Techniques for identifying flood plains in Oregon Public Deposited


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  • Seven methods for delineating flood plains were evaluated. The methods are classified : (1) Geomorphic Technique; (2) Soils Technique; (3) Botanical Technique; (4) Hydrologic Technique; (5) Hydrologic- Hydraulic Technique; (6) Historical Technique; and (7) "Combination Method". All of the methods, except the Botanical Technique and the Hydrologic-Hydraulic Technique, were compared on sample flood plain reaches throughout Oregon. Chronological sequences of natural flood plains were identified and mapped based on: (1) scarps separating two or more flood plain or terrace geomorphic surfaces and (2) the abundance and degree of photographic expression of flood plain features such as meander scrolls, point bars, oxbows, oxbow lakes, sloughs, old channels, natural levees, and sand splays. Soils were useful in delineating flood plain areas since soils cannot be older than the alluvium in which they form or older than the associated geomorphic surface. Since soils exhibit increasing development of genetic horizons with increasing age, delineation of younger bottomland soils from older soils on higher flood plains or terraces, usually results in the delineation of flood prone areas. Since floods scar bark, or prune branches or the tops of trees, botanical studies can provide information about the flood date, flood stage, and area flooded. The number of annual tree rings that have grown since scarring of the bark, or the age of sprouts that have grown since flood damage occurred, are equal to the number of growing seasons since the trees were damaged. The thickness of sediment above the base of a tree serves as a measure of the rate of sediment accumulation above the surface on which the tree was located. Historically flood plains have been recognized and delineated by (1) aerial or ground photographs taken during floods; (2) high flood marks such as float debris; and (3) locating flooded areas from field interviews. Previous hydrologic studies have related the stage of rivers at gage locations to the adjacent flood plain areas, and in this manner flood plains have been recognized and delineated. In addition, hydraulic studies have been used to determine flood stages at valley cross sections, for specific flood frequencies. Flood plain width therefore may be determined from the valley cross sections. Use of both the gaging stations and the valley cross sections results in a hydrologic-hydraulic procedure for flood plain identification. The particular flood plain mapping technique that is most applicable in any given study depends on the data available, the purpose and scale of mapping, and the field conditions. Each technique has deficiencies; therefore, the "Combination Method" is introduced to utilize the most useful information from each technique. Collective use of two or more techniques is called the "Combination Method". The relative accuracy of the Geomorphic Technique, Soils Technique, Historical Technique, Hydrologic Technique, and "Combination Method" was determined for some representative stream reaches in Oregon. Areas flooded in a 100 year average recurrence interval event were compared for all methods available in 10 study areas, which ranged from semiarid to humid environments. For the areas compared the "Combination Method" was used as the base method. The Geomorphic Technique was the method most consistently comparable with the "Combination Method". The Soils Technique was generally accurate for delimiting flood plains, with some studies showing excellent correlation. The three study areas with comparisons using the Historic Technique were generally good and most discrepancies occurred where approximate historical flood lines were drawn for inaccessible areas. The Hydrologic Technique showed by fax the poorest correlation with the "Combination Method" as well as with all other methods. The differences between the "Combination Method" and the Hydrologic Technique are explained for specific maps, but in general the major problem was the lack of field correlation of crest staff gage flood elevations, with the general flood plain area away from the river. No independent evaluations were made of the Hydraulic Technique, although the Hydrologic-Hydraulic Technique was included as part of the "Combination Method" in one study.
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Last modified: 08/23/2017

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