Orestimba Creek Feasibility Study - Stanislaus County

Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012
Hydrologic - Uncertainty factors include hydrologic data record lengths that are often
short or do not exist, precipitation-runoff computational methods that are not precisely
known, and imprecise knowledge of the effectiveness of flow regulation. 8
Hydraulics - Uncertainty arising from the use of simplified models to describe complex
hydraulic phenomena, including the lack of detailed geometric data, misalignments of
hydraulic structures, debris load, infiltration rates, embankment failures, material
variability, and from errors in estimating slope and roughness factors. For the Urban EIA
as standard deviation in stage of 0.6 feet was used, while 1.5 feet was used for the Rural
EIA (EM-1110-2-1619 guidance for minimum uncertainty standards alluvial fan vs.
channel stage).
Geotechnical – Under without project conditions, we have assumed the CCID canal fails
when overtopped for hydraulic modeling purposes. This is based on input from the SPK
geotechnical section and historical performance.
Economics - Uncertainty concerning land uses, depth/damage relationships,
structure/content values, structure locations, first floor elevations, the amount of debris
and mud, flood duration, and warning time and response of floodplain inhabitants (flood
fighting).
5.3 Probability-Stage-Damage Functions
Damages were estimated by each category, by impact area and by event based on varying depths
within the flood plain relative to individual structures. These damages were then linked to a
stage within the channel (or the floodplain) at a common index point for each impact area by
exceedance probability. For the Urban EIA, an index point in the floodplain was used at the first
point flooding within the area at the Railroad and Lundy Rd intersection. Index locations used
for can be found in Figure 5-2. These stage-damage functions were calculated within HEC-FDA
and they represent the economic relationships between the hydrology and hydraulics. These
functions can be found by event, damage category and impact area in the tables below. These
damage values differ from the calculated damages by event shown in the HEC-FDA calculated
stage-damage curves due to uncertainties in each relationship. Damages within FDA are
aggregated back to the chosen index location.
It is noted that there is a difference between the event damages calculated within HEC-FDA in
the tables below and the actual damages reported from past flood events (Section 1.3), but this is
to be expected. Historical record is not believed to be a good reference for actual damages in the
area. A 2002 national study 9 of local flood damage estimates showed that “individual damage
estimates for small floods or for local jurisdictions within a larger flood area tend to be extremely
inaccurate. It is rare to have actual cost data to compare with damage estimates. The […]
analysis of one large flood disaster indicates that, in cases where actual costs are less than $30
8 The hydrologic data record lengths (period of record) are the number of years of a systematic record of peak discharges at a stream gage. This
parameter directly influences the uncertainty associated with the frequency-discharge function shown in Figure 5-1 and consequently the project
performance statistics. In general, a longer period of record implies less uncertainty associated with this function. The period of record for
Orestimba Creek Hydrology is 45 years.
9 Pielke, Jr., R.A., M.W. Downton, and J.Z. Barnard Miller, 2002: Flood Damage in the United States, 1926-2000: A Reanalysis of National
Weather Service Estimates. Boulder, CO: UCAR.
23