Document Type

Dissertation

Date of Degree

Fall 2015

Degree Name

PhD (Doctor of Philosophy)

Degree In

Civil and Environmental Engineering

First Advisor

Larry J. Weber

Second Advisor

Keith E. Schilling

Abstract

In 2008 flooding occurred over a majority of Iowa, damaging homes, displacing residents, and taking lives. In the wake of this event, the Iowa Flood Center (IFC) was charged with the investigation of distributed flood mitigation strategies to reduce the frequency and magnitude of peak flows in Iowa. This dissertation is part of the several studies developed by the IFC and focused on the application of a coupled physics based modeling platform, to quantify the coupled benefits of distributed flood mitigation strategies on the reduction of peak flows in an agricultural watershed.

Additional investigation into tile drainage and terraces, illustrated the hydrologic impact of each commonly applied agricultural practice. The effect of each practice was represented in numerical simulations through a parameter adjustment. Systems were analyzed at the field scale, to estimate representative parameters, and applied at the watershed scale.

The impact of distributed flood mitigation wetlands reduced peak flows by 4 % to 17 % at the outlet of a 45 km2 watershed. Variability in reduction was a product of antecedent soil moisture, 24-hour design storm total depth, and initial structural storage capacity. The highest peak flow reductions occurred in scenarios with dry soil, empty project storage, and low rainfall depths. Peak flow reductions were estimated to dissipate beyond a total drainage area of 200 km2, approximately 2 km downstream of the small watershed outlet.

A numerical tracer analysis identified the contribution of tile drainage to stream flow (QT/Q) which varied between 6 % and 71 % through an annual cycle. QT/Q responded directly to meteorological forcing. Precipitation driven events produced a strong positive logarithmic correlation between QT/Q and drainage area. The addition of precipitation into the system saturated near surface soils, increased lateral soil water movement, and reduced the contribution of instream tile flow. A negative logarithmic trend in QT/Q to drainage area persisted in non-event durations.

Simulated gradient terraces reduced and delayed peak flows in subcatchments of less than 3 km2 of drainage area. The hydrographs were shifted responding to rainfall later than non-terraced scenarios, while retaining the total volumetric outflow over longer time periods. The effects of dense terrace systems quickly dissipated, and found to be inconsequential at a drainage area of 45 km2.

Beyond the analysis of individual agricultural features, this work assembled a framework to analyze the feature at the field scale for implementation at the watershed scale. It showed large scale simulations reproduce field scale results well. The product of this work was, a systematic hydrologic characterization of distributed flood mitigation structures, pattern tile drainage, and terrace systems facilitating the simulation of each practices in a physically-based coupled surface-subsurface model.

Public Abstract

In 2008 flooding occurred over a majority of Iowa, damaging homes, displacing residents, and taking lives. In the wake of this event, the Iowa Flood Center was charged with the investigation of distributed flood mitigation strategies to reduce the frequency and magnitude of floods in Iowa. This work focused on the application of a numerical model to quantify the flood reductions induced by flood mitigation strategies in an agricultural watershed. Floods were estimated to be reduced in magnitude by 4 % to 17 % at the outlet of a 45 km2 watershed. Variability in reduction was a product of soil wetness and rainfall. The highest reductions occurred in scenarios with dry soil and light rainfall. The influence of flood mitigation strategies were estimated to no longer impact stream flow at 2 km downstream of the watershed outlet.

Additional investigation into commonly applied agricultural practices led to a new method to incorporate fine scale features into coarse models. Through this approach terraces were found to delay and reduce the peak flows from the watershed at small drainage areas. Over large catchment areas the impacts of terraces were unnoticeable. Agricultural tile drainage is typically applied to reduce excess near surface water, and was found to significantly impact surface water flow. The quantity of rainfall and evapotranspiration altered the influence of tile drains over varying drainage areas. This research led to a number of unique outcomes related to large flood reductions which are applicable to agricultural catchments common in Iowa.

Keywords

publicabstract, Agricultural Practices, Flooding, Hydrology, Numerical Simulation

Pages

xxi, 239 pages

Bibliography

Includes bibliographical references (pages 222-239).

Copyright

Copyright 2015 Nicholas Wayne Thomas

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