Rhiannon Ervin
April 1, 2011
Assignment 5
The purpose of this project is to further examine suitable sites for LID projects around a section
of the Mystic River. The analysis conducted herein focuses on controlling rooftop run off. Location
screening is conducted for two different technology types for controlling rooftop runoff; green roofs and
rooftop infiltration. The analysis uses hydrography, land use, building footprint and soils data layers
obtained from MassGIS as well as housing layers from the 2000 census.
Before examining the best locations for the two different technology types, a more general
survey of the stud area was conducted. Many LID technologies attempt to infiltrate water back into the
ground. However, this is only effective if the soil drainage characteristics allow water to drain at a high
enough rate. The summarize function was used on the soils data layer to calculate the area of each type
of drainage characteristic. The results of this summary are shown in Table 1. From this table it can be
seen that 47% of the soil is Urban Fill, which means that the drainage characteristics are unclear and
could differ from place to place. 36% of the area is moderately well drained or higher, which is good for
LID implementation.
Table 1. Drainage characteristics of the Mystic River Study Area.
% of
Drainage Type
Area (m2)
Area
Somewhat Exceptionally Well Drained
3,367,462
11
Well Drained
6,026,710
20
Moderately Well Drained
1,515,716
5
Loamy Fill
31,270
0.1
Urban Fill
14,375,998
47
Filled Floodplain
5,309,701
17
Also of interest is the land use in the study area. As will be shown later on in the analysis,
certain technologies maybe more appropriate for commercial areas, as opposed to residential areas.
Also, any technologies that are infiltrating water without cleaning it may be inappropriate in industrial
areas. The summarize function was used on the land use layer to give the results shown in Table 2. The
two land use types that will be utilized herein are residential and commercial, which make up 37% and
8% of the land surface, respectively.
Table 2. Land use characteristics in the Mystic River Study Area.
% of
Land Use Type Area (m2)
Area
Open Space
1,471,192
4
Residential
15,289,206
37
Commercial
3,356,373
8
Industrial
3,528,038
9
Transportation 14,087,557
34
Water
3,411,734
8
Green roofs are a technology that creates a type of garden on building rooftops. The purpose of
this technology is to retain a fraction of stormwater that falls on the rooftops, as well as increase the
time it takes the rest of the rooftop water to reach storm drains or the river. This helps with flooding by
not allowing all of the water to reach the river at the same time. Also, some pollutant removal from the
rooftop water is also possible. For this analysis, buildings in commercial areas are selected as the best
possible locations for green roofs. This selection was made because commercial buildings tend to have
larger roof area, are more likely to have flatter roofs and it may be easier to gain access to these
rooftops for installation and maintenance than it would be with private homes.
To select buildings in commercial areas, the “select by attribute” function was used to create a
commercial land use layer from the original land use layer. Then all of the buildings that intersected the
commercial land layer were selected, shown in Figure 1. The total area of all building footprints located
in the commercial land use category is 637,332 m2 and the mean area is 616 m2. Not all of these sites
will be suitable for a green roof, and not all the rooftop area on each building will be covered with a
green roof; however this analysis narrows down possible locations to go out into the field to start
examining and gives a ballpark for the maximum area that could be covered by green roofs.
Figure 1. Building footprints of buildings in commercial areas.
Direct infiltration of rooftop runoff may be more appropriate for residential areas. This
technology collects water from rooftops and directs it all into an infiltration chamber constructed below
ground. The maintenance of this technology is somewhat less than that of a green roof. Also, it does
not require that extra weight be put on the rooftops of the houses. Therefore, it seems a reasonable
technology for residential areas. Buildings in residential areas were selected in a similar manner as
those in commercial areas, except the “select by location” feature was used to select buildings that
intersect with the created residential layer. A map of all buildings located within the residential land use
is shown in Figure 2. Because this technology discharges water directly into the ground, it is important
to consider the pollution content of the water entering the infiltration beds. Therefore, all buildings
within 500 ft of an industrial area were removed from the current selection (Figure 3). Lastly, to prevent
homeowner flooding due to these infiltration beds, buildings located in “Urban Fill” were removed from
the analysis (Figure4). As the drainage condition of this fill is unclear and a large number of potential
residences remain, it is reasonable to remove homes in “Urban Fill” from the suitable locations. This
gives a total rooftop area of 759,336 m2 and a mean area of 120 m2. Again, this rooftop area is much
higher than what can feasibly be incorporated into a low impact development plan. But it gives an
upper bound on the amount of impervious rooftop that can be used to control stormwater. More
importantly, it narrows down the number of neighborhoods site visits necessary for location selection.
Figure 2. Buildings located in residential areas.
Figure 3. Buildings within residential land use that are more than 500 ft from an industrial area.
Figure 4. Buildings within residential locations that are more than 500 ft from industrial areas and are
not located on “Urban Fill”.
For most LID techniques to be effective, they require maintenance, especially when they are
first installed. It is likely that residences that are owner occupied (instead of rented) are more likely to
receive the needed maintenance, than those that are rented. To examine the proportion of owner
occupied houses in the selection shown in Figure 4, the field calculator is used to normalize 2000 census
data of owner occupied houses by the total number of houses. The distribution is shown below in
Figure 5.
Figure 5. Frequency distribution of percent owner occupied housing (by block group) in the selection
area.
This analysis was conducted on the block group level. The average percent owner occupied was
42% with a minimum of 19% and a maximum of 83%. This analysis maybe somewhat misleading
because of the size of the block groups. The selected block groups were determined by creating a layer
from the selection in Figure 4 and then selecting the block groups that intersected this layer. The
created the block group selection shown in Figure 6. This method incorporates any block group that
touches any part of the layer created from Figure 4. Therefore, a much larger selection is made in Figure
6. A selection could have been made to specify that the block group be completely contained by the
created buildings layer, but this is also an arbitrary decision. For the screening level purposes, these
percent owner occupied statistics give a good indication of the makeup of the selected residential area.
Figure 6. Block groups selected for the calculation of percent owner occupied.
Again, this analysis should only be used as a screening tool to select LID sites. After this process,
the best sites should be narrowed down by actual site visits. The general soil characteristics are
especially important in places where water is being infiltrated. Therefore, the scale of the soils layer is
not accurate enough to ensure that the soils will drain water easily. Also, some areas that were ruled
out because they were too close to industrial lands may be good locations for LID if the industry is clean
operating. This analysis also cannot take into account any local laws about implementing LID
technologies. Overall, the potential areas for LID implementation examined in this paper should be used
as a starting place from which a better understanding can be generated.