Peter A. Nelson

Extraordinary Flood Response of a Small Urban Watershed to Short-Duration Convective Rainfall

Abstract The 9.1 km2 Moores Run watershed in Baltimore, Maryland, experiences floods with unit discharge peaks exceeding 1 m3 s−1 km−2 12 times yr−1, on average. Few, if any, drainage basins in the continental United States have a higher frequency. A thunderstorm system on 13 June 2003 produced the record flood peak (13.2 m3 s−1 km−2) during the 6-yr stream gauging record of Moores Run. In this paper, the hydrometeorology, hydrology, and hydraulics of extreme floods in Moores Run are examined through analyses of the 13 June 2003 storm and flood, as well as other major storm and flood events during the 2000–03 time period. The 13 June 2003 flood, like most floods in Moores Run, was produced by an organized system of thunderstorms. Analyses of the 13 June 2003 storm, which are based on volume scan reflectivity observations from the Sterling, Virginia, WSR-88D radar, are used to characterize the spatial and temporal variability of flash flood producing rainfall. Hydrology of flood response in Moores Run is c...

Redistribution of pyrogenic carbon from hillslopes to stream corridors following a large montane wildfire

Pyrogenic carbon (PyC) constitutes a significant fraction of organic carbon in most soils. However PyC soil stocks are generally smaller than what is expected from estimates of PyC produced from fire and decomposition losses, implying that other processes cause PyC loss from soils. Surface erosion has been previously suggested as one such process. To address this, following a large wildfire in the Rocky Mountains (CO, USA), we tracked PyC from the litter layer and soil, through eroded, suspended, and dissolved solids to alluvial deposits along river sides. We separated deposited sediment into high- and low-density fractions to identify preferential forms of PyC transport, and quantified PyC in all samples and density fractions using benzene polycarboxylic acid markers. A few months after the fire, PyC had yet to move vertically into the mineral soil and remained in the organic layer or had been transported off site by rainfall driven overland flow. During major storm events PyC was associated with suspended sediments in river water, and later identified in low-density riverbank deposits. Flows from an unusually long-duration and high magnitude rain storm either removed or buried the riverbank sediments approximately one year after their deposition. We conclude that PyC redistributes after wildfire in patterns that are consistent with erosion and deposition of low-density sediments. A more complete understanding of PyC dynamics requires attention to the interaction of post-fire precipitation patterns and geomorphological features that control surface erosion and deposition throughout the watershed. Index Terms: Carbon Cycling, Soils, Biogeochemistry.

Morphodynamic response of a variable‐width channel to changes in sediment supply

River channels commonly exhibit downstream variations in channel width, which can lead to the development of alternating shallow and deep areas known as riffle-pool sequences. The response of these channels to variations in sediment supply remains largely unexplored. Here we investigate the morphodynamic response of a variable-width channel to changes in sediment supply through laboratory experiments conducted in a straight flume in which we imposed sinusoidal variations in width. We first developed equilibrium conditions under a constant sediment supply and then eliminated the sediment feed to create a degraded, armored bed. This sediment-starved bed was subjected to two types of sediment supply increases: a return to the initial constant supply, and the introduction of a well-sorted sediment pulse (analogous to gravel augmentation). Riffles and pools formed in wide and narrow areas, respectively, and the location of and relief between riffles and pools remained the same throughout all experimental runs, regardless of the sediment supply. The primary channel response to changes in supply was adjustment of the overall slope. The sediment pulse evolved primarily through dispersion rather than translation, which contrasts with prior gravel augmentation experiments conducted in constant-width channels and suggests that width variation and resulting riffle-pool topography enhances pulse dispersion. Our results indicate that width variation is a primary control on the location and relief of riffles and pools in straight channels, and sediment supply changes are unlikely to affect riffle-pool morphology when bank geometry is fixed and water discharge is steady.

Coevolution of bed surface patchiness and channel morphology: 1. Mechanisms of forced patch formation

Riverbeds frequently display a spatial structure where the sediment mixture composing the channel bed has been sorted into discrete patches of similar grain size. Even though patches are a fundamental feature in gravel bed rivers, we have little understanding of how patches form, evolve, and interact. Here we present a two-dimensional morphodynamic model that is used to examine in greater detail the mechanisms responsible for the development of forced bed surface patches and the coevolution of bed morphology and bed surface patchiness. The model computes the depth-averaged channel hydrodynamics, mixed-grain-size sediment transport, and bed evolution by coupling the river morphodynamic model Flow and Sediment Transport with Morphological Evolution of Channels (FaSTMECH) with a transport relation for gravel mixtures and the mixed-grain-size Exner equation using the active layer assumption. To test the model, we use it to simulate a flume experiment in which the bed developed a sequence of alternate bars and temporally and spatially persistent forced patches with a general pattern of coarse bar tops and fine pools. Cross-stream sediment flux causes sediment to be exported off of bars and imported into pools at a rate that balances downstream gradients in the streamwise sediment transport rate, allowing quasi-steady bar-pool topography to persist. The relative importance of lateral gravitational forces on the cross-stream component of sediment transport is a primary control on the amplitude of the bars. Because boundary shear stress declines as flow shoals over the bars, the lateral sediment transport is increasingly size selective and leads to the development of coarse bar tops and fine pools.

Finite amplitude bars in mixed bedrock‐alluvial channels

We present a nonlinear asymptotic theory of fully developed flow and bed topography in a wide channel of constant curvature to describe finite amplitude perturbations of bottom topography, subject to an inerodible bedrock layer. The flow field is evaluated at the leading order of approximation as a slowly varying sequence of locally uniform flows, slightly perturbed by a weak curvature-induced secondary flow. Using the constraint of constant fluid discharge and sediment flux, we calculate an analytical solution for the cross-sectional profile of flow depth and bed topography, and we determine the average slope in the bend necessary to transport the sediment supplied from a straight, alluvial, upstream reach. Both fully alluvial bends and bends with partial bedrock exposure are shown to require a larger average slope than a straight upstream reach; the relative slope increase is much larger for mixed bedrock-alluvial bends. Curvature and sediment supply are shown to have a strong effect on the characteristics of the point bars in mixed bedrock-alluvial channels. Higher curvature bends produce bars of larger amplitude and more bedrock exposure through the cross section, and increasing the sediment supply leads to taller and wider point bars. Differences in the relative roughness of sediment and bedrock have a smaller, secondary effect on point bar characteristics. Our analytical approach can potentially be extended to the case of arbitrary, yet slowly varying, curvature, and should ultimately lead to an improved understanding of the formation of meanders in bedrock channels.

A Thousand Hours of GW Librae: The Eruption and Aftermath

We report the results of a worldwide observing campaign in 2007 April, which covered the second known outburst of the dwarf nova GW Librae, as well as the aftermath in 2008 and 2010. The data consist of time-series photometry obtained from several locations around the Earth. The main eruption lasted 26 days. It began with a sharp 1 day rise to maximum light at V = 8.3, after which the star dimmed slowly for a while, then dropped off suddenly, landing at V = 15. The main eruption was followed by a slow, shallow drop toward quiescence. As expected, GW Lib showed powerful and long-lasting superhumps in its light curve at a period slightly longer than Porb. These superhumps took a long time (10 days) to appear and continued for at least 90 days after the initial eruption. This time frame agrees with the general idea that cataclysmic variables (CVs) of very short Porb are actually quite old, with small secondaries that have been exhausted by mass transfer over eons of evolution. A superhump period excess of 1.3% suggests a secondary star mass near 0.06 M⊙. A bolometric accounting of accretion light during the outburst yields a fairly good estimate of the time-averaged accretion rate; for the measured distance of 100 pc, we estimate Lbol = 8( ± 2)1031 ergs s-1, which suggests dM/dt = 1.3( ± 0.3)10-11 M⊙ yr-1. We obtained an additional 102 nights of photometry during 2008 and 2010. During both years of posteruption observation, the star was near quiescence at V ~ 16.7. Its white dwarf pulsations, famous before outburst, were missing in both years. However, in 2008 a new pulsation appeared, with a period of ~20 minutes (~73 cycles day-1) and properties not previously seen in any other stars of this type. The signal wandered slightly in frequency and amplitude, suggesting an underlying poor coherence and/or unresolved multiplet structure. This might signal a new type of pulsation in the heated white dwarf, or it could arise from the accretion disk itself. The star also commonly showed the famous but mysterious 2.1 hr signal in posteruption data. It appears, however, that the dominant (fundamental) signal is actually at 4.2 hr (~5.5 cycles day-1). The origin of this signal remains as obscure as ever. This clock seems to lose phase in a few days or less; such a low coherence suggests an origin in the accretion disk.

Detection of very high altitude fallout from the comet Shoemaker-Levy 9 explosions in Jupiter's atmosphere

A comparison between visible light charge-coupled device (CCD) photometry and model light curves for several Jovian satellite eclipses reveals that significant attenuating material was present in Jupiters atmosphere up to about 300 km (relative to the 1-bar pressure level) over the comet Shoemaker-Levy 9 impact sites a month after the explosions occurred.

RETURN OF THE KING: TIME-SERIES PHOTOMETRY OF FO AQUARII’S INITIAL RECOVERY FROM ITS UNPRECEDENTED 2016 LOW STATE

In 2016 May, the intermediate polar FO~Aqr was detected in a low state for the first time in its observational history. We report time-resolved photometry of the system during its initial recovery from this faint state. Our data, which includes high-speed photometry with cadences of just 2 sec, shows the existence of very strong periodicities at 22.5 min and 11.26 min, equivalent to the spin-orbit beat frequency and twice its value, respectively. A pulse at the spin frequency is also present but at a much lower amplitude than is normally observed in the bright state. By comparing our power spectra with theoretical models, we infer that a substantial amount of accretion was stream-fed during our observations, in contrast to the disk-fed accretion that dominates the bright state. In addition, we find that FO~Aqrs rate of recovery has been unusually slow in comparison to rates of recovery seen in other magnetic cataclysmic variables, with an

Effects of Urbanization on Flow Duration and Stream Flashiness: A Case Study of Puget Sound Streams, Western Washington, USA

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T Pyxidis: death by a thousand novae

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