§ 156.026 DRAINAGE REPORT.

   A narrative report and all computations that support the design of the drainage system to include:

   (A)   Design narrative and computations.

      (1)   A statement describing the basis of design for the final drainage network components and any applicable engineering assumptions and calculations;

      (2)   A statement of the drainage system's provision for handling events greater than the 100-year, 24-hour runoff by a state licensed professional engineer. Emergency overflow paths should be provided to prevent building structure flooding for events greater than the 100-year, 24-hour storm.

      (3)   Hydraulic calculations, prepared by a professional engineer licensed by the state shall be provided for all proposed storm sewers, ditches and curbs.

      (4)   Flow rates and velocities at critical points in the drainage system (e.g., in pipes, at pipe outfalls, and in conveyance ditches) including in the supporting documentation;

      (5)   Cross-section data for open channel flow paths and designated overland flow paths;

   (B)   Detention report requirements. For detention facilities in general, the engineer must submit the following for review of the detention facility:

      (1)   Elevation vs. discharge tables or curves for all frequencies;

      (2)   Elevation vs. storage tables or curves for all frequencies;

      (3)   Inflow calculations and data for all frequencies;

      (4)   Site plan showing appropriate design information including tables (see example below) that summarize data from (a), (b), and (c) above;

“DESIGN” - SUMMARY OF FLOWS FROM BASIN AT GIVEN STORM DURATIONS
STORM FREQUENCY	DURATION	POOL ELEV.	STORAGE (CU. FT)	PRE-DEV. TOTAL FLOW (CFS)	POST-DEVELOPMENT
					LOW-FLOW (CFS)	OVERFLOW (CFS)	TOTAL FLOW (CFS)	OUTLET VELOCITY (FPS)
NORMAL POOL	2HR	526.00	0	-	-	-	-	-
2-YEAR	2HR	528.20	117,750	10.70	11.10	0.00	11.10	2.5
5-YEAR	2 HR	528.60	165,412	18.10	7.30	11.10	18.40	3.2
10-YEAR	2HR	529.50	234,796	23.95	7.54	16.44	23.98	4.1
100-YEAR	2 HR	530.50	274,938	24.00	8.60	15.45	24.05	4.8

 

      (5)   Hydraulic grade-line computations for pipes entering and leaving the basin for all frequencies;

      (6)   if the embankment contains fill material, a levee/dam, or if a retention (i.e. wet) pond is proposed, a geotechnical report is required;

      (7)   Structural calculations for the outlet control structures.

      (8)   100-year stage discharge hydrograph.

   (C)   Minimizing stormwater runoff volumes and release rates.

      (1)   Release rates. In the selection of a drainage plan for a new development or redevelopment, the applicant shall evaluate and implement site design features which prevent the increase in runoff volumes and rates from the site for the most critical design storms.

      (2)   The applicant's drainage plan submittal shall include evaluations of site design features which are consistent with the following hierarchy:

         (a)   Preserve regulatory floodplains, flood-prone and wetland areas;

         (b)   Minimize impervious surfaces on the property, consistent with the needs of the project;

         (c)   Provide infiltration of runoff on-site;

         (d)   Attenuate flows by use of open vegetated swales and natural depressions and preserves the existing natural stream channel;

         (e)   Provide stormwater retention structures;

         (f)   Provide wet or wetland retention structures;

         (g)   Provide dry detention structures; and

         (h)   Construct storm sewers.

   (D)   Water quality.

      (1)   The drainage system should be designed to minimize adverse surface and groundwater quality impacts off-site, to adjacent properties, and on the property, itself. When designers propose detention or retention pond designs, all flows from the development shall be routed through the basin (i.e., flows from any storm event shall not bypass the ponds).

      (2)   Only if allowed by the City Engineer as an exceptional circumstance, when it is not practical or feasible to route all of the project's flow to the detention basin, the design of the basin shall compensate for the bypass flow, and the designer must prove that the bypass flow will not cause downstream problems such as flooding or erosion.

      (3)   In cases where detention facilities are practical and the long-term maintenance of the facilities are provided for, detention of stormwater shall be promoted throughout the property's drainage system to reduce the volume of stormwater runoff and to reduce the quantity of runoff pollutants. For example, detention basins shall incorporate design features to capture stormwater runoff pollutants.

      (4)   The developer is encouraged to consider creative ways to manage stormwater impacts. As such, the development may provide water quality treatment for runoff from increased impervious areas to minimize impacts of post-development stormwater runoff on water quality. If used, the drainage report shall include a description of the water quality protection and volume control measures incorporated into the design. Volume control practices may be incorporated into the detention plan as a water quality control practices. If used, the first inch of runoff from the new impervious area of development on the site is considered the water quality control storage. For example, green infrastructure such as permeable pavement or infiltration basins may be used as a best management practices for water quality treatment and volume control practices.

   (E)   Multiple uses. The drainage system should incorporate multiple uses to the maximum extent practicable. Uses considered compatible with stormwater management include open use, aesthetics, aquatic habitat, recreation (boating, fishing, trails), wetlands, and water quality mitigation.

(Ord. 2017-03, passed 3-6-2017)