07-09-18 Drainage and Flooding Committee Meeting

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IML entire apron. Within this sample box, 762 feet of block cracking was counted. Extrapolating this rate to the entire apron results in about 34,900 feet of block cracking on the apron. Apron 3 Apron 3 is across the pipeline corridor from Apron 2. It also is an asphalt pavement that was seal coated with grip -flex in 2008. The surface still has aggregate and provides good friction. The apron has 38 tie -downs and 1 T-hangar building within the apron. Like Aprons 1 and 2, the large amount of cracking on the apron prevented the individual crack counting and instead was sampled. However, this time three different 50-foot square sample boxes was determined across the apron because there is some variability in the condition of the entire apron. The multiple samples provides a way to average the overall quantity of cracking for better estimating. Within the three sample boxes, 1270 feet of block cracking was counted. Extrapolating this rate to the entire apron results in 29,300 feet of block cracking on the apron. 3.2. DRAINAGE CONDITION Assumptions and Constraints The FEMA effective models were used for evaluation of tailwater in the Big Island Slough channel. The HEC-RAS model for the channel (13106-00-00) and the HEC- HMS model for the Armand Bayou watershed were obtained from Harris County Flood Control District (HCFCD) and appropriate information was used. When modeling natural channels, cross sections were taken at the mid -point of each channel and the channel was assumed to be uniform using that cross section. The maximum length for this purpose is limited to 200 LF. Texas Department of Transportation (TXDOT) intensity duration parameters were used to calculate the rainfall intensity to use in rational method for runoff calculation from each sub -basins. The XPSWMM model is calibrated at the sub -basin level against the rational method to generate the similar peak flow. Manning's Roughness Coefficient (n) values were assumed, and are displayed below in Table 2. TABLE 2 - MANNING,S n VALUES The design event was selected based on the factors discussed in Section 2.3, and is set to the 10-year storm event. This exceeds the FAA requirements. Page 114