3.3 Stormwater Management 3.3 STORMWATER MANAGEMENT This section of the Technical Background Report describes the background, identifies general drainage patterns, existing and future conditions, and related issues in the planning area. The various sources used in the preparation of this chapter include county and local resources such as Ventura County Public Works Agency Watershed Protection District and City of Simi Valley. 3.3.1 Background The City of Simi Valley is located in the southeastern portion of Ventura County immediately adjacent to Los Angeles County. The developed portions of the City are situated primarily on the valley floor with proposed development extending up to the alluvial fans coming out of canyons. The valley is defined by the Santa Susana Mountains on the north and east and by the Simi Hills on the south. The Santa Susana Mountains separate the Simi Valley from the Santa Clara River Valley and Towns Fillmore and Piru to the north. The Simi Hills separate the valley from the City of Thousand Oaks to the southwest, and Moorpark Sphere of Influence separates the western limit.19 The major drainage course through the valley is the Arroyo Simi. The Arroyo Simi is approximately 11.7 miles in length. This major creek drains from the extreme limits of the watershed in the east and northeast, then westerly through the Las Posas Valley (As Arroyo Las Posas) to the Oxnard Plain (as Calleguas Creek) and the Pacific Ocean. Tributaries to Arroyo Simi from the Santa Susana Mountains on the north are (from west to east), Alamos Canyon, Brea Canyon, North Simi Drain, Dry Canyon, Tapo Canyon, Chivo Canyon, and Las Llajas Canyon. Canyons draining the Simi Hills from the south are Sycamore Canyon, Bus Canyon, Erringer Road Drain, Runkle Canyon, Meier Canyon, and finally Black Canyon in Santa Susana area.20 3.3.2 Existing Conditions The City of Simi Valley is located within the Calleguas Creek Watershed boundary. In order to identify the drainage pattern within the City, it is important to understand the Calleguas Creek Watershed hydrology and Simi Valley Master Plan of Drainage. Calleguas Creek Watershed Hydrology The Calleguas Creek drains an area of approximately 343 square miles, predominantly in southern Ventura County and outlets into the Pacific Ocean at Mugu Lagoon. The watersheds include Arroyo Conejo, Conejo Creek, Arroyo Santa Rosa, Arroyo Simi, Arroyo Las Posas, and Calleguas Creek, as well as Revolon Slough and Mugu Lagoon. The Santa Susana Mountains, South Mountain, and Oak Ridge Mountain form the northern boundary of the watershed. The Simi Hills and Santa Monica Mountains 19 Calleguas Creek Watershed Hydrology Study Present Condition, Ventura County Public Works Agency Watershed Protection District, March 2003. 20 Simi Valley Master Plan of Drainage, Hawkes & Associates, December 1990. Simi Valley General Plan Update Technical Background Report 3.3-1 Chapter 3 Infrastructure distinguish the southern boundary.21 Ventura County Watershed Protection District (VCWPD) formally known as the Ventura County Flood Control District, (VCFCD) provides administrative services for all flood control zones throughout the County. In addition, VCWPD provides monitoring of surface water flows, issuance of flood hazard reports, land development and building permit review, watercourse and encroachment permits, waterwell permits, and monitoring and implementation of the Flood Warning System.22 Based upon the Calleguas Creek Watershed Hydrology Study, peak discharges for the 100-year frequency storm event were determined with the Calleguas Creek Watershed Ventura County Modified Rational Method (VCRAT) model. A comparison of Present Condition flow rates (from March 2003 Present Condition Calleguas Creek Watershed Hydrology Study) to Future Condition (from April 2005 Future Condition Calleguas Creek Watershed Hydrology Study) with project flowrates at various key points in the watershed shows an average increase of 1.5 times discharge flowrates between the present condition and the projected future condition for Arroyo Simi. Simi Valley Master Plan of Drainage In addition to the Calleguas Creek Watershed Hydrology Study, The Simi Valley Master Plan of Drainage was reviewed. The valley floor is located in the approximate center of the study watershed, which is essentially rectangular shaped with an average east/west length of 9 miles and an average north/south width of 10 miles. The drainage area is defined as the entire 90 square miles Simi Valley drainage basin. The study area is located within the VCWPD Zone III. Excellent north/south ground slopes up to several percent permit storm runoffs to be rapidly conveyed to the Arroyo Simi, from which they are transported westerly out of the basin. The overall gradient in the east/west direction is generally sufficient for efficient conveyance of flows. However, it must be noted that in certain localized situations east/west slopes are quite flat, necessitating special attention during the future detail design of drainage facilities.23 In addition, undeveloped areas lying west of Madera Road will receive special evaluation as part of the updated Master Plan of Drainage and with the future development projects. Stormwater runoff calculations were made in accordance with the method outlined in the VCWPD Hydrology Manual dated in 1978. This method was a modification of the Rational Method developed over a period of years by the LACFCD and adapted for use in Ventura County by the VCWPD in 1975. That method has been adapted by the City of Simi Valley. Several flood control structures which significantly affect the amount of stormwater runoff passing to flood control and drainage facilities in the study area already exist or are contemplated for construction.24 The Tapo Canyon, Dry Canyon, White Oak Creek, Tapo Hills, Bus Canyon and Sycamore Canyon watersheds were also studied by the District and the associated times of concentration, rates of imperviousness, tributary areas, delineation and sizes of subareas, soils data and conveyance roting were incorporated in the hydrologic analysis to develop discharges for 10- and 100-year-frequency storms.25 21 Simi Valley Master Plan of Drainage, Hawkes & Associates, December 1990. 22 Ventura County Watershed Protection District Website www.vcwatershed.org. 23 Simi Valley Master Plan of Drainage, Hawkes & Associates, December 1990. 24 Calleguas Creek Watershed Hydrology Study Present Condition, Ventura County Public Works Agency Watershed Protection District, March 2003. 25 Simi Valley Master Plan of Drainage, Hawkes & Associates, December 1990. 3.3-2 Simi Valley General Plan Update Technical Background Report 3.3 Stormwater Management Wood Ranch Reservoir, an existing structure, also acts as a detention basin collecting flows generated in the extreme southwestern portion of the Sycamore Canyon watershed. Sycamore Canyon Dam was constructed near the eastern limit of the watershed in 1980 and is the largest of the detention basins in Simi Valley with a capacity of 660 acre-feet. It provides significant reduction in peak flows in the Arroyo Simi and downstream.26 Prior to incorporation of City of Simi Valley, the installation and maintenance of regional facilities and storm drain systems known as the Red Line Drainage System was performed by VCWPD and the secondary storm drain system was maintained by the “Subzone” of VCWPD. VCWPD facilities currently provide substantial flood protection at a regional level, establishing an effective base level system upon which to develop future planned local facilities. However, many of these facilities provide less than VCWPD standard 50-year protection.27 3.3.3 Future Conditions Proposed drainage facilities are sized to carry 10-year frequency storm runoff generated at the hydrologic concentration points and design standards established by the City of Simi Valley. The proposed facilities are planned to meet existing and projected needs for storm drainage from initial concentration points to the downstream confluence with regional flood control facilities. The proposed facilities include storm drain pipelines and channels including initial collection structures and inlet and outlet structures. In all cases where compatible with planned land uses, natural channels will be maintained if possible. In cases of industrial land use, where the planning of an improved conveyance structure is deemed appropriate, concrete rectangular open channels will be planned. In all other cases, conveyance structures will be planned as underground storm drain conduits in the form of reinforced concrete pipe up to a diameter of 72 inches and as rectangular concrete boxes for larger sizes.28 Inflation occurs since the Master Plan of Drainage was prepared, one accepted method of revising construction cost estimates is to utilize the Engineering News Record (ENR) Construction Cost Index published weekly in the Engineering News Record, a widely circulated construction periodical. The index is a reflection of the combined costs of basic construction materials and construction labor as sampled in twenty major cities throughout the United States.29 The 1990 ENR Construction Cost Index for twenty cities was 4702 and for Los Angeles was 5795. The 2007 ENR Construction Cost Index for twenty cities is 7856 and for Los Angeles is 8873.30 The current estimate for the proposed facilities would have an approximately 60 percent increase since the Study in 1990. Reevaluation of the City’s Storm Drain System is currently underway with the updated