Evaluation of Historical Alluvial Channel Crossings


Date: 2024/01/01

Author(s): U.S. Army Corps of Engineers

Publication: Report prepared for the Middle Rio Grande Endangered Species Collaborative Program, 238 p.


Flooding and sedimentation on the Middle Rio Grande (MRG) have created dynamic adjustments of the river channel necessitating numerous flood control projects over the last century to make the land arable and safeguard the surrounding communities. As part of the flood control effort the MRG has been laterally constrained, typically by levees or high ground, but historical documentation of the channel shows historical channels occurred outside the current levee constraints. Current active channel locations are known to consist of coarser bed materials than their correlated floodplain deposits.

The hypothesis underlying this analysis was that the historical channel locations also had coarser material deposits than their floodplains. And, when historical channels became abandoned, they were subsequently filled in, and covered with finer material, as is observed in today’s floodplain system.. The crux of this hypothesis is that in certain locations, where the current river channel is in contact with coarser material from the historical channels, river water is laterally transferred through the floodplain sediments via groundwater movement.

During the 2019 spring-snow melt runoff, locations were geospatially recorded where existing levees experienced seepage and other integrity issues. These observations provide an opportunity to evaluate whether historical river planforms, compared to other conditions, such as standing water or vertical gradient change, are contributing to levee integrity issues. The historical river planforms of 1918, 1935, and 1949 are utilized for this study primarily because they are oldest planforms available on the MRG and provide a relic of where the river active channels used to be. This analysis evaluated the correlation between observed 2019 seepage issues and historical channel locations for the corridor between the Corrales siphon and the I-25 crossing south of Albuquerque, NM. To this extent the 2019 issue locations were filtered to evaluate only those locations with groundwater induced issues (e.g., seepage, sloughing in the drain, sand boils, etc.).

The conducted analyses indicates that the filtered 2019 issue locations have some correlation to the observed active channel locations in 1918, 1935, and 1949, with the earliest year having stronger visual correlations, although the active channel distance statistics favor the latter years. It was speculated that a stronger correlation may exist between the 2019 issue locations and active channel morphology that predates 1918, based on the morphological planform type changes between 1918 and 1949. The correlation between coarser sediment deposits and the 2019 issue locations is poor, however, and may be a result of correlated boreholes that were not closely associated with the 2019 issue locations.

The analysis suggested that there is a moderate correlation between the 2019 issue locations and zones identified as high to moderate risk for seepage and slope stability failures. Part of the hypothesis testing was to evaluate whether historical river planforms, compared to other conditions, such as standing water or vertical gradient change, are contributing to groundwater movement that is causing seepage and slope stability issues on the riverside drains. This analysis was inconclusive in identifying a primary cause but suggests that historical channels and traditional seepage concerns (e.g., standing water, vertical gradient change, etc.) are likely both culprits in the observed 2019 issue locations.

While the evaluation was focused on a flood control nexus because of a dataset that provided spatially diverse and observable links to groundwater movement, it does suggest that an increased groundwater connection through coarser sediment deposits from historical active
channel locations is possible. Based on the data evaluated, the relationship is moderate at best and likely more pronounced where the current active channel has incised into the stratigraphy laid down by historical active channels. Habitat restoration projects that consider the historical
active channel locations may benefit from the increased groundwater connection observed in this analysis groundwater correlation to the historical active channel locations.

Additional analyses that expand on the current evaluation may provide additional insight into this relationship between groundwater movement and historical channel locations. The 2019 data collection effort included the reach of the Rio Grande around the Belen and Los Lunas, NM area. An expansion of this analysis to that area may provide insight into whether the seepage signal is stronger. Filtering the data by proximity to boreholes may also prove advantageous and minimize uncertainties introduced by correlating borehole data that is further away from identified sites of concern. A more specific elevation analysis may also be beneficial. Correlating elevations of the historical channel elevations with the current active channel location would reveal if there is an elevation connection to the observed seepage locations. If the elevation shows a seepage head towards the drain, it may suggest that direction of the historical channels may play a role in the observance of seepage concerns that is in addition to the coarser nature of the deposits.

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