Outfall Protection

What exactly is outfall protection?  Outfall protection is an essential element of proper culvert installation and can certainly extend the life of your culvert.  Outfall protection most often consists of rip rap or other large aggregate placed at the end of the culvert to intercept and absorb the energy produced by the water exiting the culvert. 

Rock Outfall Protection

As water flows down a ditch or stream, it is gaining speed or velocity.  This velocity increases once the flow of water is concentrated in a culvert.   Culverts also tend to have more smooth edges, or less roughness than the ditch or streambed to slow this water down.  Once this water exits the culvert, it is moving relatively fast with lots of power ready to move some soil and cause erosion.  In addition to scouring out the channel down from the culvert, this water tends to swirl around as it leaves the culvert and can eventually wash out the culvert, costing you money to come in and re-install your culvert. 

Outfall protection intercepts this flow and spreads it out, thus reducing the speed of the water and its erosive power.  Outfall protection can be as simple as some old bricks, busted up concrete, old tires, or large rock.  On cross-drain culverts, where you are transferring runoff from across the road, you may want to not only put outfall protection under the end of the culvert, but also on the bank adjacent to the culvert exit, to help preserve the bank and prevent excessive erosion.   On culverts used for creek crossings it is often important to not only have outfall protection on the culvert exit, but also on the sides of the banks on either side of the culvert exit.  This will prevent swirling water exiting the culvert from eroding the stream banks and eventually blowing out the sides of your culvert. 

Outfall protection can also be an important component of wing ditches on some of your woods roads that are generally only used during forest operations.  In these instances, you may not need to invest in large, rock aggregate, but instead can prevent excessive erosion by placing slash or brush at the outlet of the wing ditch.  Another method of providing outfall protection on wing ditches could be using vegetation.  If this is an area that doesn't receive a large amount of runoff, it is a good idea to use our seeding chart on page 67 of the blue book. 

In conclusion, outfall protection is essential in protecting your culvert, ditch, stream, wallet, and water quality in general.  Outfall protection will save you money by not having to re-install culverts and reduce the frequency that you have to come and pull your ditches.  Outfall protection protects streams health and preserves water quality by reducing the amount of sediment traveling downstream that result from stream bed and stream bank scour.  

What to consider: Culverts

Poorly installed and improperly designed culverts can lead to excessive erosion, damage to streams, high maintenance costs, and road failure.  In order to prevent these issues it is important to consider culvert type, diameter, length, and location.

Excessive sedimentation from a poorly planned culvert

Type
There are two types of culverts: stream crossings and cross drains.  The first type, a stream crossing culvert, is generally placed in a location where a permanent stream crossing may be necessary, since it is often too costly both economically and ecologically to remove. These culverts allow both stream flow and aquatic wildlife to pass underneath the road.  The second type, cross drains, are used to transport upland runoff, accumulated in road ditches on the upland side of the roadway to the lower end where both flow volume and velocity can be dissipated.

Mouth of a cross drain culvert

Diameter
Culvert diameter is determined by both the soil type on the site, how steep the site is, and the acreage of the watershed that the culvert drains.  Taking into consideration these factors ensures that the culvert is sizable enough to handle the maximum volume of water that it may encounter.

Culvert size chart from page 51 of the Texas Forestry Best Management Practices Handbook 

Length
Culvert length is dictated by how wide the road is.  Culverts should be long enough so that each end extends at least one foot beyond the edge of the fill on either side.

Location
Culvert location is paramount in dictating the longevity of the culvert.  In stream crossings, culverts should be placed in a section of stream where the channel is straight and the stream bed is firm.  Cross drain culverts should be spaced out depending on slope.  Cross drain culvert spacing can be determined using the following formula:

Culvert spacing = (400'/slope%)+100
         *Slope in percent is expressed as a whole number (i.e. 15%=15)
               
            Example: Spacing = (400'/15)+100'
                          Spacing= 127'

Conditions
In addition to factoring in culvert specifications, it is also extremely important to factor in current weather conditions and trends.  Culvert installation should be done when stream flows and chance of rain are low.  Ideally, the entire installation process should be completed before a rain event.

UP NEXT:  Installing your culvert