A. To facilitate review, the drainage plan should have the drainage areas delineated on it. The drainage plan should also show the location and size of each channel and structure and the applicable flow rates (Q's) to each.
B. An engineer's report shall be submitted with drainage plans. The report shall include the following:
1. Drainage area (DA) at each structure;
2. Flow rate (Q) at each structure and the computations leading to the flow rate determination based on the rational method Q = CIA
Where: | ||
*Q | = | The peak runoff rate (cfs) |
C | = | The runoff coefficient (dimensionless) |
*I | = | The average rainfall intensity (in./hr) |
A | = | The size of the drainage area (acres) |
* A rainfall intensity (I) of 2.5 in./hr. can be used if no other historical data is available;
3. The effects that the proposed development will have on the existing storm sewer collector system.
C. The grading plan shall be to a minimum scale of one inch equals one hundred feet (1" =100') showing existing and proposed finished two foot (2') contours.
D. Lots shall be graded such that water from one lot does not drain across more than two (2) adjacent lots, before it reaches an improved channel or street. A drainageway shall be constructed to intercept stormwater, unless the drainage from all lots is sloped to the abutting streets.
E. Storm Sewers:
1. Shall carry as a minimum the Q10 flow *(Q10 = 10-year return frequency);
2. Shall have a velocity greater than or equal to 3 FPS;
3. Shall have inlet computations and nomographs submitted in the engineer's report.
F. Plans for open channels shall show:
1. Typical sections;
2. Slope of channel;
3. Profile and stationing of channel along centerline;
4. Erosion control (excelsior mat, enkamat, riprap, and slab, etc.).
5. 3 FPS < Velocity < 6 FPS for Bermuda grass lined channel;
6. Side slope on grass lined slopes of 3:1;
7. Existing flowline elevations shown for a distance of one hundred feet (100') beyond the beginning and end of storm drainage.
G. Manning's "n":
1. Grass lined channel, n = 0.030;
2. Concrete-lined channel, n = 0.015;
3. Reinforced concrete pipe (RCP), n = 0.013;
4. Corrugated galvanized metal pipe (CGMP), n = 0.024.
H. Rational formula "C":
1. "C" residential = 0.40 to 0.70;
2. "C" commercial = 0.50 to 0.90;
3. "C" industrial = 0.60 to 0.90. (Ord. 1628 §§ 1, 4, 1988)