FLOOD02366 - Laserfiche WebLink

Home Browse Search

36 I I I I I I I I I I I I I I I I I I Probably the most valuable piece of planning data following a catastrophic flood is an accurate assessment of return period, Unfor- tunately, it is also the most difficult datum to obtain. In making this computation, the hydrologist is first faced with the decision of whether to include the flood discharge in the flood frequency analysis. If it is included and is the flood of record, then a return period of N + 1 (one greater than the length of the hydrologic record) is derived. For ex- ample, if 50 years of record exist for the Big Thompson then a return per- iod of 51 years for the July 31 flood must follow. If, on the other hand, the flood discharge is not included in estimating the frequency distribution, then a higher return period will be obtained but one of the basic rules of statistics has been violated by extrapolating beyond the range of data from which the distribution was estimated. The hydrologist that follows this method is also open to the criticism that commonplace events are being used to determine a curve for predicting rare events. Numerous additional prob- lems and sources of error accompany the determination of return periods for truly large floods. The hydrologist cannot escape the question of whether it is preferable to err with too high or too Iowa return period. Too large a return period costs land owners dollars. An underestimate may be paid for in lives as well as dollars. The events of July 31, 1976 have focused attention on the need for flood plain regulation in the Big Thompson Canyon. Federal, state and local resources as well as considerable effort are being expended on this activity, and we can be sure that the results will be an improvement over the pathetic state of affairs that existed prior to the flood. In view of all the attention being directed towards the Big Thompson, one cannot help but wonder