HomeMy WebLinkAboutStorm Drainage Calculations 090723a I HOLLOWAY ENGINEERING
la SURVEYING & CIVIL DESIGN
200 Casey Drive, Maumelle, AR 72113 1 Worlc: 501.851.3366
September 7, 2023
Mr. Christopher Smith
City of Little Rock
723 W. Markham Street
Little Rock, AR 72201
RE: La Marche Place
Storm Drainage Calculations - Preliminary
16800 La Marche Drive, Little Rock, Arkansas
Mr. Smith,
Please see the following information regarding a preliminary drainage analysis for the above referenced
project located off La Marche Road. If there are any questions or concerns regarding this project, please
do not hesitate to contact me at 501-851-3366 or eric@holloway-eng.com.
Per the City of Little Rock Storm Drainage Ordinance, the Rational Method of calculating runoff was used
since the drainage area for this project is less than 200 acres. The existing site is approximately 1.5
acres with 1.33 acres being improved with this development. The area upstream of the site which has
runoff flowing to this site is approximately 12.9 acres and includes drainage from lands from the South
and West ranging down to the intersection of La Marche and Forest Lane. The stormwater enters the site
near the end of the proposed road from the South will flow through a 30-inch pipe directly to the larger
drainage conveyance and floodway along the eastern part of the property. This will divert flows that
currently flow onto the lands to the North directly into the larger channel.
We are proposing a new pipe network as stated above with a 100-year flow path within an access and
drainage easement along a road leading all the way to the larger channel within the floodway. We have
attached our preliminary pre and post development flows for both the 25-year and 100-year storm. While
we would prefer not to detain as we believe it is better for the overall system to flush our additional flows
through the larger channel before it reaches its peak. If not allowed we have ample space between the
end of the road and the 25-foot buffer along the floodway to provide detention as calculated on the
attached.
Thank you for your assistance.
Sincerely,
J. Eric Holloway, PE 188
Attached: 2023-082 Preliminary and Post Development Flows
Flow, Q, determined by the Rational Formula: Q = C * I * A
LamarchePredev
AREA = 1.33 Ac.
Overland
Channel
L High Elev. Low Elev. s n Tc
300 15 0 0.050 0.3 13.6
1 1 0 1.000 0.04 0.1
10 - Year Storm
C=
0.4
Total Tc=
13.6 Min. 0.227 Hr.
1(10)=
5.3991n./Hr.
Q(10)=
2.9 cfs
25 - Year Storm
C=
0.45
Total Tc=
13.6 Min. 0.227 Hr.
1(25)=
6.2121n./Hr.
Q(25)=
3.7 cfs
50 - Year Storm
C=
0.55
Total Tc=
13.6 Min. 0.227 Hr.
1(50)=
6.8801n./Hr.
Q(50)=
5.0 cfs
100 - Year Storm
C=
0.65
Total Tc=
13.6 Min. 0.227 Hr.
1(100)=
7.5421n./Hr.
Q(100)=
6.5 cfs
Flow, Q, determined by the Rational Formula: Q = C * I * A
Lamarchedev
AREA = 1.33 Ac.
Overland
Channel
L High Elev. Low Elev. S n Tc
25 0.25 0 0.010 0.3 5.0
450 9 0 0.020 0.02 4.5
10 - Year Storm
C=
0.6
Total Tc=
9.4 Min. 0.157 Hr.
1(10)=
6.2021n./Hr.
Q(10)=
4.9 cfs
25 - Year Storm
C=
0.65
Total Tc=
9.4 Min. 0.157 Hr.
1(25)=
7.1881n./H.r.
Q(25)=
6.2 cfs
50 - Year Storm
C=
0.7
Total Tc=
9.4 Min. 0.157 Hr.
1(50)=
7.9351n./Hr.
Q(50)=
7.4 cfs
100 - Year Storm
C= 0.75
Total Tc= 9.4 Min. 0.157 Hr.
1(100)= 8.6661n./Hr.
Q(100)= 8.6 cfs
!D)