University ofHawaii at Manoa filereviewers also questioned the practical feasibility of establishing rye/bermuda grass cover in the coralline substratum likely to be exposed through

University of Hawaii at ManoaEnvironmental Center

Crawford 317. 2550 Campus RoadHonolulu. Hawaii !l6fl22

Telephone (808) 948·7361

September 11, 1990RP:Ol32

Mr. Manabu Tagomori, Deputy DirectorCommission on water Resource ManagementDepartment of Land and Natural ResourcesP.O. Box 621Honolulu, Hawaii 96822

Dear Mr. Tagomori:

Application for a Stream Channel Alteration PermitMakakilo and Makalapa Gulches

Ewa, Oahu

The Environmental Center has reviewed the above document with the assistance ofPaul Ekern, Hydrology; and Lee Lyttle, Environmental Center. The R.M. TowillCorporation proposes to constIuct two temporazy diversions and ale permanent streamalteration within Makakilo and Makalapa gulches. The open channels will be used torealign water flows in order to provide flood protection for the development ofKapolei Village, a master planned housing project.

Our reviewers expressed concern that the permit documentation lacked sufficientdata t.o specify anticipated peak flow velocities. Lacking such specifications, itis not possible to assess the adequacy of the temporary channel designs. Ourreviewers also questioned the practical feasibility of establishing rye/bermudagrass cover in the coralline substratum likely to be exposed through excavation of.1 chr:nnel having a 15 foot invert. They also noted that erosion in vegetationlined channels can vary significantly with different types of grass.

Additional information on permissible Velocities for channels lined withvegetation is provided on the enclosed tables and figures from our reviewers'file!:.

Thank you for the opportunity to comment on this document, and we hope that youwill find our comments helpful.

tOu,,~, ~r~lY,\\ ( I II );{j\/ \. \J ) ,I'V:.v. _

John T. Harrison, Ph.D.-­Environmental Coordinator

Enclosurecc: OEQC

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