R.M. Towill Corporation
05/27/08 - present
Haseko (Ewa), Inc. proposed an 1,100-acre development at the lowermost reaches of the Kaloi Gulch watershed on Oahu, Hawai. According to the City's storm drainage standard, the 100-year storm peak flow at the lowermost reaches of the Kaloi Gulch watershed is approximately 10,800 cfs. The Kaloi Gulch watershed did not have a defined ocean outlet that was suitable for meeting the City and County of Honolulu's flood control flow requirements.kaloi
The Kaloi Gulch watershed is located in a relatively dry area on the leeward coast of Oahu. There is no regular US Weather Service meteorological station in the watershed. Two nearby stations are located at Kunia and at Honolulu International Airport. The isohyetal map of the area indicates that the upper watershed receives an average annual rainfall of about 130 inches and the lower watershed receives an average annual rainfall of less than 125 inches.
The 2004 plan of storm drainage improvement for Kaloi watershed to meet City requirement asked for the enlargement of the temporary Oneula Beach Park outlet. According to this plan, the storm flow generated in the Kaloi Gulch watershed would flow through a series of golf courses (Ewa Villages, Coral Creek, and Ocean Pointe) and eventually empty into the ocean through the beach park. Flows generated on the west side of the City's Honouliuli Wastewater Treatment Plant (HWWTP) outfall (tributary from Kalaeloa and Ocean Pointe) would be kept separate from the Kaloi drainageway and would remain on the west side of the outfall.
The drainage channel through the beach park consisted of a 500-foot wide open channel with an invert grade at the shoreline of +4 feet msl. Grading within the beach park and along the shoreline were necessary to enlarge this opening to carry the required 10,800 cfs. The 100-year storm peak flow of 10,800 cfs at the lower most reaches of the Kaloi Gulch watershed was determined by the City not based on a regional average of gaged watersheds and does not necessarily account for actual hydrologic conditions specific to the Kaloi Gulch watershed.
Storm runoff and sediment loading produced by Kaloi watershed under the 2004 plan was estimated by this team as part of a recently completed R.M. Towill project, entitled "BASINS/HSPF Modeling and Drainage Design for Kaloi Gulch Watershed in Southern Oahu."
This project was a continuation of a recently completed project. For the new project, a revised modeling analysis of storm runoff and sediment loading under the 2004 plan was conducted by adding the area west to the HWWTP outfall (tributary from Kalaeloa and Ocean Pointe).
The information produced by this project was used to aid the planning and design of flood control infrastructures as part of the lower Kaloi Gulch watershed development plan by Haseko (Ewa), Inc.
Project Scope and Approach:
This project involved the hydrologic and transport modeling of the Kaloi Gulch watershed and the Kalaeloa watershed. The model was developed based on a hydrologic simulation program - FORTRAN (HSPF), which was implemented under a U.S. Environmental Protection Agency (EPA) computer framework of BASINS. The developed model was then applied to investigate the flow and sediment transport in drainage channels in the Kaloi Gulch and Kalaeloa watersheds with respect to 20-yr, 50-yr and 100-yr design rainstorms.
HSPF is a set of computer codes that can simulate the hydrologic and associated water quality processes on previous and impervious land surfaces and in stream and well-mixed impoundments.
HSPF model consists of two MODULES:
1. Land Segment Module
This module is used to calculate runoff over a watershed receiving storm rainfall based on hydrologic mass balance equations. These equations consider various elements in the hydrologic cycle including surface runoff, interflow, infiltration, potential groundwater flux, and evapotranspiration. This module calculates runoff for both pervious and impervious land surface.
2. Reach and Storage Module
This module is used to route the storm flow through channels and impoundments based on kinematics wave equations.
(a) Land segments
(b) Drainage channels
Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) integrates geographic information system (GIS), data analysis and modeling system. BASINS was developed by US Environmental Protection Agency (EPA) and is being widely used to support watershed based analysis and total maximum daily load (TMDL) development, as required by US Clean Water Act.