Background on the Coastal Smart Growth

To help address the impacts of historic and current development patterns on the Massachusetts coast, CZM's Coastal Smart Growth Program catalogues, develops, and distributes planning, technical, regulatory, and outreach tools for real-world growth management that protects coastal resources. This web page highlights the need for better growth management, providing information on U.S. coastal population growth, the resulting sprawling development patterns, population growth and sprawl issues in Massachusetts, and the environmental impacts of sprawl.

Population Growth along the U.S. Coast

Coastal areas are crowded and becoming more so every day. More than 139 million people—about 53 percent of the national total--reside along the U.S. coast. This population is expected to increase by an average of 3,600 people per day, reaching 165 million by the year 2015.

In 1960, an average of 187 people were living on each square mile of coastal land (excluding Alaska). This population density increased to 273 persons per square mile by 1994, and is expected to reach 327 by 2015. Population densities are highest along the East Coast, especially in the Northeast.

Population Growth Has Led to Sprawl

Increases in population density have led to sprawling patterns of development in the suburbs and beyond. In addition to the 5,800 housing units in multi-unit buildings that are built every week along the U.S. coast, about 8,700 new single-family homes are also constructed. Single-family housing developments frequently include large homes on large lots. For example, almost one-third of all new home construction is for houses with more than 2,400 square feet of floor area (U.S. Bureau of the Census, 1994). Further, the median lot size in the United States is about 17,000 square feet (Culliton, 1998). Residential development patterns stimulate similar commercial development patterns, driven by the need for convenient proximity of commercial space to neighborhoods.

Population Growth and Sprawl in Massachusetts

Although population rates in Massachusetts are modest when compared to national averages (i.e., from 1960-2000, the U.S. population grew 56 percent and the Massachusetts population grew 23 percent; from 1990-2000, the U.S. population grew 13 percent and the Massachusetts population grew six percent), sprawling patterns of development are problematic in the Bay State. Losing Ground: At What Cost?, an in-depth report developed by MassAudubon, shows a state-wide trend toward low-density development characterized by large houses and large lots. The report also points to coastal impacts of sprawl, with the Southeast and Cape Cod (as well as in the I-495 corridor) seeing the greatest habitat loss in the state, and the coastal communities of Barnstable, Falmouth, Sandwich, and Plymouth having the highest rate of conversion of forest to residential development. In addition, the report shows that between 1970 and 2002, the average living area for new homes in Massachusetts increased 44 percent and the average lot sizes increased 47 percent.

Other studies looked at the 1999 Massachusetts land use data on a local and regional level. While these reports do not provide a comprehensive picture of land use change throughout coastal Massachusetts, they do serve as an indicator of localized development patterns. For example, CZM studied land use change in the Parker Watershed (on the Massachusetts North Shore) and found that while rates of residential growth are gradually decreasing, low density residential development (i.e., single-family homes on more than ½-acre lots) continues to exceed all other development types (other residential, commercial, and infrastructure) combined. Low density residential development accounted for 63 percent of all new development between 1970 and 1999 (with the proportion increasing in later years); the biggest loss was to forested lands, which accounted 60 percent of the land conversion.

Environmental Impacts of Sprawl

This development alters natural landscapes, directly impacting coastal habitats. In addition, the resulting increase in impervious surfaces (such roads, rooftops, and other impermeable materials that prevent stormwater infiltration) effects coastal water quality, habitat quality, water temperature, and aquatic life, as discussed below.

• Water Quality - Sprawling development patterns substantially increase the quantity of impervious surface, which increases the quantity of stormwater runoff, as well as the pollutants it contains (including sediment; nutrients such as nitrogen and phosphorus; organic carbon; trace metals such as copper, zinc, and lead; petroleum hydrocarbons; and pesticides [Schueler and Holland, 2000]). Although all of these pollutants directly impact coastal water quality, one of the most detrimental is nitrogen. Because this nutrient is typically the limiting factor that keeps aquatic plant growth in check, excess nitrogen in stormwater runoff can consequently cause algal blooms, which can lead to reduced oxygen levels when the algae decays. This algal growth also reduces water clarity, damaging the growth of seagrass beds and other critical habitats. The primary sources of nitrogen in most developed watersheds are fertilizer runoff from lawns and golf courses, automobile exhaust, and municipal wastewater treatment plants.
• Habitat Quality - Increased runoff from impervious surfaces to creeks, rivers, and estuaries substantially impacts these habitats. Illustrating this change, a one-acre parking lot produces about 16 times the volume of runoff than a one-acre meadow (Schueler and Holland, 2000). These magnified "pulses" of runoff alter the stream flow patterns and, consequently, the shape of the stream channel. Streams in watersheds with more than 10 percent impervious surfaces become physically unstable, causing erosion and sedimentation (Booth, 1991; Booth and Reinelt, 1993). In addition, natural habitats, such as pools, woody debris, and the wetted perimeter of the streambed, decline (Booth and Reinelt, 1993; Shaver et al., 1995). Overall, habitat quality falls below the level necessary to sustain a broad diversity of aquatic life.
• Water Temperature - As runoff flows across paved roads and parking lots into coastal marshes, water temperature rises--and the more impervious surface area in the watershed, the warmer the water (Galli, 1991). This is particularly true in small tidal creeks. Because these areas are often naturally low in dissolved oxygen, further increases in temperature can push oxygen levels toward zero, especially in the summer. The upper reaches of tidal creeks and marshes serve as nursery grounds for many finfish and shellfish that inhabit coastal waters, so the dissolved oxygen balance of these areas has great implications for the health of the marine environment.
• Aquatic Life - Early work examining impervious surface coverage in watersheds concluded that the diversity of stoneflies, mayflies, caddis flies, and other macroinvertebrates (which are an important link in the food web for fish and wildlife) falls sharply when impervious surfaces exceed 10 percent (Klein, 1979). Later studies derived similar results, as did studies of fish that indicated that sensitive species such as trout, salmon, and other anadromous fish (i.e., fish that migrate from the sea up rivers to spawn) disappeared as impervious surfaces covered 10 to 12 percent of the watershed. Developed watersheds tend to include barriers to migration for anadromous species, illustrated by sharp declines in eggs and larvae in hardened watersheds (Schueler and Holland, 2000). Research by the Maryland Department of Natural Resources also concluded that urbanization of watersheds correlates with reduced fish communities (Carmichael et al., 1992). Studies specifically focusing on coastal estuaries have confirmed that general degradation begins at the 10 percent impervious threshold (Taylor, 1993). When impervious surfaces exceed 15 to 20 percent, the variety and abundance of food available for juvenile fish is significantly reduced (Holland et al., 1996).

Booth, D. 1991. Urbanization and the natural drainage system-impacts, solutions, and prognoses. Northwest Environmental Journal. 7(1):93-118.

Booth, D., and L. Reinelt. 1993. Consequences of urbanization on aquatic systems: measured effects, degradation thresholds, and corrective strategies. In Proceedings of Watershed '93, A National Conference on Watershed Management.

Carmichael, J., B. Richardson, M. Roberts, and S.J. Jordan. 1992. Fish Sampling in Eight Chesapeake Bay Tributaries. Maryland Department of Natural Resources. CBRM HI-92-2.

Culliton, T.J. 1998. Population, distribution, density and growth. NOAA's State of the Coast Report. National Oceanic and Atmospheric Administration (NOAA), Silver Spring, MD.

Galli, J. 1991. Thermal Impacts Associated with Urbanization and Stormwater Management Best Management Practices. Metropolitan Washington Council of Governments, Maryland Department of Environment, Washington, D.C.

Holland, A.F., G.H.M. Riekerk, S.B. Lerberg, L.E. Zimmerman, D.M. Sanger, T.D. Mathews, G.I. Scott, M.H. Fulton, B.C. Thompson, J.W. Daugomah, J.C. DeVane, K.M. Beck, and A.R. Diaz. 1996. The Tidal Creek Project, Interim Report. Submitted to the Charleston Harbor Project. 230pp.

Klein, R. 1979. Urbanization and Stream Quality Impairment. American Water Resources Association. Water Resources Bulletin.

MassAudubon. 2003. Losing Ground: At What Cost?. Lincoln, Massachusetts.

Schueler, T., and H.K. Holland. 2000. The Practice of Watershed Protection. Center for Watershed Protection, Ellicott City, Maryland.

Shaver, E., J. Maxted, G. Curtis, and D. Carter. 1995. Watershed protection using an integrated approach. In Stormwater NPDES-related Monitoring Needs. Engineering Foundation. Crested Butte, Colorado. August 7-12, 1994. American Society of Civil Engineers.

Taylor, B.L. 1993. The influences of wetland and watershed morphological characteristics and relationships to wetland vegetation communities. Master's Thesis, Department of Civil Engineering, University of Washington, Seattle, Washington.

U.S. Bureau of the Census. 1994. Statistical Abstract of the United States. Washington D.C. GPO for USDOC.