stream channel, retention or detention ponds, or discharged
into buffer zones around the above-ground stream channels.
For each of these locations, the land area draining to these
points was determined, again using the stormwater inlets
(points) and facilities (polygons) as the pour point input of
the
Watershed
tool.
For the final step of our analysis
, Raster Calculator
was used
to build equations that added the new land area and subtracted
land area from the original lidar-derived watershed raster.
Results
Figure 6 shows results of delineating the watershed from the
lidar-elevation model; elevation within the watershed ranges
from 78 to 150 meters. Overlaying the
USGS
-derived watershed
(bold black outline) on the lidar image reveals that the water-
shed shapes are very similar but also reveals impacts of roads
and other impervious surfaces and sharpening some of the
edges of the lidar-watershed. The area measured in hectares is
very similar, i.e., 1,089 ha for the
USGS
-derived watershed and
1,086.6 ha for the lidar-elevation model. However, the lidar-
watershed was the most appropriate to use in the next two
steps of our analysis because of its finer one-meter
2
resolution,
and because it appears to include the influence of impervious
surfaces in some areas of the watershed delineation.
When overlaying the stormwater network shapefiles on the
lidar-elevation model several areas that met the criteria under
each situation listed above in Methods were identified : (a)
stormwater network facilities isolated from the stream net-
work, including retention ponds either not located on water
flow channels or connected only to each other by stormwater
network pipes (Plate 1), (b) stormwater pipes that discharge
outside the watershed, and (c) stormwater pipes that drain
into the watershed area.
To reiterate, we carefully evaluated each stormwater facil-
ity with all map layers, raster datasets, and the aerial photos
to ensure accuracy in identifying it as a retention pond or a
detention pond. Some areas were clearly visible on the lidar-
elevation model as separate catchment areas (please refer back
to Plate 1, Box 2 - darker shading represents higher elevations,
thus watershed divides). In some instances, stormwater pipes
and a section of the above-ground stream channel designed
to drain into a retention pond and not into the main stream
channel were identified (Plate 2, Box 1). In yet other cases, we
discovered stormwater pipes that actually connected two un-
connected stream channels, and then directed water flow into
a buffer zone around the main stream channel (Plate 2, Box 2).
Twenty-four areas of water flow disconnected from the
stream channel because of the stormwater network facilities
were identified. The total area of these independent catch-
ment areas (delineated for the stormwater network facilities
isolated from the stream channel) is 162.4 hectares, and con-
stitute locations subtracted from the Flatlick Branch lidar-
derived watershed.
Ninety-four locations where stormwater inlets were con-
nected to pipes crossing the watershed boundary (again for an
example see Figure 5) were identified. Of these, 32 stormwa-
ter inlets outside of the watershed boundary were connected
to pipes adding stormwater flow into the watershed, thus add-
ing land area; and 41 stormwater inlets inside the watershed
boundary were connected to pipes that removed stormwater
flow from the watershed, thereby removing land area from the
drainage basin.
In addition, the direction of the water flow (into or out of
the watershed) for one of the stormwater pipes could not be
determined (Figure 5). The remaining locations of stormwa-
ter network pipes that crossed the watershed boundary had
no impact as they were already removed from the watershed
when the individual catchment areas (see above paragraphs
on isolated catchment areas) were identified.
Figure 7 represents the final watershed overlaid with
the boundary of the original lidar-delineation. The original
Figure 5. Stormwater network pipes that cross the boundary of the lidar-derived watershed. Dark gray lines and polygons represent
stormwater network pipes and facilities. Within the bold black outlines are examples of stormwater pipes and facilities that cross the
lidar-watershed delineated boundary.
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
May 2015
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