REPORT REPORT 
Evaluating Losses Avoided 
Through Hazard Mitigation 


City of Centralia, Washington 



Evaluating Losses Avoided Through Hazard Mitigation 


City Of Centralia, Washington 
February 2008 


Evaluating Losses Avoided Through Hazard Mitigation 1 
Introduction 


Over the past 15 years, the Cities of 
Centralia and Chehalis, Lewis County, and 
individual homeowners, with the support 
of FEMA, Washington State Emergency 
Management Division, and the Small Business 
Administration have invested several million 

dollars to acquire or elevate many flood-

prone residential structures in Lewis County. 
With the expenditure of these large amounts 
of public and private funds, the question is 
often asked: How well are these mitigation 
initiatives working? Are we able to 
quantify the losses avoided as a direct 
result of implementing acquisition and 
elevation programs? 

In an attempt to answer these questions, 
this evaluation focused on a sample of 35 
residential structures located in the City of 
Centralia that were elevated under FEMA’s 
Hazard Mitigation Grant Program (HMGP) after 

Jim Smith (FEMA staff) visits one of the elevated homes in the City of 
Centralia. 


major floods in 1996 and 1997. HMGP is a part of the Robert T. Stafford Disaster Relief and Emergency Assistance 
Act (The Stafford Act) and provides grants for states and communities to implement hazard mitigation measures 

after presidentially declared disasters. 

Methodology 

In order to calculate the flood losses that were avoided due to an elevation project, it is necessary to obtain the 
following pre-mitigation data on each residential structure to be evaluated: 

• Location 
• Structure type 
• Structure square footage 
• First floor elevation (before and after mitigation) 
• Number of floors 
FEMA was able to obtain Elevation Certificates from the City of Centralia’s Building Department for 67 structures that 
were elevated under the HMGP after the 1996 and 1997 flooding. Detailed elevation information was obtained 
for 35 of the elevated homes that are located within the same area that was again flooded by the December 2007 
event. Flood elevations for the December 2007 flood event were established from high water marks obtained 
for each structure. The Elevation Certificates provided the data required to develop pre-elevation, depth-damage 
relationships for each structure. With these relationships established, losses avoided from the December 2007 flood 

event were calculated for each building, contents, and displacement costs avoided. With the high quality of data 
obtained on these 35 structures, this analysis will begin to answer the questions posed in paragraph 1. 

This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 

Evaluating Losses Avoided Through Hazard Mitigation 2 2 
It is important to understand the difference between a benefit-cost analysis to assess cost effectiveness, which 
is performed for FEMA-funded mitigation projects, such as the HMGP, and the methodology that is used to 
assess losses avoided. A benefit-cost analysis identifies potential future losses that will be avoided over the life of 

mitigation projects by the use of a probability analysis. Measuring the losses avoided of past mitigation projects 

applies a methodology (stage-damage functions) that can be used after a flood event, to quantify the losses that 

would have occurred had the mitigation not been undertaken. 

Economic Assumptions 

Assumptions used for the Losses Avoided calculations are: 

• 
Building replacement costs of $99.46/sq foot (2008 prices and conditions) were obtained 
from R. S. Means - CostWorks. Building costs were specific to the Centralia area. 
• 
A content value of 40 percent of the building replacement value was used. 
• 
The default depth-damage relationships for residential structures, contents, and displacement 
costs from the Riverine Full Data BCA Modules, were used in the analysis. 
• 
A building damage of 50 percent or more would result in demolition (default value specified 
in FEMA’s BCA Riverine Flood-Full Data Module). 
Computation of Losses Avoided 

Building Data 

Table 1 provides building data by address and HMGP disaster 
number, for 35 homes that were elevated after the 1996 
and 1997 FEMA flood disaster declarations. The elevation 

data came from Elevation Certificates provided by the Centralia 
Building Department and building data (structure type & 
square footage) were obtained from Lewis County Assessors 

records. The 2008 structure replacement value was based on 
$99.46 per square foot, which was derived from R.S. Means 

CostWorks. 

Depth-Damage Functions 

Data from Table 1 was entered into FEMA’s Riverine Flood - Full Data BCA Module for each of the 35 structures being 
analyzed. Table 2 provides an example of the default, model generated building depth-percent damage function, 
content percent damage function, and displacement days for a 1-story structure with no basement (all but 6 of the 
homes fall into this category), for various flood depths. 

FEMA’s Riverine BCA Module was run for each elevated structure, and the resulting monetary depth-damage 

relationship output (for buildings, contents and displacement costs) was the basis for calculating the losses avoided 

from the December 2007 flood. 

Herman Huggins (FEMA staff) indicates the high 

water mark from the December 2007 floods. 


This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 

Evaluating Losses Avoided Through Hazard Mitigation 3 3 
TABLE 1 - Building data for elevated structures in the City of Centralia. 

Property 
ID 
Base 
Flood 
Elevation 
FFE 
(Before) 
FFE 
(After) 
Feet 
Raised 
Structure 
Type 
Square 
Footage 
2008 Structure 
Replacement 
Value 
FEMA DISASTER DR-1100-WA 
1 173.5 173.3 179.0 5.7 1 Story 1,398 $ 139,045 
2 173.0 173.2 175.9 2.7 1 Story 1,319 $ 131,188 
3 174.0 172.5 177.1 4.6 1 Story 1,656 $ 164,706 
4 174.7 174.8 177.6 2.8 1 Story 1,536 $ 152,771 
5 N/A 175.0 177.8 2.8 1 Story 872 $ 86,729 
6 174.1 172.1 177.6 5.5 1 Story 1,188 $ 118,158 
7 173.0 171.5 176.7 5.2 2 Story 1,380 $ 137,255 
8 174.3 174.4 177.9 3.5 1 Story 1,630 $ 162,120 
9 169.0 168.8 172.1 3.3 1 Story 1,452 $ 144,416 
10 173.0 170.6 176.7 6.1 1 Story 1,008 $ 100,256 
11 174.2 173.5 178.5 5.0 1 Story 1,344 $ 133,674 
12 174.7 168.3 177.9 9.6 1 Story 2,364 $ 235,549 
13 174.1 174.8 177.8 3.0 1 Story 1,250 $ 124,325 
14 173.0 173.1 176.4 3.3 2 Story 1,593 $ 158,440 
15 173.0 171.9 175.7 3.8 1 Story 1,628 $ 161,921 
16 173.0 172.1 176.1 4.0 2 Story 1,945 $ 193,450 
17 175.1 175.2 177.3 2.1 1 Story 1,775 $ 176,542 
18 173.0 173.3 176.2 2.9 1 Story 845 $ 84,044 
19 174.0 173.2 177.6 4.4 1 Story 1,857 $ 184,697 
20 173.0 171.5 176.8 5.3 1 Story 1,560 $ 155,158 
21 173.0 172.8 175.6 2.8 1 Story 1,284 $ 127,707 
22 173.5 173.5 178.0 4.5 2 Story 3,302 $ 328,417 
23 173.0 174.1 176.2 2.1 1 Story 1,692 $ 168,286 
24 173.0 171.1 177.9 6.8 1 Story 1,314 $ 130,690 
25 173.5 174.5 177.6 3.1 1 Story 1,356 $ 134,868 
26 173.0 171.7 175.8 4.1 1 Story 896 $ 89,116 
27 N/A 175.3 177.3 2.0 1 Story 918 $ 91,304 
28 173.0 172.6 175.3 2.7 1 Story 1,028 $ 102,245 
FEMA DISASTER DR-1159-WA 
29 174.0 173.5 175.1 1.6 1 Story 980 $ 97,471 
30 170.0 170.9 173.0 2.1 1 Story 1,578 $ 156,948 
31 176.0 174.9 178.4 3.5 1 Story 1,044 $ 103,836 
32 169.8 170.2 172.8 2.6 1 Story 1,490 $ 148,195 
33 173.5 174.6 177.4 2.8 1 Story 1,920 $ 190,963 
34 174.3 174.3 177.7 3.4 2 Story 1,798 $ 178,829 
35 170.0 170.5 173.5 3.0 2 Story 2,116 $ 210,457 
Notes: FFE designates First Floor Elevation 

Base Flood Elevation and FFE referenced to the National Geodetic Vertical Datum of 1929 (NGVD ‘29) 

This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 

Evaluating Losses Avoided Through Hazard Mitigation 4 4 
TABLE 2 - Relation of damage to flood depth for 1-story buildings (without basements) in City of Centralia. 
[Flood depth in feet; values in building and content columns in percent] 

Flood Depth Building (DDF) Contents (DDF) Displacement 
(days) 
-1 0 0 0 
0 9% 14% 0 
1 14% 21% 62 
2 22% 33% 126 
3 27% 41% 166 
4 29% 44% 182 
5 30% 45% 190 
6 40% 60% 270 
7 43% 65% 294 
8 44% 66% 302 
>8 45% 68% 310 

Sample Calculation: ID 19 
Flood Elevation 173.9 ft 
First Floor Elevation 171.9 ft 
Flood Depth 2 ft 
Building DDF 22% 
Contents DDF 33% 
Displacement Period 126 days 
Note: DDF designates Depth Damage Function 

Considering 
that the 
estimated 
total losses 
avoided were 
$1,905,760, the 
flood damages 
prevented from 
this single 
event likely 
exceeded the 
original project 
cost by almost 
two to one. 
This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 

Evaluating Losses Avoided Through Hazard Mitigation 5 5 
Hydrologic Data 

This losses avoided analysis was based solely on the December 2007 flood event which had peak discharges ranging 
from 63,100 cubic feet per second (cfs)¹ at the Chehalis River near Doty gage (upstream of Centralia) to 79,100 
cfs² at the Chehalis River near Ground Mound gage, downstream of Centralia. These discharges equate to an 
approximate 500-year, and 200-year recurrence interval, respectively. 

December 2007 Flood elevations shown on Table 3 were derived by comparing high water marks on each structure 
to the known first floor elevation of the raised home. This peak flood stage was then compared to pre-mitigation 
first floor elevations to derive the “avoided” water depth in each home. It should be noted that 2 of the elevated 
homes had water slightly above the first floor elevations (FFE’s) and eight others had water within 1 foot of the first 
floor, and thus sustained minor flood damage. Pre-mitigation water depths would have ranged from 0.7 feet below 
to 9.0 feet above the FFE’s. 

Losses Avoided 

Losses Avoided, or flood damages prevented by these two past HMGP home elevation projects, were derived by 
taking the water depth relative to FFE’s from Table 3, and recording the corresponding damages for that water 
depth found in the Riverine Flood – Full Data BCA Model‘s depth-damage output tables. Two homes had water slightly 
above the first floor during the December 2007 event, and eight others had water within 1 foot of the first floor. 
The observed damages associated with these flood levels have been netted out of the Losses Avoided for these 
structures. A summary of the Losses Avoided for each structure, and damage category, can be found in Table 4. 
The location of each structure within the study area, and the approximate depth of flooding are displayed on 

Plate 1. 

Cost-Effectiveness 

The individual project cost for each structure elevated was unavailable, so a cost-effectiveness analysis for each 
home could not be accomplished. Good aggregate data was available however, for HMGP projects DR-1100-WA, 
and DR-1159-WA that elevated a total of 116 Centralia homes in 1996 and 1997 for a cost of $3,371,969, or an 
average cost of $29,069 per structure. Since all homes in this analysis were elevated in the same time period 
under these two disasters, and were randomly distributed throughout the floodplain, this average would be a fairly 

accurate estimate of the mitigation costs for all homes in our study. Applying an average elevation project cost of 

$29,069 to all 35 structures, results in a total mitigation cost of $1,017,415. Considering that the estimated total 
losses avoided were $1,905,760, the flood damages prevented from this single event likely exceeded the original 

project cost by almost two to one. 

¹ Provisional USGS Data 
² ibid 

This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 

Evaluating Losses Avoided Through Hazard Mitigation 6 6 
TABLE 3 - Relation of damage to flood depth for 1-story buildings (without basements) in City of Centralia. 

Property 
ID 
December 2007 
Flood Elevation 
(NGVD ‘29) 
FFE Before 
Elevation 
(NGVD ‘29) 
Water Above 
FFE (feet) 
FFE After 
Elevation 
(NGVD ‘29) 
Water Below/ 
Above FFE (feet) 
FEMA DISASTER DR-1100-WA 
1 177.5 173.3 4.2 179.0 -1.5 
2 173.1 173.2 -0.1 175.9 -2.8 
3 174.3 172.5 1.8 177.1 -2.8 
4 177.1 174.8 2.3 177.6 -0.5 
5 176.4 175.0 1.4 177.8 -1.4 
6 177.8 172.1 5.7 177.6 0.2 
7 174.9 171.5 3.4 176.7 -1.8 
8 177.7 174.4 3.3 177.9 -0.2 
9 171.1 168.8 2.3 172.1 -1.0 
10 174.9 170.6 4.3 176.7 -1.8 
11 178.1 173.5 4.6 178.5 -0.4 
12 177.3 168.3 9.0 177.9 -0.6 
13 176.6 174.8 1.8 177.8 -1.2 
14 175.2 173.1 2.1 176.4 -1.2 
15 173.9 171.9 2.0 175.7 -1.8 
16 175.1 172.1 3.0 176.1 -1.0 
17 177.1 175.2 1.9 177.3 -0.2 
18 175.2 173.3 1.9 176.2 -1.0 
19 176.4 173.2 3.2 177.6 -1.2 
20 174.8 171.5 3.3 176.8 -2.0 
21 174.7 172.8 1.9 175.6 -0.9 
22 177.0 173.5 3.5 178.0 -1.0 
23 173.4 174.1 -0.7 176.2 -2.8 
24 174.4 171.1 3.3 177.9 -3.5 
25 176.8 174.5 2.3 177.6 -0.8 
26 173.7 171.7 2.0 175.8 -2.1 
27 174.8 175.3 -0.5 177.3 -2.5 
28 174.1 172.6 1.5 175.3 -1.2 
FEMA DISASTER DR-1159-WA 
29 175.5 173.5 2.0 175.1 0.4 
30 171.9 170.9 1.0 173.0 -1.1 
31 177.6 174.9 2.7 178.4 -0.8 
32 170.6 170.2 0.4 172.8 -2.2 
33 176.4 174.6 1.8 177.4 -1.0 
34 174.7 174.3 0.4 177.7 -3.0 
35 171.8 170.5 1.3 173.5 -1.7 
Notes: FFE Designates First Floor Elevation 
NGVD ‘29 Designates The National Geodetic Vertical Datum of 1929 

This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 

Evaluating Losses Avoided Through Hazard Mitigation 7 7 
TABLE 4 - Losses avoided as results of building elevations in City of Centralia, flood of December 2007. 

Property 
ID 
Water Depth 
above FFE 
Pre-Mitigation 
(feet) 
Losses Avoided (2007 dollars) 
Building 
Repair Costs 
Content 
Losses 
Displacement 
Costs Total 
FEMA DISASTER DR-1100-WA 
1 4.2 $ 40,601 $ 24,360 $ 12,116 $ 77,077 
2 -0.1 $ 10,626 $ 6,376 $ -$ 17,002 
3 1.8 $ 33,600 $ 20,160 $ 8,635 $ 62,395 
4 1/ 2.3 $ 29,026 $ 17,416 $ 9,866 $ 56,308 
5 1.4 $ 14,917 $ 8,950 $ 4,506 $ 28,373 
6 1/ 5.7 $ 31,903 $ 19,141 $ 13,544 $ 64,588 
7 3.4 $ 25,804 $ 15,483 $ 6,792 $ 48,079 
8 1/ 3.3 $ 33,072 $ 19,844 $ 12,627 $ 65,543 
9 2.3 $ 33,938 $ 20,363 $ 9,479 $ 63,780 
10 4.3 $ 29,375 $ 17,625 $ 9,770 $ 56,770 
11 1/ 4.6 $ 32,349 $ 19,409 $ 11,982 $ 63,740 
12 1/ 9.0 $ 97,517 $ 58,510 $ 30,095 $ 186,122 
13 1.8 $ 25,363 $ 15,217 $ 7,103 $ 47,683 
14 2.1 $ 21,389 $ 12,833 $ 4,546 $ 38,768 
15 2.0 $ 35,623 $ 21,374 $ 9,438 $ 66,435 
16 3.0 $ 34,821 $ 20,893 $ 8,161 $ 63,875 
17 1/ 1.9 $ 24,716 $ 14,830 $ 9,570 $ 49,116 
18 1.9 $ 17,818 $ 10,691 $ 5,862 $ 34,371 
19 3.2 $ 50,607 $ 30,364 $ 13,793 $ 94,764 
20 3.3 $ 42,824 $ 25,694 $ 12,228 $ 80,746 
21 1/ 1.9 $ 25,924 $ 15,554 $ 7,612 $ 49,090 
22 3.5 $ 62,399 $ 37,440 $ 13,427 $ 113,266 
23 -0.7 $ 4,544 $ 2,726 $ -$ 7,270 
24 3.3 $ 36,070 $ 21,642 $ 10,827 $ 68,539 
25 1/ 2.3 $ 29,266 $ 17,560 $ 9,037 $ 55,863 
26 2.0 $ 19,606 $ 11,763 $ 6,363 $ 37,732 
27 -0.5 $ 4,109 $ 2,465 $ -$ 6,574 
28 1.5 $ 18,404 $ 11,043 $ 5,288 $ 34,735 
FEMA DISASTER DR-1159-WA 
29 1/ 2.0 $ 11,697 $ 7,018 $ 6,004 $ 24,719 
30 1.0 $ 21,973 $ 13,184 $ 4,795 $ 39,952 
31 1/ 2.7 $ 24,609 $ 14,766 $ 8,426 $ 47,801 
32 0.4 $ 16,302 $ 9,781 $ 1,845 $ 27,928 
33 1.8 $ 38,957 $ 23,374 $ 9,631 $ 71,962 
34 0.4 $ 11,803 $ 7,082 $ -$ 18,885 
35 1.3 $ 21,466 $ 12,880 $ 1,563 $ 35,909 
TOTAL $ 1,013,018 $ 607,811 $ 284,931 $1,905,760 
Note: FFE designates First Floor Elevation 
1/ These structures sustained some building and content damage from the December event. These damages have been 
netted out of the Losses Avoided figures. 

This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 

Evaluating Losses Avoided Through Hazard Mitigation 8 
Summary 

Had they not been elevated, all of the 35 homes in our study would have had inundation damage, or significant 
incremental damage from the December 2007 flood event. Fifteen of the structures would have experienced 
less than 2 feet of water, 18 would have been flooded between 2 and 5 feet, and 2 would have had over 5 feet of 
water above pre-mitigation first floor elevations. One home would have had 9 feet of inundation. The extent of 
flood damage is related to the depth of the flood within the structure, and the value of both the structure and its 
contents. Had these 35 homes not been elevated, the December 2007 flood would have caused between $6,574 
and $186,122 in flood losses for each home. For just these 35 elevated homes, the total losses avoided from the 
December 2007 flood are estimated to be $1,905,760. 

The cost-effectiveness of these Centralia elevation projects was clearly established for a single flood event. It can 

be expected that the payoff from mitigation expenditures will continue to increase over the effective life of the 

structures, as cumulative losses avoided grow with subsequent flood events. 

This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 

Appendix 



Evaluating Losses Avoided Through Hazard Mitigation 
This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 
Map A Map A 

Evaluating Losses Avoided Through Hazard Mitigation 
This document was produced at the FEMA State Joint Field Office for DR-1734-WA. City Of Centralia, WA 
Map B Map B