James E. Pizzuto

Evolution of a sediment wave in an experimental channel

The routing of bed material through channels is poorly understood. We approach the problem by observing and modeling the fate of a low-amplitude sediment wave of poorly sorted sand that we introduced into an experimental channel transporting sediment identical to that of the introduced wave. The wave essentially dispersed upstream and downstream without translation, although there was inconclusive evidence of translation late in the experiment when the wave was only 10–20 grain diameters high. Alternate bars migrated through zones of differing bed load transport rate without varying systematically in volume, celerity, or transport rate. Sediment that overpassed migrating bars was apparently responsible for dispersion of the wave. The evolution of the wave was well predicted by a one-dimensional model that contains no adjusted empirical constants. Numerical experiments demonstrate, however, that the theory does not predict sediment waves that migrate long distances downstream. Such waves can only be explained by the following processes not represented by the theory: selective bed load transport, spatial variations in bar and other form roughness, the mechanics of mobile armor, and perhaps other mechanisms.

How to Avoid Train Wrecks When Using Science in Environmental Problem Solving

I collaborations are increasingly common in many areas of science, but particularly in fields involved with environmental problems. This is because problems related to human interactions with the environment typically contain numerous parameters, reflect extensive human alterations of ecosystems, require understanding of physical–biological interactions at multiple spatial and temporal scales, and involve economic and social capital. Distilling useful scientific information in collaborative interactions is a challenge, as is the transfer of this information to others, including scientists, stakeholders, resource managers, policymakers, and the public. While this problem has been recognized by historians and philosophers of science, it has rarely been recognized and openly discussed by scientists themselves (but see NAS 1986). The participation of individuals from a diverse set of scientific disciplines has the potential to enhance the success of problem solving (USGS/ESA 1998). However, obstacles often arise in collaborative efforts for several well-known reasons. First, it is often difficult to find a common language because of disciplinary specialization (Wear 1999, Sarewitz et al. 2000). Second, existing scientific knowledge (theories, models, etc.) may reflect a historical scientific and sociopolitical context that may make it ill suited to address current environmental problems and questions (see, for example, Ford 2000, NSB 2000). Third, collaborations involving multiple disciplines may create difficulties owing to mismatches in space and time scales, in forms of knowledge (e.g., qualitative versus quantitative), and in levels of precision and accuracy (see, for example, Herrick 2000). Fourth, scientists are partly conditioned by nonscientific values. A social fabric may dictate scientists’ worldviews, lead them to favor certain assumptions over others, and underlie the way they study ecosystems (Boyd et al. 1991). In this article, we argue that the success of interdisciplinary collaborations among scientists can be increased by adopting a formal methodology that considers the structure of knowledge in cooperating disciplines. For our purposes, the structure of knowledge comprises five categories of information: (1) disciplinary history and attendant forms of available scientific knowledge; (2) spatial and temporal scales at which that knowledge applies; (3) precision (i.e., qualitative versus quantitative nature of understanding across different scales); (4) accuracy of predictions; and (5) availability of data to construct, calibrate, and test predictive models. By definition, therefore, evaluating a structure of knowledge reveals limitations in scientific understanding, such as what knowledge is lacking or what temporal or spatial scale mismatches exist among disciplines. The epistemological exercise of defining knowledge structures at the onset of a collaborative exercise can be used to construct solvable problems: that is, questions that can be an-

Ecological Forecasting and the Urbanization of Stream Ecosystems: Challenges for Economists, Hydrologists, Geomorphologists, and Ecologists

The quantity and quality of freshwater resources are now being seriously threatened, partly as a result of extensive worldwide changes in land use, and scientists are often called upon by policy makers and managers to predict the ecological consequences that these alterations will have for stream ecosystems. The effects of the urbanization of stream ecosystems in the United States over the next 20 years are of particular concern. To address this issue, we present a multidisciplinary research agenda designed to improve our forecasting of the effects of land-use change on stream ecosystems. Currently, there are gaps in both our knowledge and the data that make it difficult to link the disparate models used by economists, hydrologists, geomorphologists, and ecologists. We identify a number of points that practitioners in each discipline were not comfortable compromising on—for example, by assuming an average condition for a given variable. We provide five instructive examples of the limitations to our ability to forecast the fate of stream and riverine ecosystems one drawn from each modeling step: (a) Accurate economic methods to forecast land-use changes over long periods (such as 20 years) are not available, especially not at spatially explicit scales; (b) geographic data are not always available at the appropriate resolution and are not always organized in categories that are hydrologically, ecologically, or economically meaningful; (c) the relationship between low flows and land use is sometimes hard to establish in anthropogenically affected catchments; (d) bed mobility, suspended sediment load, and channel form—all of which are important for ecological communities in streams—are difficult to predict; and (e) species distributions in rivers are not well documented, and the data that do exist are not always publicly available or have not been sampled at accurate scales, making it difficult to model ecological responses to specified levels of environmental change. Meeting these challenges will require both interdisciplinary cooperation and a reviewed commitment to intradisciplinary research in the fields of economics, geography, quantitative spatial analysis, hydrology, geomorphology, and ecology.

Forecasting the combined effects of urbanization and climate change on stream ecosystems: from impacts to management options

Summary 1 Streams collect runoff, heat, and sediment from their watersheds, making them highly vulnerable to anthropogenic disturbances such as urbanization and climate change. Forecasting the effects of these disturbances using process‐based models is critical to identifying the form and magnitude of likely impacts. Here, we integrate a new biotic model with four previously developed physical models (downscaled climate projections, stream hydrology, geomorphology, and water temperature) to predict how stream fish growth and reproduction will most probably respond to shifts in climate and urbanization over the next several decades.2 The biotic submodel couples dynamics in fish populations and habitat suitability to predict fish assemblage composition, based on readily available biotic information (preferences for habitat, temperature, and food, and characteristics of spawning) and day‐to‐day variability in stream conditions.3 We illustrate the model using Piedmont headwater streams in the Chesapeake Bay watershed of the USA, projecting ten scenarios: Baseline (low urbanization; no on‐going construction; and present‐day climate); one Urbanization scenario (higher impervious surface, lower forest cover, significant construction activity); four future climate change scenarios [Hadley CM3 and Parallel Climate Models under medium‐high (A2) and medium‐low (B2) emissions scenarios]; and the same four climate change scenarios plus Urbanization.4 Urbanization alone depressed growth or reproduction of 8 of 39 species, while climate change alone depressed 22 to 29 species. Almost every recreationally important species (i.e. trouts, basses, sunfishes) and six of the ten currently most common species were predicted to be significantly stressed. The combined effect of climate change and urbanization on adult growth was sometimes large compared to the effect of either stressor alone. Thus, the model predicts considerable change in fish assemblage composition, including loss of diversity.5 Synthesis and applications. The interaction of climate change and urban growth may entail significant reconfiguring of headwater streams, including a loss of ecosystem structure and services, which will be more costly than climate change alone. On local scales, stakeholders cannot control climate drivers but they can mitigate stream impacts via careful land use. Therefore, to conserve stream ecosystems, we recommend that proactive measures be taken to insure against species loss or severe population declines. Delays will inevitably exacerbate the impacts of both climate change and urbanization on headwater systems.

Sediment pulses in mountain rivers: 1. Experiments

[1]xa0Sediment often enters rivers in discrete pulses associated with landslides and debris flows. This is particularly so in the case of mountain streams. The topographic disturbance created on the bed of a stream by a single pulse must be gradually eliminated if the river is to maintain its morphological integrity. Two mechanisms for elimination have been identified: translation and dispersion. According to the first of these, the topographic high translates downstream. According to the second of these, it gradually diffuses away. In any given river both mechanisms may operate. This paper is devoted to a description of three controlled experiments on sediment pulses designed to model conditions in mountain streams. Each of the experiments began from the same mobile-bed equilibrium with a set rate and grain size distribution of sediment feed. In one experiment the median size of the pulse material was nearly identical to that of the feed sediment. In the other two the pulse material differed in grain size distribution from the feed sediment, being coarser in one case and finer in the other case. In all cases the mode of pulse deformation was found to be predominantly dispersive, a result that constitutes the main conclusion of this paper. The pulses resulted in a notable but transient elevation of sediment transport rate immediately downstream. When the pulse was coarser than the ambient sediment, the bed downstream remained armored, and a migrating delta formed in the backwater upstream. When the pulse was finer than the ambient sediment, translation was observed in addition to dispersion. The presence of the finer material notably elevated the transport of ambient coarse material on the bed downstream. In part 2 [Cui et al., 2003], the experimental results are used to test a numerical model.

Sediment pulses in mountain rivers: 2. Comparison between experiments and numerical predictions

[1]xa0Mountain rivers in particular are prone to sediment input in the form of pulses rather than a more continuous supply. These pulses often enter in the form of landslides from adjacent hillslopes or debris flows from steeper tributaries. The activities of humans such as timber harvesting, road building, and urban development can increase the frequency of sediment pulses. The question as to how mountain rivers accommodate pulses of sediment thus becomes of practical as well as academic significance. In part 1 [Cui et al., 2003], the results of three laboratory experiments on sediment pulses are reported. It was found there that the pulses were eliminated from the flume predominantly by dispersion of the topographic high. Significant translation was observed only when the pulse material was substantially finer than the ambient load in the river. Here the laboratory data are used to test a numerical model originally devised for predicting the evolution of sediment pulses in field-scale gravel bed streams. The model successfully reproduces the predominantly dispersive deformation of the experimental pulses. Rates of dispersion are generally underestimated, largely because bed load transport rates are underestimated by the transport equation used in the model. The model reproduces the experimental data best when the pulse is significantly coarser than the ambient sediment. In this case, the model successfully predicts the formation and downstream progradation of a delta that formed in the backwater zone of the pulse in run 3. The performance of the model is less successful when the pulse is composed primarily of sand. This is likely because the bed load equation used in the study is specifically designed for gravel. When the model is adapted to conditions characteristic of large, sand bed rivers with low Froude numbers, it predicts substantial translation of pulses as well as dispersion.

The influence of riparian vegetation on stream width, eastern Pennsylvania, USA

We surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks. Cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches, while deposition is localized on active floodplains on the insides of bends. Our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches. Two dimensionless parameters, α and E , explain our observations. α represents the influence of grassy vegetation on rates of active floodplain deposition; it is 5 times higher in nonforested reaches than in forested reaches. E is proportional to rates of cutbank migration; it is 3 times higher in nonforested reaches than in forested reaches. Differences in width between forested and nonforested reaches are proportional to E/ α. In forested reaches, channels are wide with banks that are difficult to erode. Dense tree roots create a low value of E , and the channel migrates slowly. E/ α is high, however, because α is very low: shade from trees inhibits the growth of grass on active floodplains. In nonforested reaches, channels are narrow with banks that are easy to erode. E is high, and the channel migrates rapidly. E/ α is low, however, due to a very large value of α: grass grows readily on nonforested convex bank floodplains. Thus, differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active floodplains, implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetation-mediated rates of deposition on active flood-plains. These results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and floodplain depositional processes, rather than focusing on these processes in isolation from one another.

Mathematical modeling of autocompaction of a Holocene transgressive valley-fill deposit, Wolfe Glade, Delaware

Six thousand years of sediment accumulation and autocompaction in a transgressive valley-fill deposit are simulated numerically. Rates of accumulation of freshwater organic-rich mud, subtidal mud, and organic-rich mud of the modern salt marsh are specified by using existing sedimentologic, paleontologic, and geochronologic data. Processes of pore-water explusion and autocompaction are determined from finite-strain consolidation theory. Model parameters for subtidal mud facies are determined from laboratory consolidation tests. Model parameters for organic-rich deposits are determined by sampling equivalent modern sedimentary environments and by model calibration. Model calibration involves (1) reproducing the present distribution of void ratios measured in a 7.6 m Vibracore, (2) correctly calculating the preserved thickness of lithologic units, and (3) determining a history of elevation changes of the sediment surface that is consistent with the Delaware sea-level curve and the known sequence of paleoenvironments of Wolfe Glade, Delaware. The results indicate that preserved horizons in the 10 m Holocene section of Wolfe Glade have been lowered a maximum of 2.3 m. The rate of lowering due to autocompaction alone has been one-half to one-third of the rate of sea-level rise during most of the past 6000 yr. Numerical experiments suggest that organic-rich freshwater wetland deposits are primarily responsible for the extensive autocompaction at Wolfe Glade. Our approach may be used to correct Holocene coastal stratigraphic data for autocompaction, but careful model calibration (based in part on the stratigraphy) will usually be required.

Channel adjustments to changing discharges, Powder River, Montana

Sixteen years of annual surveys reveal how Powder River responds to varying discharges. During 1978, the second largest recorded daily mean discharge occurred. Cutbank migration, bed degradation, net bank erosion, and overbank deposition all contributed to increase the channel area at 12 cross sections by an average of 62%. During the ensuing years, the channel area decreased as sediment was stored in low-lying benches adjacent to the active bed of the channel. The survey data indicate that the balance between bank erosion and deposition varies with discharge. In years when the annual maximum daily mean discharge is 3 /s (a flow of 60 m 3 /s has a recurrence interval of ∼1.1 yr), bank erosion and deposition are approximately equal. In years when the annual maximum daily mean discharge is between 60 and ∼150 m 3 /s (a discharge of 150 m 3 /s has a recurrence interval of ∼2.7 yr), bank deposition exceeds bank erosion, and the channel contracts, often by developing benches. In years with higher discharges, the channel expands through net bank erosion. These results demonstrate that the channel of Powder River is influenced by a wide variety of formative discharges. Powder River9s recent history of expansion and contraction and the development of prominent benches cannot be explained by equilibrium models based on a single, channel-forming discharge.

Ontogeny of a flood plain

The ontogeny of five flood-plain segments is described for a period of 18 yr following a major flood in 1978 on the Powder River in southeastern Montana. The flood plains developed on relatively elevated sand and gravel deposits left within the channel by the 1978 flood. In cross section, the flood plains resemble benches with well-developed natural levees. Flood-plain growth occurred as sediment was draped onto preexisting surfaces in layers of sand and mud a few centimeters to decimeters thick, resulting in some lateral, but mostly vertical accretion. Annual and biannual measurements indicated that, as the flood-plain segments grew upward, the annual rate of vertical accretion decreased as the partial duration recurrence interval for the threshold or bankfull discharge increased from 0.16 to 1.3 yr. It is clear that a constant recurrence interval for overbank flow cannot be meaningfully assigned to this type of flood-plain ontogeny. These flood plains did not grow on migrating point bars, and vertical accretion at least initially occurred within the channel, rather than across the valley flat during extensive overbank flows. Sediments of these flood plains define narrow, elongated stratigraphic units that border the active channel and onlap older flood-plain deposits. These characteristics are considerably different from those of many facies models for meandering river deposits. Facies similar to those described in this paper are likely to be preserved, thereby providing important evidence in the geologic record for episodes of periodic channel expansion by ancient rivers.