HomeMy WebLinkAboutTask 2 - Literature Review and Data Gap Analysis
COMMUNITY-BASED HILO BAY
RESILIENCE AND WATERSHED MANAGEMENT PLAN
Task 2 - Literature Review & Data Gap Analysis
Deliverable Draft Prepared by: Lynker Corporation, Fisheries Immersed Sciences Hawaii
Inc (FISH), Arizona State University (ASU), Roth Ecological Design International (REDI),
Date: 5/5/2026
EA Engineering, Science, and Technology, Inc., PBC 2
1. Site Description And Background 2
2. Modeling Approach 2
3. Data Review 5
4. References 14
Roth Ecological Design Int. 16
1. Data Needs 16
2. Data Currently Acquired 17
Arizona State University 18
1. Purpose 18
2. Introduction 18
3. Study area 19
4. Climate, geology, flow routing, and hydrologic metrics 20
5. Land Use 22
6. Water Quality 24
7. Flooding 31
8. Sea Level Rise 31
9. Data Gaps 33
10. Proposed sampling points for suspended sediment concentration and nutrient
monitoring 35
11. References 36
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EA Engineering, Science, and Technology, Inc., PBC
EA Engineering, Science, and Technology, Inc., PBC (EA) prepared this Technical
Memorandum for Lynker Corporation on behalf of the Hawaii County Department of
Research and Development as part of the Community-Based Hilo Bay Resilience and
Watershed Management Plan. This memorandum describes the proposed hydraulic
modeling approach and summarizes findings from a desktop data review and data gap
analysis. The objective is to identify data requirements necessary for completing Task
5b: Hydraulic Modeling and Flood Hazard Maps.
1. Site Description And Background
The Hilo Bay Watershed covers approximately 470 square miles (1,200 square
kilometers). It stretches from the summits of Mauna Kea and Mauna Loa down to Hilo
Bay, and encompasses agricultural, forest, conservation, urban, and rural lands. About
69 percent of the watershed is state-owned, and it receives between 75 and 300 inches
of rainfall annually.
For decades, Hilo Bay and its sub-watersheds have been impaired, impacting
endangered habitats and local communities. In 2005, the University of Hawaiʻi and the
Hilo Bay Advisory Group created a restoration plan identifying pollutants and data gaps
(Hawaii Department of Health 2005). In 2023, the U.S. Army Corps of Engineers
(USACE) recommended comprehensive water quality and sediment analysis, mapping
sediment sources, and developing a watershed management plan with prioritized
strategies and funding pathways (USACE 2023).
The new plan will integrate these goals with EPA requirements for eligibility under
Section 319, aiming to conserve and restore biodiversity, restore water quality, protect
groundwater, manage surface water, and reflect community values. It will guide
sustainable infrastructure and resilience effor ts as Hilo adapts to climate change and
population growth.
2. Modeling Approach
Hydraulic models for Honoli’i Stream, Wailuku River, and Alenaio Stream will be
developed using the USACE Hydrologic Engineering Center – River Analysis System
(HEC-RAS). These models will simulate flood events with annual exceedance
probabilities (AEP) of 10 percent, 4 percent, 2 percent, 1 percent, and 0.2 percent
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(corresponding to 10-, 25-, 50-, 100-, and 500-year return periods). The primary
objectives of hydraulic modeling includes:
• Generating floodplain boundaries
• Estimating flood elevations and water surface profiles
• Identifying areas susceptible to erosion and sediment delivery under various
storm scenarios
• Suppor ting broader watershed management strategies
a. MODEL DEVELOPMENT
The hydraulic models will be HEC-RAS two-dimensional (2D) and will use unsteady-flow
input. Key components of the HEC-RAS 2D model will include:
• Boundary Conditions: Upstream boundaries will use flow rates from streamgage
analyses and/or hydrologic models; downstream boundaries will incorporate
sea-level with tidal influences. Future boundary conditions, including flow rates
and sea-level, will be derived from available literature regarding changing climate
conditions.
• Terrain: Derived from available light detecting and ranging (LiDAR) and
bathymetric data, with a 2D mesh applied to the model domain. Mesh cells will
be oriented perpendicular to channel flow and sized to maintain low Courant
numbers (ratio of flow velocity and time step vs cell size) for stability.
• Roughness: Manning’s roughness values will be assigned based on land use and
vegetation cover.
• Structures: Bridges, culverts, and levees will be integrated as inline structures to
accurately represent hydraulic conveyance or flow restrictions through these
features.
AEP flow rate inputs for Wailuku River, Alenaio Stream, and Honoli’i Stream watersheds
will be derived from U.S. Geological Survey (USGS) streamgage records. The Honoli’i
Stream analysis will be conducted by other project team members, and EA will obtain
streamgage flow data from online resources for the other watersheds. EA will estimate
boundary condition flow rates for the hydraulic models by applying drainage area
scaling to gaged flow data. Flow rates in streams without streamgage records will be
estimated from similar adjacent streams. Historical hydraulic analyses will be used to
validate scaled estimates, and USGS StreamStats will be used to generate supplemental
estimates of AEP flow rates for calibration and validation. In addition, the completed
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Wailoa River hydrologic and hydraulic model (USACE 2017) has been obtained and will
be reviewed and integrated to provide the hydraulic modeling results for that river.
The modeling extents will encompass the coastal plain and extend toward the
mountains (mauka) until all floodplains are fully contained within ravines. Upland areas
where flood flows remain confined within ravines and stormwater is conveyed through
multiple smaller tributaries will not be included in the modeling. The approximate
extents of proposed modeling domains are shown on Figure 1.
Figure 1 Proposed Modeling Domains
b. SCENARIOS AND CALIBRATION
Four scenarios will be evaluated:
1. Existing Conditions
2. Existing under Future Climate Conditions (2075): Increased flow rates and
sea-level rise with expected future climate conditions in 2075.
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3. Proposed Conditions: Incorporating green stormwater infrastructure adaptation
strategies (relevant/updated model inputs will be provided by other team
members to support this scenario).
4. Proposed under Future Climate Conditions (2075)
Model calibration and validation will be performed using Federal Emergency
Management Agency’s (FEMA’s) effective model flood hazard area maps as the primary
reference to ensure consistency with established flood hazard areas. In addition, any
available historical records, such as observed flood events, stream gauge data, and
high-water marks, will be incorporated to improve accuracy. Where formal data is
limited, anecdotal information from local stakeholders and community observations
may be considered to provide context and validate model performance under real-world
conditions.
DELIVERABLES
Results will be documented in a Hydraulic Modeling Technical Memorandum, including:
• Summary of model inputs and development process
• Results for existing, future, and proposed conditions
• Figures, tables, and supporting attachments
3. Data Review
Hydraulic modeling will utilize publicly available data, published literature, and
government-furnished information. The following data sources have been identified as
valuable for supporting these modeling efforts.
a. FEMA – FIS, FIRM, NFHL, EFFECTIVE MODELS
The FEMA conducts Flood Insurance Studies (FIS) to develop Flood Insurance Rate
Maps (FIRMs). These maps illustrate flood hazard information, including flood extents
for various storm events, base flood elevations, and the locations of hydraulic structures
(FEMA 2017, 2023, and 2026). FIRM data is also available in shapefile format through
the National Flood Hazard Layer (NFHL), as shown in Figure 2. FEMA develops
hydrologic and hydraulic models to establish the base flood elevations and flood
extents. These effective models have been obtained from FEMA for a fee.
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Figure 2 NFHL
The FIS report, NFHL files, and effective models were acquired and reviewed. The FIS
report provides watershed descriptions, flood protection measures, and summaries of
hydraulic analyses. It includes tables of peak discharges for flooding sources and base
flood water surface elevations at modeled cross sections. FIRMs identify special flood
hazard areas subject to inundation by the 1 percent annual chance flood for streams
that have been studied. The 1 percent annual chance floodplain is shown on the FIRMs
for Wailuku River, Alenaio Stream, and Wailoa River. Honoli’i Stream has not been
studied and therefore lacks flood hazard mapping. NFHL files contain floodplain
boundaries, stream centerlines, cross sections, waterbodies, and other relevant features
in shapefile format.
The effective model files were developed using multiple versions of HEC software. The
files obtained include HEC-1 printouts in PDF format, HEC-2 files, and HEC-RAS files.
These models were reviewed for existing structure data, such as bridge and culvert
geometry. Most files pertain to streams outside the current modeling scope, including
numerous files for the Wailoa watershed. Structure data was identified for Alenaio
Stream, but only limited data was found for Wailuku River, and none for Honoli’i Stream.
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b. GOOGLE EARTH
Google Ear th provides both current and historical aerial imagery for the project site
(Google Ear th 2026). The latest imagery can be used to verify and refine land cover
delineation, characterize surface conditions, and assign Manning’s roughness values for
hydraulic modeling. Aerial imagery can be valuable when referencing FEMA’s effective
models and other data sources such as the Hawaii Statewide geographic information
system (GIS) Program data, as it helps identify model elements (matching hydraulic
structures to road crossings). Comparing historical and current imagery can reveal
changes in topography, surface roughness, or the number of structures, ensuring that
use of the effective model data account for any changes.
c. HAWAII STATEWIDE GIS PROGRAM
The Hawaii Statewide GIS Program provides a centralized repository of geospatial data
maintained by the State of Hawaii Office of Planning and Sustainable Development
(Hawaii Statewide GIS Program 2026). This resource offers a wide range of datasets
relevant to watershed and hydraulic modeling, including stream centerlines, roads,
bridges, parcels, vegetation, and agricultural land use, as shown in Figure 3. These
layers will be integrated into the hydraulic modeling process to enhance model accuracy
and ensure comprehensive representation of physical and land-use conditions. Stream
centerlines may be used to identify and label channel thalwegs within the modeling
domains. Bridge data includes deck width and span length, but does not include deck
thickness, deck elevation, or pier information. The limited bridge data available in this
database may be used in concert with Google Earth and the terrain data to approximate
structure geometry for hydraulic models. Additional bridge and culvert data may be
available from county resources, but are not currently accessible for review. If this
information becomes available before modeling begins, it will be incorporated into the
modeling process.
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Figure 3 Hawaii Statewide GIS Layers
d. NOAA – LAND COVER, LIDAR
The National Oceanic and Atmospheric Administration (NOAA) Coastal Change
Analysis Program provides a free, publicly accessible database of land cover and land
change information for U.S. coastal regions (NOAA 2026). These datasets classify land
cover types by the following: wetlands, developed areas, forests, and cultivated lands,
with an emphasis on coastal and estuarine environments. NOAA land cover data can
suppor t the estimation of Manning’s roughness coefficients for hydraulic modeling.
High resolution land cover data from 2010 is shown in Figure 4.
LiDAR is a remote sensing method used to collect detailed topographic data. The NOAA
Office for Coastal Management curates LiDAR, including both topographic and
bathymetric data datasets, for coastal areas. These high-resolution elevation data can
be represented as a digital elevation model, which serves as the terrain input for
HEC-RAS hydraulic models. LiDAR surveys conducted in 2013, 2018, and 2023 across
different por tions of the watershed will be merged to produce a comprehensive terrain
model of the entire watershed for hydraulic analysis. The merged LiDAR terrain is shown
in Figure 4.
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Figure 4 NOAA Land Cover and LiDAR
e. USDA – WEB SOIL SURVEY
The web Soil Survey is a publicly available, free, online database of soil data produced
by the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service
(USDA 2026). Surface and subsurface soil data, such as hydrologic soil group, are
available in the Web Soil Survey database. Soil classification can be used to suppor t
estimation of Manning’s roughness coefficients for hydraulic models. A map of soil
types from the web soil survey is shown in Figure 5.
Figure 5 Web Soil Survey
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f. USGS – STREAMGAGES
The USGS operates streamgages to monitor hydrologic conditions in rivers and streams
(USGS 2026a). These instruments collect continuous data such as discharge, gage
height, water level, and velocity. Multiple gages in a region can provide contextual
information, but site-specific data are generally preferred for accurate modeling and
analysis.
Several active gages are located on streams within the project area, including Honoli‘i
Stream and Wailuku River. Historical records are also available for Waiakea Stream (at
Mountain View), which was monitored from 1967 to 1995, and reestablished by our
team in 2025. These datasets enable computation of AEP flow rates. While streamgage
coverage is robust for major streams, data gaps exist for smaller tributaries and
un-gaged locations. These gaps can be addressed using regional regression equations
and supplemental tools such as USGS StreamStats (USGS 2026b).
g. USGS – STREAMSTATS
The USGS StreamStats application is a web-based tool that provides hydrologic and
basin characteristics for user-defined points on streams across the U.S. (USGS 2026c).
The tool integrates geospatial datasets and regression equations to compute peak-flow
estimates for various recurrence intervals. StreamStats can be used to delineate
watershed boundaries and estimate flow rates for un-gaged locations within the
modeled watersheds. These estimates can supplement USGS streamgage data and
provide input for hydraulic modeling of stream locations where continuous flow records
are unavailable.
h. UPDATED HYDRAULIC AND HYDROLOGIC MODELING FOR THE WAIAKEA-PALAI
CAP 205 PROJECT
This document presents a detailed hydrologic and hydraulic analysis of the Wailoa River
system, focusing on Waiakea Stream and Palai Stream (USACE 2021a). The updated
model incorporates high-resolution LiDAR data collected between 2018 and 2023,
NOAA topobathymetric data for Hilo Bay, and field measurements of bridges and
culverts obtained in 2024. These enhancements provide a robust terrain foundation and
accurate structural geometry for the 2D HEC-RAS model. Hydrologic inputs were
calculated using a calibrated Hydrologic Engineering Center Hydraulic Modeling System
model informed by NOAA Atlas 14 precipitation frequency data and local rainfall
records. Synthetic inflow hydrographs were developed for events ranging from the
10-year to the 500-year recurrence interval. The resulting hydraulic model will provide
flood hazard maps, flood elevations, and water surface profiles of the Wailoa River.
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i. LITERATURE REVIEW
The Hawaii County Depar tment of Research and Development curated an extensive list
of literature that is relevant to the watershed planning and presented this literature on
their website (County of Hawai’i Research & Development 2026). This literature was
reviewed to identify the documents which include information relevant to hydraulic
modeling and flood hazard mapping. The following documents were identified:
• 2020 Hawaiʻi Coastal Resilience Assessment (National Environmental Modeling
& Analysis Center 2020)
— Includes a discussion of flooding hazards and incorporates the FEMA flood
extents in exposure index calculations. Includes anecdotal accounts of
flooding after tropical storms, and discussion on coastal flooding from storm
surge and sea-level rise.
• U.S. Army Corps of Engineers – Hilo Bay Watershed Planning Assistance to
States Report (USACE 2023)
— Includes description of the watershed and FEMA flood extents.
• Hilo Bay Water Circulation and Water Quality Study (USACE 2009)
— Includes baseflow estimates for Wailuku and Wailoa Rivers entering the bay.
• Hilo Bay Watershed-Based Restoration Plan (Hawaii Department of Health
2005)
— Lists average flows (million gallon(s) per day) for major rivers and provides a
summary of watershed hydrology.
• Hilo Bay Muliwai National Estuarine Reserve System Site Designation
Application (Hilo Bay Muliwai 2013)
— Includes watershed information and FEMA flood extents.
• Waiakea-Palai Streams – Section 205 Flood Risk Management Final Integrated
Feasibility Report and EA (USACE 2021a)
— Includes hydrology and hydraulics modeling for Wailoa River using HMS and
1D RAS. This modeling was updated in 2024 for the Waiakea-Calai CAP 205
Project.
• Mauna Kea Watershed Management Plan (Mauna Kea Watershed Alliance 2010)
— Provides watershed data for all Mauna Kea watersheds.
• Rapid Watershed Assessment – Hilo Watershed (USDA National Resources
Conservation Service 2009)
— Includes watershed data for the Hilo Watershed (Hydrologic Unit Code
2001000003).
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• Alenaio Stream – Harbors and Rivers in Hawaii: Final Survey Report and
Environmental Impact Statement (USACE 1982)
— Contains historical hydrology flow data. This data is outdated for current use.
• Wailuku-Alenaio Watershed Project Work Plan (Soil Conservation Service Forest
Service 1976)
— Includes historic watershed and flooding data; outdated for current use but
may inform original design of flood control structures.
• Alenaio Stream LB – Levee, Floodwall C & Lined Channel (USACE 2016a)
— Levee information which may be used for hydraulic model setup.
• Alenaio Stream – Floodwall A, B – RB & Lined Channel (USACE 2016b)
— Levee information which may be used for hydraulic model setup.
• Wailoa Stream RB – Diversion Levee 1–4 & Channel (USACE 2017)
— Levee information which may be used for hydraulic model setup.
• Hawaiʻi Sea Level Rise Vulnerability and Adaptation Report (Hawai’i State
Climate Commission 2022)
— Sea-level rise data applicable for future downstream boundary conditions.
• Hawaiian Islands National Shoreline Management Study (USACE 2021b)
— Sea-level rise data applicable for future downstream boundary conditions.
• A Water-Budget Model and Assessment of Groundwater Recharge for the Island
of Hawai‘i (USGS 2011)
— Presents rainfall and runoff modeling calculations, including future with
changing climate condition rainfall and runoff estimates.
• Agricultural Water Use and Development Plan (State of Hawaii Department of
Agriculture 2019)
— Includes agricultural water use and water control structure data.
• 2020 County of Hawaiʻi Multi-Hazard Mitigation Plan (County of Hawai’i 2020)
— Describes flood hazards, levee information, and historic flooding. Uses FEMA
floodplain mapping. Includes a table summarizing peak discharges from the
FEMA FIS.
DATA GAPS AND CONCLUSIONS
A comprehensive review of available resources indicates that sufficient data exists to
develop hydraulic models for the Hilo Bay watershed; however, several gaps remain that
require attention. These gaps and proposed mitigation strategies are summarized
below.
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j. Identified Data Gaps
• Bridge Geometry: Limited information is available on bridge dimensions and
structural details.
• Culverts: Uncertainty exists regarding the presence and size of culverts within
the modeling domain.
• Stream Bathymetry: Existing LiDAR and topographic datasets may not capture
underwater channel geometry.
• Flow Rates for Honoli‘i, Wailuku, and Alenaio Streams: Hydrologic models are
unavailable for these streams.
• Future Flow Rates: Limited literature addresses projected changes in runoff in
future with changing climate conditions scenarios.
k. Planned Solutions
• Bridges: Field data collection is currently outside the project scope. Where
detailed geometry is lacking, assumptions will be made using terrain data and
aerial imagery. While this may reduce precision, it is acceptable for project
objectives. Also, bridge data may be available from county resources and if this
information becomes available before modeling begins it will be incorporated
into the modeling process.
• Culverts: If present, culver ts are likely small and will have negligible impact on
large flood events (e.g., 100-year storms). They may be excluded from the model
if no data is available. Culvert data may be available from county resources and if
this information becomes available before modeling begins it will be
incorporated into the modeling process.
• Stream Bathymetry: In the absence of bathymetric data, riverbeds will be
approximated using water surface elevations from LiDAR. This conservative
approach will be documented.
• Flow Rates: Flow estimates will be estimated using USGS streamgage data and
drainage area scaling, with calibration and validation using StreamStats and
hydraulic data from historical resources.
• Future Flow Rates: Available literature on climate-driven rainfall changes will be
reviewed to correlate projected precipitation with runoff for the modeled
watersheds. Additional sources will be sought and EA’s internal resources will be
used to strengthen these estimates.
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l. Conclusions
Despite these gaps, adequate information is available to construct reliable hydraulic
models for the project area. Where assumptions are necessary, they will be clearly
documented. These limitations are not expected to significantly affect the overall
quality or utility of the modeling results for flood hazard mapping.
4. References
County of Hawai’i Research & Development. 2026. Creating a Community-based Hilo Bay
Resilience and Watershed Management Plan.
https://www.rd.hawaiicounty.gov/economic-development/hilo-bay. Accessed
January 2026.
County of Hawai’i. 2020. Multi-Hazard Mitigation Plan. May.
Federal Emergency Management Agency (FEMA). 2017. Flood Insurance Study Numbers
1551660902F - 1551660904F. September. https://msc.fema.gov/portal/search.
Accessed January 2026.
———. 2023. Flood Insurance Rate Maps 18089C0126F, 18089C0127E, 18089C0131E,
18089C0132E, 18089C0151E, 18089C0152F, 18089C0156F, and 18089C0157F.
———. 2026. FEMA’s National Flood Hazard Layer (NFHL) Viewer.
https://www.arcgis.com/apps/webappviewer/index.html?id=8b0adb51996444d4
879338b5529aa9cd. Accessed January 2026.
Google Ear th, Google Ear th - Imagery of Hilo Bay Watershed. Accessed January 2026.
Hawaii Depar tment of Health. 2005. Hilo Bay Watershed-Based Restoration Plan.
October.
Hawai’i State Climate Commission. 2022. Hawai’i Sea Level Rise and Vulnerability and
Adaptation Report.
Hawaii Statewide GIS Program. 2026. Geospatial Data Portal.
https://geoportal.hawaii.gov/. Accessed January 2026.
Hilo Bay Muliwai Hui: Edith Kanakaole Foundation, University of Hawai’i at Hilo. 2013.
National Estuarine Reserve System Site Designation for Hawai’i State. August.
Mauna Kea Watershed Alliance. 2010. Mauna Kea Watershed Management Plan. 15
April.
National Environmental Modeling & Analysis Center. 2020. Hawai’i Coastal Resilience
Assessment.
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National Oceanic and Atmospheric Administration. 2026. Data Access Viewer,
https://coast.noaa.gov/dataviewer/#/. Accessed January 2026.
Soil Conservation Service Forest Service. 1976. Watershed Work Plan, Wailuku-Alenaio
Watershed, Hawaii County, Hawaii. March.
State of Hawaii Department of Agriculture. 2019. Agricultural Water Use and
Development Plan Update. December.
U.S. Army Corps of Engineers (USACE). 1982. Alenaio Stream, Island of Hawaii Harbors
and Rivers in Hawaii, Final Survey Report and Environmental Impact Statement.
July.
———. 2009. Hilo Bay Water Circulation and Water Quality Study. January.
———. 2016a. Alenaio Stream LB - Levee, Floodwall C & Lined Channel (ASFC).
https://levees.sec.usace.army.mil/levees/3205010103. Accessed January 2026.
———. 2016b. Alenaio Stream - Floodwall A, B - RB & Lined Channel (ASFA).
https://levees.sec.usace.army.mil/levees/3205010101. Accessed January 2026.
———. 2017. Wailoa Stream River Basin – Diversion Levee 1, 2, 3, 4 & Channel (WSRB).
https://levees.sec.usace.army.mil/levees/3205010202. Accessed January 2026.
———. 2021a. Waiakea-Palai Streams, ———. 2021b. Hawaii Regional Assessment
National Shoreline Management Study. December.USACE Honolulu District. 2023.
Hilo Bay Watershed Planning Assistance to States. February.
U.S. Department of Agriculture (USDA). 2026. United States Depar tment of Agriculture
Web Soil Survey, https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx.
Accessed January 2026.
USDA National Resources Conservation Service. 2009. Rapid Watershed Assessment,
Hilo Watershed, Hawai’i, Hydrologic Unit Code – 2001000003. March.
U.S. Geological Survey (USGS). 2011. A Water-Budget Model and Assessment of
Groundwater Recharge for the Island of Hawai’i. May.
———. 2026a. USGS Current Water Data for the Nation.
https://waterdata.usgs.gov/nwis/rt. Accessed January 2026.
———. 2026b. StreamStats. https://streamstats.usgs.gov/ss/. Accessed January 2026.
———. 2026c. National Water Information System: Web Interface.
https://waterdata.usgs.gov/nwis/rthttps://streamstats.usgs.gov/ss/. Accessed January
2026.
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Roth Ecological Design Int.
Roth Ecological Design Int (REDI) will be undergoing geospatial analysis to identify
priority areas/parcels where green stormwater infrastructure could have the most
benefit.
1. Data Needs
This memo summarizes the data needs for Roth Ecological Design Int (REDI)’s
scope or work. This includes datasets currently available for the Hilo analysis, identifies
remaining data needs, and requests confirmation of preferred data sources where
applicable. The goal is to ensure alignment on inputs prior to final modeling and
analysis.
● Watershed Boundaries
● Preferred watershed scale (e.g., HUC12 vs. HUC8) and applicable GIS files
● Total Maximum Daily Loads (TMDLs)
● Applicable known or future TMDLs for the study area (e.g., enterococci,
turbidity, nutrients)
● Water Quality Standards
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● Applicable state or local standards to be used for assessment and
exceedance tracking
● Modeled Catchment Boundaries
● To be provided following completion of the relevant contractor scope item
● Modeled Flood Extents
● Flood extents for specified return periods; to be provided following
completion of the relevant contractor scope item
2. Data Currently Acquired
The following datasets have already been identified and/or acquired and are proposed
for use unless otherwise directed:
● Water Quality and Hydrology
● EPA and USGS water quality data (1973–2024): Water Quality Data Portal
● USGS water level data: USGS Station 16717000
● NOAA water level data: NOAA Tides and Currents Station 1617760
● Infrastructure and Land Use
● Cesspool locations: Hawai‘i GeoPortal
● County zoning data: Hawai‘i GeoPortal
● Agricultural land use: Hawai‘i GeoPortal
● Parcel data for Hawai‘i County (to be supplemented with ReGrid if
needed): Hawai‘i GeoPortal
● Land Cover and Physical Characteristics
● Land cover, impervious surface, and tree canopy: NOAA Digital Coast
● Elevation data: NOAA Digital Coast
● NHD waterbodies: Hawai‘i GeoPortal
● Stream network: Hawai‘i GeoPor tal
● NRCS Soil Web Survey
● Climate and Precipitation
● Annual rainfall: Hawai‘i GeoPortal
● NOAA Atlas 14 precipitation frequency estimates (Hilo Airport)
● Risk and Contextual Indicators
● 2020 Social Vulnerability Indicators: Hawai‘i GeoPor tal
● FEMA National Flood Hazard Layer
● Base Flood Elevations: Hawai‘i GeoPortal
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Arizona State University
Center for Global Discovery and Conservation Science, Hilo, Hawai‘i
1. Purpose
The following document details the ‘state of knowledge’ with respect to hydrologic
processes within the Hilo Bay Watershed and delivery of contaminants to Hilo Bay. The
document focuses on the identification and quantification of sources of contaminants
within the watershed and delivery mechanisms to Hilo Bay. We conclude with a list of
‘data gaps’ and our planned approach/suggested approaches to fill the data gaps.
2. Introduction
The Hilo Bay Watershed encompasses one of the six major moku-o-loko, or districts of
Hawai‘i Island and spans at least 24 ahupua‘a from ʻŌlaʻa in the south to Pāpaʻikou in
the nor th. The upper limits of the watershed begin in wao akua (realm of the gods) on
the slopes of Mauna Kea and Mauna Loa, and extend across the reefs of Hilo Bay.
Kānaka Maoli (Native Hawaiians) considered Hilo Bay to be a resource rich place
integrating systems of fresh and sea water. Here, they managed fishponds across the
varying salinities of the ‘broad waters’ of the Waiākea/Wailoa estuary (Kelly et al., 1981)
and harvested wood to construct canoes on Hilo’s broad black sand beaches (Desha ,
2000). The abundant rainfall and fer tile land allowed Hilo Paliku (Hilo of the upright
cliffs) to be ‘cultivated from the sea upward to the line of frost’ (Kalākaua,1972:284).
Nowadays, Hilo Bay serves as an economic center, linking the shipment of goods onto
and away from the island, and is used for fishing, canoeing, surfing, and in rare cases,
swimming. The Bay also provides scenic beauty for the benefit of residents and
tourists. The eastern portion of the bay, extending through Keaukaha, is rich in corals,
fish, and other forms of aquatic life and are popular destinations for traditional fishing
and tourism.
Since at least the mid 20th century, however, the Hilo community has documented
pollution in the Bay. A comprehensive study of Hilo’s sewer system in 1961 (Belt and
Collins, 1961) presented water quality data it claimed were ‘terrifying’ and concluded
that it was “remarkable that no water-borne epidemic has occurred in the past.” Hilo Bay
has exceeded state water quality standards (due to high suspended load, high levels of
contamination, or both) since the late 1970s, and portions of the bay and watershed
have been federally listed (303d) as impaired waters by the Environmental Protection
Agency (EPA).
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Pollution to Hilo Bay has two primary sources: 1) chronic and episodic groundwater
delivery of untreated sewage, which leaches from on-site-sewage-disposal systems
(OSDS) through young and highly porous volcanic rock; and 2) episodic delivery of
sediment and other contaminants mobilized from agricultural or urban surfaces via
high-intensity rainfall, overland flow, and flooding. In Hilo, only 30-40 % of the urban
population is connected to sewers (Silvius et al., 2005), while there over 10,000 OSDS
(primarily cesspools) throughout the watershed (Mezzacapo and Shuler, 2022) leaching
untreated sewage into the subsurface. Although the problem has been recognized for
many years, the collection of data, community knowledge, and political momentum
have not yet aligned to generate lasting positive change. Specifically, past studies have
noted that despite the numerous studies of contaminant sources and concentrations in
Hilo Bay, ‘none of the data sets are lengthy’ and ‘none of the data sets pinpoint the
sources of pollutants by land use type’ (Silvius et al, 2005).
Now, a new community-supported process has begun to collect data following EPA
procedural guidelines to address knowledge gaps and produce the necessary data to
inform a comprehensive watershed management plan. Our goal is to engage with the
community to learn, follow federal guidelines to collect and interpret the necessary data,
and provide specific recommendations to the County. Our hope is that the community
and the County will use the results of our study to craft a watershed management plan
to prioritize mitigation and acquire funding to restore Hilo Bay to a healthy aquatic
ecosystem. The following summarizes the state of knowledge on the problem of
pollution to Hilo Bay and identifies key knowledge gaps we aim to fill over the course of
this study.
3. Study area
The Hilo Bay Watershed, as defined by the County of Hawai‘i in their Request for
Proposal, includes three primary watersheds: Honoli‘i, Wailuku, and Waiākea/Wailoa
(Figure 1). Together the three watersheds cover 1106 km2 (273,299 acres) covering
rocky uplands, native forest, agricultural and rural, suburban, urban, and industrial land
uses. Honoli‘i is the northernmost watershed and exhibits a relatively thin and narrow
shape, discharging to the ocean at the northern end of Hilo Bay. Wailuku is the largest
watershed in the state of Hawai‘i, and discharges to the western side of Hilo Bay at the
northern boundary of Hilo’s urban center. Waiākea/Wailoa forms a large watershed
which discharges to Hilo Bay through the Wailoa Pond estuary on the southern margin
of Hilo Bay.
19
Figure 1: Map of three watersheds draining to Hilo Bay. Contours show mean annual
precipitation in mm.
4. Climate, geology, flow routing, and hydrologic metrics
4.1 Precipitation
Mean annual rainfall in Hilo Bay Watershed varies from 3000 mm in the lower
watershed around Hilo to greater than 7000 mm in the upper Honoli‘i watershed north
of Hilo (Frazier et al., 2016) (Figure 1). Mean monthly rainfall at Hilo International Airport
is 244 mm and varies from a high of 353 mm in December to a low of 142 mm in June.
May through July is typically drier, while October through December is typically wetter.
Intense rainfall can occur any time of year and sustained heavy rainfall produces flashy
stream responses throughout the watershed, resulting in large and episodic stormflow
delivery to Hilo Bay.
4.2 Streamflow and runoff generation
The USGS provides current long-term streamflow data at Honoliʻi (#16717000)
and Wailuku at Pi‘ihonua (# 16704000), as well as historical long-term streamflow data
from Waiākea at Mountain View (#16700000) and Wailuku in Hilo (#16713000). The
first three stations are located at the downstream edge of forest boundary and thus
provide good estimates of natural flow. Flow duration (for mean daily flow) and flood
frequency (annual peak flows) are shown in Figure 2.
20
The mean age of the underlying substrate in each watershed decreases from north to
south from 60 ky (thousand years) in Honoliʻi, to 31 ky in Wailuku, and 2.2 ky in Waiākea.
The routing of precipitation into surface and subsurface (groundwater) runoff appears
to be controlled by the hydrologic properties of the different aged substrate. Whereas in
the older Honoliʻi watershed, 54 % of rainfall leaves as surface flow, only 16 and 4 % of
rainfall, respectively, leaves as surface runoff from Wailuku and Waiākea watersheds
(Table 1). Although all three watersheds have similar vegetation cover in the upper
reaches and similar mean annual evapotranspiration, increasing ‘leakage’ or an
increasing percentage of rainfall leaving the watershed as subsurface flow is seen from
Honoliʻi to Wailuku and Waiākea. This difference in the ‘routing’ of precipitation into
surface and subsurface flow controls the delivery of contaminants from these
watersheds to Hilo Bay. Native Hawaiians observed this phenomenon and named the
flashy ‘Wailuku’ (waters of destruction) and ‘Wailoa’ (broad river), accordingly. Wailoa
became one of the most productive Loko I‘a as the mixing of cold spring water (wai pi‘o)
with the sea created suitable environment for fish nurseries supporting ‘ama‘ama and
awa (Maly & Maly 2003).
Despite large mean annual precipitation in the Waiākea watershed (~ 3600 mm/year)
the geologically young and highly porous substrate results in an intermittent stream in
Hilo. The vast subsurface aquifer is reflected in estimates of sustainable yield which
increase from 147 MGD in the surface water dominated Onomea aquifer (which
includes Honoli‘i) to 395 MGD in the groundwater dominated Kea‘au aquifer (DWS).
Freshwater inputs (carrying nutrients and contaminants) to Hilo Bay can thus be
conceptualized as arriving to two sections: a surface water dominated western half, and
a groundwater dominated eastern half (sensu Lucas et al., 2023). The Wailuku River is
21
the primary input to the western half of the Bay, while the Wailoa River, and submarine
groundwater discharge dominate the eastern half. Mean daily discharge from the
Wailuku River to the western side of Hilo Bay (at station 16713000, active 1977 – 1983)
was ~ 1.0 million cubic meters per day (~ 264 MGD), while submarine groundwater
discharge primarily from the Waiākea to the eastern portion of the bay is estimated at
1.8 million cubic meters per day (~ 476 MGD) (M & E Pacific, 1980).
5. Land Use
Nearly three quarters of the watersheds draining to Hilo Bay are covered in land
designated for conservation (Figure 3). These lands include native forests, grasslands,
and bare soil and lava rock (Table 2). Cropland, pasture, shrub/brush, and rangeland are
the next two most prevalent land uses. Developed use is a relatively small proportion
but is concentrated in the lower parts of all watersheds and especially in lower
Waiākea/Wailoa, in close proximity to Hilo Bay. Land use and land cover provide
information on contaminant sources to surface flow (e.g., loose soil in agricultural
lands, or urban runoff) and subsurface flow (e.g., high density urban areas with
cesspools, see below).
22
23
6. Water Quality
6.1 Suspended sediment concentration and turbidity in streamflow
Suspended sediment concentration (SSC) was measured in lower Wailuku River the
near the outlet to Hilo Bay (USGS station 16713000) from 1977 to 1983. Although these
data were collected at the height of sugar cane production, and do not identify specific
sources of sediment within the watershed, they do provide an important metric against
which to compare our forthcoming results. Over the monitoring period, the average total
daily load (TDL, metric Tons/day) was 64 T/day, and the maximum daily load was
greater than 10,000 T. There is a weak positive relationship between discharges greater
than 9.6 m3/s and SSC (Figure 4). This ‘storm sediment rating curve’, shows a positive
power law relationship between the largest flows (those with exceedance probability
greater than 30 %) and SSC. Whereas there are other scattered data points showing
large SSC on relatively low discharge days, the general trend shows particularly high
SSC occurring in less common, high flow events. For this general trend, an exponent
less than 1 (0.54) suggests sediment delivery is supply limited within the watershed.
Although the USGS station in Hilo is no longer active, mean daily discharge correlates
well with the upstream gage (Pi‘ihonua) during the period of overlap: (QHilo = 1.48QPi’ihonua
24
+ 1.08, R2 = 0.98). Our data collection will fill this gap and assess current sediment
delivery through Wailuku River.
SSC was measured in Waiākea at Hoaka Road (USGS station 16700600) from 2004 to
2006 (Presley, 2008). Over the monitoring period mean daily flow ranged from 0 (no
flow) to 12.46 m3/s. The maximum measured daily load was only about 6 T/d. There is
a weak positive power law relationship between mean daily discharge and SSC (R2 =
0.31). Over the monitoring period (745 days), SSC was greater than 0 on only 65 days (<
9%), indicating the relatively small amount of sediment transport in Waiākea stream.
The repor t also includes downstream monitoring on Waiākea stream within Hilo (USGS
station 16701300). Here, the mean daily load was only 0.65 T/d.
As part of the same USGS report (Presley et al., 2008), Alenaio stream was monitored at
two locations in Hilo (16701600 and 16701650, at the Kilauea Bridge) from 2004 to
2006. Across 15 samples, maximum SSC was 121 and maximum nitrate concentration
was 0.141 mg/L. Maximum TDL was estimated as 317 T/d. The slope of the trend line
between discharge and SSC (0.45) and discharge and nitrate (0.2) is less than 0,
indicating a stream that dilutes at higher flows. Alenaio stream is an infrequent, but
important contributor of sediment and nutrients to Hilo Bay.
There are no continuous records of suspended sediment concentration or turbidity at
the Honoli‘i gage, however there are data relating estimated discharge at the mouth of
Honoli‘i with Turbidity (NTU) measurements (Strauch, 2017). Turbidity exhibited a
non-linear and accelerating response to rises in discharge, and higher turbidity carried
higher loads of fecal indicator bacteria (FIB). These FIBs were associated with pig
activity in the upper watershed (Strauch et al., 2014).
Low levels of turbidity (NTU) were measured throughout the wet season on upper and
lower Wailuku, Pukaihae, Maile, and Honoli‘i (Young and Gadszak, 2008). The authors
measured a maximum TDL of about 16 T/d at Honoli‘i when mean daily flow was a 0.68
m3/s (65% exceedance probability). In summary, the SSC data show little sediment is
transported to Hilo Bay in Waiākea, but that Wailuku, Honoli‘i, and to a lesser extent
Alenaio, transport large amounts of sediment. There are few data to assess sediment
loads in Honoli‘i, but visual observations cite Honoli‘i as a large sediment contributor.
There are no spatial data identifying sediment sources in any of the streams.
6.2 Nutrients and contaminants
High nutrient concentrations and bacteria levels
have frequently been documented in rivers and
nearshore waters of Hilo Bay watershed
(Steadmon et al. 2024; Wiegner et al. 2017;
Waiki et al. 2025), including antibiotic-resistant
strains of Staphylococcus (Gerken et al. 2021;
Saingam et al 2021), posing ecological and
25
public health risks. Temporal and spatial patterns of nutrient and bacteria data along
the coast and in river plumes have indicated that bacteria are transported from the
watershed in surface runoff, especially during high flow events, from urban land uses
and human sources (Wiegner et al. 2017; Economy et al. 2019). Although water quality
along the coast has high variability in space and time, the Wailuku river mouth is a
common hotspot of turbidity and bacteria contamination (Wiegner et al. 2017; Saingam
et al. 2021). The Wailoa River plume is the more likely dominant source of nutrients into
the bay, evidenced by high concentrations of nitrate after storm events. The primary
source of nitrates and phosphates is leaching from on-site-sewage-disposal systems
(‘OSDS’, Whittier and El Kady, 2014). Only 30% of urban Hilo is connected to sewer
(Silvius et al, 2005). There are over 10,000 cesspools in the greater Hilo area with the
greatest concentration of cesspools per square kilometer in the south-central region of
Hilo (Figure 5). Combined, the OSDS dump an estimated 5.6 MGD of untreated effluent
into Hilo Bay (HDOH, 2017). Contamination of drinking water, as measured by DOH, led
to the closure of the Kaumana spring (one of Hilo’s first drinking water sources) in the
1960’s (Young and Godszak, 2008). Previous Water Quality Monitoring (ibid) has
identified streams with consistently elevated nutrient concentrations that isolate areas
contaminated by OSDS and includes maps of sampling locations from which to build an
effective monitoring program. The repor t also identified stream corridors subject to
periodic flooding and subsequent transport of contaminants to Hilo Bay. The urban
nature of the lower Waiākea/Wailoa watershed requires assessment of existing
floodwater engineering and a modeling assessment of predicted flooding under
different sea level rise scenarios.
Cesspools are predominantly found within the Waiākea/Wailoa watershed (Table 3).
The highest densities of cesspools (> 500/km2) are located west of the Kanoelehua
Avenue between Puainako and Haihai streets and along Kaumana Drive mauka of
Mohouli Street. Hilo’s municipal water serves over 13,000 connections and is sourced
from groundwater at Pana‘ewa Well Nos. 1, 2 and 3, Pi‘ihonua Well Nos. A, B, and C,
Saddle Road Well “A”, and the UH Hilo Well. The Hawai‘i County Department of Water
Supply Water Quality Report for Hilo (2024) found a maximum nitrate of 0.39 ppm.
However, the UH Hilo and Pi‘ihonua A & B wells are downstream of high OSDS density
areas and may be at risk of contamination. In 2017, the Hawai‘i State Legislature
passed Act 125 in 2017 to require the replacement of all cesspools by 2050 (Hawaiʻi
State Legislature, 2017). Hawai‘i County must expand sewer service and initiate a
cesspool conversion plan by 2026.
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In-stream nutrient criteria for streams (HAR 11-54) require geometric means below 30
ug/L (in dry season) and 70 ug/L (in wet season) (0.03 and 0.07 ppm), and below a 2%
exceedance value of 0.170 and 0.300 ppm (dry/wet season). High levels of nitrate were
measured at Kaumana Waiākea and Waiolama Springs and at the Kaluiiki branch of the
Waipahoehoe (Young and Godszak, 2008). Waiolama spring (489 – 850 μg/L) and
Kaumana spring (161 – 352 μg/L) showed high levels of nitrate, and nitrate was also
found at high concentrations (379 – 487 μg/L) in Waiākea Pond. The authors pinpointed
elevated spring concentrations leading to impairment of river conditions downstream.
Continuous monitoring on Waiākea stream at Hoaka (from Presley et al., 2008) shows
steady increases in nitrate and phosphate concentrations with increasing discharge, but
overall low levels of nitrate and (< 110 μg/L at all flows) and phosphate (< 125 μg/L at
all flows) typical of background flows. The approximately linear relationship between
mean daily discharge and kg/day of nitrate and phosphate shows a good fit (R2 = 0.80
and 0.76, respectively). Total maximum daily loads were 32.3 kg/d for nitrate and 75.4
kg/d for phosphate. These data suggest little to no nutrient loading upstream of Hoaka.
Analysis of stream nutrient concentrations and yields in relation to land cover and
watershed characteristics for 24 east Hawaii Island streams (Michaud and Wiegner
2011) found elevated nitrate concentrations compared to a global baseline for
unpolluted tropical streams and concluded that anthropogenic activity in the Mauna Kea
watershed has resulted in increased nitrate concentrations. This study did not find
strong relationships between water quality conditions and watershed characteristics
27
including watershed area, mean annual rainfall, population density, or agricultural land
cover. However, their sampling was limited to baseflow conditions (based on feasibility)
and did not include any spatial variability within stream networks.
6.3 Turbidity and water quality in Hilo Bay
Since at least 2021, the Pacific Islands Ocean Observing System (PACIOOS) buoy has
recorded temperature, salinity, turbidity, chlorophyll, oxygen, and saturated oxygen at
15-minute intervals less than a kilometer from the mouth of the Wailuku River. There is a
power law relationship between discharge in Wailuku River (Pi‘ihonua) and turbidity
(NTU) measured at the buoy (Mead and Wiegner, 2010). Discharges exceeding modest
flows of approximately 3.5 m3/s (~ 34 % probability exceedance) resulted in turbidity
rising above the water quality threshold of 3 NTU. Point measurements across the bay
showed turbidity to exceed the same water quality thresholds throughout the western
(‘surface-water dominant’) portion of the bay following high flow (90th percentile) events
(Lucas et al., 2023). During the same high flow events, the entire bay exceeded water
quality standards for chlorophyll. At mean flows, turbidity in excess of water quality
standards were concentrated along the western shore to beyond the breakwater. At low
flows, turbidity in excess of water quality standards was only found at the mouth of
Wailuku River; however, chlorophyll concentrations exceeded water quality standards in
many areas of the eastern (‘groundwater dominant’) portion of the bay. There is an
opportunity to align the buoy with land-based monitoring and measurements to better
understand sediment and nutrient production and the resulting impact, including peak
concentrations and residence times within the bay.
6.4 Impaired streams
A recent assessment from NWQI listed all streams in the Hilo Bay Watershed as
impaired based on nutrient or turbidity standards (Figure 6 from the USDA, NRCS, 2022).
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Minimal freshwater nutrient data exist beyond the nearshore region of the watershed;
however nutrients and other sources of contamination have been measured in Hilo Bay.
Nitrate concentrations were found at higher concentrations near the mouth of Wailuku
and increased during stormflow conditions (Wiegner et al., 2013). Chlorophyll α
concentrations were greater in the eastern por tion of Hilo Bay (dominated by
groundwater flow) during low flow conditions (Lucas et al., 2023), which suggests heavy
nutrient loading of groundwater emerging in Wailoa Pond or as submarine groundwater
discharge (SGD). This interpretation is consistent with a recent NASA DEVELOP report
showing consistently high chlorophyll α concentrations at the mouth of the Wailoa River
(Sonobe et al., 2025). A recent study employing multiple tracers in Keaukaha, east of the
Hilo Bay breakwater, documented the presence of OSDS sewage in coastal waters and
measured flow rates of effluent in groundwater to be 130 to 213 m/d (Waiki et al.,
2025). Concentrations of fecal indicator bacteria in coastal springs were similar to
measurements across the state but were orders of magnitude lower than
concentrations in Hilo Bay following storms perhaps due to dilution of effluent in the
large groundwater reservoir (Waiki et al., 2025; Wiegner et al., 2017; Economy et al.,
2017).
Across Hilo Bay, 40 – 50 % of samples tested positive for human waste (Wiegner et al
2017; HDOH, 2018). At high flow, Enterococcus concentrations exceeded HDOH
standards throughout the Bay, while at low flows, standards were exceeded in the
Wailuku and Wailoa Rivers. Concentrations of Enterococcus and Clostridium perfringes
within Hilo Bay increased dramatically following a high flow event in May 2013 (86 m3/s
peak mean daily flow, 1-year return interval flood) (HDOH, 2018).
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Under dry conditions, groundwater inputs result in decreased salinity along the southern
shore of the bay and turbidity is generally low (Badlowski et al., 2021). During storm
events, large inputs from Wailuku River results in a plume of low temperature, low
salinity, and high turbidity that hugs that western shoreline and extends into the
bay(Badlowski et al., 2021).
Hilo Bay is relatively shallow (less than 15 m deep) and despite the breakwater,
residence time is generally low. Mass balance estimates of residence time within Hilo
Bay vary from 3.5 days (dry conditions) to 2.5 days (wet conditions), and only 1.3 days
during storms (M&E Pacific 1980). Generally, the combined influence of groundwater
inputs and persistent northeast trade winds result in a ~ 5 cm/s westward surface
current (M&E Pacific 1980; USACE 2009).
6.5 Critical Source Areas
6.5.1 Critical Source Areas for sediment
The recent NWQI report (USDA, NRCS, 2022) mapped critical source areas (CSA’s)
where sediment loss vulnerability (defined by typical rainfall intensity and soil
erodibility) overlaps high transport potential (proximity to streams). The report found
CSA’s to be concentrated in agricultural areas in the northern watersheds, and in areas
of stream convergence where sediment is episodically mobilized (such as mid-Wailuku).
The Waiākea watershed has fewer CSAs because of its highly porous substrate and
gentler slope; however, there are potential sediment source areas in the pasturelands
near the middle of the watershed. Throughout the Hilo Bay Watershed, pasture was
found to be the highest-risk land use category for sediment production. The
methodology used, however, was developed for agricultural soils and does not fully
capture sediment production from forests, particularly forests inhabited and modified
by feral ungulates. Studies from other areas of Hawaii island have documented the
substantial impacts of these animals to watershed coverage of vegetation and exposed
soil area (Cole and Litton 2014). Sediment load data are needed across different land
uses to inform changes in land use practices that results in decreasing SSC below
water quality standards.
6.5.2 Critical Source Areas for nutrients
As mentioned above, the critical source areas for nutrients are the over 10,000 OSDS
within the Hilo Bay Watershed (Figure 5). We hypothesize the spatial distribution of
nutrient concentrations in streams is correlated to the upslope nutrient loading of the
OSDS. Our results will link upslope loading to groundwater and surface water
conveyance, to ongoing DOH water quality monitoring along the shoreline. Whereas the
existing water quality data on nutrient concentrations in the Hilo Bay region is largely
confined to nearshore monitoring stations (e.g., Figure 7 shows the distribution of
nitrate measurements in the national Water Quality Portal), our monitoring will
30
encompass middle and upper por tions of the stream networks in order to develop a
spatially-resolved understanding of nutrient loading hotspots.
7. Flooding
● November 1-2, 2000 as ‘flood of record’ for Waiākea.
● Peak rainfall intensity of greater than 80 mm/ h (3.2 in/ h), > 750 mm in 24 h ( 30
in 24 h)
● Waiakea – Palai flood modeling done by USACE (2024).
8. Sea Level Rise
Higher water levels resulting from sea level rise (SLR) is an important factor in
determining the long-term resiliency of coastal regions. Global SLR (GSLR) is driven by
climate change, as ocean volume is increasing due to thermal expansion of warming
oceans and due to additional water from melting ice sheets and glaciers. Global mean
sea level has increased by approximately 6.7 inches (17.0 centimeters) over the last 100
years with noted acceleration since about 1970 (Sweet et al. 2022). However, sea level
does not increase uniformly around the globe; it is spatially variable and can change
significantly by region. This more locally focused increase is referred to as relative SLR
(RSLR), which can be more or less than the global average (Sweet et al. 2022). RSLR is a
function of vertical land movement (land subsidence / uplift) and changes in regional
ocean currents in addition to GSLR.
The US Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Interagency Task
Force (Interagency Task Force), which is led by the National Oceanic and Atmospheric
Administration (NOAA) and comprised of scientists from several federal agencies and
academic institutions, published a technical report (Sweet et al. 2022) that is widely
31
used to understand SLR. This technical report provides five SLR scenarios projected to
2150 at global, regional, and local scales for all US states and territories. These five
scenarios cover low, intermediate-low, intermediate, intermediate-high, and high
projections of SLR, each with varying likelihood of occurring going forward.
Hawai’i’s State Climate Commission has developed RSLR guidance based on the 2022
Interagency Task Force technical report (Hawai’i State Climate Commission 2022). The
recommendations from the state are to evaluate two RSLR scenarios depending on the
tolerance of risk for a given project, which are the intermediate and the
intermediate-high scenarios for Hawai’i. This guidance states that RSLR of 4 feet by the
year 2100 (intermediate scenario) should be considered as the minimum for all
planning and design, and RSLR of 6 feet by the year 2100 (intermediate-high scenario)
should be considered for planning and design of public infrastructure and other projects
with low tolerance for risk (Hawaiʻi Climate Change Mitigation and Adaptation
Commission. 2017, updated 2025). Note that the RSLR projection values have a
baseline year of 2000. The project team will coordinate closely with the County to
determine the risk tolerance for this project and select the appropriate scenario for use.
For more guidance from the state on this, see Guidance for Addressing Sea Level Rise
in Community Planning in Hawai’i (Courtney et al. 2020) and Guidance for Using the Sea
Level Exposure Area in Local Planning and Permitting Decisions (Romine et al. 2020).
Additionally, to build on the State’s guidance, more site-specific RSLR projections from
the 2022 Interagency Task Force technical report are available from nearby tidal
stations. In this case, RSLR projections can be pulled from the Hilo Bay, HI tidal station
(Station ID: 1617760). The projections from this station show that sea levels are rising
slightly faster in this location compared to the state’s guidance. For this project, the
RSLR projections from the Hilo Bay, HI tidal station will be used as they are more
conservative and regionally appropriate. Under the intermediate scenario this area is
projected to experience RSLR of 1.16 feet by 2050, 2.01 feet by 2070, and 4.17 feet by
2100. Under the intermediate-high scenario this area is projected to experience RSLR of
1.43 feet by 2050, 2.97 feet by 2070, and 6.17 feet by 2100 (Sweet et al. 2022). Again,
note the RSLR projection values have a baseline year of 2000. To provide the most
appropriate representation of RSLR, the project team will shift the baseline year of the
RSLR projection values to 2025 in accordance with Appendix A in Application Guide for
the 2022 Sea Level Rise Technical Report (Collini et al. 2022) provided by the
Interagency Task Force.
The project team will use the selected RSLR values to define the boundary conditions of
the watershed hydraulic models and support the development of flood hazard maps
that consider impacts of future climate conditions. The same RSLR values will be used
to support the assessment of coastal hazards. There are sufficient literature and data
available to support these efforts; no data gaps identified.
32
9. Data Gaps
Below is a list of data gaps along with planned or proposed solution(s) in blue.
1. Unknown current relationship between discharge and SSC for background
(upstream) and impacted (downstream) reaches of three major rivers. Install
turbidity probes, autosamplers, and (where necessary) stream gauges to develop
relationships between flow and total daily load of natural and impaired streams.
Determine sediment yield to Hilo Bay from the three primary watersheds.
2. Unknown sediment yield from different portions of the watershed and different
land covers. Where are the ‘hot spots’? Measure SSC at moderate to high flows at
distributed sites throughout the watershed. Use watershed scaling to estimate
discharge and spatial distribution of sediment yield. Measure infiltration capacity
across different land uses to determine threshold rainfall intensity to mobilize
sediment and estimate frequency and magnitude of sediment yield across
watershed.
3. Unknown nutrient concentrations and spatial patterns across Hilo Bay Watershed
streams. Measure nitrate + nitrite/phosphate across streams, gulches, and
ditches to identify areas of high nutrient loading (see proposed monitoring plan
in Figure 8). Measurements to be conducted at low flows and at high flows
across wet and dry seasons. Follow EPA guidelines (see below) in order to
suppor t the State Depar tment of Health’s Clean Water Act repor ting requirements
(HDOH 2004) and its proposed design for an inland water quality monitoring
program.
o Hawaii Administrative Rules (11-54) specify water quality criteria for inland
waters based on five parameters: Total Nitrogen, Nitrate + Nitrite Nitrogen,
Total Phosphorus, Total Suspended Solids, and Turbidity. Numeric criteria
specify values that are not to be exceeded for the geometric mean, 10%, and
2% of measured values. Numeric criteria are higher for wet season
concentrations (November 1 through April 30) and lower for the dry season
(May 1 through October 31).
o Measurements of relevant parameters from the period October 1, 2022 and
September 30, 2027 should be submitted to the State Department of Health
Clean Water Branch by October 1, 2027 to be included for consideration in
the State’s 2028 Integrated Report for §303(d) and §305(b) of the Federal
Clean Water Act, along with complete metadata and documentation required
to assess the completeness and accuracy of the data. A minimum of 30
samples per waterbody over the five-year period is required for assessment.
Turbidity will be monitored continuously using in situ sensors, and water
33
samples will be collected for measurements of nutrients and total
suspended solids. Total suspended solids will be measured gravimetrically
by trained technicians at the ASU GDCS laboratory at the Institute for Pacific
Islands Forestry in Hilo and nutrient concentrations will be analyzed at the
state-cer tified water quality laboratory at that National Energy Laboratory of
Hawaii Authority (NELHA).
4. Cost-benefit analysis to identify ‘low-hanging fruit’ or low(er) cost initiatives that
might address ‘hot spots’ identified above.
5. Little is known about the suite of ‘other’ harmful substances from agricultural,
suburban, and urban sources transported to the bay in surface and subsurface
flows. There are potential/unknown risks associated with pesticides, especially
the extensive historical use of sodium arsenite on sugarcane plantations
(Falinski et al. 2014; Hallacher et al. 1985; Johnson et al. 2024), as well as
dumping of hazardous materials and toxic waste in lava tubes (Halliday et al.
2003). Previous studies have measured high levels of arsenic trapped in
undisturbed sediment layers near original sites of pollution in Waiākea Pond,
however there is little evidence that this arsenic has mobilized into biota
(Hallacher et al. 1985; Falinski et al. 2014). We do not have resources to
document and model the mobilization, transport, and fate of these contaminants
and instead will rely on sediment and nutrients as indicators of a chemical
‘cocktail’.
6. Unknown how watershed sources affect patterns and trends in water quality
monitoring observed by DOH and Hawai‘i Wai Ola along the Hilo Bay Coast.
Design our sampling effor ts to complement ongoing sampling and synthesize
data sets to expand scope of inference of proposed upstream
monitoring/sampling.
34
10. Proposed sampling points for suspended sediment concentration and nutrient
monitoring
35
11. References
Badlowski, Gina A., Jason E. Adolf, and Geoffrey Fouad. "Spatial analysis of water
quality parameters in Hilo Bay, Hawai'i, using a combination of interpolated surfaces
and hot spot analysis." Environmental monitoring and assessment193.3 (2021): 118.
Belt, Collins & Associates. (1961). A comprehensive plan for the Hilo sewer system, City
of Hilo, Hawaii. Prepared for the Board of Supervisors, County of Hawaii.
Cole, R. J., & Litton, C. M. (2014). Vegetation response to removal of non-native feral
pigs from Hawaiian tropical montane wet forest. Biological invasions, 16(1), 125-140.
Collini, R.C., J. Carter, L. Auermuller, L. Engeman, K. Hintzen, J. Gambill, R.E. Johnson, I.
Miller, C. Schafer, and H. Stiller. 2022. Application Guide for the 2022 Sea Level Rise
Technical Report. National Oceanic and Atmospheric Administration Office for Coastal
Management, Mississippi–Alabama Sea Grant Consortium (MASGP-22-028), and
Florida Sea Grant (SGEB 88).
https://oceanservice.noaa.gov/hazards/sealevelrise/noaa-nos-techrpt02-global-regiona
l-SLR-scenarios-US-application-guide.pdf.
County of Hawaiʻi Depar tment of Water Supply. (2024). Annual report on water quality:
Hilo water system.
Courtney, C.A; Romine, B.M.; Lander, M.; Hintzen, K.D.; Owens, T.M.; Pap, R.A. 2020.
Guidance for Addressing Sea Level Rise in Community Planning in Hawai'i. Prepared by
Tetra Tech, Inc. for the University of Hawai'i Sea Grant College Program and State of
Hawai'i Department of Land and Natural Resources and Office of Planning, with funding
from National Oceanic and Atmospheric Administration Office for Coastal Management
Award No. NA16NOS4730016.
Desha, S. L. (2000). Kamehameha and his warrior Kekūhaupiʻo (F. N. Frazier, Trans.).
Kamehameha Schools Press. (Original work published 1920–1924).
Economy, L. M. (2017). Pathogen and nutrient concentrations in Hilo Bay, Hawaiʻi:
Examining the effects of shoreline sewage pollution and river discharge [Master's thesis,
University of Hawaiʻi at Hilo]. UH Hilo Digital Repository.
Falinski, K. A., Yost, R. S., Sampaga, E., & Peard, J. (2014). Arsenic accumulation by
edible aquatic macrophytes. Ecotoxicology and environmental safety, 99, 74-81.
Gerken, T.J., Roberts, M.C., Dykema, P., Melly, G., Lucas, D., De Los Santos, V., Gonzalez,
J., Butaye, P. and Wiegner, T.N., 2021. Environmental surveillance and characterization
of antibiotic resistant Staphylococcus aureus at coastal beaches and rivers on the
island of Hawaiʻi. Antibiotics, 10(8), p.980.
36
Hallacher, L. E., Kho, E. B., Bernard, N. D., Orcutt, A. M., Dudley Jr, W. C., & Hammond, T.
M. (1985). Distribution of arsenic in the sediments and biota of Hilo Bay, Hawaii. Pacific
Science 39(3): 266-273.
Halliday, W. R. (2003). Raw sewage and solid waste dumps in lava tube caves of Hawaii
island. Journal of Cave and Karst Studies, 65(1), 68-75.
Hawaiʻi Climate Change Mitigation and Adaptation Commission. 2017, updated 2025.
State of Hawaiʻi Sea Level Rise Viewer. Version 1.19. Prepared by the Pacific Islands
Ocean Observing System (PacIOOS) for the University of Hawaiʻi Sea Grant College
Program and the State of Hawaiʻi Department of Land and Natural Resources, Office of
Conservation and Coastal Lands, with funding from National Oceanic and Atmospheric
Administration Office for Coastal Management Award No. NA16NOS4730016 and under
the State of Hawaiʻi Department of Land and Natural Resources Contract No. 64064.
http://hawaiisealevelriseviewer.org. Accessed January 12, 2026.
Hawaiʻi State Department of Health. (2017). Relating to cesspools and prioritizing and
reporting the cesspools in the state that should be upgraded, replaced, or closed: Report to
the 2018 legislature.
Hawaiʻi State Department of Health. (2018). 2018 State of Hawaiʻi water quality
monitoring and assessment report: Integrated report as required by the Clean Water Act
§303(d) and §305(b). Clean Water Branch.
Hawaiʻi State Legislature. (2017). Act 125: Relating to cesspools. 2017 Regular Session.
https://www.capitol.hawaii.gov/session2017/bills/HB1248_CD1_.pdf
Hawai'i State of Climate Commission. 2022. Hawaiʻi Sea Level Rise Vulnerability and
Adaptation Report. Hawaiʻi State Climate Change Mitigation and Adaptation
Commission, prepared by the State of Hawaiʻi Department of Land and Natural
Resources, Office of Conservation and Coastal Lands.
https://climate.hawaii.gov/wp-content/uploads/2025/07/Sea-Level-Rise-Adaptation-an
d-Vulnerability-2022-Update_Final.pdf
Johnson, A.G., Kennedy, J.J., and Alvarez, D.A., 2024, Pesticides in surface water
downstream of and near agricultural and developed land in Hawai‘i, 2015–19: U.S.
Geological Survey Scientific Investigations Report 2024–5071, 94 p.,https://doi.org/
10.3133/sir20245071.
Kalākaua, D. (1972). The legends and myths of Hawaii: The fables and folk-lore of a
strange people. Charles E. Tuttle Co. (Original work published 1888)
Kelly, M., Nakamura, B., & Barrère, D. B. (1981). Cultural resources overview: Hilo Bay area,
Island of Hawaiʻi. Department of Anthropology, Bernice Pauahi Bishop Museum.
37
Lucas, S. N., Fouad, G., & Adolf, J. E. (2023). Spatially distributed water quality
responses to freshwater discharge in a tropical estuary, Hilo Bay, Hawai ‘i. Environmental
Monitoring and Assessment, 195(3), 428.
M & E Pacific. (1980). Hilo area comprehensive study—geological, biological, and water
quality investigations of Hilo Bay. Hawai‘i: United States Army Corps of Engineers,
Honolulu District.
Maly, K., & Maly, O. (2003). Panae‘o: A Cultural-Historical Study of the Hilo Bay Watershed.
Prepared for the Hawaii County Economic Oppor tunity Council.
Mead, L. H., & Wiegner, T. N. (2010). Surface water quality along the longitudinal
continuum of Canoes-Waiākea Stream, Hilo, Hawaiʻi. Pacific Science, 64(4), 539–549.
https://doi.org/10.2984/64.4.539
Mezzacapo, M. S., & Shuler, C. K. (2022). Characterizing nutrient sources and transpor t in
the Wailoa River and Waiākea Pond, Hilo, Hawaiʻi. University of Hawaiʻi at Mānoa, Water
Resources Research Center.
Michaud, J. and Wiegner, T., 2011. Stream Nutrient Concentrations on the Windward
Coast of Hawai ‘i Island and Their Relationship to Watershed Characteristics1. Pacific
Science, 65(2), pp.195-217.
Presley, T.K., Jamison, M.T.J, and Nishimoto, D.C., 2008, Suspended-sediment and
nutrient loads for Waiakea and Alenaio Streams, Hilo, Hawaii, 2003-2006: Honolulu,
Hawaii, U.S. Geological Survey Open-File Repor t 2007–1429, 73 p.
Romine, B.M.; Habel, S.; Lemmo, S.J.; Pap, R.A.; Owens, T.M.; Lander, M.; Anderson, T.R.
(2020). Guidance for Using the Sea Level Rise Exposure Area in Local Planning and
Permitting Decisions. Prepared by the University of Hawaii Sea Grant College Program
with the Hawaiʻi Depar tment of Land and Natural Resources - Office of Conservation
and Coastal Lands for the Hawaiʻi Climate Change Mitigation and Adaptation
Commission - Climate Ready Hawaiʻi Initiative. (Sea Grant Publication TT-20-01). Cover
Photo Credit: Dr Shellie Habel, courtesy of DLNR-OCCL, 2020.
Saingam, P., Li, B., Sung, S. and Yan, T., 2021. Immediate impact of Hurricane Lane on
microbiological quality of coastal water in Hilo Bay, Hawaii. Environmental science &
technology, 55(5), pp.2960-2967.
Silvius, K. M. (2005). Hilo Bay watershed management plan. Prepared for the Hilo Bay
Watershed Advisory Group.
Sonobe, D., White-Murillo, S., Shimabukuro, K., & Clark, A. N. (2025). Hilo Bay Water
Resources: Monitoring Water Quality in Hilo Bay, Hawaii to Support Future Community
Planning.
38
Sweet, W.V., B.D. Hamlington, R.E. Kopp, C.P. Weaver, P.L. Barnard, D. Bekaert, W. Brooks,
M. Craghan, G. Dusek, T. Frederikse, G. Garner, A.S. Genz, J.P. Krasting, E. Larour, D.
Marcy, J.J. Marra, J. Obeysekera, M. Osler, M. Pendleton, D. Roman, L. Schmied, W.
Veatch, K.D. White, and C. Zuzak. 2022. Global and Regional Sea Level Rise Scenarios
for the United States: Up¬dated Mean Projections and Extreme Water Level Probabilities
Along U.S. Coastlines. NOAA Technical Repor t NOS 01. National Oceanic and
Atmospheric Administration, National Ocean Service, Silver Spring, MD, 111 pp.
Steadmon, M., Takakusagi, M., Wiegner, T.N., Jones, M., Economy, L.M., Panelo, J.,
Morrison, L.A., Medeiros, M.C. and Frank, K.L., 2024. Detection and modeling of
Staphylococcus aureus and fecal bacteria in Hawaiian coastal waters and sands. Water
Environment Research, 96(5), p.e11037.
Strauch, A. M., Mackenzie, R. A., Bruland, G. L., Tingley III, R., & Giardina, C. P. (2014).
Climate change and land use drivers of fecal bacteria in tropical Hawaiian
rivers. Journal of environmental quality, 43(4), 1475-1483.
U.S. Army Corps of Engineers (USACE). (2009). Hilo Bay water circulation and water
quality study. Honolulu District.
U.S. Department of Agriculture, Natural Resources Conservation Service. (2022). Hilo
NWQI watershed assessment and implementation plan.
Waiki, S. M., Colbert, S. L., Wiegner, T. N., Puniwai, N., Nakoa III, J. W., Storie, N. M., ... &
Aguiar, D. K. (2025). Sewage pollution from onsite sewage disposal systems and an
offshore wastewater treatment plant outfall in coastal waters of Keaukaha, Hawaiʻi
Island. Journal of Hydrology: Regional Studies, 57, 102122.
Whittier, R. B., & El-Kadi, A. I. (2014). Human health and environmental risk ranking of
on-site sewage disposal systems for the Hawaiian Islands of Kaua‘i, Maui, Moloka‘i, Lāna‘i,
and Hawai‘i. University of Hawai‘i at Mānoa, Water Resources Research Center.
(Prepared for the Hawaii State Depar tment of Health).
Wiegner, T. N., Mead, L. H., & Molloy, S. L. (2013). A comparison of water quality
between low-and high-flow river conditions in a tropical estuary, Hilo Bay,
Hawaii. Estuaries and coasts, 36(2), 319-333.
Wiegner, T. N., Edens, C. J., Abaya, L. M., Carlson, K. M., Lyon-Colbert, A., & Molloy, S. L.
(2017). Spatial and temporal microbial pollution patterns in a tropical estuary during
high and low river flow conditions. Marine Pollution Bulletin, 114(2), 952-961.
Young, R. H., & Godszak, L. (2008). Impact of urbanization on the quality of Hilo Bay,
Hawaiʻi. University of Hawaiʻi at Mānoa, Water Resources Research Center.
39