Sec. 21A-74. Grading plan.
(a) Grading Plan. A drainage/grading plan will be required as part of the plans submitted for primary (preliminary) plat approval. This plan shall include the following:
(1) A topographic contour map with one (1) foot contours.
(2) Delineate drainage sheds to inlets or group of inlets.
(3) Show flood route path to detention/retention basin.
(4) Show finish floor elevation for each lot.
(5) Show finish grade at each corner of each lot and each change of grade.
(6) Indicate direction of overland flow.
(7) Indicate location of swales, length, slope, cross section, etc.
(8) Show storm sewers, inlets, manholes and other structures, size, length, slope, type, rim and invert elevation, etc.
(9) Show location of culverts, size, length, type, invert elevation, end treatment, etc.
(10) Plat of subdivision including street names, lot numbers, right-of-way lines, property lines, easement lines.
(11) Show elevations of high and low points or change of slopes on streets; indicate length and slope of streets.
(12) Grading of the lot shall be indicated to extend to the lot line where the grades will match the existing grade of the adjacent property. The use of retaining walls or steep slopes to match the existing grades of the adjacent property shall not be acceptable unless approved by the Plan Commission.
|
(General Slope) | ||
|---|---|---|
(Less Than) | (More Than) | |
2% | 7% | |
Unimproved Areas | 0.30 | 0.30 |
Railroad Yard Areas | 0.30 | 0.50 |
Parks and Cemeteries | 0.20 | 0.35 |
Playgrounds | 0.30 | 0.45 |
RESIDENTIAL | ||
|
Suburban | 0.45 | 0.50 |
|
Single Family | 0.50 | 0.50 |
|
Multi Units | 0.50 | 0.70 |
|
Apartments | 0.80 | 0.95 |
COMMERCIAL | 0.80 | 0.95 |
INDUSTRIAL | ||
|
Light | 0.50 | 0.80 |
|
Heavy | 0.60 | 0.90 |
Detailed Steps for Determining Allowable Release Rate and Required Flood Storage
The following are detailed steps for the determination of allowable release rate and the required flood storage capacity: (or use of industry recognized computer program)
Name of Project ______________________
Location ______________________
Design Engineer ______________________
I. Determination of Allowable Release Rate – Undeveloped Site.
1. Area of site ______ acres
2. Average ground slope ______ foot/foot
3. Overland flow distance ______ feet
4. Overland flow time of concentration ______ minutes
5. Average slope of channelized flow (See Note a) ______ foot/foot
6. Channelized flow distance ______ feet
7. Channelized flow time of concentration (See Note a) ______ minutes
8. Total time of concentration (line 4 + line 7) ______ minutes
9. Rainfall intensity for two-year storm ______ inch/hr
10. Runoff coefficient (Use c=0.15 as maximum, see ______
11. Allowable release rate, (line 1 x line 9 x line 10) Q=CiA ______ cfs
Note a: For flow in a well-defined channel determine time of concentration from measured lengths, cross-sections and slopes and submit necessary calculations and drawings.
II. Determination of Reservoir Size—Developed Site:
12. Impervious drainage area ______ acres
13. Pervious drainage area ______ acres
14. Composite runoff coefficient ______
15. Determine flood reservoir capacity. See detailed example attached. Required reservoir capacity ______ acre-ft.
III. Permissible Bypass Rate through Development Site from Upstream Area:
A. Determination of Bypass Rate:
16. Total area upstream _______ acres
17. Future/present impervious area (cross out inappropriate case) _______ acres
18. Future/present pervious area (cross out inappropriate case) _______ acres
19. Composite runoff coefficient (must not be less than 0.15.) _______
20. Design storm frequency for the upstream area (2 yr.) _______ year
21. Time of concentration, for the upstream area at point of entry (upstream area to be considered as developed)(By same method as line 8) _______ min.
22. Design storm intensity for above duration _______ in./hr.
23. Permissible bypass rate (line 16 x line 19 x line 22) _______ cfs
B. Determination of Required Size of Bypass System:
24. Bypass system will be open channel/closed conduit (cross out inappropriate case)…………………………..
25. Water cross-section area for discharge in line 23……. _______ sq.ft.
26. Wetted perimeter for area in line 25…………………… _______ feet
27. Hydraulic radius (line 25 ÷ line 26)..…………………… _______ feet
28. Line 27 to the 2/3 power………………………………… _______
29. Invert slope……………………………………………….. _______ ft./ft.
30. Line 29 to the 1/2 power………………………………... _______
31. Manning’s roughness coefficient use (n) = ………….. _______
32. Bypass capacity [(1.49 x line 25 x line 28 x line 30) ÷ (line 31)]
(Q = 1.49 A R2/3 S1/2) / (n)
