• The watershed encompasses 19 square miles (mi2) (12,064 acres) and extends from the ridge of the Ko‘olau Mountains to the near-shore waters of Māmala Bay. It includes Makiki, Mānoa, and Pālolo streams, which flow to the Ala Wai Canal, a 2-mile-long, man-made waterway constructed during the 1920s to drain extensive coastal wetlands. This construction and subsequent draining allowed the development of the Waikīkī District.
• Overall, the Ala Wai Watershed contains approximately 200,000 residents and is the most densely populated watershed in Hawai’i. The upper portion (approximately 7.5 mi2 or 40 percent of the watershed) is zoned as Conservation District, which is intended to protect natural and cultural resources, including the island’s aquifer. The remaining approximately 11 mi2 of the middle and lower watershed is heavily urbanized, supporting a high density of single-family residences, condominiums, hotels, and businesses, as well as many public and private schools, including the University of Hawai’i at Mānoa (UH), the largest university in the state. Within this urban footprint, the population density is one of the highest in the nation with 12.36 persons per urbanized acre (Fulton et al., 2001).
• In addition to a variety of residential, commercial, and institutional development, the watershed also includes the Waikīkī District, a prime tourist destination that attracts more than 79,000 visitors per day. In large part because of the tourism industry, Waikīkī is the primary economic engine for the state, providing 7 percent of the gross state product, 7 percent of the civilian jobs in the state, and 9 percent of the state and county tax revenue (DBEDT, 2013).

• It was created in 1921 by Walter F. Dillingham's Hawaiian Dredging Construction Company, and completed in 1928. When the city was issuing permits for new buildings in Waikīkī, they required builders to build above sea level. Dillingham sold the spoil he had dredged in creating the canal so builders could increase the elevation of the newly-created land. Original design plan was a second outlet continuing from the Kapahulu library through Kapiolani Park and outlet near Kaimana Beach (near Natatorium).  The canal was constructed to drain the Waikiki marshlands to allow development was and was not designed for major flood control.  Since its construction, the drainages have been highly urbanized causing urban pollutants and runoff increase to the Canal.  The Canal is owned and maintained by the state of Hawaii. The canal has been dredged at least three times, in 1967, 1978 and 2002.
• While much of the urban areas technically belong to the City and County of Honolulu (primarily residential properties), and state land areas are limited to the University of Hawaii, public schools, and state highway, all these areas drain into a tributary/drainage that eventually flows into the canal.  So the problem starts in city jurisdictional areas into a state maintained area. There is no federal responsibility for the pollution problems.
• Early on, and certainly after the 2004 Manoa flood, the UH Office of Emergency Management and their planners became very involved in the AWCP.  Their involvement waned after the UH decided to address flooding separately on campus (diversion walls, directing surface flows to Manoa stream).  USACE has given numerous talks at the UH Water Resources Research Center (WRRC), that includes faculty and students, on USACE processes and individual projects.  Also during this time, the former USACE-Honolulu District Civil Works chief advised a senior class of the UH Natural Resources and Environmental Management (NREM) School on USACE processes and provided tours of the proposed Ala Wai Canal Project areas.
• Cost / Funding: In July 2018, Congress appropriated $345 million toward that effort. Non-federal sponsor cost share is approximately $125M. The benefit-cost ratio is approximately 3.68 to 1.
• Federal Sponsor: U.S. Army Corps of Engineers (Local: Honolulu District)
• Non-Federal Sponsor: Potential non-Federal sponsor are the state of Hawai’i DLNR, (Engineering Division) and the City and County of Honolulu (Department of Environmental Services)
• Project duration: Approximately 5 years

Partnership Coordination: This is a project for the community made possible by the state and city and county. The U.S. Army Corps of Engineers, Hawaii State Department of Land and Natural Resources, and City & County of Honolulu have been working on a plan since 2001 to mitigate flooding in the Ala Wai Watershed in the event of a major storm.

• A high risk of flooding exists within the Ala Wai Watershed due to aging and undersized flood conveyance infrastructure.  Based on the peak flows computed for the Flood Risk Feasibility study, it is estimated the Ala Wai Canal has the capacity to contain about a 20- to 10-percent annual chance exceedance (ACE) flood before overtopping the banks.  The risk of flooding is exacerbated by the flashy nature of the streams in the watershed, with heavy rains flowing downstream extremely quickly due to steep topography and relatively short stream systems.    The 1-percent ACE floodplain is the area that is inundated by a flood with a 1-percent chance (1 in 100) of occurring in any single year. These are also commonly referred to as the 100-year floodplain and 100-year flood (but does not necessarily mean that this degree of flooding occurs every 100 years). This definition also applies to floods of other magnitudes (e.g., a 20-year flood is a flood that has a 5-percent chance of occurring and, a 10-year flood has a 10-percent chance of occurring in any single year, respectively).
• Flash-flooding conitions can materialize within an hour in the upper portion of the Ala Wai Watershed.
• Army Corps of Engineers estimates a major flood in the watershed could damage 3,000 structures and cost more than $1.14 billion.
• Repairs to the Waikiki area as the result of a 100-year flood event. An October 2004 rain storm flooded Manoa Valley, described as a 25-year event caused $85 million in damage.
• The Ala Wai Canal topped its banks and caused flooding in Waikiki before — during storms in 1965 and 1967, as well as during the passage of Hurricane Iniki in 1992.

• Hawai‘i streams are flashy by nature. Within the study area, rain often starts in the mountainous areas of the upper watershed, with little precipitation in the lower elevations. The peak flow rate from mountains to sea is approximately 30 minutes. Storms typically last for 24 hours or less. With the sudden nature of the flood events and the associated high velocities, floods within the watershed threaten life safety and may result in significant damages. Rarely does the watershed experience long periods of standing water from a flood event. When heavy rains do occur over multiple days, standing flood waters become a problem. Based on U.S. Army Corps of Engineers (USACE) hydrology and hydraulic modeling, the majority of the peak flow is from the Manoa Stream, with Palolo Stream being the second highest contributor and Makiki Stream the third.
• The watershed’s highest elevation is about 3,000 feet, before dropping to 300 feet at valley floors and rolling to sea level — all over the span of about four miles. Given the combination of sheer slope, considerable rainfall — up to 150 inches a year in the Koolau Mountain Ridge — as well as the dense Waikiki population and growing climate-change concerns, the waterway is designated as “high risk” for flash flooding.