Jonathan D. Phillips

Fluvial sediment budgets in the North Carolina Piedmont

Phillips, J.D., 1991. Fluvial sediment budgets in the North Carolina Piedmont. Geomorphology, 4:231-241. Sediment budgets for four large ( > 1000 km 2) drainage basins in the Piedmont physiographic province of North Carolina were estimated from data compiled during erosion and sedimentation surveys. Budgets for the Upper Tar, Upper Neuse, Haw and Deep River basins showed broadly similar trends in allocation of eroded sediment among yield and storage. Sediment yield as a percentage of mean annual gross erosion within the basins averaged 10%. This was less than the rate of alluvial storage, which averaged 14% of annual gross erosion. About 76% of the mean annual erosion was stored as colluvium on hillslopes. There are relatively small differences in sediment delivery and storage ratios among the study basins. This suggests that the relative order of magnitude of the allocation of eroded sediment for southern Piedmont rivers is colluvial storage, alluvial storage and yield. Of the sediment that does reach the streams, more than half is stored as alluvium on an average-annual basis. Aggrading channels and floodplains, siltation of benthic habitats and sediment pollution problems are thus likely to persist unless dramatic reductions in upland erosion or in sediment delivery to streams are realized. With respect to longer-term basin evolution, results point to the need for an improved understanding of the temporal scales of colluvial and alluvial sediment storage.

Divergence, Convergence, and Self-Organization in Landscapes

Many theories, old and new, of landscape and earth-surface system development involve concepts of self-organization. There are at least eleven distinct definitions of self-organization in the literature that are relevant to landscapes. Some have profoundly different implications with respect to the nature and trajectories of landscape evolution and earth-surface system behavior, including whether evolution is convergent or divergent, whether entropy or energy dissipation is maximized or minimized, the role of chaos, and the mechanisms by which self-organized patterns are generated. Despite these differences, most self-organization concepts can be broadly aggregated into two categories: those concerned with the evolution of order and regularity in the aggregate or ensemble properties of the landscape, and those concerned with the differentiation of landscapes into more diverse spatial units. This paper presents a theory of spatially divergent self-organization related to the latter, showing that autogenic differentiation is directly linked to dynamical instability and chaos. The determination of the self-organization properties of a landscape should be a starting point rather than a goal of geographic explanation. The extent to which field-testable hypotheses are generated, or explanations provided based on process mechanics or landscape history, will ultimately determine the utility of self-organization concepts and methods in physical geography.

An evaluation of the factors determining the effectiveness of water quality buffer zones

Abstract This study examines the relative role of slope length, slope gradient, surface roughness, and soil hydrologic properties on determining the pollution control effectiveness of vegetated buffer zones. Two models describing buffer conveyance capacity are introduced. The first assumes that pollutant transport through the buffer depends on the energy of overland flow and is based on Bagnolds stream power concept. The second assumes that the buffer effectiveness is a function of total contact time of both surface runoff and throughflow and is based on Darcys law and the Manning equation. The hydraulic and detention models, respectively, are applied to the problem of estuarine shoreline buffer zone delineation in Carteret County, North Carolina. Results show that where solid-phase pollutants transported as suspended or bedload in overland flow are the major concern, slope gradient is the most critical factor, followed by soil hydraulic conductivity. Where dissolved pollutants that are transported by both surface and subsurface flow are of concern, buffer width is by far the most important factor, with soil moisture storage capacity also playing a role. Methods developed here may be applied to any water quality buffer delineation problem to determine the relative influence of soil properties, geomorphology, and surface conditions.

Nonpoint source pollution control effectiveness of riparian forests along a coastal plain river

A detention-time model of water quality buffer zones is used to evaluate the nonpoint source pollution control effectiveness of riparian forests in a two-county area of the lower Tar River basin, North Carolina. Soil map units, which represent specific combinations of soil, topography, and vegetation characteristics, are compared in terms of their relative ability to filter nitrate in agricultural runoff. All typical riparian forests provide significant water quality protection, but there is a wide variation in buffer effectiveness. This suggests a need for flexibility in determining buffer widths. A range of 15–80 m is appropriate for the soil-landform-vegetation complexes found in riparian zones within the study area. Buffer widths of 60 m — and often much less — are generally adequate on the soils likely to be used for agricultural production.

Biogeomorphology and landscape evolution: The problem of scale

Abstract Vegetation may vary continuously and rapidly, while landforms sometimes vary episodically and relatively slowly. This disparity in temporal scales creates scale linkage problems in dealing with landform-vegetation interactions. Four theoretical approaches for coping with these issues are outlined, and illustrated via applications to problems arising from fieldwork on the coastal plain of North Carolina. The first approach, the transient form ratio, is used to determine whether ecosystems and geomorphic environments preserved in Pleistocene stratigraphy represent stable landform-ecosystem combinations which should reflect landform and vegetation responses to sea level rise. The second, the information criterion, is applied to estimate the appropriate time steps for simulation modelling, and appropriate spatial resolutions for field studies, of vegetation effects on soil thickness in coastal plain uplands. The abstracted systems argument, the third approach, is used to show that alluvial sedimentation and floodplain ecological dynamics cannot be considered independently in studies of floodplain system response to post-European settlement. Finally, ratios of relaxation times and durations suggest that changes in upland erosion regimes associated with historical devegetation are best treated as a singular perturbation in the context of Quaternary landscape evolution.

Divergent evolution and the spatial structure of soil landscape variability

Abstract The spatial structure of soil variability at the landscape scale was examined on adjacent geomorphic surfaces dating from 80 to 200 ka in eastern North Carolina. The purpose was to determine whether there is evidence at broader scales (distances of 102–104 m) for the divergent evolution observed in the field at very detailed scales (distances of 100–102 m). The state probability function (SPF), which measures spatial dependence for categorical environmental data along a transect, was applied to soil series mapped at a 1:24,000 scale. The older Talbot Terrace and younger Pamlico Terrace surfaces showed distinctly different patterns of spatial variability. The range of spatial dependence was shorter on the older surface (about 200 vs. 300 m), and the SPF was higher at any given distance, indicating more variability. The SPF for the Pamlico surface also indicates a periodicity related to fluvial dissection of the landscape, which is not readily detectable on the Talbot transect despite its greater degree of dissection. The results confirm earlier field studies which suggest that pedogenesis is marked by divergence, whereby differences in initial conditions or local perturbations persist and increase to produce a more variable soil cover.

Sediment characteristics of an extreme flood: 1993 upper Mississippi River valley

The 1993 Mississippi River flood was notable for its high magnitude, long duration, summer occurrence, and low sediment discharge. A field survey of a 70-km-long reach in the vicinity of Quincy, Illinois, revealed that the event was characterized by 100 yr flood had remarkably little sedimentological or geomorphological impact on the flood plain within the study reach because the transport effectiveness of floods in large drainage basins is influenced by event sequencing in the same manner as floods in small watersheds, and the cohesive flood-plain soils were not susceptible to erosion.

Nonlinear dynamical systems in geomorphology: revolution or evolution?

Abstract Geomorphic systems typically exhibit complex, apparently random behaviors and patterns in both spatial and temporal domains. This complexity can arise from the cumulative impacts of individual process-response mechanisms which are far too numerous to be accounted for in individual detail, or due to multiple controls over process-response relationships which operate over a range of spatial and temporal scales (stochastic complexity). Nonlinear dynamical systems (NDS) theory—which includes specific techniques and concepts such as chaos, dissipative structures, bifurcation and catastrophe theory, and fractals—shows that extreme complexity can also arise due to the nonlinear dynamics and couplings of relatively simple systems represented by relatively small equation systems (deterministic complexity). While emphasis in earth science has been largely on stochastic complexity, geomorphologists have long recognized the presence of deterministic complexity as well. Recent developments in the study of nonlinear dynamical systems have implications for the ability to make reliable long-term predictions and therefore applications of NDS in geomorphology may be revolutionary to some extent. However, virtually all the basic conceptual underpinnings of NDS theory can be mapped onto existing and even traditional theoretical concepts in geomorphology. In this sense, the use of NDS concepts in understanding earth surface processes and landforms is evolutionary. The link between traditional and NDS-based concepts is illustrated by analyzing a generalized geomorphic mass-flux system. It is shown that the system is in most cases unstable (consistent with chaos or deterministic complexity). However, the three attractor states of the system correspond exactly to the states of aggradation, degradation, and steady state.

Geomorphic impacts of flash flooding in a forested headwater basin

Abstract Geomorphic impacts of a July 2001 flood in the Hungry Mother basin near Marion, Virginia, were examined to shed light on the relationships between channel characteristics and the frequency of channel-modifying discharges. Creeks in the study area have been observed for many years, with no significant channel changes since at least 1985. The 2001 flood had a recurrence interval of >200 years, and caused the only channel change, bank erosion, and transport of coarse channel material observed in recent decades. The paucity of fines in channels before or after the flood, and the absence of sub-sand sized material in the flood deposits, indicates that normal, frequent, well-below-bankfull flows are sufficient to transport the (apparently limited) supply of fines. The large particles transported during the 2001 flood after years of inactivity indicate that relatively rare floods are necessary to mobilize this material. This suggests the notion of a bimodal ‘dominant’ discharge. On the one hand frequent flows considerably below bankfull levels are sufficient to maintain the channel and prevent significant accumulation of fine sediments and organic matter. On the other hand, rare floods are necessary to transport the coarser bed material and erode channel banks. In the Hungry Mother area, bed material has no relationship to normal flows, or to flows with recurrence intervals on the order of 1–3 years. Bankfull discharge is apparently not related to either the maintenance or channel-changing dominant discharges. These results suggest that the use of channel dimensions and/or bed material as surrogate indicators of hydrologic regimes can be quite complicated, and that in some streams bankfull flow has no particular significance in terms of sediment transport and channel modifications.

The end of equilibrium

Abstract An overview of the proceedings of the 1992 Geomorphology symposium on Geomorphic Systems several trends. First, there is a pervasive concern with the presence and nature of geomorphic equilibria. However, the focus is not on detection of a single, stable equilibrium condition for geomorphic systems, but on the behavior of systems away from equilibrium, the presence and prevalence of dis- and nonequilibrium forms, and the presence of multiple equilibria for a given systems. A second major theme is the nonlinear behavior of geomorphic systems. This nonlinearity provides a bridge between existing threshold concepts in the discipline and the emerging study of earth surface phenomena as complex nonlinear dynamical systems. In connection with this, the critical role of feedback mechanisms in geomorphic systems emerges as a third general theme. Finally, a number of papers implicity or explicity raise the possibility of universality -i.e., that there are universal properties of geomorphic system behavior independent of location- and time-specific controls. Taken as a whole, the symposium papers are interpreted as signalling the possible end of classic equilibrium studies in the sense of searching for a single or typical equilibrium response to a given set of processes or controls. Conceptual frameworks which emphasize, or at least include, nonequilibrium responses and multiple equilibria are on the ascension. Historical perspectives suggest that these new developments are not necessarily inconsistent with prevailing and traditional geomorphic thought.