Orestimba Creek Feasibility Study - Stanislaus County

Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012
Time of Day: The time of day that a flood hits a population center can make a significant
difference in the impacts to the affected population. Nighttime is particularly critical as residents
normally are asleep and not aware of their surrounding situation. Additionally, darkness affecting
escape, adds time which may make the difference between life and death.
Escape Routes: Knowledge of escape routes is critical for survival. In some instances the
population may have only one escape route as topographical features may make the other escape
routes impossible. Moving to high ground quickly is of paramount importance to escape the
affects of the flood.
Distance/Time to Safety: The time it takes to actually move from the point of risk to safe
location is considered in determining the risk of loss of life. Being caught in a vehicle is possibly
the worst place to be if you encounter a flood. In some instances the best advice is to stop
driving. Although the situation may be scary and fleeing in a vehicle may be tempting, the safest
thing to do is to get out of the vehicle and move to high ground on foot. Staying in the vehicle
increases the chances that it will be swept away and a floating vehicle is difficult to stop.
Depth and velocity of Water: Depth and velocity of floodwaters play an important part in the risk
to life. Heavy objects can be literally lifted by water and moved downstream causing risk to
property and life. Trees especially can move on top of water and cause serious damage. Seeking
high ground is the best way to avoid these floating debris.
Without Project Condition – Floodplain
For purposes of this analysis, the without project 1% floodplain will be considered the
geographic area for analysis of the population at risk. The illustration below shows the 1%
event floodplain used in the analysis where no project is assumed. As indicated by the
floodplain outline, much of the flooded area is the downtown and eastern residential area of
Newman. It is assumed that the commercial and industrial occupants are aware of the general
flood situation and would be able to respond quickly to a warning order, either during the day or
at night. It should be noted that the city of Newman east of the railroad tracks would be expected
to be inundated with floodwaters when faced with a 1% event flood. The depths of flooding
would range from less than one foot to approximately two feet. Depth of flooding is critical to
the analysis relating to life issues, as it is known that the impacts occurring at night have the
highest rate of casualties.
Geo-spatial analysis indicates that there are 1,062 residential structures located within the
floodplain. Using the 2.64 residents per household figure from the US Census, the total
population at risk is calculated to be 2,804 or slightly over 26 percent of Newman’s population.
Indications are that the flood warning time for residents downstream of Orestimba Creek during
a hydrologic event caused by abnormal rainfall will allow approximately 24 hours of warning
time to Newman residents prior to peak flows arriving in Newman. Accordingly, notification
methods used by Stanislaus County and City of Newman are considered adequate to deal with
these floods. It is expected that all notified people in Newman would have sufficient time to
move to a safe area. Evacuation routes have been identified by maps provided by the Sponsor.
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