of water, causing deposition of sediments and thus filling the gully section. Sometimes, the drop structures are also used at the gully head to pass the flow safely and controlling the gully head. The different components of drop structures are shown Fig. 8.1.

Components and Functions:
  1. 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 made at the top in the headwall for easy water conveyance. Rectangular notch is most commonly used. The size of the notch should be sufficient to allow the water very safely.
  2. Head Wall Extension: It is the extended portion of head wall into the gully sides. It permits stable fill and prevents piping (due to seepage) around the structure. Its main function is to provide structural strength against sliding of the structure and also to prevent the flow of water from the sides of the drop spillway.
  3. Wing Walls: These are constructed at the rear end of the structure with some inclination, usually at 45o 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. They provide stability to the fill and protect the gully banks and surface.
  4. Cut-off Walls: These are constructed to provide structural strength against sliding of the structure. They increase frictional resistance of the structure which opposes the force causing the slide.  In other words, cut-off walls act as a key for the structure; prevent piping under the structure besides reducing uplift and sliding.
  5. Toe Walls: Prevent undercutting of apron.
  6. Side Walls: These are constructed in the side along the gully walls. They guide the water and protect the fill against erosion. The function of the side walls is to prevent splashing of water over the gully banks and also to confine the water flow within the apron.
  7. End Sills: These are the elevated portion of rear end of the apron. Its main function is to obstruct the water from directly moving into the channel below. They also raise the tail water level to create hydraulic jump and to dissipate the energy of the flowing water.
  8. Longitudinal Sills: These are constructed in the apron section. They are constructed lengthwise parallel to the side walls. The sills are useful to make the apron stable.
  9. Apron: It is one of the main downstream components of the straight drop spillway as it receives the gully flow with high velocity and changes the flow regime so as to minimize the soil erosion on the downstream channel. It includes several elevated blocks to make the apron surface rough. This feature of apron is responsible for dissipating the maximum kinetic energy of falling water by creating hydraulic jump. As a result the velocity of outflow water is significantly reduced.

Fig. 8.1. Drop spillway

The drop structure is used to control the velocity of runoff in a channel by allowing the water to fall from higher elevation to much lower elevation. The main three purposes of drop spillways are as follows:

  1. To provide a transition between a broad or flat waterways and ditch or gully section.
  2. To raise the flow line to allow formation of sufficient soil depth for vegetative growth where bottom of the gully is at risk.
  3. To raise the flow line of the waterway so as to provide drainage in case of wet waterways.

Uses of Drop Spillway

  1. To control gradient in either natural or constructed channels, To control tail-water at the outlet of a spillway or conduit.
  2. To serve as a reservoir spillway where the total drop is relatively low.
  3. To serve as inlet and outlet structures for tile drainage system in conjunction with gradient control.
  4. To use as grade stabilization in lower reaches of waterways and outlets.
  5. To use as erosion control, to protect the roads, buildings etc.
  6. Straight drop spillway as an outlet in tile drainage system and for releasing the irrigation water into the field in irrigation system.
  7. In the reservoir, for letting out the water through low height drop spillway of less than 3 m,
  8. For controlling irrigation in the water distribution system and
  9. As an outlet for disposing surface water from large areas, especially with drainage ditches.

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