Drainage Improvement Prioritization in Lincoln, NE

Virtually every municipal public works staff person has had the experience: Heavy rains are followed by calls from certain residents on particular streets complaining about drainage issues. Oftentimes these “squeaky wheels” have their concerns addressed far sooner than other potentially more serious municipal drainage problems that are not the subject of complaints. In fact, frequency of resident complaints is commonly used by many municipalities as the determining factor to establish which drainage and flood control projects take precedence over others. Unfortunately, this can result in diminished resources for improvements that are more critical to public health, safety, and welfare. Another common approach for drainage improvement prioritization is to compare the capacity of the drainage system against design flows and to assign highest priority to those channel reaches where there is the largest differential. While this approach has the advantage of being empirical, it is an oversimplification that can also result in unwise expenditures.

Historically, the City of Lincoln, NE, has used a priority list of the stormwater projects originally developed in 1966 and updated in 1979. These projects were prioritized mainly on the capacity (five-year storm for residential areas and 10-year storm for commercial and industrial areas) of the drainage system. Topographic and economic factors were also considered in the prioritization methodology. However, many other factors were not featured in the priority formula. Because the minimum design standards used by the city have been upgraded and significant zoning changes have been made since 1979, a new and more robust approach of developing capital improvement program (CIP) priorities was needed. In 2003, the city’s Public Works and Utilities Department and Watershed Management Division began a stormwater study project to develop criteria, a ranking system, and a prioritization methodology for identifying stormwater improvement projects for urban drainage system upgrades, rehabilitation, and system extensions.

Peer Engineer Review Process
The city and its consultants (including JEO Consulting Group Inc., Wright Water Engineers Inc., and Black & Veatch) assembled an engineering peer review committee to assist with this project. The committee provided input and suggestions regarding the prioritization criteria and appropriate weighting of these criteria.

The city retained the Heartland Center for Leadership Development, an independent nonprofit organization, to serve as facilitator for a series of engineer peer review committee discussions regarding the criteria, weighting factors, and format for a proposed prioritization methodology. The Heartland Center also facilitated interim work sessions between the city and the consulting team to design committee meetings, debrief and report on each meeting, and consult on follow-up strategies and on the development of the prioritization ranking tool.

The peer review committee met on three separate occasions during May and June 2004 to develop an updated prioritization methodology. Many municipal stormwater CIP prioritization methods throughout the United States were evaluated. The following broad approaches are typically used for CIP prioritization:

• Written scoring–City, county, or district has a written and well-documented scoring system for ranking projects.
• Written policy–Entity has a written policy for prioritizing projects but no scoring system for benefits.
• Engineering judgment or committee review–Projects are selected based on departmental engineering judgment or selected by varying priorities set by a city council.

Based on the advantages and disadvantages of each approach, summarized in Table 1, the city determined it would use a written scoring approach for future CIP prioritization.

Peer Review Committee–Meeting One
At its first meeting, the peer review committee established basic objectives for CIP projects:

• Protect public health and welfare
• Minimize property losses
• Enhance the floodplain
• Provide flood conveyance capacity
• Enhance the environment
• Encourage aesthetics

The peer group was instructed to consider a list of factors compiled that could be used to prioritize CIP projects. It was noted that the factors were not in any ranking order, nor was it necessarily a comprehensive list. Eventually, through discussions with the peer review committee, the city would determine a “ranking order” and “weighting” system for these and potential other factors (refer to the Glossary of Key Terms):

• Structure flooding (residence, business, critical facilities, etc.)
• Street flooding (types of street, location, depth and duration, etc.)
• Yard flooding
• Isolated ponding
• Condition of existing structures (age, size, type, damages, etc.)
• Maintenance frequency
• Complaints
• Erosion
• Inadequacy of existing system
• Undeveloped upstream area
• Developed area
• Negative impact on downstream system
• City’s responsibility
• Miscellaneous issues (aesthetics, political, water quality, etc.)

Possible Weighing Factors/Multipliers

• Risk/severity factor (loss of life, injury, etc.)
• Flood frequency factor

Following the presentations, the facilitators moderated an open discussion through which numerous comments and questions arose. The discussion was an effective tool for the city, the consulting team, and the peer review committee to refine the nature, scope, and intention of the project at hand. It was determined that the prioritization methodology designed as a result of this committee’s work should be a flexible tool that could be used as a screening device for city staff. Intentionally, the cost of a project would be considered separately. The group was also informed that the city wished to develop a methodology that was dependent primarily on information that is currently available, rather than one that would create the demand for additional information gathering.

Peer Review Committee–Meeting Two
At the second peer review meeting, participants were seated in separate table groups to facilitate individual and small group consideration of the various criteria factors introduced at the preceding meeting. Participants were given worksheets that listed 12 factors and were instructed to work individually to assign a point value to each factor, which would sum up to a total of 100 points. The higher number of points would determine the higher priority. Participants could choose to assign zero points to a criterion, and they could choose to write in additional criteria for consideration. After working as individuals, table groups were directed to discuss their scores, and then come to consensus on a table score for each criterion. The groups’ scores were tallied and are shown in Table 2.

Following this exercise, a general discussion revealed that most participants thought it would be appropriate to collapse the factors into a few broad categories, and to have other factors become weighting factors. The group also desired to have clear definitions regarding “high/low” priorities and “major/minor” flooding events.

Prioritization Categories
The following prioritization categories were developed for the purpose of project ranking:

• Structural flooding: flooding that causes structures to be inundated by floodwater. The structural flooding potential was identified through hydrologic and hydraulic analysis, study of topographic maps, field investigation, and recorded historic problems. The structural flooding category is further divided into the severity of the flooding potential by having a higher multiplier for the minor storm event structural flooding frequency as compared to the major storm event structural flood frequency.
• Non-structural flooding: flooding that causes stormwater to pond on the street or on public or private property for an extended period of time without encroaching any structure. The non-structural flooding potential in the study area was evaluated for the minor storm event. The non-structural flooding potential was identified through hydrologic and hydraulic analysis, study of topographic maps, field investigation, and recorded historic problems. The non-structural flooding category is further subdivided to account for the severity of the flooding by having a higher multiplier for the flooding on private property/arterial street rights of way.
• Existing infrastructure condition: This category includes the structural condition and maintenance frequency for the given underground stormwater drainage system. The information for this category was obtained from the city maintenance staff. The existing condition of the system was determined by field investigation and reviewing maintenance records. This category is subdivided into three categories to address the severity of the problem.
• Miscellaneous factors: Miscellaneous factors include health and safety, critical locations, community development, downstream impacts, complaints, undeveloped/developed area, cost, legal issues, and links to other improvements to be considered in the prioritization system. The ranking points for this category were provided by the watershed management staff.

The process of the project ranking system requires evaluating and identifying the pipe and inlet deficiencies, finding inadequate culverts, finding extent of the structural and non-structural flooding potential, and determining the existing infrastructure condition for any underground drainage system in a given watershed.

Following the second peer review meeting, JEO developed a draft ranking sheet and definitions. These documents were sent to peer review committee members so that they could consider the format before their final meeting.

Peer Review Committee–Meeting Three
At the final meeting, the peer review committee members were asked to study the draft ranking sheet and provide qualitative feedback by recording what they liked and did not like about the proposed ranking system, and what they would suggest as possible changes for improvement. The group also offered suggestions about how the format might be adjusted and improved.

The intent was not to create the methodology as a “black box” that the city would use strictly based on the final rankings. Instead, it would serve as a tool to guide the city while still allowing the flexibility to use engineering judgment. The committee provided input and suggestions regarding the prioritization criteria and appropriate weighting of these criteria. The city then incorporated this input into the final design of a written ranking system. The result was the Prioritization Ranking Worksheet (Figure 1). Cost considerations were left at the city’s discretion, which promoted city involvement in the final prioritization and selection of CIP projects for design and construction.

CIP Project Development
Equally important to the updated prioritization methodology was a process for developing CIP stormwater projects that would effectively address flooding and other drainage problems. An extensive process was used to analyze the stormwater system and identify potential CIP projects.

Data collection was among the most critical aspects of this process. Numerous field visits were conducted not only to gather additional information necessary for the analysis but also to verify information provided by the city. Sump locations and overland flow paths were identified and documented. Approximate limits of ponding and potential for structural flooding were noted. Open channels were observed, and cross-sections were measured. In some cases, discussions with local landowners were held regarding past flooding events. Field worksheets and digital photographs were utilized to collect and organize the field data. Various methods of quality control were implemented throughout the data collection process. The extensive data collection procedures for the watershed inventory ensured that the analysis was accurate, which was vital for a study of this magnitude.

The primary function of the inventory was to develop a hydrologic and hydraulic (H&H) model using Bentley GEOPAK modeling software. A city-specific drainage library containing information on standard inlets, intensity/duration/frequency (IDF) curves, and other pertinent features was created for H&H modeling. This software package uses the Rational Method for hydrologic analysis, Manning’s equation for hydraulic analysis, and HEC-12 for inlet analysis. The analysis was performed for the minor (five- and 10-year) and major (100-year) storm events. Though important in all aspects of the project, quality control was most critical during the modeling process. The complexity of the modeling procedures necessitated extensive quality control procedures.

After completion of the analysis, a stormwater geographic information system (GIS) database was created using ESRI ArcMap. A GIS stormwater database can serve various applications from modeling and thematic mapping to preliminary engineering design. Numerous geospatial analysis tools are available within the GIS as well. Additionally, these data can be merged with the city’s existing database, providing many data-management options.

The results of the H&H analysis were used to identify specific drainage system deficiencies, which were evaluated based on several criteria: structural flooding, non-structural and street flooding, and insufficient pipe/inlet capacity. The GIS was used to illustrate the location, severity, and extent of each deficiency. Drainage system deficiencies, together with field observations and city input, were used to develop conceptual plans for drainage improvements. Drainage improvement recommendations served to reduce and, if possible, eliminate potential for flooding while complying with city design standards. While these projects were conceptual in nature, they allow the city to plan for future improvements with the provided preliminary cost opinions.

Application
Twenty-three sub-basins from across the city were studied, encompassing nearly 6,000 acres. Analysis was performed for approximately 400,000 lineal feet of underground pipe; 20,000 lineal feet of open channel; and 5,000 inlets, manholes, and junctions. In total, 71 projects were proposed during the first two phases of the project. The final Prioritization Ranking Worksheet was applied to each of the proposed CIP projects that had been created to address flooding and other drainage system problems. Of the CIP projects proposed, 10 have proceeded to final design and construction through funding from the city’s 2005 stormwater bonds. These projects range from very small (several thousand dollars) to very large (multimillion dollars). This includes four of the 10 highest-ranked projects according to the prioritization ranking.

The CIP stormwater projects identified and prioritized through the urban drainage study have allowed the city to prepare a proactive stormwater CIP, rather than a program that merely reacts as problems occur. This study ensures that the taxpayers’ dollars will be spent on the stormwater infrastructure that is in the most need of replacement due to structural deficiencies, lack of capacity, or lack of an overland flow path, which may cause flooding. The prioritization methodology and criteria used to select the stormwater improvement projects were very helpful for the city staff in explaining the short- and long-term need for the stormwater CIP projects to the elected officials as well as the general public. The city intends to incorporate the remaining stormwater improvement projects identified though this study into its future CIP for design and construction at a later date.

Glossary of Key Terms
Minor storm: a storm event having a 20% or 10% chance of being equaled or exceeded in magnitude in any given year (also known as the five- or 10-year storm). As per the city’s design criteria manual, a minor storm event is the five-year storm for residential area and the 10-year storm for industrial/commercial area.

Major storm: a storm event having a 1% chance of being equaled or exceeded in magnitude in any given year (also known as the 100-year storm event). As per the city’s design criteria manual, a major storm event is a 100-year storm event for residential area and industrial/commercial area.

Structural flooding: flooding that causes structures to be encroached with floodwater

Structural flooding frequency: The term structural flooding frequency is used to describe the regularity of flooding to which a particular structure is exposed.

Minor storm structural flood frequency: a recurrence of structural flooding during a minor storm event

Major storm structural flood frequency: a recurrence of structural flooding during a major storm event

Non-structural flooding: flooding that causes stormwater to pond on the street or on public or private property for an extended period of time without encroaching any structure. The non-structural flooding potential was evaluated for the minor storm event only. As per the city design standard, the non-structural flooding is expected to occur during the major storm event.

Non-structural flooding potential–high: The non-structural flooding potential is considered high if it meets any one of the following criteria:

• Ponded depth at street inlet is greater than 1 foot.
• For pipes less than 24 inches in diameter, minor storm event discharge is greater than 15 cubic feet per second (cfs) over the pipe capacity.
• For pipes greater than or equal to 24 inches in diameter, minor storm event discharge is greater than 40 cfs over the pipe capacity.
• The street culvert overtopping frequency is less than the minor storm event.
• Sump area overland flow through private property due to drainage system deficiencies

Non-structural flooding potential–low: The non-structural flooding potential is considered low if it meets any one of the following criteria:

• Ponded depth at street inlet is between 0.5 foot and 1.0 foot.
• For pipes less than 24 inches in diameter, minor storm event discharge is less than or equal to 15 cfs over the pipe capacity.
• For pipes greater than or equal to 24 inches in diameter, minor storm event discharge is less than or equal to 40 cfs over the pipe capacity.
• Minor storm event is less than street culvert overtopping frequency of less than the 50-year event.
• Pipe deficiencies on private property (no sump area overland flow)

Inlet deficiency–high: The inlet deficiency is considered high if the ponded depth of the inlet is greater than or equal to 1.0 foot.

Inlet deficiency–low: The inlet deficiency is considered low if 0.5 foot is less than the ponded depth of the inlet, which is less than 1.0 foot.

Pipe deficiency–high: The pipe deficiency is considered high if:

• For pipes less than 24 inches in diameter, minor storm event discharge is greater than 15 cfs over the pipe capacity.
• For pipes greater than or equal to 24 inches in diameter, minor storm event discharge is greater than 40 cfs over the pipe capacity.

Pipe deficiency–low: The pipe deficiency is considered “low” if:

• For pipes less than 24 inches in diameter, minor storm event discharge is less than or equal to 15 cfs over the pipe capacity.
• For pipes greater than or equal to 24 inches in diameter, minor storm event discharge is less than or equal to 40 cfs over the pipe capacity.

Overland flow path: path where stormwater runoff in excess of pipe and inlet capacity flows, whether planned or not

Ponding limits: the limits of flooding in a sump area as determined by the ponded depth of an inlet or the existing topography

Sump area: a low-lying area with potential for ponding