Assessment Flooding and Rainwater Harvesting in Kaiaka Bay Watershed, O‘ahu, Hawaii

Abstract

The purpose of this project was to assess potential flood control through water harvesting in Kaiaka Bay Watershed (KBW) located in north central O‘ahu, Hawai‘i. Rainwater harvesting not only provides the benefit of helping to meet the fresh water demand through periods of drought or summer seasons, but the system also diminishes the downstream energy from surface runoff during extreme precipitation events. Water harvesting can be achieved through the implementation of flood retention basins and groundwater-recharge injection wells. Modeling was used in the analysis by two software programs, the Watershed Modeling System (WMS), a model user interface combining a number of watershed models, and WELL, a simple groundwater analytical model. Within WMS, the watershed models used were HEC-1, for flood simulations, and HEC-RAS for flood zone delineations. Site models were developed to test the success of the harvesting system. First, HEC-1 was calibrated through comparing observed and simulated streamflow from five dates of precipitation—3/2/12, 3/4/12, 3/24/12, 4/27/12, and 1/5/13. Two different data sets of rain distributions were employed and compared. Values for HEC-1 model-parameters were initially attained based on reviewed literature of a 2008 storm calibration. The parameters were then adjusted to achieve the best fit between observed and simulated data. Next, the model HEC-RAS was used in assessing flooding zones based on streamflows estimated by HEC-1 under a number of scenarios. Cases with and without harvesting were compared. The assessment included delineating floodplains and estimating flood levels. Each delineated floodplain was compared to areas delineated by the Federal Emergency Management Agency (FEMA), based on a 100-year flood analysis. Finally, the WELL model was used to determine how water table levels could be affected through artificial groundwater recharge from ten injection wells in each of four hypothetical flood basins. Modeling results showed inaccuracies regarding streamflow, and when compared to FEMA’s zones, seemed to overestimate flood depths produced from surface runoff under conditions of heavy rainfall. With the application of the harvesting system, the model showed a depth reduction of nearly seven feet in areas of the floodplain that generally exhibit the greatest impact from flooding. One third of streamflow predicted by HEC-1 showed the best match with the FEMA flood zones. Inaccuracies were due to the lack of data and accurate parameters, but the results were acceptable for an initial assessment of water harvesting in KBW. In addition to watershed parameters, the results were sensitive to rainfall data, including amounts and distribution, which requires site rain gauges. The results generated from the WELL model showed an increase of up to about 7 feet in the water table level, a welcome contribution to water resource sustainability. A detailed and site-specific groundwater model should be used in future assessments.