Soil Hazards

Erosion is the removal and simultaneous transportation of earth materials from one location to another by water, wind, waves, or moving ice. Deposition is the placing of the eroded material in a new location. All material that is eroded is later deposited in another location (Colorado Natural Hazards Mitigation Plan, 2013, p. 3-166). In Colorado, erosion and deposition are typically initiated by water or wind.

• Riverine erosion and deposition is the process whereby river beds, banks, and floodplains are shaped by the force of moving water as it mobilizes, transports, and deposits sediments and organic debris. This process is best described as a river’s tendency for constant course alteration, shape and depth change, and the balancing act between the water’s sediment transport capacity and its sediment supply. Swiftly moving floodwaters cause rapid local erosion as the water carries away earth materials. Deposition occurs where flood waters slow down, pool or lose energy in other ways, and materials settle out. See Fluvial Hazards for more information related to the hazards associated with these natural processes.
• Wind erosion occurs when wind is responsible for land removal, movement, and deposition and most commonly occurs in exposed areas such as fields, tailings, and deserts. Another factor that controls the amount of erosion is the ease with which material can be dislodged and transported. Hard granites erode very slowly while soft silts and sands erode very quickly.

Expansive (or swelling) soils are soils or soft bedrock that increase in volume as they get wet and shrink as they dry out. Expansive soils contain a high percentage of certain kinds of clay particles that are capable of absorbing large quantities of water. Soil volume may expand 10 percent or more as the clay becomes wet, and the powerful force of expansion is capable of exerting damaging pressures on foundations, slabs, or other confining structures. Subsurface Colorado swelling soils tend to remain at constant moisture content in their natural state and are usually relatively dry at the outset of disturbance for construction on them. Exposure to natural or human-caused water sources during or after development results in swelling, and in many instances the soils do not regain their original dryness after construction, but remain moist and expanded due to the changed environment.

Ground subsidence is the sinking of land over man-made or natural underground voids, which can result in serious structural damage to buildings, roads, irrigation ditches, underground utilities, and pipelines. In Colorado, the type of subsidence of greatest concern is the settling of the ground over abandoned mine workings. Collapsing and settling soils are relatively low-density materials that shrink in volume when they become wet and/or are subjected to great weight such as from a building or road. Human activities that lead to subsidence include underground mining, pumping groundwater or petroleum, hydrocompaction, and draining organic soils. Natural causes of subsidence include the development of sinkholes, rock sliding downward along faults, natural sediment compaction, and melting of permafrost. Subsidence may occur virtually instantly or gradually over many years. It may occur uniformly over a wide area or as local depressions or pits separated by areas that have not visibly subsided. In Colorado, it is most common in the sedimentary rocks over abandoned coal and clay mines. Although less common, subsidence can also occur where underground water has dissolved subsurface materials or has been withdrawn by wells.