Task 5. Incorporate Paleoflood Information, Compute New LPIII Parameters, and Compare the Results

The information presented within this workshop is for instructional purposes only and does not represent any actual studies undertaken by the United States Army Corps of Engineers.  All dates presented are relative to the completion date (2021) of the paleoflood analysis.

A paleoflood analysis was completed for Bald Eagle Creek near Sayers Dam.  This study was undertaken in an effort to provide geologic and hydrologic information related to large historic and pre-historic discharges.  The work was completed through a collaborative effort among geologic, geomorphic, hydraulic, and hydrologic disciplines.  The analysis yielded evidence of a high-stage flood deposit in addition to features that have not been eroded or deposited upon during the past several thousand years.  The analytical approach involved field identification of geologic and geomorphic evidence of past large floods (i.e., paleostage indicators, PSI) or evidence of surface stability and a lack of inundation (i.e., non-exceedance bounds, NEB).  The elevations of two river terraces (i.e., called the “Qt1” and “Qt2” terraces, from oldest to youngest) were delineated using high-resolution topographic data and compared with discharge estimates from HEC-RAS two-dimensional hydraulic modeling.

Geomorphic and stratigraphic evidence show that the Qt1 terrace has not been eroded or deposited upon since its formation approximately 2500 – 3500 years ago, with a best estimate age of 3000 years ago.  The age estimate of the Qt1 terrace deposit is based on relative soil-profile development, radiocarbon analyses of charcoal fragments, and Optically Stimulated Luminescence analyses of river sand deposits.  The estimated paleodischarge capable of inundating the Qt1 terrace, in terms of a 3-day duration flow, is within a range of 57,500 to 115,000 cfs, with a best estimate of 85,000 cfs.  The Qt1 terrace can be interpreted as a NEB.

Additionally, deposits associated with the Qt2 terrace, which is inset into and younger than the Qt1 terrace, may represent a subsequent, intermediate flood.  The Qt2 terrace is interpreted to have been formed approximately 450 years ago (with a range of about 375 – 550 years ago), from the same suite of dating techniques. The estimated paleodischarge needed to inundate this surface and result in deposition of silty sand is within a range of 25,000 to 37,500 cfs (in terms of a 3-day duration flow).  The Qt2 terrace can be interpreted as a PSI (i.e. discrete event not within the historical record).

Using the skills that you have acquired, you must now interpret the results of this paleoflood analysis and include them to estimate an improved LPIII distribution.  This information will be used to size and design critical infrastructure related to an ongoing dam safety study.

• To begin, right click on the existing “3DAY_Extended_B17C” analysis.
• Click Save As…
• Name the new analysis “3DAY_Paleo_B17C”.
• Move to the EMA Data tab.
• The first row of the Perception Thresholds table controls the time window of the entire analysis.  Since we’d like to include paleoflood information possibly dating to the year -979 BCE (i.e. study completion date – 3000 years ago), the first row of the Perception Thresholds table needs to be modified.  Change the first row in the Perception Thresholds table so that the analysis spans your interpreted date range of the NEB.

It is possible that your interpretation of the age of the NEB will differ from other groups.

• Ensure that the low and high perception thresholds for this first row are left at 0 and “inf”.

Now, information needs to be added to the Perception Thresholds table to account for the NEB and PSI.  Two perception thresholds will need to be used since the perceivable range of flows during these periods are markedly different (one for the NEB and one for the PSI).  One possible interpretation is:

• For the NEB, enter a perception threshold of [100,000 – inf] for the period between -979 to 1571.
• For the PSI, enter a perception threshold of [45000 – inf] for the period between 1572 to 1910.
• The perception thresholds for this interpretation are shown in the following figure.  However, this is by no means the only way to interpret the available data.

• Once all of the information has been entered to the Perception Thresholds table, click Apply Thresholds. This will fill out the Flow Ranges table with the complementary information.

Now, the PSI (i.e. discrete event) must to be added to the Flow Ranges table.

• Enter a Peak, Low, and High flow value for the PSI event.
• Change the Data Type for the PSI event to Historical.

This information, along with the year in which you think the PSI event occurred, should be entered based upon your interpretation of the paleoflood data presented earlier.  It is possible that your interpretation of the age of the PSI will differ from other groups.

• Ensure that the Flow Ranges table contains a low and high value for every single year in the analysis period.
• A possible interpretation of the paleoflood data is shown within the following figure.

• Click Compute.
• Move to the Tabular Results tab.  Note the computed curve, 5-, and 95-percent confidence limits for all of the desired frequency ordinates, the moments/parameters of the LPIII distribution fit to the data, and other data related to the analysis are shown.
• Select the 3DAY_Extended_B17C and 3DAY_Paleo_B17C analyses and click Results | Graph to plot the results of both analyses in the same window.

Question:  How do the results of this analysis compare to the 3DAY_Extended_B17C analysis?  Why are they different?  How much information content was added by including the paleoflood information?

Download the final project files here: Sayers_Dam.zip

Continue to Task 6. Create a Volume Frequency Analysis and Develop a Family of Volume-Frequency Curves.