Drop Inlet Spillways

Agdex#:	751
Publication Date:	05/85
Order#:	85-057
Last Reviewed:	05/85
History:	Original Factsheet
Written by:	D. Hilborn - Engineer (By-Products)/OMAFRA

Table of Contents

1. Introduction
2. Uses of Drop Inlet Spillways
3. Types
4. Design
5. Installation
6. Maintenance
7. Assistance

Introduction

A drop inlet spillway is a mechanical system which lowers water through a box or pipe structure. This system internally dissipates most of the energy produced by the water. Concrete Catch Basins, Plastic Drop Pipes and Steel Sloped Culverts are all examples of drop inlet spillways.

Figure 1. View of an erosion control demonstration project which utilizes several drop inlet spillways. Grate structure in the foreground is an inlet for the central drop pipe.

Uses of Drop Inlet Spillways

A drop inlet spillway is normally used to drop low to medium volumes of water over a sharp incline (30%). The incline height is normally greater than 1 m with no upper limit. Common applications are gully control, surface water inlets to open ditches and terrace inlets.

Advantages:

1. Once water is in the spillway it is totally contained. The water does not rely on local soil conditions for erosion resistance.
2. The spillway unit is normally prefabricated, reducing on site construction time.
3. The unit has a limit to capacity. This makes it very suitable for floodwater storage systems. Also, it normally can not reach a self-destructing flow rate.
4. The system is very competitive in installed cost with other comparable methods. This is especially true at low flow rates.
5. Flow-rate characteristics are readily available for design purposes.
6. For some cohesionless soils, a drop inlet spillway is the only proven erosion control method.

Disadvantages:

1. The entry point of the spillway normally concentrates the water-flow to a small area. This point can often plug with debris or local scouring can occur due to the high velocity of the water.
2. A proper design is required to prevent water from channeling along the sides of the pipe.
3. A head (or stage) of water is normally required to obtain full capacity of the inlet (this may make the berm height unreasonably high).
4. Unit has a limit to capacity. Once capacity is reached, another system is required to store the water or handle the overflow.
5. Spillway systems can be more expensive than comparable system for high flow rates.

Types

Drop Pipe Structure

This structure consists of two components, a vertical pipe and a horizontal pipe. The drop pipe can be square or round in cross-section. It can be constructed of concrete, steel or plastic. The limiting flow factors are the flow over the crest of the vertical pipe and the capacity of the horizontal pipe.

Figure 2. View of a drop pipe structure during construction. Note the use of anti-seepage collars on the horizontal pipe. Caution should always be taken when working in a trench to avoid injury from collapse of the sidewalls.

The flow over the crest is dependent on the circumference of the vertical pipe and the head (stage) of water over it. The inlet guard should be as non-restrictive as possible since any obstructions will adversely affect the flow rate.

The horizontal pipe is installed into the bottom side of the vertical pipe. The flow through this pipe is dependent on the head from the vertical pipe and the length and roughness of the pipe material. The horizontal pipe is normally smaller in diameter than the vertical pipe since the water running through it is under a higher pressure due to the increased head.

The kinetic energy in the dropping water is normally dissipated in three locations:

1. The bottom of the vertical pipe must break the falling energy of the water. Normally the horizontal pipe outlet is located several inches above the bottom so that the trapped water will absorb and distribute the energy. 
2. Also energy will be dissipated by the friction in the horizontal pipe. This is especially true during peak flow situations. Under most circumstances the energy is transferred from the pipe to the surrounding soil by the friction bond between the soil and the outside of the pipe. However, especially with a sloped horizontal pipe (often used on high drop structures), this friction bond should be determined to be adequate. 
3. Energy is also dissipated below the horizontal pipe outlet. The lining under the outlet must be adequate to handle the energy. Under most circumstances rock rip-rap, gabion mattresses or equivalent should be installed. 
   

Sloped Pipe Structure

This structure consists of one component, a sloped pipe. Capacity is normally determined by the length and internal roughness of the pipe. Slope of the pipe has very little effect since under most circumstances the "critical slope" (slope at which flow capacity does not increase with increase of slope) is exceeded.

Since the water is not forced into the pipe under a high head (as is the case with the horizontal pipe in a drop pipe structure) this structure has a much lower capacity with comparably sized pipes. It is used for low drop, low to medium flow applications.

The kinetic energy of the dropping water is dissipated in two locations:

1. By internal friction in the pipe. Again an adequate friction bond with the soil must exist. 
2. Below the pipe outlet. With the sloped pipe structure this area is always critical since most of the energy must be dissipated here.
   

Design

To design a drop inlet spillway one should complete the following steps:

1. Estimate the peak flow of water entering the structure. This quantity depends on the watershed's topography, soil type size, vegetation, cropping practices and water storage capacity. Technical expertise is often required for this step.

2. Measure the approximate drop and horizontal distance in which the drop structure is to be installed.

3. Evaluate whether flood water storage should be incorporated into the system. This depends on: 

   1. Size and shape of the potential flood water storage area (a highly sloped basin will not hold much water)
   2. Reduction of size and cost of drop structure when a floodwater storage is used. If an extremely long horizontal pipe is required, the economics of floodwater storage will be greatly improved.
   3. Duration of peak flow. If the peak flow occurs and subsides very quickly a flood water storage will have a greater effect in reducing the size of drop pipe required.
   4. Potential of crop damages. Some crops can't withstand a 24 hour flooding period. A smaller allowable flooding period will lower the feasibility of the flood water storage system.
      

4. Determine the type and size of structure required. Figure 3 and Figure 4 shows capacities of several types and sizes of structures. Floodwater storage is not included in these Figures.

5. Determine the height and length of the berm. The height is dependent on the head or stage of water required to obtain maximum flow of the spillway structure (see top section of Figures 3 and 4 for design) or the maximum height of floodwater storage. An additional 10 to 20 cm of height should be added to allow for settlement or shifting of the berm. An emergency spillway should be incorporated into the design. Often this spillway can be a notch of rip-rap (underlaid with filter mat) over the crest of the berm. The emergency spillway is used to handle flows which exceed the capacity of the drop pipe and floodwater storage. This occurs when the storm exceeds the design storm, when the inlet plugs, or if improper judgment of watershed characteristics occurs. 

6. If necessary, the velocity of the exiting water should be determined. If the velocity exceeds the erosion resistant capacity of the natural channel, rip-rap or equivalent will have to be added. 

7. Complete the layout of the structure, berm, slopes, inlet, etc. Consideration must be given to problems with water tracking down the outside of the pipe. Often anti-seepage collars are installed to increase resistance to external water flow (see Figure 5 for sketches of example structures). Another important feature is the design of the inlet grate. The inlet is required to stop entry of debris that has the potential of plugging the structure. It should also prevent human and animal entry. A riser type of inlet is normally recommended since it increases the overall filter area. Often the inlet should be marked with a high pole to increase visibility especially during deep snow conditions.
   
   
8. Incorporate spillway with other erosion works. The erosion must be controlled above and below the spillway to prevent eventual failure of the spillway. Other erosion works could be a grassed waterway, conservation tillage, terraces or a well designed water-course. 

Figure 3. Graph for sizing of sloped pipe and drop pipe systems for capacities below 0.5 m³/s

Figure 4. Graph for sizing of sloped pipe and drop pipe systems for capacities above 0.5 m³/s

Figure 5. Sketches of Drop Inlet Structures

Figure 6. View of riser type of inlet. The filtering area is much greater with this inlet than the flat type of inlet, reducing chance of plugging.

Figure 7. View of sloped, flat type of inlet on a concrete riser. Note rip-rap around the inlet. This type of inlet has a much greater chance of plugging than the riser style of inlet.

Installation

The following points should be followed to ensure good, failproof construction:

1. Hire a contractor who has knowledge and experience in construction of these structures or provide supervision by a qualified person. 
2. Formulate a sequence of construction. Often the spillways are constructed to grade before other works such as grassed waterways are installed.
3. Prevent or control differential settlement. Complete removal of organic matter from the fill material is almost always necessary. The pipe trench should be widened out to prevent a cavity from occurring under the compacted berm. Additional height will have to be added to the berms in high fill areas to allow for extra settlement.
4. Try to complete the work at a favourable time of the year. This is usually a time when peak flows are least expected and soil handling conditions are ideal. Also if a grassed waterway is to be incorporated into the system timing should be ideal for quick grass growth. Never complete construction during frozen or very wet conditions.

Maintenance

Any erosion control system needs regular attention to prevent any weak points and consequent failure. A checklist for spillways should be followed as below:

1. Obstructions in the inlet or the spillway should be removed. If these obstructions reoccur frequently a different inlet should be installed.
2. Watch for cracks in the berm or spillway foundation. If cracks occur, immediate repair will be required. Often the back slope will have to be decreased to prevent further failure. 
3. A path should be dug through the snow or ice to the inlet just before peak flow is expected.

Maintenance and inspection is especially important in the first couple of years after installation since the vegetation will not have developed fully and earth settlement may still be taking place.

Assistance

Some technical assistance is available from the Agriculture Engineer at your local office of the Ministry of Agriculture, Food and Rural Affairs. The local offices of the Conservation Authority and the Ministry of Natural Resources may offer both technical advice and construction supervision. Also a Consulting Engineer could be hired for design and supervision.