ATTACHMENT C1: Alternative Stormwater Strategy 1. Overall Site Strategy The alternative stormwater project consists of a comprehensive water quality approach including underground base of public parking structures public streets of the development. stormwater management, storage, and storage and water quality measures integrated as well as a comprehensive The total investment network of permeable in the Alternative into the paving along the Stormwater Project is about $13.7M. Below is a list of expected benefits when employing permeable pavement at the Liberty Center project. the project. One basin will be constructed extended detention-water Two extended dry detention within the footprint quality basins and basin are proposed for of a parking garage dedicated to Drainage Area A and the other beneath a parking garage dedicated to Drainage Area B as indicated in the attached illustration. The 'bottom floor of each garage will serve as the storm water basin. street parking spaces will be constructed attached illustration. The permeable On- of a permeable pavement and storage system as shown in the pavement will consist of an interlocking paver system. Adjacent impervious areas such as the roadway and sidewalks will slope to direct drain storm water runoff to the permeable pavement. " Detention Basins & Water Quality Volume: • The detention basins will serve as a flood control measure by attenuating storm water runoff. This is accomplished basin while releasing it at a controlled by storing the excess storm water runoff volume in the rate. The system will reduce peak flow rate generated from a 100 year storm event occurring under post-development conditions to the peak flow rate generated from a 1 year storm event occurring under pre-development • the peak flow rate of conditions. Capture the "first flush" storm event (0.75 inches) and: o Reduce pollutant load in the storm water runoff through settling of suspended solids (TSSremoval 50%-80%). o Reduce erosive velocities of storm water runoff released to' the receiving streams by maintaining drawdown an extended time between 24 and 48 hours. This practice enhances the integrity and.bank stability of the receiving stream. • Capture and re-use of storm water for irrigation purposes. This practice reduces the demand on the public drinking water system and reduces the net volume of storm water runoff from the project. Irrigation water will be stored within the footprint • Storm Water Basin Metrics: o Northwest Parking Garage Storm water Basin: • Post-development, Q 100 Year = 276.1 cfs • Allowable release rate, q 1 year = 3.3 cfs of the detention basin. I I I' • Detention Volume = 15.8 acre-feet • Water Quality Volume = 2.7 acre-feet • Irrigation Volume • Estimate footprint o I I ! = to be determined 5 acres of development property for storm water management preserved by utilizing garage preserved by utilizing garage purposes. Southwest Parking Garage Storm water Basin: • Post-development, • Allowable release rate, q 1 year = 1.6 cfs Q 100 Year = 125.0 cfs • Detention Volume = 7.1 acre-feet • Water Quality Volume = 1.2 acre-feet • Irrigation Volume = to be determined • Estimate 2.1 acres of development footprint for storm water management property purposes. Permeable Pavement: Table 2.11.1 Anticipated performance of permeable pavements. Category Subcategory Runoff Water Ouality Suspended Solids' i-' Full WOvInfiltration Partial Infiltration No Infiltration >90% 80-90% 80% Phosphorus' Medium Medium Medium Nitrogen/Nitrates' Low Low Low High High High Heavy Metals Bacteria Not clear at this time. Other practices using media filtration do treat bacteria. Using a sand layer may enhance this. Thermal Permeable pavements with a reservoir storing the WQv or most of that volume are expected to provide good thermal attenuation, but this will vary based on the particular design (i.e. material, the storage volume, outlet configuration etc.) Oil and Grease PolyAromatic Hydrocarbon High High High Comparable to conventional pavements, reduced compared to . asphalts sealed with coal-tar based coatings. Chlorides & Sodium~ Not controlled. RunoffVolume Reduction 85-90% %WQv-captured • 85% Recharge High Medium .. Not at all. Runoff TIme of Concentration Improved lag time, but varies with design. Peak Row Attenuation Significant peak flow attenuation, but varies with design. * There would be an expected improvement with the addition of sand layers and/or vegetative systems. ** May be a significant groundwater concern depending upon winter application practices. • Reduce volume of storm water runoff through subgrade storage and some infiltration into underlying soils. • Reduce peak runoff rates from the development area through increased lag time and infiltration into underlying soil. • Reduce suspended solids from runoff by filtration base aggregate layers. through the permeable pavement and sub- • Permeable pavement media, including sub-base aggregates, provide an environment for supporting micro-organisms reducing these pollutant • Provide some moderating known to bio-degrade metals and petroleum hydrocarbons thus loads in the storm water runoff. of water temperature. 2., 3. 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