Not all riprap is created equal

The Glen Oak Hills stream near Oakwood Village is an example of what an undisturbed stream looks like; it also shows 40-year-old riprap.  The upstream portion is natural, while the downstream portion has riprap on the left bank.  The stretch of riprap looks good, because it has large stones, uniform in size, and carefully stacked.  So whether riprap looks good depends on the craftsmanship and design.  It can be done very badly, or skillfully.

Residents seem in agreement that the stream should be open.  But will it really be "open," if riprap is used?  A small stream's water will disappear if riprap is laid thickly on the streambed.   If our stream is riprapped, you won't see any water, except during heavy storms or spring runoff.

I support Option 3 (keep streambed mostly natural) because:
  • Far fewer trees are cut, and residents want to save the trees.
  • Water will still be visible in pools.  With the other options, water pools will disappear, and wildlife will not be able to drink. Riprap is a desert for wildlife. Residents value the wildlife. 
  • Option 3 is the most natural and the most pleasing. Stormwater greenways have great potential as wildlife refuges, quiet places for human enjoyment, and for education of schoolchildren.  But they can never realize this potential if we destroy them.
Those are my conclusions based on a rainy day of scouting.  Below are my observations.  I'll take you there.  You can click on a photo to enlarge it.

Glen Oak Hills Neighborhood stream

Thanks to Kathy McElroy for finding a stream in the Glen Oak Hills neighborhood that's a good example for us to study.  It's located on Masthead Dr. near its junction with Island Dr.  If you stand on Masthead and look West, upstream is on you left.  Upstream, the stream is natural, no riprap. There has been little or no disturbance of the streambed.  The boulders are all rounded from the glacier.  Many of them are granite. 
Upstream portion--natural streambed, no riprap

Upstream--we can tell this isn't riprap because many large boulders are granite, and large trees come to the very edge of the stream.

This part of the stream looks good, so it's important that readers realize that it hasn't been riprapped.  Riprap was not needed, because the stream is not incised into a ravine.

Downstream from Masthead Dr., the stream now runs in a ravine, so the bank needed reinforcement. In the photo above (W bank,downstream), you can see a steep bank of riprap limestone. (The riprap blocks have a greenish-yellow or ochre color.)

Above: As you look further downstream, the riprap has ended.  It has given way to the natural, rounded boulders you see here--a natural streambed.

The riprapped bank on the downstream west side, now about 40 years old, looks good for three reasons:
  • The stones have weathered some, and are covered with moss.
  • The stones are large, averaging about 2 feet (some 3 ft)--with relatively uniform sizes
  • The stones are carefully stacked
Flowing water is still visible in the riprapped section because either the riprap was not thickly laid, or sediment has filled in the spaces between rocks--or it's possible that no riprap was laid in the streambed.  This stream also has a lot more water than our creek.

Ravines between Glenway Golf Course and Bikeway

The straight ditch of riprap running parallel to the bikeway was finished a year ago. 

No water visible here during a rainstorm. A biological desert.

There are many large blocks, but here's how it differs from the riprap at Glen Oaks Stream.  It...
  • Has some large blocks but a lot more small stones.  Sizes not uniform.
  • Was not laid carefully--just jumbled in place--a pile of rubble.
  • Was laid more thickly.
This stretch of riprap is ugly because of poor design and poor execution.  Basically, overkill.

But the key point I want to make is--the water goes under the stones!  The water flows through spaces below the thick layer of rocks.  I visited during the rain, and I saw no water flowing anywhere on riprap, except for...

Wildlife needs variety plus pools for drinking.

...this spot.  And the water pools here only because the outlet in the rear forms a dam.

Riprap, with unstable blocks and sharp edges, is more hazardous for children.

This section of riprap along the bikeway shows how haphazardly the stones wrere placed. Lack of uniformity in size makes it worse.

Metal Rock vanes

Option 3 for our greenway suggests 3 metal rock   vanes, or dams, at the E end.   Imagine these metal dams as rock dams in our greenway:
Below the Glenway Golf Course, in a ravine leading to the bikeway, riprap and metal dams were constructed about 12 years ago.  The goal was to control erosion.  The riprap here looks better because it was not laid so thickly.  Because it's shady, moss has had a chance to grow on the stones.  Update: This might be the best example of what our greenway might look like under Option 2: DRY.

It's important to note: during the rain, NO WATER WAS VISIBLE.

Pools for wildlife

Wildlife need places to drink and variety.  Pools are habitat for plants and aquatic insects--they attract small animals, which attract larger animals.

Our stream is starved for water because there aren't enough places for water to sink in.  If we can create enough rain gardens upstream, there is hope for a natural stream, and a chance to show our kids how to restore nature. 

But for now, the stream is dry most of the time.  But it's possible even now to restore some small pools.  If shallow basins are scooped out, and a waterproof barrier is placed under the little basin, water will remain for a few days, attracting more wildlife.

An ideal place for pools would be above or below each of the three dams suggested for Option 3.  Imagine a little waterfall, rushing into a pool!

At the International Crane Foundation, a wetland for the Whooping Crane exhibit was created in this way, even though the "wetland" was far above the water table.

For this project, we have a chance to think about how to realize the wonderful potential of our stormwater greenways.  It's a slow process, but a pilot project here would be good for the whole city.

What Option 1 (bury stream) will look like

This is Westmorland Stream, just below Tokay Blvd., buried long ago.  Looking toward bikeway.
Because the ravine is partially filled to bury the pipe, it is shallower than before.

For a slide show of these photos and more, click here.  Move cursor off screen to see photos full screen.


  1. What risk is there of increased mosquitoes and mosquito-born diseases if standing water is significantly increased?

  2. There is a risk of increased mosquitoes in certain kinds of standing water pools. For this reason, the city treated the pool at the east end of the greenway last summer. Normally, fish, tadpoles and other small critters eat the mosquito larvae if the stream has enough water. Probably, we don't have those critters. But on the other hand, if there's a storm, it will flush out all the mosquito larvae. So, if the pool flushes or dries up in less than two weeks, no mosquitos. Otherwise, mosquitoes. That's why mosquitoes are so variable from year to year--they need just the right conditions.

    Riprap doesn't eliminate the risk of mosquitoes. Water could still pool, but out of sight, in the cavities below the rocks. When this happens, there's no way to treat the invisible pools.

  3. Monitoring for mosquitoes in the little pools of the greenway would make an ideal project for a middle school student. They keep track of how long the pools have existed, use a little dipper to strain out larvae, to see if any are present. Learning opportunity! Turn a liability into an asset. But we kill the pools, and the opportunity is gone.


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