§ 19-144. Major facility design elements.

(a)

Facility design criteria.

(1)

For design or modification of stormwater facilities, the following criteria shall be followed:

a.

All hydrologic data and computations shall be based on NRCS (formerly SCS) methodology. Computer modeling may be completed using HydroCAD, TR20/TR55, SWMM or comparable city approved modeling software.

b.

Hydraulic computations shall be completed using the rational method for storm sewer design, or in commonly used software packages including FHWA HY-8, Geopack, Eagle Point or SWMM compatible software.

c.

Water quality modeling shall be completed using Pondnet, P8 Urban Catchment Model, SLAMM or comparable software.

d.

Outlet structure designs shall provide rate controls that limit post-project rates to not exceed existing rates for 1- or 2-year, 10-year, 100-year events and 100-year/10-day snowmelt.

e.

An emergency overflow spillway shall be identified and designed to convey storm flows from events greater than the 100-year event. Extreme events (on the order of a 10.0-inch event) shall be analyzed to ensure the emergency overflow spillway will function as designed. If an emergency overflow route is adjacent to the property, the lowest building opening must be minimum one foot above the emergency overflow.

f.

Maximum 3:1 (H:V) side slopes (see diagram that follows).

g.

10:1 (H:V) safety bench from normal water level (see diagram that follows).

h.

For basins intended to have permanent water levels, a minimum of four feet of standing water (dead storage depth) and a maximum of ten feet shall be provided.

i.

Proper allowance shall be made for future access and maintenance. Easements shall be greater than or equal to 20 feet wide.

j.

Separation between the inlet(s) and outlet shall be maximized to prevent short-circuiting.

Water Level Diagram

(2)

The facility design shall provide adequate live storage to provide protection from the design storm, consistent with lowest building elevation standards. Lowest building floor elevation is defined as the lowest slab elevation for a home or building, including basements and crawl spaces. The lowest building floor elevation for structures adjacent to wetlands and water bodies shall be an elevation three feet above the 100-year high water level.

(3)

Newly constructed or modified detention basins shall provide storage volume below the outlet (dead storage) to allow for water quality treatment in accordance with the following, whichever is most restrictive:

a.

Water quality features meeting the MPCA NPDES construction permit for permanent stormwater management systems;

b.

Water quality treatment consistent with NURP criteria (90 percent removal of TSS and 60 percent removal of TP for a standard NURP particle size distribution); and/or

c.

For proposed projects in a watershed that directly discharges to a "Preserve" water feature (Table 20 of SWMP), NURP plus enhanced treatment is required as listed below:

1.

Stormwater ponds are required to include a sediment forebay at the inlet. The volume of the sediment forebay should be five to 15 percent of the permanent pool surface area. The sediment forebay shall be a minimum of three feet plus the depth for sediment storage.

2.

Projects in watersheds that discharge directly to Assumption Creek must incorporate BMPs as outlined in Appendix A C.1, C.2, C.3 and C.5 of the MPCA NPDES construction general permit, and as listed below:

i.

During construction.

aa.

All exposed soil areas with a slope of 3:1 or steeper, that have a continuous positive slope to a special water must have temporary erosion protection or permanent cover within three days after the area is no longer actively being worked. All other slopes that have a continuous positive slope to a special water must have temporary erosion protection or permanent cover within seven days after the area is no longer actively being worked.

bb.

Temporary sediment basin requirements described in Part III.B.1-5 of the NPDES construction general permit must be used for common drainage locations that serve an area with five or more acres disturbed at one time.

ii.

Post construction. The water quality volume that must be treated by the project's permanent stormwater management system described in Part III.C of the NPDES construction general permit shall be one inch of runoff from the new impervious surfaces created by the project.

iii.

Buffer zone. An undisturbed buffer zone of not less than 100 linear feet from the special water (not including tributaries) shall be maintained at all times. Exceptions from this requirement for areas, such as water crossings or limited water access, are allowed if the permittee fully documents in the SWPPP the circumstances and reasons that the buffer encroachment is necessary. All potential water quality, scenic and other environmental impacts of these exceptions must be minimized and documented in the SWPPP for the project.

iv.

Temperature controls. The permanent stormwater management system must be designed such that the discharge from the project will minimize any increase in the temperature of trout stream receiving waters resulting from the 1-, and 2-year 24-hour precipitation events. This includes all tributaries of designated trout streams within the section that the trout stream is located. Projects that discharge to trout streams must minimize the impact using one or more of the following measures, in order of preference:

aa.

Minimize new impervious surfaces.

bb.

Minimize the discharge from connected impervious surfaces by discharging to vegetated areas, or grass swales, and through the use of other nonstructural controls.

cc.

Infiltration or evapotranspiration of runoff in excess of pre-project conditions (up to the 2-year, 24-hour precipitation event).

dd.

If ponding is used, the design must include an appropriate combination of measures such as shading, filtered bottom withdrawal, vegetated swale discharges or constructed wetland treatment cells that will limit temperature increases. The pond should be designed to draw down in 24 hours or less.

ee.

Other methods that will minimize any increase in the temperature of the trout stream.

ff.

For proposed projects in a watershed that directly discharges to an "Improve 1" (Table 20 of SWMP) water feature NURP plus enhanced treatment is required as listed below:

A.

Stormwater ponds are required to include a sediment forebay at the inlet. The volume of the sediment forebay should be five to 15 percent of the permanent pool surface area. The sediment forebay shall be a minimum of three feet plus the depth for sediment storage.

B.

Projects in watersheds that discharge directly to Riley or Bluff Creeks must analyze the downstream system to ensure there is no increase in runoff rates. One way to determine the downstream effects is to extend the analysis downstream to a point where the proposed development represents less than ten percent of the total watershed draining to that point.

(b)

Skimmers and outlets.

(1)

Skimming devices should be designed to remove oils and floatable materials up to a one-year frequency event. The skimmer should be set 12 inches below the normal surface water elevation and should control the discharge velocity to 0.5 fps.

(2)

Outlets shall be evaluated for the need to dissipate energy so as to reduce velocities to permissible levels as allowed by the soil and vegetation. At a minimum, flared end sections should be provided with riprap consistent with Minnesota Department of Transportation standards. For areas with high flows or where excessive erosion occurs or is anticipated, energy dissipation per Federal Highway Administration standards shall be followed.

(3)

Riprap shall be provided below the channel grade and above the outfall or channel bottom to insure that riprap will not be undermined by scour or rendered ineffective by displacement. Riprap consisting of natural angular stone suitably graded by weight shall be designed for anticipated velocities. Riprap shall be placed over a suitable filter material or filter fabric to insure that soil particles do not move through the riprap and reduce its stability.

(4)

For outlets through berms or roadway embankments and all culverts under public traveled streets, anti-seepage collars shall be used (see diagram in this section). The collars shall be installed so as to increase the creep distance or seepage line along conduit by 15 percent. The locations for the use of collars include:

a.

All water and pond structures with a pool depth of two feet and two-day duration.

b.

Two hundred fifty acre watershed or more.

c.

Design head of ten feet or more.

(Ord. No. 444, § 3, 2-12-07; Ord. No. 472, § 1, 8-25-08)