(A) Applicability. Projects disturbing more than one acre shall follow the most current general permit to Discharge Stormwater Associated with Construction Activity No. MN R100001 (Permit) issued by the Minnesota Pollution Control Agency (MPCA), as amended.
(B) Investigation. After a building permit has been applied for and prior to the issuance of the permit, the city shall thoroughly investigate the existing drainage features of the property to be used.
(C) Obstruction of natural drainage prohibited. No building permit shall be issued for the construction of any building on which construction or necessary grading thereto shall obstruct any natural drainage waterway.
(D) Undrainable lands. No building permit shall be issued for the construction of any building upon ground which cannot be properly drained.
(E) Protection of existing drainage installations. Where application is made for a building permit and subsequent investigation shows that the property to be occupied by the building is adjacent to a portion of a public road or street containing a drainage culvert, catch basin, sewer, special ditch, or any other artificial drainage structures used for the purpose of draining that property or neighboring property, the applicant shall specifically agree in writing to protect these waterways in such a way that they shall not be affected by the proposed building construction or grading work incidental thereto.
(F) Order to regrade. The city may order the applicant to regrade property if existing grade does not conform to any provision of this section, if the grade indicated in the preliminary plan has not been followed, or if the grade poses a drainage problem to neighboring properties.
(G) Construction site stormwater runoff control. Owners and operators of a construction activity shall develop site plans that must be submitted for review and approval, prior to the start of construction activity. Site plans must be kept up-to-date by the owners and operators of a construction activity with regards to stormwater runoff controls. Site plans shall incorporate erosion and sediment controls, waste controls, and best management practices as described in the permit.
(H) Post-construction stormwater management.
(1) All owners and/or operators must submit site plans with post-construction stormwater best management practices, for review and approval, prior to the start of construction activity.
(2) Stormwater runoff rates for proposed activities, development or redevelopment conditions, shall not exceed the existing conditions. Runoff rates may be restricted to less than the existing rates when necessary for the public health and general welfare of the watershed.
(3) For new development projects there shall not be a net increase from pre-project conditions (on an annual average basis) of:
(a) Stormwater volume, unless precluded by the limitations and exceptions in the permit;
(b) Stormwater discharges of total suspended solids (TSS); and
(c) Stormwater discharges of total phosphorus (TP).
(4) For redevelopment projects there shall be a net reduction from pre-project conditions (on an average annual basis) of:
(a) Stormwater volume, unless precluded by the limitations and exceptions in the permit;
(b) Stormwater discharges of total suspended solids (TSS); and
(c) Stormwater discharges of total phosphorus (TP).
(5) Regional detention basins shall be utilized to manage peak flow rates and meet water quality objectives when feasible.
(6) Stormwater calculations shall be provided for the two-year, ten-year and 100-year, 24-hour storm events. The precipitation depths for these storm events shall be obtained from National Oceanic and Atmospheric Administration (NOAA) Atlas 14 Volume 8, published June 2013, or its successor, using the online NOAA precipitation frequency estimates. The applicant must document the location and event depths used.
(a) The hydrologic and hydraulic design calculations shall include the design storm frequency, time of concentration, soil curve numbers, peak runoff rates and volumes for each watershed area, flow velocities, and infiltration rates where applicable.
(b) A calculation summary report shall be submitted.
(7) Storm sewer pipes shall be designed for a ten-year storm event, unless a lower intensity event is allowed by the City Engineer. The use of the rational method shall be used for pipe design calculations.
(8) If a stormwater management control measure depends on the hydrologic properties of soils (e.g. infiltration basins), then a soils report shall be submitted. The soils report shall be based on on-site boring logs or soil pit profiles.
(9) Permanent sedimentation and water quality ponds shall be designed to meet Nationwide Urban Runoff Program (NURP) standards per Protecting Water Quality in Urban Areas (MPCA 2000). Provide an outlet skimmer to prevent migration of floatables and oils for at least the one year storm event; baffled weirs are not allowed.
(10) The owner of a detention basin, water quality pond, infiltration facility, or a water quality treatment device, if not a governmental unit, shall provide to the city a recordable agreement detailing an operations and maintenance plan that assures that the structure(s) will be operated and maintained as designed.
(11) Structure elevations. Any new residential, commercial, industrial or other habitable structures shall be constructed with the low opening elevation at least one foot above the 100-year flood level. The low floor shall be at least one foot above the normal water level. Low openings shall be at least one foot above the emergency overflow.
(12) Development that creates one acre or more of new impervious surface must infiltrate runoff from impervious areas and address the use of best management practices to limit the effect of the loss of previous area.
(a) One inch of impervious surface runoff must be infiltrated within 48 hours using accepted BMPs for infiltration, such as infiltration trenches, rainwater gardens, or infiltration basins. Infiltration volumes and facility sizes shall be calculated using the appropriate hydrological soil group classification and saturated infiltration rate from the table below. Documented site-specific infiltration or hydraulic conductivity measurements completed by a licensed soil scientist or engineer can be used in place of the values in the table.
Hydrologic Soils Group | Infiltration Rate (in/hr) | Soil Texture |
A | 0.30 | Sand, loamy sand, or sand loam |
B | 0.15 | Silt loam or loam |
C | 0.07 | Sandy clay loam |
D | 0.03 | Clay loam, silt clay loam, silty clay, or clay |
(b) Prior to infiltrating runoff, pretreatment shall be required before the runoff enters the infiltration practice.
(c) Prohibitions and/or restrictions may be placed on the infiltration techniques used to achieve conditions for post-construction stormwater management, without higher engineering review, sufficient to provide a functioning treatment system when the infiltration device will be constructed in areas as outlined in the permit.
(d) Linear projects. Where lack of right-of-way precludes the installation of volume control practices that meet the conditions for post-construction stormwater management, a lesser volume control may be approved as allowed in the permit. A reasonable attempt must be made to obtain right-of-way during the project planning process for volume control practices.
(I) When the post-construction stormwater management for total suspended solids and total phosphorus cannot be cost effectively managed on the site of the original construction activity, mitigation measures shall be met as required by the permit.
(J) A legal mechanism(s) must be established for long-term maintenance of structural stormwater best management practices, constructed after August 1, 2013, that are not owned or operated by the city, but that are within the city's jurisdiction. The legal mechanism(s) shall meet the permit requirements.
(K) Project review thresholds. No person or political subdivision shall commence a land disturbing activity or the development or redevelopment of land without first submitting plans to and obtaining approval from the city or commission as required in the Rules and Standards of the Shingle Creek and West Mississippi Watershed Management Commissions Rule D - Stormwater Management.
(Ord. 2015-2, passed 4-27-2015)