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South Dakota Department of Transportation
Project Synopsis

Title: Applicability of Paleoflood Surveys to the Black Hills
Project Researcher: Dan Driscoll, USGS SD Water Science Center
Project Manager: Christina Bennett
Research Period: 9/26/2005 - 9/30/2007
Cost: $200,000.00

Problem Statement:Flood frequency analyses for the Black Hills area generally carry extremely low confidence levels due to contributing factors that are atypical as compared to other regions of the country. These factors include unique geologic formations, diverse topographic conditions, variable precipitation patterns, and sparseness of historical data to relate hydrologic conditions over time. The massive rain storm that occurred near Rapid City, SD on June 9-10, 1972 represents an extreme rainfall event that added uncertainty into the datasets used to perform flood frequency analysis in the Black Hills area. The corresponding flood flows recorded at various U.S. Geological Survey (USGS) gaging stations as a result of this catastrophic event introduced extreme outliers into the datasets used for flood studies that utilize the standard log-Pearson III frequency analysis as recommended in Bulletin 17B of the Interagency Advisory Committee on Water Data (IACWD, 1982). Floods documented at these stations come from mixed populations (snowmelt, cloudburst, wildfire-effected, etc.) Consequently, flood frequency analysis rates for the highly localized area directly affected by the 1972 storm became largely inconsistent with flood frequency rates of other adjacent areas not as intensely affected by the 1972 storm.

Paleoflood studies collect and investigate a broader range of information related to the hydrologic history of an area, and thereby significantly increase the potential for improving flood frequency analyses in the Black Hills area. In essence, Paleoflood hydrology is the study of previously unmeasured floods, whether fairly recent in occurrence or even ancient in time. A paleoflood survey typically consists of: 1) identifying paleostage indicators (PSIs), which are indicators of flood stage associated with a previous, large hydrologic event; 2) estimating the peak discharge for that event; and 3) estimating the age of the event. A variety of approaches can then be used to incorporate the paleoflood data (number of floods, magnitude, and associated timeframes) into flood frequency analyses. Paleoflood techniques have proven to be very useful for improving the flood frequency estimating of relatively recent events where recurrence intervals are in the range of 50- to 500-years, and are also applicable to studies involving much older floods when the flood events have been more significant.

Improved confidence in flood frequency estimates for the Black Hills area would initially require paleoflood surveys at a number of sites. The surveys would, at the least, need to include paleohydrologic investigations of distinctive sedimentary deposits, botanical evidence of past large flows, erosional indicators of paleostage on channel margins, and indicators on landforms in streams and floodplains. The paleohydrologic data gathered during the surveys would then require careful analysis prior to inferring the maximum stage of past floods or developing estimates on the age of those floods. If the surveys and analysis can be successfully accomplished, the resultant data would be highly useful to improving the flood frequency estimates of Black Hills watershed areas.

Findings: Sites along French Creek and Spring Creek were eventually chosen to conduct comprehensive paleoflood surveys. The surveys consisted of detailed stratigraphic analysis of flood sediment layers in slack-water coves along the stream reaches. Samples of organic material were extracted from the layers for carbon-dating to produce historical flood records going back several hundred years. Searches for flotsam at higher elevations were also conducted and additional carbon-dating of that material allowed correlation with the samples extracted from sediment layers. Using these flotsam indicators as high-water marks, approximate peak discharges can be estimated at the survey sites.

Research Objectives:
1  Assess the applicability of paleoflood survey techniques to generate better historical records on the magnitude and frequency of peak flood events in the Black Hills area.

Research Tasks:
1  Meet with the project's technical panel to review the project scope and work plan.
2  Review and summarize paleoflood literature that can be directly related to the conceptual performance of paleoflood surveys in the Black Hills.
3  Perform interviews with individuals from the South Dakota School of Mines & Technology (SDSM&T), scientific entities, and other government agencies that may have intimate knowledge of the Black Hills relative to flooding, hydrology, geology, and othe
4  Scrutinize the estimation of the peak flood data for the Rapid City storm event of 1972 in efforts to more fully affirm said data.
5  Investigate records of flooding and gaging stations unaffiliated with USGS to assess the applicability of any stage indicators arising from that data.
6  Based on a set of well conceived criteria that includes streams with historical gage records, select sites around the entire Black Hills area deemed as likely candidates for successful paleoflood surveys.
7  Meet with the project's Technical Panel to present findings and propose the list of candidate survey sites for review and approval.
8  From the list approved by the project's Technical Panel, sample the most promising sites through field investigations aimed at defining the select locations where comprehensive paleoflood surveys will be instigated.
9  At the selected locations, conduct comprehensive paleoflood surveys that include: finding distinct paleostage indicators (PSIs), gathering discernible paleoflood evidence, recording surveyed cross-sections of the drainage basins, and permanently docu
10  Through accepted state-of-the-practice methods, perform absolute and relative dating techniques on the paleoflood evidence gathered during the field surveys to gain the best possible timeframe estimates for past significant flood events.
11  Develop a dataset that combines the historical flood frequency estimates, including stage indicators, for all locations selected during the study and perform cross-analyses to determine if preliminary conclusions can be drawn as to whether Black Hill
12  Prepare a final report summarizing research methodology, findings, preliminary conclusions, and recommendations relative to subsequent phases of the planned research efforts.
13  Make an executive presentation to the South Dakota Department of Transportation Research Review Board at the conclusion of the project.

Documents Available:
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