Drop Inlet Spillways
Table of Contents
- Introduction
- Uses of Drop Inlet Spillways
- Types
- Design
- Installation
- Maintenance
- 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:
- Once water is in the spillway it is totally contained. The water does
not rely on local soil conditions for erosion resistance.
- The spillway unit is normally prefabricated, reducing on site construction
time.
- 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.
- The system is very competitive in installed cost with other comparable
methods. This is especially true at low flow rates.
- Flow-rate characteristics are readily available for design purposes.
- For some cohesionless soils, a drop inlet spillway is the only proven
erosion control method.
Disadvantages:
- 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.
- A proper design is required to prevent water from channeling along
the sides of the pipe.
- A head (or stage) of water is normally required to obtain full capacity
of the inlet (this may make the berm height unreasonably high).
- Unit has a limit to capacity. Once capacity is reached, another system
is required to store the water or handle the overflow.
- 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:
- 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.
- 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.
- 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:
- By internal friction in the pipe. Again an adequate friction bond
with the soil must exist.
- 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:
-
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.
-
Measure the approximate drop and horizontal distance in which the
drop structure is to be installed.
-
Evaluate whether flood water storage should be incorporated into
the system. This depends on:
- Size and shape of the potential flood water storage area (a highly
sloped basin will not hold much water)
- 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.
- 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.
- 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.
-
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.
-
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.
-
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.
- 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.
- 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:
- Hire a contractor who has knowledge and experience in construction
of these structures or provide supervision by a qualified person.
- Formulate a sequence of construction. Often the spillways are constructed
to grade before other works such as grassed waterways are installed.
- 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.
- 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:
- Obstructions in the inlet or the spillway should be removed. If these
obstructions reoccur frequently a different inlet should be installed.
- 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.
- 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.
For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca
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