Save for Title Page
Download 4.32 Mb.
|
- Navigate this page:
- 4.10.1 Subsidence Risk
- Table 63: Subsidence Risk by City in Valley County
- 4.10.2 Subsidence History in Valley County
- 4.10.3 Presidential Declared Disasters for Subsidence
- 4.10.5 Vulnerability
- 4.10.6 Subsidence and Climate Change
- 4.10.7 Relationship to Other Hazards
4.10 SubsidenceSubsidence was not identified in the 2008 Valley County Pre-Disaster Mitigation Plan, but was identified as one of the hazards to be included in this plan update. The analysis included in this plan update includes an in-depth look at what subsidence is, the history of it within Valley County, and the potential it has to impact residents. Subsidence is the sinking or settling of the ground surface. It can occur by a number of methods. Ground subsidence can result from the settlement of native low-density soils, or the caving in of natural or man-made underground voids. Subsidence may occur gradually over many years as sags or depressions form on the ground surface. More infrequently subsidence can also occur abruptly as dangerous ground openings that could swallow any part of a structure that happens to lie at that location, or leave a dangerous steep-sided hole. The types of subsidence of greatest concern are settlement related to collapsing soils, sinkholes in karst areas, and the ground subsidence over abandoned mine workings. A trigger mechanism is the change in the local environment affecting the soil mass causing subsidence and sinkholes which causes the collapse. Water is the main factor affecting the local environment that causes subsidence. The main triggering mechanisms for subsidence are water level decline, changes in groundwater flow, increased loading, and deterioration (abandoned coal mines). Water level decline can happen naturally or be human induced. Factors in water decline are pumping water from wells, localized drainage from construction, dewatering, and drought. Changes in the groundwater flow include an increase in the velocity of groundwater movement, increase in the frequency of water table fluctuations, and increased or reduced recharge. Increased loading causes pressure in the soil leading to failure of underground cavities and spaces. Vibrations caused by an earthquake, vibrating machinery, and blasting can cause structural collapse followed by surface settlement. 4.10.1 Subsidence RiskThe overall probability for subsidence within Valley County is unlikely, its relative impact is low, and thus the overall risk for subsidence is little to none. The risk for subsidence is different for each city and was determined based upon the specific data collected and outlined in the history section of this hazard profile. In assessing subsidence for the 2015 update, data from 2009-2014 was used to determine the risk for each of the cities and the county as a whole. The table provided below provides the name of each of the cities in the county, the probability that subsidence will have an impact on that jurisdiction, the impact potential, along with the overall risk calculated by the probability and impact ratings. Table 63: Subsidence Risk by City in Valley County
*Valley County information takes into account unincorporated areas of the County. 4.10.2 Subsidence History in Valley CountyAccording to the 2008 Valley County Pre-Disaster Mitigation Plan, subsidence has the potential to impact areas of Valley County if there is flooding or high groundwater levels. Excessively wet soil can also lead to land subsidence in the county. There was no specific history of specific areas of land subsidence in Valley County in the past five years. Subsidence has occurred previously in the county following flood events which makes land subsidence a potential risk for Valley County. 4.10.3 Presidential Declared Disasters for SubsidenceThere were no declared disasters related to subsidence in Valley County. 4.10.4 Mitigation Actions for the Past Five YearsGeneral education and code enforcement are some of the mitigation projects involving subsidence that have been conducted within the last five years. 4.10.5 VulnerabilityAny residential land which lies in Valley County has the potential to be impacted by subsidence. All roadways, houses or businesses built have the potential to be vulnerable to subsidence. Houses or businesses located near a body of water or waterway, especially the rivers in Valley County, and anything located on the top of a considerable slope is also more vulnerable. Large volumes of rain and the spring thaw increase the vulnerability of areas in the county. 4.10.6 Subsidence and Climate ChangeChanges in climate have the potential to impact subsidence in Valley County. Periods of excessive and prolonged rainfall can cause ground water levels to rise and swell prone soils, particularly cohesive soils with a high clay content (and to a lesser extent silt), are particularly susceptible to volumetric change. Conversely, excessive and prolonged dry periods cause shrinkage. In winter, thewaterlogged ground can move further by frost heave. 4.10.7 Relationship to Other HazardsSubsidence can be related to other hazards such as summer storms, because they can cause excessive or prolonged periods of rain which can cause the ground to become susceptible to volumetric change. Drought also has the potential to be related to subsidence because periods of drought can cause shrinkage of soils, which can impact subsidence. Lastly, flooding can cause excessive water on the ground which can cause volumetric changes. Download 4.32 Mb. Share with your friends:
| ||||||||||||||||||||||||||||||||