SGER: Evaluation of the Rating Curve Hysteresis Due to Unsteady Channel Flows Using Non-Intrusive Measurements Acquired During the Iowa 2008 Flood

University Of Iowa, Iowa City IA

Investigators

Abstract

0843798 Muste The PI proposes to better understand and refine river discharge estimates using improved measures of rating curves. Traditional river discharge estimates are conducted at times when flow is relatively normal and steady, allowing safe and convenient measurement. These measurements serve as the basis for calculating rating curves. The curves, in turn, are used to estimate flow and discharge during flood events ? possibly leading to significant uncertainty due to hysteresis. Understanding the discharge-flow hysteresis relationship requires measurements during high flow events. The 2008 Midwestern flood offers a unique opportunity to couple flow and discharge measurements in order to better understand and model rating curve hysteresis during extreme events. With the goal of coupling river stage and discharge, measurements will be made in a non-contact manner developed at IIHR-Hydroscience & Engineering which uses Large-Scale Particle Image Velocimetry (LSPIV). This technique was initiated on June 10, 2008. Funds are requested to 1) finalize the on-going measurements; 2) perform topographic and bathymetric surveys; 3) evaluate deployed instruments by conducting baseline flow measurements; 4) analyze data and report findings; and 5) produce conceptual process models and define future research needs. The unique aspect of the proposed research is the use of non-intrusive measurement techniques in a natural extreme event provided by the 2008 Midwestern flood. The series of measurements will allow the PI to assess the technique and its ability to provide improved rating curve estimates under rare unsteady flow situations in natural channels. The expectation is that LSPIV will provide a simple, compact, cost- and effort-effective package for providing these critical measurements. Unsteady flows will be investigated in detail and conceptual frameworks will be developed in order to determine what ancillary data are needed to answer discharge hypotheses. The transformative, exploratory nature of this project will provide field testing for technical aspects of the LSPIV technique to include specular reflection for free-surface tracing, optimal velocity distribution laws in the water column testing as a function of stream characteristics, flood wave raising and falling data capture, and uncertainty analysis. Rating curves are the basis of stream stage measurements, yet their uncertainty during high-flow events obviates the ability to fully understand unsteady fluvial processes. River processes hypotheses regarding extreme and unusual flow regimes can be tested and the results of this study will enable river stage and discharge estimation in regions without gauging stations. Integrated hydrologic-geomorphic-biological studies will be possible through a better understanding of key temporal and spatial processes in flood propagation. Uncertainty in rating curves calculated during stable river flow regimes will hinder flood stage forecasting whereas this project will provide data useable by national mission agencies for more accurate flood estimates. With billions of dollars lost each year, more accurate flood data are timely and of broad interest. ***