Habitat Assessment and Mitigation Accounting: Ala Wai Watershed, Hawaii

Habitat Assessment, Mitigation Accounting, Scenario Testing, Habitat Suitability Modeling, Flood Control Modeling, Barrier Assessment, Infrastructure Assessment

The goal of the USACE Ala Wai Canal Flood Mitigation Project was to reduce the risk of flooding within in the Manoa area of Honolulu, HI. The project was focused on holding back or diverting peak flood flows to lessen the impact of a flooding event. The infrastructure needed to do this was expected to have an impact on aquatic habitat and native Hawaiian stream animals. We used the Hawaiian Stream Habitat Assessment Procedure (HSHEP) model to determine the impact and quantify mitigation scenarios. In addition to supporting the HSHEP model, long stretches of Manoa, Palolo and Makiki streams were surveyed using the High Definition Stream Surveys (HDSS) approach to better understand instream conditions both at the impact sites and throughout the stream in general. The application of the HSHEP model and HDSS approach to habitat quantification for the assessment of current conditions with project impacts and mitigation scenarios proved very successful. HDSS habitat availability data were collected throughout the streams and allowed very detailed understanding of where and what type of habitat was present in different stream segments. Prior to the HDSS fieldwork, the presence of over-hanging drops within Manoa and Palolo Streams were unknown. Covering extensive stream reaches also allowed us to see that the upper reaches of all of the streams still have suitable habitat for native amphidromous species and in many locations in the highly developed lower and middle reaches suitable habitat still exists.