Unless granted a waiver by the City, applicants shall meet the storm water standards established in this chapter.
1. The site design shall provide on-site treatment during construction and postconstruction to ensure no increases over pre-settlement conditions (meadow not in good condition, CD=58) for the one-year, 24-hour storm event, the five-year, 24-hour storm event, and the 100-year, 24-hour storm event.
2. The site design shall provide for on-site water quality treatment to reduce potential increases in downstream water pollution. Practices or techniques shall be employed that capture and treat runoff from a 1.25” rainfall event, as further defined within the ISWMM. This standard would address approximately 90% of the rainfall events that occur in Central Iowa.
3. For the purposes of channel protection and as urban developments occur, some of the largest increases in runoff rate and volume (by percentage increase) occur during the smaller, more frequently occurring storm events. For this reason, practices or techniques shall be employed that provide extended detention of the 1-year, 24-hour storm event – with release rates established as per methods defined within the ISWMM manual to provide a minimum drawdown period of 24 hours. This standard would address approximately 98% of the rainfall events that occur in Central Iowa.
4. For the purposes of overbank protection and to minimize surcharge of downstream storm sewer systems and reduce the frequency of flash flooding along urban streams and tributaries, practices and techniques shall be employed that limit allowable peak release rates that are anticipated to occur post-development during the 2-, 5- and 10-year, 24-hour storm events to levels no greater than those expected to occur from natural conditions a given site from a similar storm event (e.g. the post-development release rate from 5-year storm event will be no greater than the natural release rate from a 5-year storm event).
5. For the purposes of extreme protection and to reduce the frequency and impacts caused by larger flood events practices and techniques shall be employed that limit allowable peak release rates that are anticipated to occur post-development during the 25-, 50- and 100-year, 24-hour storm events to levels no greater than those expected to occur from natural conditions on a given site from a similar storm event (e.g. the post-development release rate from a 100-year storm event will be no greater than the natural release rate from a 100-year storm event).
6. The site design shall retain on-site for recharge a portion of the water quality treatment volume calculated as a soil specific recharge factor multiplied by the volumetric runoff coefficient multiplied by the area and all divided by 12. The soil specific recharge factor is given as 0.51 for Hydrologic Soil Group (HSG) A soils, 0.34 for HSG B soils, 0.17 for HSG C soils, and 0.08 for HSG D soils. The volumetric runoff coefficient is calculated as 0.05 + 0.009 multiplied by the site impervious percentage. See the Iowa Storm Water Management Manual for additional clarification on the calculation. For areas of the site where there is no feasible way to achieve the recharge requirement, other options may be considered by the City if the options meet the performance standard listed for sites with restrictions in Subsection 4 below.
7. Applicant shall fully attempt to comply with the standards in Subsections 1 through 3 above. Options considered and presented shall examine the merits of relocating project elements to address varying soil conditions and other constraints across the site. If full compliance is not possible, the following flexible treatment options shall be used:
A. Applicant shall document the flexible treatment options sequence starting with Alternative #1. If Alternative #1 cannot be met, then Alternative #2 shall be analyzed. If Alternative #2 cannot be met then Alternative #3 shall be met. When all of the conditions are fulfilled within an alternative, this sequence is completed.
B. Recharge techniques considered shall include infiltration, reuse and rainwater harvesting, and canopy interception and evapotranspiration and/or additional techniques included in the Iowa Storm Water Management Manual.
C. Higher priority shall be given to BMPs that include volume reduction. Secondary preference is to employ filtration techniques, followed by rate control BMPs.
D. Factors to be considered for each alternative will include:
(1) Karst or Coal geology.
(2) Shallow bedrock.
(3) High groundwater.
(4) Hotspots or contaminated soils.
(5) Excessive cost.
(6) Poor soils (infiltration rates that are too low or too high, problematic urban soils).
E. Alternative #1: Applicant attempts to comply with the following conditions:
(1) Achieve recharge to the maximum extent practicable, and
(2) Treat by means of a filtration-based storm water treatment facility, the water quality volume determined in standard 2 above in order to provide removal of fine particles, and
(3) Options considered and presented shall examine the merits of relocating project elements to address varying soil conditions and other constraints across the site.
F. Alternative #2: Applicant attempts to comply with the following conditions:
(1) Achieve recharge to the maximum extent practicable, and
(2) Remove 80% of the annual Total Suspended Solids load, and
(3) Options considered and presented shall examine the merits of relocating project elements to address varying soil conditions and other constraints across the site.
G. Alternative #3: Off-Site Treatment. Off-site mitigation, as outlined in Section 161.07(2), Waivers, of the required treatment volume that cannot be provided on site can be used to protect Walnut Creek or North Walnut Creek.
8. The site shall be designed to provide vegetated buffers for water quality protection adjacent to receiving channels and waters. Buffers shall commence at “top of bank,” or at the delineated boundary of the water body. Buffer width as based on land use and are as follows:
A. Residential: 30 feet
B. Industrial: 50 feet
C. Mid/High Density Residential & Commercial: 50 fee
D. Redevelopment of infill parcels that are surrounded by existing development shall be considered on a case by case basis. The intent of this section of ordinance is not to make existing lots undevelopable.
9. The site shall be designed using the Better Site Design process. Better Site Design involves techniques applied early in the design process to preserve natural areas, reduce impervious cover, distribute runoff and use pervious areas to more effectively treat storm water runoff. Site design should address open space protection, impervious cover minimization, and runoff distribution and minimization, and runoff utilization through considerations such as:
A. Open space protection and restoration.
(1) Conservation of existing natural areas (upland and wetland).
(2) Reforestation.
(3) Re-establishment of prairies.
(4) Restoration of wetlands.
(5) Establishment or protection of stream, shoreline and wetland buffers.
(6) Re-establishment of native vegetation into the landscape.
B. Reduction of impervious cover.
(1) Reduce new impervious cover through redevelopment of existing sites and use of existing roadways, trails etc.
(2) Minimize street width, parking space size, driveway length, sidewalk width.
(3) Reduce impervious surface footprint (e.g., two-story buildings, parking ramp).
C. Distribution and minimization of runoff.
(1) Utilize vegetated areas for storm water treatment (e.g., parking lot islands, vegetated areas along property boundaries, front and rear yards, building landscaping).
(2) Direct impervious surface runoff to vegetated areas or to designed treatment areas (roofs, parking, driveways drain to pervious areas, not directly to storm sewer or other conveyances).
(3) Encourage infiltration and soil storage of runoff through grass channels, soil compost amendment, vegetated swales, rain gardens, etc.
(4) Plant vegetation that does not require irrigation beyond natural rainfall and runoff from the site.
D. Runoff utilization.
(1) Capture and store runoff for use for irrigation in areas where irrigation is necessary.
10. The following general criteria shall be incorporated in site design for storm water runoff to protect surface and ground water and other natural resources:
A. Reduce impacts on water.
B. Protect soils.
C. Preserve vegetation.
D. Decrease runoff volume.
E. Decrease erosion and sedimentation.
F. Decrease flow frequency, duration, and peak runoff rates.
G. Increase infiltration (groundwater recharge).
H. Maintain existing flow patterns.
I. Reduce time to peak flows by increasing the time of concentration to and through storm sewers.
J. Store storm water runoff on-site.
K. Avoid channel erosion.
11. Topsoil Requirements:
A. Preservation: No topsoil shall be permanently removed from any construction site in accordance with the Iowa Department of Natural Resources General Permit 2. The preservation of topsoil shall be met only when the depth of topsoil after soil disturbing activities have been completed and final stabilization achieved for the permitted activity is equal to, or greater than, 4.0 inches on all areas of the site where the surface of the ground disturbed for the permitted land disturbing activities is exposed and not covered by concrete, asphalt, gravel or other such impervious material. If 4.0 inches of on-site topsoil is not available, imported topsoil meeting the requirements below or the amendment of existing low-quality on-site material may be used to comply with this requirement. Three inches of low-quality on-site material may be incorporated with a minimum of 1.5 inches of compost meeting the requirements below to achieve an acceptable equivalent alternative. Topsoil shall be defined as the soil material excavated from the upper 12-inches of the soil profile that has a uniform quality free from debris, hard clods, roots, sod, stiff clay, hard pan, stones larger than 1 inch, has a high degree of fertility with an organic matter content of at least 2%, is free of herbicides that prohibit plant growth, has a pH level between 6.0 and 8.0, and is friable with a clay content less than 25%. Compost shall be defined as stable, mature, decomposed organic solid waste that is the result of the accelerated, aerobic biodegradation and stabilization under controlled conditions. The result is a uniform dark, soil-like appearance with 100% of the material passing through a 1 inch sieve (3/8 or ½ inch screen preferred), a pH range between 5.5 and 9, a minimum organic matter content of 35% dry weight and a soluble salt content of less than 4.0 mmhos/cm.
B. Compaction: For the purposes of compliance with the Iowa Department of Natural Resources General Permit #2 requirements, the minimum standard for “minimizing soil compaction” shall be defined as working the soil prior to seeding/sodding such at a penetrometer can be inserted into the upper 6 inch with less force than 200 psi. As an alternative to the penetrometer test, a soil with a bulk density of less than 1.6 grams/cubic centimeter shall be deemed compliant with this requirement.