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.
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| 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.
