​Don’t let stormwater dig a hole you can’t get out of!

​Fast moving water pouring out of a pipe can quicky lead to erosion. Planning ahead and utilizing rock outlet protection can save future maintenance headaches. In this month’s article we will see where this practice is useful, consider design, and review maintenance considerations. Let’s start by identifying where the practice is helpful.
Rock outlet protection can be useful where discharge velocities from a channel, storm drain, or culvert are high enough to cause erosion. The practice can be applied for the following outlet types:

​For design of the rock outlet protection, assume the most severe soil and vegetation cover conditions. The size of the watershed and topography should also be given serious consideration. Rock outlet protection is not intended for slopes greater than 10% or at the top of cut or fill slopes.
Caution should be used if flow rates out of the discharge pipe will exceed 100 cubic feet per second (cfs) for a 10-yr.-fequency storm. Utilize the NRCS Technical Release 55 (TR 55) or other suitable method to determine peak rate of runoff. The outlet protection needs to be stable for the velocity of flow expected from a 10-year frequency storm event. While level spreaders are helpful in conjunction with outlet protections, we will focus on the design of the rock outlet. The width of the outlet should be the width of the headwall or 4 feet wider than the pipe diameter (2 feet on each side of the pipe). The elevation of the downstream end of the outlet protection needs to be equal to the elevation of the receiving stream. The necessary length of the outlet protection and the rock size can be determined from the following figure. 

Larger diameter pipes will require larger rock diameter, a greater thickness for the rock layer, and additional length for the rock channel. The rock riprap needs to be well graded and be placed to obtain a solid and compact layer. Filter or granular bedding is needed beneath all riprap to prevent underlying soil from eroding. Larger size riprap will likely require a thicker bedding layer or 2 different sizes of bedding. Geotextile use can help prevent piping of soil. Care should be taken to properly anchor the geotextile. A properly designed and installed rock outlet protection will help to reduce future maintenance needs.
 
Maintenance will help to protect the riprap, vegetation cover, and associated structural components. The following are key to proper maintenance.

• Determine a responsible party to inspect and maintain the outlet protection.
• Missing riprap should be replaced as soon as possible.
• Protect the outlet from damage by equipment and traffic.
• Fertilize and mow area vegetation to keep a healthy cover.
• Seed and mulch any bare areas that develop.
• Remove sediment and debris.

 
If properly designed, installed, and maintained the rock channel outlet protection should function for decades to come. Lack of proper outlet protection will require more extensive and costly repairs in the long run. Feel free to reach out to our office at (513) 695-1337 if you have any questions regarding rock outlet protection.
References

1. Urban Hydrology for Small Watersheds (TR-55), United States Department of Agriculture Natural Resources Conservation Service. June 1986. Web link for this publication is available at:

https://nationalstormwater.com/wp-content/uploads/2020/07/Urban-Hydrology-for-Small-Watersheds-TR-55.pdf

1. Rainwater and Land Development Manual (Chapter 6.1), Ohio Environmental Protection Agency. May 2025. Web link for this publication is available at: https://dam.assets.ohio.gov/image/upload/epa.ohio.gov/Portals/35/storm/technical_assistance/6.1_Outlet_Stablization.pdf

Article by Travis Luncan, Urban Technician

Water! Water is all over the place, and it moves wherever it wants. Water has the ability to cause major problems at high velocity during certain times such as high rain events that cause flooding. Erosion is an unfortunate result of the damage that moving water can cause. Erosion can be especially present on construction sites due to the exposed sediment and piles of material, and so it is important to know the different types of erosion and the best management practices that can be used to prevent sediment from eroding off site and into the waters of the state. In this month’s Development Digest, we dive into the most common types of erosion found on construction sites.
Generally, there are 4 common forms of erosion that are caused by the movement of water on exposed sediment on construction sites. These 4 forms are splash erosion, sheet erosion, rill erosion, and gully erosion.

• Splash erosion: Splash erosion is very simple and straight forward. Anytime a raindrop hits the ground, specifically exposed sediment, it can hit the sediment and loosen up the soil particles. This is an issue because when the soil particles get loosened, flowing water can easily carry them on downstream. Splash erosion is the first step in the process that if unchecked can lead to very large and costly amounts of erosion.
• Sheet erosion: Sheet erosion is defined as the uppermost layer of the soil being gradually removed do to some environmental factor. The most common cause for sheet erosion on construction sites is rainfall. Rain hits the bare soil loosening up the soil particles and the water then carries those particles off site through runoff.
• Rill erosion: Rill erosion is similar to sheet erosion; however, the erosion is a little more defined into smaller channels. In this type of erosion, the entire surface of the soil is not necessarily being eroded away because the water is forming into small paths and eroding sediment from those concentrated areas. Rill erosion can lead to much worse erosion if left unfixed.
• Gully erosion: Finally, the last type of erosion is the biggest and most easily seen type of erosion. Gully erosion forms from all of those little rills that have been carved into the ground by the water. Eventually, all of those rills meet somewhere and all of the water from the rills get combined into one big channel. This channel is called a gully. A gully has a higher velocity than rill erosion due to the greater amount of water flowing through, and because of this, a gully can be, and in most cases is, the deepest type of erosion. In general, gully erosion can be about 6-12 inches minimum, and at most can be many feet deep depending on the underlying soil and the different soil layers. Once gully erosion starts, it can get much bigger and bigger, and if left untouched and can cause damage to the landscape and anything that could be affected by the land being lost to flowing water.
  

During construction, it is very important to utilize the BMPs (best management practices) available to all of us to prevent erosion from happening. The most important practice available is stabilization. Whether it be through phased grading to minimize disturbance, keeping straw covering the ground during temporary pauses in work, or immediate stabilization of a site at final grade, getting exposed sediment covered up and stabilized is the best thing we can do to minimize the risk of erosion. There are also other practices that we can use if stabilization is not an option due to ongoing construction. BMPs such as silt fence, mulch berm, and filter sock are great for the perimeter of a site and can be used to slow down water and prevent any eroded sediment from moving further downhill and causing more damage. Check dams consisting of rock are great in areas of concentrated flow to slow down the velocity of water and prevent erosion. Overall, limiting the area of disturbed soil on a site can go a long way to limit the amount of erosion that may occur. Being mindful and aware of the work that we do to the land, and what effects it might have, is important. The less care we have about sediment and erosion control, the more damage erosion can do to the land and to the water, so hopefully by now having a basic understanding of the types of erosion out there, we can do our part as humans to protect the soil and keep it in place.

Have questions about sediment and erosion control, or just about erosion in general? Please contact our office and we’d be happy to answer any questions you may have!
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Article written by Urban/Ag Technician Seth Byerly

Rainwater rolls off the landscape as a trickle, stream, or river. For a light rain event the drainage channel may not matter that much. For a moderate or heavy rain events the design and condition of the drainage channel becomes more critical. A grass swale is one of the stormwater conveyance methods identified in Ohio EPA’s Rainwater Land Development Manual. In this article we will look at good design, stabilization and maintenance of grass lined swales. Many drainage assistance calls we go on involve a swale. Common issues we receive calls on are erosion in the swale, overflow of the swale, and modification/blocking of the swale. Most of these issues are preventable with proper design, building, maintenance of the swale and education of residents/HOA’s.

Design starts with looking at runoff. The area drained, soil types, land use, topography and historic rainfall data all play a role in determining the swale size and how it is stabilized. A swale should be designed to carry the peak rate of runoff from a 10-year flood before it overtops.  If the swale is designated as a Flood Route, it must be designed to handle a 100-year rain event. While it would be well beyond the scope of this article to go through all of the calculations, NRCS Technical Release 55 (TR 55) is one suitable method to determine peak rate or runoff. Grassed swales designed to protect residences and businesses need to have out of bank capacity to carry the peak rate of runoff before water can flow inside adjacent residences or businesses.

Shapes of drainage swales can be parabolic or trapezoidal. The parabolic channel closely approximates natural flow conditions, but design and construction are more complex. Trapezoidal channels are preferred where there will be a large quantity of water or high flow rates. Side slopes that are 3:1 or flatter are recommended.

Next to consider is the channel’s stabilization and resistance to erosion:
Clay soils, which are common in Warren County, offer moderate protection from erosion but good protection once vegetation is established. Sod or seed and matting are preferred to establish vegetation. The grassed swale should be vegetated as soon as possible after reaching final grade. Waiting makes it more difficult to get good stabilization. Delays in stabilizing the slopes will also cause maintenance issues that will later need fixed. Stabilize upslope areas to prevent sediment from washing down and filling in the swale.

Check dams may be incorporated to decrease water flowrate, to reduce erosion, and to allow grass to establish. The rock check dam should be lower in the center so that water does not flow around and erode the edges of the swale. Check dams are often temporary measures. It is good to plan to determine when the check dam will be removed.

For sites with prolonged flows, a high-water table or seepage problems; a subsurface drain or rock-lined waterway may be incorporated. Grassed swales should also have a stable outlet with adequate capacity to prevent ponding or flooding damage.

Beyond design and construction of the swale, owners should consider ongoing maintenance. Most important would be to ensure that residents don’t block, modify or build in the swale. Grass growth should be monitored to insure a vigorous stand of grass. Protect the swale from compaction or damage due to equipment or traffic. Fix damaged areas immediately. With good design maintenance and stabilization, a swale will protect property and prevent future maintenance needs.
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References:

1. Urban Hydrology for Small Watersheds (TR-55), United States Department of Agriculture Natural Resources Conservation Service. June 1986. Web link for this publication is available at:

https://nationalstormwater.com/wp-content/uploads/2020/07/Urban-Hydrology-for-Small-Watersheds-TR-55.pdf
   2. Rainwater and Land Development Manual (Chapter 6.7), Ohio Environmental Protection Agency. May 2025. Web link for this publication is available at: https://dam.assets.ohio.gov/image/upload/epa.ohio.gov/Portals/35/storm/technical_assistance/6.7_Open_Channels.pdf

Steep slopes on a construction site can lead to some problems when talking about erosion control and water runoff. Slopes greater than 3:1 allow water to quickly run down the slope leading to erosion and possible water damage downhill. In this month’s development digest, we discuss slope surface roughening techniques and why they are a beneficial practice on an exposed slope during construction.

What is slope surface roughening and what is the purpose? Slope surface roughening is the act of roughening up or raking the surface of a slope horizontally to create grooves, bumps, and depressions. These bumps and depressions on the slope serve multiple purposes including:

• Slowing down water velocity flowing down slope
• Trapping sediment that may wash down the slope
• Providing a better base to grow vegetation

Water running down a slope at a high velocity creates the potential for large soil erosion and damage, and by working grooves into the slope, the water is slowed down at every bump it reaches, resulting in a decreased velocity. The same idea is used when talking about trapping sediment. Like flowing water, the grooves and bumps create somewhat of a barrier for sediment moving downhill and some of the sediment gets trapped on the hill, which means the sediment is not flowing into the creeks and streams. Slope roughening is also good for vegetative growth because the depressions create a great place for the vegetation to establish. A flat, compacted slope creates a difficult surface for the vegetation to establish, and by providing depressions and loosening up some of that soil, the vegetation is more likely to survive and provide ground cover. In fact, according to the Ohio Rainwater and Land Development Manual, any slopes that are steeper than 3:1 are required to be grooved or tracked if vegetation will eventually be installed, so you might as well create those grooves before construction starts for soil erosion control as well!
 
Two main methods to roughen the slope are commonly used:

• Dragging a bucket with teeth along the slope
• Driving tracked machinery horizontally across the slope
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Using a bucket with teeth to drag along the slope is a very simple and effective method to create the grooves and depressions needed to slow down the water velocity. Using tracked machinery is also a good method that can lead to uneven grooves along the slope that will also help slow down water velocity. Both methods can be an effective way to roughen up the soil, however the track method may not be as good of an option depending on the soil because it could lead to soil compaction, which would not be good for vegetative growth. It is best to determine the soil type before choosing one of these methods.
 
Slope surface roughening is a tried and proven practice that works effectively to assist in the battle against soil erosion and water runoff. Whether using a bucket with teeth, equipment tracks, or any other method that roughens the slope, the creation of the grooves and depressions is a great practice to protect the soil and the water during the construction phase of a project. More information and specifications can be found in the Ohio Rainwater and Land Development Manual which is linked below.
 
Have questions about this practice, other best management practices, or other questions in general? Feel free to reach out to our office by calling us at (513) 695-1337 or emailing at [email protected].
 
Ohio Rainwater and Land Development Manual: https://epa.ohio.gov/wps/portal/gov/epa/divisions-and-offices/surface-water/guides-manuals/rainwater-and-land-development
 
Article Written by Seth Byerly, Urban/Ag Technician