gscotchie
Civil/Environmental
- Dec 10, 2015
- 8
I have a subcatchment described in the attached photo.
The subcatchmet has a roof with five disconnected roof drains that spill onto stone (open graded ASTM #2 stone with no fines) areas. Each stone area has a catch basin. The catch basins are part of a longitudinal network which outfalls into an existing storm water pond. The stone depth is approximately 12". Assuming a porosity of the stone to be 0.54 I could assign a special CN value of 61 for the stone.
S=1000⁄(CN-10)
Where: S = depth in inches
If S is calculated as the available voids in the ASTM #2 stone, the CN can be estimated by rearranging the equation as:
CN=1000⁄(S+10)
CN=1000⁄(0.54*12"+10) = 61
I would like to take advantage of exfiltration (1.02 in/hour) beneath the stone. Should I use a CN value of 96 for the stone, use a pond depth of one foot, the surface area of the stone, the volume of the voids as my storage volume, and 1.02 in/hour for exifiltration. I'm afraid I'll be overestimating my storage if I use a value of 61 while incorporating the storage of the stone into a exfiltrating pond. If I use a pond for exfiltration before the flow enters the catch basins what CN value should I assign the stone, 96 or 61? The stone areas I would like to incorporate storage are about 0.1 acres in size each.
What is the best way to account for the storage and exfiltration the stone areas provide?
Thank you,
The subcatchmet has a roof with five disconnected roof drains that spill onto stone (open graded ASTM #2 stone with no fines) areas. Each stone area has a catch basin. The catch basins are part of a longitudinal network which outfalls into an existing storm water pond. The stone depth is approximately 12". Assuming a porosity of the stone to be 0.54 I could assign a special CN value of 61 for the stone.
S=1000⁄(CN-10)
Where: S = depth in inches
If S is calculated as the available voids in the ASTM #2 stone, the CN can be estimated by rearranging the equation as:
CN=1000⁄(S+10)
CN=1000⁄(0.54*12"+10) = 61
I would like to take advantage of exfiltration (1.02 in/hour) beneath the stone. Should I use a CN value of 96 for the stone, use a pond depth of one foot, the surface area of the stone, the volume of the voids as my storage volume, and 1.02 in/hour for exifiltration. I'm afraid I'll be overestimating my storage if I use a value of 61 while incorporating the storage of the stone into a exfiltrating pond. If I use a pond for exfiltration before the flow enters the catch basins what CN value should I assign the stone, 96 or 61? The stone areas I would like to incorporate storage are about 0.1 acres in size each.
What is the best way to account for the storage and exfiltration the stone areas provide?
Thank you,