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IML <br />Culverts <br />Culverts resulting in major head loss during the 10-year storm event were deemed to <br />have insufficient capacity. These structures are upsized to lower the water level <br />upstream of the structure. To ensure cost effectiveness of drainage improvements, <br />improvements to structures crossing runways, taxiways or other pavements were <br />avoided wherever possible. The exception was structure 110, crossing Taxiway A <br />adjacent to T-Hangars, (See Exhibit 3.1) which is currently an 18" crossing. <br />However, no other utility work interfering with the pavement is slated for this section <br />of the taxiway. It is recommended a parallel 18" diameter pipe be installed at this <br />location. However, this can be deferred to combine with the future taxiway <br />rehabilitation work as the 5-year storm event barely encroaches the Taxiway A in the <br />existing conditions. <br />The proposed ditch and culvert improvements are shown in Exhibit 10. <br />The following structures are proposed to be replaced with bigger pipes. <br />TABLE 5 - PROPOSED CHANGES TO DRAINAGE STRUCTURES <br />Proposed System Analysis <br />No changes were made to hydrologic parameters from the existing model as there <br />are no significant anticipated land use changes. <br />Two catchments were altered to reflect proposed changes in grading in the northern <br />section of the airport. 9.8 acres will be diverted from ditch segment 149 to ditch <br />segment 16, which will be expanded to provide linear detention. Figure 11 displays <br />the change in catchment areas. <br />Page 128 <br />