May 10 th , 2007
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The revised universal soil loss equation (RUSLE) was used to analyze potential high erosion risk areas in Northampton, Massachusetts, with a geographic information system (GIS) program. Soil units and land use features were reclassified with numerical values. Slope length and steepness were grouped together under one single index and calculated by means of GIS. These three factors were multiplied together and a map indicating high erosion risk areas was derived. Slope length and slope steepness have the biggest influence in determining where erosion will take place, and rivers and streams are at the highest risk for erosion. This erosion map should be considered for construction areas, as these areas produce high amounts of sediment, and based on this map the proper prevention or control should be implemented.
Erosion is one of the world’s most serious problems, some people have even gone as far as to call it a problem as big as global warming (Radford 2004). Soil erosion is a process caused by water, wind, ice (natural erosion) or by living organisms (bioerosion) and starting in the twentieth century, humankind. Today, people induce about 60 to 80% of all soil erosion and soil degradation currently affects one-third of the world’s land surface (McNeill 2000). Erosion currently costs the United States economy $30 to 44 billion a year and costs Indonesia $400 million a year in Java alone (Morgan 2005).
These high costs are due to both on-site and off-site impacts of erosion.
On-site impact refers to the reduction in soil quality that results from the loss of the nutrient-rich upper layer as well as reducing the soil moisture, which can create more drought-prone conditions. The effects of on-site problems are very important on agricultural land, where soil degradation can influence crop yields.
Off-site erosion impacts lead to contamination of drinking water, disruption of marine and lucustrian ecosystems and many other problems (Favis-Mortlock 2005).
Sedimentation in rivers and drainage ditches can reduce their capacity, increasing the risk
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of flooding and many hydroelectricity and irrigation project have been ruined because of erosion (Morgan 2005).
Through the use of a geographic information system (GIS) erosion can rapidly be assessed. A GIS assessment allows erosion risk to be incorporated into the design of development project layouts (Boggs et al 2001). An assessment of areas can also show high-risk areas for erosion and lead to programs, which monitor these areas appropriately.
For this project GIS was used to evaluate the city of Northampton, MA risks for erosion and the factors that have the highest contribution to high erosion risk areas were determined. After assessing the erosion risk for the area the question was asked, is
Northampton taking proper erosion risk management actions at construction zones around the city?
Geological History
The Connecticut Valley originated in the Mesozoic Era (225 – 265 Ma). As
Pangea split, forming the Atlantic Ocean, several rift valleys or smaller faults also occurred due to the stretching stresses. Such a rift valley formed the initial drainage basin of the Connecticut Valley. During this time, mountains were created around the basin, lava flows influenced the landscape and dinosaurs roamed the area. The mountains have since been eroded into the basin forming the sedimentary rocks found in western
Massachusetts today (Little 2004).
In the time of the Cenozoic Era (65 Ma – present day) glaciation highly affected the region as continental glaciers moved southward from Canada. During the retreat of the ice meltwater deposits created a dam in the New Britain – Rocky Hill area. Water
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filled the dammed area as the ice continued to melt, forming Lake Hitchcock which lasted for about 4,000 years extending a distance of about 250 miles (Little 2004).
Throughout the existence of Lake Hitchcock deposits filled the lake. Deltas were created from stream tributaries and glacial meltwater, leaving sand and gravel deposits.
While the lake bottom accumulated mud and clay layers in the form of varves. After the lake drained the Connecticut River cut through the deposits creating the terraces and flood plains seen around the Connecticut Valley today (Little 2004)).
These deposits from glacial lake Hitchcock are primarily responsible for the soils in the Valley today. Soils develop according to parent material, slope, climate and time
(Little 1986), and the glacial and fluvial processes have left a variety of parent materials for new soil development.
Human History
Native Americans first settled western Massachusetts, before European settlers came into the area in 1638. The people in this area have always relied heavily on the rich agriculture from the nutrient abounding soils. The climate in western Massachusetts averages about 22°C (71°F) in the summer and about -3°C (26°F) in January and the average precipitation is 1,120 to 1,140 mm (44 to 45 inches, www.britannica.com). In
2000 the counties of Berkshire, Franklin, Hampshire and Hampden (all in western
Massachusetts) had 834,358 residents, 13.1 percent of the population of the entire state
(US Census 2000).
In order to assess erosion risk for Northampton the revised universal soil loss equation (RUSLE, A = K x S x L x C, Boggs et al 2001) was used. This equation takes
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into account soil erodibility (K), slope steepness (S), slope length, and cover factor or land use (C). The dataset used for this project could not be directly plugged into the
RUSLE equation, all data was first manipulated to fit the correct parameters of the model.
Soil Erodibility factor (K)
Erodibility of soil is its resistance to both detachment and transport. Several factors work into how erodibable soil is, including topographic position, slope steepness and amount of disturbances, but the most important factor are the properties of the soil.
Texture, aggregate stability, shear strength, infilitration capacity and organic and chemical content all vary the soil’s erodibiliy (Morgan 2005).
The surficial geology layer from which soil information was derived from, was downloaded from the MassGIS website. The available data was soil type (i.e till, alluvial...) and the soil unit name. Soil unit names were use to reclassify the soil to the K values for the soil erodibility factor. There are 94 different types of soil units in the
Northampton area. The Soil survey of Hampshire County, Massachusetts, Central part
(USDA, 1981) was used to find the K factor for the soils in Northampton.
Six units had no K values in the Soil Survey, these units with no data were given a negative value. Bodies of water received a K value of 0. After the reclassification was completed the Raster Calculator was used to separate K values greater than 0 and less than 0. This produced a layer where all values less than 0 were assigned a value of 0 and all values greater than 0 were assigned a value of 1. The K factor layer and the calculated layer were multiplied together with the Raster Calculator to produce a layer with only positive K values. This layer was converted to a raster file, named K_values and made permanent.
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Land Use factor (C)
In most RUSLE models C represents the crop management factor, but the city of
Northampton has more than just cropland. Land use for Northampton includes, urban areas, recreation areas, industrial and commercial land, forest, cropland and more (figure
2a). These land use features were reclassified based on their effects on erosion with consideration to sediment availability (a parking lot or street lot has no soil), canopy cover - how much precipitation would reach the ground (forested areas are much more sheltered from precipitation than open land areas) and vegetation cover. The land use feature layer was reclassified to C values, named C_values, made permanent and converted to a raster file.
Slope Length and Steepness factor (LS)
Slope length and steepness factors can be combined into a single index. This index is a ratio of soil loss under a given slope steepness and slope length to the soil loss.
These ratios are from the standard USLE condition of a 5° slope, 22 m long, for which
LS = 1.0 (Morgan 2005). In order to calculate the LS factor slope, flow accumulation and flow direction had to first be determined. Total directional flow accumulation was calculation with the equation:
FlowAccumation(FlowDirection([elevation]) where elevation is the digital elevation model (DEM,), this was built into the raster calculator. This layer was made permanent and named flowacc.
To calculate slope length and steepness the expression:
LS(r) = (m + 1) [A(r)/a
0
] m
[sin b(r)/b
0
] n
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was used, where r=(x,y), which is a given point on a hill slope. A is the resolution, b is the slope and the parameters are m and n .
The expression:
Pow ([flowacc] * resolution / 22.1, 0.6) * Pow ( Sin ([slope] * 0.01745) / 0.09, 1.3)) was built in the Raster Calculator, where 30 is the cell resolution, 22.1 represents the 22 m long slope and 0.09 = 9% = 5.16° is the slope of the standard USLE plot. The parameters were set for a slope length <100 m and slope angles <14 m, where it has been shown that the vales of m = 0.6 and n = 1.3 have the consistent results with the RUSLE
LS factor (Mitaova 1999). This layer was calculated, named slope_length, and made permanent.
Rainfall Factor (R)
R is the rainfall erosivity index, this usually calculated as mean annual EI
30
/100, where E is rainfall energy in foot-tons per acre and I
30
is the maximum 30-minute rainfall intensity in inches per hour (Morgan 2005). These values were hard to obtain for a small area such as Northampton, therefore an attempt was made, based on equations provided by Morgan (2005), to use a best fit regression equation. To approximate mean annual R values for Northampton, the mean annual rainfall total (mm) was multiplied by 0.5.
Precipitation data was downloaded from Prism Group (www.ocs.orst.edu). This data was converted to a raster file and the projection was defined as NAD83
Massachusetts Mainland State Plate, to match the other data files. Because this raster file’s resolution was 4 km x 4 km and the other data layer’s resolution was 30 m x 30 the rainfall layer had to be resampled, breaking up the 4 km grid to 30 m. The raster file was then extracted to mask all of the United States except for Northampton.
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The units for rainfall were originally 0.1 m, this was converted to millimeters by multiplying by 100. This was then multiplied by 0.5 to approximate the mean annual R values. These calculation were performed on rain data from January 2007. This layer was named R_values and made permanent.
Erosion Risk
The reclassified K, C, SL and R layers were built into the equation:
[K_values]*[C_values]*[slope_length]*[R_values]
in the Raster Calculator (figure 4). This layer was also recalculated without rain data named erosion risk and made permanent.
Nineteen percent of Northampton’s soil has erodibility K values above or equal to
0.35 (figure 1 & 2). Sixty-six percent of the land use features create high risk for erosion which C values are greater or equal to 0.3 (figure 3 & 4). Seven percent of the slope length and steepness factor is at high risk (figure 5). The RUSLE map including the rain data and excluding the rain data are very similar (figure 6 & 7). Because the rain data is only for one month of one year and is extremely variable, the erosion risk equation without rain data will be used to analyzed high risk areas. Eleven percent of Northampton is at high potential risk for erosion.
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Major Contributing Factor
After analyzing all three factor’s maps (K, C and SL) it was concluded that slope length and slope steepness is the major factor causing erosion. The erosion risk map looks very similar to the SL map (figures 5 & 7). Most in most areas where there is a steep slope or a long slope there is a high erosion risk. While the K and C factors both contributed to the equation, as the erosion risk map and slope length map are not the same, neither factor is as important as slope length and steepness, in determining where high erosion will be.
This can be seen in the cropland area by the Connecticut River (figures 8a-d).
This area includes the Hadley, Limerick and Winsoski soil units which area all similar to each other. The Hadley series consists of well drained soils on flood plans, the Limerick series is poorly drained and the Winsoski series consists of moderately well drained soils on flood plains, all are formed in medium textured alluvium (USDA 1981). This alluvium is the silty flood deposits on top of the varved clay from Lake Hitchcock. These soils are the best agricultural soils in the Valley (Little 1986), which leads to high erosion factor values. The soil erodibility factor is 0.5, the highest values for the K factor, and therefore extremely erodable. The land use, C values is also 0.5 due to the high amount of sediment availability, lack of canopy cover – it is not protected from participation and the rotating vegetation cover, which depends on the time of year. Although both K and C values are at the most vurerable state for erosion (figures 8a-b), the erosion risk is still low because there is no slope in this area (figures 8c-d).
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Therefore, according to this revised universal soil loss equation, gravity – in the form of slope length and steepness, is the most important factor when considering erosion risk.
Rivers and Streams
A majority of the high erosion risk sites are on the banks of rivers and streams
(figure 9). This is due mainly to the high, steep slopes along the banks these streams and rivers. The steep slopes are created by water eroding the banks and shorelines. As the banks become more eroded they become steeper creating a cycle, leading to high amounts of soil lost. As was seen in the equation and on the erosion risk map, the steeper a slope the more susceptible to erosion it becomes, therefore river and stream banks are going to be highly vulnerable to erosion.
Smith College and Hospital Hill
In the Smith College and Hospital Hill area there is a large amount of high risk
(figure 10a). The soil erodibility (K) values are at high around the river (figure 10b). The land use (C) values are also high on campus with a value of 0.4, because the land use feature is urban area (figure 10c). This area was assigned a value of 0.4 due to medium amounts of sediment availability and high amounts of open land in the form of fields and lawns. While, again, K and C do influence the erosion risk factor, the slope length and slope steepness play the biggest role, and SL is high around the Smith campus and
Hospital Hill area.
The Mill River runs through campus and around Hospital Hill, this river is responsible for steep slope along the banks of the river leading to high risk erosion areas
(figure 10d). Where erosion is occurring on the Smith College campus - by the Mill River
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below the dam - stabilization structures have been erected to prevent this problem. This erosion is occurring exactly where the equation predicted, which adds validation to the
RUSLE.
Another high erosion risk site on campus according the equation is next to and below the presidents house. When this area was actually checked out, erosion processes were found to be taking place, but only where there was no vegetation. The whole high risk cell indicated by the equation was not eroding.
It is very important to be aware of where high erosion risk areas are when planning on construction, as construction sites open up the earth, increasing the sediment supply and disturb the ground cover, increasing erosion risk. The building site for Ford
Hall was analyzed and it is not in a high-risk erosion area (figure 8a), due to the fact it is being constructed on a previous building site. While the erosion risk for this site is low, short-term erosion prevention should still be taken as the site is within close proximity to the Mill River.
Rain and Wind
As with all modeling equations choices must be to include and exclude data. This equation did not include rain or wind energy, which has large effects for determining high-risk areas. While rain and wind were not incorporated directly, they were taken into account when establishing K and C factors. For the K factor, the properties of the soil were considered including infiltration capacity, which is an effect of rain and aggregate stability and shear strength, which deal with how the soil reacts to both rain and wind.
Rain was incorporated into the C factor values, through consideration of impermeable surfaces and sewer systems.
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In the future a model should be created using GIS which would make possible to plug in rain data from year of any month into equation. GIS models can be made to eliminate many tedious steps, thereby making a quick and easy process to compare several years worth of rain data. These maps including rain would give a better idea of where to monitor for erosion, but they are still not perfect. The erosion risk equations which do incorporate rainfall, still do not take into account rivers and streams that flows out of their banks during floods. These high waters could have a large effect on eroding soils, for example the cropland by the Connecticut River where the K and C values are very high (figures 8b-c).
Other sources of error
The Soil survey of Hampshire County, Massachusetts, Central part (USDA 1981) used soil depth to help classify the K factor, but the depth of the soils from the surficial geology layer was unknown. Therefore all soil units were assumed to have the same depth of 0-7 inches, this assumption adds error to the methods. In order to add accuracy to the equation the soil depths in Northampton should be measured.
While the land use feature layer can help classify the land use it is still very broad.
For a more accurate equation every building, road, sidewalk, tree, plot of grass, etcera should be consisted, instead of the large grouping system used.
There are many different and effective ways to control and prevent erosion. The technique used depends on whether the objective is to reduce the velocity of runoff and wind or to increase the surface water storage or to safely dispose of excess water
(Morgan 2005).
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Terraces
The purpose of a terrace is to intercept surface runoff, slow it down to a nonerosive velocity and shorten slope length. Terraces are earth embankments cut across the slope; layouts can differ on the spacing and length of the terrace, location of outlets, gradient and dimensions of the channel (Morgan 2005). A reasonable area to construct a terrace could be the hill southwest of the President’s house and the admission building on the Smith College campus. The problem with building a terrace on campus is that it is costly and could produce potential off-site erosion problems down stream in the Mill
River.
Waterways
Like terraces waterways covey runoff to a non-erosive velocity, to a suitable disposal point. The dimensions of a waterway should be able to hold the runoff from a storm with a ten-year return period, and therefore they must be carefully designed.
Typically waterways are placed upslope of farm and cropland areas, intercepting water coming from the above slope, diverting it across the slope (Morgan 2005). Waterways would be a good way to control off-site erosion in Northampton due to the number of cropland areas below slopes. This type of erosion; erosion due to runoff, is not strongly shown in the erosion risk map, but still a large source of erosion problems.
Stabilization Structures
Structures used to control erosion on steep slopes are known as stabilization structures, these can be plants or anthropogenic constructions. Plants have advantages as
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they provide immediate reinforcement of the soil, and offer the basis for long-term slope stability as the vegetation grows.
Steeper slope usually need stabilization from the construction of a retaining wall.
Gabions work well for this; these are wire-mesh baskets that are packed tightly with stones. They also work better than concrete structures as they allow water to run through the facing without lost of the structural efficiency (Morgan 2005). A stabilization structure is present on the Smith campus southwest of Sage Hall. This structure is created with many boulders placed within the steep slope. The boulder wall has the same advantages as gabions, allowing water to steep into the cracks between boulders, as well as reducing the runoff.
Construction Sites
It is especially important to monitor construction sites for erosion. Construction sites open up the earth creating a large source of sediment to be potentially washed away.
These sites may also clear away trees, grasses and other vegetation that previously acted as a stabilization structure for the soil.
The city of Northampton monitors and controls construction by means of making patrons or contractors who want to build submit a notice of intent to the Northampton
Conservation Commission. These notices of intent must be submitted if the site is within
200 ft of a body of water or wetland area or if the site is larger than an acre. The notices include, but are not limited to, a site description, wetland resources, proposed work, stormwater management, pre and post construction site runoff rates, stormwater operation and maintenance plan and an erosion control plan. The erosion control plan contains several stabilization practices to reduce the time soil is exposed to the elements – these
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are implemented to cover exposed soil to minimize the discharge of sediment. Structural practices are employed to reduce the amount of stormwater entering into a disturbed area, or to trap sediment before it leaves the site (Notice of Intent 2007).
The sites have been analyzed prior to the submission of the notice and include illustrations of the areas of work where the erosion and sedimentation controls will be used. Some of the stabilization practices include mulching or hydroseeding and surrounding with a silt fence and hay bales, soils that are exposed or stockpiled and are inactive for 60 days or more. Vegetation is also planted in used in areas that have been disturbed, this may be temporary or permanent. Structural practices include hay bales and silt fences that are entrenched into the ground, preventing underflow. These controls remain in place during the entire process of the construction or until all disturbed areas are re-vegetated and stabilized (Notice of Intent 2007).
It is significant that the city of Northampton use controls on construction sites such as the notices of intent. These make the builders aware of potential areas of harmful erosion before the construction begins and take all the means to control erosion. It is also significant that the city requires submission of the notices for site that are close to water because the erosion risk map indicates that most erosion in Northampton takes place around streams and rivers.
The erosion map should be used when building new sites even if the site is not within 200 ft of water or over an acre, because small sites can still cause significant erosion. A hard copy of this report and an informative poster will also be given to the
Office of Planning and Development.
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It is also important to note that not all erosion is bad erosion. Erosion has been occurring for millions of years, shaping the landscapes we know today, but human accelerated erosion needs to be closely monitor and controlled.
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Figure 1. Northampton soil units reclassified as soil erodibility values (K).
Figure 2.
Frequency distribution for soil erodibility (K), note the -0.5 means no data.
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Figure 3. Northampton’s land use features reclassified as C values.
Figure 4. Frequency distribution for land use features (C).
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Figure 5. Slope length and slope steepness’s of Northampton.
Figure 6. Erosion risk map for Northampton, derived from multiplying soil erodibility (K), land use features (C) and the slope length and slope steepness factor.
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Figure 7. Erosion risk map for Northampton, derived from multiplying soil erodibility (K), land use features (C), the slope length and slope steepness factor and rainfall data from January 2006. Note, part of
Northampton is missing due to the high resolution of the rainfall data.
Figures 6a-d. Cropland in eastern Northampton by the Connecticut River.
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6a. Soil erodibility values. 6b.
Land use values 6c . Slope length and steepness . 6d.
Erosion risk.
Figure 9 . Erosion risk and streams in Northampton. Note the high erosion risk areas around the streams.
Rivers are not shown.
Figures 8a-d.
Smith College and Hospital Hill area and site of Ford Hall.
Figure 8a.
Erosion risk. Figure 8b. Soil erodibiliity (K).
Figure 8c. Land Use (C) . Figure 8d. Slope length and steepness (SL).
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I would like to thank Jon Caris for spending so much time patiently working with me in the GIS lab; this project would not have been possible without him! I would also like to thank Bruce Young from the Northampton Office of Planning and Development for meeting with me and for graciously letting me borrow a ‘notice of intent’. And last, but not least, Bob Newton for talking to me about K and C values for the city.
Boggs, G., Devonport, C., Evans, K., Puig, P. 2001. GIS-Based Rapid Assessment of
Erosion Risk in a Small Catchments in the Wet/Dry Tropics of Australia. Land
Degradation & Development. v. 12, p. 417-434.
Chavez, R. 2006. Modeling Soil Erosion Risk in Los Maribios Volcanic Chain,
Nicaragua. Tropical Resources Bulletin, v. 25, p. 50-56.
Favis-Mortlock, D. 2005. Soil Erosion Site. http://soilerosion.net/. Accessed March 4 th ,
2007.
Little, R. 2004. Earth View LLC. http://www.earthview.pair.com. Accessed May 6 th ,
2007.
Little, R. 1986. Dinosaurs, Dunes, and Drifting Continents. Valley Geology Publications,
Greenfield, MA 01301.
Encyclopedia Britannica Article, Massachusetts Climate. www.britannica.com. Accessed
May 9 th
, 2007.
McNeill, R. J. 2000. Something New Under the Sun. W.W. Norton & Company, Inc.
New York, NY.
Notice of Intent. 2007. New England Environmental, Inc. Prepared for Microcal, LLC, 22
Industrial Drive, Northampton, MA. April 9 th
.
Radford, T. 2004. Soil erosion as big a problem as global warming, say scientists. The
Guardian.
U.S. Census Bureau, 2000. www.census.gov. Accessed May 9 th
, 2007.
U.S. Department of Agriculture, Soil Conservation Service in cooperation with
Massachusetts Agriculture Experiment Station. 1981. Soil survey of Hampshire County,
Massachusetts, Central part.
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