Krest Engineers performed the detention basin design and analysis for the development of the North Airport Logistics Center on a 55.87-acre tract of land located at Rayford Rd in Humble, Texas. The property consists of two tracts: Tract I (28.88 acres) is located to the west of Rayford Road while Tract II (26.99 acres) is on to the east of Rayford Rd. The project site falls within the Turkey Creek (HCFCD Unit No. K111-00-00) watershed, a tributary of Cypress Creek (HCFCD Unit No. K100-00-00).
The project area is 55.87 acres, so the site runoff curve and the small watershed method were used to calculate the runoff peak flows and hydrographs according to HCFCD PCPM. The detention analysis was carried out using HCFCD PCPM Method 2. To design the detention basin, a PCSWMM model was established to route the proposed condition hydrographs (2-year, 10-year, and 100-year) through the detention basin. The calculated storage volume under 100-year event from PCSWMM modeling is 37.28 ac-ft, which provides a detention rate of 0.67 ac-ft/ac, greater than the minimum detention rate of 0.65 ac-ft/ac required by HCFCD. The 100-year peak outflow rate is 105.33 cfs, less than the maximum allowable outflow rate of 108.65cfs.
In the project area, the effective 100-yr and 500-year WSEs are influenced by backwater from Spring Creek. Since the proposed floodplain fill impacts storage rather than conveyance, a conveyance study was not conducted. Furthermore, the project site is completely outside of any delineated FEMA floodway. The total fill volume due to site development between the natural ground and the 500-year WSE 85.5' is calculated as 109.58 ac-ft (the net fill volume is 40.58 ac-ft within Tract I and 69.00 ac-ft within Tract II). The provided floodplain mitigation volume in the detention basin between 100-yr WSE 65.97' and the natural ground is 109.67 ac-ft, which is greater than the total fill volume of 109.58 ac-ft.