People will hike for miles up mountains, travel to foreign countries, and plan family vacations to stand in the presence of waterfalls. There’s something about water steadily rushing towards a precipice, hurling itself fearlessly over the edge, and tumbling mesmerizingly through the air before it calmly carries on its steady journey downstream. In the Coastal Plain of Maryland, home base of Underwood & Associates, the landscape slopes gradually towards the Atlantic, with less than half of the Coastal Plain surpassing 10 feet in elevation. Rivers and their tributaries flow at a gentle, meandering pace, and historically wetlands and bogs abounded in this low and slow region of Maryland. When water in these tributaries encounters steep terrain, it descends over irregularly stepped gradients of sandstone boulder outcrops and native cobbles and gravels, generally keeping contact with the rock throughout the decline. We refer to these vertical elevation drops as cascades and have studied them in-depth in order to mimic these natural features in our restorations.
Cascades are a tool we use in regenerative design when uplifting ecosystems, creating nature-based solutions to stormwater, and restoring streams. Our RSC (Regenerative Stream Conveyance) and SPSC (Step Pool Stormwater Conveyance) techniques aim to store water as high as possible in the landscape to allow for maximum infiltration and processing in order to produce higher quality habitat and cleaner water. When we come across a steep drop in elevation on a project, instead of mechanically removing a significant amount of material to level the grade and consequently disturbing a greater area, we utilize cascades to elegantly traverse the grade. Cascades allow you to store water high in the treatment train, and then drop grade quickly instead of spreading the fall over a long linear distance. The cascade provides the stable stairsteps for elevation drop and then the series of wetland pools can resume their gentle 1 ft falls. It is important to note that we only use cascades when necessary and always aim to design gradually-stepped projects when possible.
A cascade must be constructed properly in an RSC or SPSC in order to function successfully. When Underwood & Associates invented these regenerative techniques, we created specific parameters for building cascades. For example, when the longitudinal slope of a project exceeds 5% (20:1), cascades must be used, yet we will often employ them around 4% to allow for larger wetlands. Cascades should be built in a pronounced amphitheater shape, which lends itself to resilience and allows the cascade to be integrated seamlessly into the surrounding landscape rather than inserted abruptly in the middle of flow. Cascades should be built multiple layers of boulders deep to ensure they are a true landscape feature that can handle high-energy flows rather than just a veneer of rock at the surface. This is critical since they are intended to replicate a natural outcropping of boulders that has been exposed due to the powerfully erosive forces of water. Additionally, the footer of the cascade must be buried below the bottom of the receiving pool to provide security and ensure the cascade is not subject to shifting.
In a cascade, water shouldn’t slip and slide gently down a rock chute, but rather the stream should trip and tumble dynamically down stairsteps. Using native boulders will allow vegetation to establish on the face of the rock, creating a natural appearance and promoting true ecological restoration. Another important feature of constructed cascades is that filter cloth is not needed since multiple layers of native stone outcroppings allow sand and gravel particles to filter through and choke up the interstices, eliminating the need for synthetic fabrics. A correctly designed and built cascade should be natural, resilient, and enduring.
Cascades are a wildly important feature in RSCs and SPSCs that should not be overlooked nor undertaken without a deep understanding of proper procedure. For technical guidelines on building a cascade read Anne Arundel County’s Design Guidelines for Step Pool Stormwater Conveyance (SPSC) Systems and Maryland Department of Natural Resources’ RSC Construction Guidance. Still have questions? Underwood & Associates is always happy to advise on cascade design. You can reach us via the contact form on our website ecosystemrestoration.com.
Cascades may not have the height of Angel Falls, the width of Victoria Falls, or the volume of Niagara Falls, yet cascades serve a distinct purpose in our restoration projects on the Coastal Plain and provide beauty and function in their own vertically humble way.