Gully Erosion Control
Table of Contents
Throughout Ontario, especially in areas with deep sand plains, high stream banks or long sloping valleys, extensive gully erosion problems occur. Large quantities of valuable agricultural soils are lost each year to gully erosion.
Gully erosion is an advanced stage of rill erosion where surface channels have been eroded to the point where they can not be smoothed over by normal tillage operations.
Often, gullies can be prevented if good land conservation measures are practiced on the farm. Good tillage and cropping practices increase the absorptive capacity of the soil resulting in less run-off and also protect the land surface from erosion. Surface and tile water should be conveyed from lands through proper waterways so as not to create potential gully problems. Buffer strips should be located at potential gully start points such as open ditches or deep depressions.
The saying "A Stitch in Time Saves Nine" is also valid for gully erosion control. Often a potential large gully problem can be solved if discovered and controlled early in its formation.
Increase in flow of surface (or subsurface) water
An increase in water flow can be due to several reasons such as improvement of drainage channels upstream, removal of water holding areas upstream (such as a swamp or bush), enlargement of the watershed or a change in tillage and cropping program in the watershed.
Decrease of soils resistance to erosion
There are several ways that a soils resistance to erosion can be reduced such as elimination of a vegetative cover, breakdown of soil structure due to poor tillage and cropping practices, constant saturation by a trickling tile or compaction by tractor wheels.
Sloughing and mining at the bottom of the gully
Sloughing occurs at the base of a gully due to a strong spring, a constant saturation of the area by a pond or the eroding power of a waterfall from a higher to lower elevation, e.g. cultivated field to eroded channel. As the undermining process continues, the gully head moves upstream and enlarges in width and depth.
In the design of a control system for a gully problem the following steps are to be followed.
After this information is obtained, a control system can be selected. There are two types of control systems available: (a) the low to medium gradient systems utilize soil linings, drop structures or a buried pipe to run the water down the gully slope; and (b) the high gradient system uses drop pipe inlets to carry the water to a satisfactory outlet.
If the gully slope is small (less than 5%) and the available channel is quite wide (more than 20 feet) a grassed waterway can carry a medium to large flow of water.
Figure 3. An established grassed waterway controlling gully erosion in a corn field.
Initial establishment of the vegetation can be a problem. Maintenance practices such as fertilization and mowing of the vegetation as well as land management practices above the gully area to prevent sedimentation of the waterway are necessary for this control system to be successful.
Grassed Waterway With Drop Structures
If a channel slope is too steep (5-10%) for a grassed waterway, a series of drop structures can be used to reduce the effective slope of the waterway below the upper limit of 5% for a grassed waterway installation. Drop structures can either take the form of a chute spillway or a grade control structure.
Chute spillways are constructed with rock riprap, interlocking or cable connected concrete blocks or other materials and are designed to carry a concentrated flow of water down an inclined structure.
Figure 4. A chute spillway absorbs the energy of the flowing water.
They blend in well with grassed waterways since their shapes are similar. These structures must be underlain with a filter cloth material.
Grade Control Structures
Grade control structures are a type of drop structure that reduces waterway grade by providing vertical drops up to 3 feet at selected locations along the channel.
Figure 5. Gabion basket grade control structures move water down slopes.
There are two main types of grade control structures used in agriculture: (a) the gabion basket type; and (b) the wood log type. These structures should always be backed with a filter cloth material to prevent soil from washing through and causing failure. For further information on grade control structures see Factsheet Gabion Basket Drop Structures Along Waterways.
If the channel slope is in the range of 10-25% throughout a significant portion of its length, the entire steeply sloping section could be lined with an erosion resistant material. Acceptable lining materials are rock riprap, gabion mattresses and interlocking or cable-connected concrete blocks. These structures must be underlain with a filter cloth material. An inflexible channel lining such as poured concrete does not have the ability to move with the base material shifts caused by settling, frost heaving or possible erosion. Thus, concrete is not recommended as a waterway lining.
The velocity of water exiting this structure may be excessive, causing further erosion problems downstream. A deceleration device such as large rock riprap may be required.
Surface Water Inlet and Buried Tile Line
A surface water inlet such as a catch basin or perforated riser pipe connected to an underground tile line will remove run-off water from small watersheds.
Figure 6. A perforated riser pipe which intercepts surface water and moves it underground.
Most gully gradients are acceptable with this system; however, the recommendations regarding maximum and minimum grades in OMAFRA Publication 29, Drainage Guide for Ontario should be adhered to. A clay berm should be constructed slightly downstream of the surface inlet to pond the water and allow the surface inlet to operate at full capacity.
When the topography of the land is suitable, a larger berm may be constructed that will pond water for periods up to 24 hours.
Figure 7. A floodwater storage (water and sediment control basin) holding back water.
The construction of this floodwater storage for smaller watersheds (or water and sediment control basin) results in a significantly reduced outlet tile size. Watershed areas up to 50 acres can feasibly be designed using a floodwater storage system. Proper design of this system is critical since failure could cause dramatic erosion problems.
When excessive gully slopes are encountered, drop pipe inlet systems provide acceptable solutions to erosion problems.
Drop pipe inlets are normally used for high drop, high gradient locations. A concentrated flow of water is carried through an enclosed pipe structure made of steel, plastic or concrete to a satisfactory outlet. Most of the components of the drop pipe inlet, if steel or plastic, can be prefabricated to reduce field construction time. A clay berm is constructed in conjunction with the drop pipe inlet to allow the run-off water to pond at the intake and force the pipe to flow at full capacity.
Figure 8. A drop pipe inlet system with a high berm to allow temporary floodwater storage.
The structure should be constructed similar to a dam except instead of holding water, it holds back soil. For further information on drop pipe inlets see Factsheet Drop Inlet Spillways.
The following types of drop pipe inlets are commonly used in agriculture:
Drop Pipe Structure
This structure consists of a vertical pipe fabricated to a horizontal
pipe which is installed at approximately 1% grade to the outlet.
Figure 9. Sketch of drop pipe structure.
Although this type of drop pipe can handle elevation differences up to 10 feet it is preferable to limit the drop height to 5 feet for safety construction reasons.
High-Drop Inlet Structure
The high-drop inlet is similar to the drop pipe structure except that the vertical pipe is about 5 feet high and the horizontal pipe follows the surface of the gully once it intercepts with the gully face.
Figure 10. High-drop inlet structure with seepage drain.
This structure is used for steep gully drops greater than 5 feet in height.