Agenda - Planning Commission - 07/06/2017

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Appendix 4A Low Floor Elevation Guidance Overview of Lowest Floor Issue There seems to be two reasons for establishing a minimum lowest floor elevation in the vicinity of a pond — to prevent flooding of the structure by surface water and to prevent seepage or damage from uplift pressures that could result from a rise in the water table elevation. The first reason (direct flooding) can easily be established with knowledge of the maximum flood elevation of a pond (or the 100-year elevation, if this is used) and ground surface topography. The second reason (a rise in the water table clue to increased pond elevations) is not so straight forward. This second area is the subject of this memo. When a formerly dry pond becomes wet (or when a wet pond's water elevation increases) due to a storm event, downward seepage of the ponded water begins. The rate of seepage through the bottom of the pond is dependent upon: 1) The elevation of the water surface above the pond bottom 2) The soil type at the bottom of the pond (i.e. the pond bottom's thickness and permeability) 3) The type of soil underneath the pond (e.g., clay, silt, sand, gravel) 4) The degree of saturation of the soils beneath the pond 5) The depth to the water table In general, higher seepage through the bottom of the pond will occur when the water surface elevation is high, the pond's bottom sediments are thin and/or sandy, the soils underneath the pond are permeable (such as sand or gravel), the soils underneath the pond have a high moisture content (i.e, they are at field capacity or higher), and the water table is well below the bottom of the pond (i.e. the soils are freely draining). Higher seepage rates through the bottom of the pond will cause the water table elevation to rise by creating a "mounding condition" below the pond. How high and how widespread the water table mound becomes are contributing factors to whether or not basements will be affected. However, the single most important factor that will determine if seepage from a pond will cause wet basement problems is the depth to the water table, below the basement.