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Purpose <br />This report was commissioned by the City of La Porte to investigate the occurrence of sheet now <br />runoff from the airport to neighborhoods to the west and north of the airport during significant <br />storm events. The City of La Porte asked Civil PEs and HT&J to conduct this separate study <br />alongside a TxDOT Aviation Division study of drainage within the airport. The purpose of this <br />report is to evaluate any drainage impacts resulting from the airport draining into bordering <br />neighborhoods to the north and west of the airport property. The problem areas along the north <br />and west borders of the airport were identified, which coincide with the two major drainage <br />systems of the airport. Overflow rates to these areas were estimated and potential improvements <br />were evaluated to mitigate the severity of flooding to the neighboring offsite areas. <br />Assunipluons and. Constraints <br />For the evaluation of the airport drainage system, the Federal Emergency Management Agency <br />(FEMA) effective models were used for evaluation of tailwater in the Big Island Slough channel. <br />The HEC-RAS model for the channel (B 106-00-00) and the HEC-HMS model for the Armand <br />Bayou watershed were obtained from Harris County Flood Control District (HCFCD) and <br />appropriate information was used. <br />For evaluating the drainage system of the offsite neighborhood to the north, tailwater conditions <br />were assumed to be at the top of the natural channel. This was used as an estimate of the 25-year <br />tailwater conditions that are recommended in the City's design standards. The numerous small <br />culverts through the residential driveways were not modeled due to the instability this would create <br />in the model. <br />When modeling natural channels, cross sections were taken at the mid -point of each channel and <br />the channel was assumed to be uniform using that cross section. The width of cross sections varied <br />from 50 ft to 400 ft. <br />Texas Department of Transportation (TXDOT) intensity duration parameters were used to <br />calculate the rainfall intensity to use in rational method for runoff calculation from each of the sub - <br />basins. The XPSWMM model is calibrated at the sub -basin level against the rational method to <br />generate the similar peak flow. TxDOT parameters were used to be consistent with on -site and <br />off -site evaluation. <br />Manning's Roughness Coefficient (n) values were assumed, and are displayed below in Table 1. <br />Table 1 Manning's n Values <br />Material Type <br />Manning's n <br />Reinforced Concrete Pipe (RCP) Culvert <br />0.014 <br />Concrete Pilot Channel <br />0.025 <br />Natural Channel <br />0.035 <br />Natural Channel Overbanks <br />0.040 <br />La Porte Municipal Airport — Offsite Drainage Analysis <br />HT&J, LLC, January 2018 Page 2 <br />