Journal of Hydrologic Engineering

In response to the destructive floods of 1983, 1986, 1995, and 1997, the U.S. Army Corps of Engineers and the Reclamation Board of the State of California are partnering a study to investigate flood damage reduction and ecosystem restoration opportunities in the Sacramento and San Joaquin River Basins, California. This paper provides a short background on the study and details the methodology used to develop the baseline technical hydrology needed to support ongoing system analyses and modeling efforts. Discussion emphasizes conceptual relations between rain flood hydrology and floodplain delineation, a short retrospective of Central Valley flood events, and a method for developing synthetic flood hydrographs. Conclusions are drawn regarding the effective use of gaged flow data in flood frequency analyses, benefits of performing flood frequency analyses from a watershed perspective, and potential of Comprehensive Study methodologies for use in other macroscale studies.

A computer application and national geospatial database have been developed to support the calculation of flooding flow $\left(Q_f\right)$ and threshold runoff across the conterminous United States and Alaska. Flooding flow is the flow required to cause a stream to slightly overflow its bank and cause damage. Threshold runoff [L], defined as the depth of runoff required to cause flooding, is computed as flooding flow divided by the unit hydrograph peak flow. A key assumption in this work is that the two-year return flood $\left(Q_2\right)$ is a useful surrogate for flooding flow. The application described here computes flood magnitude estimates for selected return periods $(Q_2\text{,}$ $Q_5\text{,}$ $Q_{10}\text{,}$ etc.) using regression equations published by the U.S. Geological Survey for each of 210 hydrologic regions. The application delineates basin boundaries and computes all basin parameters required for the flood frequency calculations. The geographic information system database that supports these calculations contains terrain data [digital elevation models (DEMs) and DEM derivatives], reference data, and 89 additional data layers related to climate, soils, geology, and land use. Initial results indicate that there are some practical limitations associated with using $Q_2$ regression equations to estimate flooding flow.

Conventional methods for estimating the average return period, $E\left(T\right)\text{,}$ and failure risk, R, generally ignore the impact of persistence in annual streamflows on associated probabilistic statements and streamflow statistics. Recent evaluations of streamflow observations indicate statistically significant serial correlations (persistence) associated with annual low flows in the United States. We define the average occurrence interval $E\left(T\right)$ as the expected time to the first event, and we present a method for estimating $E\left(T\right)$ and R in the presence of persistence. We show that for observed ranges of persistence, $E\left(T\right)$ can be nearly 100% greater and R more than 20% lower than conventional estimates. This implies that the expected design life of a system is longer when persistence is taken into account. Also, by ignoring persistence, low flow quantiles may be underestimated by 50% or more. An evaluation of the effect of persistence on drought risk estimation across the U.S. is presented.

This paper focuses on the regional impact of climate change on several critical water quantity and quality issues. The study area is the Ali Efenti Basin in central Greece. The soil and water assessment tool daily step conceptual model has been applied to simulate the water cycle and the nitrogen transport within the catchment. The outputs of six general circulation models have been used to perturb the time series of precipitation and temperature. For all of the scenarios a decrease in streamflow is observed and there is an increase in the magnitude of floods for certain return periods. Changes in surface water runoff influence nitrogen losses, resulting in an annual reduction of the nitrogen flux to the water body.

Changes in land use can have significant impacts on hydrology and the environment. The Long-Term Hydrologic Impact Analysis model and a geographic information system-based Soil Conservation Service curve number method were used to estimate average annual runoff and runoff for rainfall events with varying return periods, respectively, for NASA’s John F. Kennedy Space Center (KSC) within the Indian River Lagoon watershed (IRL), Florida, for 1920, 1943, and 1990 land uses. Changes in estimated runoff depths due to land-use alteration between 1920 and 1990—for design rainfall events ranging from a 24 h/1 year to a 24 h/100 year return period—ranged from 7–17% for KSC to 22–55% for the IRL watershed. The percentage changes in average annual runoff due to land-use changes were much greater. Between 1920 and 1990, estimated average annual runoff for the KSC increased about 49%, and the IRL runoff increased nearly 113%. For the KSC and IRL, most of the increase in runoff between 1920 and 1990, both for single events and annually, came from landscape urbanization, although increased agricultural land uses in the IRL also contributed to increased runoff. Large differences in estimated runoff between the KSC and IRL are due to differences in the amount of urban land use within the respective areas (1990 urban land uses for the IRL are 34.8 versus 20.5% for KSC). Land-use change can have a dramatic impact on annual runoff volume, as demonstrated in this study; thus the effects of land-use change on annual or long-term runoff should be considered in land-use planning.

The physics-based sediment transport equations are derived from the assumption that the sediment transport rate can be determined by a dominant variable such as flow discharge, flow velocity, slope, shear stress, stream power, and unit stream power. In modeling of sheet erosion/sediment transport, many models that determine the transport capacity by one of these dominant variables have been developed. The developed models mostly simulate steady-state sheet erosion. Few models that are based on the shear-stress approach attempt to simulate nonsteady state sheet erosion. This study qualitatively investigates the applicability of the transport capacity models that are based on one of the commonly employed dominant variables—unit stream power, stream power, and shear stress—to simulate nonsteady state sediment loads from steep slopes under different rainfall intensities. The test of the calibrated models with observed data sets shows that the unit stream power model gives better simulation of sediment loads from mild slopes. The stream power and the shear stress models, on the other hand, simulate sediment loads from steep slopes more satisfactorily. The exponent $\left(k_i\right)$ in the sediment transport capacity formula is found to be 1.2, 1.9, and 1.6 for the stream power model, the shear stress model, and the unit stream power model, respectively.

In the literature approximate and exact theoretical equations are presented for steady-state flow toward a discharging well, which is located eccentrically in a circular aquifer along whose outer cylindrical boundary either the drawdown or the flux is zero. This paper presents the results of using a physical model to test the validity of an analytical solution that describes drawdown for a well, which is placed eccentrically in a circular island aquifer. It is constructed in a sandbox model with several wells to conduct numerous steady-state tests. This study shows that the analytical equations are valid for small drawdowns. However, the equations do not represent higher drawdowns in the wells that are eccentrically located in the circular island model under steady-state flow in an unconfined aquifer.