Presenter/s: Fred Meyer
Symposium Session: 2023 - 12 All that glitters is not Gold: Restoring Dredged Rivers
Topics covered: california, dredged rivers, floodplain, and riparian
ABSTRACT
Floodplain connectivity on many California rivers have been impacted by the combination of historic gold dredge operations and flow regulation. Flow regulation typically reduces peak flow magnitude, frequency, and duration, which correspondingly reduces frequency and duration of overbank flows. As historic gold dredge operations moved across the valley bottomlands, they have inverted the grain size of these bottomlands by bringing the coarse material to the top (cobbles-to-boulders) and depositing fines at the bottom or between the dredge tailings (slickens). These dredge tailings have also reduced floodplain connectivity by 1) destroying the natural floodplain morphology, 2) destroying the natural grain size gradation of the floodplains (fines displaced, now dominated by cobbles and boulders), and 3) the remnant dredge tailings often confine and entrench the low flow channel between tailings piles. The post-dam high-flow flow regime is usually reduced to the point where the river is no longer able to re-work these tailings and heal itself. In most cases, improvements in the high flow regime to re-enable the river to rework the tailings is infeasible; therefore, physical rehabilitation of the channel is required.
There are typically three ways to conduct channel rehabilitation to reverse the confinement and entrenchment caused by the dredge tailings to re-create floodplain connectivity: 1) Lower the dredge tailings down to create new floodplains that are scaled to the post-dam flow regime, 2) Raise invert of the mainstem channel to increase water surface elevations and connectivity to higher overbank surfaces, or 3) a combination of lowering tailings and raising the invert of the mainstem channel. Solely relying on dredge tailing lowering is extremely expensive due to the large earthworks involved, so raising the channel invert is a way to substantially reduced rehabilitation project costs via reduced earthworks and less off-site hauling of excess cut materials. One typical constraint is that the elevation of the 100-year flood cannot be raised by the rehabilitation project; therefore, a careful balance between dredge tailing lowering and mainstem channel raising is needed to meet flood management requirements and minimize implementation costs. This presentation will provide an overview of design strategies for regulated rivers with extensive dredge tailings, and the physical, riparian, ecological, and cost tradeoffs of these strategies that may be informative for other rehabilitation projects on regulated rivers with extensive dredge tailings.