The provisions of this chapter apply where the owner or developer is required to control excess waters within a development site. A Stormwater Management Plan is required for all developments or redevelopment equal to or greater than 1.0 acres.
Developments or redevelopments less than 1.0 acres are required to submit the simplified impervious calculations as required in the building permit application and are required to retain excess water on-site when the impervious surface is increased equal to or greater than 10%. All developments and redevelopments subject to review under Title 13.08 (as they apply to multi-family, commercial, retail, and mixed-use) are subject to meeting the onsite stormwater management requirements for the 100-yr storm event. Design criteria within this chapter shall apply to all on-site retention systems regardless of the acreage of the development or redevelopments.
(Ord. 2020-05, 4-16-2020; amd. Ord. 2022-10, 6-2-2022)
At a minimum, the Stormwater Management Plan shall include the following:
A. Exhibits or plans showing basins for the existing and proposed conditions showing:
1. Topographic base map;
2. Basin areas and drainage patterns;
3. Existing watercourses;
4. Drainage systems and facilities;
5. Wetlands and environmentally sensitive areas;
6. Irrigation systems and facilities
7. Basin peak flowrates;
8. Property lines;
9. Any FEMA floodplains and flood elevations;
10. Roads;
11. Easements;
12. Natural hazard areas as identified on city maps; and
13. All other existing significant natural and artificial features.
B. Hydrologic and hydraulic design calculations for the pre-development and post-development conditions for the design storms. These calculations must show that the proposed stormwater management measures are capable of controlling runoff from the site in compliance with this title. Such calculations shall include, but are not limited to:
1. A description of the design storm frequency, duration, and intensity where applicable;
2. Time of concentration;
3. Soil curve numbers or runoff coefficients;
4. Peak runoff rates for each watershed area;
5. Velocity of peak flowrates;
6. Infiltration rates, where applicable;
7. Culvert, storm drain, ditch and/or other stormwater conveyance capacities;
8. Facility sizing calculations; and
9. Documentation of sources for all computation methods and field test results.
C. Stormwater quality treatment.
Design standards for controlling excess waters within the development site are:
A. Runoff Analysis: Storm drainage designs are to be prepared using the Rational Method or Hydrograph Method.
1. The Rational Method may only be used for design of systems with contributing areas less than 20 acres.
2. Drainage designs using the Hydrograph Method require computer models capable of modeling the hydrologic characteristics of the watershed, a series of storms, and routing the hydrographs through the drainage pipe network, channels and storage facilities.
B. Rainfall Data:
1. Rainfall Intensity-Duration-Frequency (IDF) curves are to be obtained from the National Oceanic and Atmospheric Administration (NOAA) Hydrometeorological Design Studies Center.
2. Historical Daily Rainfall data is to be obtained from the Salt Lake Airport rainfall gauge.
C. Design Storms:
1. Redevelopment with an increase in imperviousness equal to or greater than 10%, must manage rainfall onsite for the increased area of imperviousness and prevent the off-site discharge of the precipitation from all rainfall events less than or equal to the 24-hour 85th percentile rainfall event, being equal to a depth of 0.52 inches.
2. New development of vacant land must manage rainfall on-site and prevent the off-site discharge of the precipitation from all rainfall events less than or equal to the 24-hour 85th percentile rainfall event, being equal to a depth of 0.52 inches.
3. The maximum discharge may not exceed 0.2 cfs for the 100-yr 6-hr storm event, unless otherwise approved by the city engineer.
D. Rainfall Intensity:
1. The value of the design rainfall intensity (I) for the Rational Formula is selected from the appropriate Intensity Duration Frequency (IDF) curve, with a duration chosen to coincide with the time of concentration. The time of concentration is the time required for stormwater runoff to become established and reach the design location from the furthest point within the contributing basin.
2. The time of concentration is the sum of two components, the "inlet time" and the "travel time". The inlet time is the overland flow time for stormwater runoff to enter the storm drain system. In developed urban areas where paved surfaces drain directly to catchbasins, an inlet time of 10 minutes shall be utilized for assessment of 10-yr storm events. An inlet time of 5 minutes is to be utilized for the 100-yr storm event. Appropriate formulas are to be used when the surface overland route to a catchbasin is not paved or is longer than 500-ft.
3. The travel time is the length of time required for stormwater to travel within the storm drain system from the point of inflow to the location being analyzed.
E. Runoff Coefficient:
1. C values should be established based on the proposed land uses, proposed developments and hydrogeological information. Default C values shown in the following table should be used unless otherwise justified.
Land Use | C Value for ≤ 10-yr Storm*
|
Land Use | C Value for ≤ 10-yr Storm*
|
Asphalt or Concrete | 0.70 - 0.95 |
Roofs | 0.70 - 0.95 |
Lawns: | |
sandy soil, flat, 2% | 0.05 - 0.10 |
sandy soil, avg., 2%-7% | 0.10 - 0.15 |
sandy soil, steep, > 7% | 0.15 - 0.20 |
heavy soil, flat, 2% | 0.13 - 0.17 |
heavy soil, avg., 2%-7% | 0.18 - 0.22 |
heavy soil, steep, > 7% | 0.25 - 0.35 |
Unimproved | 0.10 - 0.30 |
*Notes:
1. Lower values are for SCS soil group A; higher values are for SCS soil group D.
2. For 100-yr storm events, multiply coefficient by 1.25, not to exceed 1.0.
2. In a case of applying the Rational Method to a mixed land use in a drainage area, a weighted average C value should be used.
F. Infiltration Rates:
1. Sites with land disturbance area equal to 1.0 acre or more and which use an infiltration rate that depends on the hydrologic properties of soils require an onsite infiltration test to determine the infiltration rate. The infiltration test is to be conducted at the depth where the soils will be receiving the water (i.e. bottom of the infiltration structure) and at a distance of not more than 100-ft horizontally between the infiltration test and the infiltration structure.
2. A borehead permeameter - cased (fixed) test method per ASTM 6391 / USBR 7310 is required to determine the infiltration rate. Other constant head permeability tests that utilize in-situ conditions and are accompanied by a recognized published source reference may be approved by the city engineer.
3. Safety Factor: A safety factor of 2.0 is to be applied to an infiltration rate determined by field or lab tests to account for decreased infiltration rates over time.
4. Design Saturated Hydraulic Conductivity rates (Ksat Rate) shown in the following table are to be used in the absence of a site specific infiltration test:
Soil Textures | K Hydraulic Conductivity Rate (ft/sec) |
Soil Textures | K Hydraulic Conductivity Rate (ft/sec) |
Gravel, Sandy gravel, Coarse sand | field testing is required |
Sand | 5.77 x 10-4 |
Loamy | 5.13 x 10-4 |
Sandy Loam | 1.13 x 10-4 |
Silty Loam | 2.36 x 10-5 |
Loamy | 2.28 x 10-5 |
Sandy Clayey Loam | 2.07 x 10-5 |
Silty Clayey Loam | 5.57 x 10-6 |
Clay Loam | 8.04 x 10-6 |
Sandy Clayey Loam | 7.11 x 10-6 |
Silty Clay | 3.34 x 10-6 |
Clay | 4.21 x 10-6 |
G. Low Impact Development BMPs:
1. Onsite retention is to be accomplished by the use of Low Impact Development (LID) BMP that are designed, constructed, and maintained to infiltrate, evapotranspire and/or harvest and reuse rainwater. Allowable LID BMPs are:
a. Rain Garden
b. Bioretention Cell
c. Bio Swale
d. Vegetative Strip
e. Pervious Surfaces
f. Infiltration Basin
g. Infiltration Trench
h. Dry Well
i. Infiltration Galleries
j. Harvest and Reuse
2. Design and sizing of the LID BMPs are as per the document “A Guide to Low Impact Development within Utah” prepared for Utah Department of Environmental Quality, dated December, 2018. Darcy’s Law may be used as an alternate design method for dry wells.
3. Infiltration structures are to be located a minimum of 20-ft from a building foundation or as otherwise recommended by a geotechnical engineer.
4. Surface ponding retention/infiltration systems are not permitted on lots or parcels less than 0.5 acres in size unless the lot or parcel is part of a collection of lots or parcels equal to or greater than 0.5 acres and is limited to one residential dwelling structure. Measurements for parcel or lot size calculations excludes any hard surface area used for a roadway or lane.
5. The bottom of an infiltration structure is to be greater than 2.0 feet above the seasonal high groundwater table.
6. Void space ratio in drain rock used for storage calculations is not to exceed 0.35.
H. Water Quality:
1. Stormwater runoff from roadways and parking surfaces with a surface area of 0.25 acres or more must be routed through a stormwater quality treatment system (i.e. hydrodynamic separator) prior to discharging into an infiltration system, storm drain, canal or creek. Surface areas to be measured from edge of pavement or an equivalent edge. The stormwater quality treatment system must be designed to:
a. Treat all flows up to 50% of the 2 year (3-hour duration) post-development flowrate; and
b. Reduce the average annual Total Suspended Solids loadings by a minimum of 80%.
2. Facilities that contain fueling areas and chemical and hazardous storage areas require containment systems designed by a professional engineer.
3. For sites with a disturbance area of 1.0 acres or more, a Stormwater Quality Plan is required with the following minimum requirements:
a. Description of long-term stormwater BMPs and how they were selected;
b. The pollutant removal expected;
c. The technical basis which supports the performance claims for the selected BMPs; and
d. Design, installation, operation and maintenance standards.
I. Existing stormwater management and irrigation facilities must be protected in place during construction. Changes or disturbance of these systems require a design to be completed by a professional engineer and approval by the city engineer.
J. Site Grading Requirements:
1. Sites are to be graded to route stormwater runoff to onsite BMPs.
2. Bare soil shall be planted in a manner that will prevent erosion of the ground.
3. When bare soils are at risk of eroding, erosion control matting, temporary mulches, or other stabilization is required to prevent the soil from eroding.
4. Sags or low points in roads or subdivisions are to be designed with a safe overland outlet flow route, designed for the 100-yr storm event.
5. The maximum depth of flooding in a roadway or parking lot during a 100-yr storm event is not to exceed 1.0-ft.
6. Site grading is to be designed and constructed to route stormwater runoff from discharging onto neighboring properties with the following exceptions:
a. A drainage easement is recorded on the property receiving the stormwater discharge for the purpose of conveying the stormwater on the receiving property.
b. The stormwater discharge maintains a historical drainage pattern and the discharge does not raise the water surface elevation on the receiving property.
7. Lots shall be graded to drain surface water away from the foundation walls. The grade shall fall not fewer than 6 inches within the first 10 feet. Where lot lines, walls, slopes or other physical barriers prohibit 6 inches of fall within 10 feet, drain or swales shall be constructed to ensure drainage away from the structure. Impervious surfaces within 10 feet of the building foundation shall be sloped not less than 2 percent away from the building.
8. Driveways sloping from a perimeter street toward a structure must be graded such that any stormwater runoff will be safely routed around the structure without flooding the structure or causing any water damage to the structure or contents within the structure. Properties utilizing this method must have a professional engineer seal the design.
9. Swales shall have a minimum slope of 1 percent.
K. Catchbasins:
1. To ensure that the capture or inlet capacity matches the storm main capacity, the spacing of catch basins on streets may be varied; however, they shall generally meet the following criteria:
a. Spacing: Road grades less than or equal to 3%, space 500 feet maximum or 7,000 square feet of paved area. Road grades greater than 3%, space 300 feet minimum or 5,000 square feet of paved area.
b. Space at intersection to not interfere with ADA ramps.
c. Side inlet catchbasins are required for all curbed roads.
2. Catchbasin outlets are to be a minimum diameter of 12 inches and sized to convey the design inlet capacity.
3. Materials: Boxes with "knock outs" for out outlet connections are not allowed for public catchbasins.
4. Water Quality: New catchbasins to be constructed with a minimum 3.0-foot deep sump and a trapping hood cover (i.e. ACF Environmental Trash Guard Hood) on the outlet pipe unless there is a downstream water quality treatment unit (i.e. hydrodynamic separator).
L. Storm Mains:
1. Capacity: Hydraulic capacity shall be calculated using Manning's formula. A roughness coefficient of 0.013 shall be used for concrete and 0.011 shall be used for smooth plastic pipe.
2. Velocity: Minimum velocity shall be 2.5 ft/s at the design flow rate.
3. Minimum Size: 12 inches inside diameter.
4. Joints: Joints shall be water tight unless the storm drain is designed as an infiltration or exfiltration pipe (wrapped in drain rock and filter fabric) then joints may be soil tight.
5. Minimum Depth of Cover: Provide two (2) feet minimum cover.
6. Junctions: The crown elevations for each storm drain pipe should match, where feasible.
7. Materials: Materials are to be approved by the city engineer. Corrugated metal pipes are not acceptable.
M. Manholes:
1. Maximum Spacing:
a. 12" to 24" diameter - 400 feet
b. over 24" diameter - 325 feet
2. Manholes are required at:
a. All grade or alignment changes
b. Pipe size changes
c. All intersection mains
d. All upstream end of mains
e. All catchbasin connections, unless otherwise approved by the city engineer
f. Outfalls to creeks in order to isolate the upstream main to facilitate cleaning. The manhole is to be located as close as possible to the point of discharge.
N. Dry Well:
1. Where dry wells are used as means for infiltration, dry well wall surface area (interface with receiving soils) shall be sized using Darcy's empirical law:
Q = A K i
Where: Q = rate of flow (cfs)
A = cross section area of soil through which flow takes place (consider wall area interfacing with receiving soils)
I = gradient of headloss over a given flow distance (assume 1.0 unless otherwise known)
Upon determination of permeability factor, a safety factor of 2 shall be applied.
Hydraulic Gradient
I = h/l
Where: h = average available head
L = flow distance
Depth of the dry well will vary in accordance with the requirements derived from Darcy's law.
O. Floodplains:
1. The following is to be shown on a grading plan:
a. FEMA floodplain boundary
b. Floodplain zone
c. Floodplain Base Flood Elevation (BFE)
d. Firmette panel number and date
e. Lowest habitable floor elevation
2. The lowest habitable floor elevation of a structure must be 1.0 feet above the mapped Base Flood Elevation. Exceptions may be allowed as per the most current FEMA's guidelines and policies.
P. Maintenance Access:
1. All components for a drainage system must be designed to allow for future maintenance, including inspections, cleaning, and replacement.
2. Where vehicular access is required to access manholes and outfalls, the surface is to be designed to support maintenance vehicles in accordance to AASHTO H20 loading.
(Ord. 2020-05, 4-16-2020; amd. Ord. 2022-10, 6-2-2022)