Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 37

Significant
Moderate
Stable
FIGURE 2.16
Severity of coastal erosion in the United States. (From United States Global Change Research Program, Climate
change impacts on the United States: The potential consequences of climate variability and change, Overview:
Coastal Areas and Marine Resources, http://www.usgcrp.gov/usgcrp/Library/nationalassessment/overview-
coastal.htm, (accessed July 4, 2010), 2000.)
• Dredging channels may change the bathymetry offshore, leading to an increase in
the energy of waves striking the shoreline
• Mining beach sand
• Construction of sea walls and spits
• Rising sea levels caused by global warming
Figure 2.16 is a map of the United States showing the significance of coastal erosion. The
generally higher severity of erosion along the eastern and southern coastlines can be
attributed to sandy beaches; the western beaches tend to have higher rock content.
2.3.1.3.2 Groundwater Erosion
A significant type of erosion often overlooked results from the movement of groundwa-
ter. Groundwater erosion frequently occurs in areas of significant deposits of non-clastic
chemical sedimentary rocks such as limestone, dolostone, and evaporite deposits.
Groundwater erosion can eventually lead to a type of landform called karst topog-
raphy. Karst topography is a landscape created by groundwater slowly dissolving
sedimentary rock such as limestone. The resulting landforms from this type of erosion
include sinkholes and caves. A sinkhole is a rounded depression of the ground and typi-
cally forms when the surface rock and soil above a cave collapses. A cave is a natural
feature beneath the surface formed by groundwater dissolving away the rock. At many
locations throughout the United States, extensive cave systems have formed (Davies and
Legrand 1972). Figure 2.17 shows the most common features of a karst area, and Figure
2.18 shows areas of the United States with sedimentary-type deposits prone to karst fea-
ture formation.