A brief project update…
As you all know, we met our 30% completion deadline a week and a half ago. We just received revisions from our engineer consultant to improve our drawings and move our design closer to completion. Our team has also decided on a stair and overlook design for access to the creek. We are currently working on drafting up design plans and drawing renders. Once our access design is fully complete, we will submit it for approval. We only have two more weeks left until school starts, so we’re really getting into crunch time.
And now we move on….
So what exactly is fluvial geomorphology? Well, let’s break it down. “Fluvial” comes from the latin term fluvius, which means river, and geomorphology is the study of landforms, the processes that created them, and the history of their development. So, fluvial geomorphology is the study of the physical processes in river systems and the landforms which they have created, are creating, and will create.
Much like the fingerprints of two humans, no two rivers are exactly alike. While their formations can vary tremendiously, geologists have classified these formations into several different categories, such as straight, meandering, and braided. But what causes these differences?
One of the main drivers in river channel formation is the river’s slope. In steep mountainous regions, rivers tend to be much straighter and faster flowing. As a river flows out of the mountains, it becomes less steep and gradually slows down and begins to meander. Once it reaches the nearly flat valley floor, the river can become braided. Then eventually, the rivers joins up with another river, or finally reaches the ocean.
The slope of a river also plays an important role in the size and structure of the materials that make up its channel. Steeper, cascading channels are dominated by boudlers and large woody debris. These types of channels are typically found at the top of watersheds.
Step-pools, like the ones we are designing to stabilize the banks of the north fork of Strawberry Creek, are naturally found at slopes of about 3-10%. The materials that make up step-pools are boulders and cobble, and are naturally more organized than the materials found in cascading reaches. The length between two pools in this type of channel is typically 1 to 4 times the channel width, depending on the slope and amount of water in the river.
Once the river reaches a slope of about 1-3%, it starts to flatten and widen, creating a plane bed. This type of channel has smaller material than the previous two, consisting of gravel and smaller cobble.
As the river reaches a slope of about 1-2%, its flows slow down. It begins to meander back and forth with up and down sequences of riffles and pools. The materials found in these rivers are usually gravel and some sand. The length between two pools in this type of channel is typically 5 to 7 times the channel width.
When a river is stable and in “equilibrium”, its slope and the amount of water flowing through it are in balance with the size and quantity of the materials in the river. If one or more of these factors are altered, the river can start to degrade (erode and incise) or aggrade (deposit and rise).
Widespread urbanization has led to an increase in impervious surfaces in the Strawberry Creek watershed, which has increased the volume of water flowing through the creek during storm events. This has thrown the creek out of eqilibrium and towards erosion and incision which can eventually lead to infrastucture damage and steep, unsafe banks. One of the main goals of our project is to mitigate this unbalance by laying back these steep banks and stabilizaing the channel with rocks and native vegetaion.