HomeMy WebLinkAboutPA2023-0051_20230222_Hydrology Report dated 2-07-2023
HYDROLOGY STUDY
FOR
TRACT 907, LOT 473
APN 423-172-08
413 Via Lido Soud
NEWPORT BEACH, CA 92663
Feb 7, 2023
PREPARED BY
Core Civil Engineering, Inc
23172 Plaza Pointe Drive, $145
Laguna Hills, CA 92653
(949)954-7158
HYDROLOGY STUDY
INTRODUCTIOIN
The hydrology study presented in this report discusses the design criteria and methodology
utilized for the hydrologic analysis of the subject property. The design criteria and
methodology used in this study follow the Orange County Hydrology Manual. The 2-yr, 24-
hour precipitation depths specified in the Orange County Hydrology Manual are used for
hydrologic analysis of the 2-year, 24-hour storm.
RATIONAL METHOD
Runoff calculations for this study were accomplished using the Rational Method. The Rational
Method is a physically based numerical method where runoff is assumed to be directly
proportional to rainfall and area, less losses for infiltration and depression storage. Flows were
computed based on the Rational formula:
Q= CIA
Where Q = peak discharge (cfs)
C = Runoff coefficient based on land use and soil type
I = Rainfall intensity (in./hr.)
A = Watershed area (acre)
The runoff coefficient represents the ratio of rainfall that runs off the watershed versus the
portion that infiltrates to the soil or is held in depression storage. The runoff coefficient is
dependent on the land use coverage and soil type.
For a typical drainage study, rainfall intensity varies with the watershed time of concentration.
The watershed time of concentration at any given point is defined as the time it would
theoretically take runoff to travel from the most upstream point in the watershed to a
concentration point, as calculated by equations in the Orange County Hydrology Manual.
DRAINAGE AREA
For this project, a total of one drainage area (DMA #1) was utilized for hydrology study
of the subject property.
DMA #1
Existing Conditions
Total
Area
(sq. ft.)
Total
Impervious
Area
(sq. ft.)
Total
Pervious
Area
(sq. ft.)
Length
of Initial
Area, L
(ft.)
Difference in
Elevation b/w
Ends of Initial
Area, H
(ft.)
3,602
(0.08 acre)
3,480 0 90 1.94 ft.
Difference in elevation ends of initial area, H = 1.94 ft.
From Figure D-1, Orange County Hydrology Manual Time of Concentration Tc = 5.8 min.
From Figure B-3 for return frequency 2 yr. Intensity I(t) = atb = (5.702)(5.8-0.574) = 2.08 in/hr.
Q = CIA
Where Q = Runoff in cubic feet per second (cfs)
C = Runoff coefficient representing the ratio of runoff to rainfall
I = The time-averaged rainfall intensity in in./hr. corresponding to the time of
concentration
A = Drainage area in acre
C = 0.9 (Ai + (I – Fp)Ap/I) for I > Fp
C = 0.9Ai for I < Fp
Where Fp = Infiltration rate (0.4-Soil Group A, 0.3-Soil Group B, 0.25-Soil Group C, 0.20-Soil
Group D.
Ai = Ratio of impervious area to total area
Ap = Ratio of previous area to total area, Ap = 1-Ai
C = 0.9
Q1exist(2yr.) = 0.9 x 2.08 x 0.08 = 0.15 cfs
Proposed Conditions
Total
Area
(sq. ft.)
Total
Impervious
Area
(sq. ft.)
Total
Pervious
Area
(sq. ft.)
Length
of Initial
Area, L
(ft.)
Difference in
Elevation b/w
Ends of Initial
Area, H
(ft.)
3,602
(0.08 acre)
3,480 122 90 2.72 ft.
Difference in elevation between ends of initial area, H = 2.72’
From Figure D-1, Orange County Hydrology Manual Time of Concentration Tc = 5.5 min.
From Figure B-3 for return frequency 2 yr. Intensity I(t) = atb = (5.702)(5.5-0.574) = 2.14 in/hr.
Q = CIA
Where Q = Runoff in cubic feet per second (cfs)
C = Runoff coefficient representing the ratio of runoff to rainfall
I = The time-averaged rainfall intensity in in./hr. corresponding to the time of
concentration
A = Drainage area in acre
C = 0.9 (Ai + (I – Fp)Ap/I) for I > Fp
C = 0.9Ai for I < Fp
Where Fp = Infiltration rate (0.4-Soil Group A, 0.3-Soil Group B, 0.25-Soil Group C, 0.20-Soil
Group D.
Ai = Ratio of impervious area to total area
Ap = Ratio of previous area to total area, Ap = 1-Ai
C = 0.9
Q1new(2yr.) = 0.9 x 2.14 x 0.08 = 0.154 cfs
n= 0.009 for pvc pipe
V= (1.49/0.009) x (R) 2/3 x (S) 1/2 ft/sec
Q= V x Ap cfs
A= 0.087 R=0.083 S=0.01
V= (1.49/0.009) x (0.083) 2/3 x (0.01) 1/2 = 3.15
Q= 3.15 x 0.087
Q= 0.274 cfs > 0.154 cfs
OK for 4” PVC pipe with 1% slope