Drift Dynamics of Early Life-stage Invasive Carps

Authors

Saurav Karki, University of Nebraska-LincolnFollow

First Advisor

David Admiraal

Committee Members

Richard Wood, Tirthankar Roy

Date of this Version

8-2025

Document Type

Thesis

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Civil Engineering

Under the supervision of Professor David Admiraal

Lincoln, Nebraska, August 2025

Comments

Copyright 2025, Saurav Karki. Used by permission

Abstract

Asian carp species pose significant ecological threats to North American freshwater systems due to their invasive nature and prolific reproduction. Their semi buoyant eggs develop while drifting downstream with the river current. Understanding early life-stage transport is therefore critical for predicting recruitment potential and guiding management efforts. Existing fluvial egg drift models, including those relying on 1-D hydraulics and fully 3-D CFD-based models, are either inadequate for complex braided rivers or too computationally demanding for large-scale application. The Platte River in Nebraska, characterized by wide, shallow, multi-threaded channels, is dominated by two-dimensional flow conditions, making a depth-averaged 2-D modeling approach both appropriate and efficient.

In this study, a 12-mile reach of the Platte River, from Ashland to Louisville, Nebraska, was modeled using HEC-RAS 2D to generate depth and velocity fields under low (1,500 cfs), moderate (5,000 cfs), and high (10,000 cfs) flow conditions. The hydrodynamic model was calibrated and validated against water surface elevations from three 2022 flow events, showing good agreement with USGS gage observations. A three dimensional Lagrangian egg tracking framework was then developed by integrating the 2-D hydrodynamic outputs with empirical vertical velocity distributions, stochastic turbulent diffusion, and time-dependent egg growth functions to account for buoyancy and fall velocity. For each flow scenario, a total of 5,000 virtual silver carp eggs were tracked individually to assess egg dispersion.

Results demonstrated that egg transport and dispersion are highly sensitive to both flow magnitude and channel morphology. Under low-flow conditions, eggs remained largely confined in the upstream reach, reflecting high retention caused by shallow depths, low velocities, and the presence of multi-braided subchannels. Moderate and high flows promoted efficient downstream advection, reducing retention and enabling faster, more uniform downstream transport. Longitudinal dispersion increased with discharge, while lateral dispersion was dominated by morphology-driven pathways (e.g., sandbars and islands) at low flows and by turbulent diffusion at higher flows, with broader lateral spread observed under low-flow conditions. Sensitivity analyses showed that egg fall velocity had minimal influence on lateral dispersion but noticeably affected longitudinal transport by reducing travel times, highlighting the importance of vertical egg positioning in overall drift dynamics.

Advisor: David Admiraal