Mechanics of sediment plug formation in the Middle Rio Grande, NM
Date
2013
Authors
Park, Kiyoung, author
Julien, Pierre Y., advisor
Thornton, Christopher I., committee member
Venayagamoorthy, Subhas K., committee member
Wohl, Ellen E., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The Rio Grande is a dynamic river system which has experienced significant hydraulic and geomorphic changes through recorded history from the early 1900's to the present. These changes stem, for the most part, from natural and human interventions to the river system, which experienced channel bed elevation changes, lateral migration, straightening, channel realignment, etc. Sediment plugs have formed in the Tiffany area near San Marcial in 1991, 1995, and 2005, and in the Bosque Reach 14 miles upstream from the Tiffany plug location in 2008. Many authors have investigated the cause of sediment plugs in the Middle Rio Grande but the previous studies do not provide a complete criteria for sediment plug formation. Better understanding of the complex mechanics of plug formation on the Middle Rio Grande is therefore pursed. Based on the historic flow and geometric characteristics of plug areas, seven parameters were identified as major causing factors of sediment plug formation in the Middle Rio Grande: (1) two geometric factors: variability of channel widths and roughness; (2) two water and sediment loss factors: perching/overbanking and sediment concentration distribution profiles; and (3) three backwater effect factors: backwater effects from a reservoir, a bridge, and sharp bends. The purpose of this research is to analyze possible sediment plug parameters and to assess the primary causing factors. The specific objectives are to: (1) investigate the mechanics of sedimentation effect due to each factor; (2) simulate the historic sediment plugs using a numerical aggradation/degradation program; and (3) determine which factors contribute the most to the formation of sediment plugs. Geometric factors show that the channel has narrowed 40% between 1962 and 2002 and channel capacity has 77% decreased over time. The representative composite roughness increased 50 % between 1992 and 2002. Accordingly sediment transport capacity has decreased 45%. The narrowing (40%) with increase in roughness (50%) causes considerable loss of sediment transport capacity (45%). Therefore geometric factors induce more overbank flows and channel bed aggradation. Sedimentation factors show that the perching ratio increased from 13% to 87% between 1992 and 2002. Bank depth has decreased 51% between 1992 and 2002. The perching and lower bank depth facilitated more overbank flows and 13 ~ 20% loss of water. As particle sizes have coarsened (0.2mm in 1992 → 0.25mm in 2002) and width/depth ratios have increased (129 in 1992 → 229 in 2002), leading to higher rouse numbers and more near-bed concentration profiles. High Rouse number (Ro >1.2) and near-bed sediment concentration profile speed up the aggradation rates (4 ~ 7 times faster) than for a uniform-concentration profile. The high near-bed concentrations shorten the plug formation time from 90 to 20 days. Since snowmelt floods exceed bankfull discharges less than 2 months, the acceleration factors are essential for sediment plugs to form. Backwater effects from the Elephant Butte Reservoir influenced the upstream channel bed elevation over time. At an average flow discharge (1,550cfs), the aggradation (up to 7ft) time to fill the 25.5 mile long channel is roughly 10 years. The historic Tiffany plug area has been influenced by the reservoir levels, but with a lag time of several years. Around the San Marcial Railroad Bridge, channel bed elevation has aggraded consistently (12ft increased between 1979 and 1987). The pier contraction and congested abutments generate about a 1ft high backwater propagating to the Tiffany plug area. Sharp bends caused a 1.6ft high backwater which propagates roughly 1 mile upstream. As the beginning point of the Bosque plug is located 0.6 mile upstream of the sharp bends, backwater does influence the channel aggradation of the Bosque plug. The time to fill the main channel up to the bank crest was estimated as approximately 17 days. In terms of significance, perching/overbank flow and sediment concentration profiles can be evaluated as the primary causing factors of sediment plugs, followed by the backwater effects from bridge and sharp bends. Backwater effect from the reservoir has influenced the upstream channel elevation on a long-term basis (7 ft / 10 years). Channel narrowing and higher roughness promote overbank flows and decrease of sediment transport capacity. Owing to the increase of overbank flows, sediment concentration profiles speed up the rate of channel aggradation, causing a sediment plug within a matter of weeks, thus these two factors are the most significant factors (1.2 ft / 20 days). Two other factors, the backwater effect from the railroad bridge and sharp bends, explain why the historic sediment plugs formed at particular areas, therefore these two parameters can be classified as local triggering factors (1~1.6 ft / 20 days). On the other hand, causal factors can be divided into two groups depending on the plug location. The Tiffany plugs have been more affected by the backwater effect from the reservoir and railroad bridge. On the other hand, the Bosque plug was more influenced by the decrease of channel width/channel capacity, roughness, and sharp bends.
Description
Rights Access
Subject
perching
Rio Grande
Rouse number
channel width
numerical models
sediment transport capacity