While TXDOT widened the main lane of SH 99 Grand Parkway from Kingsland Boulevard to FM 1093, Fort Bend County was responsible for the design and construction of the southbound frontage road (SBFR). Krest Engineers was tasked to review and revise the preliminary drainage design previously completed by another engineering firm to get the drainage design and analysis approved by TxDOT Houston District.
In the proposed condition, the SBFR design included 2 through lanes and 1 turn lane integrates four storm sewer systems which drains to a Willow Fork Buffalo Bayou (WFBB) tributary. The drainage design incorporates four in-line underground storage systems and one detention pond to mitigate the excessive surface runoff due to SBFR construction.
The Rational Method was employed for peak flow rate calculations due to contributing drainage areas being under 200 acres. Peak flow calculations were integrated into GEOPAK to analyze the storm sewer network under 10-year design, ensuring that ponding depth and width, as well as HGL, remained within the allowable limits to meet TxDOT Houston District Drainage Design Criteria.
Krest Engineers was also tasked to model the proposed in-line detention of System Outfall-1 through a supplemental work authorization. HEC-HMS and PCSWMM models were established respectively for hydrology and hydraulics. Through trial and error, to keep the current design of 120' - 5' x 4' box culvert with a 24" RCP, it was found out that the outflows need to be further constrained by a 6-inch diameter restrictor which will be installed at the downstream end existing Inlet. Accordingly, the PS&E package was revised by adding the new design feature of a 6-inch diameter restrictor. The proposed HGL under 10-yr design still remain within acceptable ranges (below inlet throat or manhole cover) with the proposed restrictor addition.