Like countless municipalities across the nation, the city of Norman, OK, has had to contend with increased flooding and erosion and diminished water quality resulting from urbanization. Home to the University of Oklahoma and a population of approximately 112,000, Norman seeks to address these problems, particularly a decline in water quality in Lake Thunderbird, the city’s primary source of drinking water. A recently completed Storm Water Master Plan (SWMP) will greatly facilitate Norman’s efforts to reduce dangers associated with flooding, protect water quality, comply with federal and state stormwater quality regulations, enhance the environment, improve recreational opportunities, and outline funding options for related program activities.
Developed by the city’s Public Works and Planning Departments with assistance from PBS&J, a multi-services engineering, environmental, and architectural consulting firm, the SWMP was based on detailed assessments of 15 major watersheds that convey stormwater into, through, or within the city limits. Complementary hydrologic and hydraulic modeling approaches were used to establish design flows for the SWMP. Additional information used in the development of the assessments included GIS data, past water-quality studies, and public input. Stormwater problems then were identified and solutions were developed, taking into account expected future build-out conditions.
Nearly 60 problem areas associated with flooding, stream erosion, impaired water quality, or local drainage were identified. Proposed solutions were developed to address each of the problem areas at a total estimated cost of $83 million. Wherever possible, integrated solutions were developed to address stormwater issues as comprehensively as possible. Other considerations included methods that use bioengineering and natural channel design techniques to improve or protect stream channel integrity, as well as efforts to create or enhance greenways.
In addition to identifying the need for $83 million in capital stormwater improvements, the project examined five key issues: protecting stream planning corridors for water-quality purposes, utilizing structural and nonstructural water-quality controls, addressing a severe shortage of drainage easements and rights of way, enhancing maintenance of stream areas and stormwater detention facilities, and alleviating dam safety concerns. The acquisition of drainage easements and/or rights of way is needed to enable the city to gain access to riparian areas for stream condition monitoring and maintenance purposes, as well as to enable capital improvements in certain areas. Dam safety concerns relate primarily to the need to better define risks downstream of dams and ensure consistent maintenance of dam structures, spillways, and other appurtenances.
To the extent feasible, integrated solutions were developed to address stormwater issues as comprehensively as possible. Generally, problems in a given location tended to take the form of one major type, such as stream flooding. However, even in locations in which only one problem predominated, the proposed solution was developed in such a way that it would also improve other stormwater aspects. For example, a conceptual solution for addressing stream flooding would be designed in such a manner so as to protect the stream from future erosion. Whenever possible, bioengineering and natural channel design techniques were incorporated to improve or protect a stream’s environmental integrity. Meanwhile, solution development targeted future watershed development conditions projected in the city’s 2025 Land Use Plan. In this way, solutions will better help the city address future stormwater needs and provide a more complete “blueprint” for managing stormwater.
Of the 59 projects proposed as part of the SWMP, 33 would mitigate stream flooding through the use of such approaches as bioengineered stream modifications, storm sewer improvements, and stormwater detention. Of the 33 projects, 26 are intended to address flooding of structures. All told, the 26 projects would remove 652 structures from the 100-year baseline floodplain. Of the 33 projects for mitigating stream flooding, 29 would include upgrades to road crossings that are routinely overtopped during flood events. In fact, 36 road crossings would be protected to design levels. Although an effort was made to minimize property buyouts, 12 of the 33 projects would rely on buyouts of flood-prone structures. The SWMP identifies 62 properties as possible candidates for a buyout.
The level of protection for most stream flooding solutions varied somewhat. However, improvements associated with channel capacity and roadway bridge openings used projected 100-year baseline (future) peak discharges, while roadway culvert openings used 50-year peak flows. Exceptions occurred in special cases where 10-year protection was judged to be preferred because of limited space and the costs associated with larger improvements.
Twelve projects called for in the SWMP would address local drainage problems, while another 14 projects would use stabilization measures to address stream erosion. Overall, the SWMP identifies 10,050 feet of eroding streams to be stabilized by a combination of geomorphically stable techniques, including channel grade control, streambank armoring, slope flattening, and bank toe protection. Various combinations of materials were recommended for achieving these techniques, including rock riprap, erosion protection fabric, geogrids to hold certain specific structures together, and select vegetation.
General cost estimates for each recommended project solution were developed using unit costs and estimated quantities for the construction bid items required to construct the respective projects. The SWMP also includes cost estimates for new drainage easements and/or rights of way needed to ensure construction of project improvements on property owned by the city or made available through city easements. Costs were obtained from city staff based on historical costs, location of the problems, and adjacent local land use.
Another important element of the SWMP was the integration of the recommended stormwater solutions with proposed greenbelt routes. During development of the SWMP, Halff Associates, a member of the consultant team, prepared a plan for greenbelt trails in Norman. Throughout the project, team members coordinated to ensure that stormwater projects could be integrated with greenbelts whenever possible. During the design effort for any particular project, its integration with greenbelts can be considered further and incorporated into the project if the city desires.
Prioritizing the Solutions
Two critical aspects of the SWMP involved prioritizing the solutions and developing optional financing methods to help the city decide which projects to conduct first and how to finance them. The system developed for prioritizing solutions evaluates, scores, and ranks each one, in terms of its ability to solve the problem under consideration, provide for public safety, provide sustainability, utilize funding advantages, positively affect neighborhoods and the environment, and benefit other functions such as transportation. This prioritization identifies the most critical projects for addressing the stormwater needs in Norman and provides an important tool for the city as it determines the order in which solutions might be implemented or how they might be financed.
Each prioritization factor was given a weight based on its importance. Factors were grouped and classified in four categories. The factors in the most important category were given a weighting of four, the factors in the second category were given a weighting of three, the factors in the third category were given a weighting of two, and the factors in the fourth category were given a weighting of one. The various factors are shown in Table 2 along with scoring examples for hypothetical projects.
To evaluate a project using this prioritization “matrix,” each factor then was assigned a project-specific rating between zero and three, with three being the highest, two being moderate, one being low, and zero indicating the degree to which the factor had relevance, or a positive impact on, the project. Once each factor was rated for a project, the factor weighting was multiplied by the rating to give a factor score. The individual factor scores were then totaled to give a total prioritization score for the project. The higher the score, the greater the importance of the subject project. This process was followed for each identified project. Once project prioritization scores were obtained, the project rankings were then compared on the basis of watersheds, wards, and citywide.
Protecting Water Quality in Lake Thunderbird
Individual projects aside, the SWMP also evaluated what Norman should do to improve water quality throughout the city and especially in its drinking water supply, Lake Thunderbird. The Oklahoma Department of Environmental Quality (DEQ) has designated Lake Thunderbird as a sensitive water supply lake. However, elevated levels of chlorophyll a–an accepted measure of algal content–have been found in the reservoir, prompting the Oklahoma DEQ to add Lake Thunderbird to its list of impaired water bodies, in accordance with Section 303(d) of the Clean Water Act.
Unless significant steps are taken to reduce the influx of pollutants to the lake, further degradation of the lake’s water quality can be expected as land development progresses in the Lake Thunderbird watershed. Current high loadings of nutrients–the main factor contributing to algal growth in the lake–are only expected to increase with urbanization. The prospect of more algal growth includes an increased threat of toxins being produced in the lake from algal masses, exacerbating taste and odor problems with drinking water and decreasing recreational opportunities. Although other urbanized or urbanizing areas to the north contribute significant stormwater to the lake, Norman is the largest municipal area draining to Lake Thunderbird. In fact, roughly half of the area that drains to the reservoir is in the city limits. Therefore, the SWMP includes recommendations for management practices that can help protect water quality in Lake Thunderbird.
Because limiting nutrient loadings will require a combination of structural and nonstructural measures, the SWMP included recommendations for particular approaches expected to confer the most benefit. Although implementing controls in previously developed areas would be difficult, using such controls in future developments will greatly assist Norman in its efforts to improve water quality in Lake Thunderbird.
Measures recommended in the SWMP include stream planning corridors (SPCs), various structural and nonstructural controls, fertilizer use education, fertilizer use controls, a continuation of present development density controls, and low-impact development practices. If implemented properly, these management practices will significantly assist in preserving and protecting Lake Thunderbird’s water quality and the city’s primary water source.
The SWMP proposes the dedication of SPCs within drainage areas greater than 40 acres in watersheds that contribute to Lake Thunderbird. SPCs are defined as the area of land along both sides of a stream or natural drainage corridor that encompasses the area projected to be inundated by the 1% chance flood (i.e., the 100-year floodplain) in any given year, assuming full build-out watershed conditions. As proposed, an SPC could possibly include an additional buffer width to aid in further filtering runoff and providing environmental protection of stream riparian areas. Such corridors are particularly useful in headwater areas because here the features have the best opportunity for filtering runoff and facilitating infiltration. Of course, the city will have to make certain legal and political changes before SPCs may be implemented.
Evaluating Structural and Nonstructural Controls
Norman already is implementing programmatic water-quality solutions in its urbanized areas as part of efforts to comply with the Oklahoma DEQ’s permit requirements for municipal separate storm sewer systems (MS4s). As a supplement to these efforts, the city will need to require that new developments incorporate certain structural and/or nonstructural water-quality controls.
In general, the SWMP recommends that Norman require structural stormwater controls in the same manner and locations as required for stormwater detention throughout the city. Such controls include extended detention basins, wet ponds or retention basins, filtration basins, porous pavement, and grassed swales. These structural controls can be constructed in conjunction with stormwater detention facilities in most instances. Because of maintenance costs and concerns regarding public safety and nuisance considerations, the city should encourage the use of dry detention facilities rather than wet detention facilities in most, but not all, cases.
In terms of nonstructural controls, the SWMP recommends that the city continue to ensure that the minimum control measures conducted as part of the MS4 program be met. Such measures include fertilizer application controls, street sweeping, oversight of septic system installation and operation, and area-specific development density limitations.
The SWMP recommends new ordinance requirements pertaining to structural and nonstructural controls. For example, the SWMP suggests that Norman require that water-quality facilities be constructed to capture and treat runoff from all proposed developments exceeding 1 acre (or some other size selected by the city). The runoff “capture and treatment volume” should be set to 0.5 inch of runoff from the development area, unless otherwise specified for a special condition. Furthermore, the SWMP recommends that the city allow and encourage developers to use low-impact development techniques such as rain gardens and biofilters to meet a portion or all of their water-quality control requirements, assuming that the developers provide sufficient technical justification for the techniques.
Presenting Financial Options
It is anticipated that many of the recommended solutions will be included in a capital improvement program (CIP) to be implemented by the city. Funding for this CIP program and other stormwater costs are anticipated to come from a stormwater utility that the city of Norman proposes to establish. If approved by city voters, stormwater utility fees will be based on the amount of impervious cover on each respective property within the city regardless of land use type. In fact, the SWMP serves as a basis for the creation of the utility.
Financial analyses were performed to determine how best to meet the funding needs for the programs and activities associated with the SWMP. The revenue required for the stormwater management activities and improvements outlined in the SWMP can be divided into several categories of need. These include needs for debt service, creation of a reserve fund less any non-operating revenues such as interest earnings, continued general overall operation and maintenance, shared city services, minimum control measures for stormwater MS4 regulatory compliance, enhanced maintenance for streams and stormwater detention facilities, trail construction, easements and rights-of-way acquisition, and cash-financed stormwater capital improvement projects.
In addition to reducing funding requirements from a stormwater utility, the city decided to propose funding a portion of the stormwater capital improvements with general obligation bonds to provide necessary projects more quickly in areas of critical stormwater needs. Three rate options were developed to fund the stormwater capital improvements using the split between general obligation bonding and stormwater utility rates over a 20-year program, as defined by the city. As shown in Table 3 and consistent with the CIP costs for proposed solutions, the total 20-year CIP needs in 2008–2009 dollars were estimated to be approximately $83 million. To cover these costs, three options for financing this portion of the overall program were developed, with varying amounts of general obligation bonding and stormwater utility user fees.
Considering all revenue requirements identified, monthly stormwater rates for a median single-family home having approximately 3,100 square feet of impervious cover were determined to be $6.26, $5.85, and $5.78 for options 1, 2, and 3, respectively.
Conclusion
Although the SWMP is complete, the city of Norman still must decide how to implement and finance the plan’s recommendations. This process will require the sustained involvement of stakeholders and approval of a stable funding source by Norman’s citizens, along with efforts by the city to continue to refine its future needs and goals regarding stormwater and watershed protection. Using the SWMP as a solid foundation, Norman will be able to satisfy its regulatory requirements, enhance recreational opportunities, protect the environment, and meet local challenges relating to flooding, stream erosion, drainage problems, and water quality.
