HomeMy WebLinkAboutDRAINAGE CALCULATIONS - 11-00093 - Premier Performance - Remodel1
DRAINAGE CALCULATIONS
Given: The calculation of runoff from the project site hand the sizing of the
drainage pipe) is determined using the hydrograph for Rexburg, ID.
Required; Calculate draIs
inage using the RATIONAL METHOD, Q = MA in ac-
in/hr. Lot size =1.24 acres, i = rainfall intensity in inches/hour dor a
25 -year storm.
Rainfall intensity for a I hr. duration '1 00 -dear stois 0.9 'inches
according to the hydrograph to be used. These values are a good
average within the Rexburg city limits. Use a 10 minute duration (2.9
inches/hr) for sizing pipes.
Drainage zona consists of the asphalt areas, the roof area of a
future building, 3a0' of sidewalk, and some small orass/planted
areas. This drainage is coflectecf into a catch basin (24"x 24" x 24"
deep) connected to a 12" diameter pipe connected to the Rexburg
pity storm drain syst.
Permeability coefficients used for calculating run-off are:
1 .00 for roofs
x}.95 for asphalt and concrete surfaces,
Q. 1 5 for grass/planted areas.
Runoff areas are:
the area of the future bldg roof = 16,600 sf
asphalt and concrete surfaces = 19,300 sf
grass/planted areas = 1 6. 900 sf
52,800 sf
Using the 2.9 in/hr {i value] for suing pipes,
Total runoff flow per hour = [(1.0)(16,600 s� + (0.95)(19,300 s� +
(0. 15) (16,900 sffl(2.9/12 ft) (7.48 g a 1/cfl = (67,7 33 g a I lons/h r) or 1, 129
gpm after time of concentration
Size the pipe lateral between catch basin and manhole. (Using the
Manning equation,
Q = AV., with A = 3.14d2/4, or 0.79 sf
V = 1.49/n (R using using 12" PVC SDR21 pipe (n = 0.010), and
the slope of (0.0059),
V = (1.49/0.01 0)(1.00/4)2/3(0.0059)"2 = 4.52 ft/s,
Therefore,
Q = AV = (0.79)sf(4-52 ft/s) = 3.57 cfs or 1,600 gpm.
1, 600 > 1, 129, therefore okay.