Civil Engineering ETDs

Publication Date

Summer 8-1-2023

Abstract

The continuous alteration of flow regimes in rivers has led to negative impacts on habitats and biotic communities, causing irreversible effects on river biodiversity. Implementing e-flows in management strategies can repair historical mismanagement, prevent further degradation, and restore ecosystem services. In the case of the Rio Chama, damming has altered its hydrological regime, affecting sediment transport. Experts recommend specific e-flow conditions for the Rio Chama to improve ecological conditions, in particular a bankfull discharge of 70 m3s-1 every 2 years for sediment flushing and channel maintenance. The Río Gallina, a tributary of the Rio Chama, plays a significant role as a sand source. Sites like Big Bend are directly influenced by the sediment transported by Río Gallina to the main channel. This raises concerns about the effectiveness of the established e-flow upstream of the Río Gallina and Río Chama confluence specifically at this site. To test it, a two-dimensional hydrodynamic model, HEC-RAS version 6.3.1, was used to simulate unsteady-state river hydrodynamics. Flow simulations ranged from 3 to 170 m3s-1, with increments of 3 m3s-1 up to 60 m3s-1 and increments of 10 m3s-1 beyond 60 m3s-1, main outputs were riverbed shear stress and velocity. We developed a grid layer which represent the d50´s particle spatial variations of the upper riverbed on the study site, which was then utilized for simulating incipient motion. Incipient motion analysis was performed by identifying the needed flushing flow threshold utilizing the standard relationship proposed by Shields (1936).

Our results showed that the primary d50 fraction in the upper riverbed, varies from fine sand (0.05 to 0.1 mm) to fine sand (0.1 to 0.25 mm) and that larger ranges of shear stress become more evident with increases in flow, observed in in the central sectors of the channel. It was demonstrated that recommended e-flow overestimate the flushing requirements for this site. It was shown that 74 % of the surface is flooded and 82% of it is subject to incipient motion (62% of the total surface) at a bankfull discharge of 70 m3s-1, when compared with flow values close to 15 m3s-1, approximately 61% is flooded and 77% of it presents movement (47% of the total surface), and at values close to 45 m3s-1, 70.4% of the surface is flooded and 82.1% of it shows movement (58% of the total surface). Therefore, either 15 or 45m3s-1, are substantially less than the recommended 70 m3s-1.

Keywords

Rio Chama, Sediments Transport, Modelling, HEC RAS, Ecological Flow

Document Type

Thesis

Language

English

Degree Name

Civil Engineering

Level of Degree

Masters

Department Name

Civil Engineering

First Committee Member (Chair)

Mark Stone

Second Committee Member

José Cerrato

Third Committee Member

Anjali Mulchandani

Fourth Committee Member

Smriti Chaulagain

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