Paleoflood Studies and 

their Application to 

Reclamation Dam Safety

The Problem for Dam Safety Office -

- More than half the inventory does not pass 

traditional deterministic criteria used as an 

estimate of flood hazard (i.e., the PMF)

- Costs associated with modifying the inventory 

to meet this standard were considered prohibitive

- Deterministic criteria varied through time and 

with subjectivity of the practitioner (individual 

and organizational)
A Solution -

- Utilize Probabilistic Flood Hazard Analysis 

(PSHA)

- Highly adaptable to evaluate institutional 

exposure to specific hazards or address 

particular safety concerns (e.g., spillway gates)

- Allows DSO to better categorize its inventory 

and prioritize corrective actions on the basis of 

risk
A Problem for PFHA -

- Short or non-existent records

100 years is wonderful; not common; still inadequate 

to make an accurate estimation of the large 

magnitude infrequent floods

- Records with extreme outlier(s)

What does the outlier represent exactly (extremely 

rare event or is it simply a less frequent large flood 

and what is its return period)

Paleoflood Data -

- Extends short or non-existent records

Extending record to include the last 1000 years is 

common, up to and including much of the last 10,000 not 

unusual

Addition of any paleoflood data to a frequency analysis 

has been shown to improve accuracy of the estimate

- Places extreme outlier(s) into temporal context

Time interval between events of similar size (if they are 

recurrent) or the time since the event last occurred can 

often be determined 

- Cost effective and robust





Soil Geomorphology

– Soil is a natural body that consists of horizons of 

mineral and/or organic constituents of variable 

thicknesses, which differ from the parent material in 

their morphological, physical, chemical, and 

mineralogical properties and their biological 

characteristics

– Pedogenic processes are responsible for the form 

(morphology) of most soils. The extent of this 

development is in part time dependent and occurs at 

a predictable rate; allows estimates of age

– Soil stratigraphy is the relationship between soils 

and Quaternary geologic units (bridge between 

stable landforms and active fluvial processes)
Evidence of recent flooding

Evidence of stability
Screening-Level (CFR or CR) –

- Limited field data collection

- Generally based on a single site assessment

- Discharge estimated from slope-conveyance 

calculations

- Age Estimates based on soil correlation

- Used to provide information that can be used in 

screening-level risk assessments for prioritizing 

risk profile of inventory 

- Cost: $15K to $30K
Issue Evaluation (IE) -

- Extended field data collection 

- Includes assessment from multiple sites

- Discharge estimated from one-dimensional 

hydraulic model

- Numerical age determinations (commonly 14C), 

soils 

- Used to primarily to reduce the uncertainty in 

screening-level risk assessments and for assessing 

risk related to specific dam safety issues

- Cost: $50K to $100K
Corrective Action Studies (CAS) -

- Extensive field data collection

- Assessment at multiple sites on different tributaries in 

the basin or on adjacent rivers within a region

- Two-dimensional hydraulic modeling

- Numerical age determinations (commonly 14C), soils

- Used for design-level assessment of flood hazard; 

attempts to reduce uncertainty in data as much as is 

reasonably possible given available technology

- Cost: >$150K
Conclusions -

- Paleoflood data is an extremely powerful addition to 

PFHA and a critical component in Reclamation risk 

analyses

- Vastly improves the accuracy of estimates for the 

magnitude and frequency of large floods

- Is cost effective and studies can be patterned to fit 

informational requirements

- Utilizes wide-used scientifically-based methodologies 

that have been adapted over time to take advantage of 

advances made in the fields of remote sensing, isotopic 

geochronology, computer-assisted hydraulic modeling, 

and statistical hydrology
Questions?