14 Two model scenarios illustrating the effects of land use and climate change on gravel riverbeds of suburban Maryland, U.S.A.

doi:10.1016/S0928-2025(07)11133-0

Developments in Earth Surface Processes

Volume 11, 2007, Pages 359-381

https://doi.org/10.1016/S0928-2025(07)11133-0 Get rights and content

Abstract

We model two scenarios consisting of 10 years of daily discharges to illustrate the effects of changing land use and climate on gravel riverbeds. The managed growth/no climate change (MGNCC) scenario represents minimum effects of land use and climate change, while the urban sprawl/climate change (USCC) scenario represents more extensive effects. We apply our scenarios to a 30 m reach of the Northwest Branch of the Anacostia River. We use downscaled precipitation estimates from the Hadley2 global circulation model (GCM) to account for climate change, and use a continuous hydrological model to produce discharge estimates. A sediment transport model, combined with empirical formulae to specify upstream sediment inputs, computes changes in grain-size distribution, bedload and suspended material discharge, suspended sediment concentration, the areal fraction of the bed in motion, bed elevation and slope, the silt–clay content of the active layer, and something new: the fraction of exposed bedrock in the active layer. The USCC scenario is characterized by larger and more frequent storm flows than the MGNCC scenario, which in turn creates increased bedload and suspended load transport, increased bed mobility, and higher suspended sediment concentrations. The USCC scenario is also characterized by extreme variability in mud and bedrock content of the active layer. Convincing model predictions of the influence of climate and land use changes on gravel riverbeds will require additional study of (1) upstream sediment supply, (2) mud storage and remobilization in gravel streams, and (3) the controls on exposed bedrock in the active layer.

Introduction

Changes in climate and land use both have profound effects on rivers (Palmer et al., 2002; Macklin et al., 1992; Leopold, 1968). Climate changes influence precipitation and temperature, two variables that either directly or indirectly control the supply of water and sediment to stream channels. Land use changes, particularly conversion from agriculture to urban land uses, also influence storm discharges and sediment supply (Yorke and Herb, 1978). Because storms and sediment supply represent two of the most significant controlling variables on stream morphology, land use, and climate changes are clearly important drivers in fluvial geomorphology.

Driven by the industrial revolution and population growth, anthropogenic influences on stream channels have probably never been greater (Hooke, 2000). These influences are primarily exerted by changes in climate and land use. For example, predictions of global climate change suggest future doubling of atmospheric CO₂, increases in average temperatures by 2–6°C, and increased variability in precipitation (International Panel on Climate Change (IPCC), 1997). At the same time, agricultural land in developed countries is increasingly being converted to urban and suburban land uses (e.g., Irwin and Bockstael, 2004).

The profound influence of changes in climate and land use on streams has been widely noted, but methods for assessing the effects of these changes are poorly developed. In this study, we describe a preliminary effort to determine, in a specific study area, how these drivers acting together collectively influence the bed characteristics of a gravel-bed river. Our study is primarily motivated by the ultimate goal of predicting the ecological effects of changing land use and climate, so we particularly focus on variables that are important to organisms living in gravel-bed streams of our humid temperate study area (though this paper does not explicitly address the connection between hydraulic and sediment transport processes and ecological processes).

This paper is designed to present our approach to modeling these processes and to present the results of modeling specific scenarios that illustrate the combined effects of both land use and climate change on gravel riverbeds. This paper does not address the relative contributions of each driver separately to changes in streambed morphology and composition. A future publication is planned to address this interesting issue.

Section snippets

Regional setting

The Northwest Branch of the Anacostia River provides the field setting for our study. The Northwest Branch has a drainage area of 54.6 km², and it is located northwest of Washington, DC in Montgomery County, Maryland (detailed location maps are provided by Moore and Palmer, 2005; Palmer et al., 2002; Yorke and Herb, 1978). A United States Geological Survey (USGS) gaging station at the outlet of the basin has been active since 1924. The watershed is underlain by metamorphic rocks and is of low

Methods

The goal of our research is to evaluate the influence of specific land use and climate change scenarios on stream channel bed sediment characteristics in the mid-Atlantic region. To accomplish this goal, we first defined two scenarios with differing land use and climate characteristics. One scenario we term the “Managed Growth/No Climate Change Scenario (MG/NCC)”, while the other is referred to as the “Urban Sprawl/Climate Change Scenario (USCC)”. These two scenarios reflect extreme end members

Results

The total precipitation varied considerably between the MGNCC and the USCC scenarios. For the MGNCC scenario, a total of 2601 mm occurred in the 10-year period between 1960 and 1969. For the USCC scenario, a total of 3193 mm occurred between 2090 and 2099.

The greater precipitation estimated for the USCC scenario is reflected in flow duration curves of daily discharges obtained from hydrological modeling (Fig. 14.3). Only the upper ends of the flow duration curves are illustrated in Fig. 14.3,

Interpretations of the results

The results presented above can be divided into two categories. One set of results may be derived directly from the higher discharges associated with the USCC scenario. Not surprisingly, these include increased bedload transport, increased bed mobility, and increased suspended sediment concentrations. All of these results are easily associated with higher discharges. The other set of results includes the increased variability in mud and bedrock fractions in the active layer. These results are

Conclusions

In this paper we have reported model results for two scenarios of combined land use and climate change. The models are designed to predict changes in bed elevation, slope, grain-size distribution, bedrock exposure, and sediment transport processes for a study area of the Northwest Branch of the Anacostia River in Montgomery County, Maryland, just northwest of Washington, DC. The models provide input for predicting the effects of land use and climate change on stream ecology, so particular

Acknowledgments

Funding and support for this project was provided by the U.S. Environmental Protection Agency, including the Science to Achieve Results (STAR) Program (EPA number: R828012) and the Global Climate Change Program (EPA numbers: 1W0594NAEX and R83038701). Two anonymous reviewers provided very helpful comments.

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14 Two model scenarios illustrating the effects of land use and climate change on gravel riverbeds of suburban Maryland, U.S.A.

Abstract

Introduction

Section snippets

Regional setting

Methods

Results

Interpretations of the results

Conclusions

Acknowledgments

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Deposition of fine and coarse sand in an open-work gravel bed

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Effects of agriculture on erosion and sedimentation in the Piedmont Province, Maryland

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Numerical simulation of aggradation and downstream fining

J. Hydraul. Res.

Deposition and removal of fines in gravel-bed streams