§ 23.31.044 DRAINAGE REPORT.
   (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.