A loose jumble of large stones. While riprap is commonly used in Madison because it's inexpensive, it has numerous disadvantages:
- Not aesthetic--looks very disordered
- Not entirely permanent--stones can shift or wash downstream
- Too thick for vegetation to grow through
- Bulky for use in tight places
- Dangerous for children--who can trip or twist ankles
This riprap along the SW Bikeway is an eyesore
More costly than riprap, and commonly used in Madison. It can be very permanent, but under some circumstances can shift or crack from freeze-thaw, or from water pressure behind it.
Poured concrete looks like a gutter.
Articulated concrete blocks
Some blocks can be laid by hand, and remain in place because they have interlocking parts. Other systems use blocks linked into large mats by cables. I don't know whether they make small mats, but large mats are laid by cranes--out of the question here. Articulated blocks are inexpensive, and resist cracking and shifting. Some blocks have holes through which vegetation can grow. In photos like this one, usually the vegetation looks ratty.
Natural stone with masonry
Tim Kessenich, owner of a property through which the stream flows, has used this technique. It looks beautiful, but requires some maintenance--for example, when a growing root causes a block to shift. Obviously the labor involved makes this an expensive option. But I think we should get an estimate.
A technique recently employed along Pheasant Branch Creek in Middleton (Wis. State J. 8/9/09). The masses of tangled roots from a dead tree are salvaged and moved to the bank that needs reinforcement. I'll investigate this--but it looks bulky and not very aesthetic--and you need a source of roots nearby.
There are a few trees in the ravine growing in the stream bed or very close to it. Trees are nature's way of controlling erosion. So why not leave them, and combine them with other kinds of bank protection? Trees could be damaged during construction, and will eventually die, requiring the gaps to be filled. But leaving them for now could save some construction and landscaping costs.
Which method is best?
In choosing, we have to consider:
- Future maintenance of stream bed and sewer
- Aesthetics & landscaping (natural stream curves, vegetation, surface appearance)
- Damage during construction to existing contours & trees
- Safety for children
It may be a mistake to limit ourselves to one technique. Let's instead consider a combination. For example, for the stream bed and places where erosion is severe, use articulated blocks. Other problem spots might employ low walls of concrete, or concrete blocks, or masonry--set back as terraces. In much of the ravine not threatened by rushing water, we could plant ostrich ferns donated by neighbors, and irrigate them during dry times with soaker hoses from rain barrels. All these techniques could work around the few trees--close to the stream--which are left in place.
This combination of masonry, concrete, and curves looks good.
I've heard talk of using techniques--like articulated blocks--that allow vegetation to grow through. But photos I've seen usually show ratty-looking vegetation. Remember, our ravine is shady, so not many plants will grow there. We'll have to do some careful landscaping with good soil. Don't just assume that plants are going to look at concrete blocks with holes in them filled with gravel, and say "whoopee!"
For more information on articulated blocks Photo of a crane laying a mat of blocks: www.flickr.com/photos/40020160@N03/3712980141/