Joshua R. Wyrick

Flood-driven topographic changes in a gravel-cobble river over segment, reach, and morphological unit scales

Regulated rivers generally incise below dams that cut off sediment supply, but how that happens and what the consequences are at different spatial scales is poorly understood. Modern topographic mapping at meter-scale resolution now enables investigation of the details of spatial processes. In this study, spatial segregation was applied to a meter-scale raster map of topographic change from 1999 to 2008 on the gravel-cobble, regulated lower Yuba River in California to answer specific scientific questions about how a decadal hydrograph that included a flood peak of 22 times bankfull discharge affected the river at segment, reach, and morphological unit scales. The results show that the river preferentially eroded sediment from floodplains compared to the channel, and this not only promoted valley-wide sediment evacuation, but also facilitated the renewal and differentiation of morphological units, especially in the channel. At the reach scale, area of fill and mean net rate of elevational change were directly correlated with better connectivity between the channel and floodplain, while the mean rate of scour in scour areas was influenced by the ratio of slope to bankfull Froude number, a ratio indicative of lateral migration versus vertical downcutting. Hierarchical segregation of topographic change rasters proved useful for understanding multiscalar geomorphic dynamics. This article is protected by copyright. All rights reserved.

Modeling Stormwater Basins for Potential Retrofit Designs

Outflow structures for stormwater detention basins can vary widely within a single watershed. The designs are often left to the whim of the local engineer. Why should there be such differences? This research attempts to discern the performance and efficiency of various typical and atypical weir shapes. To accomplish this task, a scale model of a stormwater detention basin was constructed with removable outlet weirs. The physical model was calibrated using typical weir structures (e.g. rectangular, V-notch, submerged orifice, etc.) in which accepted numerical models exist to calculate outflow discharge. Empirical stage-discharge relationships were then calculated for the atypical weir structures (e.g. stepped notch, combinations of typical shapes, etc.). The atypical weirs simulated herein represent the range of outlet structure designs located in the Upper Mantua Creek watershed, Gloucester County, New Jersey. This area of New Jersey has experienced large urban growth in the past 40 years, and many of the existing basins were not designed for the current runoff volumes they experience. Within this 7.3 square mile watershed, there are currently 61 stormwater basins and more than 50 different outlet structure designs. The ability of each stormwater basin to pass or detain a given flow event can now be determined empirically. From these data, appropriate retrofit designs can be implemented.