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HomeMy WebLinkAbout05/21/1996, 4 - STORM DRAINAGE: PROBLEMS AND POSSIBLE SOLUTIONS 4 f council aGEnaa Repoin C I T Y O F S A N L U I S O B I S P O FROM: Nfiichael D. McCluskey, Public Works Director Prepared By: Wayne Peterson, City Engineer-f SUBJECT: Storm Drainage: Problems and Possible Solutions CAO RECONIIVIOVDATIONS: By motion, direct staff to prepare a detailed study regarding the feasibility of forming a drainage utility. DISCUSSION: Background During the winter of 1995 the City experienced serious flooding. This was the result of intensive storms that impacted most of California. As a result of damage that was experienced, the City and County were declared a Federal Disaster Area and the agencies and public received assistance from the Federal and State government to aid in recovery. At the February 28, 1996 Council meeting, staff was asked to prepare a report proposing ways the City could reduce the threat of flooding on its residents. history The City of San Luis Obispo has developed considerably over the past 40 years. During this time of development there has been a changing view of the City's responsibility for storm drainage and an increasing understanding of the amount of runoff that the storm drainage system must accommodate. During most of the century, the traditional engineering standards for calculating the amount of rainfall runoff that must be accommodated in the drainage system was based on models developed in the eastern part of the United States. Engineers working in southern California have found that the soil types, vegetative cover and slopes of our area contribute to a much larger runoff rate than what was the basis of traditional design. Engineers are becoming aware of the tremendous impact a factor called bulking can have on runoff and the resulting erosion that occurs in our creeks. This bulking factor relates to the amount of silt that the creeks carry during high flow periods. The bulking factor can vary tremendously. The highest rates probably only occur in years immediately following major fires in the upland areas. In the past, the City has had a very liberal view towards what people did to the drainage systems crossing their own property. Some property owners culverted the drainage way, others realigned the creeks to one side of their property, while still others felt the drainage way was too large and Council Agenda Report -.Storm Drainage: Problems and Possible Solutions Page 2 proceeded to encroach on the edges of it by filling to create a more usable area for building. The theory of flood protection was everyone protecting themselves as long as they did not increase the risk for their neighbor. Little evaluation was made as to the far reaching effect of the drainage modification, nor would the effects be understood if an evaluation had been attempted because engineers are only now beginning to see the long-range impacts caused by the changes. Now at the end of the twentieth century it is time for the City and its residents to review the problem and prepare a plan for the next several decades that will address the flooding problems experienced by this community. Description of Drainage System The City's drainage system consists of three principal parts. First, the source of the drainage (about 42.6 sq. miles of surface area); second, the system that collects the runoff (the small creeks, swales, streets and culverts); and third, the major creek system that carries the bulk of the runoff through and out of the City (San Luis, Stenner, Old Garden and Prefumo Creeks). The total watershed of San Luis Creek at the ocean is about 84 sq. miles. Only about half of this watershed passes through the City. There are three major drainage sheds that lead into the City: San Luis Creek, 11.5 sq. miles; Stenner Creek, 8.3 sq. miles; and Prefumo Creek, 3.4 sq. miles. By the time San Luis Creek has passed through the City and collected the flows from Stenner Creek, Prefumo Creek and other tributary areas, the drainage shed includes 42.6 sq. miles of land. Since less than 10 sq. miles of drainage shed actually lies within the City, there is another 10 sq. miles of land contributing runoff that flows through the City using the many small creeks and drainage ways that are dry for much of the year. Most of the hillside land above the City that contributes runoff to the creeks and the drainage systems flowing through the City is steep, rocky and poorly vegetated. The water in the creeks rise and fall very fast during heavy rain. Within the City most rainfall runoff is collected along the paved City street system. It is carried to the most convenient drainage way or creek and from there to one of the major creeks. The system of drainage collection and transmission has been under development for as long as the City has existed. Except for the Edna Islay area, it does not include any planned detention system. Much of the system, outside of the public streets, was built under private ownership and continues to be under private ownership. Some portions of the private system were dedicated to the City in the past. The City has only a partial inventory of the total system. San Luis Obispo Creek carries all of the runoff from the City and surrounding areas south to Avila Beach and the ocean. Problems There are several different kinds of flooding problems experienced in this community and they can be addressed differently both from technical and financial view points. Council Agenda Report- Storm Drainage: Problems and Possible Solutions Page 3 Problems Experienced in the Drainage System Soume Problems: The hillsides above the City contribute a significant amount of runoff that must be handled by the City drainage system. This affects the design and character of the needed collection and transmission system we need. Some of the hillsides are very steep and are composed of unstable material that slide and wash out during storms. The loose material is carried into the City and must be handled by the storm drainage system. Rock and debris must be carried through the system or caught before it enters the enclosed portions. The rocks which do go through the pipes wear out the pipes. The rocks that are prevented from going'through the system must be removed by maintenance crews before the upper ends of the pipes become plugged. Collection System Problems: Drainage collection points within the City are pipe inlet structures, catch basins or drop inlets. These devices are a source of flooding when they become plugged by rocks, leaves or other debris, or when the system they connect to becomes overloaded with too much water, or when the system is failing. The City has a series of rock catchers at the upper end of many of the culverts leading into the City. Most of the time these systems work, but on occasion when a large slide or instability occurs in the watershed above the rock catcher, the system is not able to handle the volume of debris and problems result. Much of this system was not designed for a particular capacity or was designed under a lower standard than that being used today. Major Creek Problems: During normal flows these facilities have few problems as long as they are kept clear of blockage. During high flows the water can flood out of the banks, damaging adjacent improvements or cause areas of creek bank to erode. This is aggravated when large trees die or become weakened and fall into the creek. They leave the bank exposed to erosion and tend to cause problems wherever they go down stream. Problem Solutions: There are several possible ways the City can begin a program that will reduce the flood risk for its residents. National Flood Insurance Program-Option 1 First, the City could more actively participate with the National Flood Insurance Program (NFIP). This program is similar to the fire rating program in which the City has been a traditional participant. Currently, the City participates in the NFIP and residents are able to buy flood insurance at a subsidized rate from their insurance provider. The rate they pay for the insurance varies dependent on the flood risk identified for their property and the rating the City has established through the CRS (Community Rating System). The City has been participating in the NFIP program since the 1970's. By conforming with.certain requirements of the CRS the City this year was granted a rating of 8, which means that residents receive a 10% reduction in their flood insurance. A 2 rating, similar to the one we have for fire protection, would provide a 40% reduction in the insurance cost to the public. y3 f � Council Agenda Report- Storm Drainage: Problems and Possible Solutions Page 4 More participation in the program by the City will not necessarily reduce flooding but will help residents reduce risk of property damage and buy insurance at a more affordable price. Currently city residents buy $48.5 million in coverage at a cost of$192,000. The current 8 rating results in an annual savings of$20,000 to our residents. Every year more residents are required to buy flood insurance as a condition of their Federally backed mortgages. The total value and cost of flood insurance will increase considerably in the future. Examples of programs that the City could undertake to strengthen its participation in the NFIP and gain additional reductions in insurance cost include the following: develop an automatic early warning system; adopt a management program with an implementation plan to reduce flooding; provide an annual outreach program to promote flood protection and flood insurance; implement a flood protection program similar to the seismic retrofit program to require that all properties in flood zones install flood protection devices; acquire, remove and not rebuild improvements from flood prone areas; raise the existing freeboard requirement from one to two feet. Change Development Criteria-Option 2 A second way the City can address flooding is by not allowing new development to make the existing flooding situation any worse. The City can do this by adopting more rigid design standards that would require developed properties to detain runoff on their property. The City can also do this by establishing more specific limitations on how flood prone property can be used. Examples of new criteria would be as follows: a requirement that developments cannot create any additional runoff, new developments must mitigate all or portions of downstream deficiencies related to the development, similar to how we handle traffic and utility impacts; or increase the required freeboard from one to two feet. Four Part Comprehensive Program-Option 3 Third, the City can take a proactive stance to reduce and eliminate flooding by addressing existing deficiencies, drainage system maintenance and by establishing a comprehensive storm drainage program. This should be done in addition to adopting a NFIP program and establishing higher standards of development. It would consist of four parts as follows: 1. The City can, by way of the CIP, determine what portion of the City's existing resources can be use to construct and maintain the facilities necessary to reduce flooding. This would be, in essence, the General Fund contribution to the comprehensive program. 2. The City can involve new development by setting up an Impact Fee Program similar to the traffic fee program. Every development would pay a fee related to its generation of runoff and its impact on the drainage system. The money generated would be used to construct improvements in the drainage system and help to bring the system to current desired standards. This would be the developers contribution to a comprehensive program. 3. In conjunction with the Impact fee program, there should be established meas of special benefit or Assessment Districts. In these areas, the City could construct projects that provide Council Agenda Report- Storm Drainage: Problems and Possible Solutions Page 5 special benefit to the property owners in the area and using revenues developed from the sale of special benefit district bonds, that are to be repaid over time, from assessments collected from property owners. Projects paid for by Assessment District should be local in nature. This would be the private, directly affected property owner's contribution to a comprehensiveprogram. 4. The City can develop a Drainage Utility. This Utility could bill each property based on the property's generation of runoff, collect the amount monthly with the water/sewer/solid waste bill, and use the revenue to maintain the drainage system. This would be the private, non- directly affected, property owner contribution to a comprehensive program. These funds would be used to fund maintenance efforts only, as construction would be handled by one of the other three above referenced programs. New development could be required to buy into the system with an Impact Fee and install development related improvements as with our utilities. Drainage Utility-Option 4 A fourth way to provide a storm drainage program is to adopt only the last element of the above option--a drainage utility. Instead of providing for maintenance only, the utility would include both construction and maintenance responsibility. All costs would be shared City-wide for a City-wide benefit. Incentive Program-Option 5 From the context of the February 1996 Council meeting it appeared that the Council wanted staff to be "creative" or "innovative" in finding new ways of addressing neighborhood and City-wide flooding. None of the above four options are really "innovative" ways to address storm drainage issues. Each is commonly used throughout the nation. However, staff.has come up with one idea for a new way of approaching the problem. Such a plan would mimic the Federal/State grant programs by providing a "matching" fund storm drainage program. In this plan, for every four dollars raised by a neighborhood the City would "match" with one dollar. This 80/20 plan would be similar to an assessment district but would provide an "incentive" for each neighborhood because their overall cost would be 80% of the project cost versus 100% in a full assessment district. Thus staff would offer the "80/20" grant program as a fifth option for Council consideration. Conclusion: In conclusion, given the state of repair of the City's existing storm drainage system (poor), and the forecast for substantial increases in capital spending by the City (poor), the outlook for a fully functioning, adequately maintained drainage system is not good. The only option which provides a healthy future is a storm drain utility (Option 4). In this system all citizens pay equally to help all areas of the City, with no undue financial burdens. To seriously address the needs of the present and the future, staff recommends that the Council direct the pursuit of formation of such a utility, beginning with a more detailed study of feasibility. imvmw.<%. �1 s'