Katy kindly gave me permission to reprint her Mt. Times commentary. She simply points out the reality of building along wild, rivers.
Riprap is a band-aid
By Katy Hanna
This may fall on deaf ears considering how many excavators I have already seen along our rivers, but I wanted to offer a different perspective on flood control to discourage riverfront homeowners from installing riprap along their stream banks. I understand the desire to protect one’s home from potential flood damage but everyone should make sure they are well informed about how riprap functions in rivers before they hire an excavator and take a deep sigh of relief, believing their properties are safe.
For those of you who are unfamiliar with riprap, it is the placement of very large boulders on steep stream banks, which aims to armor the bank against flooding. It is essentially an armored levee. What riprap does is speed up the velocity of river flow so that it passes by that particular spot swiftly without lingering and spreading over the stream bank. The rock creates very little friction, so the water flows by much more quickly than it would if it encountered a natural stream bank with vegetation, wood, and natural substrate. Sounds good you say? Isn’t that the goal, to get the river by my house as efficiently as possible? That may be YOUR goal but let’s take a couple steps back and look at the big picture. The first thing that comes to mind is how does the riprap I put in front of my house affect my neighbor’s stream bank downstream? The answer varies because river dynamics are notoriously hard to predict, but generally speaking, you are not doing your neighbor any favors. Most likely the river will approach your neighbor’s stream bank with increased force and erode it away at an unnaturally accelerated pace. So why doesn’t my neighbor join the club and riprap their bank too? If your neighbor and your neighbor’s neighbors all follow suit, what is the inevitable result? We are left with a river that resembles a ditch with uniform 20-foot high boulder walls on either side for miles and miles. If you have never seen a river like this, look up images of the Los Angeles River on the internet, it no longer resembles a river at all but rather a concrete-lined ditch.
When rivers are converted into ditches they become very efficient water transporters, but they lose most of their inherent functions as ecological systems. A river is not simply a water passageway in the same way that a forest is not simply a stand of timber. Rivers are living ecosystems that are constantly changing. They move sediment and organic matter from mountaintops to the ocean and they are home to countless forms of life from fish to otters. In fact 80% of all wildlife species depend on rivers & riparian areas for their survival. Riprap removes riparian vegetation & shade from rivers, therefore removing food sources and cover for wildlife. It creates a sort of desert, where very few species can survive.
We have to put these flooding events into context. River flow fluctuates greatly with the seasonal inputs of precipitation and snowmelt. Most years the high flows only reach the so-called ordinary high water mark but in other years rivers receive so much additional water from precipitation and snowmelt they overtop their banks and flood. The degree of flooding is directly related to the combination of existing snow pack, the amount of precipitation in a short time frame (say 24-72 hours), and the temperature at higher elevations. A given river’s floodplain has been established over thousands of years of flood events. It is very difficult for the average person to see where a 100-year floodplain lies. Only experts who can read the landscape, often with the aid of aerial photos and lidar imagery, can identify the true extent of a floodplain. This is one of the reasons why so many of us have unwittingly built our homes in floodplains or have never fully grasped what the purpose of a floodplain is. Floodplains are the mechanism by which a river spreads out over the landscape and slows itself down, all the while adding important sediment and nutrients to the surrounding forest floor. So while floodplains slow down, spread out, and dissipate a river’s energy, riprap confines a river to a narrow channel, and INCREASES a river’s energy and speed.
At best riprap is a band-aid, a temporary fix that gives homeowners a false sense of security. The Army Corps of Engineers’ response to the 1964 flood was to channelize the rivers with bulldozers, building levees that made flooding worse and encouraged property owners to build up infrastructure in the floodplain. It is well understood that these “flood control” methods are outdated and ineffective. The current science in river restoration is to remove dams (once widely hailed as flood control mechanisms) and to restore floodplains, returning rivers to a more natural state. In other words, undoing the damage of the Army Corps’ antiquated techniques.
What can be done to prevent natural catastrophes like flooding? In a word, nothing. The Sandy, Zigzag, and Salmon Rivers all originate on the side of an active volcano which has the power to make it’s own localized weather patterns. The glacial melt and loose, volcanic soils descend from the mountain to the ocean via these powerful, wild rivers. When conditions are ripe for flooding, there is nothing that will stand in their way, not houses, not concrete, and certainly not riprap. I think most mountain residents understand that living here comes with a higher threat level from natural “catastrophes.” We are more susceptible to forest fires, snowstorms, windstorms, and yes, flooding. We understand that living here has both privileges and risks. We cannot mitigate every potential threat from nature with technology. We cannot cut down every tree in the forest so they won’t catch on fire or fall on a citizen. We cannot plug the active volcano that soars above us to prevent an eruption. We live among uncertainty, unpredictability, and the wonder of what nature is capable of.
Flooding can be scary, it can be destructive to our infrastructure, and it can disrupt our lives. The best thing we can do is work with nature, learn from nature, and mimic nature, rather than trying to control it, especially when our efforts have failed time and time again. Trust me, that riprap won’t hold for long…the rivers decide their own course, engineers will never be able to control them.
