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Tahoma Creek: Aggradation and resource management

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Author(s): Scott W. Anderson

Document Type: Unpublished Internal Document
Publisher: National Park Service
Published Year: 2013
Pages: 43
DOI Identifier:
ISBN Identifier:

Aggradation directly beneath the Tahoma Creek Bridge has required repeated dredging to maintain adequate freeboard. Given that this span is an integral part of the Parks main access route, damage to or destruction of this structure would be a major disruption to Park functioning. This research was initiated with a goal of documenting and understanding the aggradation within Tahoma Creek, with the hope that such information would provide a better sense of how such hazards may evolve in the future and how best to manage these hazards. Specifically, the research attempts to a) document the rates of vertical channel change along the length of Tahoma Creek, b) determine, if aggradation is observed, whether this represents a systemic or transient disturbance, and c) discuss the potential options for mitigating these hazards.

Using LiDAR surveys flown in 2002, 2008 and 2012 to directly measure aggradation and incision over the entirety of the basin, I find that the lower reaches of Tahoma Creek, as a whole, have not aggraded over this period of record. While over 106 m3 of sediment was transported through the lower five kilometers of the creek, the net change in storage within these reaches was about -5x104 m3. Averaged over this area, this represents around 10cm of incision. Local exceptions exist, including a zone of net aggradation between the bridge and the Nisqually confluence for the 2008-2012 period.

Alder stands growing on the bare-gravel surfaces within the channel were observed to have established primarily in the years immediately following the debris flows of the early '90s, indicating that the lower channel does respond to such upstream sediment loading. However, the vertical position of these stands within the channel places a low upper limit on the extent of aggradation that may have occurred during these years, and further indicate that the modern channel has not risen above the high-stand position obtained in the mid-'90s. Taken together, it appears that debris flows do cause increased channel activity in the lower reaches, but that this activity largely manifests as increased lateral mobility and sediment transport rates, with only minor associated aggradation. The increased activity appears to subside within several years of the end of significant upstream sediment loading.

A variety of sources were used to investigate channel activity over the past 100 years. The methods are not exact, but broadly suggest that the channel has seen several intervals of increased activity or aggradation over this period, while maintaining a long-term stability.

Tree-ring records were used to reconstruct debris flows over the past 500 years. This records shows that there was a suite of events, similar in extent to the modern debris flows, in the mid-19th century. This coincides with the onset of glacial retreat out of the Little Ice Age (LIA). This suite of debris flows, along with several other isolated events that occurred during earlier periods of retreat, suggest a connection between negative glacial mass balance and debris flow frequency. However, the exact mechanisms of this connection are unclear, making it difficult to predict how this frequency will evolve in the coming decades. Regardless, these records show that the modern debris flows are not without precedent, increasing the odds that Tahoma Creek is already in an equilibrium defined by semi-regular intervals of such elevated sediment loading. That being said, the extent of forest mortality in the upper basin, and particularly above the old campground site, does appear to be unprecedented since the forests were last cleared by the Tahoma lahar c. A.D. 1500.

Taken together, these findings indicate that the lower channel has been dynamically stable over the period of record, and the recent debris flows do not appear to be significantly more frequent or intense than those of the past 500 years. The aggradation at the bridge appears to be a function of unique local conditions, which may include a) the narrowness of the bridge opening relative; b) a dynamic overshoot of aggradation as the stream infills the dredged reach; c) augmented local sediment influxes from dredging spoils placed on local gravel bars; or d) an increased local base-level caused by aggradation at the Nisqually-Tahoma Creek confluence. All four options are plausible readings of the available data, nor are they mutually exclusive.

While uncertainty remains, it is my belief that dredging provides little benefit to the long-term maintenance of the Tahoma Creek Bridge, and likely plays a role in the persistence of the local aggradation. Near the bridge, Tahoma Creek transports an average of 45,000 m3/yr of bed-material, and may transport an order of magnitude more than this during a single large flood. In contrast, dredging efforts generally reposition between 2,000 and 25,000 m3 of material. As such, even if the dredged material was removed from the channel, the channel transports enough material to re-obtain the pre-dredging profile in, at most, several years. The lower local channel slope created by dredging causes material to preferentially deposit in this reach, and may cause transient aggradation above the equilibrium profile as the channel attempts to dynamically re-obtain balance. This situation is exacerbated by the practice of placing dredging spoils on top of gravel bars within the active channel. This sediment is readily re-entrained by the river, creating a lateral sediment influx of a magnitude that greatly exceed the natural lateral inputs that are derived from the erosion of vegetated floodplain banks.

These results put the fate of the bridge in something of a grey area. While there is no evidence to suggest that the recent aggradation will continue unabated, it is very likely that periodic, local conditions will reduce the freeboard of the bridge below acceptable margins of safety with some regularity. This is simply the nature of dynamic, mountain streams. Given the regularity with which channel maintenance has been performed near the bridge reach, and the very short-term nature of the improvements, it still seems reasonable to consider an investment in a longer-term solution that will reduce the need for emergency operations.

There exists a suite of techniques designed to increase local sediment transport rates, which theoretically could reduce the potential for aggradation at the bridge. However, none of these methods are likely to be effective in Tahoma Creek, given the high energy of the stream and coarse sediment being transported. Modifying the bridge to accommodate the creek is a more reasonable solution, and seems most inline with the mission of a National Park. While the entire Park is classified as a National Historical Landmark District, the Tahoma Creek Bridge itself is considered a non-contributing structure, reducing the bureaucratic overhead needed to modify it. The major drawback to this solution is the cost involved. This cost must be weighed against that of repeated dredging, which provides only marginal benefits in the short-term and may actually exacerbate the situation in the long run. Regardless of which mitigation strategy is used, simple long profile surveys of the low-flow wetted channel, taken annually between the Nisqually-Tahoma confluence and a point somewhat upstream of the bridge, would provide valuable insight into the processes at play. Such surveys could be easily obtained within the parks current framework of resource management.

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Suggested Citations:
In Text Citation:
Anderson (2013) or (Anderson, 2013)

References Citation:
Anderson, S.W., 2013, Tahoma Creek: Aggradation and resource management: Unpublished Internal Document, National Park Service, 43 p..