Sinkhole hazards and risk assessment in a planning context By Kimmerly, Phillip R
INTRODUCTION
Sinkholes can be broadly defined as closed topographic depressions, generally elliptical to circular in landscape view, resulting from the settlement or collapse of soil or rock into solution openings beneath the ground surface, such as caves or enlarged rock fractures. Examination of a topographic map is the quickest way to detect sinkholes. How well a sinkhole shows on a topographic map depends on the depth of the sinkhole and the contour interval of the map. Sinkhole terrains, formally termed karst topography, are an environmentally sensitive landscape category characterized by sinkholes, caves, disappearing streams, springs, and by poor surface drainage yet well-developed subsurface drainage.SINKHOLE FORMATION
Sinkhole development involves either bedrock or soil. Bedrock collapses are rare events and generally involve the roof of a cave. Most sinkholes or sinkhole collapses occur in the soil. Soil thickness overlying bedrock in sinkhole terrains typically varies on a magnitude virtually unknown in any other geologic setting.Following intense yet uneven weathering of bedrock, soil migrates downward into the subsurface through vertical openings in the bedrock. Vertical openings, termed joints, are bedrock fractures found in sets with distinct directional trends throughout a given locality. In soils, sinkholes can result from ground water backflooding the fractures beneath the soil-bedrock contact, surface water infiltrating downward through the soil and fractures in the bedrock, or ground-water withdrawal.
Sinkhole collapse can be triggered by water ponded in a surface depression, filling of the sinkhole with earth, excavation of a sinkhole, dynamic loading by construction machinery, disruption of natural surface drainage to accommodate development, excessive ground-water pumping, or ground motion accompanying large earthquakes.
SINKHOLE FLOODING
Flood hazards posed by sinkholes are not widely recognized by planners, urban policymakers, and local governments because they do not occur along perennial stream courses. Urban flooding in sinkhole terrains results in costs to both the property owner and the taxpayer.
Field inspection in undeveloped rural landscapes suggests that many sinkholes are capable of temporarily storing the increased runoff and higher peak flows generated by increased areas of impervious surface, such as streets, roads, driveways, parking lots, and roofs. In urbanizing sinkhole terrains chronic flooding not only persisted after development but typically increased in both frequency and volume of runoff ponded in sinkholes. Chronic sinkhole flooding will become measurably greater in moderate-to high-density residential, commercial, or industrial development where the percent of impervious surface becomes significantly higher.
GROUND-WATER CONTAMINATION
Routing of urban runoff into sinkholes produces a general deterioration of ground-water quality downstream in urban areas (Ruhe, Clark, and Epstein 1980). Each sinkhole not only acts as a collecting basin for runoff, but also as a point of ground-water recharge. The origin and general geologic character of sinkholes ensures that surface water will enter the subsurface by infiltration through depression bottoms. Surface water carries whatever soluble contaminates are available.
Rapid movement of contaminates through the sinkhole and into the ground water affords little opportunity for filtration by the soil. Once in the ground water, contaminates move rapidly toward nearby springs and streams as a function of the hydraulic gradient. Ground-water protection measures are even more important where sinkholes become integral components of an urban drainage system.
DESIGNING A PLANNING PROCESS MODEL
PHASE 1: RECOGNIZING SINKHOLE COLLAPSE AND FLOODING AREASPhase one requires identifying sinkhole collapse and flooding problem areas. Efforts should begin with the identification of all sinkholes located in prime development areas bordering urban areas. They should study aerial photographs with either pocket or mirror stereoscopes. Planners can then map depressions noted in stereovision on clear mylar sheets at the largest map scale available. Ideally aerial photographs at the same scale from different periods will allow time lapse comparisons of the sinkholes.
During this phase, planners should try to locate all sinkholes known to flood chronically. Generally sinkholes that have collapsed in the past have a higher probability of recurrent collapse. Planners should select a stratified random sampling of sinkholes subject to flooding and collapse from both rural and urban settings for further study.
PHASE 2: FIELD OBSERVATION
Planners should make an aerial photographic survey at low altitude of the sinkhole terrain, both in rural and marginal urban settings, during the rainy season. Following each storm, the staff gage and continuous recording water-level data should be analyzed to determine the volume of runoff collected in the sinkholes. Extensive ground-level photography produces a record of all flooded sinkholes and any subsequent sinkhole collapses. The staff should evaluate sinkhole floodstage, depression storage capacity, and subsurface drainage rate data, using an appropriate hydrologic model. Planners should then correlate the water level data with precipitation records from each major storm. Important data include historic precipitation records and rainfall intensity-time duration-frequency curves.
An accurate sinkhole distribution map is paramount in planning residential and commercial development, surface drainage networks, storm sewers, and urban transportation routes. A distribution and drainage map (SDD) enables the planner to identify flood-prone areas in the sinkhole terrains while they are still rural and to incorporate planning and engineering considerations in approving the type and density of development. A sinkhole collapse risk map (SCR) enables planners to assess relative collapse risk. The SCR, as a mylar overlay, is a compilation of the sinkhole distribution map (SDD), photographic and field evidence of sinkholes with previous collapse episodes, and a map showing any relationship between collapse tendency and the trend of the sinkhole long axes.
The locality needs to establish a separate drainage authority or storm water management district as part of the adoption of a storm water management master plan. The authority or management district's function is to review development plans and to provide geological and engineering data and advice on sinkhole flooding and sinkhole collapse risks attendant with development. The authority's technical staff should focus on pertinent geologic and engineering data collection, analysis, and interpretation. The staff should include civil engineers, hydrologists, and geologists trained in karst terrain processes.
The planning commission can also take steps to reduce the risk of sinkhole flooding and collapse risk. The commission should give special consideration to the environmental sensitivity of the general subdivision and commercial development regulations in karst terrains. A significant first step would be to restrict development within five hundred feet of a sinkhole and to prohibit the placement of earthen fills within sinkholes.
Where commercial development is planned with sinkholes as an integral part of urban drainage, storm water runoff from all impervious surfaces should be routed to on-site water treatment facilities to protect ground-water quality.
Reference
www.uny.ac.id
PHASE 3: DERIVATIVE MAP CONSTRUCTION
Based upon flooding and collapse data, staff can prepare derivative maps that pinpoint potential sinkhole problem areas. A derivative map presents one or more types of closely related geologic and hydrologic data on a scale commensurate with planning, engineering, and development needs. By overlaying the derivative map on the topographic base, planners can locate and evaluate the distribution of geologic and hydrologic factors affecting sinkhole flooding and the relative influence of each factor with respect to topography.An accurate sinkhole distribution map is paramount in planning residential and commercial development, surface drainage networks, storm sewers, and urban transportation routes. A distribution and drainage map (SDD) enables the planner to identify flood-prone areas in the sinkhole terrains while they are still rural and to incorporate planning and engineering considerations in approving the type and density of development. A sinkhole collapse risk map (SCR) enables planners to assess relative collapse risk. The SCR, as a mylar overlay, is a compilation of the sinkhole distribution map (SDD), photographic and field evidence of sinkholes with previous collapse episodes, and a map showing any relationship between collapse tendency and the trend of the sinkhole long axes.
PHASE 4: IMPLEMENTATION
The final step in the design of a planning process to minimize sinkhole-flood and collapse risk is the collective will to implement the program.The locality needs to establish a separate drainage authority or storm water management district as part of the adoption of a storm water management master plan. The authority or management district's function is to review development plans and to provide geological and engineering data and advice on sinkhole flooding and sinkhole collapse risks attendant with development. The authority's technical staff should focus on pertinent geologic and engineering data collection, analysis, and interpretation. The staff should include civil engineers, hydrologists, and geologists trained in karst terrain processes.
The planning commission can also take steps to reduce the risk of sinkhole flooding and collapse risk. The commission should give special consideration to the environmental sensitivity of the general subdivision and commercial development regulations in karst terrains. A significant first step would be to restrict development within five hundred feet of a sinkhole and to prohibit the placement of earthen fills within sinkholes.
Where commercial development is planned with sinkholes as an integral part of urban drainage, storm water runoff from all impervious surfaces should be routed to on-site water treatment facilities to protect ground-water quality.
Reference
www.uny.ac.id
Kemmerly, P. R. (1993). Sinkhole hazards and risk assessment in a planning context. American Planning Association.Journal of the American Planning Association, 59(2), 221. Retrieved from https://search.proquest.com.ezproxy.uny.ac.id/docview/229597026?accountid=31324
