Tibebu B. Ayalew

Analyzing the effects of excess rainfall properties on the scaling structure of peak discharges: Insights from a mesoscale river basin

Key theoretical and empirical results from the past two decades have established that peak discharges resulting from a single rainfall-runoff event in a nested watershed exhibit a power law, or scaling, relation to drainage area and that the parameters of the power law relation, henceforth referred to as the flood scaling exponent and intercept, change from event to event. To date, only two studies have been conducted using empirical data, both using data from the 21 km2 Goodwin Creek Experimental Watershed that is located in Mississippi, in an effort to uncover the physical processes that control the event-to-event variability of the flood scaling parameters. Our study expands the analysis to the mesoscale Iowa River basin (A = 32,400 km2), which is located in eastern Iowa, and provides additional insights into the physical processes that control the flood scaling parameters. Using 51 rainfall-runoff events that we identified over the 12 year period since 2002, we show how the duration and depth of excess rainfall, which is the portion of rainfall that contributes to direct runoff, control the flood scaling exponent and intercept. Moreover, using a diagnostic simulation study that is guided by evidence found in empirical data, we show that the temporal structure of excess rainfall has a significant effect on the scaling structure of peak discharges. These insights will contribute toward ongoing efforts to provide a framework for flood prediction in ungauged basins.

Classical and generalized Horton laws for peak flows in rainfall-runoff events

The discovery of the Horton laws for hydrologic variables has greatly lagged behind geomorphology, which began with Robert Horton in 1945. We define the classical and the generalized Horton laws for peak flows in rainfall-runoff events, which link self-similarity in network geomorphology with river basin hydrology. Both the Horton laws are tested in the Iowa River basin in eastern Iowa that drains an area of approximately 32 400 km(2) before it joins the Mississippi River. The US Geological Survey continuously monitors the basin through 34 stream gauging stations. We select 51 rainfall-runoff events for carrying out the tests. Our findings support the existence of the classical and the generalized Horton laws for peak flows, which may be considered as a new hydrologic discovery. Three different methods are illustrated for estimating the Horton peak-flow ratio due to small sample size issues in peak flow data. We illustrate an application of the Horton laws for diagnosing parameterizations in a physical rainfall-runoff model. The ideas and developments presented here offer exciting new directions for hydrologic research and education.

Exploring the Effect of Reservoir Storage on Peak Discharge Frequency

AbstractIn this paper, a simple hydrologic example is employed to illustrate the important features of reservoir regulated flood frequency. Despite its practical significance, the estimation of reservoir regulated flood frequency is largely dominated by empirical methodologies containing assumptions that could lead to incorrect results. The goal of this paper is to show by means of a continuous rainfall-runoff simulation how several reservoir variables, including the reservoir storage capacity, the size of release structures, operation rules, and the statistical variability of inflows to the reservoir, quantitatively control the regulated flood frequency. Although the example presented in this paper does not encompass the full complexity of the problem, it reveals important features of the regulated flood frequency. The study also highlights how specific assumptions in the traditional and widely used inflow volume-duration-frequency (VDF)–based methodology could lead to underestimation of flood risk for l...

Attitudes toward post-earthquake water and sanitation management and payment options in Leogane, Haiti

The Haitian government passed a law in 2009 to decentralize water utility management and improve cost recovery. This study identifies the attitudes of the public towards payment for and management of water and sanitation, several months after the 2010 earthquake, through a survey (N = 171) and semi-structured interviews (N = 19) in Leogane, Haiti. A majority of survey respondents were willing to pay for water and sanitation, which aligns with the fee-based approach of the 2009 law. Significant differences were found between geographic locations, suggesting that a neighbourhood-level approach to water and sanitation is appropriate.

Effect of Spatially Distributed Small Dams on Flood Frequency: Insights from the Soap Creek Watershed

AbstractDams are ubiquitous in the United States, with more than 87,000 influencing streamflow across the nation. The significant majority of these dams are small and are often ignored in real-time...

Effect of River Network Geometry on Flood Frequency: A Tale of Two Watersheds in Iowa

AbstractAlthough numerous studies conducted over the past four decades have shown the significant role that drainage network geometry plays in determining the streamflow response and hence the peak...

Assessing preferences regarding centralized and decentralized water infrastructure in post-earthquake Leogane, Haiti

BackgroundThough the benefits of centralized water systems (e.g. improved publichealth, environmental protection, streamlined operations, economy of scale, reliability) are well known, these systems are not always feasible or appropriate. In developing world settings there has been growing interest by infrastructure experts,researchers, and international lending institutions in decentralized means of improving access to drinking water. While decentralized water systems with independent components may be less vulnerable to systemic failures, hazards, and extreme environmental events, centralized water systems are often associated with a higher quality of life. This study investigates stakeholder preferences regarding water infrastructure issues in Leogane, Haiti (population ~300,000), a town situated at the epicenter of the January 2010 earthquake.MethodsThe methodology included a paper survey, semi-structured interviews, and a participatory workshop.ResultsWhile most of the study participants relied on decentralized water sources prior to the earthquake, a majority also expressed a preference for a centralized water system going forward. However, the study participants articulated an integrated vision for the future of local water management. Study participants indicated an interest in alternative decentralized solutions, for example featuring artesian wells with homewater treatment, and saw linkages between water supply decisions and local environmental protection, agriculture, and deforestation.ConclusionsThese results are discussed within the context of sustainable infrastructure reconstruction efforts in Haiti, specifically as an example of how local preferences can be integrated into the visioning of infrastructure design.

Participatory engineering for recovery in post-earthquake Haiti

Participatory engineering has been called for after major catastrophes, yet is often bypassed due to countervailing implementation of ‘quick fixes’. While immediate expert-driven solutions may be attractive, in the long-term they may be ineffective and inconsistent with the goals and capacities of local stakeholders. This article discusses the findings of National Science Foundation research by a team of three engineers and one social scientist who visited Haiti twice, four and seven months after the January 2010 earthquake, to investigate community participation in water and sanitation engineering processes in Léogâne. Methods included interviews with local inhabitants, water-sector actors, and government agencies; inspections of the engineering of the existing water and sanitation system; surveys of the affected population; and a participatory workshop to which numerous community-based organizations were invited. The research tests the potential for engineers to develop stakeholder-based participatory processes in a post-disaster context, which is hypothesized to produce better outcomes than traditional top-down authoritative planning processes. Focusing on the sanitation sector within a multi-stakeholder arena, the article analyzes the potential for various kinds of interactions amongst actors during unfolding decision-making processes at multiple scales, and assesses how each might contribute to better post-disaster engineering and ultimately more sustainable water and sanitation systems.

Can floods in large river basins be predicted from floods observed in small subbasins

Recent results from the analysis of peak floods observed in nested watersheds have revealed the existence of a scale invariant relationship between peak floods and drainage area at the scale of a single rainfall-runoff event. The relationship follows the power law E[Qe| A] = α(e)A θ(e) where E[Qe| A] is the expected value of peak flood at a given drainage area A, α(e) is the intercept, and θ(e) is the exponent for a given rainfall-runoff event ‘e’. These results also revealed that α(e) and θ(e) change from one rainfall-runoff event to another. In this article, we show that a log-linear relationship between α(e) and θ(e) can be used to simplify the problem of predicting α(e) and θ(e) from the physical characteristics of the catchment and rainfall. In particular, we show that α(e) can be predicted from peak floods observed in the smallest gauged subcatchment in the basin and its log-linear relationship with θ(e) can be used to predict peak flood at any location in the basin. We demonstrate this using observed peak floods from the Iowa River basin in the Upper Midwest part of United States.