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Stormwater Management Considerations City of Coralville Stormwater Management Criteria

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Stormwater Management Considerations City of Coralville Stormwater Management Criteria Stormwater Management Considerations City of Coralville Stormwater Management Criteria The City of Coralville is a regulated MS4 Community (Municipal Separate Storm Sewer System), 1 inch = 2,000 feet meaning that they are mandated by the US EPA and Iowa DNR to comply with specific portions Legend West Land Use Area 0 1,000 2,000 of the Clean Water Act and National Pollutant Discharge Elimination System. One of the MS4 Coralville City Limits Floodplains Drainage Basins requirements pertinent to new developments, such as the West Land Use Area, is the institution Feet of a Post-Construction Stormwater Management Ordinance, which Coralville executed in 2014. West Land Use Area A - 100-YR Coralville’s ordinance is based on the requirements set forth by the State and Federal authorities WLUA Stream Lines AE - 100-YR in an effort to protect the quality of surface waters to the maximum extent possible without City of Coralville excessive encumbrance upon development. Requirement highlights include: Drainage Area 500 - YR Coralville, Iowa Floodway • Infiltration of the Water Quality volume (runoff from a 1.25-inch rain event) onsite • Infiltration or extended detention (24 hours) of the Channel Protection volume (1-year rainfall event; 2.4 inches over 24 hours) ® • Detention of the 5 to 100-year runoff, with release at no more than the 5-year, 24-hour pre- development rate HRG PLOT: 3:36:57 PM 12/21/2015 BY: agwinnu FILE: O:\10140120\GIS\MXD\Report_exhibits\WLUA_Streams_and_Floodplains.mxd • Preservation of existing topsoil • Vegetated riparian buffers (30-ft for low density residential; 50-ft all other land uses) Although onsite infiltration of small storm runoff is stipulated in the ordinance, there is a general understanding that the large storm detention requirement may be applied development-wide, as discussed in the Development Standards for Stormwater Management section below. Additional conditions and exclusions can be found in the City’s Post Construction Stormwater Ordinance. Figure 100: The West Land Use Area showing delineated watersheds, drainage basins, stream lines, and regulatory floodplains 1 inch = 2,000 feet Legend West Land Use Area 0 1,000 2,000 Coralville City Limits Floodplains Drainage Basins 69 Stormwater Management Considerations Feet West Land Use Area A - 100-YR WLUA Stream Lines AE - 100-YR City of Coralville Drainage Area 500 - YR Coralville, Iowa Floodway ® HRG PLOT: 3:36:57 PM 12/21/2015 BY: agwinnu FILE: O:\10140120\GIS\MXD\Report_exhibits\WLUA_Streams_and_Floodplains.mxd Stormwater Management Recommendations for the West Land Use Area Area-Wide Development Recommendations Site Development Recommendations topsoil under lawns, and using small distributed practices such as rain gardens can sequester rainfall onsite, reduce the need for storm infrastructure, and also reduce municipal and landowners’ These stormwater management considerations contain an overview of recommendations by The existing Municipal development criteria, primarily the City’s Post Construction Stormwater maintenance burden. HR Green. See HR Green’s December 2015 West Land Use Area Stormwater Report for more Ordinance, should be strictly followed for the best ultimate results regarding protection of runoff detailed information. water quality, reduction of post-development runoff quantity issues, and reduction of municipal With proactive land use planning and Low Impact Development (LID) techniques, stormwater maintenance effort and cost. runoff from low density development can often be directed to green spaces, or other marginal The Master Plan for development and land use allocation in the project area includes guidance land where natural infiltration is provided essentially for free. With this design philosophy, care must with respect to overall stormwater management and maintenance of regional hydrologic function. Transportation Related Drainage Recommendations always be taken to ensure that runoff from heavy rain events is not concentrated in a manner that Approximately 18% of the study area is currently planned to be “open space” consisting primarily damages these natural areas. Some type of hard infrastructure is often provided for conveyance of green space, parks, or stream corridor and flood plain buffers. These locations have been The transportation network has been designed in a way to minimize waterway crossings and, of larger events only, allowing small event runoff to bypass and infiltrate naturally. strategically chosen based on existing natural and unique features that should be maintained in where possible, to follow ridgelines. Stormwater runoff from the roadways will discharge into natural functioning condition. Some open space may require active maintenance or constructed the existing waterway areas zoned as open space. Per the City’s Post Construction Stormwater The single most important variable in choosing a stormwater management technique is the type of measures to prevent or remove invasive species encroachment, prevent disruption of natural Ordinance, roadway drainage systems should be designed to include some detention, retention, soil on the site in question, and to a lesser extent, the uphill watershed to that site. This is because function by upstream hydrologic modification (increasing, contaminating, or warming of runoff), or infiltration of the Water Quality Volume and Channel Protection Volume to further minimize the soil’s ability to infiltrate rainfall directly dictates how much runoff there will be and how quickly or mitigate other impacts typically associated with urban development adjacent to outstanding impacts on the waterways. Examples of BMP techniques applicable to roadway design include ponded water will percolate into the soil. The water that doesn’t infiltrate is the water that must be natural features. Some areas, particularly to the west of the existing Iowa Medical Classification ditch or median bio-swales, curb-cut rain gardens, permeable paving systems, stormwater curb- managed with piping, detention, etc. Releasing runoff too quickly results in increased downstream Center, have such unique and outstanding natural properties that they should be stringently cut planter boxes, minimizing pavement width, and underground storage. costs: flooding, urban stream degradation, reduced water quality, and infrastructure damage. The protected and perhaps celebrated and promoted by establishing a formal preserve, nature soil’s infiltration potential is a combination of geophysical traits that control the volume and rate of education center, or other means for protecting the area while also deriving public benefit from it. Local roads with less traffic can be designed with permeable paving systems that can greatly infiltration, and this metric is the key to preliminary stormwater management practice selection. reduce – in some cases eliminate – the need for conventional stormwater infrastructure. This option Areas planned to be maintained as open or green space in perpetuity that include streams, ponds, should be investigated on a site by site basis because it is possible that the total infrastructure cost wetlands or other surface water features shall be protected from increased runoff rate and runoff may be less, while simultaneously providing significant water quality and quantity improvements. polluted by chemical, biological or physical constituents (including heat). Runoff from developed Additional benefits may include enhanced aesthetic appeal, reduced wintertime salt use and ice areas to said sensitive natural areas shall be treated by means meeting the City’s Stormwater hazard, functional appeal, and downstream detention reductions. Ordinances and / or Iowa Stormwater Management Manual to limit runoff rate and pollutants such that the natural areas are not degraded due to the changes in runoff characteristics. Along roadways with ditches, if applicable, simply using amended soils and a native seed mix in ditches can significantly reduce downstream peak runoff volumes and flows. Another creative Particular care should be taken for the design and implementation of University Parkway, Jones option is the installation of open-bottom manholes in areas where high infiltration rates exist. Boulevard, Oakdale Boulevard and any other development adjacent theses valuable natural open Often the cost of these simple practices is offset by cost savings in smaller storm sewer piping. spaces buffers and creeks. Pavement, grading and drainage design as well as best management practices should all be thoughtfully coordinated to provide the most environmentally sensitive Stormwater Management Technique Decision Making design and implementation solutions. As indicated above, various practices exist for the management of stormwater, and the reasoning Mapped floodplains should be considered when locating constructed development; the City for selecting each type varies by site conditions and the scale of contributing drainage area. In of Coralville’s development restrictions for floodplains are the same as state (Iowa DNR) and general, the more dense the development, the smaller and more unobtrusive the practice must federal (NFIP / FEMA) requirements. Mapped floodplains consist of the regulatory “Floodway” be. However, higher-density development correlates with increased fraction of the land surface within the 100-year floodplain, the 100-year floodplain itself, and the 500-year floodplain. The that is made up of impermeable surfaces, thereby increasing the total volume of stormwater runoff Floodway
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