Flooding - A Kailua problem!
HĀMĀKUA MARSH COMPLEX MASTER PLAN(HHF, 2014)
An Overview of the area (Jim, 2009)
Kailua is squeezed in between Kailua Bay and the wetlands Kawainui Marshland, and the southern residential area of Enchanted lake. Behind the city is a large mountain range and smaller hillsides that all feed into streams running down towards the two wetlands. The sea at the other side have since the 1970's caused erosion problems. This has forced the city and state to ensure its residents with different ways to control all this water. (koolau.net, 2019)
From the past two posts, I emphasised that the city, along with the US Army Corp of Engineers (USACE), in 1952 built the first flood control system, separating Kawainui Marsh from the city with a 6300 feet long levee. This was unfortunately flooded in 1987, and it caused the city to pay for all the damages - sued by a large part of the population. Therefore, the levee was re-constructed and raised to 4 feet including a retaining wall at the top. Today a trail goes along the full length of the levee (US Army Corps of Engineers Honolulu District, 2019). This levee is what you see in the first picture above, sliding down with the town of Kailua on the left side. To the right is Kawainui marshland - and in the far distance Maunawili Valley and the Koolau Mountain Range (Central-top in the picture). In the foreground, but behind the marsh land is O'lo Mana mountain (3 peaks), a famous and dangerous hiking trail, where hikers try to conquer all three peaks.
Maunawili Valley feeds into Kawainui marshland at around 6.8 million gallons of water every day.
(koolau.net, 2016). At the same time, Kawainui marshland flows into Kailua bay through Oneawa Channel with 10 million gallons of water per day (koolau.net, 2019).
"Kailua's shallow groundwater table is responsible for major flooding that tends to occur
during large storm events. The Kawai Nui Flood Control Project, completed in 1966 to move
water from Kawai Nui Marsh to Kailua Bay, and the Ka‘elepulu Canal, originally built by early
rice farmers to control water flow from the marsh to their pond fields, have eased flooding to some
extent, although urbanization and associated creation of impervious, paved surfaces have created
localized ponding problems."(koolau.net, 2019) |
Flooding and tracking
A
resource provided from the course (Watershed Stewardship at Virginia Tech) shows the varies watershed gauges around the United States. In this case, when you enter the link, it will show you Hawai'i, and a closer look at O'ahu is presented below.
Local image of watershed gauges at O'ahu, Hawai'i (Nova, 2019)
It shows that the only relevant gauge station located in the area of Kailua Watershed is Makawao Stream, which is a tributary to Maunawilli Stream. Derived from the picture, you can see that the station is marked green at the moment of writing, which means that no flooding occur. This goes for all of the 39 defined stations, whereas two lie in the Not Defined category.
Measures at the station only revealed observed data. No projections are made. However, what can be seen from the picture below, is that the regular water level is 6 ft., and that for this particular area, above 10 ft. is considered a minor flooding, and above 12.4 ft. a severe flooding level. September 26 was a day with a higher amount of precipitation, which caused the level to raise to nearly 8 feet.
Flooding graph of Sep 29 2019 (Nova, 2019)
Flooding events
The flooding event that needs to be covered is the flooding that happened on New Years Eve 1987, the event that is still in most people's mind. The area was on the paper protected well enough from the event that occurred, by the flood protection system finished in 1966, mentioned in the beginning. Nevertheless, the measures from that day shows a dramatically increase in cubic-feet-per-second (cfs)
Summary of significant Historical discharges (Dracup, 1991)
Of the table, you can see that 5 events have been used to create an understanding of what happened that night at 10:45pm. Nearly twice as much water ran through the area of gauge station 2540 in the 6 hour measure, with 2010 cfs compared to the the december flooding in 1967 with 1480 cfs.
The peak instantaneous discharge cfs was not in itself the cause of the flooding, ranking only third compared to the 1965 discharge event. The overall 24-hour discharge was what caused the high amount of flooding and damages, having nearly three times as much water running through the area, as compared to other events of the period.
Another reason is that the present levee had sunk nearly a feet from when it was constructed.
(Dracup, 1991) For more information, see this Link.
Drought - current and historical
One of the sites that you can use to explore the local drought, are through the government site, simply put
drought.gov.
Drought in Hawaii September 24 2019 (National Integrated Drought Information System, 2019)
The example shown here are from September 24 2019. It shows that coastlines of O'ahu lies within a normal range, not seeing any problems with drought at the moment, whereas higher altitudes sees an abnormal drought. Looking towards Molokai and Maui, it is shown that they experience severe drought at the moment - an area equal to 6.1% of the total state.
The next picture shows the historic figures of drought in Hawai'i. Integrating the D0-D4 ratings of how severe the drought is from above picture. The period from 2009-2014 was very dry, having 2011 as the driest year. 2005 saw only a slight drought with 40% of the entire state.
Drought in Hawaii 2000-2019 (National Integrated Drought Information System, 2019)
References:
HHF. (2014, June 18). HHF.com. Retrieved from Environmental Planning - KAWAINUI-HĀMĀKUA MARSH COMPLEX MASTER PLAN: https://hhf.com/wp-content/uploads/2014/06/kawainui_marsh_aerial-700x411.jpg
US Army Corps of Engineers Honolulu District. (2019, September). Civil Projects - Kawai Nui Marsh. Retrieved from US Army Corps of Engineers Honolulu District: https://www.poh.usace.army.mil/Missions/Civil-Works/Civil-Works-Projects/Kawai-Nui-Marsh/
National Integrated Drought Information System. (2019, September 24). NIDIS. Retrieved from Drought in Hawaii: https://www.drought.gov/drought/states/hawaii
koolau.net. (2019, September). koolau.net. Retrieved from Kailua Watershed: http://www.koolau.net/watershed_10.html
Nova. (2019, September 29). National Oceanic and Atmospheric Administration's. Retrieved from National Weather Service: https://water.weather.gov/ahps/region.php?state=hi
Dracup, J. A. (1991). The New Year's Eve The New Year's Eve Flood on Oahu, Hawaii: December 31, 1987 - January 1, 1988. Retrieved from Chapter: 4 HYDROLOGY AND HYDRAULICS: https://www.nap.edu/read/1748/chapter/6#36
J. O. (2009, December 11). Hawaii VA Loans. Retrieved from kailua-homes-will-be-re-zoned-to-highest-flood-zone-risk-if-action-is-not-taken: https://hawaiivaloans.com/kailua-homes-will-be-re-zoned-to-highest-flood-zone-risk-if-action-is-not-taken/
