(A) Required. For subdivisions larger than two acres, a drainage report shall be submitted with the submittal of the improvement plans. Subdivisions less than two acres will be checked on a case by case basis with requirements determined by the Engineer.
(B) Format. The following format shall be followed in preparation of the drainage calculations:
(1) Title page.
(a) Type of calculations (preliminary or final);
(b) Project name;
(c) Preparer’s name, firm and date; and
(d) Stamp and signature of engineer who prepared the calculations.
(2) Introduction.
(a) Site location:
1. City, county, street grid; and
2. Adjacent development.
(b) Site description:
1. Existing topography, land use and the like; and
2. Existing drainage facilities.
(c) Proposed project description; and
(d) Reference to flood hazard reports or other related drainage reports or flood insurance rate maps (FIRMs).
(3) Existing drainage system.
(a) Basin drainage characteristics;
(b) Existing drainage patterns; and
(c) Off-site flows and effects of historic flows or adjacent properties.
(4) Proposed drainage system.
(a) Discussion of criteria:
1. Basin and sub-basin sizes;
2. Hydrologic method;
3. Storm frequencies analyzed;
4. Proposed flow patterns; and
5. Hydraulic controls for terminal drainage outfall.
(b) Major facilities:
1. Discuss location, size and configuration of major trunk lines, open channels, culverts and the like and relate to existing or proposed terminal drainage facilities; and
2. Discuss need for and sizes of detention or retention facilities and means of releasing stored runoff.
(5) Conclusions. Discuss impacts of improvements:
1. Benefits - Does the improvement reduce existing drainage problems; and
2. Adverse impacts with solutions to mitigate impact.
(6) Appendices containing hydrologic and hydraulic computations.
(a) Runoff calculations:
1. On-site map shall be a reproducible watershed map, which includes:
a. Sub-area acreage;
b. Sub-area design flow rates;
c. Sub-area runoff rates; and
d. Plan view of improvements with respective design flow rates.
2. Off-site watershed shall be shown on USGS quad map and shall indicate:
a. Watershed acreage;
b. Design flow rate and location of discharge;
c. Flow patterns; and
d. Runoff coefficients used.
3. Table of runoff for each sub-area showing acres, runoff coefficient, time of concentration and intensity used.
(b) 1. Hydraulic calculations:
a. Main line pipe and channels; and
b. Calculation sheets shall include:
(i) Contributory drainage areas;
(ii) CFS in each pipe or channel reach;
(iii) Invert elevations of each pipe or channel reach;
(iv) Hydraulic grade line control;
(v) Hydraulic gradient;
(vi) Hydraulic grade line elevations;
(vii) Pipe sizes, lengths and slopes; and
(viii) Channel dimensions and water surface profile computations. The standard storm drain design sheet shall be per standard drawings.
2. Detention or retention ponds. Calculations must include:
a. Volume or routing computations for ten-year storm;
b. Ponding elevations;
c. Discharge rates to show that ponds will regain adequate capacity; and
d. Ponds with no outlets for retention shall include soil information and percolation rates from a certified geotechnical engineer showing that the necessary volume will be regained in the specified time.
3. Pump stations:
a. Pump sizing calculations;
b. Sump sizing criteria; and
c. Sketch of station layout.