Flooding Problem Area Assessment and Improvement Concepts

For the 18 high priority flooding project locations where improvements were conceptualized, a cost-benefit analysis was conducted in order to determine the overall economics of each project for prioritization and justification purposes. The benefit that each project provides to the County was evaluated in terms of its ability to reduce the occurrence of flooding for roadways and property. In some cases property impacts were also differentiated between lot flooding and structure flooding where more detailed evaluation was required to establish benefit. The financial benefits to roads and property of each project improvement concept were evaluated for financial justification using the methodology in the following sub-sections.

The financial validity of each proposed conceptual project was evaluated by comparing the estimated project cost to the estimated flooding benefit cost that the project would be estimated to provide. The prioritization of projects for implementation was then based on the cost to benefit ratio. Ideally for a project to be considered for a higher ranking, the estimated benefit should be greater than the estimated project cost (i.e., ratio > 1). In some cases the ratio can be less than 1 but the project may still be retained for further consideration due to the conceptual nature of the current efforts. As more detailed study, evaluation, and cost analysis are completed in the future, the project may warrant a higher ratio and be further justified.

The cost benefit of reduced road flooding for a proposed project was estimated as the reduction of the cost of time lost by drivers detoured around the flooded road. Delay cost is the first variable needed for estimation purposes. The delay cost per vehicle per hour was referenced from data from the BCA Reference Guide (FEMA, 2009) at $38.15 per vehicle-hour.

The next variable needed is the actual detour time that an average vehicle would need to make to reach its intended destination. This is highly variable depending on the configuration of the flooding relative to the roadways and would require a detailed evaluation which was considered beyond the scope of this study. As such, a conservative value of 1 hour was used as the average detour time a vehicle would have to get around a flooded roadway segment.

Traffic volumes are the next variable required for estimation. For many of the roads in the project areas, the County provided daily traffic count data. For roads where traffic count data was not available, the number of parcels that would normally use the road was multiplied by the average rate per unit referenced from the Institute of Transportation Engineers (ITE) Trip Generation Manual (2003, 7th Edition) to estimate the average daily traffic. The values used for this project are tabulated below:

Daily Trip Assumptions for Flood Delay Estimation

The last variable that was required was the duration of flooding. The reduced flooding duration was estimated from the ICPR model time-stage results for the node nearest the road flooding referenced from the existing conditions model results presented in the November 2009 Final Hydrologic & Hydraulic Model Summary Report prepared for the Master Plan. If the time-stage results indicated flooding for an indefinite amount of time (i.e., land locked areas where recovery was not taken into account in the modeling), the reduced flooding duration was assumed to relate directly to the number of additional storm events protected from road flooding by the proposed project in terms of level of service (LOS). For example if a project goes from protection of the 10 year storm event past the 25 year and 50 year to the 100 year event, it would be 3 LOS increases. The flooding duration information is not available from the modeling data in these instances so an empirical relationship had to be assumed.

Reduced Flooding Duration Relationship for Land-Locked Areas

Road Flooding Cost Benefit Impact =  Vehicles/Day x Average Detour Time (Hours) x

Reduced Flooding Duration (Days) x

$38.15/vehicle-hour

Note that a cost benefit was not considered for roads unless at least the minimum LOS storm event (i.e., 25 year/24 hour storm event) did not show flooding for that event under proposed conditions. Also, note that the frequency of flooding (i.e., how many times per year, etc.) was not considered in this analysis due to the additional complexity of estimating frequency of flooding for each road during each LOS storm event.

The cost benefit of reduced property flooding for a proposed project was estimated as the reduction of the cost of damages to structures and contents plus the cost of displacement/disruption losses. For residential land use only the financial impact of property damage and resident displacement due to flooding was considered. For commercial and institutional land uses the costs associated with disruption of business due to flooding was also considered. Estimating costs associated with the above factors is intensive, and a detailed analysis was considered beyond the scope of this project. Based on the foregoing, simplifying assumptions were made in order to meet the purposes of this evaluation. The damage and displacement/disruption costs were assumed to be related to readily available property value data referenced from the Alachua County Property Appraiser. In other words, the damage cost impact for a given flooding instance is related to the value of the property impacted. Additionally, flooding duration was assumed to not have a significant impact on flooding costs. In other words, once flood damages occur the remedial cost would be similar regardless of the duration. Based on these assumptions, property flooding cost benefit impacts were estimated using the following equation:

Property Flooding Cost Benefit Impact =  Property Value * (Reduced Structure Damage % + Reduced Displacement Cost %)

The actual damage a property incurs is highly variable based on the depth of flooding and contents. The duration of an impact or displacement is also highly variable depending on the amount of time residents require alternate housing or commercial property are unable to do business. Simplifying assumptions had to be made in order to provide an empirical relationship that could be applied for this project’s purposes.

Each flood prone property in a problem area was evaluated to determine the greatest modeled storm event at which the proposed conditions model does not show property flooding. This was compared to the existing conditions model results to identify the increase in the number of modeled LOS storm events that do not show flooding for each structure due to the proposed project. Therefore, the reduced damage or displacement cost % was assigned based on the increase in the number of LOS storm events that do not show property flooding. The assumed reduced damage % and reduced displacement % are shown on the following table:

Assumed Cost Reduction Relationships for Residential Property Impacts

For commercial and institutional properties an additional 70% was added to the Total % to account for disruption of business.

Note that cost benefit was not considered for properties unless at least the minimum LOS storm event (i.e., 25 year/24 hour storm event) did not show flooding for that event under proposed conditions. Also, note that the frequency of flooding (i.e., how many times per year, etc.) was not considered in this analysis due to the additional complexity of estimating frequency of flooding for each property during each LOS storm event.

Note also that an initial analysis of each project area was done considering only impacts to structures. Impacted parcels were only considered in the cost benefit if structure flooding was indicated by the model results. In this case, damage costs were based on the structure (building) market value from the Property Appraiser’s database. If the resulting cost benefit ratio was more than 1, then the analysis was concluded and left as conservative even though other lot impacts (yard flooding, etc.) may exist that were not incorporated into the calculations. For projects where this analysis resulted in a ratio of less than 1, additional analysis was done incorporating property lot flooding impacts. In these cases, the benefit impact was based on just the property value and not including the parcel’s building value. In these cases where structure flooding was indicated, both impacts were taken into account and the total fair market property value (lot + building) was used for benefit calculations. This differentiation is reflected on the benefit cost table associated with each individual project. 

The stormwater management problems in these areas have been evaluated in detail sufficient to conceptualize improvements solutions to reduce flooding and increase level of service (LOS). These concept solutions were developed to a sufficient detail to support the estimate of approximate final design/permitting and construction costs that would likely be incurred during the implementation of Capital improvements Program (CIP) for flooding projects under the proposed Stormwater Management Program (SMP). In addition, the level of detail was sufficient to approximate the cost benefit to the County of implementing the projects.

A summary of the high priority problem areas and associated projected improvement implementation costs is included on Table 4-1, listed in County ranking order. The total estimated cost to implement the projects is approximately $20,720,389. This information will be utilized in the Program Recommendations and Cost Analysis Summary Report prepared for the SWMP. This information will be used to project costs that a SMP would need to accommodate to implement a flooding CIP program to address these high priority areas.

Note that the costs are provided in what would be considered 2010 dollars. In the event that many of the projects are not implemented for several years, a re-evaluation of the costs would be in order.

A cost benefit analysis of each project was completed comparing the implementation cost to the expected flood reduction benefit in terms of flood impact reduction costs. The results of this effort are summarized on Table 4-1 as well. On Table 4-2 the projects are prioritized according to their benefit cost ratio which provides a guide in terms of what projects may warrant implementation first in order to maximize the County’s return on cost. It should be reiterated that the improvement conceptualizations included herein for the high priority flooding problem areas are based on limited detail assessment of existing conditions based on available information such as plans and 1’ contour topography, preliminary hydrological & hydraulic modeling, and preliminary evaluation of corrective actions. In all cases further design/permitting efforts would be necessary to support the preparation of plans for construction of the projects. Constraints or other unforeseen issues may be determined during further analysis that may make the recommendations unfeasible or may dictate alternate improvement measures be undertaken. Nevertheless, the conceptual improvements recommendations presented in this report are considered reasonable for the purposes of projecting costs for a flooding CIP program to implement the projects. The low priority flooding project areas summarized in this report should be monitored in future years for conditions which may warrant them to be investigated in more detail. No improvement concepts have been developed for these areas at this time.