Loading...
HomeMy WebLinkAbout05/14/1991, 1 - FLOOD MANAGEMENT POLICY C51-1-11- p111 (� MEETING DATE: ��iI��H�INIIIIIIIII III � 1 ?991�1II� city o san Luis OBISPO � COUNCIL AGENDA REPORT ITEM NUMBER: _'ROM: David F. Romero, Public Works Director SUBJECT: Flood Management Policy CAO RECOMMENDATION: Receive staff report and public testimony. By motion, A. Reaffirm Flood Management Policy, B. Direct staff to pursue flood control improvements considering, 1) upstream detention, 2) elimination of tight spots, 3) widening of San Luis Creek below confluence, and 4) incorporation of trails and retention of riparian habitat. INTRODUCTION: On October 30, 1990 the City Council reviewed a report updating the status of the 1983 Flood Management Policy, Standards and Action Plan. The staff report is attached. The Council directed staff to investigate the number of structures subject to flooding and the cost of floodproofing them. Staff was also directed to set the item for discussion at a public hearing. SUPPLEMENTAL INFORMATION ON INSURANCE AND FLOODPROOFING OF BUILDINGS INSURANCE: A. INSURANCE The Federal Emergency Management Agency (FEMA) has estimated that there are approximately 4000 * commercial and residential buildings in San Luis Obispo subject to a 100 year flood. This is based on preliminary information shown on Flood Insurance Rate Maps (FIRM) . The San Luis Obispo Land Conservancy is preparing a more detailed study which should be available within the next several months. The City has no way of determining exactly how many buildings in the flood areas carry flood insurance, since some insurance is covered under "private" insurance policies, not through the Flood Insurance Administration of FEMA. Properties which have government backed loans must carry flood insurance if they are in a flood hazard zone. An in-depth study, requiring many hours of research, would be necessary to produce specific numbers accurately estimating the true cost of flood insurance premiums. There are so many different scenarios (e.g. depth of flooding, type of construction, different building and contents values, etc. ) that it makes it difficult to estimate. However, based on information supplied by FEMA and local insurance brokers, there appears to be up to 75 percent savings on premiums for structures protected (raised or floodproofed) in compliance with the City's flood ordinance. * City records indicate 2138 structures in Flood Hazard Zones. ouh1��►►►lu►Illlll�p��u1p�►�UIII city of San Luis OBlspo COUNCIL AGENDA REPORT Flood Mgmt. Policy Page Two Utilizing data supplied by FEMA, assuming $ 100, 000 coverage with $ 500 deductible, the average premium may be $ 325 per year. With 4000 structures, the annual cost of flood insurance would be approximately 1. 3 million dollars per year. B. FLOODPROOFING Most properties can be floodproofed, though many building entrances of existing buildings can be floodproofed only by use of manually installed gates when flood waters threaten. This is not an entirely satisfactory answer and may result in a high failure rate during a flood. Properties which are newly constructed or "substantial remodels" (50% or greater cost of remodel to present value) are required by City ordinance to bring the structure into compliance. Of the 4000 structures in the flood hazard zones, approximately 100 (44 residential & 47 commercial/other) have been documented as having met the requirements of the ordinance. The remaining 3900 structures are subject to various levels of flooding, although some may already be elevated above a 100-year flood. There is no fixed cost to floodproof buildings, therefore estimates must be used; $5000 commercial, $2000 residential. Assuming that a means could be found to require all effected buildings (approx. 70 % residential, 30% commercial) to install floodproofing (or elevate the structure) , cost is calculated as follows: .70 x 3900 x $2000 = $5,460, 000 (Residential) .30 x 3900 x $5000 = $2, 340, 000 (Commercial) TOTAL = $7,800, 000 Requiring all these properties to floodproof would require a massive effort on behalf of the City in ordinance revision, hearings, notices, approvals of submittals, issuing permits and inspecting installations. Undoubtedly, a number of staff years of effort would be required. GENERAL DISCUSSION: The insurance program does nothing to minimize physical losses, it merely helps reimburse for a portion of the cost of damage. The floodproofing program will minimize damage to buildings which are protected (if manual installation of gates is timely) , however it does not prevent damage to public facilities, vehicles, injury to persons, or pain and suffering. Emergency access for police, fire or ambulance is also restricted during floods. city of San tins OBISPO COUNCIL AGENDA REPORT Flood Mgmt. Policy Page Three The current City program requiring floodproofing when buildings are newly constructed or are remodeled, the cost of which equals or exceeds 50% of their value, results in approximately 15 existing buildings being floodproofed per year. At this rate it could be over 200 years before all buildings susceptible to flooding are protected. This could be speeded considerably by a lowering of the 50% figure to 25% or even 10-15%, thus including a significant additional number of properties. There are legal and ethical issues regarding the expenditure of public funds for private flood insurance or flood protection. CONCLUSION AND STAFF RECOMMENDATION: Staff believes that the City must follow all available avenues in providing flood protection for its citizens. An insurance program will help compensate for damages but does nothing to prevent flooding. The floodproofing of buildings is a long slow process and, because of the short drainage basin and short advance notice, is like to be only partially effective upon completion. Staff feels that it is most important that the City continue with an aggressive creek widening program, following policies adopted by the Council. floodrpt/dfr#27 MEETING DATE: «�hn��ll(VNN���� IUI`I BISPO October X990 IIIAI Cl� Of san LUIS OITEM NUMBER: COUNCIL AGENDA REPORT FROM: David Romero, Public Works Director SUBJECT: Flood Management Report Update and Policy Revision CAO RECOMMENDATION: By motion, receive staff report and set a public hearing to receive testimony regarding Flood Policy and basic approaches to be followed for flood protection INTRODUCTION -In 1983 the City prepared two booklets, a Flood Management Report and a Flood Management Policy. On June 21, 1983 the City Council made certain environmental findings and adopted The Flood Management Policy, Standards and an Action Plan (pink book) . Since 1983 many of the projects designated by the Action Plan have been completed, others are in various stages of design, still others have been delayed for various reasons. In the intervening years the City philosophy regarding floods and flood protection has altered somewhat, and staff has been directed to restudy the basic approaches to flood protection with the aim of eventually adopting a new Flood Protection Policy and Action Plan. PURPOSE The purpose of this report is to provide an update of waterway improvement work conducted since 1983 , and to initiate a dialogue which will lead to adoption of a new action plan to protect against flood damage. This is to be done while keeping in mind a primary goal of preserving the natural beauty and character of the creek system. SUMMARY COMPARISON Key to a decision to change the approach to flood protection is a clear understanding of the 1983 approach as compared to the approach recommended in this report. Several studies have shown that after the City has done all that it can in creek management, the city will still flood in major storms. The 1983 approach called for widening of most of the major creeks to handle flows anticipated in design storms. The approach recommended in this report calls for a combination of partial retention upstream of the city and a smaller creek widening below the confluence, with no widening of San Luis Creek above the confluence. �-y i '""�� ►�Hlllfl�lli►���III city of San Luis OBlspo COUNCIL AGENDA REPORT Flood Update Page Two BACKGROUND In the 1983 Flood Management Report certain background information was presented, some of which is summarized and updated as follows: a) The major creek system includes San Luis Obispo Creek, Old Garden Creek, Stenner Creek and Prefumo Creek, with many smaller tributaries, most of which are unnamed. b) The major creek system contains a total of 12. 6 miles within the city limits. c) Approximately 92% of the creeks within the community are privately owned, approximately 8% are publicly owned. Approximately 50% of the privately owned creeks are within drainage easements, due in large measure to the City efforts in recent years to acquire easements. d) The total drainage basin at the downstream city limits is 42. 6 square miles of which 9.5 square miles lies within the city limits. The 100 year flood plain within the city is 1.5 square miles (0.4 square miles of which are within Laguna Lake) . e) The largest floods of recent history occurred in 1969 ($1,628,000 damage) and in 1973 (around $4, 000, 000 damage) . ACTIONS SINCE 1983 In 1983 the City Council adopted a flood management policy, standards and an action plan. In adopting these the City Council attempted to achieve a balance between a) full flood protection, resulting in major alteration to the creek environment, and b) minimal flood protection, with preservation of existing creek amenities. The flood design criteria recommended by staff and adopted by the Council was based on the existing 40 year storm capacity of the culverts crossing 101 into and out of Cuesta Park and the 40 year storm capacity of the undercity culvert. By using these structures as a design basis, the City could achieve a relatively cost effective level of protection for downtown. Minor creeks were to be designed on a 10-25 year storm basis, Old Garden Creek on a 25 year storm basis, San Luis Creek above the confluence on a 40 year storm basis, and Stenner Creek, Prefumo Creek and San Luis Creek below the confluence on a 50 year storm basis. i l city of San tins OBISPO g COUNCIL AGENDA REPORT Flood Update Page Three In order to achieve a balanced capacity on each of the creeks, staff submitted a prioritized list of restricted capacity (tight spots) on each of the creek systems. Since 1983 , the city has carried out an aggressive program to correct these tight spots. Of the 26 tight spots listed, 17 have been corrected, 5 are in some stage of design, and 4 have had no action. Seventeen street drainage improvement projects were listed. Of these 14 have been completed, 1 is ongoing annually and 2 have had no action. Five minor waterway modifications were listed. Of these 3 have been completed, 1 is ongoing annually and 1 has been corrected by improvements installed in a nearby subdivision. Nine major waterway modifications were listed. None has been completed, though a number of right-of-way easements have been acquired and one project (San Luis Creek---Bianchi Lane bridge to downstream of South Street) has been designed. A second project (San Luis Creek, sewer plant to Prado Road) will be completed as part of a subdivision now in process of obtaining approvals. Improvements completed on San Luis Obispo Creek to relieve tight spots have resulted in increased capacity at 9 locations as shown on Exhibit A . The major uncompleted work remaining from the 1983 report relates to enlarging San Luis Obispo Creek to a 5o year capacity between the sewer plant and the confluence, and enlarging San Luis Creek to a 40 year, and Stenner Creek to a 50 year, capacity from the confluence to Nipomo Street. RECENT STUDY (Exhibit B - Summary of Report) In October 1987 the City Council requested a City study of the I efficacy of upstream detention of flood waters, thus providing a more cost effective solution to flood protection and resulting in lesser damage to the creek environment. The City hired James Schaaf, president of Schaaf & Wheeler, to conduct the study. Mr. Schaaf studied 7 alternative sites on San Luis Obispo Creek, 3 alternative sites on Stenner Creek, various dam heights at each of these locations and various combinations of alternatives, a total of 31 possibilities overall. Mr. Schaaf studied detention dams (with a hole in the bottom, thus allowing some water to pass through) , modification of culverts and flood diversion dams �' l r 11111111111111101111111N city o� san Luis OBlspo Mmmltnw COUNCIL AGENDA REPORT milmm Flood Update Page Four leading to off-channel storage. Of all the alternatives, none had a positive benefit-cost ratio*. The best alternative was a 56' high dam somewhat upstream of U.S. 101 above Cuesta Park, at a site previously reviewed by the Corps of Engineers. This facility had a benefit-cost ratio of 0.80. This design would allow a 40 year storm to pass and would require that the entire downstream creek system be brought up to that capacity and more as one goes further downstream. This closely approximates the policy adopted in 1983 . Schaaf's closing statement in the conclusion portion of the report is as follows: With no immediate prospect of providing some flood protection by utilizing upstream detention, the City will have to concentrate its flood control efforts on improving channel capacities through the populous area to minimize the flooding problem and encouraging both the flood proofing of individual structures and the individual purchase of adequate amounts of federal flood insurance. Schaaf's basic approach to the study was to construct a detention dam large enough to retain a 100 year storm while releasing a limited flow (25-50 year) through the dam and into the city creek system. ALTERNATIVE APPROACH One of the primary reasons for the study was to determine whether some upstream detention would allow the City to minimize creek widening within the community. With this goal in mind, staff has conducted considerable additional analysis, with a slightly modified approach, to see whether some alternative might better meet the goal and at the same time be cost effective. Staff's approach was to construct a dam designed for minor storms to pass through without restriction, to partially restrict the flow for intermediate and major storms, but to be overtopped by major storms. This design results in a lower dam, but more importantly, provides some level of downstream protection for all significant storms. Preliminary calculations indicate a good *A positive benefit-cost ratio indicates that benefits to be derived from the project are greater than the cost of doing the project. 1' 7 f °u��i�u►�Illllfl��l►g��U�U city of San Luis OBlspo COUNCIL AGENDA REPORT Flood Update Page Five possibility of obtaining a favorable benefit/cost ratio for the dam (Exhibit C) . Construction of such a dam would not eliminate the need for creek widening in San Luis Creek, particularly below the confluence. However required widening would be minimized, reducing acquisition and construction cost and environmental damage. DISCUSSION All of the dam alternatives require property acquisition, flood easement acquisition, an environmental impact study and funding of a fairly large capital project so it can be constructed at one time. All call for some fairly large magnitude of flow passing on through the city creek system. Of all the dam alternatives, the staff approach (a dam with a small, perhaps 8 ' diameter, opening in the bottom) is most cost effective (B/C ratio 0.95) . The dam would restrict flows entering the city on San Luis Creek and the opening would be sized so as to meet the capacity of the existing creek through the downtown. Except for tight spots, San Luis Creek through town could remain in its current natural state. Below the confluence, where unrestricted flows from Stenner Creek and Old Garden Creek join San Luis Creek, the dam will restrict flows by approximately 23%. The remainder flow, while substantially reduced, can only be carried within the creeks below the confluence if the creeks are widened. Although the width of widening would be reduced by the dam, virtually all portions of the creek would require widening. Since most of this area of the community is not densely developed, it appears practical that this portion of the creek could be widened without excessive cost or excessive environmental damage. j i This has led staff to the conclusion that the most feasible means i of providing flood protection for the community involves 3 i concurrent programs a) development of a detention dam on San Luis Creek above the city, b) elimination of tight spots in each waterway so as to achieve a balanced hydraulic capacity on each creek, and c) widening of. San Luis Creek from the confluence to the southerly city limits. These conclusions were presented to the Zone 9 board at the December 28, 1989 meeting and unanimously concurred with. i i I i ����n�►��►►�iilllll�l°�����II city of San tins OBlspo COUNCIL AGENDA REPORT Flood Update Page Six FLOOD MANAGEMENT POLICY In 1983 , the Council adopted a Flood Management Policy (Exhibit D) . Because the policy is so basic to updating the entire report, staff requests that the Council review, modify it as desired and adopt or readopt a policy. Staff concurs with the current policy with the minor amendments shown. ADDITIONAL DISCUSSION In recent years, the citizens and the Council have expressed a desire to develop a trail system along the creeks and for development of additional parks and open space. Development of a dam above the city would necessitate acquisition of property for the construction of the dam and flood area. It would also be highly desirable to acquire the remainder property between the dam and the culvert leading under 101 (site of old reservoir and training facility) . Because flooding of all of this property would be rare and only for short time intervals, all parcels could be developed to whatever recreational purpose was desired by the city. This is an especially beautiful riparian area. With a widened San Luis Creek between the confluence and the southerly city limits, it would be quite feasible to incorporate a trail system into the facility. Both flood control measures therefore, could and should serve a dual purpose for the community, and the existing riparian habitat could largely be preserved. FISCAL IMPACT Property acquisition and improvements are estimated at $2 .2 million for the upstream detention dam. Tight spot improvement for San Luis Obispo and other major creeks is estimated to cost $0.5 million. Property acquisition below the confluence is estimated to cost $4. 2 million if it must be purchased, though most of it may be exacted as properties develop. Creek widening below the confluence is estimated to cost $8.5 million, though much of it may be required as property develops. All figures are exclusive of costs to develop trails, recreational facilities, or environmental mitigation. i- 9 �����1 city of San tins OBISPO j COUNCIL AGENDA REPORT Flood Update Page Seven ALTERNATIVES A) Do Nothing Alternative The city could determine that since none of the flood control projects are cost-beneficial, the city should not use taxpayers funds for improvements. i Advantages The city would save millions of dollars in direct expense and untold staff hours. Creeks below the confluence would remain at their current width. Disadvantages Without some positive improvement to the creek system, substantial rains will result in major flooding of downtown and lower Higuera, similar to that which occurred in 1969 and 1973 . The benefit-cost formula does not take into account human suffering, risk to life, or major inconvenience when the city is incapacitated by, and recovers from, a heavy storm. i City would lose an excellent opportunity to develop a creek trail system, and recreation facilities at the dam site. B) The 1983 Plan Alternative The City could determine that the current approach to .flood protection should be followed. Current approach calls for no upstream dam but widening of all creeks to sufficient capacity to i handle the design storm. Advantages Major one-time capital cost of acquiring property and constructing the dam can be avoided. Disadvantages Most of San Luis Obispo Creek through the city would have to be widened (or if not widened, lined) to accommodate design flows. Because of additional creek widening compared to the upstream detention dam approach, property acquisition problems are compounded and total costs are greater. - r city of san Luis oBispo COUNCIL AGENDA REPORT Flood Update Page Eight C. The Upstream. Detention Dam, together with minimal widening on major creeks, Alternative (staff recommendation) Advantages 1) The upstream detention dam recommended by staff provides the most cost-effective solution of all the alternatives studies. 2) Riparian habitat upstream of the city would be purchased and developed for park purposes. 3) San Luis Creek upstream of the confluence would not have to be widened, thus preserving existing habitat. 4) San Luis Creek downstream of the confluence would be widened by a lessor amount than under the current plan, thus minimizing damage to the riparian habitat and costs. 5) A trail system can be developed in conjunction with widening below the confluence. 6) Citizens within the community will be protected when all improvements are completed. 7) If desired, the upstream detention dam can be managed so as to retain some water at the end of the rainy season which can then be released during the summer to enhance the creek habitat. Disadvantages I 1) Acquisition of property and development of the dam will require a lump sum expenditure of $2.2 million, which may create funding problems. 2) Widening of San Luis Creek below the confluence will still be required. 3) Construction of the dam and widening below the confluence will create environmental damage which will require some time to completely mitigate. 4) There is always some risk of dam failure, though the dam would be empty or would retain only a small amount of water most of the time. ���n�►b►►►(VIIIIIIIIII(► IUIII city of san Luis osispo COUNCIL AGENDA REPORT Flood Update Page Nine RECOMMENDATION Staff feels that the pain and suffering and the risk to life and property during flooding of downtown is a dominant consideration, of much greater importance than a judgement based only on a benefit-cost ratio. Staff feels that the best answer lies in the recommended program, which after it is carried out over a number of years, and completed, will result in a high level of flood protection for the city. If the program is conducted sensitively the natural creek environment will be protected, and the city can develop a trail system along much of the creek and a major recreation facility at the dam site. Specifically, staff requests the following Council action. A. Reaffirm Flood Management Policy B. Direct staff to pursue flood control studies and reports with the following as basic approaches: 1) Development of a detention dam upstream of the city on San Luis Creek following staff concept of restricting all flows greater than a 10 year storm. 2) Eliminate tight spots in each of the City's waterways, so as to achieve a balanced design which will handle design flows. 3) Widen San Luis Creek from the confluence to the southerly city limits to handle design flows. This widening will be accomplished as opportunities present themselves, based on an adopted creek widening plan line and creek widening policy. Most of the acquisition and widening would be conducted as properties are developed. 4) Incorporate in all waterway improvements, where feasible, trails, retention of riparian habitat and other amenities to create an attractive environment. Because of the major implications of this action, staff recommends that the matter be set for public hearing. Attachments: A) Tight Spot Creek Improvements B) Summary-Schaaf C) Staff Study of Upstream Detention Alternatives D) Flood Management Policy NOTE: Copy of the Flood Management Policy (pink book) is available in the Council reading file. floodldfr#24 /�/� RLI I Fla! 1 �`� � � oD I o �� oao� � 1 - X0000000 1 •:� �c v o0� o OBD°o°�-�o :4�� � ooODo°�o�a� ��, �o � �.- 00. 'IN o\ er. =F-313EIDEID =1jEj11E1= dd L.0,fir , � �.-•---._.� .�\ �y EXHIBIT "A" Lr.-JAI C r SUMMARY One strategy for reducing flood damages in San Luis Obispo is to create one or more upstream detention facilities. The theory behind these facilities is that they would hold back. the peak portions of flood flows and release them slowly enough to reduce or prevent downstream flood damage. These detention facilities could be one of two types: The first would be a dry dam. This would essentially be an embankment made of compacted earth with an outlet structure in the bottom which would allow a limited amount of flow to pass. The dry dam would also have a concrete spillway so that in the event of an extremely large flood the dam would not be overtopped. This type of detention facility would look similar to the U.S. 101 culverts on San Luis Obispo Creek near Cuesta Park. In most cases the embankments would be higher than the approximately 35-foot high embankments and the outlet works smaller than the 19-foot by 15.25-foot arched culvert under each U.S. 101 crossing. There is very little if any permanent pool behind these culverts under U.S. 101. This would be similar for the dry dam detention facilities. The second type of detention facility would be a diversion with off-channel storage. This type of facility would consist of a diversion structure on the creek, a transmission facility usually consisting of a pipe line or an open channel and a dam which forms a reservoir not on the main stem of the creek but on a smaller branch. This type of facility would be similar to the diversion structure, pipe line, open channel, dam and reservoir on Stenner Creek operated by Cal Poly as a water supply facility. However, because so much water has to be stored to reduce downstream flooding, the dams considered in this investigation would be significantly higher than Cal Poly's and the pipe line and the open channel would need to be capable of handling a much larger capacity of flow than the existing pipe and channel. A number of different sites above the City were selected for consideration of upstream detention of flood flows. On Stenner Creek three different alternatives were considered. These consisted of a dry dam in the narrow canyon upstream of the railroad crossing, and two diversions with off-chan- nel storage. On San Luis Obispo Creek eight different alternatives were considered. These consisted of six one-facility alternatives, one three- facility alternative and one four-facility alternative. A two-facility alternative was not feasible. The six one-facility alternatives consisted of a dry dam located immediate- ly upstream of the U.S. 101 crossing which is just upstream of Cuesta Park, a modification to the existing culvert under U.S. 101, a dry dam located at the Corps of Engineers' site considered in the 1987 feasibility study, and three diversions with off-channel storage. Two of the diversions were for high flows only. The shorter of these two diversions had to convey water diverted from San Luis Obispo Creek 5,000 feet to a reservoir formed behind a dam built near the mouth of the canyon just to the north of Reservoir Canyon. The longer diversion had to convey water diverted from San Luis Obispo Creek a distance of 8,000 feet to reach the reservoir. The longer diversion had a steeper slope than the shorter diversion. The low flow diversion was 5,000 feet long. Schaaf & Wheeler i The three-facility alternative consisted of a dry dam on San Luis Obispo Creek just upstream of the Reservoir Canyon Road crossing, a dry dam in the canyon just to the north of Reservoir Canyon and a larger dam on Reservoir Canyon. The four-facility alternative consisted of the same three dry dams as for the three-facility alternative with the addition of a dry dam on San Luis Obispo Creek along the Old Cuesta Grade Road. The three-facility and the four-facility alternatives were investigated to determine if there was any merit to considering smaller but more numerous dams as a flood damage reduction strategy. The principal method used to evaluate the alternatives was a benefit to cost ratio. In this .method, if this ratio of benefits to costs is greater than or equal to one, then the benefits of constructing the alternative outweigh the costs. Conversely, if the ratio of benefits to costs is less than one, the costs outweigh the benefits. Benefits considered were flood damages prevented. These were updated from the 1977 Master Flodd Control and Drainage Plan even though some structures in the flood plain have been flood proofed since the 1977 report. No new flood damages were considered as all new construction must meet City stand- ards requiring that structures be built so as to be free from flood damage. Water supply benefits were also considered, particularly for the diversion alternatives. The benefits for the high flow diversions proved to be quite low principally because the diversions did not operate until the flow in the main creek reached a particular threshold value like the 50-year or the 40- year peak flow. Thus there would not be too many times that such a facility would be expected to be used and not too much water supply would be expected on the average. The low flow diversion alternative would provide much more significant benefits. However, these were not sufficient to make the total project achieve a benefit to cost ratio greater than one. While the benefit to cost ratio is one important consideration in the analysis of alternatives, it is also important to consider other aspects such as financing. Detention facilities generally require a large, initial outlay of money to design and construct the facility as opposed to some channel enhancement works which may be constructed incrementally as funding allows. Thus detention projects which have more benefits than costs may be more difficult to fund than projects which can be implemented over a period of time. The results of the benefit to cost ratio analysis indicated that the best alternative was the dry dam at the Corps' site. This alternative would limit the flows downstream of Cuesta Park to a rate approximately equal to the capacity of the under-city culvert. The benefit to cost ratio, however, was only 0.80 indicating that the costs would outweigh the benefits. The costs included construction costs, right-of-way costs, and operations and maintenance costs but did not included any costs for environmental mitiga- tion. Even with this dry dam in place, there would still be potential flooding along San Luis Obispo Creek downstream of the Stenner Creek confluence. For example, given this dry dam with a release rate which matched the capacity of the undercity culvert, the 100-year peak discharge downstream of the Stenner Creek confluence would be reduced from 13,700 cfs to 12,200 cfs. Schaaf Wheeler ii r Therefore, significant channel work would still be required to provide even a 50-year level of protection in this reach of San Luis Obispo Creek. Also some improvements upstream of Stenner Creek would still be needed to elimi- nate the remaining bottlenecks and very low capacity sections. When considering the one-facility versus three-facility and four-facility combinations, the costs went -up as the number of facilities increased. Placing dams higher up in the watershed meant that the height of the dam had to increase in order to achieve a given level of storage and thus the facil- ity became quite costly. Of all the flood damage mitigation plans over the years, the alternatives with the highest benefit to cost ratios always included some upstream - detention facility. However, none of the previous studies found a project with a ratio'greater than or equal to I.A. Given the low benefit to cost ratio, there does not appear to be an immediate prospect -of providing flood protection by usiiU,_pstream detention. The City can still pursue a reduction in flood damages: ou¢h_'� increasing channel capacities through the City, by encouraging flood proofing of exist- ing structures, and by conforming to the guidelines of the National Flood Insurance Program. The concept of an upstream detention facility could be reconsidered if the City found another substantial benefit which could be obtained at the dry dam or in the reservoir area such as a large recreational use which could withstand periodic inundation. In particular, if the City purchased large amounts of land particularly in the Alternative 2 pool area for other purposes, it might be appropriate to reconsider the concept of a dry dam at that site. Schaaf & Wheeler iii MEMORANDUM PUBLIC WORKS DEPARTMENT ENGINEERING DIVISION Date August 22,1990 To David F. Romero, Director of Public Works From Wayne Peterson, City Engineer Subject Alternative Up-stream Detention Systems Two alternate scenarios to Jim Schaaf Is proposals for upstream detention have been explored by staff. First, is to detain all creek flows above the City and to provide a controlled release that results reduces flows in San Luis Creek to the confluence so there is no flooding during a 40 year storm or smaller storm. Storms that are larger than the 40 year design standard would cause increasing flows and downstream flooding would occur. The apparent result would be similar to that which would have resulted if the creeks channel had been improved to handle a 40 year storm all the way to the confluence. An added advantage of the upstream retention over. the creek improvement is that the creek below the confluence will also see somewhat less water in all storms and will require less widening to handle a 50 year design storm. The dam structure would be designed to provide a continual release through a pipe at a high rate so that when the storm ends the level of water would rapidly drop leaving no water stored behind the dam. The weir overflow of the dam would should be broad so that when the 40 year flood is exceeded little additional water is stored behind the dam. This reduces the height of the dam and the cost. Some reduction in flood flow below the dam is still achieved because the dam must store some water above the crest of the spillway for the spillway to work. This concept would require no manual control at the dam. In the case of storms exceeding a 100 year event there would be little benefit. Technical Information The release rate up to a 40 year design storm would be throttled to appear to the downstream area as a 10 year storm and all of the creek to the confluence would contain the water. A 100 year event would be equal to a 50 year storm at the confluence. A second scenario that has been suggested is to build the dam with a very small release system at the bottom of the dam and a larger release systems higher up on the face of the dam. This dam would not release all the water stored immediately after a storm. It would store enough water so as to be able to release it 4 _( / '7 2 throughout the summer. This system might help recharge downstream water tables during the summer. (While the storm retention would be in the magnitude of 900 to 1000 acre feet, this retention for later release would be in the magnitude of 400-500 acre feet in order to provide a continuous 1 cfs flow in the creek during the summer and early fall. ) This dam would have a very small pipe releasing water from the bottom of the reservoir area, a larger pipe similar to the first scenario releasing water from a higher level during and after storms and a very wide spillway as in the first scenario releasing flood flows for large storms. Location of Dams 1� Two different retention sites were considered for the two scenarios. The first site is on San Luis Obispo Creek directly above the City's police shooting range. This is a site that the Corps of Engineers considered in their analysis as did the Nolte Report. The dam for scenario one would be about 60 feet high and have a spill way at 48 feet. An 8 foot diameter pipe would be the low flow outlet. The following chart indicates the depth of water in the dam for each storm and the flow in the creek at the confluence with Stenner Creek. size Depth or Q in SLO CK Q in SLO CK of Storm Water in Dam Above Confl. below Confluence 25 year storm 38 ft. 2082 cfs. 4796 cfs. 40 year storm 45 ft. 2541 cfs. 6366 cfs. 50 year storm 51 ft. 2831 cfs. 7461 cfs. 100 year storm 55 ft. 5550 cfs. 10464 cfs. Storage behind the two 101 culverts is almost nothing for all but the 100 year flows. A 60 foot high dam will prevent overtopping by a standard project flood. 2} An alternate to the dam on the main thread of San Luis Obispo Creek is two dams on the tributary called Reservoir Canyon and the adjacent branch entering from the east of Reservoir Canyon. Reservoir Canyon is very narrow and has little capacity. For this alternative to work most of the flow from Reservoir Canyon needs to be diverted into the adjacent creek or the dam in Reservoir Canyon needs to be exceptionally high. This diversion must be at a high flow. The following chart shows the advantages of these retention with two different release rates. As can be seen, even with the lowest release rate the flows in San Luis Obispo Creek at the confluence are higher than the flows resulting from the main stream retention dam. I 3 100 cfs release 400 cfs release Cuesta Confl. Cuesta Confl. cfs. cfs. cfs. cfs. 10 YEAR 854 1145 25 YEAR 1617 2160 40 YEAR 2261 3021 2458 3380 50 YEAR 2722 3637 100 YEAR 3873 5079 4129 5377 Storage for 400 cfs release at 100 years is 900 ac-ft and 427 ac- ft at 40 years. Storage for 100 cfs release at 100 years is 1113 ac-ft. The height of each dam for the lower release rate is 50 feet in Reservoir Canyon and 56 feet in the East Fork of Reservoir Canyon Creek. Attached are two maps showing the proposed dams and the areas that would be flooded as a result of their construction. Dam Size to Provide 1 cfs summer release. In order to provide the additional storage necessary to allow for summer releases or for water supply the dams shown would need to be about 10 to 15 feet higher than those for just flood protection. In addition their may be more stringent construction requirements for dams which store water throughout the year as compared to those that are only holding water a short time. This is an area which has not been investigated. An alternate way to provide for the summer release program would be a managed gate on dam proposed in scenario one. This dam would be the same size but would have a gate across the outlet pipe which would allow the release rate to be lowered or stopped. In this way the dam could be made to store water year round. Most really large dams operate this way. The operator would need to be aware of weather conditions and lower the water prior to any storm that may generate flows requiring retention. BENEFITS DUE TO PREVENTION OF FLOOD DAMAGES Location: Upper San Luis Obispo Creek Location Rate Costs Benefits B/C SITE 4 8 ft. pipe $2.2 Million $2. 1 Million 0.95 ON MAIN STREAM RESERVOIR 100 CFS. 3 .8 Million 2. 0 Million 0.53 CANYON 400 CFS. 3 .0 Million 2 .0 Million 0. 53 c /-!9 4 The additional cost to provide added storage to allow a summer release of 1 cfs was also investigated. The estimated additional cost due to the higher dams at either site is $1 Million. Cost figures are calculated using information in Schaaf's report. Benefits are calculated in the same manner as they were in Jim's report also. Jim's report of cost and benefit analysis is based upon methods used by the Corp of Engineers and does not include costs of ecological requirements or the emotional impact on people of a flood. Additional geological studies should also be of the various sites. Advantages and disadvantages of these two scenarios Scenario One-Only Flood Retention and floodwater reduction Advantages 1. Smaller lower cost facility. 2. Less water stored during a large flood. 3. Less area inundated and for a shorter period of time. 4. Maybe simpler dam construction. 5. No standing water to manage during the summer. 6. Entire flood plain of dam can be used for recreation when not being used for flood control. Disadvantages 1. Possible problems with water rights claims. Scenario Two-Flood Retention and summer release. Advantages 1. A water feature that could be worked into a recreation area. 2. Potential water recharge for downstream basins. 3 . May gain more support since it will provide benefit every year and not just during floods. Disadvantages 1. More costly and complicated alternative with either operating system. 2 . Possible problems with water rights claims. 3 • A managed outlet may be less costly due to a lower dam structure and a smaller flooded area but would require staff management and decisions as to the size of the pool to maintain during various times of the year in order to maintain a safe facility. 5 4. Concern for safety of downstream persons due to a permanent upstream retention of water. Recommendation: Engineering Staff recommends that first scenario be further explored with the option of gating the outlet pipe to maintain a minimal pool for possible summer release. Staff does not recommend that the dam size be enlarged for the simpler operation due to the extraordinary extra cost. f890rept.wp v ' 10 f- o lip_ LLI LLJ LLI uj co LLI L I N � I •may O z � I ell - •, i .. . )� � . �. 1•• _ �' •:fir �'�. r. ' ' r�, , LLI • 1 � j r '� - --•�o. �;. _ � 'v �- tib aaAA .; "� ���•�J�.��y W Z 4% O U r`� 00 9 0 Lu `.� Ch sMAN UJ 0 Z.� /J � , /,vim .• � .___^.� � • - .� •;•.`J:. H � 4C LLI LIJ J y f�jY ..✓ r % i�,�& . � � R ```��- �� . 1"� � f �S r� rpt • ��_, /-1 � � .. • C.-' ��lJ I SECTION II. Flood Management Policy. A. In April 1983 a comprehensive flood management report was prepared by staff and presented to the Council. This report included valuable background information and provided a framework for the City's flood management policy and program. B. Policy Obiectives. It shall be the policy of the City that waterways and adjacent lands be generally managed to: 1. Maintain the creeks in a natural state te the maxim extent feasible; 2. Prevent the loss of life from flooding; and a . Minimize damage C-L-em fleeding. a::isfk.R#{-:.{....:,�.;�;v:;fa:.RS::a:;�'xxxf�au::;.w�;:w;.waxxyaif!n'tiab✓.cti...........iwvi»rr �r..,.�:Nc+x,'4aH.a','�ya,.NR..�ir: .•!� '::<2.�''��''i,��+,��rs m,,,,.wj:.w:,www:.a�w{ussx.,w,n:;:;:awaa..vaao-twS.fnvoa»>::v:ncr;:xa� �!Aw:.u.::iv::vx�:c3A5>:n'�uw:isw::<axwsu;uxw�:tisxx v':.:tt.:ci.!.vkiiakio na+.v.B:Kvmw..gmfi:{K;htwirn'F,.. mm�.':v'<avvLrv.:iLx4m.iii:LtGCYn'fv'Cv:xi.}miifivw:i:':ri:;Ltiv.�riiLS:A\iiiA�fti,.A:ia�vnwlxvi:4:{:1iK:i;i:v�:in'xvxvxx vaw.w.wy.. .fX :.. _ av'.^?yy^.'v^:.^CM. ::.�y�•�:p•.'Mvvyvy uin.iwr w. ,.::...a. t�p�n n � �cen�."�. � •h"�'c�i ��u";�`d�nQ� `b�.:.�e�e"'�c�.e' ......... :::..Yxki..r.ii:l.iW:^: .%^P....v..VCha::A.::nwMCSa\vIS:.Sw.vnv>.v:NJM:NW'IXv..a....•M:.x.vrvxvS:.ifvirv::%vw,vv:�wn.wMn:nw.axawJ.wlavnivvv .. yr;:::.:. ..N> ::,.'fie: <:::�:.• 'e>:;�cree'::,:�:«:: �.,... .,. 4. Projects designed to alter a creek shall consider solutions which result in as natural a creek corridor as is possible. C. Storm Design Capacities. Pending focused environmental evaluations, the following storm design capacities are adopted to contain the creeks within their banks (with s: 2:< oa freeboard) ; 1. San Luis Creek - 40-year storm to confluence; 50-year storm below confluence; 2. Old Garden Creek - 25-year storm; 3 . Stenner Creek - 50-year storm; 4 . Prefumo Creek - 50-year storm; and 5. Other Minor Waterways - 10 to 25-year storm dependent on watershed area. D. Development in Creek Area. New development in waterway areas shall not be permitted to encroach in any manner which would diminish or otherwise constrict the specified storm design capacity of the waterway in that areMINIMUM. elk q >;; s� es. New development projects .::cab:wv..�w.::.�:v^:�*}is8:<w:S.zJ:av�::nc.:�::wA...acwn:.`w:..��:03,:o�c>'u::o:an.:;a:w:.<.. .�+a:xaavkww:«u in waterway areas, including governmental projects, shall be required to participate equitably in waterway 23" e zCMM bests to attain the storm design capacity of the waterway at the project location. E. City Waterway Program Emphasis. The City shall undertake, on a planned basis and subject to funding, integrated structural and non-structural waterway alteration programs which shall include: 1. Annual, environmentally-sensitive creek maintenance; 2. Removal, from the creeks, of obstacles which inhibit natural flow; and 3. Enlargement of creek channels, to attain specified storm design capacity standards, only when no other alternative is found to be feasible. :... 4> :<ire v. <:::....;.._,.,.,..:...:::g.:.: �.,rif .:.. .f�eareat�t�ria�. �.a�pxr�rt`e�t�n�s xrixere pt; The City shall actively seek to obtain easements and/or ownership for pMMVF creek access, maintenance and construction whenever possible and appropriate. Joint creek improvement projectsjde �t IffifflN shall be encouraged by the City when related to proposed development in areas adjacent to waterways. The e t j shall develep—and—implement guidelines=whieirwill help assure that the ereele system is maintained in a er-an, sensitive manner. S lsoh guidelines shall 1 F and P3pr--- the appy =nate respenslbiiity fems-stteh eree3esystem maintenanee betifeen the—public and private-seeters waa,mcoa by 0 - 3 � Z„