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32891 Parcel 1DRAINAGE STUDY AND HYDRAULIC CALCULATIONS PREPARED FOR: PRECISE GRADING PLAN PARCEL 1 OF PARCEL MAP 32891 GWN PROPERTIES LLC LA OUINTA, CA PREPARED BY: LAND ENGINEERING CONSULTANTS, INC. P.O. BOX 541, 650 AVENUE K CALIMESA, CA. 92320 PH: (909) 795-8882 STEVEN H. RITCHEY R.C.E. 51129 7//z:/7/6 DA DA E DISCUSSION INTRODUCTION This study has been prepared in support of a Precise Grading Plan for a proposed single family residential parcel and is located approximately 1100 feet easterly from the intersection of Coyote Canyon Ct. and Citation Ct. in the City of La Quinta. The purpose of this study is to determine the rate of storm water runoff from the steep rocky hillside to the east that drains through this property for the design of related drainage facilities for this project, to detain/minimize that runoff, to provide for a desilting/debris basin to eliminate any sediment in the street, and to address any water quality conditions of concern. EXISTING WATERSHED DESCRIPTION There is approximately 1.14 acres of tributary off-site area east of this development which consists of a steep rocky hillside. This property is currently vacant and being maintained with grass vegetation on a regular basis. The entire site plus the tributary areas total approximately 1.44 acres that sheet flow westerly toward Peerless Place and are then collected in the street and conveyed northerly toward about 600 feet to a catch basins on both sides of the street. The total 100 year 1 hour runoff generated from the tributary off-site area was determined to be 5.46 cfs in its existing condition. METHODOLOGY Rational Method Hydrology Rational Method Hydrology calculations were undertaken in conformance with the Riverside County Flood Control & Water Conservation District Hydrology Manual dated April 1978. These calculations are based on a 1 hour duration storm and used for the design of all on-site drainage facilities. The following scenarios have been modeled: Development Condition: 100 -year frequency storm Rainfall depth was derived from the precipitation frequency Atlas, NOAA Atlas 14. Rational method computations were performed using Civilcadd/Civildesign Engineering Software, ver. 7.1, based on the Hydrology Manual. Intensity, runoff coefficients, and discharge were calculated at each node by the software, based on user input of rainfall, soil type, acreage, and land use parameters. The rational method calculations, as well as applicable plates from the Manual, are included in this report. Unit Hydrograph Method Hydrology Unit hydrograph method calculations were also undertaken in conformance with the Riverside County Flood Control & Water Conservation District Hydrology Manual dated April 1978. These calculations are based on a 100 year frequency storm and used for the design of the detention basins. The following scenarios have been modeled: Existing Condition: 1 -hour, 3 -hour, 6 -hour, 24-hour duration storms The unit hydrograph method calculations, as well as applicable plates from the Manual, are included in this report. PROPOSED WATERSHED DESCRIPTION This project proposes to develop one single family home with driveways, patio areas and an underground drainage system used to convey storm flows through the site. To protect this home from the steep rocky hillside to the east, a rock fall wall is proposed to prevent falling rocks and debris from coming in contact with the house and to create a drainage channel along the toe of slope and a detention basin to reduce storm flows and to eliminate debris/sediment from entering the street. The off-site runoff from the hillside will fill up in the basin, settle out any sediment and overflow through a storm drain and then bubble up over the curb into the street. The on-site runoff is collected by many catch basins throughout the site and convey through underground drain pipes and then bubble up over the curb into the street. The 100 year 1 hour runoff generated from the tributary off-site area, after development of the site and construction of the drainage channel wall and detention basin, that enters the street is 2.89 cfs which is Tess than the pre -development 100 year 1 hour runoff. CONCLUSIONS This drainage study and the calculations presented herein demonstrate that the onsite flows are collected and carried through the project in a controlled manner within drainage structures adequately sized. All off-site flows are directed toward the drainage channel detention basins at the north and south sides of the property. A reduction in peak runoff is created by the detention basins to mitigate for any increase in run-off generated by the development of this site. This bubble up catch basins used for the runoff exiting the site are sized to retain and infiltrate the first flush flows to mitigate any water quality concerns but also allow for the peak runoff to bypass through and continue down the street. Also attached to this study is the original Hydrology and Hydraulics Report prepared by Essi Engineering Inc. in June of 2008 for parcel 1 of Parcel Map 32891 and lot 27 of Tract 28470-1. That report supports the proposed construction of the drainage channel, rock fall wall and detention basin as proposed on this site. Hydrologic values used herein are consistent with that report.