Lincoln 270 City of Lincoln Stormwater Management Plan April 2, 2013 # 2005.48 Prepared By: Civil Engineering Solutions, Inc. 590 E Street Lincoln, Ca 95648 (916) 645 5700
1.0 Background: The project site is located in the City of Lincoln in Placer County, California. The project site lies within the Orchard Creek drainage shed and is located South of the Lincoln Crossings development, extending to Athens Avenue with Highway 65 to the East and Industrial Road to the West. The proposed development consists of business professional, commercial, medical campus, paved parking with driveways, landscaping, open space and existing open areas totaling to approximately 275 acres. The Project area is located in four drainage basins: 1. Orchard Creek Tributary #3: The drainage basin is located in the northern portion of the Project area and consists of approximately 135 Project areas. Tributary #3 crosses under the existing culvert at State Route 65 and drains southwest within the Project area, underneath Twelve Bridges Drive and discharges to the Orchard Creek Main Channel upstream of the existing culvert crossing under Industrial Avenue. The project development areas will occur along the right bank of Tributary #3, upstream of the 12 Bridges Drive crossing and along both the right and left banks, downstream of 12 bridges drive. 2. Orchard Creek Main Channel: The drainage basin is located in the middle portion of the Project area and consists of approximately 48 Project acres. The Main Channel crosses under the State Route 65 existing culvert south of Twelve Bridges Drive and drains west within the Project area to an existing culvert crossing under Industrial Boulevard and the Union Pacific Railroad. The project development areas will occur north of the right bank of the Orchard Creek Main Channel, through the project areas. No development will occur south of the Main Channel. 3. Orchard Creek Tributary 2-1: The drainage basins located in the southern portion of the Project area consist of approximately 40 Project acres. Tributary #2-1 crosses under the State Route 65 culvert and drains west within the Project area to an existing culvert crossing under Industrial Avenue. No development is proposed for the project lands at Orchard Creek Tributary 2-1. 4. Orchard Creek Tributary #2: The drainage basin located in the southwest portion of the Project area consists of approximately 52 Project acres. Tributary #2 drains north into the southwest end of the Project area and then west to meet with Orchard Creek Tributary #2-2 which is also draining from the south along Industrial Avenue before leaving the Project site at an existing culvert crossing under Industrial Avenue. No development is proposed for the project lands near Orchard Creek Tributary #2.
2.0 Hydrology: This section summarizes the methodology used to develop the hydrologic model for the study area. The modeling was performed using the United States Army Corps of Engineers (USACE) Hydrologic Engineering Center s program HEC-1. Existing and post-condition models were developed. Flows were computed for the 2-year through 500-year 24-hour storm events. Pre-project and post-project watershed limits and flow directions are shown on exhibits SH-1 and SH-2 respectively. 2.0.1 Rainfall Frequency and Duration Precipitation input into the HEC-1 models was developed using the Placer County Design Precipitation Program (PDP2). This program uses elevation and subbasin centroid x and y coordinates to center a storm with a given return period over a specified subbasin, which produces HEC-1 precipitation input (PI cards) for each subbasin in the model. A 24-hour rainfall, in 5 minute increments was developed using the PDP program, and input into a HEC-1 basin calculation. The PDP program includes the peak incremental rainfall that occurs. Since the Project drainage subbasins studied have a relatively short time of concentration, the peak rainfall and peak runoff occurs within the 24-hour storm event. A storm event longer than 24 hours will not increase peak runoff values. 2.0.2 Land Use The project site is proposed to be developed into subdivisions consisting of business professional, commercial, industrial, medical campus, paved parking with driveways, landscaping, open space and existing open areas. With Hydrologic Soil Group D being the underlying dominant soil condition, the Placer County Stormwater Management Manual was utilized to obtain land use values such as infiltration losses (constant and initial) and impervious cover for pre-project and post-project conditions. These parameters are used to determine precipitation loss due to interception and infiltration. 2.0.3 Hydrograph Transformation and Channel Routing Channel lengths and slopes were measured and calculated from tentative maps provided by UBORA, Engineering and Planning. The kinematic wave method was used to transform the excess rainfall into runoff. The subbasin flow from one location was routed using the Muskingum-Cunge and Kinematic Wave methods and combined from upstream to downstream.
2.0.4 South Lincoln Master Drainage Plan The project s parameters were input into the City s regional South Lincoln Master Plan model for Auburn Ravine, Ingram Slough and Orchard Creek (SLMP-AIO). Watersheds in the area of the project were corrected and subdivided for local features in the updated analysis The South Lincoln Master Drainage Plan (SLMP) previously studied the regional impacts of development in the South Lincoln Areas, including Orchard Creek, Ingram Slough and Auburn Ravine. This project is located along Tributary 3, and the Main Channel of Orchard Creek. The property also includes segments of tributaries 2, 2-1, and 2-2, but these tributaries will not be affected hydraulically by the project. The SLMP assumed the undeveloped condition for this property as at the time it was outside the City Limits and study areas. The SLMP maximized the amount of detention within the Main tributary and tributary 3 of Orchard Creek, so that the combined hydrographs of Orchard Creek, and Ingram Slough would not be timed at peak discharge so as to worsen the conditions in Auburn Ravine. Actually a net reduction in peak flows was predicted, as required. The optimization of the amount of detention placed in the two Orchard Creek tributaries, was such that if additional detention of the peak flows were proposed, a concern over potential coincidence of the peak flows of Orchard Creek and Auburn Ravine might be realized (a timing issue) which could result in an increase to peak flow rates in Auburn Ravine downstream of the City of Lincoln. A copy of the South Lincoln Master Drainage Plan can be downloaded from: http://www.civilsolutions.com/workspaces/lincoln/slmp/ The analysis for this project indicates, that the development of the project may increase flows within tributary 3 of Orchard Creeks within the project limits. However, the preliminary hydrology analysis indicates that no increases within the Main Channel of Orchard Creek, onsite or downstream of the project will occur, including Auburn Ravine. As this project will be able to demonstrate through this type of analysis that no downstream impacts to peak flows would result from the development, it is not recommend detention be placed with this project for the purpose of peak flow mitigation in order to avoid peak flow impacts to hydrograph timing at Auburn Ravine, downstream of the confluence with Orchard Creek. TABLE 2.0.4A Pre-Project Peak Flow Rates(cfs):
TABLE 2.0.4B Peak Flow Rates(cfs) with Lincoln 270 Without Mitigation: TABLE 2.0.4C Peak Flow Rates(cfs) Changes, Pre- to Post-project: 2.0.5 Volumetric Impacts The project s addition of impervious surfaces has the potential to increase the total volume of runoff from the project during a flood event, exacerbating the existing flooding problems of the Cross Canal Sump in Sutter County. As a result, the City of Lincoln has agreed to mitigate runoff volume impacts for the 8- day 100-year event, by storing runoff volume increases in a regional retention facility. Currently, the City maintains an operational facility near the City s wastewater treatment plant at Fiddyment Road. The City is also building a new facility at Lakeview Farms at Coon Creek. The City will collect a development fee from the project for these facilities and has included the project s potential impacts (based on land use), in the design of those facilities.
3.0 Hydraulics: 3.0.1 Main Channel Orchard Creek The project does not propose to have any hydraulic impacts to the 100-year floodplain of the Main Channel of Orchard Creek. No fill is proposed within the 100-year floodplain boundary of this channel. 3.0.2 Tributary 3 Orchard Creek Floodplain Fringe fill improvements are proposed along tributary 3 of Orchard Creek. Both sides of Tributary 3 south of Twelve Bridges Drive would be impacted, and the west side of Tributary 3 would be impacted between State Route 65 and Twelve Bridges Drive. Our hydraulic evaluation of this reach indicates that an increase in peak 100-year water surface elevations would occur upstream of the project, if these fills were placed without mitigation. The project will add conveyance capacity(excavated overbank) within the areas shown on Exhibit DR-1 as OFFSET CONVEYANCE CREATION. Example cross sections at these locations area also shown on this exhibit, which identify the overbank excavation depths. The hydraulic analysis for the project demonstrates that the proposed excavations will reduce water surface elevations through the project such that a no adverse impact solution is obtained and no offsite properties will be impacted by increased water surface elevations in the 100-year event, resulting from the proposed project fills within the floodplain. 3.0.3 12 Bridges Drive Bridge Modifications A bridge replacement for the widening of 12 bridges drive will occur in the future, not as a part of this project. It is expected that the bridge will be widened and lengthened in that effort. Those improvements are not considered a part of this project and have not been factored into the project hydraulic analysis. It is assumed that project which will have impacts on the regulatory floodway, will be required to be designed for a no adverse impact condition.
4.0 Stormwater Quality: The Lincoln 270 project will install improvements that comply with the City of Lincoln s Stormwater Quality criteria. The City of Lincoln is a Phase II community and subject to the State s MS4 permit. The State Water Quality Control Board has drafted new requirements for storm water management permits administered under the Nationwide Pollution Discharge Elimination System (NPDES) regulations. The new MS4 permit will become effective July 1, 2013, and is posted at: http://www.waterboards.ca.gov/water_issues/programs/stormwater/phase_ii_municipal.s html 4.0.1 Stormwater Management During Construction Activities The release of on-site stormwater runoff during Construction activities is regulated by the State General Construction Permit issued by the Regional Water Quality Control Board for all commercial and residential construction sites greater than one acre. The General Construction permit requires that a Storm Water Pollution Prevention Plan (SWPPP) be developed and implemented to prevent the transport of pollution and sediments from the site by runoff. The SWPPP identifies the Best Management Practices (BMPs) that will be implemented during the construction process. Erosion and sediment control BMPs typically include such things as applying straw mulch to disturbed areas, the use of fiber rolls and silt fences, sedimentation basins, drain inlet protection, stabilized construction accesses, and construction equipment fuel and maintenance requirements. The final sizing and selection of BMPs will consider requirements specific to the Orchard Creek watershed and proposed construction activities. A Stormwater Pollution Prevention Plan (SWPPP) will be required to describe the BMPs which will be used to prevent erosion and to clean site discharge waters before entering State Waters. A permit with the Central Valley Regional Water Quality Control Board of the State of California will be obtained for the proposed construction activities. If construction occurs during the wet season, additional winterization improvements will be required to stabilize the disturbed areas of the site, prevent erosion, and clean discharge waters. All construction related BMP improvements must comply with the NPDES General Permit for Storm Water Discharges Associated with Construction Activities, NPDES No. CAS000002, Order No. 99-08DWQ.
4.0.2 Post-Construction (Permanent) Stormwater Management Exhibit DR-1 shows the drainage management areas (watersheds) for the postproject conditions of the project. Post construction stormwater management is intended to treat in perpetuity the urban runoff generated on-site. The BMP techniques within the project area will reduce and/or eliminate the pollutants from the urban stormwater runoff and prevent the contamination of receiving waters. Lincoln 270 will apply the then current permit criteria applicable at the time of development and in conformance with the City of Lincoln Improvement Standards, the Placer County Flood Control Agency s Stormwater Management Manual, the open space preserve Operations and Maintenance (O&M) Plan, to design and address post construction stormwater treatment. Post construction stormwater treatment is composed of three general elements: source control, runoff reduction and treatment of runoff. All three elements will be used in the Lincoln 270 stormwater management plan. The basic practice of source control is to minimize the potential for constituents to enter runoff at the source. An example of a source control BMP would be stamping of drainage inlets to inform residents that waters flow to the Creeks. Examples of typical Source Control BMPs that may be applied at this project are included in the attached document sourcebmps.pdf The tool the project will employ towards the goal of runoff reduction, is the use of Low Impact Development (LID) measures. Implementation of LID includes the construction of decentralized small scale improvements that provide for local infiltration and treatment opportunities that reduce the quantity of runoff which enters the storm drain systems during a rainfall event. LID will be implemented to offset runoff increases that occur when development converts native ground to impervious cover. Additional Treatment control BMPs will be located at the end of the pipe and provide further treatment of the stormwater before it enters into the natural creek system, including: Grassy swales; and, Structural or proprietary BMPs The final selection of Best Management Practices (BMPs) shall consider requirements specific to the Orchard Creek and Auburn Ravine watersheds. Examples of flow based and volume based LID/BMPs that may be employed by this project are included in the attached documents: flowbmps.pdf and volumebmps.pdf respectively.
4.0.3 Low Impact Development Measures (LID) Low impact development (LID) is an approach to stormwater management that emphasizes the use of small-scale, natural, constructed and proprietary drainage features integrated throughout a development site. The intent of LID measures is to slow, clean, infiltrate and evapo-transpire runoff, to reduce the quantity of urban runoff entering the storm drain systems. The added opportunities for infiltration offered by the use of LID can add water to local aquifers, increasing water reuse. It is a sustainable practice that benefits water quality protection, stream stability and can contribute to water supply. Unlike traditional storm water management, which collects and conveys storm water runoff through storm drains, pipes, or other conveyances to a centralized storm water facility, LID within Lincoln 270 will use site design elements to minimize changes to the site s pre-development runoff rates and volumes. Lincoln 270 s LID elements will assist with the goal of optimizing to the site s predevelopment hydrology by using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to where it originates. Key principles of low impact development include: Decentralize and manage urban runoff to integrate storm water management throughout the watershed Preserve the ecosystem s natural hydrological functions and cycles. Account for a site s topographic features in its design. Reduce directly connected impervious surfaces to slow runoff and provide additional infiltration opportunities. Reduce impervious ground cover and maximize infiltration on-site. Examples of LID measures which may be used in this project are described in TABLE 4.0.3. This is only a partial list of the types of measures which may be selected for this project. TABLE 4.0.3 Applicable LID Measures: LID Measure Descriptions Benefits Description Disconnected roof drains * Pervious or partially paved driveways & Porous pavement areas, and soil confinement Separated sidewalks & Pavement Disconnection and eliminated pavement Water running off of the impervious roof system is treated by biological filtration, and the runoff gains an opportunity to partially infiltrate. Pavement alternatives offer the opportunity for partial or complete infiltration of runoff. Runoff from the impervious sidewalk, driveway, and pavement areas can be treated and infiltrated in landscape areas before entering the gutter pan and storm drain systems. (including residential walkways) In some areas of the development, un-necessary pavement may also be eliminated for stormwater benefit.
Tree Planting and Canopy Preservation Soil amendments in landscaped areas and Storm water planters. Stream Buffer Vegetated Swales Stormwater Retention The creation and preservation of tree canopy reduces the rate and amount of total runoff which enters the storm drain systems. The addition of organic material to impervious soils can add voids which can absorb runoff preventing it from entering storm drain systems. In residential areas, this may include amending a landscape strip adjacent to the street or pavement areas where large amounts of runoff can be intercepted from the lots. In commercial areas this is likely to be limited to stormwater planter areas. At roadways this will be used where roadway flows are diverted into the landscape areas. Sheet flows can be discharged into the stream corridors (at the surface overbank) directly providing significant treatment and infiltration opportunity prior to entering the streams. Discharge of runoff into vegetated swales provides additional treatment in the in the treatment train, and opportunities for additional infiltration of runoff waters These measures remove stormwater from the system, and trap constituents at the stormwater retention location such that it is not discharged. * The use of pervious pavement and other infiltration oriented paving systems are dependant on infiltration capacity of the underlying soils, and may not be used everywhere. Geotechnical investigations are necessary to support the use of these systems.
4.0.4 End of Pipe Treatment The schematic Post-Project Drainage Systems Map is shown in Exhibits DR-1. The map identifies storm drain outfall locations. Treatment Facilities (BMPs) will be required upstream of discharge to Orchard Creek or any other Regulated Water of the State such as wetlands. Based on the plan shown in DR-1, treatment consisting of a section of Vegetated Swale will be the most common form of outfall BMP. The optimum design length of Vegetated grassy swale and the associated calculations are provided in TABLE 4.0.4. The design lengths represented in this table assume complete treatment via the Vegetated Grassy Swale, as if LID and source control measures were ineffective. In final design it would be permissible to apply benefits for the LID constructed with the site improvements, to reduce the sizing for the treatment swale. To comply with the requirements of the local Mosquito/Vector Abatement District, all BMP s will be required to be designed to discharge all waters within 96-hours of the completion of runoff from a storm event. All graded areas must drain so that no standing water could accumulate for more than 96-hours within water quality facilities. Information regarding the storm drain watershed areas tributary to each outfall location are included on exhibit DR-1. TABLE 4.0.4 Minimum Vegetated Swale Sizing at Outfalls: