T.F. Green Airport Improvement Program - FEIS Chapters - PVD

T.F. Green Airport Improvement ProgramEnvironmental Impact Statement and Final Section 4(f) Evaluation6.9.2.1 Mitigation Site DescriptionsThe following sections provide descriptions of each mitigation site.Mitigation Site 1This mitigation site (Figure 6-3) is located on the Airport within the Buckeye Brook watershed south ofRunway 34. It has been designed to compensate for the flood storage lost from fill that would be placed inWetland A13 for the Runway 34 RSA. This compensatory wetland area would be constructed in uplandsadjacent to Wetland A13. Soil investigations conducted in this area revealed sandy outwash deposits restingover slowly permeable glacial lakebed deposits composed of thinly interbedded very fine sand and silt. Thislakebed deposit forms a restrictive layer that inhibits surface water infiltration. Groundwater elevationmonitoring in the uplands revealed an apparent flow direction toward Wetland A13. Surface water elevationdata were collected at the outlet of Warwick Pond and Tributary A at the outlet of Wetland A13 from May toDecember 2010. These data revealed that the surface water elevation in Wetland A13 is outlet-controlled by theculvert that restricts the flow in Tributary A between Wetland A13 and A10. The water level in Wetland A13was never recorded below elevation 12.1 feet NAVD 88, approximately 0.1 feet below the culvert invert. Itremained at the culvert invert elevation or lower for 55 percent of the monitoring period. Water levels inWetland A13 rose to elevation 13.0 feet NAVD 88 or above a total of 14 times during the 189-day monitoringperiod; the longest duration it remained at or above this elevation was 7.5 hours, with an average time of only3.5 hours. This information is important as the mitigation area within the Airport cannot be designed to pondwater for prolonged periods, which could create a wildlife hazard to aviation. The mitigation area has beendesigned to only hold water during the duration of larger storms and will be graded to drain positively backinto Wetland A13.A low shrub cover consisting of species which do not produce fruits that attract wildlife (e.g., willow or sweetpepperbush) would be planted to obscure the brief periods of shallow flood. Crushed stone beds would beinstalled beneath the manufactured high organic content soil in portions of the created wetland to enhancesubsurface flow in the direction of the outlet culvert. This would enhance the opportunity for plants growing inthis substrate to sequester nutrients even when surface water is not present. Other water quality benefits may bederived by the installation of stone check dams to create pool and riffle morphology in Tributary A11. Thesefeatures would be installed in eroding segments of this stream and in segments to be relocated for the project.The oxygenation promoted by this practice should lower levels of dissolved iron and manganese in surfacewaters reaching Wetland A13.Mitigation Site 2This site (Figure 6-4) involves replacing the existing restrictive culvert under Lakeshore Drive. This actionwould correct flooding resulting from cumulative impacts from development and enhance fish and wildlifepassage between Warwick Pond and Wetland A5. The existing culvert, which consists of three parallel 30-inchdiameter reinforced concrete pipes installed with inverted slopes, cannot handle flows generated by a one-yearstorm. This existing culvert is approximately 85 feet long.The conceptual design for a new culvert consists of three box culverts, two 4-foot wide by 3-foot high unitssandwiching a single 10-foot wide by 5-foot wide box that is to be embedded two feet into the stream bed. ThisChapter 6 – Mitigation 6-28 July 2011\\Mawatr\ev\09228.00\reports\FEIS_Final_July_2011\PVD_CH06_Mitigation_JUL_2011.doc