ASDSO Dam Safety Toolbox


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"Water exists in the atmosphere, on the Earth’s surface (the hydrosphere), and under the Earth’s surface (the lithosphere). A continual interchange produces a closed system that is known as the hydrologic cycle. Water in the lithosphere is in the form of free water, water vapor, or ice, or is chemically combined with earth materials. Uncombined water occupies the void spaces. The interstices may be occupied by air or other gases or by water or other liquids. A rock or soil is said to be porous or to have porosity if it contains interstices or voids. Porosity can be quantitatively expressed as the ratio of the total volume of voids to the total volume of the rock or soil. It is usually given as a percentage. Primary porosity is a result of the processes that formed rock or soil. Secondary porosity is produced by fracture, solution, or recrystallization. Groundwater occurs in both primary and secondary voids."[1]

Groundwater is not typically considered in hydrologic analysis of dams other than the conservative assumption that the basin soils are saturated prior to the precipitation event. However, some states are recognizing the influence of groundwater on runoff, particularly for basins with well-drained, but thin soil profiles where saturation excess runoff (i.e., interflow) becomes important in driving runoff during the peak of the hydrograph. This differs from the more traditional assumption of infiltration excess runoff where the soil profile is considered an infinite sink.

Best Practices Resources

National Engineering Handbook: Chapter 30 - Groundwater Hydrology and Geology, NRCS


Revision ID: 7372
Revision Date: 07/21/2023