Delineating Basins with Coastal Data

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Delineating Basins with Coastal Data

• Because flash floods do not occur over the ocean,
  FFMP calculations should not include
  precipitation estimates over the water.
• Coastal data must be employed to properly
  delineate these basins.

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Delineating Basins with Coastal Data
Basins and streams derived from the DEM without
additional coastal data.
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Delineating Basins with Coastal Data
The NHD ocean/waterbody shapefile is used to mask
out areas of the DEM over the ocean.
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Delineating Basins with Coastal Data
The new DEM-derived basins and streams extend no
further than the coast as outlined by the NHD.
This version will yield more accurate FFMP
calculations.
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Closed Basin and Natural Sink Processing

• As part of the delineation process, the DEM is
  filled so all flow drains out of an area.
• In the real world, naturally-occurring sinks and
  closed basins (basins without an outlet) exist.
• The closed basin processing techniques developed
  at the EROS Data Center result in delineated
  basins that are closer to reality in these areas.

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Closed Basin and Natural Sink Processing
The Summer Lake closed basin (17120005) in Oregon
with NHD stream lines and water bodies overlaid.
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Closed Basin and Natural Sink Processing
The delineated stream lines before closed basin
processing are clearly erroneous.
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Closed Basin and Natural Sink Processing
From the filled DEM, flat areas (areas of
constant elevation) are identified and sink
polygons are defined.
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Closed Basin and Natural Sink Processing
Based on the NHD stream lines and water bodies,
points are selected as "sink outflow" locations.
At these points, the DEM grid cell will be
assigned a "no data" value so all surrounding
drainage will be directed toward this point.
Thus, a natural sink is emulated.
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Closed Basin and Natural Sink Processing
The delineated stream lines after closed basin
processing more closely resemble the actual flow
patterns into the sinks.
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Closed Basin and Natural Sink Processing
Before Closed Basin Processing
After Closed Basin Processing
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Difficulties Encountered During Delineation
Streamflow Reversed
Because an adequate buffer of DEM data was not
available along the U.S./Canada border, the
streamflow was reversed in several
instances. Possible Solution Redelineate and
burn in the NHD stream network.
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Difficulties Encountered During Delineation
HUC 13090002 absorbed into adjacent HUC 12110208
Very small HUCs or HUCs over water may have been
absorbed or included in with other HUCs in the
NED delineation. Possible Solution None
necessary unless you have operations strictly
tied to the original HUC basins.
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Difficulties Encountered During Delineation
Because the NED is a dynamic data set, various
tiles were updated during the course of the
delineation project. If adjacent HUCs were
processed with NED tiles from two different
dates, the delineations along the shared boundary
may differ. This results in small gaps or
overlapping sliver polygons.
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Difficulties Encountered During Delineation
Overlapping Sliver Polygon
0.25 mi2
Generally, these are very small in area (on the
order of several grid cells). The largest gap
shown here is only 0.25 mi2.
Shown above, gaps and/or overlapping sliver
polygons may exist.
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Difficulties Encountered During Delineation
Although some gaps and overlapping sliver
polygons may coincide with radar bins, their
small areas make this an infrequent
occurrence. Possible Solution Redelineate
basins with the most current NED tiles. However,
redelineation of these HUCs may create the need
for redelineation of surrounding HUCs as well.
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Difficulties Encountered During Delineation
Disconnected Stream Lines
Occasionally, errors in the NED data or natural
occurrences such as bifurcated streams translate
to significant problems in the delineation. For
example, an erroneous elevation value(s) can
result in misrouted or disconnected
streams. Possible Solution Correct the
erroneous NED value(s) and redelineate.
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Difficulties Encountered During Delineation
Flat areas along the coastal regions often result
in strange delineations.
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Difficulties Encountered During Delineation
These basins often occur in areas defined as
swamp/marsh. Possible Solution In addition to
masking out ocean areas, mask swamp/marsh areas
out of the DEM and redelineate.
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When to Consider Redelineation

• Basins should be redelineated if
• improved DEM data becomes available,
• it is necessary to make basin calculations at a
  specific point (for example, at a stream gage),
• it is necessary to eliminate sliver polygon gaps
  and overlaps that may exist between HUCs
• it is necessary to eliminate large swamps and
  marshes, or
• it is necessary to "burn in" stream lines (such
  as those in the NHD) to improve delineation
  results.
• Note You must have the Spatial Analyst
  Extension to delineate basins. Please contact
  Ami Arthur at NSSL (Ami.Arthur_at_noaa.gov) if you
  have a redelineation request.