with stormwater inlets and in other areas by the grading of
impervious surfaces. For example, in Figure 4, polygon 1 is a
stormwater network retention pond and polygon 2 is the wa-
tershed delineated for this retention pond (using the retention
pond area in raster form as the pour point). In this figure, the
stormwater inlets (curbside drains and manholes) are repre-
sented by small white circles, the stormwater network pipes
represented by black lines, and the light gray lines represent
the lidar-flow accumulation raster dataset. Stormwater inlets
(white circles) were clearly installed for stormwater flow loca-
tions as they are placed directly on the flow accumulation. The
stormwater network pipes were installed to allow stormwater
flow straight into the stormwater retention pond. This addi-
tional land area covered by these flows needed to be included
in this delineation, so these specific stormwater inlets served
as additional pour point inputs into the
Watershed
tool. This
evaluation and delineation was conducted multiple times, for
all isolated stormwater network facilities within the watershed,
and any stormwater inlets connected to stormwater pipes clear-
ly allowing additional water flow into the retention pond. Each
time we verified our decisions by visual evaluation using the
lidar-elevation model, lidar-contour lines, and aerial photos.
The next step was evaluation of any stormwater pipes that
cross the lidar-delineated boundary of the watershed under
situations 2 and 3 (above) and as depicted in Figure 5. For
each of these locales, we carefully examined each with the
lidar-elevation model, lidar-contour lines, and aerial photos
to determine if the stormwater pipes were connected to the
Figure 3. Steps using the
Spatial Analyst/Hydrology
toolset to delineate watershed and flow accumulation raster datasets from the
lidar-elevation model. Gray ovals represent original raster dataset or shapefile input. Rectangles represent the GIS tool. Parallelograms
represent the output from the tool and also the input raster dataset into the next tool. White ovals represent the final datasets.
Figure 4. Stormwater drain pipes adding additional land area to a retention pond’s watershed.
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May 2015
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
