IMPROVED STORMWATER MANAGEMENT BMPS AND FLOOD PREPAREDNESS FOR 2016 EL NIÑO STORMS

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1 IMPROVED STORMWATER MANAGEMENT BMPS AND FLOOD PREPAREDNESS FOR 2016 EL NIÑO STORMS Denise Yaffe, Southern California Edison, Rosemead, CA Julia Lakes, Southern California Edison, Rosemead, CA Chijioke Akunyili, Southern California Edison, Rosemead, CA Randy Bick, SRJR Consulting, Upland CA INTRODUCTION Southern California Edison (SCE) has initiated storm water management improvements at its gas-fired power generating facilities in Southern California in preparation for the 2016 El Niño storms. A storm water management and flood preparedness improvement initiative was developed during the third quarter of 2015, with over 90% of the work completed by December 31, Protecting human health and the environment from storm water pollution, protecting SCE assets and power blocks from impacts associated with flooding and excessive water damage, promoting employee and public safety, and ensuring the reliability for generating power are the benefits of this initiative. SCE S STORM WATER PROGRAM SCE has storm water management plans (SWMPs) at facilities to protect surface water quality by controlling flow and runoff, and preventing the release of storm water pollutants from contaminating the environment. The SWMPs provides guidance for SCE facility personnel involved in storm water management, including structural treatment controls, specific best management practices (BMPs) and administrative procedures that are unique to the facility. The combination of structural controls and BMPs are designed to eliminate polluted runoff from leaving the site or entering a storm drain to the maximum extent possible. The SWMP describes the drainage at the facility, identifies people responsible for on-site storm water management, lists the activities that occur on the site, and lists the associated pollutants that may result from those activities. It also describes the BMPs at the site, both structural and non-structural, while providing clear instructions and schedules for maintaining the BMPs, conducting facility inspections, and tracking all storm water related activities. SCE POWER GENERATING FACILITIES SCE s fleet of gas fired power generating facilities includes one combined cycle power plant, and five simple cycle Peaking power plants. Mountainview Generating Station (MVGS) is SCE s combined cycle power plant with four natural gas fired turbines and two steam turbines totaling 1,104 Mega Watts (MW) of electric power. MVGS is built on 54.8 acres of developed land located in Redlands California. MVGS is a water cooled plant with two cooling towers. A High Efficiency Reverse Osmosis (HERO) water treatment system is used to reclaim 85% of the plants wastewater for cooling water. Make-up cooling water is a blend of City of Redlands municipal effluent, and local contaminated groundwater. Drawing groundwater from the non-potable mid aquifer helps stabilize the movement of the contaminated plum to the potable lower aquifer. MVGS has two 5 million gallon water storage tanks. MVGS also has a 3.85 million gallon water conservation basin sized for a 100 year storm event and is used to contain all storm water from the industrials areas. Industrial areas are delineated from the non-industrial areas by grading, trenches and berms. Storm water from industrial areas of the site, drain to a system of underground pipes that discharge the water to the conservation basin. Storm water flows are slowed by course gravel that is distributed over all pervious areas of the site. Oil-filled equipment and tanks are housed in secondary containment that manually drains to an oil/water separator connected to the water conservation basin. All storm water from industrial areas is contained on site. Storm water from non-industrial areas drain into detention basin or catch basin inlets that discharge to the Santa Ana River. 1

2 SCE has five Peaking power plants that are located in Southern California, with each Peaker generating 49 MW of electric power from a natural gas-fired General Electric LM 6000 combustion turbine. Barre Peaker is located in the City of Stanton (Orange County). Center Peaker is located in the City of Norwalk (Los Angeles County). Grapeland Peaker and Mira Loma Peaker are located in the cities of Rancho Cucamonga, and Ontario (San Bernardino County), respectively. McGrath Peaker is located in the City of Oxnard (Ventura County). The Peakers are built on approximately 2 acres of developed land. Each Peaker has a 10,500 gallon aqueous ammonia aboveground storage tank, a waste water aboveground storage tank, two gas compressors, transformers, a gas-fired black start generator, a power control module, an oil/water separator, a covered hazardous materials and waste storage area, a trash container, an office and a maintenance shop (Operations Building). Gravel cover is used throughout each Peaker site allowing for infiltration of storm water from adjacent impervious surfaces. The gravel also prevents erosion and controls dust. Gravel covered pervious surfaces provides significant stabilization and allows storm water to infiltrate into the ground. Gravel covered areas surround the impervious areas of the facility to minimize runoff from leaving a Peaker facility. Non-industrial storm water runoff from each Peaker facility collects in a local conveyance system consisting of concrete and earthen swales and subsurface culverts. This system exists around the entire site, and ends in the retention basin or natural retention areas outside the fence lines. Most of the storm water infiltrates into the ground due to the pervious nature of the gravel covered areas. Each Peaker facility has four concrete swales within the paved areas. The concrete swales extend across the access roads and direct runoff to the gravel covered areas along the outer edges of the facility and eventually into the earthen swale drainage system. The outer edges of each facility have been covered with gravel to reduce erosion. STORM WATER MANAGEMENT AND FLOOD PREPAREDNESS IMPROVEMENT INITIATIVE In preparation for the 2016 El Niño storms, SCE developed a storm water management and flood preparedness improvement initiative. The objectives of this initiative were to assess the effectiveness and condition of current storm water management systems, identify mitigation needs and improvement opportunities, and then implement improved engineering controls and best management practices to protect water quality from storm water pollutants, control flooding and soil erosion, and minimize damage to SCE assets from storm water. This initiative was developed to provide a process using a four phased approached, that includes an assessment and ranking phase, mitigation planning phase, implementation phase and evaluation phase. The immediate goal was to complete at least 90% of the BMP repairs and new installations/construction by December 31, A description of the initiative is provided below. The first phase of the process begins with the Assessment and Ranking (Phase 1) of each generating facility s storm water management program. The condition of existing storm water controls, including drainage systems, containment structures, retention / catch basins, soil and sediment erosion controls and various other structural and administrative controls are inspected and reviewed to assess performance and determine mitigation measures to improve performance or address deficiencies of the facility s storm water management program. Site conditions, topography, elevation, areas of impervious surfaces, vegetation, inadequate soil conditions, and various other structures that potentially impact storm water management are included in the assessment. The facilities are also assessed for evidence of soil or sediment erosion, ponding of storm water, and potential flooding. Once the assessments are completed, the facilities are ranked based on storm water BMP deficiencies, flooding potentials, and evidence of soil or sediment erosion. Facilities are ranked low, moderate or high based on the number 2

3 and type of storm water deficiencies observed, and mitigation measures recommended. The ranking allows SCE to prioritize the work based on immediate needs to address deficiencies, add new BMPs for improved performance, or take no action but continue monitoring. The second phase of the process is developing Mitigation Plans (Phase 2) to address findings from the facility assessments. During this phase, new engineering controls and BMPs or maintenance and repairs to existing storm water controls and BMPs are identified to mitigate the deficiencies observed in Phase 1. Other mitigation actions can include additional storm water monitoring, topographic studies, or soil analysis. Phase 3 of this process is the implementation of the recommended actions identified in the Mitigation Plans. During this phase, the installation or construction of new BMPs, or the maintenance and repairs of existing storm water controls and BMPs are scheduled and completed. The installation and construction of new BMPs, and repairs to existing BMPs are inspected to verify all work was completed as designed or as specified in the Mitigation Plans. Phase 4 of this process is the performance evaluation phase. Once all of the storm water BMP improvements have been completed and implemented, the site is monitored. The effectiveness of the engineering controls and BMPs to protect water quality, control flow, and prevent soil erosion, flooding and other type of damage from storm water are evaluated. Further opportunities for improvement or BMPs that require additional repairs are then determined in Phase 4. ASSESSMENT RESULTS Storm water management systems at the six gas-fired power plant facilities were reviewed. The condition of existing storm water systems including drainage systems, containment structures, retentions basins, catch basins, soil/sediment erosion controls and storm water BMPs were inspected and assessed to determine if mitigations measures were needed. The assessments included a review of available site drawings and existing documentation, including the current SWMPs, aerial topography, site grading plans, hydrology reports, and underground utility drawings. Drainage and erosion control issues and other BMP deficiencies were observed at five of the six gasfired power plants. McGrath Peaker was the only site that showed no evidence of soil erosion, flooding or threat of storm water encroachment into the Operations Building and Power Block areas during heavy rainfall. The assessments of storm water management systems and BMPs at the five power plants are summarized below. MOUNTAINVIEW GENERATING STATION (MVGS) Deterioration of the storm water earthen berm s protective geotextile material at the north perimeter of MVGS was identified. The stability of the earthen berms to effectively manage storm water could be compromised by worn and damaged material during heavy rain storms (Figure 1). Stones were wedged into the inlet grates of the storm drain catch basins located within the plant site, and the basins were filled with sediment and debris. The presences of stones in the grates, and debris in the catch basins could impede storm water from draining at the system s designed rate and potentially cause flooding within the plant. 3

4 Rill erosion was identified on the unpaved Southern California Gas Company s Gas Yard access road which was a result of localized yard drainage that concentrated at the gate. This erosion was also transporting sediment offsite and onto the adjacent street (Figure 2). The unpaved area outside of the plant property line along Mountain View Avenue was not maintained in anticipation of a city street widening project. However, non-industrial storm water run-off from the plant appeared to be causing excessive sediment accumulation in the street gutters (Figure 3). Silt and sediment build up was observed in the storm water infiltration open channel. Leaves and debris had accumulated in front of the flood control gate. These materials have the potential to prevent flow and impede drainage. Excessive debris was observed at the inlet to the storm drain catch basin located at the visitor parking lot, adjacent to San Bernardino Avenue. A sump pump within the catch basin, pumps the collected water to a storm drain that discharges into San Bernardino Avenue at the curb face. This parking lot is subject to flooding, caused by poor drainage, a clogged catch basin grate inlet, and sump pump failures under normal rain conditions. The issue could be exacerbated under extreme rain conditions predicted with El Niño. MVGS was ranked moderate, requiring repairs and some upgrades to existing BMPs. MIRA LOMA PEAKER The existing grade elevations at the southeast corner of the site, where the Operations Building is located, are lower than the southern access road to the Peaker site. The Operations Building also sits on the western limits of a retention basin (Figure 4). This retention basin was built during the initial construction of the Mira Loma Peaker Plant to contain storm water from the Peaker site as well as the adjacent substation property to the north and east. The storage capacity of the retention basin was also reviewed and determined to be of insufficient size to contain a 100 year storm, adding concern about the potential for localized flooding from heavy storms. If the retention basin reaches its storage capacity, the storm water has the potential to encroach into the Operations Building areas. Due to potential environmental and physical limits for increasing the size of the basin (storage capacity) at this time, it was decided to route the storm water from the retention basin to the south side of the access road into an empty lot on the property to minimize a potential overflow condition. Mira Loma Peaker was ranked high to address potential flooding resulting from an undersized retention basin lacking the capacity to contain a 100 year storm. CENTER PEAKER The areas surrounding most of the Center Peaker Plant site are relatively flat and not conducive to adequately controlling storm water runoff. The gravel areas around the perimeter on the north and east sides of the Plant have no slope, causing localized ponding of storm water that accumulates at the perimeter of the access road. The adjacent substation property along the northern perimeter is at a higher elevation than the Plant which prevents storm water drainage to the north, and results in potential flooding of the area. The existing grades around the area of Sound Wall that runs along the eastern portion of the Power Block prevents Plant storm water from draining east, beyond the paved access road and gravel area (Figure 5). Photo documentation of recent storm events suggest the storage capacity for the existing retention basin would be too small to contain the substantial volume of storm water that could accumulate from the El Niño Storms (Figure 6). A preliminary survey of the existing elevations around the perimeter of the retention basin, indicated elevations to the south are higher than those at the north which is adjacent to 4

5 the Operations Building. Accordingly, there is a threat of storm water encroachment into the Operations Building area during heavy rainfall. Center Peaker was ranked moderate to high to address the poor drainage conditions and potential for flooding and encroachment of storm water into the Operations Building area. GRAPELAND PEAKER The areas surrounding most of the Grapeland Peaker Plant site are relatively flat and not conducive for effective control of storm water runoff. The gravel areas downstream of the existing Portland Concrete Cement (PCC) swales that discharge storm water beyond the security fence to the north were not properly sloped during construction resulting in localized ponding of storm water between the access road and security fence. The areas outside and to the north of the security fence are not adequately sloped to convey storm water coming from the Plant to the existing culverts located along the western and eastern sides of the property. There is a low spot at the northwest corner of the parking area for storm water to accumulate and pond after a rain event which encroaches into the parking stalls (Figure 7). The areas to the north and west of the Operations Building and fencing are flat causing storm water to accumulate and consistently pond during rain events (Figure 8). The area to the west is especially problematic since it receives additional storm water that discharges from the roof downspouts of the Operations Building. The retention collection area to the south of the Operations Building is very flat and the existing rip rap (rock) material at the invert slightly impedes drainage flow. Overall, the slope for drainage away from the building is not adequate, creating a potential for water encroachment at the building in the event of a heavy rain storm. Some light soil erosion was observed throughout the south slope areas adjacent to the southern perimeter access road. There are two existing storm water PCC swale discharge points on the south side of the Plant. Evidence of soil erosion and washed out gravel along the discharge points at the southeastern and at the southwestern points were observed. The amount and placement of rip-rap material used to lower the velocity of the storm water for minimizing soil erosion and gravel wash out were insufficient. The southwestern storm water discharge point also showed signs of erosion at the low point near the security fence. Grapeland Peaker was ranked high to address the poor drainage conditions due to the flat topology, soil erosion and potential for flooding and encroachment of storm water into the Operations Building, access roads, and parking area. BARRE PEAKER The areas surrounding most of the Barre Peaker Plant site, including areas outside the perimeter are relatively flat and not conducive to managing a large volume of storm water runoff efficiently. Barre Peaker does not have any structural BMP s to collect and convey the Plant s storm water offsite and into a public storm drain system. Storm water runoff collects in two natural retention areas located offsite, to the west and to the south of the Barre Peaker Power Plant. The Plant has PCC swales to convey storm water from the north and south sides of the site. The gravel areas downstream of the two existing northern PCC swales (northeast, northwest) that discharge to and beyond the security fence to the north were not properly sloped during construction resulting in localized ponding of storm water between the access road and security fence. Similar to the existing conditions at Grapeland Peaker, the elevations at the fence line were higher than the discharge elevations on the existing swales, impacting storm water drainage. 5

6 Evidence of localized storm water ponding and erosion of the existing gravel cover were observed on the east and south Plant areas between the access road and the Sound Wall. The existing grades within these areas are very flat with virtually no defined flow lines. Erosion of the existing gravel surface at the discharge points from the two existing PCC swales on the south side was also observed. Storm water in this area is routed under the Sound Wall at two locations and ends up collecting in a natural retention area between the Sound Wall and the Railroad Right-of-Way. The area between the Visitor parking and the entrance gate had evidence of ponding resulting from no natural or structural BMP or drainage mechanism for conveying storm water from the area. Barre Peaker was ranked moderate to low, based on the poor drainage conditions, evidence of soil erosion and localized ponding of storm water. BMP MITIGATION PLANNING / IMPLEMENTATION MOUNTAINVIEW GENERATING STATION (MVGS) The mitigation plan for MVGS involved repairs and maintenance of the existing structural BMPs that were currently in place, with some minor upgrades. All of the repairs and/or improvements were for those areas where non industrial storm water leaves the Plant with the exception of an isolated onsite storm drain collection system for the industrial areas which discharges into the existing Water Conservation Basin (WCB) and not offsite. To maintain the structural integrity and stability of the storm water earthen berms along the northern property line, the deteriorated geotextile material was removed and replaced with Mirafi FW700 material for greater durability. The new material was extended a minimum of 12 inches beyond the existing rock surfacing and secured with proper nails at 3 to 5 feet on center (Figure 9). Debris and mud from all onsite drain inlets were cleaned using a vacuum truck. The embedded aggregate and debris from top inlet grates were removed. All open channels and grates were also cleaned of debris and mud using a vacuum truck. Durawattles were installed along the unpaved frontage area at the inside of the concrete curb face of Mountainview Avenue. Durawattles were chosen for this location over traditional fiber rolls for its durability in preparation of El Niño rain events. The wattles, made of synthetic materials that do not easily degrade or absorb water, allow water to efficiently flow through without sediment, and thus prevent ponding. The existing grade at the curb face was lowered slightly as needed to install the Durawattle and eliminate soil transfer to the street (Figure 10). The unpaved eroded Southern California Gas Company s Gas Yard access road was repaired. The existing grade elevations across the access road and the entrance gate were raised to create a low profile berm. A Durawattle was installed at the high point inside the Gas Yard entrance gate berm to help control sediment from the Gas Yard towards the access road and ultimately onto Mountainview Avenue. The Durawattle BMP was chosen to control sediment run-off for its properties to withstand vehicle traffic and physical and environmental (i.e., ultra violet rays) degradation. At the Visitor s Parking lot, the existing perforated steel drain inlet top was replaced with a conventional galvanized bar grating. A low profile 3-stage KATCHALL bio-filter system was installed in the catch basin to filter debris and hydrocarbons, and prevent clogging of the submersible pump. MIRA LOMA PEAKER The potential for the retention basin to overflow was addressed by installing High Density Polyethylene (HDPE) pipe culverts from the retention basin, under the access road, to an open lot for infiltration. Work began on December 21, 2015 and was completed on January 12, An area of approximately 6

7 1 ½ feet deep by 55 feet long was excavated for the placement of HDPE piping (Figure 11). The depth and slope of the pipes were limited by an existing 480V duct bank located below grade of the construction area. The HDPE drainage pipes under the access road were encased with reinforced concrete to ensure stability (Figure 12). Reinforced concrete headwalls were also constructed at the north and south terminations of the pipes and rip rap was placed at both points of the culvert to slow the flow of storm water (Figure 13). CENTER PEAKER Based on the observations and assessments, the use of PCC concrete swales were proposed to alleviate the localized storm water ponding in the area surrounding the north and east perimeters of the site. After surveying the existing grades throughout the areas, it was determined that underground storm drain piping would be needed to address the storm water ponding issues along the eastern portion of the site. The work began on December 14, 2015 and was completed on January 7, A new 3 feet wide, 270 feet long and six inch thick reinforced concrete swale was constructed along the north perimeter access road to facilitate storm water movement from the area (Figure 14). About 105 feet of existing asphalt paving had to be saw-cut to place the new PCC swale. The new swale of approximately 270 linear feet extends to the western end of the site and discharges onto the adjacent substation property. A French Storm Drain was constructed along the Sound Wall (located along the eastern side of the site) to alleviate ponding from soil that becomes saturated (due to poor drainage conditions) in this flat area. A trench of approximately 2 feet wide by 180 feet long was excavated for the placement of 6 inch diameter HDPE storm drain perforated pipe (Figure 15). The depth of trench varied from 6 inches to 24 inches due to irregular existing grades and pipe slope. The perforated pipe which has a shallow cover is comprised of geotextile fabric and crushed rock to allow the infiltration of storm water in flat areas. Three 12 x 12 inch shallow HDPE drain inlets were also installed to facilitate drainage to the new pipe (Figure 16). The new perforated pipe is discontinued at the southeast corner of the flat area near the end of the Sound Wall, and the storm drain continues as a new solid wall HDPE pipe which connects to an existing catch basin that ultimately discharges to the existing retention basin. The solid pipe is approximately 160 feet long and was backfilled with 2 sack cement slurry. To protect the shallow pipe areas from truck traffic, four new concrete filled steel bollards were installed near the truck loading slope. Warning signs were also installed to keep traffic off of the gravel in shallow covered areas subjected to possible vehicle loads. GRAPELAND PEAKER The results of the assessment suggested the need to construct new PCC swales, regrade and resurface areas within the site, and concrete the storm water discharge points to improve drainage and minimize ponding. The scope of work was divided into Phase 1 and Phase 2. To be prepared for the 2016 El Niño storms, Phase I work was generally limited to only those improvements and/or repairs that could be successfully implemented by December 31, Work began on November 16, 2015, and the first phase was completed on December 1, As the work on this facility was occurring, additional storm water improvements were identified after a rain event occurred. The second phase of work was scheduled to begin on April 20, 2016 and should be completed in early May

8 Approximately 230 feet total of reinforced PCC swales were constructed and placed in three locations on the Peaker site to improve storm water flow to the drainage areas (Figure 17). Earthen swales were also regraded and resurfaced (Figure 18). Grade elevations of the rip rap in the drainage area south of the Operations Building were adjusted to improve flow to the existing culverts and drainage areas. Impeding rip rap at the Northwest parking lot area was removed to eliminate localized ponding. The area was regraded and additional concrete was installed at the security fence for closure (Figure 19). Phase 2 work will address the localized ponding issues on the north and west sides of the Operations Building. BARRE PEAKER The storm water improvement work began on November 13, 2015 and was completed on November 20, The general scope of work primarily consisted of replacing gravel with several inches of new larger 1½ to 2 inch rock cover, and regrading surfaces to improve drainage, control erosion and mitigate localized ponding. The existing gravel was removed and the areas between the two existing PCC swales and the security fence, and areas beyond at the North end of the Plant were regraded to improve storm water drainage off the property and to mitigate localized ponding (Figure 20). At two locations, the existing security fence was extended to match the new lower grade by securing the bottom of the fence to wires that were embedded in 8 inch thick concrete. The former gravel rock cover was then replaced with 1½ to 2 inch rock cover for better erosion protection. The existing gravel cover at the south east and south west corners of the Plant between the access road and the Sound Wall was removed. Remedial grading of the area was done to improve the storm water flow line to the low points at the Sound Wall, and then offsite into the natural retention collection area. The former gravel rock cover was replaced with 1½ to 2 inch rock cover for better protection against erosion. The following improvements were made to mitigate the potential for ponding of storm water accumulating near the entrance gate and south of the Visitor parking area. The existing rock cover was removed and up to 12 (twelve) inches of existing subgrade material was excavated at the area between the access road and the Sound Wall south of the Visitor parking and the entrance gate. The removed gravel and subgrade materials were then replaced with 1½ to 2 inch rock to contain localized ponding while improving the permeability for water to infiltrate downward. Since no distinct flow line to convey storm water out of the area could be established, remedial grading, to enhance further drainage, was not possible at this location. PERFORMANCE & EVALUATION PHASE The storm water repairs and new improvements were completed by the first week of January 2016 with the exception of Phase 2 for Grapeland. Site visits to our facilities were scheduled in 2016 to assess BMP performance and effectiveness. The structural BMPs were monitored during and after the rain events that occurred from January through March 2016 to evaluate performance in conveying storm water flow without causing erosion, storm water ponding or flooding. Southern California received a small to moderate amount of rainfall and not the large volume of precipitation that was predicted from the El Niño 2016 storms. For Southern California, the average rainfall during the months of January, February and March 2016 was approximately 3 inches, 0.5 inch, and 1.5 inches, respectively. These averages are based on data collected at rain stations located in Los Angeles (Los Angeles Civic Center, LCC), Santa Ana (Santa Ana Fire Station, ANA) and Oxnard (OXN). Based on the amount of rainfall that occurred, all of the storm water BMP improvements were effective by improving drainage and preventing flooding. Some observations during the rain events from December 2015 through February 8

9 2016 lead to minor adjustments to some of the installed BMPs, specifically at MVGS and Grapeland Peaker. Additional improvements were recommended for Grapeland Peaker as described below. After a rain event on January 5, 2016, additional drainage issues were observed at the Grapeland Peaker. The contractor returned to the site to alleviate some ponding resulting from poor drainage issues not initially identified during the site assessment phase. On January 11, 2016, the contractor performed some remedial grading to the existing earthen swale, east of the new PCC swale to improve flow to the eastern culverts, and moved some of the rip rap at the drainage area south of the Operations Building to improve flow away from the site. The additional Phase 2 work began on April 20, 2016 and includes constructing an earthen swale on the North side of the Operations Building to convey storm water away from the building and into an existing culvert, and a new PCC drainage swale along the west side of the Operations Building to address localized ponding of storm water. Based on the limited amount of rainfall during this period, it was not possible to evaluate the performance of the HDPE pipe culvert constructed at the Mira Loma Peaker Power Plant in handling a large volume of rainfall from a severe storm event. The overall slope of the pipes had to be limited to about a 0.6% slope due to a shallow 480V offsite electrical duct bank immediately adjacent to the access road where the culvert was to be placed. The potential impact of a relatively flat slope limits the velocity for storm water to flow at a rate high enough to prevent the retention basin from exceeding its capacity and potentially flooding the Operations Building. In the event that heavy rains were to be expected, the placement of gravel bags at the retention basin perimeter, southeast of the Operations Building, where the elevations are lower, was identified as a temporary mitigation measure. However, based on the amount of rainfall that occurred from January through April 30, 2016, this action was not necessary as the level of water within the retention basin never reached the HDPE pipe culvert invert elevation. CONCLUSIONS The implemented BMP repairs and/or improvements increased the performance and effectiveness of storm water conveyance and drainage systems, minimized and/or eliminated ponding of accumulated storm water, and should minimize potential erosion. These BMP improvements will mitigate possible damage around the power plants and to SCE assets from storm water. The installation of the new HDPE pipe culvert at Mira Loma Peaker should help mitigate damage from potential flooding of the Operations Building and other nearby SCE assets by improving the conveyance of storm water collected and then discharged from the existing retention basin to the open area south of the Peaker Site. This open area ultimately drains into an existing storm water concrete swale that surrounds the nearby Substation. However, the new HDPE pipe culvert is considered an interim improvement. As previously discussed, the overall slope of the pipes had to be limited to about a 0.6% which may limit the flow rate of storm water that needs to be discharged from the undersized retention basin (to prevent potential flooding) during heavy rains. The placement of gravel bags at the retention basin perimeter, southeast of the Operations Building where the elevations are lower, would then be use to protect the building from potential flooding, The concrete swale constructed along the north perimeter, and French Storm Drain system constructed along the eastern side of Center Peaker are two new BMPs that will improve the conveyance of storm water collected and discharged from the Plant. These BMPs should also improve and eliminate the localized ponding that occurs on the north and east sides of the Plant s perimeter. 9

10 The new concrete swales, regraded and resurfaced areas and concrete paved storm water discharge points at Grapeland Peaker Power Plant will improve the conveyance of storm water discharged from the Plant and minimize or eliminate localized ponding along the perimeter of the Plant. The new BMP improvements at Barre Peaker should minimize or eliminate the localized ponding in areas along the northern perimeter of the site, and improve the conveyance of storm water discharged offsite from the existing PCC swales. The remedial grading and addition of heavier rock cover at the southwest & southeast areas adjacent to the Sound Wall should help minimize soil erosion and should improve the discharge of storm water beyond the Sound Wall and into the natural southern retention area. The removal of small diameter rock and 12 inches of compacted subgrade material in the area south of the Visitor parking and replacement with larger 1½ to 2 inch rock should minimize localized ponding in this area. RECOMMENDATIONS It is recommended to continue inspecting and performing scheduled maintenance of BMPs to effectively control drainage, minimize flooding, reduce erosion, and prevent pollutants from entering storm water. Continued monitoring of the BMPs at Mira Loma Peaker is recommended. In the event that heavy rains are to be expected, the placement of gravel bags at the retention basin perimeter, southeast of the Operations Building where the elevations are lower, may be required as a temporary mitigation to protect the Operations Building from potential flooding. Further analysis to identify long term solutions for addressing the inadequate storage capacity of the retention basin is recommended. Further review of the site hydrology and entire drainage area, and assessment of the capacity of the existing retention basin are recommendations for Center Peaker. The performance of the BMP improvements at Grapeland Peaker and Barre Peaker Power Plants should be monitored during storm events, especially after severe rain events to assess the adequacy of the controls, and determine whether or not additional BMPs may be needed. The existing grades and slopes around these Plants are very flat, and as a result, the storm water doesn t efficiently exit the area. 10

11 FIGURES Figure 1. MVGS deteriorating geotextile material on storm water containment berms. Figure 2. MVGS rill erosion at Gas Yard access road gate. Figure 3. MVGS sediment accumulation in street gutter. Figure 4. Mira Loma Peaker shallow storm water retention basin bordering the Operations Building. Figure 5. Center Peaker ponding of collected storm water between sound wall and perimeter access road Figure 6. Center Peaker storm water retention basin. 11

12 Figure 7. Grapeland Peaker low grade causing ponding in parking lot. Figure 8. Grapeland Peaker Operations Building storm water ponding. Figure 9. MVGS new geotextile material protecting storm water berms. Figure 10. MVGS new Durawattles installed at unpaved frontage to control sediment. Figure 11. Mira Loma placement of HDPE pipes for emergency drainage from the retention basin. Figure 12. Mira Loma access road with concrete over drainage pipes. 12

13 Figure 13. Mira Loma retention basin new emergency drainage outlet. Figure 14. Center Peaker new PCC swale added to prevent ponding and convey storm water offsite to Substation Yard. Figure 15. Center Peaker perforated pipe installation for French drain. Figure 16. Center Peaker drain inlet for completed French drain system. Figure 17. Grapeland Peaker finishing of new PCC drainage swale outside northern site perimeter. Figure 18. Grapeland earthen swale regarding & resurfacing. Figure 19. Grapeland parking lot low point after regrading. Figure 20. Barre regraded North Gravel Area with 1½ to 2 in rock cover downstream of existing northeast PCC Swale 13