Better Characterizing Uncertainty in Geologic Paleoflood Analyses Dean Ostenaa Ostenaa Geologic LLC October 24, 2017 Presentation for 2017 GSA Annual Meeting Seattle, WA

Benefits of a Structured, Formal Approach to Estimating Uncertainty Replaces adhoc and overly simplified uncertainty descriptions “discharge uncertainty is ±25%”…………..based on the hydraulic model? “stage uncertainty is ±1 m”………based on the field observation? Adds transparency Identifies dependencies within complex conclusions Brings focus to critical strengths and weaknesses of interpretations and results Geologic insights are essential to recognition of most epistemic uncertainties (dominant type of uncertainty for paleofloods)

Bring Geologic Perspectives into Engineering Analyses As geologic information is brought forward into this practice, take the opportunity to bring geologic perspectives of how uncertainty related to that information should be conveyed Geologic experience from conveying paleoseismic information for seismic hazard practice has potentially useful parallels and lessons Precedents for event identification and site-to-site correlation Stratigraphic completeness at an individual site Uncertainties associated with age dating Sedimentary processes and site-scale depositional/erosional models Alternatives to continuous distributions for flood series SSHAC process and models from UCERF3

Accepted Practice Procedures for incorporation of paleoflood data in standard flood frequency estimates, used for flood insurance map estimates and most standard engineering practice, are in the final steps of approval for publication.

Uncertainty -Relationships and Dependencies Topographic inputs affect everything Resolution – both for hydraulics and for geomorphic characterization Conveyance Local hydraulics – is the geologic site likely to see erosion or deposition in a large flood Slope – both local and reach scale Stream Power Stable through time? Hydraulic models Different issues for 1D and 2D models PSI’s and Geomorphic Inputs Do site characteristics that affect uncertainty change with increasing stage/discharge; or within the study reach?

Geologic Paleostage Indicators (PSIs) Geologic PSI define the contrast between evidence of fluvial modification and evidence of landscape stability

PSI Variability and Uncertainty Types of PSI and Sources of Uncertainty Flotsam/botanic (tree scars) (Stage) ± depends on size, material, etc. Could be emplaced above HWM Koenig et al. (2016) has nice summary Scour/trim lines (Stage + flow) Soil vegetation resistance Erosion criteria; Flow velocity/depth Measurement uncertainty Top vs bottom; variability along length Sediment deposition (Stage + flow) Always a minimum Impacted by hydraulic setting (particle size) Sediment supply Modification of geomorphic surfaces (Stage + flow) Erosion criteria Topographic input resolution Hydraulic model resolution Reach scale natural variability Hydraulic variability Varies with sub-reach hydraulics Smaller in backwater areas Cross channel Larger in through-flowing reaches Dependent on stage/discharge? Captured in hydraulic model outputs? PSI location relative to hydraulic model cross section or calculation grid Less in 1D models; more in 2D models Depends on # of x-sections, topographic resolution, hydraulic model mesh size

Illustrating Uncertainties of PSI Water Surface Profiles 510 Elevation, in feet above arbitrary datum 509 508 Left bank HWMs Right bank HWMs Average water surface 507 100 200 300 400 500 600 Distance upstream, in feet Modified from Koenig et al., 2016

Some Initial Questions Identify the Discharge Uncertainty Associated with Identified PSI Some Initial Questions Multiple PSI Locations? Same Hydraulics at Locations? Same Models at all Locations? Yes No No No Yes Yes

Identify Factors Associated with Hydraulics Add Branches for Single or Multiple Models as Needed Hydraulic Model Choices 2D Model Choices Add Branches for Single or Multiple Models as Needed Parameter Set 2 (Roughness, etc.) 1D Model Choices Model A Scenarios and Parameters Parameter Set 2 (Roughness, etc.)

Identify Factors Associated with Characteristics of PSI Add Branches for Values as Needed Stage Shift for Location off X-Section Alternate Values PSI Stage A PSI Stage B PSI Stage C Stage Uncertainty for PSI

Combine Results for Aggregate Discharge Uncertainty Distribution Discharge A w/ Aleatory Uncertainty Q PSI Stage A PSI Stage B PSI Stage C Q Discharge B w/ Aleatory Uncertainty Q Combine Distributions from Multiple Branches Discharge C w/ Aleatory Uncertainty Q

PSI Hydraulics Combine Results Hydraulic Model Choices 1D Model Choices 2D Model Choices Add Branches for Single or Multiple Models as Needed Model A Scenarios and Parameters Parameter Set 2 (Roughness, etc.) Hydraulics Stage Shift for Location off X-Section Stage Uncertainty for PSI Alternate Values Add Branches for Values as Needed PSI Stage A PSI Stage B PSI Stage C PSI PSI Stage A PSI Stage B PSI Stage C Discharge A w/ Aleatory Uncertainty Discharge B w/ Aleatory Uncertainty Discharge C w/ Aleatory Uncertainty Q Combine Distributions from Multiple Branches Combine Results

Better Approaches to Exploiting Geologic Data Minimize Topographic Uncertainty Lidar; SFM; various digital topographic tools Exploit Spatial Information Detailed geomorphic mapping at reach-scale Maximize geomorphic resolution Two-dimensional hydraulic modeling Evaluate key spatial sub-areas defined by geomorphic mapping Link individual deposits, geomorphic surfaces or landforms Use multiple empirical relations/approaches as a quantitative definitions for deposition or erosion-based inundation limits Criteria based on reach-specific soils and vegetation Evaluate and develop separate uncertainty estimates for each geomorphic sub-area through multiple lines of evidence. Combine multiple sample areas for global best estimates and uncertainties Thresholds for deposition by floods are lower than thresholds for erosion Not all sites of similar geomorphic age will have erosion/deposition at the same discharge

Stream Power and Big Lost River Criteria From Ostenaa and O’Connell, 2005

Big Lost River Channel Stream Power Profile From Ostenaa and O’Connell, 2005

Concluding Thoughts Geologic paleoflood investigations can significantly extend flood history records Can reduce uncertainty of discharge and probability estimates of infrequent events Accepted in current practice and regulatory framework Complimentary to other flood hazard estimation approaches Structured and formal evaluations of uncertainty Improve transparency and acceptance by public and regulatory stakeholders Highlight dependencies and bring focus to strengths and limitations