Appendix K.3: Hydromodification Management Exemptions

Appendix K.3: Hydromodification Management Exemptions

Appendix K.3: Hydromodification Management Exemptions INTRODUCTION The purpose of this appendix is to identify areas within the South Orange County (San Juan Hydrologic Unit) Watershed Management Area where it is appropriate to allow Priority Development Projects to be exempt from the hydromodification management BMP performance requirements. Exempt areas are defined as areas that drain directly or via a continuously exempt flow path (i.e., not susceptible to hydromodification impacts or of negligible risk to hydromodification impacts) to a water storage reservoir, lake, enclosed embayment, or the Pacific Ocean. Per Provision E.3.c.(2)(d) of Permit Order R9-2015-0001: Each Copermittee has the discretion to exempt a Priority Development Project from the hydromodification management BMP performance requirements of Provisions E.3.c.(2) where the project discharges storm water runoff to: (i) (ii) (iii) Existing underground storm drains discharging directly to water storage reservoirs, lakes, enclosed embayments, or the Pacific Ocean; Conveyance channels whose bed and bank are concrete lined all the way from the point of discharge to water storage reservoirs, lakes, enclosed embayments, or the Pacific Ocean; or An area identified by the Copermittees as appropriate for an exemption by the optional Watershed Management Area Analysis incorporated into the Water Quality Improvement Plan pursuant to Provision B.3.b.(4). The South Orange County Copermittees are using their discretion to allow for the exemptions described above. The following sections provide justification for the exempt receiving waters within South Orange County. These receiving water exemptions are based on: (1) a detailed review of the exempt channels listed in Table 3-1 of the South Orange County Hydromodification Management Plan (HMP) (County of Orange, 2015); (2) confirmation of the exempt reservoirs listed in Table 3-2 of the HMP; and (3) a detailed review of the exempt large river reaches listed in Table 3-3 of the HMP, which are consistent with Provision E.3.c.(2)(d)(i) and (ii) and Provision E.3.c.(2)(e) of the permit. Exempt Channels Major storm drains, concrete lined conveyance channels, and engineered channels considered exempt were listed in Table 3-1 of the HMP. A subsequent detailed review of Table 3-1 has been conducted as part of the Watershed Management Area Analysis (WMAA) to define the material lining each channel based on: available storm drain GIS layers; aerial imagery review, using Google Earth; street level imagery review, using Google Maps Street View; and a field assessment of specific reaches of interest to fill data gaps. The result of this effort is a detailed channel bed and bank material map for South Orange County June 2018 Water Quality Improvement Plan 1

Appendix K.3: Hydromodification Management Exemptions selected channels listed in HMP Table 3-1 evaluated for a permanent exemption in the SOC WQIP. A vicinity map of these channels is provided in 1. Maps of each individual channel evaluated are provided in s 2 to 12. Based on the review, six of the eleven channels reevaluated in the WQIP from the HMP Table 3-1 are fully hardened and the other five have segments of erodible earthen bed or bank material. These include: San Juan Creek, North Creek, Segunda Deshecha Canada Channel, Trabuco Creek, and Trafalgar Storm Drain (outfall). San Juan Creek, between Caspar Park Road and the Pacific Ocean, is included as a large river exemption in HMP Table 3-3. Thus, this channel has been deleted from the list of exempt channels since the large river exemption includes the reach downstream of Caspar Park Road. The remaining four identified channels with segments of earthen channel (North Creek, Segunda Deshecha Canada Channel, Trabuco Creek and Trafalgar Storm Drain) have the potential to be susceptible to channel erosion due earthen banks and beds, however, their tributary watersheds are fully built out or are very close to that level, as shown in Table 1, thus reducing the potential for excess erosion beyond baseline conditions. Each of these channels shows signs of past instability 1, to varying degrees. However, field observations indicate that each channel has reached a state of quasi-equilibrium. Per the Southern California Channel Evolution Model (CEM) (SCCWRP, 2013), Type I and Type V channels on the top and bottom rows of 13 are considered to be in dynamic equilibrium. The CEM conditions observed in the field is summarized in Table 2. Considering the lack of additional development at buildout conditions and the relative stability of the current channels, these receiving waters have negligible likelihood of instream hydromodification impacts caused by future urban development and warrant an exemption. Attachment 1 of this Appendix provides the basis for designating negligible hydromodification impact based on cumulative future buildout. The levels of additional buildout development, summarized in Table 1, are below the threshold for impact, even if the additional development footprint were 100 percent impervious. 1 Instability occurs when the controls on channel form are perturbed to a point that the fluvial system must adjust to a new equilibrium channel form. South Orange County June 2018 Water Quality Improvement Plan 2

Appendix K.3: Hydromodification Management Exemptions Table 1. Tributary Land Use Conditions for Existing and Buildout Conditions in Channels of Interest Channel of Interest Tributary Area (sq mi) Existing Tributary Impervious Cover (%) Existing Tributary Urban Development (%) Future Build out Tributary Urban Development (%) % Increase in Tributary Development (Buildout Existing) North Creek 0.15 45.5 56.7 56.7 0.0 Segunda Deshecha 6.86 27.7 45.7 46.3 0.6 Trafalgar 0.51 55.8 72.2 72.2 0.0 Trabuco 0.9 51.7 77.7 77.7 0.0 Table 2. Southern California CEM Condition Observed for Channels of Interest Channel of Interest Southern California CEM Condition North Creek Segunda Deshecha Trafalgar Trabuco CEM Type V Quasi-equlibrium & Phase 1Veg Vegetated CEM Type V Quasi-equlibrium & Phase 5C - Constructed CEM Type V Quasi-equlibrium & Phase B5 Braided quasi-equilibrium CEM Type V Quasi-equlibrium & Phase 5C - Constructed Based on the detailed review performed, Table 3 below provides a summary for the justification of each channel exemption. Field photographs are provided in Attachment 2 for the five exempt channels which have earthen segments. South Orange County June 2018 Water Quality Improvement Plan 3

Appendix K.3: Hydromodification Management Exemptions Table 3. Summary for the Justification of Channel Exemptions Justification for Channel Exemption Channel Downstream Limit Upstream Limit Fully Hardened Dominant Coastal Processes Negligible Future Development in Watershed / Quasiequilibrium Sleepy Hollow Storm Drain Pacific Ocean Park Avenue X Bluebird Storm Drain Pacific Ocean Glenneyre Street X Salt Creek Channel Pacific Ocean 300 ft north of Pacific Coast Highway San Juan Creek Channel Pacific Ocean Paseo Michelle Prima Deshecha Canada Channel Pacific Ocean Portico del Norte and Diamante 1 North Creek Pacific Ocean Dana Harbor Drive 2 X Cascadita Canyon Storm Channel Prima Deshecha Canada Channel Via Cascadita X Segunda Deshecha Canada Channel Pacific Ocean Calle Frontera X Marquita Storm Channel Pacific Ocean Encino Lane X Trafalgar Storm Drain Pacific Ocean South Ola Vista X Trabuco Creek Confluence with San Juan Creek Avenida De La Vista 1 HMP Table 3-1 incorrectly indicated Avenida Vaquero as the upstream limit of Prima Desheca s channel exemption. The correct upstream limit is Portico del Norte and Diamante. 2 HMP Table 3-1 incorrectly indicated Doheny Park Road as the upstream limit of North Creek s channel exemption. The correct upstream limit is Dana Harbor Drive. X X X South Orange County June 2018 Water Quality Improvement Plan 4

Appendix K.3: Hydromodification Management Exemptions Exempt Reservoirs HMP Table 3-2 included a summary of exempt water storage reservoirs and lakes in South Orange County. These water bodies include Sulphur Creek Reservoir, El Toro Reservoir, Rancho Santa Margarita Lake, and Dove Canyon Lake, as shown in 1. No revisions are necessary to HMP Table 3-2. Exempt Large Rivers HMP Table 3-3 provided a summary of large river exemptions in South Orange County based on interim criteria. A subsequent detailed review of HMP Table 3-3 has been conducted as part of the Watershed Management Area Analysis (WMAA) to justify or appropriately modify the large river exemptions. A vicinity map of the two interim large river exemptions evaluated is provided in 1, which also includes the appropriate modifications to HMP Table 3-3. A Map of San Juan Creek is provided in 13. The large river exemption for San Mateo Creek is being eliminated from HMP Table 3-3 because no South Orange County development is anticipated to discharge directly to this river, which is situated entirely outside the WMA, in San Diego County. Thus, a San Mateo Creek exemption is considered unnecessary and no justification is provided. Justification for San Juan Creek s large river exemption is provided in the Rancho Mission Viejo (RMV) Runoff Management Plan (ROMP) (PACE et al, 2013). This reference is directly applicable given that the majority remaining urban development planned in the San Juan Creek watershed is associated with the RMV project. Chapter 15 (see Attachment 3 of Appendix K.3) of the ROMP states the following: The Conceptual WQMP [Water Quality Management Plan] (Geosyntec, 2004) established that, although the tributaries to San Juan Creek are susceptible to hydromodification impacts, the mainstem of San Juan Creek is not susceptible to hydromodification impacts from direct discharges from the planned Ranch Plan development. As stated in the Conceptual WQMP, San Juan Creek, given its watershed and channel characteristics, is considered to be able to accept additional flows without causing erosion. This conclusion was based on the watershed/sub-watershed-based hydrologic and fluvial geomorphologic studies and planning principles used to develop the Ranch Plan (Geosyntec, 2012). The resiliency of San Juan Creek is described by Balance Hydrologics (2005) as follows: The intrinsic resilience of these larger channels [San Juan Creek and lower Gabino channels] which have a very similar channel form as that reflected in 1938 aerial photographs or 1947 topographic maps (PCR [et al], 2002) is coupled with the proportionately smaller project-related discharges to minimize hungry-water effects on these two channels. In addition, these larger watersheds are inherently more dynamic systems than the side canyons, and better able to modulate the effects of any slight downstream erosion that may occur. South Orange County June 2018 Water Quality Improvement Plan 5

Appendix K.3: Hydromodification Management Exemptions Conclusion In conclusion, while the justifications for receiving water exemptions presented in this WMAA differs slightly from that provided in the South Orange County HMP, the extent of the exempt receiving waters is consistent with those used to create the HMP Exemption Maps ( 3-2 and Appendix F of the HMP). Thus these exemption maps are consistent with the E.3.c.(2)(d) exemption criteria and can continue to be used as a basis for evaluating whether hydromodification management requirements apply to areas in South Orange County. These Exemption Maps should be considered a living document to be updated by the Copermittees if more accurate information on drainage infrastructure and subcatchment delineation is obtained in the future. REFERENCES Balance Hydrologics, Inc. 2005. Geomorphologic Factors Affecting Sediment Generation and Transport Under Pre- and Post-Urbanization conditions at Rancho Mission Viejo and in the San Juan and San Mateo Watersheds, Orange County, California. June 2005. County of Orange. 2015. South Orange County Hydromodification Management Plan (HMP). OC Watersheds. April 1. County of Orange. 2016. Zoning Map. Planning Division. Geosyntec Consultants. 2004. Rancho Mission Viejo Conceptual Water Quality Management Plan. Prepared for Rancho Mission Viejo by Geosyntec Consultants, June 2004. Geosyntec Consultants. 2012. RMV ROMP Water Quality and Hydromodification Analysis, Memorandum. February 21, 2012. PACE, Geosyntec Consultants, and Huitt-Zollars. Rancho Mission Viejo Runoff Management Plan. 2013. Chapter 15: Rancho Mission Viejo Hydromodification Management Plan. PCR Services Corporation, Philip Williams and Associates, and Balance Hydrologics, Inc. 2002. Baseline Geomorphic and Hydrologic Conditions Report, Ranch Mission Viejo: Portions of the San Juan and Western San Mateo Watersheds, prepared for Rancho Mission Viejo. Southern California Association of Governments (SCAG). 2005. Land Use Map. South Orange County June 2018 Water Quality Improvement Plan 6

El Toro Reservoir Dove Canyon Lake Rancho Santa Margarita Lake Sleepy Hollow Storm Drain 2 Bluebird Storm Drain 3 Sulphur Creek Reservoir San Juan Creek 5 Salt Creek Channel 6 Trabuco 4 Prima Deshecha Canada Channel 7 Cascadita Canyon Storm Channel 9 North Creek 8 Segunda Deshecha Canada Channel 10 Marquita Storm Channel 11 Trafalgar Storm Drain 12 Reservoirs Underground Conduit Hardened Concrete Hardened Concrete 0 2.75 5.5 Miles Orange Channels and Rivers Evaluated for Hydromodification Exemption Vicinity Map County, California SW0266 March 2017 1 \\oldtown-01\data\projects\sw0266 - South Orange County WQIP B.3\Orange County NPDES Support\Project\Fig1_VicinityMap.mxd 3/28/2017 12:08:44 PM

Park Avenue Underground Conduit Hardened Concrete Hardened Concrete P:\GIS\Orange County NPDES Support\Project\Pace_Data.mxd 7/22/2016 5:34:39 PM Sleepy Hollow Storm Drain Pacific Ocean Park Avenue 0 75 150 225 300 Orange Feet Exempt Channels from Hydromodification Sleepy Hollow Storm Drain County, California SW0266 September 2016 2

Glenneyre Street Underground Conduit Hardened Concrete Bluebird Storm Drain Pacific Ocean Glenneyre Street Hardened Concrete 0 125 250 375 500 P:\GIS\Orange County NPDES Support\Project\Pace_Data.mxd 7/21/2016 3:58:40 PM Feet Exempt Channels from Hydromodification Bluebird Storm Drain Orange County, California SW0266 September 2016 3

Avenida De La Vista Trabuco Underground Conduit Hardened Concrete Trabuco Confluence with San Juan Creek Avenida De La Vista Hardenened Concrete 0 1,250 2,500 3,750 5,000 \\oldtown-01\data\projects\sw0266 - South Orange County WQIP B.3\Orange County NPDES Support\Project\Fig14_Trabuco.mxd 3/23/2017 10:42:47 AM Feet Exempt Rivers from Hydromodification Trabuco Creek Orange County, California SW0266 March 2017 4

Casper Park Road Paseo Michelle San Juan Creek is removed from the list of exempt channels (HMP Table 3-1). It is considered a large river exemption (HMP Table 3-3) because of its documented intrinsic resilience. Underground Conduit Hardened Concrete Hardened Concrete San Juan Creek Outfall to Pacific Ocean Caspar Park Road 0 1 2 3 4 Miles Orange Exempt Rivers from Hydromodification San Juan Creek County, California SW0266 September 2016 5 P:\GIS\Orange County NPDES Support\Project\Fig13_SanJuan.mxd 9/13/2016 2:21:54 PM

300 Ft North of Pacific Coast Highway Underground Conduit Hardened Concrete Hardened Concrete P:\GIS\Orange County NPDES Support\Project\Fig6_SaltCreek.mxd 9/2/2016 4:33:44 PM Salt Creek Channel Pacific Ocean 300 Ft North of Pacific Coast Highway 0 150 300 450 600 Orange Feet Exempt Rivers from Hydromodification Salt Creek Channel County, California SW0266 September 2016 6

Avenida Vaquero Underground Conduit Hardened Concrete Hardened Concrete P:\GIS\Orange County NPDES Support\Project\Fig7_PrimaDeshecha.mxd 9/13/2016 2:13:31 PM Prima Deshecha Canada Channel Pacific Ocean Diamante * * Limit not shown in map extents 0 1,000 2,000 3,000 4,000 Feet Orange Exempt Rivers from Hydromodification Prima Deshecha Canada Channel County, California SW0266 September 2016 7

Channel exemption supported based on lack of additional tributary urban development at buildout condition. Dana Harbor Drive Channel exemption supported based on lack of additional tributary urban development at buildout condition. Underground Conduit Hardened Concrete Hardened Concrete North Creek Pacific Ocean Dana Harbor Drive 0 300 600 900 1,200 Orange Feet Exempt Rivers from Hydromodification North Creek County, California SW0266 September 2016 8 P:\GIS\Orange County NPDES Support\Project\Fig8_NorthCreek.mxd 9/13/2016 2:15:34 PM

Via Cascadita Underground Conduit Hardened Concrete Hardened Concrete P:\GIS\Orange County NPDES Support\Project\Pace_Data.mxd 7/21/2016 4:32:10 PM Cascadita Canyon Storm Channel Prima Deshecha Canada Channel Via Cascadita 0 75 150 225 300 Orange Feet Exempt Channels from Hydromodification Cascadita Canyon Storm Channel County, California SW0266 September 2016 9

Calle Frontera Channel exemption supported based on lack of additional tributary urban development at buildout condition. Underground Conduit Hardened Concrete Hardened Concrete P:\GIS\Orange County NPDES Support\Project\Pace_Data.mxd 7/22/2016 6:07:05 PM Segunda Deshecha Canada Channel Pacific Ocean Calle Frontera 0 750 1,500 2,250 3,000 Orange Feet Exempt Channels from Hydromodification Segunda Deshecha Canada Channel County, California SW0266 September 2016 10

Encino Lane Underground Conduit Hardened Concrete Hardenened Concrete Marquita Storm Channel Pacific Ocean Encino Lane 0 100 200 300 400 Feet Orange Exempt Rivers from Hydromodification Marquita Storm Channel County, California SW0266 September 2016 11 P:\GIS\Orange County NPDES Support\Project\Fig11_Marquita.mxd 9/2/2016 4:47:10 PM

South Ola Vista Channel exemption supported based on lack of additional tributary urban development at buildout condition. Underground Conduit Hardened Concrete Hardened Concrete P:\GIS\Orange County NPDES Support\Project\Fig12_Trafalgar.mxd 9/2/2016 4:54:31 PM Trafalgar Storm Drain Pacific Ocean South Ola Vista 0 200 400 600 800 Orange Feet Exempt Rivers from Hydromodification Trafalgar Storm Drain County, California SW0266 September 2016 12

13: Southern California Channel Evolution Model (CEM) June 2018