FLOOD10390

• County: Boulder
• Community: Boulder
• Stream Name: Fourmile Canyon
• Basin: South Platte
• Title: Fourmile Canyon Creek - Major Drainageway Planning - Phase A Report - Alternatives Analysis
• Date: 6/1/2000
• Prepared For: UDFCD, Boulder
• Prepared By: Love & Associates
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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The total damage costs are estimated by computing the area under the curve depicted in the Average
Annual Flood Damage Potential Graphs, found in Figures 5-1 through 5-10. A composite graph of
all reaches is provided for comparison, see Figure 5-11. This damage analysis was calculated as
recommended in the Urban Drainage and Flood Control District's Methodology for Evaluation of
Feasibility: Multiiurisdictional Urban Drainage and Flood Control Projects. Later in this document,
alternates will be proposed, which reduce damages through the construction of flood mitigation
improvements

Table 5-1
Relationship of Recurrence Interval of Flood
To Probability of Exceedence

of Exceedence
50
20
10
2
1
0.2

Probabilit

Flood Frel
2-year
S-year
1 a-year
50-year
lOa-year
SOO-year

DAMAGE ANALYSIS

One of the tools that can be utilized by the Project Sponsors in their evaluation of the Fourmile
Canyon Creek Major Drainageway Planning Alternates is a damage analysis. Through this damage
analysis, flood frequency information is developed under current existing floodplain conditions.
This information is formulated using the Fourmile Canyon Creek hydrology for multiple frequencies
for future basin conditions (refer to Chapter 3 - Hydrologic Analysis) and the floodplain hydraulics
for multiple flood frequencies (refer to Chapter 4 - Hydraulic Analysis). The hydrologic and
hydraulic analyses are used to develop stage - frequency flood data. This information is then used to
develop flood damage - frequency data through the use of depth damage curves developed by the
Federal Emergency Management Agency (FEMA) and Federal Insurance Administration (FIA).
The damage - frequency data is then converted to an annual damage cost by means of an economic
study. More detailed information on the damage analysis is presented below

METHODOLOGY

5.

All property, structures and infrastructure within the Fourmile Canyon Creek SOO-year floodplain
was inventoried. Information from the Boulder County Assessor's Office was then obtained. This
information included: value of land, value of structures, type of structure (one story, no basement,
two story with basement, mobile home, etc.) type of construction for structures, current property
owner and address, and other pertinent information.

INTRODUCTION

5.1

5.2

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Benefits for the proposed alternates are realized through damage reduction. Comparison of benefits
and costs must be made for the same time frame. Benefits stemming from reduced flood damages
occurring annually over the life of the project cannot be compared directly with construction costs,
which occur over a short period of time at the beginning of the project. All benefits and costs must
be converted to either present or annual amounts before comparison, using appropriate interest and
inflation factors, which account for the time value of money. For this project, a 50-year project life
was chosen, which corresponds to the typical structural life. A 6% annual interest rate was selected
which reflects the cost of borrowed capital for the entities involved and a 3% annual rate of inflation
was assumed. The net discount rate used in the preset value calculations was 3%. Table 5-2
summarizes the annual damages and the present worth of annual damages for each study reach and
for the total floodplain

A field reconnaissance of all properties located in the SOO-year floodplain was then undertaken and
first floor elevations of structures above the adjacent ground elevation was noted. Then, using the
City's 1993 topographic mapping, the adjacent ground next to the structure was given an elevation
and the elevation of the first floor was determined.

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Using the Fourmile Canyon Creek flood profiles for the 2-, 5-, 10-, 50-, 100-, and SOO-year storms,
flood water surface elevations were assigned to each structure or property within the SOO-year
floodplain. Depth damage curves for structures and contents developed by FEMA-FIA in 1998 were
then used to determine this component of damages. Damages to open land and public infrastructure
were also estimated.

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More than one third (1/3) of the average annual damage for the entire project is associated with
Reach 4, which is mostly located in Boulder County. Most damages are from flooding of the homes
throughout the entire reach. Even the two-year storm event damages some of the buildings and/or
contents in this reach.

DESCRIPTION OF RESULTS

5.3

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Approximately one third (1/3) of the average annual damages is associated with Reaches 5 and 6a.
Most of these costs come from flooding of the houses in the flow path of the water that spills from
Fourmile Canyon Creek to Wonderland Creek.

Using the methodology described above, flood damages were developed for 2-, 5-, 10-, 50-, 100-,
and SOO-year flood events. The results of this analysis are graphically plotted on Figures 5-1
through 5-10 in the form of flood damages (dollars) vs. probability of exceedence (% per year). The
following Table 5-1, shows the relationship of recurrence interval of flood vs. probability of
exceedence.

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5-1

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