“We’ll Get The Permit” 1702 4 th Street • Santa Rosa, CA 95404 • T 707.544.2104 • F 707.522.2105 • hoganls.com Surveying • Civil & Structural Engineering • Construction Management • Violation Resolution Drainage Analysis – The Weiss & Lavine Residence Prepared December 1, 2020 Site: Owners: 40 Montecito Road Tania Weiss & Rich Lavine San Rafael, California 40 Montecito Road APN: 016-054-10 San Rafael, CA 94901 HLS Project # 3987 (415) 883-8000 Analysis Prepared by: Hogan Land Services, Inc. 1702 4 th Street Santa Rosa, CA 95404 Jim Conklin, PE C92029 [email protected]707-544-2104
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“We’ll Get The Permit”1702 4th Street • Santa Rosa, CA 95404 • T 707.544.2104 • F 707.522.2105 • hoganls.com
SUBJECT: 10-Year event hydrology calculations and reasoning for drainage network sizing
PROJECT: Drainage Analysis – The Weiss & Lavine Residence
ADDRESS: 40 Montecito Road, San Rafael, California
APN: 016-054-10
PROJECT DESCRIPTION:The subject parcel is located at 40 Montecito Road in San Rafael, California. Existing onsite infrastructure consists of a single-family dwelling, detached garage, concrete driveway, patio, and retaining walls. The proposed development under this permit includes a replacement main residence, detached garage, concrete driveway, concrete porch, wood deck, and site retaining walls. Only earthwork, grading, and drainage activities are covered under this preliminary civil plan set and preliminary drainage analysis. Topography of the area consists of moderate to steep slopes (10-30%) with the majority of the lot vegetated with native trees and grasses. This preliminary drainage analysis evaluates the potential drainage impacts of the proposed site redevelopment.
EXISTING AND PROPOSED DRAINAGE PATTERN:Runoff from the site and upslope watershed generally sheet flows diagonally across the subject parcel in a northeast direction before converging at an existing roadside ditch. Flow within the roadside concrete curb and gutter enter a public storm drain inlet along Point San Pedro
DESIGN REQUIREMENTS:The subject parcel is located within Marin County and is subject to BASMAA Post Construction requirements (See associated Preliminary Stormwater Control Plan). This analysis focuses strictly on the 10-year event sizing of the proposed drainage improvements. BASMAA Post Construction requirements are addressed within the Preliminary Stormwater Control Plan.
DESIGN CRITERIA & ASSUMPTIONS: This evaluation is based on the policies and procedures detailed in Flood Management Design Manual, by the Sonoma County Water Agency, Revised March 2020. The hydraulic design of the individual drainage improvements is based on the rational formula (Q = C i A ) and is based on the 10-year, 10-minute initial time of concentration storm. A precipitation intensity of 2.45 was used in the analysis and was derived from NOAA Atlas 14, Volume 6, Version 2. Minor downspout storm drain network was evaluated only for the worst case run within the network. The worst-case pipe sizing and slope is then conservatively used throughout the network. METHODOLOGIES USED:Rational Formula calculations for the 10-year storm are included based on the composite runoff coefficients for each tributary for sizing of the individual drainage improvements. The calculation spreadsheets are attached with this submittal. Pipe sizing and swale sizing were determined using Hydraflow Express, a design tool which uses Manning’s equations for circular channels to determine pipe sizing and triangular channels for swale sizing. 100 YEAR STORM IMPACT AND FLOODING CONCERNS:The parcel lies outside of the 100-year flood zone therefore flooding is not expected. A 100-year storm event would likely result in overflow of the culverts and swales and runoff returns to sheet flow downhill.
CONCLUSION:The calculations and analysis provided herein is based upon the Sonoma County Water Agency Flood Management Design Manual and generally accepted engineering principles. The proposed drainage improvements for the development have been sized to accommodate the 10 year storm event. Drainage network calculations are based on free flow and should be routinely inspected by the property owner. The drainage pattern at a regional scale remains unchanged from pre to post development conditions.
Sanja Perica, Sarah Dietz, Sarah Heim, Lillian Hiner, Kazungu Maitaria, Deborah Martin, SandraPavlovic, Ishani Roy, Carl Trypaluk, Dale Unruh, Fenglin Yan, Michael Yekta, Tan Zhao, Geoffrey
Bonnin, Daniel Brewer, Li-Chuan Chen, Tye Parzybok, John Yarchoan
NOAA, National Weather Service, Silver Spring, Maryland
PF_tabular | PF_graphical | Maps_&_aerials
PF tabularPDS-based point precipitation frequency estimates with 90% confidence intervals (in inches/hour)1
DurationAverage recurrence interval (years)
1 2 5 10 25 50 100 200 500 1000
5-min 1.85(1.64‑2.09)
2.28(2.03‑2.59)
2.89(2.57‑3.30)
3.43(3.01‑3.94)
4.20(3.54‑5.03)
4.82(3.97‑5.93)
5.51(4.40‑6.97)
6.24(4.82‑8.16)
7.31(5.38‑10.0)
8.17(5.77‑11.7)
10-min 1.32(1.18‑1.50)
1.63(1.45‑1.85)
2.08(1.84‑2.36)
2.45(2.15‑2.83)
3.01(2.54‑3.61)
3.46(2.84‑4.25)
3.95(3.16‑4.99)
4.48(3.46‑5.85)
5.23(3.85‑7.19)
5.86(4.14‑8.38)
15-min 1.06(0.948‑1.21)
1.32(1.17‑1.50)
1.67(1.48‑1.90)
1.98(1.74‑2.28)
2.42(2.04‑2.90)
2.79(2.30‑3.43)
3.18(2.54‑4.02)
3.61(2.79‑4.72)
4.22(3.10‑5.80)
4.72(3.34‑6.75)
30-min 0.784(0.698‑0.888)
0.970(0.862‑1.10)
1.23(1.09‑1.40)
1.46(1.28‑1.68)
1.78(1.50‑2.14)
2.05(1.69‑2.52)
2.34(1.87‑2.96)
2.65(2.05‑3.47)
3.10(2.28‑4.26)
3.47(2.45‑4.97)
60-min 0.560(0.499‑0.635)
0.693(0.617‑0.787)
0.880(0.780‑1.00)
1.04(0.914‑1.20)
1.27(1.08‑1.53)
1.47(1.21‑1.80)
1.67(1.34‑2.12)
1.90(1.47‑2.48)
2.22(1.63‑3.05)
2.48(1.75‑3.55)
2-hr 0.420(0.374‑0.476)
0.520(0.462‑0.590)
0.661(0.586‑0.752)
0.782(0.687‑0.900)
0.958(0.808‑1.15)
1.10(0.907‑1.35)
1.26(1.00‑1.59)
1.42(1.10‑1.86)
1.67(1.22‑2.29)
1.86(1.32‑2.66)
3-hr 0.355(0.317‑0.403)
0.440(0.392‑0.500)
0.558(0.495‑0.636)
0.661(0.580‑0.760)
0.808(0.682‑0.968)
0.929(0.764‑1.14)
1.06(0.845‑1.34)
1.20(0.925‑1.57)
1.40(1.03‑1.92)
1.56(1.10‑2.23)
6-hr 0.262(0.233‑0.297)
0.325(0.289‑0.369)
0.411(0.365‑0.469)
0.486(0.427‑0.559)
0.592(0.499‑0.709)
0.678(0.558‑0.833)
0.769(0.615‑0.973)
0.868(0.670‑1.13)
1.01(0.741‑1.38)
1.12(0.792‑1.60)
12-hr 0.185(0.165‑0.209)
0.231(0.205‑0.262)
0.294(0.260‑0.334)
0.347(0.304‑0.399)
0.422(0.355‑0.505)
0.481(0.396‑0.591)
0.544(0.435‑0.688)
0.611(0.472‑0.798)
0.704(0.518‑0.967)
0.779(0.551‑1.12)
24-hr 0.126(0.113‑0.143)
0.159(0.143‑0.180)
0.203(0.182‑0.231)
0.240(0.214‑0.275)
0.291(0.252‑0.344)
0.332(0.281‑0.399)
0.374(0.310‑0.459)
0.418(0.338‑0.526)
0.479(0.373‑0.626)
0.527(0.398‑0.712)
2-day 0.084(0.076‑0.095)
0.106(0.096‑0.121)
0.136(0.122‑0.155)
0.160(0.143‑0.184)
0.194(0.168‑0.229)
0.220(0.186‑0.264)
0.247(0.205‑0.303)
0.274(0.222‑0.346)
0.312(0.244‑0.409)
0.342(0.259‑0.462)
3-day 0.064(0.058‑0.073)
0.081(0.073‑0.092)
0.104(0.093‑0.118)
0.122(0.109‑0.140)
0.147(0.127‑0.174)
0.166(0.141‑0.200)
0.186(0.155‑0.229)
0.207(0.167‑0.261)
0.234(0.183‑0.307)
0.256(0.194‑0.346)
4-day 0.053(0.048‑0.060)
0.067(0.060‑0.076)
0.086(0.077‑0.098)
0.101(0.090‑0.116)
0.121(0.105‑0.143)
0.137(0.116‑0.165)
0.153(0.127‑0.188)
0.170(0.137‑0.214)
0.192(0.150‑0.251)
0.209(0.158‑0.282)
7-day 0.037(0.033‑0.042)
0.047(0.042‑0.053)
0.060(0.053‑0.068)
0.070(0.062‑0.080)
0.084(0.073‑0.099)
0.095(0.080‑0.114)
0.105(0.087‑0.129)
0.116(0.094‑0.146)
0.131(0.102‑0.171)
0.142(0.107‑0.192)
10-day 0.030(0.027‑0.034)
0.038(0.034‑0.043)
0.049(0.044‑0.055)
0.057(0.051‑0.065)
0.068(0.059‑0.080)
0.077(0.065‑0.092)
0.085(0.070‑0.104)
0.093(0.076‑0.118)
0.104(0.081‑0.137)
0.113(0.085‑0.152)
20-day 0.020(0.018‑0.022)
0.025(0.023‑0.029)
0.032(0.029‑0.037)
0.037(0.033‑0.043)
0.044(0.038‑0.052)
0.049(0.042‑0.059)
0.054(0.045‑0.067)
0.059(0.048‑0.074)
0.065(0.051‑0.085)
0.069(0.052‑0.094)
30-day 0.016(0.014‑0.018)
0.020(0.018‑0.023)
0.026(0.023‑0.030)
0.030(0.027‑0.035)
0.035(0.031‑0.042)
0.039(0.033‑0.047)
0.043(0.036‑0.053)
0.046(0.038‑0.058)
0.051(0.039‑0.066)
0.054(0.041‑0.072)
45-day 0.013(0.012‑0.015)
0.017(0.015‑0.019)
0.021(0.019‑0.024)
0.025(0.022‑0.028)
0.029(0.025‑0.034)
0.032(0.027‑0.038)
0.034(0.028‑0.042)
0.037(0.030‑0.046)
0.040(0.031‑0.052)
0.042(0.032‑0.057)
60-day 0.012(0.011‑0.013)
0.015(0.014‑0.017)
0.019(0.017‑0.022)
0.022(0.019‑0.025)
0.025(0.022‑0.030)
0.028(0.024‑0.033)
0.030(0.025‑0.037)
0.032(0.026‑0.040)
0.034(0.027‑0.045)
0.036(0.027‑0.049)
1 Precipitation frequency (PF) estimates in this table are based on frequency analysis of partial duration series (PDS).Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% confidence interval. The probability that precipitation frequency estimates (for agiven duration and average recurrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper bounds are notchecked against probable maximum precipitation (PMP) estimates and may be higher than currently valid PMP values.Please refer to NOAA Atlas 14 document for more information.