Technical Memorandum No. 8 June 3, 2013 Page 2. FEMA Floodplain Mapping Flood Elevations at WWTP

Transcription

1

2 Page 2 FEMA Floodplain Mapping Flood Elevations at WWTP Existing Flood Control Facilities The City of Davis WWTP is located immediately north of the Willow Slough Bypass and west of the Yolo Bypass (see Figure 1). Cache Creek and the Cache Creek Settling Basin are located north of the WWTP near Interstate 5. These waterways and facilities are part of the state-federal flood protection system in the Central Valley. Each of the waterways includes federally authorized project levees that protect the adjacent areas, including the WWTP, from flooding during large flood events. Historic Performance Historically, the Yolo Bypass and Willow Slough Bypass and the associated levees have performed well during large flood events. There has never been a levee failure along these two waterways. The two largest storms of record for the Sacramento Valley near Sacramento occurred in February 1986 and January Based on the maximum 24-hour flow total of the Sacramento River, American River, and the Yolo Bypass, the 1986 storm was estimated by the U.S. Army Corps of Engineers to have had a return period of 50 to 80 years. For the 1997 storm event, the return period was estimated to be 90 to 110 years. Cache Creek experienced a levee failure in 1983 when a levee along the south side of the creek failed just upstream of the Cache Creek Settling Basin. This resulted in flooding of mostly agricultural lands north of the City of Woodland. The Cache Creek levees were also overtopped in 1995 and 1998, but flood fighting efforts by the State of California Department of Water Resources (DWR) prevented complete levee failures. None of the major flood events along Cache Creek caused flooding near the WWTP. FEMA Floodplain Mapping The levees along the Yolo Bypass and Willow Slough Bypass were thought to provide 100-year flood protection to the WWTP and the surrounding area until June At that time, the Federal Emergency Management Agency (FEMA) published new floodplain maps that placed the WWTP in a Zone A floodplain (see Attachment 1). This floodplain zone is assigned to areas that have at least a one percent chance of flooding in any given year, but floodplain elevations are not defined because the floodplain limits are established using approximate methods rather than a detailed analysis. The newly defined floodplain is not the result of a new study or new data that predicts increased flows and stages in the surrounding waterways, but is the result of a new approach used by FEMA for determining the flood protection offered by levees. For this approach, FEMA now requires that levee owners provide technical documentation that demonstrates that a levee meets federal design, construction, maintenance and operation standards to provide protection from the 100-year flood. Without such documentation, FEMA assumes that a levee does not provide flood protection during a 100-year storm and the areas that are protected from flooding by the levee are mapped into the floodplain. Because the technical data for the levees protecting the WWTP is not available, the levees were de-accredited by FEMA and the area was mapped into the floodplain.

3 Page 3 Flood Elevations at WWTP Area North of the Willow Slough Bypass Although FEMA has not established the floodplain water surface elevations at the WWTP site, they have established water surface elevations in the Yolo Bypass and the Cache Creek Settling Basin. Due east of the WWTP, the 100-year water surface in the Yolo Bypass is approximately 30.4 feet based on the North American Vertical Datum of 1988 (NAVD88). Further north near the Cache Creek Settling Basin, the water surface elevation in the Yolo Bypass is approximately 32.7 feet (NAVD88). Along the southern levee of the Cache Creek Settling Basin, the 100-year water surface elevation varies from 40 feet to 49 feet (NAVD88). Thus, a levee failure along the south side of the Cache Creek Settling Basin would produce the most critical flooding in the vicinity of the WWTP north of the Willow Slough Bypass. The City of Woodland is evaluating alternatives to improve flood protection along Cache Creek. As a part of that study, Wood Rodgers, Inc. (Wood Rodgers) has prepared modeling to define the 200-year floodplain resulting from a levee failure along the south side of Cache Creek just upstream of the Cache Creek Settling Basin. For that scenario, the modeling indicates that the 200-year water surface elevation at the WWTP would be feet (NAVD88). Wood Rodgers was contracted by West Yost to revise the modeling developed for the Cache Creek study to produce the 100-year water surface elevation at the WWTP that could result from a Cache Creek levee failure. Based on that study, the 100-year water surface elevation at the WWTP was determined to be approximately 32.8 feet (NAVD88). The elevations at the main portion of the WWTP generally vary from 26 to 29 feet (NAVD88) with lower elevations in the adjacent ponds and ditches. Willow Slough Bypass Definitive water surface elevations for the 100-year storm have not been established for the Willow Slough Bypass. The State DWR prepared a HEC-RAS hydraulic model of the Willow Slough Bypass in the late 1990 s or early 2000 s. DWR staff provided a copy of the model to West Yost, but they could not provide documentation of the method and assumptions used to prepare the model and DWR staff could not attest to the accuracy of the model. The model configuration appears to be reasonable based on a review of channel widths and manning s n values. In addition, West Yost assessed the accuracy of the model as follows: According to documents prepared by the U.S. Army Corps of Engineers, the Willow Slough Bypass was originally designed for a flow 6,000 cfs. A design profile based on that flow rate was provided by the Central Valley Flood Protection Board. A flow of 6,000 cfs was inserted into the hydraulic model provided by DWR and the starting water surface elevation at the downstream end of the model was revised to match the water surface elevation from the original design profile. The computed water surface elevations from that model configuration were compared against the original design profile for the bypass and found to be within approximately 0.1 feet. The peak 100-year flow used in the DWR HEC-RAS model is 10,022 cfs. The peak flow in the Willow Slough Bypass was calculated by Borcalli & Associates in 1992 for their Covell Drainage System Comprehensive Drainage Plan. For that study, the peak 100-year flow was calculated to be 10,700 cfs, which is reasonably close to the DWR value.

4 Page 4 Based on the results of the comparisons described above, the DWR HEC-RAS model appears to be configured in a reasonable manner. The one major uncertainty for using the model to compute the 100-year water surface profile in the Willow Slough Bypass is the starting water surface elevation at the downstream end of the model near the Yolo Bypass. As provided, the DWR model calculates a starting water surface elevation in the Willow Slough Bypass based on normal depth in calculations. For this approach, no backwater influence from the Yolo Bypass is considered. Using that model configuration, the water surface elevations in the Willow Slough Bypass adjacent to the WWTP range from 25.6 to 27.8 feet (NAVD88), which are 5.6 to 7.3 feet below the top of the levee. However, these elevations are lower than the 100-year water surface elevation in the Yolo Bypass at the confluence, which is 29.8 feet (NAVD88). Therefore, the absolute minimum 100-year water surface elevation in the Willow Slough Bypass is 29.8 feet (NAVD88). The worst case 100-year water surface elevations would occur if the peak flow in the Willow Slough Bypass were to coincide with the peak 100-year stage in the Yolo Bypass. Although this is unlikely, we are not aware of any study that establishes the coincidence between flows in the Willow Slough Bypass versus flows in the Yolo Bypass. Therefore, the HEC-RAS was used to establish an estimated worst case water surface profile based on 100-year flows in the Willow Slough Bypass occurring at when the Yolo Bypass is at its peak 100-year stage. Based on this configuration, the 100-year water surface elevations in the Willow Slough Bypass adjacent to the WWTP range from 30.2 to 30.7 feet (NAVD88), which are 1.5 to 3.0 feet below the top of the levee. Figure 2 presents the bracketing minimum and maximum 100-year water surface profiles described above along with a profile of the Willow Slough Bypass north levee. For each of the water surface profiles, there is a corresponding freeboard line that is 3 feet higher than the associated profile. This represents the required levee freeboard to meet FEMA criteria for 100-year protection. The freeboard will be required for the portion of the plant that is proposed to be protection from the 100-year flooding. Currently, this is believed to be the Central Plant area plus the first pond to the west. The freeboard line is only shown along the area that is currently envisioned to be protected. As shown on Figure 2, for the worst case 100-year flood profile, the levee needs to be raised from 0.1 to 1.5 feet to achieve 3 feet of freeboard. To only protect against the 100-year backwater from the Yolo Bypass, portions of the levee would need to be raised up to a maximum of 1.1 feet. The actual 100-year water surface likely lies somewhere between the worst case profile and the Yolo Bypass backwater elevation, but without more definitive information, it is recommended that the worst-case profile be used for design. As discussed below, on-going studies by DWR may produce information that can be used in the future to improve the estimated 100-year water surface in the Willow Slough Bypass. Ongoing Activities that Could Affect the Floodplain Status The following federal, state, and local government activities are described below: FEMA Levee Analysis and Mapping Procedures State of California Department of Water Resources Activities Lower Cache Creek Feasibility Study

5 Page 5 FEMA Levee Analysis and Mapping Procedures At the request of Congress, FEMA is currently reevaluating their approach to floodplain mapping for areas protected by non-accredited levees. A draft approach has been developed by FEMA that will allow more community input during the mapping process and is intended to better reflect the actual protection offered by a levee system, even if it is not fully accredited by FEMA. For levee systems, FEMA will use one of five mapping procedures: Sound Reach Procedure for a levee system that has been designed, constructed, and maintained in accordance with sound engineering practices, FEMA will model the flood hazards as it would for an accredited levee system. However, the area protected by the levee will be mapped as a Zone D, which is an area of possible, but uncertain flood risk. Freeboard Deficient Procedure for a levee system that lacks adequate freeboard, but is structurally sound, FEMA will also map the protected area as a Zone D. Overtopping Procedure if a levee system will overtop without failure, a highly unlikely scenario, FEMA will model the levee as a lateral weir and will determine the area inundated by the volume that spills over the levee. Structural-Based Inundation Procedure if a levee system lacks structural integrity, FEMA will model the system assuming that the levee will fail and will map the inundation area into a Zone AE flood zone, which has flood elevations defined. Natural Valley Procedure FEMA will use this procedure when a levee system does not prevent flood waters from spreading across the natural valley of the waterway. For this approach, FEMA will include the levees in the model, but will allow the discharge to flow on both sides of the levee. FEMA intends to reassess their floodplain mapping in the Central Valley using one of the above procedures. The specific procedure to be used in a given area will be dependent on an assessment of levee conditions in that area. Over the next year, FEMA plans to engage the public to determine whether the community agrees with the current flood mapping or believes changes are warranted. This process could ultimately lead to revised floodplain mapping for the area around the WWTP, but this is uncertain. State of California Department of Water Resources Activities Central Valley Flood Protection Plan The Central Valley Flood Protection Act of 2008 directed the DWR to prepare the Central Valley Flood Protection Plan (CVFPP). The CVFPP is a comprehensive flood management planning effort that is intended to reduce flood risks and improve ecosystem functions in the Central Valley. A draft CVFPP was published in December The draft plan recommends a number of physical improvements including levee improvements, new bypasses, expansion of existing bypasses, bridge improvements, and other structural improvements (see Attachment 2). Among the improvements considered by the plan is an expansion of the Yolo Bypass. This expansion would include construction of new setback levees to widen the existing bypass. A preliminary schematic included in the CVFPP shows the expanded bypass covering the area currently occupied by the WWTP. It is

6 Page 6 acknowledged in the plan that additional detailed studies are required to fully define the elements of the project. It seems likely that the ultimate configuration of the expanded Yolo Bypass would be configured to avoid the WWTP rather than absorb the cost of relocating it. The plan is intended to be incrementally implemented over the next 20 to 25 years. Central Valley Floodplain Mapping As part of the CVFPP effort, DWR is developing floodplain maps for the entire Central Valley. DWR s focus is on identification of the 200-year floodplain limits and they are not currently planning to map the 100-year floodplain. According to DWR staff, their hydraulic modeling and 200-year floodplain maps will be completed by mid Although they are preparing 100-year floodplain mapping, the models produced by DWR will be made available to other public agencies and these models may provide the opportunity to produce better estimates of the 100-year design water surface elevations at the WWTP, particularly in the Willow Slough Bypass. To assist with their floodplain mapping effort and with preparation of the CVFPP, DWR performed an assessment of the physical conditions of the facilities included in the state-federal flood protection system in the Central Valley. This included an assessment of levee conditions, including those along the Yolo Bypass and Willow Slough Bypass. The levees were evaluated based on a geotechnical assessment and levee performance history. According to DWR, the levees along the Yolo Bypass near the WWTP and along the north side of the Willow Slough Bypass east of the WWTP are classified as high concern levees (see Attachment 3). The Willow Slough Bypass north levee adjacent to the Central Plant and to the west are low concern levees with no significant issues identified. DWR is coordinating their modeling and mapping efforts with FEMA and they are using FEMA s new levee mapping approach to guide their work. It is anticipated that FEMA will ultimately use the hydraulic models developed by DWR to produce new 100-year floodplain maps within the Central Valley. The timing of this mapping is uncertain, but it would occur after FEMA s public outreach process, which is scheduled to extend into the summer of Lower Cache Creek Feasibility Study As discussed above, the City of Woodland is evaluating flood protection alternatives along Cache Creek with the ultimate goal of defining a project to protect the City of Woodland from flooding. A project that protects the City of Woodland from Cache Creek flooding would also provide some flood protection to the WWTP. Although it is not believed that such a project will occur prior to the WWTP improvements, this project should be monitored. Flood Hazard Mitigation Options Although FEMA s efforts to update their floodplain mapping in the Central Valley could change the floodplain limits and water surface elevations at the WWTP, it seems unlikely that the WWTP will be removed from the FEMA floodplain given DWR s categorization of the levees in the area as high concern. If the WWTP remains in the floodplain, it may be necessary for the City of Davis to implement flood protection measures at the WWTP to comply with the requirements of the NPDES permit. Three potential options for providing flood protection are discussed below:

7 Page 7 Construct a Floodwall or Levee around the WWTP Improve Existing Levees Rely on CVFPP and Lower Cache Creek Improvements Construct a Floodwall or Levee around the WWTP Flood protection at the WWTP could be achieved by constructing a floodwall or levee around the key facilities at the site. There are uncertainties regarding the required limits of the floodwall or levee. One of the uncertainties is related to which facilities must be protected. The current NPDES permit for the WWTP states that the treatment ponds and biosolids storage facilities must be protected from a 100-year storm, but it does not mention any other facilities. Logically, it seems that the main WWTP area should be protected because this is the location of the highest value facilities. Also, despite the fact that the NPDES permit requires protection of the biosolids storage area and treatment ponds, there may be arguments against building a floodwall around these facilities. Currently, the biosolids storage area and treatment ponds have berms or levees surrounding them that are six to 10 feet above the ground elevations outside of the WWTP. These berms provide adequate protection against flooding that could occur from local runoff during a 100-year storm, but could be overtopped if a nearby levee failed along the Yolo Bypass, Willow Slough Bypass, or Cache Creek during a 100-year storm. Because the volume of flood water associated with a levee failure would be very large, any pollutants washed from the WWTP would be diluted to concentrations that could be considered less than significant. Therefore, it may be possible to make a case for protecting just the main WWTP area. This will need to be negotiated with the RWQCB. As discussed above, the Willow Slough Bypass north levee lacks adequate freeboard against the 100-year water surface elevations. Therefore, for this approach the Willow Slough Bypass north levee would be raised along the area to be protected. Improve Existing Levees Flood protection could be provided to the WWTP by improving the existing state-federal project levees that currently protect the site. This would require improvement of several miles of levees, which would make it more costly than constructing a floodwall around the WWTP. In addition, it would require significant coordination with the State, Reclamation District 2035, and other agencies. It may be possible to cost share with other agencies; however, because the State has proposed to relocate the levees along the Yolo Bypass as part of their CVFPP, they may not be inclined to invest in the existing levees. Rely on Central Valley Flood Protection Plan and Lower Cache Creek Improvements The State s CVFPP proposes to expand the Yolo Bypass and construct new levees along it. As mentioned above, the CVFPP currently shows the WWTP site within the expanded bypass. These expansion limits would require the WWTP to be relocated. Another more likely scenario is that the limits of the expanded bypass would be adjusted to avoid the WWTP. In either case, the WWTP would end up protected by new levees that would provide at least 100-year protection against flooding from the Yolo Bypass and Willow Slough Bypass. At this time, it is unclear if the CVFPP would improve flood protection against Cache Creek flooding. However, the City of Woodland is studying flood control alternatives as a part of their Lower Cache Creek Feasibility Study with a goal of defining a project to protect the City of Woodland against flooding from Cache Creek.

8 Page 8 A request could be made to the RWQCB to allow the City of Davis to rely on the existing levee protection in the area with the understanding that a greater level of protection will be provided by the CVFPP and the City of Woodland Cache Creek project in the future. However, there is some uncertainty as to when and if the expanded bypass will be implemented. The overall schedule for implementation of the CVFPP is 20 to 25 years. There is also significant uncertainty about the feasibility and timing of a project along Cache Creek. Future Planning Activities As planning and design activities continue, the following activities should occur to ensure that the procurement documents and subsequent design documents for the project are based on the best available information: Monitor the floodplain mapping efforts of DWR and FEMA. These floodplain mapping efforts could result in a revised design water surface elevation at the WWTP and they should be monitored closely. Monitor DWR activities related to the CVFPP. As DWR begins to better define the facilities to be included in the plan and the schedule for implementation, it will be clearer whether the City of Davis can rely on this plan to provide flood protection at the WWTP. Monitor the Lower Cache Creek Feasibility Study to insure that the latest Cache Creek floodplain information is considered when defining the flood protection requirements at the WWTP. Negotiate the facility protection requirements with the RWQCB with a goal of obtaining an agreement for the City of Davis to protect only the main WWTP area. DESIGN CRITERIA The cost of improving the existing levees along the Yolo Bypass, Willow Slough Bypass, and Cache Creek would be prohibitively expensive, and the timing and scope of the CVFPP improvements is uncertain. Therefore, without relief from the RWQCB it is assumed that a floodwall and/or levee will need to be constructed around the WWTP to provide flood protection. Minimum Requirements All wastewater treatment facilities must be protected from flooding. In addition, all biosolids handling and storage areas will be included in the area to be additionally protected from flooding for compliance with federal biosolids regulations. The City of Davis will work with RWQCB staff to determine if additional flood protection is needed for the existing facultative ponds if they are used to store off-spec water. Upon construction of the planned secondary and tertiary improvements, the existing WWTP facultative treatment ponds will no longer be used for wastewater treatment, at least for the foreseeable future. However, the ponds may be used for occasional shaving of peak wet weather flows in order to control the size of downstream facilities. In the most likely scenario, clarified secondary effluent peaks would be diverted to the existing facultative ponds for evaporative disposal. Any primary effluent flows diverted to storage for equalization purposes would be contained in a separate storage area that is provided with the same level of additional flood protection as the treatment facilities.

9 Page 9 The existing ponds have been protected from all historical flooding events by the existing pond levees as well as the existing Yolo Bypass, Willow Slough Bypass, and Cache Creek levees. Given that a very large flood event would be required to overtop the existing pond levees, it is recommended that the City of Davis propose to the RWQCB that ponds used in the future to accept infrequent diversions of secondary clarified effluent or filtered effluent do not warrant additional flood protection. Nevertheless, pending a definitive indication from the RWQCB that existing protection is adequate for the proposed pond uses, current facilities planning will proceed based on the assumption that all secondary peak shaving ponds and treatment facilities will need to be protected. All biosolids storage areas will be included in the area to be additionally protected from flooding for compliance with federal biosolids regulations. Any area devoted to storage of fully treated effluent suitable for unrestricted reuse under Title 22 will be excluded from the additionally protected area. Performance Requirements Flood protection facilities must: Not inhibit plant operations or increase the level of effort required to perform any plant maintenance or operation activity. Design Criteria The following design criteria will apply to flood protection for the WWTP. Flood protection facilities must: 1. Provide a minimum of two feet of freeboard for the specified design flood level of 32.8 feet (NAVD88) for the area north of the Willow Slough Bypass. 2. Provide a minimum of three feet of freeboard for the specified design flood level of 30.2 to 30.7 feet (NAVD88) in the Willow Slough Bypass. For the area north of the Willow Slough Bypass, the highest potential flood elevation at the WWTP would occur upon failure of a levee along Cache Creek or the Cache Creek Settling Basin levee, which would release water to the south toward the WWTP. The 100-year design flood elevation for this scenario was determined to be 32.8 feet (NAVD88). It is recommended that the flood protection structure for the WWTP provide at least two feet 1 of freeboard, which would require a top elevation of 34.8 feet (NAVD88). 1 FEMA typically requires three feet of freeboard for both levees and floodwalls. The freeboard provides a safety factor against uncertainties. In certain cases, FEMA will allow less than three feet of freeboard down to a minimum of two feet. This occurs in cases where some or all of the uncertainties are eliminated. This can occur in cases where water surface elevations can't rise higher than a certain level because the flows will spill out of the system in another direction. Although the RWQCB doesn't require a FEMA-approved level of protection, that freeboard approach has been adopted here. Flows can spill over the Willow Slough Bypass levee once they reach a certain elevation, which limits the potential flood elevations at the WWTP. The City of Davis may determine the appropriate freeboard, with the caveat that the RWQCB will need to be satisfied that it is appropriate. The RWQCB will generally accept the opinion of a registered civil engineer regarding adequacy of flood protection meeting the 100-year criteria.

10 Page 10 Within the Willow Slough Bypass, it is recommended that the design water surface elevations be the worst case 100-year profile, as described above and shown on Figure 2. The water surface elevation along the area to be protected varies from 30.2 to 30.7 feet (NAVD88). It is recommended that the Willow Slough Bypass north levee be raised to provide at least three feet of freeboard, which would require a top of levee elevation ranging from 33.2 to 33.7 feet (NAVD88). REFERENCE PROJECT The Reference Project will include a flood wall similar in design to that described in earlier planning documents or a levee. Design-build teams will be free to explore and recommend other structural solutions in lieu of the flood wall or levee. The selected design-build team will need to prepare a complete design of whatever type of structure they recommend. Reference Project facilities are further described in the City of Davis Wastewater Treatment Plant Secondary and Tertiary Improvements Project Reference Project Report by West Yost Associates and ARCADIS, dated May 2013, and are subject to modification. Demonstration Requirements The following criteria related to flood protection will be considered in evaluating design-build proposals: Present worth cost of maintaining the flood protection facilities over the specified life cycle period.

11

12 34 FIGURE 2 Willow Slough Bypass Water Surface Profiles 33 Elevation (NAVD88) North Levee Est. Worst Case 100 Year Water Surface Worst Case Freeboard Line (3 feet) Yolo Bypass Backwater Elevation Central Plant East Boundary & East Floodwall Tie In (Approx.) Yolo Bypass Backwater Freeboard Line (3 Feet) Distance from Central East Boundary West Floodwall Tie in (Approx.) West Plant Boundary (Approx)

13 Approximate Location of WWTP ATTACHMENT 1

14 SECTION 3.0 STATE SYSTEMWIDE INVESTMENT APPROACH Key: SPFC = State Plan of Flood Control Figure 3-1. State Systemwide Investment Approach Sacramento River Basin Major Capital Improvements PUBLIC DRAFT DECEMBER 2011 PAGE 3 5

15 Flood Control System Status Report Figure ES-2. Composite Map of Physical Levee Conditions Based on ULE and NULE Results ES-10 December 2011