Sinkhole flooding is one of the more tragic karst-related geologic hazards because it seems to affect private residences the most, but could easily be avoided. This house was damaged because the large karst valley it was built in was not recognized as a closed depression. The swallow hole of the karst valley cannot accommodate peak runoff from upstream. Because the karst valley is large, its closed nature is not apparent at ground level. Without maps or other methods of warning of flood potential, the only way a prospective (and very astute) buyer of this house could be warned of the flooding is by consulting a topographic map. Sinkhole flooding usually occurs during the same storms that cause rivers to flood, so it is sometimes not recognized as karst related.
The Kentucky Department of Emergency Services estimates that a March 1997 flood cost more than $1 million in mitigation costs alone -- including buyouts and construction of storm water detention basins for a few counties in the central and western Kentucky karst areas. The Kentucky Geological Survey has been part of efforts to address karst-related flooding problems in several cities in the Inner Bluegrass and Western Pennyroyal since 1990. Sinkhole flooding in Bowling Green (Warren County) is well known and has prompted the county to enact strict zoning regulations and building codes. The city of Versailles has spent over $500,000 to purchase flood-damaged property in order to take remedial action. These sites suggest the average annual loss statewide exceeds $1 million from flood damage alone.
Sinkhole Flooding: Why it happens
There are two general causes of sinkhole flooding, both of which have two variations. Most sinkhole flooding occurs when the rate of inflow (such as gallons per minute) of runoff from rain or snowmelt exceeds the rate of water flow that the conduits and cave passages can transmit. The other type happens when flooding in another part of the watershed reverses the flow direction and causes water to back up into the sinkhole. Unlike an above-ground stream channel, the karst conduit has fixed dimensions, which prevents the cross-sectional area of the water stream from increasing as flow increases. Because of friction with the conduit walls, and the fixed cross-sectional area of the conduits, large increases in head (water pressure) are needed to achieve comparatively small increases in flow.
Locally conduit water-flow capacity can be limited when a short section of conduit somewhere between the sinkhole and the spring has a smaller cross section than the remainder of the conduit. The location and length of the constricted section is very important. The sinkhole throat may be blocked at the surface by trash and junk, soil eroded from fields or construction sites, and by rock fall in the shallow subsurface. Other possible blockage causes are a natural roof fall and thin beds of relatively dissolution-resistant rock. Whatever the cause of the restriction, the reach of a conduit downstream from a constriction could carry a higher-rate flow. The sinkhole and section of conduit upgradient from the blockage is flooded, but the downgradient cave or conduit is mostly air-filled. The length of the flooded conduit is dependent on the position of the localized blockage. A partial collapse of the cave ceiling can block flow anywhere along the conduit, frequently very close to the spring. The accessibility of the blockage determines the success of efforts to remove it.
Sometimes the conduit has an uniformly small cross section along most of its length because it has recently (in the geologic sense) captured a larger drainage basin. A conduit develops its diameter to accommodate the most common high rate of runoff from a given watershed. If it extends itself head ward until it captures drainage from an adjacent watershed, it may be temporally undersized in relation to the combined drainage area. The cross-sectional area of the conduit or cave will be too small to accommodate the sudden increase in flow. Such conduits can have their flow capacity further reduced by any of the causes of blockage mentioned above, particularly excessive sedimentation.
A flow reversal is the second general cause of sinkhole flooding. Examples are known from the Western Pennyroyal. One specific case happens when the surface- flowing stream to which the spring discharges is in flood from a storm in its headwaters, but not in the spring's watershed. The flooded stream blocks discharge from the spring, and in some conditions can actually reverse flow into the spring mouth and into the conduits, resulting in flooding of the sinkhole. An example is the flooding of Echo River spring and cave passage in Mammoth Cave by the Green River, which itself has flooded from heavy precipitation upstream in the surface watershed. River water flows into the cave under these conditions.
A second specific cause of flow reversal is when the main conduit is blocked downgradient from a sinkhole, but the drain from the sinkhole is not blocked. Another sinkhole in the basin, and at a slightly higher elevation than the first, floods during a localized thunderstorm. The water pressure of the flooded sinkhole reverses flow in the conduits because of the blockage, and results in the water flowing into the lower-elevation sinkhole. This is analogous to when one basin of a double kitchen sink is filled with water. If the waste pipe is clogged and then the drain is opened on the filled side, water flows out of the drain into the side that was empty.
During prolonged, widespread rainfall, the entire karst aquifer can become saturate with water and almost all sinkholes will flood. This is an uncommon event, but occurred over much of the state in March of 1997.
How to Avoid Sinkhole Flooding
The first impression is that it is very simple to avoid sinkhole flooding -- do not build structures in a sinkhole. Being aware that a site is a sinkhole and that the sinkhole floods, however, sometimes requires a more direct warning. Some sinkholes are so large they are not readily recognized at ground level as closed depressions. Others flood so rarely that the potential for flooding is unrecognized. Some sinkholes at higher elevations, or higher-elevation areas within a compound sinkhole or karst valley, may be unaffected by flooding. Sinkholes flood more frequently when there is development in their watershed because roofs, parking lots, and highways increase both the total runoff and the rapidity of runoff from a storm. Prohibiting construction in these areas may be necessary, if information on the flood history of a specific sinkhole is not available. Every example of sinkhole flooding has variations and circumstances that will make it unique and slightly different from the simplified cases described above.
An informed person can take precautions to avoid having property damaged by flooding of sinkholes. When buying real estate in a karst area, pay close attention to the topography. Consult topographic maps, inspect the area surrounding the property to determine its relative elevation, look for previous signs of water damage to buildings, and ask neighbors if the property has ever been flooded. Do not assume, however, that because a few people don't know of any flooding in the sinkhole, that it can never occur.
Once a building has been erected, it is usually too late to make any cost-effective corrections. The choices are to elevate the structure on pilings, tear it down and build elsewhere, or physically move the structure. Rarely will the topography and hydrogeology allow the construction of an overland ditch, or storm sewer to relieve flooding. These types of engineering solutions are frequently only partly successful, lead to other environmental and legal problems, and often cost more than the value of the property to be protected. The most cost-effective way to prevent property damage from sinkhole flooding is to recognize the hazard and avoid it.