Reclamation of Flood Plain Mining Pits to Improve Habitat and Reduce Potential for Flood Damage
* What is the appropriate way to proceed with incorporating flood plain gravel pits into the normal flow of the river?
* What are the potential drawbacks to incorporating flood plain gravel pits into the normal flow of the river and when is appropriate to do so?
* What benefits will be realized by incorporating flood plain gravel pits into the normal flow of the river?
* What is the best method for hazard classification of individual pits?
* What are the risks associated with existing flood plain gravel pits?
* What should be done with pits that are not suited for incorporating into the normal flow of the river?
* Are there existing alternate locations for future gravel extraction on terraces rather than within the flood plain?
Need and Benefit
Available information regarding flood plain gravel pits highlights the impacts of flood plain mines that exist along many rivers throughout the Western United States. However, there is currently little or no information detailing specific methods or citing examples for reclaiming flood plain gravel pits. This research will develop procedures for determining the feasibility of and methods to reconnect flood plain gravel pits to enhance habitat and to reduce the likelihood of future flood damage.
When the river migrates and breaches a dike or levee that protects the pit from the river, all or part of the river flows into the pit. This "pit capture" is an inevitable occurrence from the river channel naturally meandering and changing its position over time (Kondof 2002). Many of the levees separating the Yakima River from gravel pits are reinforced with gravel that is mobilized by the river during a 10-year event (Dunne and Leopold 1978). In 1995 and 1996, ten pit captures were documented in Washington (Norman et al., 1998).
When the river captures a flood plain gravel pit, that portion of the river is converted from lotic (flowing) to lentic (stillwater) habitat (Kondolf 2002). The pit acts as a sediment trap, preventing the natural continuous transport of sediment delivery from headwaters to downstream reaches. Portions downstream of the captured pit become sediment starved, resulting in some combination of increased meandering, incision, bank erosion and bed armoring. A nickpoint created by the sudden drop in bed elevation at the entrance of the pit can travel upstream, degrading the riverbed. Because of the likelihood of unintended pit captures, a proactive approach to creating a pit capture could prevent damage to existing infrastructure and further degradation of habitat. While many of the gravel pits along the Yakima River are too deep to consider for intentional pit capture, others are relatively shallow, with depths between 4 and 15 feet.
Shallow pits present a unique opportunity to regain lost habitat while decreasing the chances for an unintended pit capture and the unpredictable nature of such an occurrence. Causing a pit capture at a suitable site would create a situation where upstream migration of a nickpoint can be controlled with either permanent or temporary grade control. Downstream sediment starvation can be controlled by providing a means for a portion of the sediment load to enter the pit, allowing the remaining load to continue downstream. The study site will likely require the removal of existing levees, allowing material from the old levee to aid in filling the pit.
Many of the dikes and levees separating the river from the pit were constructed of local alluvium extracted from the pit at the onset of the mining process. Setting levees back creates a wider river corridor, thus reducing the potential for flood damage and allowing for more natural fluvial processes. Increasing river-flood plain interaction will improve habitat for both fish and wildlife as well as reinvigorate hyporheic zones in portions of the Yakima Valley, thereby providing cool, clear ground water discharge to the river during times of low flow (Ring and Watson 1999). Because the disturbances that currently exist are the result of decades of flood plain mining and levee building, recovery of the river system should be measured on a similar time scale. Sediment transport rates in the vicinity of the intended pit capture will be the key to the feasibility of the project and a recovery timeline. The Yakima River has a relatively low sediment transport rate for rivers of similar size (Dunne and Leopold 1978).
This Science and Technology (S&T) Program research project will focus on the determination of how to proceed with an intentional pit capture and whether or not an intended pit capture is feasible for a specific location. Methods will be developed for a comprehensive study that effectively incorporates a gravel pit(s) i
Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Rehabilitation of Floodplain Mining Pits: Interim Report Detailing Initial Plans and Procedures (final, PDF,
By Jeanne Klawon
Report completed on September 30, 2005