3.12 HYDROLOGY AND WATER QUALITY EXECUTIVE SUMMARY This section describes the drainage features, stormwater quality, flooding hazards, and flood-protection improvements within the City’s Planning Area. Regulatory agencies governing stormwater quality and flooding hazards are also discussed. The City’s Planning Area is comprised of the City’s boundaries and adopted Sphere of Influence (SOI). The County’s Planning Area consists of unincorporated land within the One Valley One Vision (OVOV) Planning Area boundaries that is outside the City’s boundaries and adopted SOI. Together the City and County Planning Areas comprise the OVOV Planning Area. With implementation of the proposed General Plan goals, objectives, and policies potential impacts on hydrology and water quality would be less than significant. EXISTING CONDITIONS Surface Water Drainage Patterns within City’s Planning Area Surface water drainage patterns are dependent on topography, the amount and location of impervious surfaces, and the type of flood control that is located in an area. The size, or magnitude, of a flood is described by a term called a “recurrence interval.” By studying a long period of flow record for a stream, hydrologists estimate the size of a flood that would have a likelihood of occurring during various intervals. For example, a five-year flood event would occur, on the average, once every five years (and would have a 20 percent chance of occurring in any one year). Although a 100-year flood event is expected to happen only once in a century, there is a 1 percent chance that a flood of that size could happen during any year. The magnitude of flood events could be altered if changes are made to a drainage basin, such as by diversion of flow or increased flows generated by additional impervious surface area. Additionally, flood control would help manage the surface runoff that occurs in a particular area. The Federal Emergency Management Agency (FEMA) has mapped most of the flood risk areas within the United States as part of the National Flood Insurance Program (NFIP). Most communities with a 1 percent chance of a flood occurring in any given year have a floodway depicted on a Flood Insurance Rate Map (FIRM). Figure 3.12-1, 100-Year Flood Zone of the City’s Planning Area, shows the 100-year flood event boundaries for the major watercourses in the Planning Area. These areas are indicated on Figure 3.12-1 as Special Flood Hazard Areas (SFHA). These SFHA are generally located within and directly adjacent to the Santa Clara River and its tributaries, and along portions of Castaic Lake. Impact Sciences, Inc. 3.12-1 One Valley One Vision Draft Program EIR 0112.023 City of Santa Clarita September 2010 3.12 Hydrology and Water Quality The City‘s Planning Area contains many natural streams and creeks that function as storm drain channels, conveying surface runoff into the Santa Clara River. From its headwaters in the San Gabriel Mountains to its mouth at the Pacific Ocean, the Santa Clara River drains a watershed of 1,643 square miles, approximately 80 miles in length and about 25 miles in width. Ninety percent of the river’s watershed consists of mountainous terrain; the remaining portion is a mix of valley floor, floodplain, and coastal plain. Within the headwater areas of the City of Santa Clarita, discharge during rainfall events tends to be rapid due to the steep terrain. High intensity rainfalls, in combination with alluvial soils, sparse vegetation, erosion, and steep gradients, can result in significant debris-laden flash floods. The Santa Clara River and its tributary streams play a major part in moving the large volume of runoff generated from the Valley and surrounding foothills and mountains. The drainage system, including natural streams as well as constructed storm drain infrastructure within the City’s Planning Area, is adequate to handle normal precipitation in the region (8 to 22 inches per year). With the rapid urbanization of the Valley since 1960, stormwater volumes have increased due to increased impervious surface area from parking lots, rooftops, and streets. Flood control facilities have been constructed to mitigate the impacts of development on drainage patterns throughout the City’s Planning Area. Throughout the central portion of the Planning Area, some streams have been channelized into soft bottom channels with concrete sides to allow for development in the floodplain of the Santa Clara River. Rainfall The 100-year average annual rainfall within the City‘s Planning Area is between 8 and 22 inches. However, annual rainfall fluctuates from year to year as well as from site to site. Drainage Facilities Los Angeles County formed a special district to provide flood control services throughout the County and to enable the County to collect a fee for these services. The Los Angeles County Flood Control District (LACFCD) was formed by an act of the State Legislature. Parcels in the County of Los Angeles pay a fee on their annual tax bill for these services. Currently, the fee provides additional funding to meet the mandates of the Federal Clean Water Act as implemented through the National Pollutant Discharge Elimination System (NPDES) permits that are issued by the Regional Water Quality Control Board (RWQCB). The NPDES permit is the responsibility of the cities within the County and LACFCD. Projects are reviewed by the City of Santa Clarita for compliance with NPDES. Impact Sciences, Inc. 3.12-2 One Valley One Vision Draft Program EIR 0112.023 City of Santa Clarita September 2010 JO SO AVE L HN N RD W Lancaster T R S I D G H E T 0 R 7 O U COLUMBIA WY T E R D D R N O NY CA O IT U OAD Q S R IS E N C H RA EL G F IZA U AN BET Palmdale H S H LA KE K E R L A D k S ek PU e re N C K re c Y C i C F W a AN ish t Y e s ON ILL RD s a R D E H t D D B C O r G INTERSTATE a n P c i h ru 5 C a C li r fo ee rn k ia A q u e d u c NYO N RD t C A T UE UQ BO R ID Palmdale G E D OA R R O N YO U AN T C E O IT U R Q O S C I A AN G D FR O N OVOV Planning Area LD SA EN Upper Piru D R S ES t T H uc A HU G d T E e Legend: K u City of Santa E A L q D A A F O R R Clarita Boundary E s CASTAIC S e FEMA DFIRM E E l LAKE W H e G U g A H n County Boundary Y E n A K A o L SIERRA HWY y Special Flood Hazard Areas D RI n s R DG T E a o N H Mint Canyon SOI Boundary E L O R C O Y (SFHA) High-risk Zones L O N A D U Eastern D C R T o D t A E i E O CASTAIC C RO u R L A LAGOON U 100-Year Floodplain Special Flood Hazard D q D s Santa Felicia i N A U O LIFORN CA IA c Y G n N A A a C Areas (SFHA) Low r O C CONDID ANYON RD 14 GIS Projection - CA State Plane, Zone 5, NAD83, Feet. F ES T E U to Medium-risk Zones n Q U a n O o Source: City of Santa Clarita - Planning, S B RT y Y DAVE N PO R n A O D a A DA RT R City Boundary, 2007, Watersheds, 1999; W D VE NPO Es C RAILROAD AVE c H ondi QU G d o HIL VAS EZ I Thomas Bros., Hydrology, Waterbodies, LCREST PK CANY Watershed Boundary H WY ON H A RO S A RA and Streets; LA County - Planning, OVOV Boundary L D R (Calwater v. 2.2) EY SIE CA National Forests; FEMA: FIRM 100 Year Layer 2008. N Y C ON R OPPER HILL DR (Name) OAD E D U R D HAS L N A EY C RD N O AN E Y N S O O R V E Y E A L C L N L A O A S V C L Y A L A C N E O I P D W L L D H C U C M R K L M E A A D R P E OVOV Planning Area M NYO Y P N N N A A A NEWH D K C A RD O P E R Y O k A S E L NCH C R Y W L RA O e C O RO D O R R O A E e A N C N H E N r D LIFORN R N CA IA RD A E A R S NT C R F NYON A A R VE O C E S CA C R D DAD LA I Y E O A S E T B R O D OL c N R H C O N i T L N A E A City of Santa Clarita Boundary a E M L 14 E O FOR L E Y C t LI N Y CA IA R D A DA n s D O V D N o A R L CA T a I D y I V C E PE n U R NT O 126 A E L a O Q I SANTA C E H A V A MA SOI C I D G R R IC A RA R M ANYON RO I IL n L C A C R D OU AD D V OA o N C ED I D TA L E IF ls O IN O PAC S Y S R A e A PAR N N KW D M A BOULEVARD A Incorporated Areas RY I Y C R N C E N A U IO N N A H VAL VIA E I L PR N Y R IN Y A C LIFORNI O E CA A S O W S A Santa Clarita A INTERSTATE Angeles National Forest K D N R A Y A A 126 5 V W R P IG H O H A M N D D CBEA ROA A County Boundary R N O R E Y I N MAIN ST S A C Waterbody and Y IL E N ROAD W O T Y C ITTLE E AVENUE D A L AN LYONS O TA NY D P E I ON Perennial Stream IC C V C R RDPLACE A C A T O R I T O R K E A A W PLAC NY R U T O L E N J U S NEWHALL AVE I L L E N E R Y D Y R G A E S Aqueduct L T L A C V A N Y GIS Projection - CA State Plane, Zone 5, NAD83, Feet.