This growth projection study builds upon another study completed by the Berkeley-Charleston-Dorchester Council of Governments (BCD COG), the University of South Carolina and the South Carolina Department of Natural Resources. That study investigated urban growth in the greater Charleston metropolitan area from 1973 to 1994 and found that over the 21-year period, urban land use growth has exceeded population growth by a 6:1 ratio. As South Carolina is targeted as one of the top 10 retirement states in the US, and Charleston one of the top two regions in South Carolina, a fairly rapid growth scenario in the Charleston area is expected. Continued land transformation has certain negative impacts and eventually will fundamentally change the three-county area. This rapid urban growth is a major concern of resource managers, policy-makers and citizens of the State.

The present study, a partnership with the South Carolina Coastal Conservation League and funded by the NOAA Coastal Services Center through the South Carolina Sea Grant Consortium, seeks to model and predict the spatial extent of future urban growth for the Berkeley - Charleston - Dorchester area by the year 2030. The prediction is based on the historical trends of the 1973-1994 study, under the current policy constraints, and the physical environment. It is hoped that such a model will give decision-makers better information from which to implement good growth policy for the BCD area as well as South Carolina.

For the prediction of urban transition probabilities, four techniques including statistical modeling, rule-based modeling, focus group mapping, and integrated GIS modeling were used in the project. Because the size of the region is too large for high-resolution (parcel-based) modeling, analysis units were set to 200X200 meters. All the source data were resampled at this resolution before further processing. Some information losses associated with this resampling were expected.

Under the current modeling scenario, there are two assumptions involved. The ratio of overall urban land use change (255%) to overall population growth (41%) from 1973 to 1994 occurred at a ratio of about 6:1. Since this ratio is one of most important indices of urban growth, it is used here to determine the urban size for the future. For modeling purposes, a slightly more conservative ratio of 5:1 was used to predict future growth. Secondly, it is assumed that population for the three county area will grow to 795,879 by the year 2030 as predicted by projections of the BCD COG compiled with information from the US Census Bureau, SC Department of Commerce and the BCD COG. It should also be noted that the year 2015 road network is embedded in the model, even though the presentation map does not display the new roads; and the prediction does not directly reflect any zoning or land use policy decisions, as it is based on historical on-the-ground changes. In particular, the prediction does not account for new policies recently adopted or under discussion that might limit the extent of future urbanization. However, the model does exclude currently protected lands (forests, parks, wetlands, conservation easements, etc.) from future conversion to urban land use.

For the statistical modeling, component of the overall model, a multivariate logistic regression model was selected because of the non-linear nature of urban growth problems. Urban growth was measured only by change of urban area or urban land use. Urban land use is the dependent variable that is binary while independent variables are a mix of continuous, discrete, and dichotomous variables that represent the major physical, economic, and social factors that have influences on urban growth or land use. A rule-based model was developed to derive the relative transition probabilities of urban growth. This model was designed to complement the pure statistical model primarily through subjective weighting of variables. The third technique used was focus group mapping. The South Carolina Coastal Conservation League (SCCCL) conducted this component of the research. A group of experts, local officials, planners, developers, conservationists and other people who have profound knowledge of the area and urban growth were invited to a number of meetings, or interviewed individually, to express their opinions on where growth may occur during the next 30 years. Finally an integrated GIS model was designed to fully take advantage of the above three models by integrating them into one. In this model, expert prediction was weighted 10% while the other two predictions weighted 45% each in order to eliminate the arbitrary boundary of expert prediction but keep the spatial differentiation of transition probabilities predicted by the logistic model and rule-based model. The predicted urban growth mainly takes the pattern of urban sprawl and by the year 2030 consumes 868 square miles within the BCD area. The visual representation of this growth is found in the maps following this text.

If the current growth trends continue and the predictions hold true, the future urban growth will sprawl considerably outward from the current urban boundaries. This has several significant economic, environmental, and social implications in policy-making and urban planning. While these implications are too numerous to list here, their importance cannot be underestimated and the issues cannot be left unaddressed. It is hoped that this modeling project can help inspire decision-makers and citizens to become involved in the planning processes and land use decisions for areas like Berkeley-Charleston-Dorchester.

View the Map Show:

The Charleston Urban Growth Project

Reports:

Modeling and Prediction of Future Urban Growth in the Charleston Region of South Carolina: a GIS-based Integrated Approach
Jeffery Allen and Kang Lu

Other reports on urban sprawl from Conservation Ecology