Final Report: Mapping and environmental science to support clean reefs of West Maui, Hawaii
Dates
Date Reported
2017-10-13
Citation
Final Project Report: Mapping and environmental science to support clean reefs of West Maui, Hawaii: .
Summary
Episodic runoff carries suspended sediment to the nearshore, where it blocks light used for photosynthesis, smothers corals, inhibits coral recruitment, and triggers increases in macroalgae. Even small rainfalls create visible plumes over a few hours. Sediment affects coastal user enjoyment by deteriorating both ecosystem quality and visibility. Sources of erosion include unimproved roads, fallow and active agricultural fields, disturbed forests, local development, and streambanks. In this project, USGS used mapping, field experiments and monitoring, and analysis of recent (July 19–20, 2014) and historic rainfall to estimate sources of land-based pollution for watersheds in West Maui, Hawaii. USGS constructed an overall sediment budget [...]
Summary
Episodic runoff carries suspended sediment to the nearshore, where it blocks light used for photosynthesis, smothers corals, inhibits coral recruitment, and triggers increases in macroalgae. Even small rainfalls create visible plumes over a few hours. Sediment affects coastal user enjoyment by deteriorating both ecosystem quality and visibility. Sources of erosion include unimproved roads, fallow and active agricultural fields, disturbed forests, local development, and streambanks. In this project, USGS used mapping, field experiments and monitoring, and analysis of recent (July 19–20, 2014) and historic rainfall to estimate sources of land-based pollution for watersheds in West Maui, Hawaii. USGS constructed an overall sediment budget for the watersheds, describing the proportion of sediment coming from each source. Findings indicate that streambank erosion of legacy fill terraces likely contributes most of the annual fine (< sand) sediment to the nearshore. Terraces are the legacy of agricultural land management practices where soil and rock waste was pushed from the fields into stream channels. Surveys show these terraces occupy ~40% of the bank length along former agricultural land, and erode at median rates of 5-14 mm/year. A reconnaissance sediment budget for bank erosion of historic fill terraces indicates that just over 90 dumptruck loads of fine sediment are entering the nearshore environment each year from West Maui’s watersheds. Annual sediment loads from this source may be up to 10-50 times the pre-human load, and 40-300% of annualized loads from large decadal storms. In short, a case can be made that the amounts of fine sediment arriving at the nearshore every year, from even small storms, are unprecedented.
We used findings from this field assessment to inform a decision-support model. We developed an opinion-based Bayesian Belief model to estimate streambank erosion from rainfall events that can quantify sediment delivery to the coasts during rainfall events. The model can estimate sediment reduction from implementing specific management measures, such as pig removal and repairing stream crossings.
