Public UtilityENGINEERINGINTERMOUNTAIN UTILITY
UTILITY REPRESENTATIVES
REPRESENTATIVES PHONE
WATER
EROSION CONTROL
CITY OF MERIDIAN, ADA COUNTY, IDAHO
PARCELS C,D & E AND LOT 7 UNA MAS SUBDIVISION
VERRASO NORTH
TITLE SHEET FOR
GOVT LOT 4 SECTION 4, T. 3 N, R.1 E., B.M.
“
”
ENGINEERINGINTERMOUNTAIN
ENGINEERINGINTERMOUNTAIN
ENGINEERINGINTERMOUNTAIN
(208) 941-1245
AS PREPARED BY:
Intermountain Engineering
2587 SouthSide Blvd..
Melba, Idaho 83641
March 20, 2020
STORM DRAINAGE
CALCULATIONS FOR:
VERRASO VILLAGE NORTH
Meridian, Idaho
3/20/20
CALCULATIONS
FLOW CALCULATIONS
PIPE SIZING AND HYDRAULIC GRADE CALCULATIONS
DETENTION VOLUME CALCULATIONS
POND VOLUME CALCULATIONS
GENERAL NOTES
The pond volume calculations are based on an average of the water surface area and the pond bottom area,
applied over the design pond depth.
Runoff water will enter the drop inletrs after being collected and flowing in the gutter plate. Water then flows
through a sand and greese trap before entering the sepage bed. All water will be retained on site and percolate
into the underlying soils. There will be no discharge off site.
Flow and volume for the basin areas were calculated using the Rational Method. The "C" coefficient used in the
calculations is based on weighted values as shown. Since historical drainage is not available, a 100-yr event is
examined.
These calculations employ the Manning Equation. The hydraulic grade is based on calculated flow and selected
pipe size.
Since an outfall is not provided, detention volume is based on the rational method using the 100 year storm over
the worst case time of concentration & 1 hour 100 year storm.
RATIONAL METHOD
Q=CiA where: Q = Runoff Rate, cfs
C = Runoff Coefficient
i = Storm Intensity, in./hr.
A = Basin Area(s), acres
MANNING EQUATION
V=1.49R2/3S1/2 / n where: V = Velocity, fps
R = Hydraulic Radius, ft.
S = Channel Slope, ft./ft.
n = Manning Roughness Coefficient
PERCOLATION VOLUME (SCS TRIANGULAR UNIT HYDROGRAPH METHOD)
V=(Area)(Perc. rate)(t )/ (12)(60)
where: V = Volume, cu. ft.
Area = Infiltration bed area, sf
Perc. rate = percolation rate, in/hr
t = worst-case duration, min.
ORIFICE EQUATION
Q=(Coefficient)(3.1416 x Radius2)(64.4 x Head)1/2
where: Head=W.S.E. - Center of Orifice
=81.50-78.14
=3.4'
OTHER EQUATIONS USED
Q=VA where: Q = Flow, cfs
V = Velocity, fps
A = Cross Sectional Area, sq. ft.
EQUATIONS USED IN CALCULATIONS
2015 Engineering Division Development Policy Manual Division 100
Rev. Date: February 17, 2015 Section 106 - 19 of 20
INTERMOUNTAIN ENGINEERING Verraso Village North
2587C Southside Blvd.
Melba, Idaho 83641 Meridian, Idaho
(208) 941-1245 (FAX) 495-2469 DATE 3/20/2020
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
PRE- DEVELOPED FLOW AND VOLUME CALCULATION Area 1 FILE: Verraso North PAGE
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
The weighted runoff Coefficient for the developed site is calculated.
C = RUNOFF COEFFICIENT (EXIBIT "B" Recommended "C" Coefficients for "Rational
Method Equation" - Peak Rate of Discharge)
Cu = UNIMPROVED AREA RUNOFF COEFFICIENT
= 0.10
Cd = DEVELOPED SINGLE FAMILY RESIDENTIAL RUNOFF COEFFICIENT
= 0.50 (or use calculated (weighted) value if higher)
BLDG & PAVED ACRES (0.95) + LANDSCAPED ACRES (0.10)
= -------------------------------------------------------------------------------------------------------------------------------------
TOTAL ACRES
0.885 * (0.95) +0.375 * (0.10)
= ----------------------------------------------------------------------------------------------------------0.70
1.260
PRE-DEVELOPED FLOW AND VOLUME CALCULATIONS
The pre-developed flow for the project site and the upstream area from the site are calculated using
the Rational Method. The time of concentration is based on overland flow to a point of channelization
and then the travel time in the existing channel to the downstream end of the concerned area.
The pre-developed volume is calculated using the SCS Triangular Unit Hydrograph. Pre-developed
volume and flow are based on the 100-year storm event and site caracteristics.
Note the basin area is only accounting for the area of the basin that we will be able to discharge from.
BASIN CHARACTERISTICS
BASIN AREA=0.00 acres
OVERLAND FLOW LENGTH=345.00 feet
OVERLAND SLOPE=1.21%
RUNOFF COEFFICIENT (C)=0.10
FLOW AND VOLUME CALCULATION RESULTS
OVERLAND TIME OF CONCENTRATION 31.4 min
100 YEAR STORM INTENSITY (i)= 1.57 in/hr
100 YEAR PEAK FLOW (Q)=0.00 cfs NO DISCHARGE
100 YEAR VOLUME (from SCS triangular unit hydrograph)= - cf
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0FLOW (cfs)TIME (min)
SCS TRIANGULAR UNIT HYDROGRAPH
PRE-DEVELOPED FLOW
INTERMOUNTAIN ENGINEERING Verraso Village North
2587C Southside Blvd.
Melba, Idaho 83641 Meridian, Idaho
(208) 941-1245 (FAX) 495-2469 DATE 3/20/2020
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
POST- DEVELOPED FLOW AND VOLUME CALCULATIO Area 1 FILE: Verraso North PAGE
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Post-developed flow for the basin areas are also calculated using the Rational Method. The
time of concentration is based on a combination of lot to street time, gutter time and applicable
pipe time prior to reaching the retention area. The lot time is calculated using
Manning's kinematic solution (Overton and Meadows 1976). This equation is also used for sheet
flow of less than 300 feet. The gutter time is calculated using the travel length along the gutter
and the gutter velocity at the design slope. Pipe time is calculated using the pipe travel length
and an average pipe velocity calculated using manings formula.
STORM RETURN PERIOD = 100 YEAR
BASIN CHARACTERISTICS
BASIN AREA= 1.260 acres
2 YEAR 24 HOUR RAINFALL (in)= 1.20 in
OVER GRASS FLOW TO CURB & GUTTER 20.00 feet
LOT SLOPE =0.01 FT/FT
MANNING'S n FOR GRASS= 0.24
CONCRETE CURB & GUTTER roll
GUTTER FLOW LENGTH=0.00 feet
GUTTER SLOPE=0.40%
MANNING'S n FOR CURB= 0.016
PIPE LENGTH=- feet
PIPE DIAMETER= 12 IN
PIPE SLOPE 0.0022
MANNING'S n FOR PIPE= 0.013
AVERAGE PIPE VELOCITY= 1.94 ft/sec
RUNOFF COEFFICIENT (C)=0.70 (weighted)
FLOW AND VOLUME CALCULATION RESULTS
LOT TRAVEL TIME= 8.5 min.
GUTTER TRAVEL TIME= 0.0 min.
PIPE TRAVEL TIME= 0.0 min. Method 1 Method 2
uses Tc 1hr, 100 yr. Tc
TOTAL TIME OF CONCENTRATION 8.5 min 60.00 min
100 YEAR STORM INTENSITY (i)= 4.11 in/hr 1.00 in/hr
100 YEAR PEAK FLOW (Q)= 3.61 cfs 0.88 cfs
100 YEAR VOLUME (from SCS triangular unit hydrograph)= 2,453 cf 4,221 cf
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0 5.0 10.0 15.0 20.0 25.0FLOW (cfs)TIME (min)
SCS TRIANGULAR UNIT HYDROGRAPH
POST-DEVELOPED FLOW
INTERMOUNTAIN ENGINEERING Verraso Village North
2587C Southside Blvd.
Melba, Idaho 83641 Meridian, Idaho
(208) 941-1245 (FAX) 495-2469 DATE 3/20/2020
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DETENTION/RETURN VOLUME CALCULATIONS Area 1 FILE: Verraso North PAGE
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
The method calculates the required detention volume by subtracting the calculated
pre-developed volume from the post-developed SCS Triangular Unit Hydrograph. Then the
volume is increased by 15 percent to compensate for detention volume losses due to
sediment between maintenance periods.
STORM RETURN PERIOD = 100 YEAR
UNIT HYDROGRAPH METHOD
PRE-DEVELOPED SITE DISCHARGE = 0.00 cfs (worst case design)
POST-DEVELOPED VOLUME = 4,221 cf (worst case design)
DETENTION VOLUME REQUIRED = 4221 cf
15% VOLUME FOR SEDIMENT = 633 cf
TOTAL VOLUME REQUIRED = 4854 cf
POND VOLUME CALCULATIONS
The pond volume calculations are based on the equasion s Also included in this section are
infiltration bed calculations. Infiltration bed volume calculations are based on void space
in the drain rock and percolation volume which occurs over the time base for the
post-developed SCS Triangular Unit Hydrograph. If infiltration beds are not used, then only the
pond bottom percolation is considered.
REQUIRED DETENTION =4854 cf
POND VOLUME CALCULATION
POND BOTTOM AREA =3329.00 sf
POND WATER SURFACE AREA =4138.00 sf
POND TOP BANK ELEV =2561.86 ft
POND WATER SURFACE ELEV =2561.86 ft
POND INVERT ELEV =2561.46 ft
SEASONAL GROUND WATER =2540.00 Testhole showed no water at 10' BELOW GROUND
POND FREEBOARD = 0.00 ft
INVERT TO GROUND H2O = 21.46 ft
POND DEPTH = 0.40 ft
POND STORAGE= 1493.40 cf
INFILTRATION BED CALCULATION
INFILTRATION BED BOTTOM AREA =480.00 sf (entire bottom L = 60.0 W= 8.0
INFILTRATION BED DEPTH =8.00 ft
VOID SPACE IN DRAIN ROCK= 40%
PERCOLATION RATE =8.00 in/hr
VOLUME IN VOIDS = 1536.00 cf
PERCOLATION VOLUME = 854 cf
INFILTRATION STORAGE = 3884 cf
TOTAL DETAINED VOLUME = 5377 cf
> 4854 cf (required)
THEREFORE STORAGE IS ADEQUATE
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0 5.0 10.0 15.0 20.0 25.0FLOW (cfs)TIME (min)
SCS TRIANGULAR UNIT HYDROGRAPH
POST-DEVELOPED FLOW
INTERMOUNTAIN ENGINEERING Verraso Village Three
2587C Southside Blvd.
Melba, Idaho 83641 Meridian, Idaho
(208) 941-1245 (FAX) 495-2469 DATE 4/05/2017
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
PRE- DEVELOPED FLOW AND VOLUME CALCULATION Area 1 FILE: Verraso Three PAGE
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TIME REQUIRED TO DISSIPATE VOLUME
TIME CALCULATION ASSUMES A DISCHARGE OFFSITE
EQUAL TO THE CALCULATED PRE-DEVELOPED FLOW OF 0.00
CFS AND PERCOLATION OVER THE POND BOTTOM AREA
EQUAL TO 8 in./hr.
TIME = 15.17 hours