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A GIS Based Watershed Analysis System
for Tillamook Bay, Oregon
By Patrice Angelle Melancon, M.S.E., Michael E. Barrett, Ph.D.,
P.E. and David R. Maidment
ABSTRACT
The goal of this research is to develop a GIS based watershed scale
water quality model of the Tillamook Bay watershed on the coast of Oregon
that can be used as a decision support system. A method is developed
using a Geographic Information Systems (GIS) platform, specifically
Arc/Info and ArcView. A 100 foot grid digital elevation model is used
to establish connectivity within the watershed. Raster maps of runoff
and baseflow are determined from a raster map of annual precipitation.
Non-point source loads of bacteria and sediment are determined for each
grid cell as the product of discharge and expected mean concentration
(EMC). EMC values are based on land use. These non-point loads are accumulated
down to the bay segments. Point source loads from wastewater treatment
plant effluent have been included in the model. Implementation of Best
Management Practices (BMPs) result in load reductions on a per cell
basis. These reductions are based on user input data related to BMP
effectiveness and level of implementation. Predicted concentration grids
are calculated based on accumulated loads and flows. Tools are available
to determine: (1) loads, flows, and resultant concentrations at points
of interest, (2) percent reduction of load to each bay segment as a
result of BMP implementation, and (3) constituent concentration profiles
along the length of a river. Predicted flows and concentrations reasonably
match values reported in earlier studies. Model results indicate that
the majority of the bacteria load comes from dairy lands, and sediment
loads appear to be strongly linked to channel erosion processes, particularly
in the lowland river reaches.
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