Riprap--pros and cons for bank reinforcement

Stopping erosion is one of the goals of the greenway project.  It's likely that City Engineering will propose riprap as the method for stabilizing the stream bank.  Riprap is essentially stone rubble that is laid down in a jumble to line a channel.

In one of my previous posts, I showed a photo of riprap installed only six months earlier along the SW bikeway. Lisa Coleman of City Engineering pointed out that as riprap ages, it begins to look better.  She submitted the photo below to make her point.

Riprap about 12 years after construction

Lisa told me: Here's "a photo of another riprap-lined channel in the Forest Hills Cemetery that was installed in the mid '90s. This photo was taken last summer (2008). The look of the riprap does change over time as it weathers, and it does fill in a little with vegetation (not always the most desirable vegetation, but vegetation still) over time."

I agree that Lisa has a point--that we have to think about how the riprap will look in, say, 10 years.

Based on that, I'm going to modify my pro-con list that I published earlier.

Advantages of riprap
  • Low cost initially
  • Low maintenance (if rocks are large enough)
Disadvantages of riprap in our greenway
  • Questionable aesthetics--looks like rubble at first, later looks better, with more vegetation.
  • Our stream requires relatively large stones because of large storm flow. The larger the stones, the more problems for children and the harder to fit into a narrow ravine.
  • Too bulky for use in tight places.  Cannot follow the twists and turns of our present stream.  We'll end up with a ravine partially filled in with stone.  Dangerous in the first years for children--who can trip or twist ankles
  • Emplacing riprap of large stones may require large equipment that will damage the area
  • The stream will become dry most of the time.
Let me explain this last point.  The riprap will be very porous, especially until all the spaces fill with sediment.  Hence the stream will flow underground more of the time.  Children like to play in water and in puddles.  For the first 5-10 years, there probably won't be any puddles except during heavy rain.  I'm not sure about after 10 years, but I think puddles will still be less likely.  In other words, the trickling stream we have in the upper portions will be gone.  And later if we can restore the groundwater, the stream will be less likely to come back.

The bulkiness of riprap means the channel will have to be wider, causing more trees to be cut.  Where the channel now curves around trees, it will be straightened, again eliminating more trees.

We get what we pay for.  If we take the cheapest option, we get the ugliest outcome.  I personally would like to hold out for something more attractive.   I would like to see our restored steam include:
  • Natural curves and natural variety.  All parts of the bank don't have to be the same.
  • A few little plunges that make sounds of rushing water
  • A few pools
  • Banks that are terraced, perhaps reinforced with stone
  • Woodland wildflowers and ferns growing on these terraces.
  • A narrow pathway, perhaps just dirt or stepping stones
  • As many trees preserved as possible
Most of these features are impossible with riprap--so that's why I am against riprap.  Riprap and landscaping are incompatible in tight places.

The following photos show new riprap at the corner of Fish Hatchery Rd. and West Beltline Highway, during a prolonged light rain.

No comments:

Post a Comment

Please feel free to comment on the article above, or on other watershed issues.