HomeMy WebLinkAboutMS 06-01; MCCARTHY RESIDENCE; PRELIMINARY DRAINAGE REPORT FOR MCCARTHY RESIDENCES; 2006-06-01Preliminary Drainage Report
for
McCarthy Residences
Prepared By:
LEPPERT ENGINEERING CORPORATION
5190 GOVERNOR DRIVE, SUITE 205
SAN DIEGO, CA 92122
June, 2006
Job No.: CAR 8.01-29.05
Of E Ssi0
ell
No. 31829 1 \ EXP.CA C
L I.Ot P3
OF A
By: Norman S. Kasubuchi, RCE 31829
Exp.: 12-31-06
(D/ %&
Date
RECEIva
JUN 26 2006
ENGINEERING
DEPARTMENT
/A' O&-O(
Table of Contents
Introduction..................................................................1
Existing Condition .............................................................1
Proposed Condition ............................................................1
Future Widening of Carlsbad Boulevard ............................................2
Method of Calculation ...........................................................2
Hydrologic Criteria ............................................................2
Drainage Calculations ..........................................................4
Comparison between Existing and Proposed Conditions ...............................5
Hydraulic Analysis of Private Drain ...............................................5
Control of Pollutants ...........................................................6
Conclusion...................................................................7
Appendices
Appendix "A" .....................................................Location Map
Appendix "B" ................................Site Drainage Map of Existing Conditions
Appendix "C" ...............................Site Drainage Map of Proposed Conditions
Appendix "D" ...........................................Hydrology Summary Table
Appendix "E" ....................Excerpts from the San Diego County Hydrology Manual
Appendix "F" ......................Rainfall Intensity Duration Chart and Isopluvial Maps
McCarthy Residence
Drainage Study
CAR 8.01-29.05
Introduction
The purpose of this drainage study is to estimate the quantity of storm water runoff from the
McCarthy Residence, located at 5115 and 5117 Carlsbad Blvd., in the City of Carlsbad,
California (See Appendix "A"- Location Map). The study will analyze the existing and
proposed conditions to determine the increase in runoff, if any.
Existing Condition
The site consists of a 0.138 acre parcel of land that is currently developed with a duplex
residence. The site is surrounded on three sides with residential developments. The structure is
located at an elevation that is lower than Carlsbad Boulevard. The site slopes from Carlsbad
Blvd. To the Westerly property line at an average grade of approximately 5.4% (See Appendix
"B" - Existing Drainage Conditions). Surface runoff is forced to make its way around the
existing building to reach the rear yard. An existing masonry fence, located on the adjacent
property to the West, prevents runoff from draining further to the West, effectively creating a
retention basin in the rear yard of the subject property.
Proposed Condition
The proposed development consists of the demolition of the existing duplex, and the construction
of a new duplex condominium, in the same location and at approximately the same elevation as
the existing building (See Appendix "C" - Proposed Conditions). The drainage pattern for the
proposed development will be similar to the existing conditions, with the following difference.
Unlike the existing conditions, the proposed development will intercept a portion of the runoff
that drains towards the proposed garages, and direct the runoff around the new structure through
a 4" private drain pipe, where the runoff will discharge through rip rap energy dissipaters to the
rear yard.. The runoff that is not intercepted by the private storm drain system will surface drain
around the proposed building and flow to the rear yard, similar to the existing conditions.
Intuitively, the time of concentration for combined overland flow and pipe flow will be less than
McCarthy Residence
Drainage Study
CAR 8.01-29.05
overland flow, given equal or similar flow lengths, due to superior hydraulic conditions for pipe
flow (e.g. lower coefficient of friction and higher velocities). Therefore, the time of
concentration for overland flow will be used to compute the runoff for the proposed condition.
Future Widening of Carlsbad Boulevard
Carlsbad Boulevard fronting the subject property is currently improved with A.C. pavement only
(no curb and gutter). A review of the plans for the future widening of Carlsbad Boulevard (Dwg.
No. 331-7) indicates that the street will be widened approximately 15 feet from the existing edge
of pavement adjacent to the subject property. Improvements will consist of new A.C. pavement,
curb, gutter and sidewalk. In addition, the plans show the construction of a new curb inlet near
the Northerly corner of the subject property. The impacts of the proposed widening on the
drainage characteristics of the subject property will result in a nominal reduction in runoff across
the site as a result of the construction of the curb and gutter, combined with the reduction in the
drainage basin area as a result of the widening. Depending on the ultimate depth of the storm
drain inlet and facilities to be constructed as a part of the ultimate widening, a portion of the site
runoff could be directed to the future storm drain system, further reducing the surface runoff
draining to the West across the subject property.
Method of Calculation
The total runoff from the site is calculated using the guidelines set forth in the San Diego County
Hydrology Manual, dated June 2003. The specific calculation used is the Rational Method. This
is the standard practice for watersheds under 0.5 square miles. Excerpts from the San Diego
County Hydrology Manual (Manual), are located in Appendix "E".
2
0
McCarthy Residence
Drainage Study
CAR 8.01-29.05
Hydrologic Criteria
The following criteria were used in computing runoff from the existing and proposed project.
S Design storm: 10-year and l0O-year.
Hydrology method: Rational method (Q=CIA) per Section 3 of the San Diego
County Hydrology Manual.
S Time of Concentration (Tc) for Overland Flow: For overland flow, the time of
concentration is determined from the equation, Ti = (1.8)(1.1-C)(D)1/2/(S)"3,
included in Figure 3-3 of the San Diego County Hydrology Manual.
Intensity of Rainfall I: The rainfall intensity was calculated using the Rainfall
Intensity-Duration-Frequency Curves - Figure 3-1, and the Rainfall Isopluvial
Maps of the San Diego County Hydrology Manual (See Appendix "F").
Soil Group B. The County of San Diego's "Soil Hydrologic Groups" map
indicates that the site is located within Soil Group B (See Appendix "E").
According to Section 4.1.2.1 of the San Diego County Hydrology Manual, soils in
group B have a moderate infiltration rate when thoroughly wetted. The rate of
water transmission is also moderate. Other characteristics of soils in group are
that they are moderately deep to deep, moderately well drained to well drained,
and have a moderately coarse texture.
Runoff coefficient C: The runoff coefficient used for this study was based on the
Soil Group and Land Use Density per Table 3-1 of the San Diego County
Hydrology Manual. For an existing and proposed density of 14.5 du/ac (Medium
Density Residential & Soil Group B), C=0.58.
10 3
McCarthy Residence
Drainage Study
CAR 8.01-29.05
Drainage Calculations
Existing Conditions: (See Appendix "D")
Drainage Basin Area, A=0.19 ac.
Ti = (1.8)(1.1-C)(D)"2/(S)"3 = (1.8)(1.1-.58)(107)/ (5.4)1/3
= 5.52 mm., rounded to 6 mm.
110 = (7.44)(P6)(D°5 = (7.44)(1.7)(6)-o-64' = 3.98 in/hr
1100 = (7.44)(P6)(D)05 = (7.44)(2.5)(6).0645 = 5.86 in/hr
C-0.58
Q=(C)(I)(A)
Q10 = (0.58)(3.98)(0.19) = 0.44 cfs
Q100 = (0.58)(5.86)(0.19) = 0.65 cfs
Proposed Conditions: (See Appendix "D")
Drainage Basin Area, A=0.19 ac.
Ti = (1.8)(1.1-C)(D)"2/(S)'3 = (1.8)(1.1_.58)(107)h/2/ (5.4)1/3
= 5.52 mm., rounded to 6 mm.
'10 = (7.44)(P6)(D)OM5 = (7.44)(1.7)(6)°" = 3.98 in/hr
1100 = (7.44)(P6)(D)oM5 = (7.44)(2.5)(6)4 = 5.86 in/hr
C=0.58
Q=(C)(I)(A)
Q10 = (0.58)(3.98)(0.19) = 0.44 cfs
Q100 = (0.58)(5.86)(0.19) = 0.65 cfs
4
McCarthy Residence
Drainage Study
CAR 8.01-29.05
Comparison Between Existing and Proposed Conditions
Based on the above calculations, it can be concluded that the proposed development will not
increase the amount of runoff when compared to the existing conditions. This can be attributed
to the fact that the proposed development maintains the same drainage patterns and conditions as
the existing conditions. In addition, the runoff coefficient is based on the soil type and land use
density, which are the same for both existing and proposed conditions.
Hydraulic Analysis of Private Drain
The proposed development utilizes trench drains and private 4" drain pipes to intercept runoff
that drains towards the garages from Carlsbad Boulevard. For purposes of this analysis, it is
assumed that a portion of the roof runoff will be tied into the drain pipe and therefore, will be
included in the drainage basin area. The basin area for each private system has been estimated to
be approximately 0.05 ac. (See Appendix "C"). Applying the hydrologic criteria determined
above for the proposed conditions, the 100 year runoff will be calculated to analyze the capacity
of the proposed 4" drains, as follows:
Basin No. 1:
Area, A= 0.05 ac
Runoff Coefficient, C = 0.58
Rainfall Intensity, I100 = 5.86 in/hr
Runoff, 0 = 0.17 cfs
Pipe Diameter, d = 0.33 ft.
Slope, S = 3.7%
Manning's, n = 0.013
Depth of Flow, D = 0.16 ft.
Velocity, V = 4.12 ft/sec
5
F]
McCarthy Residence
Drainage Study
CAR 8.01-29.05
Basin No. 2:
Area, A= 0.05 ac
Runoff Coefficient, C = 0.58
Rainfall Intensity, I100 = 5.86 in/hr
Runoff, 0 = 0.17 cfs
Pipe Diameter, d = 0.33 ft.
Slope, S = 1.9%
Manning's, n = 0.013
Depth of Flow, D = 0.20 ft.
Velocity, V = 3.20 ft/sec
Conclusion: Based on the above calculations, the 4" private storm drains have
sufficient capacity to carry the flow generated by a 100 year storm event.
Summary
Based on the above calculations and hydrologic conditions for this area of San Diego County,
storm water runoff will neither increase nor decrease as a result of this proposed development.
Storm water runoff generated from the Eastern half of the lot will be collected by two trench
drains located in front of each dwelling unit. The collected runoff will be conveyed by private
storm drain pipes into the landscaped back yard.
Runoff collected on the Western half of the site via the roof may be discharged into planters or
landscaped areas by the use of rain gutters and down spouts. These systems, along with the drain
pipes from the trench drains will disperse runoff in a manner consistent with existing conditions.
McCarthy Residence
Drainage Study
CAR 8.01-29.05
Results from the above calculations indicate that the proposed development will not produce an
increase in storm water runoff. Also, the proposed drainage exhibit shows that the site
topography and proposed drainage improvements will enable surface runoff to follow the natural
drainage patterns for this location. It is our contention that the proposed development of the
McCarthy Residences do not pose significant adverse affects to the site, the surrounding parcels,
and to downstream conditions. .
7
Appendix "A"
LOCATION MAP
-1 Freeways
Highways
Ramps
,V Major Roads
j/ Collectors Roads
All Roads
Parcels
40 foot topo
/ Rivers
Watershed
Management
Areas
LII
El CARLSBAD
LI] CLARK
El i N1 PLR Wt L
LII SON BAY
El PEWAWUfTOS
LII SAN DIEGO BAY
El] SAN DIEGO RMR
El SAN
El SLAM jUAN
El SAN LUIS REY
LII SANTA MARGARITA
11111
LIII
El WHITLWAMR
Zip Codes
Municipal
\-2O26RiInc
Appendix "B"
Site Drainage Map of
the Existing Conditions
EXISTING DRAINAGE CONDITION
IMPROVEMENTS
hEM SYMBOL
EXISTING MAJOR CONTOUR
EXISTING INDEX CONTOUR
EXISTING BLOCK WALL
EXISTING WOOD FENCE
PROPERTY BOUNDARY
EXISTING LIMITS OF PA VEMEN T
EXISTING LOT LINE
EXISTING CONCRETE AREAS
EXISTING ASPHALT DRI VEWA Y
LONGEST PA TH OF TRA VEL
EXISTING DRAINAGE FLOW
DRAINAGE BASIN
SPOT ELEVATION
BASIN
0.19 AC.
C=O.58
10 0 10 20
1t,= 10*
L epp ert Engineering
OC>FR F=C>R ,ATtcDN3
/ 5/90 Covernor Drive, Suite 20 Son 0/ego, Cci. 92122-2548
Phone: (51) 597-2001 Fox: (6r9) 597-2009
Appendix "C"
Site Drainage Map of
the Proposed Conditions
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Appendix "D"
Hydrology Summary Table
McCarthy Residence Hydrology Summary Table
Existina Conditions
Isopluvial Information Basin Data
6 hour 24 hour Adjusted 6 hour Duration, min Rainfall intensity, in/h Area, ac C Value Total Flow, cfs
10 Year Storm 1.7 3.0 1.7 6.00 3.98 0.19 0.58 0.44
100 Year Storm 2.5 4.4 2.5 6.00 5.86 0.19 0.58 0.65
Post DvIonmnnt Conditions
lsopluvial Information Basin Data
6 hour 24 hour Adjusted 6 hour Duration, min Rainfall Intensity, in/h Area, ac C Value Total Flow,cfs
10 Year Storm 1.7
1
3.0 1.7 6.00 3.98 0.19 0.58 0.45
100 Year Storm 2.5 4.4 2.5 6.00 5.86 0.19 0.58 0.65
Appendix "E"
San Diego County Hydrology
Manual Excerpts
33CD0 30'
33'15' 15
33OO' 00'
3245 45'
32CD0'
(0
County of San Diego
Hydrology Manual
Soil Hydrologic Groups
Soil Groups
Group A
Group B
Group C
Group 0
Undetermined
71 Data Unavailable
DP\V GIS SanGIS
. H, S.v
4 E
3 0 3MiIes
San Diego County Hydrology Manual Section: 3 Date: June 2003 Page: 1 of 26
SECTION 3
RATIONAL METHOD AND MODIFIED RATIONAL METHOD
3.1 THE RATIONAL METHOD
The Rational Method (RM) is a mathematical formula used to determine the maximum
runoff rate from a given rainfall. It has particular application in urban storm drainage, where
it is used to estimate peak runoff rates from small urban and rural watersheds for the design
of storm drains and small drainage structures. The RM is recommended for analyzing the
runoff response from drainage areas up to approximately 1 square mile in size. It should not •
be used in instances where there is a junction of independent drainage systems or for
drainage areas greater than approximately 1 square mile in size. In these instances, the
Md1fied Rational Method (MRM) should be used for junctions of independent drainage
NRCS Hydrologic Method should be used for watersheds greater than approximately 1
square mile in size (see Section 4).
The RM can be applied using any design storm frequency (e.g., 100-year, 50-year, 10-year,
etc.). The local agency determines the design storm frequency that must be used based on
the type of project and specific local requirements. A discussion of design storm frequency
is provided in Section 2.3 of this manual. A procedure has been developed that converts the
6-hour and 24-hour precipitation isopluvial map data to an Intensity-Duration curve that can
be used for the rainfall intensity in the RM formula as shown in Figure 3-1. The RM is
applicable to a 6-hour storm duration because the procedure uses Intensity-Duration Design
Charts that are based on a 6-hour storm duration.
3.1.1 Rational Method Formula
The RM formula estimates the peak rate of runoff at any location in a watershed as a function
of the drainage area (A), runoff coefficient (C), and rainfall intensity (I) for a duration equal
to the time of concentration (Ta), which is the time required for water to
3-1
_EQUATION _IItIIll
Intensity (in/hr)
P6 D-0-645 IIIIII1 IIIIIIfl
su
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San Diego County Hydrology Manual Section: 3 Date: June 2003 Page: 3 of 26
flow from the most remote point of the basin to the location being analyzed. The RM
formula is expressed as follows:
Q=CIA
Where: Q = peak discharge, in cubic feet per second (cfs)
C = runoff coefficient, proportion of the rainfall that runs off the surface (no
units)
I = average rainfall intensity for a duration equal to the Tc for the area, in
inches per hour (Note: If the computed T is less than 5 minutes, use 5
minutes for computing the peak discharge, Q)
A =drainage area contributing to the design location, in acres
Cibining the units for the expression CIA yields:
(1 acre x inch) 143560 ft2 (1 foot ' ( 1 hour '\
= l.008cfs hour acre J 12 inches) 3,600 seconds) I
For practical purposes the unit conversion coefficient difference of 0.8% can be ignored.
The RM formula is based on the assumption that for constant rainfall intensity, the peak
discharge rate at a point will occur when the raindrop that falls at the most upstream point in
the tributary drainage basin arrives at the point of interest.
Unlike the MRM (discussed in Section 3.4) or the NRCS hydrologic method (discussed in
Section 4), the RM does not create hydrographs and therefore does not add separate subarea
hydrographs at collection points. Instead, the RM develops peak discharges in the main line
by increasing the Tc as flow travels downstream.
Characteristics of, or assumptions inherent to, the RM are listed below:
The discharge flow rate resulting from any I is maximum when the I lasts as long as or
longer than the T.
3-3
San Diego County Hydrology Manual Section: 3 Date: June 2003 Page: 4 of 26
The storm frequency of peak discharges is the same as that of! for the given T.
The fraction of rainfall that becomes runoff (or the runoff coefficient, C) is independent
of I or precipitation zone number (PZN) condition (PZN Condition is discussed in
Section 4.1.2.4).
The peak rate of runoff is the only information produced by using the RM.
3.1.2 Runoff Coefficient
Table 3-1 lists the estimated runoff coefficients The cqncePts related to
runoff coefficient were evaluated in a report entitled Evaluation, Rational Method "C"
Values (Hill, 2002) that was reviewed by the Hydrology Manual Committee. The Report is
available at San Diego County Department of Public Works, Flood Control Section and on
the San Diego County Department of Public Works web page.
The runoff coefficients are based on land use and soil type. Soil type can be determined from
the soil type map provided in Appendix A. An appropriate runoff coefficient (C) for each
type of land use in the subarea should be selected from this table and multiplied by the
percentage of the total area (A) included in that class. The sum of the products for all land
uses is the weighted runoff coefficient (E[CA]). Good engineering judgment should be used
when applying the values presented in Table 3-1, as adjustments to these values may be
appropriate based on site-specific characteristics. In any event, the impervious percentage
(% Impervious) as given in the table, for any area, shall govern the selected value for C. The
runoff coefficient can also be calculated for an area based on soil type and impervious
percentage using the following formula:
3-4
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 5 of 26
C = 0.90 x (% Impervious) + C, x (1 - % Impervious)
Where: C, = Pervious Coefficient Runoff Value for the soil type (shown in
Table 3-1 as Undisturbed Natural Terrain/Permanent Open Space,
0% Impervious). Soil type can be determined from the soil type map
provided in Appendix A.
The values in Table 3-1 are typical for most urban areas. However, if the basin contains rural
or agricultural land use, parks, golf courses, or other types of nonurban land use that are
expected to be permanent, the appropriate value should be selected based upon the soil and
cover and approved by the local agency.
3-5
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 6 of 26
Table 3-1
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use Runoff Coefficient "C"
- Soil Type
- A B C D NRCS Elements County Elements % IMPER.
Undisturbed Natural Terrain (Natural) Permanent Open Space 0* 0.20 0.25 0.30 0.35
Low Density Residential (LDR) Residential, 1.0 DU/A or less 10 0.27 0.32 0.36 0.41
Low Density Residential (LDR) Residential, 2.0 DU/A or less 20 0.34 0.38 0.42 0.46
Low Density Residential (LDR) Residential, 2.9 DU/A or less 25 0.38 0.41 0.45 0.49
Medium Density Residential (MDR) Residential, 4.3 DU/A or less 30 0.41 0.45 0.48 0.52
Medium Density Residential (MDR) Residential, 7.3 DU/A or less 40 0.48 0.51 0.54 0.57
Medium Density Residential (MDR) Residential, 10.9 DU/A or less 45 0.52 0.54 0.57 0.60
Medium Density Residential (MDR) Residential, 14.5 DU/A or less 50 0.55 0.58 0.60 0.63
High Density Residential (HDR) Residential, 24.0 DU/A or less 65 0.66 0.67 0.69 0.71
High Density Residential (HDR) Residential, 43.0 DU/A or less 80 0.76 0.77 0.78 0.79
Commercial/Industrial (N. Corn) Neighborhood Commercial 80 0.76 0.77 0.78 0.79
Commercial/lndustrial(G. Corn) General Commercial 85 0.80 0.80 0.81 0.82
Commercial/Industrial (O.P. Corn) Office Professional/Commercial 90 0.83 0.84 0.84 0.85
Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84 0.84 0.85
Commercial/Industrial (General I.) General Industrial 95 - 0.87 0.87 0.87 0.87
*The values associated with 0% impervious may be used for direct calculation of the runoff coefficient as described in Section 3.1.2 (representing the pervious runoff
coefficient, Cp, for the soil type), or for areas that will remain undisturbed in perpetuity. Justification must be given that the area will remain natural forever (e.g., the area
is located in Cleveland National Forest).
DU/A = dwelling units per acre I
NRCS = National Resources Conservation Service
3-6
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 7 of 26
3.1.3 Rainfall Intensity
The rainfall intensity (I) is the rainfall in inches per hour (in/hr) for a duration equal to the T
for a selected storm frequency. Once a particular storm frequency has been selected for
design and a Tr calculated for the drainage area, the rainfall intensity can be determined from
the Intensity-Duration Design Chart (Figure 3-1). The 6-hour storm rainfall amount (P6) and
the 24-hour storm rainfall amount (P24) for the selected storm frequency are also needed for
calculation of I. P6 and P24 can be read from the isopluvial maps provided in Appendix B.
An Intensity-Duration Design Chart applicable to all areas within San Diego County is
provided as Figure 3-1. Figure 3-2 provides an example of use of the Intensity-Duration
Design Chart. Intensity can also be calculated using the following equation:
I = 7.44 P6 D °645
Where: P6 = adjusted 6-hour storm rainfall amount (see discussion below)
D = duration in minutes (use T)
Note: This equation applies only to the 6-hour storm rainfall amount (i.e., P6 cannot be
changed to P24 to calculate a 24-hour intensity using this equation).
The Intensity-Duration Design Chart and the equation are for the 6-hour storm rainfall
amount. In general, P6 for the selected frequency should be between 45% and 65% of P24 for
the selected frequency. If P6 is not within 45% to 65% of P24, P6 should be increased or
decreased as necessary to meet this criteria. The isopluvial lines are based on precipitation
gauge data. At the time that the isopluvial lines were created, the majority of precipitation
gauges in San Diego County were read daily, and these readings yielded 24-hour
precipitation data. Some 6-hour data were available from the few recording gauges
distributed throughout the County at that time; however, some 6-hour data were extrapolated.
Therefore, the 24-hour precipitation data for San Diego County are considered to be more
reliable.
3-7
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San Diego County Hydrology Manual Section: 3 Date: June 2003 Page: 9 of 26
3.1.4 Time of Concentration
The Time of Concentration (Ta) is the time required for runoff to flow from the most remote
part of the drainage area to the point of interest. The I is composed of two components:
initial time of concentration (Ti) and travel time (Ti). Methods of computation for T1 and T1
are discussed below. The Ti is the time required for runoff to travel across the surface of the
most remote subarea in the study, or "initial subarea." Guidelines for designating the initial
subarea are provided within the discussion of computation of Ti. The Tt is the time required
for the runoff to flow in a watercourse (e.g., swale, channel, gutter, pipe) or series of
watercourses from the initial subarea to the point of interest. For the RM, the Tc at any point
within the drainage area is given by:
Tc = T T T1
Methods of calculation differ for natural watersheds (nonurbanized) and for urban drainage
systems. When analyzing storm drain systems, the designer must consider the possibility
that an existing natural watershed may become urbanized during the useful life of the storm
drain system. Future land uses must be used for Tc and runoff calculations, and can be
determined from the local Community General Plan.
3.1.4.1 Initial Time of Concentration
The initial time of concentration is typically based on sheet flow at the upstream end of a
drainage basin. The Overland Time of Flow (Figure 3-3) is approximated by an equation
developed by the Federal Aviation Agency (FAA) for analyzing flow on runaways (FAA,
1970). The usual runway configuration consists of a crown, like most freeways, with sloping
pavement that directs flow to either side of the runway. This type of flow is uniform in the
direction perpendicular to the velocity and is very shallow. Since these depths are ¼ of an
inch (more or less) in magnitude, the relative roughness is high. Some higher relative
roughness values for overland flow are presented in Table 3.5 of the HEC-1 Flood
Hydrograph Package User's Manual (USACE, 1990).
3-9
San Diego County Hydrology Manual
Date: June 2003 Section: 3 Page: 11 of 26
The sheet flow that is predicted by the FAA equation is limited to conditions that are similar
to runway topography. Some considerations that limit the extent to which the FAA equation
applies are identified below:
Urban Areas - This "runway type" runoff includes:
Flat roofs, sloping at 1% ±
Parking lots at the extreme upstream drainage basin boundary (at the "ridge" of a
catchment area).
Even a parking lot is limited in the amounts of sheet flow. Parked or moving
vehicles would "break-up" the sheet flow, concentrating runoff into streams that
are not characteristic of sheet flow.
Driveways are constructed at the upstream end of catchment areas in some- -
developments. However, if flow from a roof is directed to a driveway through
a downspout or other conveyance mechanism, flow would be concentrated.
Flat slopes are prone to meandering flow that tends to be disrupted by minor
irregularities and obstructions. Maximum Overland Flow lengths are shorter
for the flatter slopes (see Table 3-2).
Rural or Natural Areas - The FAA equation is applicable to these conditions since
(.5% to 10%) slopes that are uniform in width of flow have slow velocities consistent
with the equation. Irregularities in terrain limit the length of application.
Most hills and ridge lines have a relatively flat area near the drainage divide.
However, with flat slopes of .5% ±, minor irregularities would cause flow to
concentrate into streams.
Parks, lawns and other vegetated areas would have slow velocities that are
consistent with the FAA Equation.
The concepts related to the initial time of concentration were evaluated in a report entitled
Initial Time of Concentration, Analysis of Parameters (Hill, 2002) that was reviewed by the
Hydrology Manual Committee. The Report is available at San Diego County Department of
Public Works, Flood Control Section and on the San Diego County Department of Public
Works web page.
San Diego County Hydrology Manual
Date: June 2003 Section: 3 Page: 12of26
Note that the Initial Time of Concentration should be reflective of the general land-use at the
upstream end of a drainage basin. A single lot with an area of two or less acres does not have
a significant effect where the drainage basin area is 20 to 600 acres.
Table 3-2 provides limits of the length (Maximum Length (LM)) of sheet flow to be used in
hydrology studies. Initial Ti values based on average C values for the Land Use Element are
also included. These values can be used in planning and design applications as described
below. Exceptions may be approved by the "Regulating Agency" when submitted with a
detailed study.
Table 3-2
MAXIMUM OVERLAND FLOW LENGTH (LM)
-&INITIALTIME OF CONCENTRATION(T
Element* DU/
Acre
.5% 1% 2% 3% 5% 10%
LM Tj LM T1 LM TiLM Tj LM T, LM T1
Natural 50 13.2 70 12.5 85 10.9 100 10.3 100 8.7 100 6.9
LDR 1 50 12.2 70 11.5 85 10.0 100 1 9.5 100 8.0 1001 6.4
LDR 2 50 11.3 70 10.5 85 9.2 100 8.8 100 7.4 100 5.8
LDR 2.9 50 10.7 70 10.0 85 8.8 95 8.1 100 7.0 100 5.6
MDR 4.3 50 10.2 70 9.61 80 8.1 95 7.8 100 6.7 100 5.3
MDR 7.3 50 9.2 65 8.4 80 7.41 95 7.01 100 6.0 100 4.8
MDR 10.9 50 8.7 65 7.9 80 6.9 90. 6.4 100 5.7 100 4.5
MDR 14.5 50 8.2 65 7.4 80 6.5 90 6.0 100 5.4 100 4.3
HDR 24 50 6.7 65 6.1 1 75 5.1 90 4.9 95 4.3 100 3.5
HDR 43 50 5.3 65 4.7 75 4.0 85 3.8 95 1 3.4 100 2.7
N. Corn 50 5.3 60 4.5 75 4.0 85 3.8 95 3.4 100 2.7
G. Corn 50 4.7 60 4.1 75 3.6 85 3.4 90 2.9 100 2.4
O.P./Com 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 100 2.2
Limited I. 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 100 2.2
General I. 50 3.7 60 3.2 70 2.7 80 2.6 90 2.3 100 1.9
*See Table 3-1 for more detailed description
3-12
San Diego County Hydrology Manual Section: 3 Date: June 2003 Page: 13 of 26
3.1.4.1A Planning Considerations
The purpose of most hydrology studies is to develop flood flow values for areas that are not
at the upstream end of the basin. Another example is the Master Plan, which is usually
completed before the actual detailed design of lots, streets, etc. are accomplished. In these
situations it is necessary that the initial time of concentration be determined without detailed
information about flow patterns.
To provide guidance for the initial time of concentration design parameters, Table 3-2
includes the Land Use Elements and other variables related to the Time of Concentration.
The table development included a review of the typical "layout" of the different Land Use
Elements and related flow patterns and consideration of the extent of the sheet flow regimen,
the effect of ponding, the significance to the drainage basin, downstream effects, etc.
3.1.4.111 Computation Criteria
(a) Developed Drainage Areas With Overland Flow - Timay be obtained directly from the
chart, "Rational Formula - Overland Time of Flow Nomograph," shown in Figure 3-3
or from Table 3-2. This chart is based on the Federal Aviation Agency (FAA) equation
(FAA, 1970). For the short rain durations (<15 minutes) involved, intensities are high
but the depth of flooding is limited and much of the runoff is stored temporarily in the
overland flow and in shallow ponded areas. In developed areas, overland flow is
limited to lengths given in Table 3-2. Beyond these distances, flow tends to become
concentrated into streets, gutters, swales, ditches, etc.
3-13
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 14 of 26
(b) Natural Or Rural Watersheds - These areas usually have an initial subarea at the
upstream end with sheet flow. The sheet flow length is limited to 50 to 100 feet as
specified in Table 3-2. The Overland Time of Flow Nomograph, Figure 3-3, can be
used to obtain T. The initial time of concentration can excessively affect the
magnitude of flow further downstream in the drainage basin. For instance, variations
in the initial time of concentration for an initial subarea of one acre can change the
flow further downstream where the area is 400 acres by 100%. Therefore, the initial
time of concentration is limited (see Table 3-2).
The Rational Method procedure included in the original Hydrology Manual (1971) and
Design and Procedure Manual (1968)_ included alO minute value to be_ added Jp the initial ___.
time of concentration developed through the Kirpich Formula (see Figure 3-4) for a natural
watershed. That procedure is superceded by the procedure above to use Table 3-2 or Figure
3-3 to determine Tj for the appropriate sheet flow length of the initial subarea. The values for
natural watersheds given in Table 3-2 vary from 13 to 7 minutes, depending on slope. If the
total length of the initial subarea is greater than the maximum length allowable based on
Table 3-2, add the travel time based on the Kirpich formula for the remaining length of the
initial subarea.
3.1.4.2 Travel Time
The Tt is the time required for the runoff to flow in a watercourse (e.g., swale, channel,
gutter, pipe) or series of watercourses from the initial subarea to the point of interest. The T
is computed by dividing the length of the flow path by the computed flow velocity. Since the
velocity normally changes as a result of each change in flow rate or slope, such as at an inlet
or grade break, the total Tt must be computed as the sum of the It's for each section of the
flow path. Use Figure 3-6 to estimate time of travel for Street gutter flow. Velocity in a
channel can be estimated by using the nomograph shown in Figure 3-7 (Manning's Equation
Nomograph).
3-14
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 15 of 26
(a) Natural Watersheds - This includes rural, ranch, and agricultural areas with natural
channels. Obtain Tt directly from the Kirpich nomograph in Figure 3-4 or from the
equation. This nomograph requires values for length and change in elevation along
the effective slope line for the subarea. See Figure 3-5 for a representation of the
effective slope line.
This nomograph is based on the Kirpich formula, which was developed with data
from agricultural watersheds ranging from 1.25 to 112 acres in area, 350 to 4,000 feet
in length, and 2.7 to 8.8% slope (Kirpich, 1940). A maximum length of 4,000 feet
should be used for the subarea length. Typically, as the flow length increases, the
depth of flow will increase, and therefore it is considered a concentration of flow at
__ppJits_beyond lengths listed in Figure 3-2: However, because the JgMick formula
has been shown to be applicable for watersheds up to 4,000 feet in length (Kirpich,
1940), a subarea may be designated with a length up to 4,000 feet provided .the
topography and slope of the natural channel are generally uniform.
Justification needs to be included with this calculation showing that the watershed
will remain natural forever. Examples include areas located in the Multiple Species
Conservation Plan (MSCP), areas designated as open space or rural in a community's
General Plan, and Cleveland National Forest.
(b) Urban Watersheds - Flow through a closed conduit where no additional flow can enter the
system during the travel, length, velocity and Tt are determined using the peak flow in
S
the conduit. In cases where the conduit is not closed and additional flow from a
contributing subarea is added to the total flow during travel (e.g., street flow in a
gutter), calculation of velocity and Tt is performed using an assumed average flow
based on the total area (including upstream subareas) contributing to the point of
interest. The Manning equation is usually used to determine velocity. Discharges for
small watersheds typically range from 2 to 3 cfs per acre, depending on land use,
drainage area, and slope and rainfall intensity.
Note: The MRM should be used to calculate the peak discharge when there is a junction
from independent subareas into the drainage system.
3-15 I
Appendix "F"
Rainfall-Intensity-Duration Chart
and
Isopluvial Maps
1' 15! 25 5 j554 6
Duration I I I I I I I I I I I
5 2.63 l3.955.27.6.597.9o,9.22 10.5411.86 I3.l74 l4.49I5.8I
7 2.12 3i84245.3O.6.367.4f 8.48 9.54 'I0.60111.66j 12.72
10 168 2533742I O559O 674 758*8421927 101%
15 1.30 0.95 2.59 3.24 3.89 4.54 5.19 5.84 6.49 713778
20 l.08l.622 15*2.693.233.77431 4.85 5.391 5.93 6.46
25 0.93 1 40; 1.87l2.332.80327 3.73 4.20 4.67 5.13 4 5.60
30 083 124 1.66!;i.07'2.49:2.90, 3.32 373 415'455498
40 0.69 103138 1.72 2.07 2.4112763.1O .345; 3.79 413
50 0.60 .0.90 119:149 1.79,42.09 2,39 2.69 2.981 3.28
".58 o :0.80 I.W.1.06;
1. 3
3 591862 39 2.65 292 3.18
90 0.41_0.61* 082l02 123:1.43, 1.53 184 2041225 245
120 0,34 O.5l:0.68'0.85 1.02 1.19 136 153 1.70 1.87 2.04
iso 0.29 0.440.690.73 :0.88 I 03 1.18 . 1.32 1 471 1.62 1.76
150 0.26 10.39O.S2t0.651 0.780.91 104 1.18: 1.31 1.44 '1.57
'240 0.22 :0.33; 0.43 1 9.54, 0.6510.76. 6.87 0.98 1.08 1.19 1.30
300 0.19 .6 2810.38,0.47.0.56O.66 0,75 0.850.94 I 1.03 1.13
360 0.17 •0.20.330.420.5010.58 0.67 0.75. 0.84 0.92 100
FIG U RE
3 H
C
- - • • uIk!IuuImIllhuIIaliIutllInlnllHInnmfl1LHIFuuulnuIurnnugIIwi1
,ftIllIHhII1QllIHfflIllIffluhIllfflu1llUIfflhIIIIIIUu1IIHlllllllflIfl
1Ii1!I!illHIIllhJllllUuIflHllhIllhIJEQUATIONIIllI
• JIllI
offlifl.111S ~1 10041 P6 = 6-Hour Precipitation (in)
Duration
IM
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iLqlq • IIhII!Ihi!!Ill !iIllllIllfflflllOIll1llI
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• IffIOIIUIllIIllfl
.
Minutes Hours
Duration
Intensity-Duration Design Chart - Template
Directions for Application:
From precipitation maps determine 6 hr and 24 hr amounts
for the selected frequency. These maps are included in the
County Hydrology Manual (10. 50, and 100 yr maps included
in the Design and Procedure Manual).
Adjust 6 hr precipitation (II necessary) so that It is within
the range of 45% to 65% of the 24 hr precipitation (not
applicaple to Desert).
Plot 6 hr precipitation on the right side of the Chart.
Draw a line through the point parallel to the plotted lines.
This line is the intensity-duration curve for the location
being analyzed.
Application Form:
Selected frequency tO year
P6 = Li in.. P24 = • 0 !. = % %(2)
_ • P24
o : (c) Adjusted P6t2 = Fl in.
' (d) l = (a min. 5.0
6.5 (e) I = in./hr.
3 !. .5 - Note: This chart replaces the Intensity-Duration-Frequency
3.0 curves used since 1965.
6.(
5.0
4.0
3.0
2.0
:r a C -I
03 C)
6.0 V.
5.5 9L
5.0
4.5 .
4.0
3.5
3.0
2.5
2.0
III
Him
IMBROU DIE
IllIftIIfllllluIllhIIIlluIllhI ___ II1ll
11111111
_6-Hour Precipitation (in)
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11"M I mq M11111HURRHIS
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slou 9 NOR
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IIIIIIIllhIIllll1llhIllhIllI011hIllflIllUIHh1OIhhll!Illui?llfl
=1101MMMIUM11MIMPIMMI i!ioIIfluiini 1E
IN-- RU mmii U UUIMU1IW 11W III flHflffl .muuiii, I
1III Mill IllN Mill mill IfflhIIIOIfflllh1U
IIUilIllhIllllfflMllIllllIllllhIfflIlluIllllllllhIllhIIIiilllh1OHllHillllfl
15 20 30 40 50 I 2 3 4 5 6
Minutes Hours Duration
Intensity-Duration Design Chart - Template
0.1
5 6 78910
Directions for Application:
From precipitation maps determine 6 hr and 24 hr amounts
for the selected frequency. These maps are included in the
County Hydrology Manual (10. 50. and 100 yr maps included
In the Design and Procedure Manual).
Adjust 6 hr precipitation (if necessary) so that it is within
the range of 45% to 65% of the 24 hr precipitation (not
applicaple to Desert).
Plot 6 hr precipitation on the right side of the chart.
Draw a line through the point parallel to the plotted lines.
This line is the intensity-duration curve for the location
being analyzed.
Application Form:
Selected frequency OO year
6 = ______ 24 = 4.4. ..!. = %.8 %(2) P24
Adjusted P6 (2) = 15-in.
Ix Co mm.
I = 5. in/hr.
Note: This chart replaces the Intensity-Duration-Frequency
curves used since 1965.
.1................ P6 111.522.533 3.SJ 4 4.5 515.516
Duri',onp iII'Iip _I _tip 1ip1p
2.63 ''5!5?7:6.S9i?.00O.22 I0.54'!1.86iI3.1714.49,l5.8I
7 .3 9.54 jlO.60111.661 12.72
10 1.68 2.53,3.371'4.21 f05t5.9O6.74 7.58; 8.421 9.27 'i6li
15 13095 2932438945415l9 584 6.40!7131778
20 1.08 1.622.1S'2.69j3.233.771 4:334.855.39 t 5.93 16.46
25 093 140 187,233 280327,3fl 420,467 513;560
30 0.83 l.24! 1.6Sf 2.07'2.491290' 3.32 L73 :4.15 455! 4.98
40 0.69 103'1.131 1.72:2.072.411 276 3.10 4.33 so o.00 'o.o 1.19:1.49' i.792.09, 2:39 ' 2.69 j.248*t 3.28
60 0.53:0.81Y l.O6 13I 1.59:1 8612 32 2.39 . 2.SS1 2.92 3.38 éO 0.41 0:610 82 3.02 1231.43 3.63 4 : 2.04 1 2261 2.45
*20 0.34 O.5I9.68;0.85 1.02 1.19: 1.36j 3.53 1.70I.87 .O.1
ISO 0.29 0.440.59!0.730.88 i.031 3.38 1 321 147 3.62 1
380 026 039 052 065 078;0913 104 138 3l 144H57
240 022 6331043:O5496SfO761087 098 108 319 130
300 0.19 028.0.38; 047'10.56i0.661 o.is 0:65 1ó.g '3.03 113
360 0.17O.250.33,0.420.500.58_0.75 _0.840.021.0o
FIGURE
34