The information contained in this section is summarized from March, 1998
Hydrology and Hydraulics Study and the December, 1997 East Garden Grove -
Wintersburg Channel (COS) 100-Year Inundation Study prepared by Hunsaker
& Associates. The studies are provided as Technical Appendix F of the
EIR.
The natural direction of drainage of the project site is from the northeast to the southwest. A majority of the project site is at 0.07 percent slope. Exhibit 42 depicts the existing drainage system for the site and adjacent areas. Existing runoff from the site surface drains southwest; however, there are high points along the southwest boundary of the site which under a 100-year storm prevent flow from exiting the site.
For the purpose of the drainage analysis, a total of 186.9 (off-site) acres was included. The extra acreage is contained within four (4) of five (5) watershed areas tributary to the Graham Street storm drain system. A description of the watershed areas is provided below. Exhibit 43 identifies the site location and indicates the size of each watershed area tributary to the existing Graham Street storm drain system. All areas referred to below are reflected on Exhibit 43 .
The area surrounding the site consists of Area Nos. 1 and 2 to the north, and Area No. 5 to the east. These areas currently flow into the Graham Street storm drain system. It should be noted that Area 1 (Tract No. 10853) presently drains into the site at Greenleaf Lane and flows into an existing 60-inch RCP along the northern site boundary where it connects into the Graham Street storm drain. Additionally, Area No. 3, north of Warner Avenue, is tributary to this system and combines with the flow from Area No. 4 (Meadowlark Golf Course) to add to the flow in the Graham Street storm drain system.
The upstream end of the Graham Street storm drain system begins at the
intersection of Warner Avenue and Graham Street, where Area No. 3 (after
retaining some of its flow within Area No. 4) flows south on Graham Street.
The system picks up flows within Area No. 2 at Kenilworth Drive; Area No.
1, at 100± south of Kenilworth (from a 60-inch storm drain located
along the north property line of the site) (see Exhibit 42
); and Area No. 5, just south of the proposed entry of this site. The flow
from Area No. 5 enters the existing storm drain in Graham Street about 400
feet south of Kenilworth Drive. The combined flow from all these areas is
carried through the existing system under the East Garden Grove - Wintersburg
Channel and flows into the Slater Channel, which flows into the Slater Pump
Station and is ultimately pumped into the East Garden Grove - Wintersburg
Channel.
The existing condition runoff volumes for a 100-year storm event (Q100) for
the existing 186.9-acre drainage area and the 49-acre project site are shown
in Table W
. Advanced Engineering Software (AES) was utilized for estimation of the
flows for 100-year return frequency. The results of these calculations are
included in Appendix F under sections "100-year Off-Site or On-Site Hydrology"
for existing and ultimate conditions. Water Surface Hydraulic Gradient "WSPG"
was utilized for hydraulic calculations and the results and an explanation
of methodology is included in Appendix F under title "Hydraulic Calculations".
The existing drainage area boundaries and node numbers which relate to the
calculations in the drainage analysis are shown in Section 3, which is contained
in Appendix F of the EIR.
The National Flood Insurance Program (NFIP) prepares Flood Insurance Rate Maps (FIRMS) which depict flood hazard areas. The federal program enables property owners to purchase flood insurance based on identified flood hazards in the area. The FIRM map for the area shows that the project site is located within the A99 Flood Zone (Protected by Federal Project under Construction). This A99 Flood Zone is the result of a revision to the previous zone AO. This revision was made by the Federal Emergency Management Agency (FEMA) on April 30, 1996. The Zone A99 designation is used to identify areas that are protected by a Federal flood protection system under construction from a flood having a 1-percent chance of being equaled or exceeded in any given year (base flood) with no base flood elevations (BFEs) determined. The revision to the zone is based on construction of the Santa Ana River Mainstem flood control project, which includes two critical features — channel and bridge widening and channelization of the Lower Santa Ana River Channel Reaches 1 through 4, and construction of the Seven Oaks Dam. The existing residential uses north of the project site currently experience flooding due to existing deficiencies in the existing Graham Street storm drain system.
Historically, the East Garden Grove - Wintersburg Channel has reached
capacity during larger rain storms in the area according to City staff. Based
on an analysis performed by Hunsaker & Associates and contained in Appendix
F, the East Garden Grove - Wintersburg Channel may experience overtopping
in the area from Goldenwest Street westerly to Warner Avenue during a 100-year
storm event. The computer program "HEC-2, Water Surface Profiles" by the
US Army Corps of Engineers was used in this analysis for computing the 100-year
expected water surface evaluation. A split-flow option in the HEC-2 Program
was utilized for computing the runoff amounts which overtop the existing
COS Channel in the 100-year expected peak discharge. For computation procedures,
see HEC-2 Water Surface Profiles User Manual, September 1990, and Application
of the HEC-2 Split Flow Option Training document No. 18, April 1982.
The results of the HEC-2 computer study on a 100-year flood event on a fully
developed East Garden Grove - Wintersburg Channel watershed are described
below:
Water quality in California is regulated by the US Environmental Protection
Agency's National Pollution Discharge Elimination System (NPDES), which controls
the discharge of pollutants to water bodies from point and non-point sources.
NPDES permits are required for any commercial and/or industrial construction
sites. As stated above, the existing site is currently undeveloped. The site
has also been in agricultural production since the 1950's. It is anticipated
that the existing runoff (minimal) from the site contains concentrations
of fertilizers and pesticides associated with the past agricultural uses.
These include particulate solids (total suspended solids), nutrients (total
nitrogen compounds and phosphates) and oxygen demanding substances (BOD).
Appendix G of the CEQA Guidelines serves as a guideline/general example
of consequences that are deemed to have a significant effect on the environment.
A project may be deemed to have a significant effect if it will:
For the purposes of this EIR, a significant impact would occur if implementation
of the proposed project would cause or expose people and property to substantial
flooding or make worse existing drainage deficiency problems. The impacts
related to the above stated criteria are discussed below. Additionally, a
significant impact would occur if implementation of the project would cause
a substantial degradation of water quality.
Implementation of the proposed project will alter existing drainage patterns on-site. Surface flows will be redirected to allow for the development of the residential project. The proposed drainage system will convey runoff from the site (approximately 49 acres) through new storm drain lines directly to the existing Slater Pump Station. Beneficial impacts will occur to the Graham Street storm drain system and are discussed below. The proposed Q100 figures for the drainage area of the project site are presented in Table W . The hydraulic calculation results and methodology are included in Appendix F under the title "Hydraulic Calculations". Under a 100-year storm event, the proposed project will result in a total flow increase of 126.1 cfs.
Increased project runoff will be affected by the introduction of an additional 20.5 (50% of residential acreage) acres of impermeable surface areas such as streets, residential driveways, and building pads. Additionally, the proposed condition will include intercepted off-site flows (472.5 cfs) from the Graham Street storm drain. Under a 100 year storm event the proposed project will result in a total increase of 126.1 cfs into the Slater Pump Station. Because the existing areas north of the project currently experience drainage deficiencies, this increase is considered to be significant. The existing facilities are currently not adequate to handle existing runoff and therefore are not adequate to accommodate the increase in project runoff. This is considered a significant impact. An expanded discussion of the necessary mitigation is provided below.
The required hydrology analysis performed by Hunsaker & Associates to determine the increase in storm flow into the Slater Pump Station as a result of the development of this site is discussed below. Several methods were used to determine this increase. The different methods are described in Appendix F of the EIR. Please refer to Section 6.7 to Section 6.10 , which includes the updated hydrologic study, "FEMA Detailed Flood Insurance Study" prepared by Exponent. The following describes the method which provides a more conservative estimate, which yields the most realistic solution.
The flows that were determined in the hydrology study to enter the site from Graham Street (Area Nos. 2, 3, and 4 = 405.5 cfs) were taken along with the flows from Greenleaf Lane (Area No. 1 = 77.3 cfs) and traveled the Greenleaf Lane flows over to Graham Street, where they were confluenced based on their times of concentration. This resulted in a peak flow rate of 472.5 cfs (for Area Nos. 1, 2, 3 and 4). When this is subtracted from the total peak flow that the entire drainage area, including this site, discharges to the Slater Pump Station (Area Nos. 1, 2, 3, 4 and the site = 598.6 cfs), the result is an increase of 126.1 cfs. The normal peak efficiency of each pump in the Slater Pump Station is about 66,000 gpm (147 cfs). Therefore, one additional pump this size should be more than able to offset any increase in flow caused by the development of this site.
Preliminary pipe sizes required to convey calculated 100-year flows are shown on Exhibit 42 of this EIR and with the implementation of the on-site storm drain system (described below) on-site flows are adequately handled. A proposed 60-inch new storm drain line will intercept flows from Watershed Area 1, thereby eliminating off-site flows of 77 cfs into the Graham Street system. The proposed 102-inch storm drain line (parallel to the existing 60-inch RCP in Graham Street) from Kenilworth south to "A" Street and then through the site will convey all off-site flows from Areas 2-4 (472.5 cfs). These lines will connect to a 120-inch storm drain line on-site which will connect to the Slater Pump Station via a RC box under the C05. These lines with their flow interceptions eliminate the existing deficiencies in the Graham Street storm drain.
The above improvements have been required as mitigation for the proposed
project. Implementation of these improvements will mitigate proposed drainage
impacts to a less than significant level.
The proposed project is located within a flood hazard area. Due to the
location of the project site within the A99 zone which is not subject to
NFIP development standards and the results of the Hunsaker & Associates
Channel Inundation Study, significant flooding impacts to the project are
not anticipated. The results of the Channel Inundation Study indicate that
under the future with project condition, no change occurs in the current
condition of the channel upstream of Graham Street. Additionally, implementation
of Mitigation Measure 1 which required the above discussed drainage improvements
(including improvement to C05 along the site) will further reduce any potential
flooding impact to a level less than significant. Additionally, please refer
to Section 6.7
to Section 6.10
, which includes the updated hydrologic study, "FEMA Detailed Flood Insurance Study" prepared by Exponent.
The proposed project has the potential to result in a long-term impact on water quality due to the addition of pollutants typical of urban runoff. Volatile solids in urban runoff can originate: from accidental spills or deliberate dumping of lubricating oils or fuel oils; from emissions of engines during normal operations such as vehicle exhaust particulates or drippings of crankcase oil; from dustfall or rainout of atmospheric particulates; from spilling of crude or refined petroleum products; from leached or eroded pavement; from natural seepage on land; or from natural biogenic sources. The proposed project has the potential to result in an impact on water quality due to the addition of volatile solids to the runoff.
Stormwater flows from the future buildout of the residential project will
be subject to the NPDES permit process. Through the NPDES Permit process,
the City currently requires contributors to non-point runoff pollution to
establish Best Management Practices (BMPs) to minimize the potential for
pollution. Under this program, the developer is responsible for identification
and implementation of a program of BMPs which can include special scheduling
of project activities, prohibitions of certain practices, establishment of
certain maintenance procedures, and other management practices to prevent
or reduce the pollution of downstream waters. Typical elements of such a
BMP program would include addressing the use of oil and grease traps, detention
basins, vegetated filter strips, and other common techniques in order to
preclude discharge of pollutants to local storm drains and channels. Mitigation
Measures 2 and 3 will reduce potential water quality impacts to a less than
significant level.
The proposed project, in conjunction with other past, present, and reasonably foreseeable future projects will result in a cumulative impact related to flooding. The project's incremental contribution to this impact can be mitigated to a level less than significant. Water runoff will cumulatively increase due to the introduction of impervious surfaces. The proposed mitigation measures will reduce the project's incremental cumulative impact to a level less than significant.
Buildout of the proposed project in conjunction with future related projects
will incrementally contribute to a cumulative increase in the total amount
of surface runoff erosion and water quality impacts. Construction related
activities that require grading and vegetation removal will increase runoff,
causing greater erosion and downstream siltation. Implementation of proposed
mitigation and standard City policies and requirements will reduce the project's
incremental contribution to cumulative impacts to a level less than significant.
With implementation of standard City policies and requirements and proposed Mitigation Measures 1, the potential impacts to drainage will be reduced to a level less than significant.
With implementation of standard City policies and requirements and proposed Mitigation Measures 1, the potential impacts associated with flooding will be reduced to a level less than significant.
With implementation of standard City policies and requirements and proposed Mitigation Measures 2 and 3, the potential impacts to water quality will be reduced to a level less than significant.
Implementation of proposed Mitigation Measures 1 through 3 and standard City policies and requirements will reduce the project's contribution to potential cumulative drainage, flooding, and water quality impacts to a level less than significant.
