blog post by jonas procton, design engineer
Once Upon a Time…
Once upon a time, there was a stream. It was not the biggest stream, but to the fish, beavers, and snakes that called it home, that did not make it any less special. Perhaps it was the water, kept cool under the shade of evergreens even through the summer. Or maybe the meandering banks, winding their way through a deep wooded valley blanketed under the soothing still of a comforting quiet. Whatever it was, something just made this brook a cozy spot to call home.
One day, things started to change for the brook. People started stacking big rocks, until eventually there was a small stone dam in the middle of the brook! The water in the brook could not meander like it used to and was forced to turn into a big pond just to get over the dam. Because of the dam, the water in the brook had to sit around in the hot, hot sun. The water was not as cold as it used to be, and the brook became a little sad.
As time passed, people wanted to build a road to get across the brook, so they installed a culvert. A culvert is a pipe or a tube that helps a brook keep flowing, but also can support a road on top of it. They do a similar job to a bridge. Some culverts are helpful as they allow the brook to flow like it normally would.
Some culverts though… some culverts are are too small. Much too small! Small culverts cost less and are easier to install but all the water cannot fit inside, so it ends up making another pond on the upstream side of the culvert. The small culvert also makes a traffic jam for critters trying to swim up or downstream, sort of like going from a wide-open highway to a one lane dirt road.
In our story, the culvert in the brook was much too small. The fish would have to wait in line if they wanted to go through it. Some days when the water was too low, they could not even go through at all! The fish would have to wait and wait in the hot, hot water just to get through. That made the brook very sad…
How to Get to Happily Ever After
Fortunately, years later, scientists and engineers learned a lot about rivers since people put a dam and a culvert in the brook. There are a few rules or Stream Crossing Standards that help determine how big to make a culvert to keep a stream flowing like normal and keep critters moving along with it.
One of the most important things: make it wide and tall, so it feels “open”. At brooks like the one in our story, our scientists and engineers take measurements at what is called a “reference reach” (a natural- section of the stream) to make an estimate of what the brook looked like before it was damaged.
One of the more important geometric metrics is how wide the stream is from top of bank to top of bank. This measurement is known as the “bankfull width” and shows how wide the stream would be to hold water during a really big storm – the type that might come only every couple of years. When we design a replacement culvert, we aim to be at least 20% wider than the bankfull width, just to be sure. This makes sure that there is enough space for fish, salamanders, and even teeny tiny worms to get through the new culvert under all but the biggest of floods.
We also put at least two feet of native river bed materials on the bottom of the culvert to let animals get across the culvert safely. One other important rule is to make sure that the new culvert is both wide and tall enough so that it doesn’t “feel” so confined as to deter critters from passing through. This is based on a rule called the “openness ratio,” where we can do a calculation that tells us if the culvert will feel too long and skinny.
With these rules as our guide, we do our best to design a new culvert and to remove the stone dam in order to help the brook from our story transform back closer to its fairy tale beginnings, all with the hope that the critters that live in the brook will get to live happily ever after.
This story plays out in many brooks, streams, and marshes across MA. Ecological restoration services aid in the recovery of ecosystems that have become impacted or degraded by past human activities. Our ecological restoration experts work with the state, town officials and nonprofit organizations to come up with a plan including funding options to help improve the area.
Dam Removal Feasibility Study-Ipswich