Streambed scour is defined as fluctuation in the vertical position of a streambed, or the depth of the stream, as material is eroded and/or degrades. Some degree of streambed fluctuation is a natural process within the types of gravel-bedded rivers that we see; however, scour can also occur as a result of a change in the natural streambed conditions. Hoyle Tanner is currently assisting the New Hampshire Department of Transportation (NHDOT) with providing scour stabilization measures using an innovative system: A-jacks.
The History of Stream Crossing Design – If Only we Knew Then What we Know Now!
When the initial network of roads and highways was developed in New Hampshire, there was a different thought process towards designing infrastructure such as bridges, culverts or pipes, that crosses rivers and streams than there is today. Currently we would examine a stream from all angles to determine how to best approach designing a stream crossing that will not change the stream’s natural flow, depth or substrate (riverbed material). Stream crossing designs in the 1970s did not prioritize this stream information, and, as a result, in some situations the crossing structure has changed the stream’s parameters such as width, depth and flow.
The Results: Scour Pools & Stream Channel Changes
The most common example of this is where a stream crossing is too small to meet the stream’s bankfull width, or the width the stream needs when it is at a maximum flow and creates a pinch-point in the stream. Think of a water hose: When you pinch it you can create more pressure as the water comes out. In those situations, as water is forced through the smaller opening, water flows increase in speed and energy, and the water exiting the crossing can erode, or scour, the streambed, banks, or both. This can often result in a small area immediately downstream of the crossing that is deeper than the stream is upstream of the bridge or culvert – this is called a scour pool. If the amount of scour comes close enough to the culvert, pipe or bridge foundations, it can erode the ground under the crossing and risk destabilizing the crossing, including the road on top of the crossing.
When faced with these situations, stabilizing the stream bed and banks while protecting the culvert/pipe and road from being affected are interconnected goals.
A-Jacks: New Technology to Address an Old Problem
NHDOT routinely surveys stream crossings to determine if they are stable or if work should be done to either prevent scour from occurring or resolve scouring that is currently happening and may impact the crossing structure. In Woodstock, the stream crossing of Interstate-93 over Eastman Brook that was installed in 1972 is composed of a twin cell (or 2-sided) concrete box culvert; each side is 18’ wide. Original installation included riprap (stone) at the inlet and outlet of the culvert to prevent scouring. This riprap has washed away at the downstream outlet, and despite repairs of adding riprap on several occasions, the stream continues to scour downstream. Over time, this scour will jeopardize the stability of I-93, which is not an acceptable situation. Eastman Brook carries water that flows out of the White Mountains that can seasonally flow fast enough to carry even the largest riprap boulders downstream, particularly in spring, due to snowmelt combined with heavy rainstorms. Is there a different solution?
Hoyle Tanner’s experienced bridge design engineers proposed the use of A-Jacks in this location. They consist of two concrete T-shaped pieces joined perpendicularly at the middle, forming six legs. A-Jacks are designed to interlock into a slightly flexible, highly permeable matrix that will remain in the streambed. The highlight of this design is the ability of the A-Jacks system to spread out the energy that comes from water flowing quickly out of the culvert, allowing for increased resistance to the erosive forces of flowing water. Because they lock together in place, they can flex yet effectively stay put where they are installed.
The patented, two component design allows economical transport and on-site assembly. Just as you would picture a pile of jacks when you dump them onto the floor to play the game, A-Jacks interconnect and are assembled by sliding one half into another to form a complete unit. Rows of A-Jacks are assembled to interlock in horizontal as well as vertical directions. A-Jacks can be installed either randomly or in a uniform pattern. NHDOT has previously installed A-Jacks in four locations across the state and was open to the idea of using this alternative for the Woodstock scour stabilization project.
A-Jacks were installed downstream of the stream crossing in Woodstock this summer for approximately 87 feet. As shown in the photos, the streambed was excavated to a depth to allow for installing a double row of 48” A-Jacks that raised the streambed elevation to meet the bottom lip of the outlet of the culvert and tied into the natural grade of the stream downstream of the crossing; this will allow for improved aquatic organisms and fish passage through the crossing by preventing the situation shown in the before photo, which is called a perched outlet. Thus, the design will accommodate the highest stream flows and will keep water running through the culvert during the low flows of summer so the stream doesn’t disconnect, and wildlife and fish can pass freely. Can’t see them? Clean washed gravel and stone was placed on top of the A-Jacks to fill the small voids (or spaces) between the individual units, resulting in simulation of a natural streambed.
Hoyle Tanner’s Environmental Coordination team effectively coordinated between NHDOT, the bridge designers, the NH Department of Environmental Services (NHDES), the US Army Corps of Engineers (USACE) and the NH Fish and Game Department (NHF&G) to obtain agreement from each respective permitting agency that the A-Jacks, despite technically being viewed as “fill” in the streambed, were necessary in this location and would result in the best overall result that met the goals of the agencies involved.
By using innovative design techniques, our team was able to effectively stabilize an important piece of infrastructure, prevent future scouring of the stream and damage to the stream crossing, and re-create a natural streambed with improved functionality for fish and wildlife. Just a day’s work for our talented bridge designers and environmental coordinators. Let us know if you have a tricky scour issue that you would like us to take on!