Technical Drainage / Hydrology Study
I. INTRODUCTION
The Technical Drainage Study shall discuss, at a detailed level, the existing site hydrologic conditions, and the proposed drainage plan to accommodate or modify site drainage conditions in the final development plan for the site. The Technical Drainage Study shall address both on-site and off-site drainage analysis and improvements necessary to mitigate the impact of the proposed development on downstream properties.
The Technical Drainage Study shall be in accordance with the outline listed under Section 6.5 of the Douglas County Design Criteria and Improvement Standards dated September 17, 1998.
The Round Hill Vacation Resort is located within the Tahoe Basin southwest of Elks Point Road and U.S. Highway 50. The address is Assessors Parcel Number APN: 05-230-11 an undeveloped 11.34-acre parcel (Figure 2A). Developer intends to develop the Project Site as a vacation ownership resort on a multi-phase basis consisting of approximately 144 vacation ownership units (consisting of one-bedroom and two-bedroom types comprising a total of approximately 220 bedrooms) together with certain recreational amenities, laundry, sales facilities, and related facilities.
The Resort is within the southwest one-quarter of Section 15, Township 13 North, Range 18 East, M.B.D. & M, Douglas County, Nevada. Described as Parcel “D” as set forth in that certain parcel map for ROUND HILL, LTD. filed for record in the office of the County Recorder of Douglas County, State of Nevada on September 20, 1979 in Book 979, at page 1667 as Document No. 36918. The Vicinity map that is included as “Figure One” shows the Resort and adjacent area.
Areas north and east of the site are densely developed (to Tahoe standards) while other areas surrounding the site have relatively sparse development. Most of the development in this area occurred in the 60’s and 70’s before TRPA’s oversight.
The existing topography of the site is shown on “Figure 2A” (8.5x11 reference format). This site has an overall east to west cross slope of approximately 3%. The existing land is 100% pervious. The landscape on the site is predominantly tree and grass covered with natural forest litter. Soils on the site vary from rock outcroppings to sandy loam. All soils on site are classified by the Soil Conservation Service to be within Hydrologic Group C, soils with slow infiltration when saturated and have a moderately high runoff potential. Surface soils have permeability of 0.6 to 2.0 inches per hour. Regional and Site Soils Characteristics Maps are attached as a part of this report. Detailed descriptions and Estimates of Soil Properties Significant in Engineering for each soils group are also attached. For use in calculations in this study, a moderate permeability rate of 0.833 inches per hour was used.
The water table under the site is expected to be at depths greater than 7 feet in accordance with the South Lake Tahoe Folio & Hydrologic Map, which is attached as Figure 3.
The site to the east of Highway 50 has been previously developed and conveys runoff to the eastern property line in three locations.
The site is not within a flood plain or within a debris flow hazard area as presented on the “Flood Rate Insurance Map”, panel 15 dated September 30, 1992 and on the Flood and Related Debris Flow Hazards Map” dated 1978. Both are attached for reference as Figures 4 and 5.
The project will be developed in strict accordance with TRPA Ordinances which require “Best Management Practices” to preserve water quality as well as Section 25.5.A which requires that runoff from impervious surfaces be discharged to groundwater. It specifically requires infiltration facilities designed to accommodate the volume from a 2-Year 6-Hour storm.
II. discussion of design engineering
Per “Douglas County Design Criteria & Improvement Standards” (DCIS) Section 6.1.2.5 “Storm Runoff Detention”. The “Allowable Peak Discharge” may not exceed “Pre-Development” discharge for 2, 10 & 25 year – 24-hour duration storms. Provide emergency flow paths for 100 year – 6-hour storms. The “Design Storm Event” (per Table 6.3 DCIS) shall be for a 25 year – 6-hour storm event. No curb “overtopping in a “Design Storm Event”. “ Residential Dwellings shall not be inundated at the ground line, and the depth of water over the gutter flow line shall not exceed 12” in a 100 year 6 hour storm event.
Per a memo dated March 23, 1999 from TRPA to the Governing Board, “Drainage conveyances through the project shall be design to a 10 year 24 hour storm event and in addition, “Drainage conveyances through the project SEZ” shall be designed to a 50 year storm event. No hourly reference was given for the 50-year storm so for this report the rainfall intensity was calculated using an adjusted, “I” valued based on Sub area Time of Concentration (Tc).
2, 10, 25, 50, & 100-year rainfall calculations and supporting isopluvial maps and charts are included in this report.
III. Existing and Proposed Hydrology
A. EXISTING
The watershed, which includes the site, is bounded on the north by Round Mound “Area A - Figure 2C - Offsite Watershed Boundaries” up to the saddle that crosses U. S. Highway 50 to the east, continuing east to include “Area B - Figure 2C”. The eastern property line receives drainage flow from “Area C - Figure 2C” and hydrologic data was obtained from the “R.O. Anderson Engineering, Inc.” report for “Round Hill Shopping Center” dated September 13, 1996. All drainage flow onto the project site from Area ‘B’ and ‘C’ are conveyed via pipe in three locations. Area B via 30” line at the north, Area C “Commercial Site” at mid eastern property line via 18” line and Area C “Residential” at southeastern property line via 18” line. Maximum flow (“Q”) was calculated and used for these three discharge lines based on size and slope by “Equation 1” listed at the end of this study.
The existing runoff from the various watersheds, which includes the site, is estimated using the Rational Method. The on-site “C” factor is estimated to be 0.20 based on 100% pervious. Rainfall intensities are estimated from NOAA Atlas 2 isopluvial mapping and equations for estimating intensities of storms of different durations. Times of concentration are from Equation 2. Because of the large number of uncertainties and no established methods of calculation the runoff from rain upon snow was not estimated.
The existing runoff is conveyed through the site by a combination of methods including overland flow and cross drains. The runoff is conveyed across the property to the south entering the natural drainage course, which leads to Lake Tahoe (detailed discussion follows).
Because the site is within the Tahoe Basin there is concern over even the smallest amount of erosion. However there are no areas on site where there have been erosion problems. Off-site flows follow historical drainage courses.
TABLE ONE: EXISTING SITE RUNOFF
Q in cfs Historical Year calculated using rational method: Q = CIA
Where: C = .20
I = intensity one hour rainfall (per historical storm event adjusted per actual Tc)
Tc = 23.59 minutes
A = Acres
|
Storm |
C |
I |
A |
Flow in CFS |
|
Q2 |
.20 |
1.10 |
11.34 |
2.49 |
|
Q10 |
.20 |
1.69 |
11.34 |
3.83 |
|
Q25 |
.20 |
2.03 |
11.34 |
4.60 |
|
Q50 |
.20 |
2.25 |
11.34 |
5.10 |
|
Q100 |
.20 |
2.46 |
11.34 |
5.58 |
All offsite influences per historical event remain given and constant and are provided, as follows, by area of delivery from Figure 2C “Offsite Watershed Boundaries”. Actual Tc for offsite watersheds are from historical documents or as listed on Figure 2C.
Table Two: Total Offsite Contributing Storm Flows
|
Storm |
Area A |
Area B |
Area C |
Total CFS |
|
Q10 |
11.82 |
22.04 |
44.20 |
78.06 |
|
Q50 |
15.72 |
29.46 |
50.17 |
95.35 |
|
Q100 |
17.22 |
34.20 |
53.80 |
105.22 |
Note: The 18” RCP from Residential Area C was calculated to be flowing full at discharge for Q50 & Q100. Q10 for Area C (commercial site) was arrived at by interpolation of Q50 & Q100 data obtained from the R. O. Anderson Report for Round Hill Shopping Center.
Table Three: Pre-Development
Combined Storm Water Discharge from Site
The pre-development storm flows listed below have been conveyed to the downstream culvert at “Our Lady of Tahoe Catholic Church” property line and continue to conveyances at Elks Point Drive.
|
Storm |
Site |
Offsite |
Total CFS |
|
Q10 CFS |
3.83 |
78.06 |
81.89 |
|
Q50 CFS |
5.10 |
95.35 |
100.45 |
|
Q100 CFS |
5.58 |
105.32 |
110.90 |
B. PROPOSED
The proposed improvements include infiltration galleries around all buildings at roof drip lines, under the street and parking pavement (“Infiltrator Systems Inc.” cut sheets attached) and 3 onsite retention ponds which will store and infiltrate 100% of the runoff from impervious areas in the “Design” storm. The site after development will be split into seven sub-basins labeled one though seven on the “Grading and Drainage Plan” which is included with this study. The Rational Method is used to estimate the runoff from each sub-area with ‘c’ at 0.95.
It is noted that the peak “Post-Development” flow is less than the “Post-Development” flow. There will be no runoff generated by the project in any Historical Storm event as calculated and analyzed through Q100. We have reviewed the impact of the 100-year flood line and have determined that we are within all guidelines of “Douglas County Design Criteria and Improvement Standards - Division 6 - Storm Drainage”.
In order to protect the quality of groundwater as well as preserve the infiltration rates of the system, the runoff entering each system will first be treated through sand-oil separators (SOS). All onsite “Design” flow from paved traffic surfaces will be conveyed through the “Stormceptor” SOS units (cut sheets attached) or per detail DC-D09 “Flow Restrictor / Oil Pollution Control Device”. Channel landscape planting of dense willow scrub brush per the approval of TRPA and Douglas County will be used to mitigate sediment and oil flows entering the site from Area B - figure 2C. Design flows from Area C - Figure 2C “Residential” shall be treated using a Jensen “High Velocity Stormwater Interceptor” SOS unit capable of treating flows of 30 cfs.
Areas indicated by ”SEZ” (Stream Environmental Zone) on the “Grading and Drainage Plan” shall be protect and remain undisturbed except for two driveway crossings. The driveway crossings will allow cross flow via “Culvert Headwalls” per a modified version of Douglas County drawing number DC-D10. It is the intent of the project to use the historical drainage courses as a project “amenity”. Presenting the drainage course as a maintained landscape while protecting the SEZ. The design enhancements of the historical flow line running through the site (served by Area A and Area B - figure 2C) will enhance the natural channel components to minimize erosion and filter out pollutants that may be historically flowing to Lake Tahoe. This watercourse is 944 LF @ 3% with onsite time of concentration (Tc) of 23.59 minutes (Equation 2). Additional Tc’s are listed in Figure 2C. 100-year storm flows entering the site via pipe conveyance vary in velocity from 7 fps (30” line from Area B), 15 fps (18” line from Area C Commercial) to 16 fps (18” line from Area C Residential) calculated using Equation 3. Pipe discharge from off-site conveyance is assumed to be flowing full and at maximum capacity in a 100-year storm. It is intended that the velocities from the 18” lines be dissipated or reduced with controlled release onto the site at their discharge point. It is the intention of the design engineering to maintain and control permissible velocities for well-maintained landscaped channels.
Table Four: Onsite Retention – Area’s per Plan
Rainfall intensity calculated per 2-Year Storm Data with adjustment for actual Tc. All flows indicated on the following chart are conveyed through 900-Gallon Sand/Oil Separators then conveyed to retention.
|
Area # |
Acres |
C |
Tc (minutes) |
Rainfall in/hr |
Flow CFS |
Depth of Flow at Curb 2A/W = inches |
Retention CF |
|
Area 1 |
0.28 |
0.95 |
8 |
1.75 |
0.47 |
0.05 |
2250 |
|
Area 2 |
0.21 |
0.95 |
6 |
1.93 |
0.39 |
0.04 |
1030 |
|
Area3 |
0.25 |
0.95 |
7 |
1.84 |
0.44 |
0.04 |
1399 |
|
Area 4 |
0.25 |
0.95 |
9 |
1.66 |
0.71 |
0.07 |
2173 |
|
Area 5 |
0.10 |
0.95 |
5 |
2.02 |
0.19 |
0.02 |
513 |
|
Area 6 |
0.33 |
0.95 |
8 |
1.75 |
0.55 |
0.06 |
1386 |
|
Area 7 |
0.24 |
0.95 |
7 |
1.84 |
0.42 |
0.04 |
1336 |
Roof Structure Drip line & Pool Area Infiltration/Retention Trench Design Calculations
A x I/ P (W +(2D/3)) + WD/3= Length of Trench Required @ One CF retention/foot
Where:
P = Percolation Rate
A = Impervious Area in Square Feet
W = Width of Trench
D = Depth of trench
I = Intensity of rainfall in inches per hour for 25 year Storm event
82,250 x .083 / .833(1.5 + (2 x 1/3)) + 1.5 x 1/3 = 2968.15 LF & CF retention
Summary of On-Site Retention
1. Total Site Acreage = 11.34 (100%)
2. Total Impervious Acreage = 3.75 (33%)
3. Total Pervious Acreage = 7.59 (67%)
4. Total Cubic Feet of On-site Storm Runoff Retention = 13,055 Cubic Feet
5. Factor for moderate percolation in Retention Structures = 1.833
13,055cf x 1.833 = 23,929.82 Cubic Feet Per Hour Storm Water Retention On-site
23,930 cubic feet / 60 minutes / 60 seconds = 6.65 CFS leaves site via percolation
Table Five: Summary of Post-Development Flows
|
Storm |
Site |
Offsite |
Total CFS |
|
Q10 CFS |
-2.82 |
78.06 |
75.24 |
|
Q50 CFS |
-1.55 |
95.35 |
93.80 |
|
Q100 CFS |
-1.7 |
105.32 |
104.25 |
Downstream flow paths are conveyed via open culvert and closed conduit. As flow exits the site at the Church site along the south property line it is conveyed via a single 18” CMP capable of a maximum of 2.4 cfs @ 3% slope and n= .22. Historical information supports this limited flow capacity, since the parking lot floods on a regular basis. Once the 18”CMP is flowing full storm flow breaks out over the parking lot and continues to ELKS POINT DR. where it continues under, over and around to the continuation of the open culvert which continues on to Lake Tahoe.
Douglas County Standard Details for Public Works construction will be used for the final working drawing plan submittal. Examples of these details specific to storm drain system construction are attached and a part of this report.
Erosion and sediment deposition and mitigation measures during construction will be in accordance with Section 7.2.11 of DCIS. This section will also govern the development of permanent stabilization via landscape methods.
Flood elevations are within the limits and guidelines as related to FFE requirements and with Douglas County Title 20.
Please see the 30 scale “Grading and Drainage Plan” for additional onsite drainage structures and design engineering concepts and mitigation.
IV. CONCLUSION
This “Technical Study” has incorporated the best available information and criteria established by Douglas County and the TRPA. Detailed calculations are included with this submittal. In general, it is intended that the project will improve the water quality and decrease water quantity over the existing conditions. This is a result of the extensive use of infiltration for on-site disposal, the use of Best Management Practices and attention to detail during design. Therefore, the impacts of the project should only be positive and a model for future projects within the Tahoe Basin.