HomeMy WebLinkAboutDrainage Computations SummaryDRAINAGE COMPUTATIONS SUMMARY
for
BRIGHT HOUSE COMMUNICATIONS BUILDING
352 Gradle Drive, Carmel, Indiana
PROJECT DESCRIPTION
Bright House Communications proposed to construct a 1,295 sf communications utility building at
352 Gradle drive. The site is currently developed and contains an existing building and asphalt
parking lot. Bright House will remove some of the existing pavement where the proposed
building is to be placed and install a small amount of new pavement. A portion of the existing
parking lot at this site will be re- striped to provide the required number of parking spaces for this
new building and the existing building current located on the property.
EXISITNG AND PROPOSED CONDITIONS
The project will disturb approximately 0.93 acres of the existing site. Approximately 768 sf of
the existing parking lot will be removed. The new building and pavement will total
approximately 2,022 sf. The net increase of hard surfaces on the site will be 1,254 sf (0.03
acres). Because the disturbed area and addition of impervious surfaces are less than 1 /4 acre, we
do not propose to provide storm a storm water detention facility or storm water quality BMP.
Refer to Exhibit A for a drawing showing the existing and proposed impervious surfaces.
DRAINAGE DESCRIPTION
The existing site and a portion of the property to the south of the site drains to an eight inch pipe
at the northeast corner of the site.
The following sections contain calculations for the existing and proposed discharges from the
project site. Based on these calculations we propose to remove the existing eight inch pipe and
replace it with a new 12 inch pipe.
EXISTING DISCHARGE
Drainage Area
We have assumed that the entire roof of the existing building on the property south of the site
drains to the inlet at the northeastern edge of the parking lot via the 6 inch pipe under the
parking lot. The assumption is based on our observation that the existing building does not have
external downspouts and there are no apparent roof drain outlets in the drainage ditch along
Gradle Drive. In addition to the southern building, approximately half of the roof of the existing
building to the east of the proposed building drains to the existing inlet. Refer to the attached
drawing for the boundary of the assume drainage area.
A 40,075 sf 0.92 ac.
Time of Concentration
The longest flow path is from the southern edge of the existing building through the building's roof
drain pipes to the existing inlet at the northeast corner of the property. Refer to the attached
drawing for an illustration of the assumed travel path.
The time of concentration from the roof of the southern building is calculated as the inlet time (time
for storm water to reach the first roof drain inlet on the south edge of the existing building) plus
the travel time through the roof drain pipes. The inlet time is assumed to be 5 minutes. The travel
time through the roof drain pipes is calculated using 3 fps as the velocity in the pipes. Therefore:
T 5 minutes ((290 ft. x 3 fps)) /60) 6.6 minutes
The time of concentration from the roof of the eastern building is the time for runoff to reach the
ground via gutter and downspouts (assumed to be 5 minutes) plus the travel time across the lawn
to the inlet. The travel time across the lawn is calculated using methods in TR -55 for sheet flow
across a dense grass surface. Therefore:
Sheet Flow time (0.007 (nL) 0.8)/((P) 0.5)(S 0.4))
Where n 0.24, L 90 feet, P 2.66, S 0.06
Sheet flow time (0.007 (0.24"90) 0.8)/((2.66 0.5)(0.06 0.4)) 9.25 minutes
Rainfall Intensity
The longest time of concentration was used to select the 10 -yrar rainfall intensity. This time of
concentration is 9.25 minutes and the corresponding rainfall intensity is approximately 5.36 in /hr
Runoff Coefficient
The runoff coefficient is a composite of the building roofs, pavement and lawn.
Roof area 1 1,797 sf
Pavement and sidewalk area 22,110 sf
Lawn area 6,168 sf
C ((1 1,797)(0.90) (22,1 10)(0.85) (6,168)(0.25))/40,075 0.77
Existing 10 -year Discharge
The existing 10 -year discharge is:
Q GA (0.77)(5.36)(0.92) 3.80 cfs
PROPOSED DISCHARGE
Drainage to proposed Structure No. 1
Drainage Area
A 0.78 ac. 33,977 sf
Time of Concentration
The time of concentration is the time calculated from the roof of the southern building.
T 5 minutes ((290 ft. x 3 fps)) /60) 6.6 minutes
Rainfall Intensity
The rainfall intensity for a 6.6 minute time of concentration is: 5.83 in /hr
Runoff Coefficient
The runoff coefficient is a composite of the building roofs, pavement and lawn.
Roof area 1 1,740 sf
Pavement /sidewalk area 21,084 sf
Lawn area 1,153 sf
C ((1 1,740)(0.90) (21,084)(0.85) (1,153)(0.25)) 33,977 0.85
10 -Year Discharae
Q GA (0.78)(5.83)(0.85) 3.87 cfs
Pipe Size
Manning's equation was used with Slope 0.018 and n 0.01 1
The required size is: 12 inches
Drainage to proposed Structure No. 2
Drainage Area
A 0.14 ac. 6,098 sf
Time of Concentration
The time of concentration from the roof of the eastern building is the time for runoff to reach the
ground via gutter and downspouts (assumed to be 5 minutes) plus the travel time through the
proposed swale to the inlet. The travel time through the swale is estimated based on an assumed
velocity of 3 fps.
T 5 minutes (1 1 3 ft /3 fps) 5.63 minutes
The time of concentration for Structure No. 1 (6.6 minutes) will be used to calculate the discharge
since it is longer time of concentration.
Rainfall Intensity
The rainfall intensity for a 6.6 minute time of concentration is: 5.83 in /hr
Runoff Coefficient
The runoff coefficient is a composite of the building roofs, pavement and lawn.
Roof area 1,312 sf
Pavement /sidewalk /gravel area 810 sf
Lawn area 3,976 sf
C ((1,31 2)(0.90) +(810)(0.85) +(3,976)(0.25)) 6,098 0.46
10 -Year Discharge
Q (sum CA)i ((0.78)(0.85) (0.14)(.46))(5.83) 4.24 cfs
Pipe Size
Manning's equation was used with Slope 0.019 and n 0.1 1
The required size is: 12 inches