SDP 2016-0007SSI
ENGINEERING
77-570 SPRINGFIELD LANE, SUITE C
PALM DESERT, CA 92211
(760) 772.8400 OFFICE
(760) 772.8421 FAX
CML AND STRUCTURAL ENGINEERING - PLANNING - SURVEYING
IN THE CITY OF LA QUINTA, STATE OF CALIFORNIA
FINAL HYDROLOGY STUDY
FOR
E.J. LA QUINTA, LLC, CONTOUR
DERMATOLOGY OFFICE
IN THE NW 1/4 OF SECTION 28, T 5 S, R 7 E, SBM
SITE DESCRIPTION:
The site consists of 1.27 acre of land located in La Quinta, California at the southeast corner of the
intersection of Jefferson Street and Westward Ho Drive and is bounded on the south by the private street
known as Palm Circle Drive and on the east by the Bermuda Dunes Golf Course. The site is the location
of the original club house and parking lot, which still exist. The site is to be cleared and a new doctor's
office constructed on the southerly pad. The site is situated several feet above the adjacent streets and a
retaining wall lines the back of walk on Jefferson Street. This will be removed and the site lowered. A
central parking lot is proposed with the entry centered on the Jefferson Street frontage. The existing entry
on Westward Ho Drive and on Jefferson Street will be removed and replaced by curb and gutter. An
existing catch basin is located on Jefferson Street at the southwest corner of the site and collects storm
flows for that street. It is interconnected to a main system running down the thoroughfare. The general
flow of Jefferson Street is southerly and Westward Ho Drive flows steeply east towards the Whitewater
River Channel for approximately 0.25 miles where it is intercepted by a pair of Type 300, side -entry catch
basins on either side of the street. It is unclear from City records and Staff exactly where this off-site
system discharges. Presumably either onto the existing golf course to the north or directly out to the
White Water River Channel. The golf course to the east is set well below the site and a cart path crossing
at Westward Ho Drive runs southerly along the site boundary. A proposed retaining wall will adjoin the
west side of the cart path and a 3:1 slope rises to meet the pad area with a flow line running at the base
of the slope to the grassy areas either side. This area designated Area "C" is considered a self -treating
area since stormwater runoff will flow over slopes of permeable areas consisting of grass in the golf
course area and decomposed granite planters in the new areas. A system of underground storage
chambers and a Maxwell Plus system are proposed to contain the on-site flows. No off-site flows are
proposed to be intercepted since the existing streets have been fully improved with their own storm drain
systems. The main parking area drains towards a single catch basin, type 300, 4 feet in width. This
interconnects to the Maxwell primary chamber and then connects to the secondary chamber. The north
building area will connect its future storm drain network into the northern primary chamber located north
of the entry drive and the southern building site's storm drain system connects directly into the secondary
chamber located on the south side of the entrance. The existing site currently drains to the offsite streets
and the golf course. The proposed design greatly reduces the flows. However, it is impossible to capture
the entire site due to the higher vertical location of the pad in comparison to the surrounding land to the
north and east.
PURPOSE OF STUDY:
The existing site is already developed with a parking lot and club house. There would be no need to
determine the preconstruction runoff since the two would be the same number. The post construction
runoff from the site shall be analyzed for the design of the proposed underground retention basin and we
shall prove the design widths of the catch basin and analyze the hydraulic grade lines within the proposed
storm drains, assure that the retention basin provides adequate freeboard and capacity to retain the
entirety of all of the 100 -Year Storm flows for the 1 -Hour, 3 -Hour, 6 -Hour and 24 -Hours events and
determine that the basin can be free of storm water within a 72 hour period. An above ground basin and
below ground perforated pipeline shall be used in combination with an overflow basin at the base of the
above ground basin as a backflow to allow excess storm water to enter and leave the basin as the
pipeline reaches full capacity and then later percolation via the Maxwell Plus units will allow the basin to
be drained, ebbing and flowing as necessary during the storms. For the purposes of this report, we have
made no additional percolation losses within the either the below ground or above ground basins.
ANALYSIS:
The soils report prepared by Earth System Southwest, report # 16-02-712 dated 2-12-2016 and
percolation update letter 17-02-217 dated 2-27-2017, included at the end of this report, conflicted on the
percolations rates. Originally stating that testing nearby yielded between 4.6 inches (label P-2) per hour
and 11.4 inches per hour (mislabeled in the report P-2 but really P-3 per the diagram), the testing method
used was not one accepted by the City. The original report mentioned the possibility of a perched water
table at depths greater than 40 to 50 feet with the free-flowing layers above that. Earth System Southwest
subsequently did addition testing located more closely to the area designated for the underground
storage system. The subsequent retesting led to two revised numbers for the new borings. The revised P-
1 percolation rate was 0.6 inches per hour at a depth of 18 to 23 feet and 2.4 inches per hour for boring
hole P-2 at 3 to 8 feet of depth. The incredibly wide deviation from the original numbers a few yards away
leave questions that cannot be easily answered or accounted for with confidence in what would happen
over time. This study was revised, based upon this radically deviating data, to account for no percolation
in the calculations and account for a steady drawdown via the Maxwell Plus system being proposed. The
drawdown rate allowed by the City of La Quinta for this system is 0.1 CFS. The additional losses column
of the spread sheet for the synthetic unit hydrograph accounts for the 0.1 CFS over that period's duration
being analyzed, varying anywhere from 3.33 minutes to 15 minutes. This is also the rate used to calculate
the 72 hour maximum emptying of the entire system. The required depth for the Maxwell's "stinger"
section is 60 feet below natural existing grade or at approximate elevation 3.0 feet above sea level.
The soils type is "A" with a runoff index (RI) of 32 and an infiltration rate (Fp) of 0.74 inches per hour. The
development type is commercial. Both the Riverside County Flood Control and Water Conservation
District's (RCF&WCD) rational method and synthetic unit hydrograph method were utilized to determine
the Q's and volumes for the different storms. All rainfall data was derived from the NOAA website utilizing
the longitude and latitude of the center of the site. The intensities were derived from the Zone II Intensity
Curve from the US Department of Commerce. The Washington DC Public Roads Department nomograph
for side entry catch basins in a sump condition (1070.03) was utilized to determine the ponding elevation
for the 100 -Year, 1 -Hour storm flows. This area is within the blow sand region of the valley and
percolation testing exceeds test capacities, in many cases. The percolation area, as mentioned
previously, will be zero. A 40% void ration for the gravel bedding is used to the calculations and the bed is
calculated to 1 foot above and below the pipeline.
Area "C" will remain, for the most part, unchanged. The addition of low water usage plantings in the new
slopes will virtually eliminate nuisance runoff. The area consists of existing concrete and asphalt golf cart
paths and a 3:1 slope rising to a top at the rear of the proposed parking lot sidewalk running north and
south. It also consists of a portion of the parkway along Westward Ho Drive. Area "C" drains mainly
across the golf course area of the adjacent property prior to exiting onto Westward Ho Drive and is
considered as self -treating. Since the lack of changing values for runoff for the synthetic unit hydrograph