Background

In the Virtual Tour section of the Toolbox, we observe significantly more water volume in the downstream sections of the tributary during storm events. This is typical of a residential (or commercial) development which does not treat or manage the stormwater it generates. The cumulative effect of stormwater runoff from many developments can have tremendous downstream impact.

Through a comparison of upstream and downstream locations on the tributary, we are investigating the dynamics and effects of stormwater runoff which is generated by these two communities.

The topic of storm water management is of concern to the Butler Farms and Oak Creek Village for a variety of environmental reasons.

Maps and preliminary data

All the spatial data below are derived from Vermont Center for Geographic Information (VCGI) Warehouse site.

Stream network and watershed delineation have been created with the ARC/INFO Hydro-module based on the ElevationDEM_DEM24 NED data. This layer has been processed by VCGI in order to create new ElevationDEM_DEM24 NED based grid, from original composite grid in spherical coordinates (Geographic), to have a 30m cell size. Grid extent is the outline of the state of Vermont buffered 2,000 meters. Data is in Vermont State Plane meters, NAD83, units are feet. Reprojected using the bilinear interpolation option. This map is prepared based on digital elevation data available from VCGI.

Note that the elevation-based stream network (red) does not quite match the stream network derived from digital orthophoto (blue). Especially in the area that we are mostly concerned, in the Butler Farm development, there is a significant discrepancy. Most likely this is because the stream has been chanellized to flow along the pathway that does not quite match the natural flow. Another explanation is that the ground in this area is quite flat and some minor errors in the NED data caused the water to flow along a non-existent path. In any case lack of a well defined flow path increases the likelihood of flooding in this area during high flow.

The Soils coverage derived from the Natural Resource Conservation Service county survey (National Soils Information System (NASIS) database) provides information about the groundwater table height.

This coverage identifies the range in depth (feet) to seasonal high water table from the surface of the ground. This helps determine if the water table may be present in some part of the map unit at or above a critical depth below the mineral soil surface. If depth equal "6.00" then the depth to seasonal high water table for the map unit is greater than 6 feet. If the depth is a negative number, water is ponded on the surface (ie. "-1" indicates that the ponded water is 1 foot deep).

Depth of groundwater at its lowest point Depth of groundwater at its highest point. The negative values stand for above ground values. Note that according to this map a considerable part of the development may have standing water.

The citizens have reported their concerns that there has been a considerable increase of flooding after the golf course has been constructed in the neighborhood. To evaluate the potential impact of the golf course, let us look at its location relative to the catchment area.

The golf course area drains into two streams, one of which runs through the Butler Farm subdivision, whereas the other one joins Trib 7 lower and is unlikely to affect the flooding in the residential areas. On this map both subwatersheds have been delineated based on the digital elevation data available.

From the landuse map it is clear that a larger portion of the golf course area (described as the "Leisure activities") indeed drains into the Butler Farm stream and therefore whatever land modifications occur in the golf course area they will influence the hydrologic regime in the subdivision. However it still needs to be confirmed to what extent the "leisure" land use category really matches the boundaries of the Golf course. What was the change on the land cover that the Golf course construction has caused?

Let us take a look at the soil map itself:

MUID MUSYM MUNAME HYDROGROUP
007Cv Cv COVINGTON SILTY CLAY D
007Cv Cv COVINGTON SILTY CLAY D
007FaC FaC FARMINGTON EXTREMELY ROCKY LOAM, 5 TO 20 PERCENT SLOPES D
007Lh Lh LIVINGSTON CLAY D
007SuB SuB STOCKBRIDGE AND NELLIS STONY LOAMS, 3 TO 8 PERCENT SLOPES C
007VeB VeB VERGENNES CLAY, 2 TO 6 PERCENT SLOPES D
007VeC VeC VERGENNES CLAY, 6 TO 12 PERCENT SLOPES D

Note that most of the soils are clay. Clay soils have low infiltration rates.

The following Table presents the different soil types and landuse activitites and calculates the infiltration capacity for the whole watershed.

MUID ACTIVITY SumOfAREA (m2) Infiltration rate (m/day) Fraction of Impervious Area Porosity Existing Infiltration Capacity (m3/day) Potetntial Infiltration Capacity (m3/day)
007Cv Residential High 13456.2 0.15 0.38 0.42 529.35 853.79
007Cv Residential Low 136137.6 0.15 0.15 0.42 7342.24 8637.93
007Cv Construction 15718.5 0.15 0 0.42 997.34 997.34
007Cv Roads 32132.2 0.15 1 0.42 0.00 2038.79
007Cv Golf 25374.0 0.15 0 0.42 1609.98 1609.98
007Cv Farming 137458.7 0.15 0 0.42 8721.76 8721.76
007Cv No human activity 10175.3 0.15 0 0.42 645.63 645.63
007FaC Residential High 142.1 0.15 0.38 0.35 4.62 7.46
007FaC Construction 778.3 0.15 0 0.35 40.86 40.86
007FaC Golf 8898.1 0.15 0 0.35 467.15 467.15
007FaC Farming 2739.3 0.15 0 0.35 143.81 143.81
007Lh No human activity 1385.8 0.15 0 0.39 80.03 80.03
007SuB Residential High 39071.1 0.31 0.38 0.35 2628.31 4239.21
007SuB Construction 561.1 0.31 0 0.35 60.88 60.88
007SuB Roads 942.5 0.31 1 0.35 0.00 102.26
007SuB Golf 1309.2 0.31 0 0.35 142.05 142.05
007SuB Farming 50107.8 0.31 0 0.35 5436.69 5436.69
007VeB Residential High 123191.8 0.15 0.38 0.39 4410.88 7114.33
007VeB Residential Low 152973.9 0.15 0.15 0.39 7509.10 8834.24
007VeB Office 39.4 0.15 0 0.39 2.27 2.27
007VeB Construction 46099.1 0.15 0 0.39 2662.22 2662.22
007VeB Roads 49154.9 0.15 1 0.39 0.00 2838.70
007VeB Golf 107337.2 0.15 0 0.39 6198.72 6198.72
007VeB Farming 221721.6 0.15 0 0.39 12804.42 12804.42
007VeB No human activity 36578.2 0.15 0 0.39 2112.39 2112.39
007VeC Farming 855.3 0.15 0 0.39 49.39 49.39
007VeC No human activity 660.0 0.15 0 0.39 38.12 38.12
Total for Watershed 1214999.1       64638.23 76880.42

From this analysis we can see that all the development on the watershed has decreased the potential infiltration capacity by about 16%. Though this is a considerable amount of water it is unlikely to assume that all the flooding we see is a result of the development only. If we assume a fully forested watershed, there would still be a further decrease in runoff due the canopy interception and higher evapotranspiration by trees. Still, with the types of soils that we have in this watershed, flooding is probably a natural feature of this landscape which has been somewhat enhanced by the human interference so far.

Based on these rough estimates we may conclude that in natural conditions on this watershed a rainfall of 2.49 inches was causing flooding conditions. Under current development conditions it will be 2.09 inches that will cause flooding. Note that this is most likely an overestimate, since we are assuming that the rain is evenly distributed over a day period (which is wrong in most cases), and we also use the upper bound for the infiltration rates (in reality they are probably smaller). In any case we see that under undisturbed conditions flooding events were still quite possible (rainfall events over 2.5 inches are quite likely in Burlington at least once a year). The flooding became even more likely due to the Butler Farm subdivision built on the watershed.

Possible Action Plan

(This is just to through out a few for discussion purposes)