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Prepared By Integrated Land Management Inc.

for the White Clay Watershed Association

October 3, 1997

The White Clay Watershed Association (WCWA) has been monitoring the health of the tributaries of the White Clay Creek (WCC) in Chester County since 1991. This has been accomplished through biological monitoring involving the collection and counting of the insects and other small arthropods. These organisms found in the rocks and gravel of the streambed are known as benthic macroinvertebrates.

Since the life cycles of benthic macroinvertebrates integrate many factors affecting the overall health of streams over time, their diversity and relative abundance can reveal much about the condition of the stream ecosystem. Certain orders are able to tolerate higher levels of pollution, lower oxygen levels and/or higher temperature regimes indicative of impaired streams. Other orders are much less tolerant of such stresses. Therefore, the relative distribution of these organisms can tell much about the overall health of the stream.

Biological monitoring contrasts with discrete physical and chemical measurements used to reveal the parameters of the stream or outfall at a specific time and place. Water chemistry measures the levels of "end of pipe" pollutants found in point source discharges from land uses such as industrial factories, sewage treatment plants etc. Since the passage of pollution control legislation in the 60's, point source discharges from such uses have been significantly reduced in most, but certainly not all, cases.

It is recognized that many streams are still impaired by human land uses, and non point source (NPS) pollution is acknowledged to be a serious problem for stream ecosystems. NPS pollutants are conveyed by the runoff from agricultural, suburban and urban land uses. These pollutants include sediments, pesticides and herbicides as well as and nutrients such as nitrogen and phosphorus from firm fields and lawns, in addition to a variety of heavy metals and hydrocarbons from paved surfaces. Chemical monitoring can quantify the levels of such pollutants at a particular time and place. However, since levels of NPS pollutants typically fluctuate substantially over the course of a year, discrete sampling may overstate a problem, or miss it altogether.

NPS impacts are not only chemical in nature; they also include physical changes to the stream hydrology and thermal regime from land uses. Such impacts result from impervious surfaces (e.g. roofs, roads, parking lots) which substantially increase runoff from storms. Under natural conditions, bankfull flooding nornally occurs every year or two, a frequency for which the ecosystem is adapted. Once impervious surfaces exist at moderate densities, bankfull flooding occurs many times in a single year. This repeated stress affects the organisms directly, and causes the streambanks to become heavily eroded since there is not enough time for riparian vegetation to recover between storms. Sediments stirred up by frequent bankfull floods not only smother organisms directly, but habitat is eliminated as point bars of sand and silt replace the more productive rocky substrate.

A further impact of impervious surfaces is that they prevent groundwater recharge, so base flow between storms is reduced where impervious surfaces are extensive. Where low flow conditions of temperature and dissolved oxygen are already marginal, further reductions in base flow will substantially stress the benthic community. Furthermore, impervious surfaces get quite hot during the summer, generating pulses of very warm runoff with every thunderstorm. Dissolved oxygen will decrease as temperature increases, so its level may drop below the critical threshold needed to sustain the more sensitive families during a drought. This will place considerable stress upon the ecosystem of the receiving stream.

The threshold at which the effects of NPS impacts become important seem to occur at fairly low land use intensities. Studies of Piedmont streams in Delaware suggest that NPS impacts can be substantial when the imperviousness of the watershed exceeds 10 to 20 percent (Shaver et al 1996, Maxted and Shava 1997). Below 10 percent impervious cover, the streams were observed to have a benthic macroinvertebrate community typical of healthy ecosystems. However, once the impervious cover in the watershed exceeded 30 percent, only benthic communities typical of impaired ecosystems were observed.

Twelve monitoring sites were established by the WCWA and scientists at Stroud Water Research Center, (the fresh water field research laboratory of the Academy of Natural Sciences of Philadelphia). These sites were selected for a long term baseline study to document trends in water quality throughout the watershed.

As shown in Figure II-1, each major tributary has several different sites, starting at the headwaters and proceeding downstream. Each site was chosen to have relatively similar habitat features in terms of riffle and pool structure, bottom substrate, stream flow velocity and streamside forest cover. Variations in the benthic macroinvertebrate reflect local in-stream habitat as well as habitat and water quality above each site.

Samples are collected at each site, from four different locations in the stream bed. In order to prevent disruption of the riffle, collecting begins at the farthest location downstream of the riffle, or tailrace, and continues to the highest location or the headrace. The collecting is done with a tool called a Surber sampler. The Surber sampler is a net of fine nylon mesh that is attached to a one foot square frame. This frame is placed upon the bottom of the stream bed to precisely define the collection area, while the net collects organisms as they flow downstream.

Stones within the frame are removed and the organisms that are tightly attached are carefully removed with forceps. This procedure is done in a bucket of water, from that location, to prevent the loss of any of the organisms. The rocks are then measured and returned to the stream. After the rocks are clean, the net of the Surber sampler is turned inside out over the same bucket to remove the insects collected from the stream. The organisms are preserved in alcohol in the field and brought back to the lab for sorting. Sample collection is completed in one day. Sample sorting and counting takes from six to nine months. The procedure is detailed and exacting. All of the insects are sorted to the family level. Some non-macroinvertebrates are sorted to the family level, as well.

The Stream Watch volunteers use dissecting microscopes to ensure that no organisms in the sample are overlooked. The sorted organisms are then verified, counted, and placed in a vial, labeled, and recorded by a trained technician. The samples are then archived.

Since there can be a ten-fold variation from year to year at each site, it is necessary to establish a baseline of data for the results to be scientifically valid. The first five years (1991-95) of data collected have established a baseline for the parameters tested at each collection site. After seven years of biological monitoring the WCWA has one of the most exhaustive and thorough volunteer monitoring programs ever conducted for a particular watershed.

Several different parameters were used to evaluate the results. Of particular significance is the identification and counting of organisms sensitive to NPS impacts, the total number different taxa or diversity, and the ratio of the more sensitive orders to more tolerant of NPS impacts. The sensitive orders are listed as Ephemeroptera (Mayflies), Plecoptera (Stoneflies) and Trichoptera (Caddisflies). Known as EPT in the literature, these more sensitive orders found in unpolluted, free-flowing cold streams comprise the major food of trout. The more pollution tolerant order evaluated in this study is Diptera (Flies and Midges).

The results for the average number of EPT for the period 1991-1995 are presented below in Table 2:

Note that the headwaters have rather high densities of these more sensitive orders, as emphasized in bold. ANOVA (a statistical analysis) indicates that sites 11, 12, 0 and 19 are not statistically different. The densities in these sites are typical of high quality streams, and support the designation of the East Branch above Avondale as an "Exceptional Value Water" Stream, as well as supporting the "High Quality" designation for the remaining headwaters. These EPT counts indicate water quality at those locations could support a breeding trout population. This has been confirmed in the East Branch headwaters, but we do not have any data from the Middle or West branches.

However, the EPT densities decline markedly in downstream tributary reaches. By the time the White Clay approaches the state line at Site 14, the EPT density has decreased by 96%. Of even greater concern, is the dramatic decline in the East Branch below Avondale, where a 99% decrease in EPT density occurs. This steep decline in the EPT counts indicates a serious impact between sites 12 and 18.

The decline in EPT densities of the Middle Branch also raises concerns. Compared to the headwater site 0, EPT densities at site 4 (in Franklin Township) have declined 88%. A similar reduction also occurs in the West Branch. These results are also graphically presented in Figure III-1, "Density Of Sensitive Orders". Healthy headwater EPT densities are displayed without hatching, while the lowest EPT densities in sites 4, 14, 16 and 18 are crosshatched in proportion to their low densities. Intermediate densities, shown in diagonal hatching, are found in the lower reaches of the Middle and West Branches.

When the ratio between EPT and Diptera is calculated, a similar picture emerges. In the headwaters, relatively few Diptera are found and the proportion of EPT is well over 50 percent. As the sites proceed downstream, the relative proportion of EPT decreases to one percent at site 18, and shows a minimal rebound to 5 percent at the state line. Table 3 presents the ratio of EPT to Diptera throughout the watershed.

As was mentioned above there are a wide variety of NPS and point sources which could affect population of macroinvertebrates. In this section and the next we identify point and NPS factors which could potentially contribute to changes in abundance and distribution. However, it is important to note that correlation or association of changes in benthic macroinvertebrate fauna and potential sources of pollutants does not represent causation.

There are a number of permitted point sources in the watershed (e.g. Avondale Sewage Treatment Plant upstream of site 18, West Grove STP upstream of site 18, and Penn-London STP upstream of site 19). Some changes in the populations occur in the vicinity of these sources. Detailed studies by PA DEP in the last decade indicate that effluent from the Avondale STP plays a role in the degradation of the benthic macroinvertebrate community above site 18 (Steele, 1986, and Boyer, 1992 and 1993). While there appears to be a relationship between point discharge and conditions at Station 18 there are a number of NPS factors also contributing to the habitat of benthic macroinvertebrates at this and other locations throughout the watershed.

Steele, (1986) also reported that Trout Run, which joins the East Branch in Avondale, was extremely stressed. Draining the commercial/industrial corridor of Route 1, Trout Run was observed to be almost devoid of benthic macroinvertebrates of any kind, and its sediments showed elevated levels of chlorinated hydrocarbon pesticides (DDT, DDE, DDD and Chlordane). Water Column measurements showed levels of Chlordane and Lindane in levels high enough to affect aquatic life (Steele, 1986).

Aquatic life in Egypt Run, which drains into the East Branch above site 16, was also observed to be stressed in this report. In light of these observations, it is apparent that more detailed monitoring of Trout Run and the Avondale treatment plant is called for to explain the situation affecting site 18. Currently, this segment of the East Branch is the subject of a detailed monitoring study by PA DEP, with results anticipated by the end of 1997.

Beyond the point source impacts suggested in site 18, the Stream Watch results correlate with possible NPS impacts in the more heavily developed areas along Rt. 1 below the headwaters. As the terrain steepens at the fall line where there is less land disturbance, a minor rebound seems to occur. However, where the tributaries converge just above the state line, a further decline occurs with an additional 2 to 3 fold reduction in EPT densities.

To better understand the Stream Watch results, they need to be related to the NPS impacts of human land use activities in the upstream watershed. A Geographical Information System (GIS) is an effective tool to correlate the monitoring results with the estimated NPS impacts of human activities. Using the GIS, NPS impacts are projected by relating the inherent sensitivity of the landscape to the location and type of human disturbance. Two underlying data sets are used in the GIS, a Soils Mapping Unit layer, and the 1990 Land Cover layer. The Soils layer includes information on slopes and other factors that affect the inherent sensitivity of the landscape to human land use activities. The Land Cover layer delineates where these activities occur in the watershed.

Three parameters have been used in this analysis to classify NPS impacts:

While levels of nutrient pollutants can be readily projected using the GIS, their effect on the benthic community is thought to be relatively minor in the fast moving low order streams that comprise the majority of the watershed. On the other hand, sediment losses directly impact the benthic macroinvertebrates by smothering them, as well as embedding the substrate to the extent that much less habitat is available. Runoff increases affect the hydrology by reducing base flow and increasing stormflow. This stresses organisms by increasing both streambank erosion and critical summer temperatures, as discussed in Section I. Streamside forests can help mitigate the effect of the preceding NPS pollutants by reducing flood peaks, filtering runoff end shading streams.

A listing of the various Land Cover Categories is set forth below in Table 4, along with their relative NPS pollutant potential for the parameter involved.

A factor strongly implicated in the loss of benthic macroinvertebrate diversity is the extent of impervious surfaces, as discussed in Section I. A substantial reduction in biotic indices has been observed when the imperviousness of the watershed exceeds 30 percent, with the reduction occurring at a threshold of 10 to 15 percent imperviousness (Shaver et al., 1996, Maxted and Shaver 1997). Watersheds with an impervious cover below 10 percent had streams with very high biotic indices, while nearly all watersheds with an impervious cover over 30 percent were substantially impaired.

Using the impervious allocations set forth in Table 4 the overall imperviousness in the WCC is roughly 6 percent. This is largely due to the predominance of agricultural land uses. As a result, the cumulative percentage of impervious surfaces does not exceed a 10 to 15 percent threshold in any of the subwatersheds. However, if increased runoff is the main NPS impact of impervious surfaces in WCC, the relative amount of runoff from each subwatershed becomes a useful basis for comparison.

According to the Soil Cover Complex Method described in Section VII, the GIS was used to determine relative rates of runoff from the different land cover categories. Runoff amounts from forest are 1.3 inches per year, well below the average of 5 inches per year for the WCC watershed as a whole. Since this value is similar to that of low density residential land uses which are 12 percent impervious, while runoff from conventional tillage is nearly 9 inches, the criteria of relative runoff amounts may be more viable in agricultural watersheds. On the higher end, runoff from the highest intensity kind uses is from 16 to 24 inches per year.

Figure V-1 "Land Cover Storm Runoff,' displays how the land cover dataset is coded to allocate the annual storm runoff amounts in meters according to the various land cover categories as set forth in Table 4. Note the concentration of runoff amounts in the range of 0.40 to 0.60 meters associated with the commercial, industrial, and high density uses along the Route 1 corridor and around Avondale, as well as that associated with the concentration of mushroom facilities south of Avondale.

The GIS was then used to extract the average annual runoff for the watershed associated with each Stream Watch site to provide the results shown in Figure V-2, "Watershed Runoff'. Note that sites 11 and 14 have annual runoff amounts in the range of 3.4 to 3.7 inches, reflecting their low intensity land uses. Sites 0, 6, and 7 generate from 4.0 to 4.4 inches, while sites 4, 12, 16 and 17 fall within the watershed average of 4.7 to 5.1 inches.

Sites 3 and 19 have runoff amounts of 5.7 inches. Site 18 along the Route 1 corridor in the Avondale area has the highest runoff amount of 7.2 inches, double that of the lowest sites, and a value nearly six times that of a wooded watershed. Due to the extensive area of low density uses included in these watersheds, subwatersheds in the vicinity of Route 1 would actually have much higher runoff amounts than the 7.2 inches for the watershed as whole. This would create localized flood pulses in the minor tributaries that directly impact site 18.

For the most part, the distribution of increased runoff corresponds with the observed decline in EPT counts. However, note that the high EPT count for site 19 occurs in subwatershed where the runoff averages 5.7 inches. This seems to support the threshold phenomenon discussed above. Although increased runoff would not seem to account for the entire decline at site 18 due to other point source effects, the high intensity land uses in the Route 1 corridor and south of Avondale cannot be ignored. Since runoff events propagate downstream, it can be anticipated that site 16 would also be stressed by the increased runoff.

The Universal Soil Loss Equation (USLE) is the accepted method to determine projected erosion rates. The USLE projects sediment losses as a function of slope, soil texture and land cover, as discussed in more detail in Section VII. Steeper slopes have a significant effect on the underlying potential for soil erosion for a given land cover. Figure V-3, "Inherent Erosion Potential", graphically displays how the steeper slopes found in the vicinity of the fall line in the southern part of the watershed have the highest potential for erosion.

The USLE also relates erosion to actual Cover and Practice (CP) factors, which vary widely according to the land cover and type of tillage practice. For conventional tillage, the CP factor is the highest, with a smaller factor allocated to conservation tillage. Farmsteads also are projected to generate high sediment losses, given their high concentration of livestock. Other land covers generate much lower annual sediment losses. Table 4 sets forth the relative CP values of each land cover, multiplied by a factor of 100,000 for clarity.

Figure V-4 "Cover/Practice Factors" graphically displays the extent of conventional tillage, farmsteads and conservation tillage, the only categories with a CP value over 0.020. All other categories have such low CP values that they are not displayed. Note the concentration of agricultural activities north of Route 1 and south of West Grove.

The GIS overlays the two maps to project relative soil losses on an annual basis. Individual losses for each land use within the subwatersheds are averaged to highlight the subwatersheds where accumulated soil losses are the greatest. Figure V-5, "Watershed Erosion", displays how the susceptibility of the land to soil erosion relates to human land cover activities.

Note that the headwater sites 0, 11 and 19 have annual erosion rates from 0.76 to 0.82 ton/ac/yr, reflecting their low intensity land uses. Sites 3 and 18 also have relatively low erosion rates from 1.04 to 1.20 ton/ac/yr due to the preponderance of nonagricultural uses. On the other hand, erosion rates in sites 6, 14 and 16 vary from 1.36 to 1.41 ton/ac/yr, while site 17 has rates of 1.55 ton/ac/yr. Sites 4 and 7 generate 1.78 to 1.83 ton/ac/yr, double that of the headwaters, reflecting the presence of agricultural activities. These latter sites are located in the steeper terrain south of Route 1, where earth disturbance activities will generate more erosion.

Note how the geographical distribution discussed above results in relatively high soil losses in the lower reaches of the West and Middle Branches. This corresponds to monitoring sites 3, 4, 6, 7 and 17, where a substantial decline in EPT populations is documented in the Stream Watch results. However, East Branch watersheds where declines are observed seem to have considerably lower soil losses, so projected erosion rates cannot explain all of the Stream Watch results, although they are suggestive.

There has been much interest in recent years about the positive effects of streamside forests, known as Riparian Forest Buffers (RFBs). In certain situations, RFBs have been shown to remove nutrients from groundwater before it enters the streams. RFBs also filter sediments in runoff from upslope areas to a varying extent, and floodplain RFBs can substantially attenuate flood peaks.

Notwithstanding the NPS pollutant benefits, RFBs also offer very significant instream benefits. The tree species found in RFBs provide the essential organic inputs of leaf litter and dissolved carbon. The roots and deadfall provide a complex physical structure for optimal benthic habitat. Stream reaches in RFBs are much wider as well, thus affording much more habitat. The shading provided by RFBs can help to prevent instream temperatures from exceeding thresholds that affect sensitive EPT orders.

Given the potential for such benefits, the GIS was used to evaluate whether the extent of stream buffering could be correlated with the Stream Watch results. For appreciable buffering, mature forests were rated as being twice as effective as emerging forests. Likewise, smaller headwater streams were rated as being twice as responsive to RFBs as the wider streams of third order and higher. By overlaying the forests upon the streams in this manner and normalizing the results, the percent of buffered steams in each subwatershed can be determined. Figure V-6, "Stream Buffering", displays the extent of effective stream buffering throughout the entire watershed.

Streamside buffers on headwater streams extended along 53 to 64 percent of the streams. Immediately downstream in sites 3, 12 ,17 and 18 the buffering declined to range from 36 to 48 percent, following the trend in EPT results. This is to be expected, given the preponderance of agricultural and/or high intensity uses in these watersheds. Among the steeper forested slopes along the fall line to the south, the extent of buffering increases from 58 to 73 percent, where a minor rebound of the EPT density is found.

While the extent of buffering increases in sites where the EPT density stabilizes, these results suggest that RFBs may not be effective in mitigating NPS pollution once the stream system is impacted by upstream activities. While this is disappointing, it is not surprising, as accepted design principles for RFBs emphasize that RFBs be used to prevent NPS pollution from impacting the stream in the first place. Therefore, these results suggest that RFBs be established in the critical first order tributaries affected by runoff from conventional tillage and urban land uses along the Route 1 corridor.

The results of the Stream Watch monitoring effort are not reassuring. Five years of exhaustive collection, counting and analysis confirm that, while the headwaters are in relative good health, the White Clay Creek has been substantially impacted by both point source and NPS pollution. These results are similar to Mill Creek, another suburban watershed outside of Philadelphia, which also shows very poor EPT diversity (Mill Creek Watershed Association, 1997). Residential density and commercial uses in the Mill Creek watershed are similar to the more developed Route 1 corridor section of the WCC watershed.

Since the headwaters are located where runoff amounts and erosion rates are generally low, increases in erosion and runoff from conventional tillage and high intensity land uses seem to substantially impact the WCC. The GIS graphically demonstrates that runoff increases correlate well with the declines in the East Branch. However, runoff increases do not correlate as well with EPT counts in the West and Middle Branches. On the other hand, erosion rates seem to correlate well with the observed declines in the West and Middle Branches, while having less of an effect upon the East Branch.

Taken together, it is quite possible that the Stream Watch results reflect the cumulative impacts of both processes, where EPT counts decline substantially once a certain threshold is exceeded. Stream buffering moderates these effects somewhat, however the accumulated impacts of both point and NPS pollution by the state line is such that the EPT density has irretrievably declined.

In terms of land uses, agricultural land uses by themselves seem to have a lesser impact. The decline from site 11 to site 12 is suggestive, as soil losses and runoff amounts increase, while stream buffering decreases. The pronounced decline at site 4 on the Middle Branch also may involve agricultural impacts in synergy with adjacent residential uses. This site also happens to lie below a reach of the stream that has very little riparian forest. Conventional tillage does raise the runoff amount and erosion rate substantially, approaching the threshold discussed above. While present agricultural practices have far less NPS impacts than in the past, deep floodplains now exist due to erosion over the past centuries. Additional runoff from high intensity land uses then continually erodes these unstable streambanks. As a result, the critical threshold is exceeded and EPT densities decline.

However, old time residents confirm that the entire watershed did support a breeding trout population in the 1940s, when agricultural practices were far more disturbing and less of the watershed was forested. This suggests that agricultural practices may not be a primary cause for the decline seen in the present results. The deep and persistent decline in sensitive orders in the East Branch may involve the residual effects of persistent pesticides used in agricultural and mushroom land use practices since the end of World War II.

Given that there seem to be minimal changes in the physical habitat in the sites below the headwaters, there is no reason to think that the entire watershed should not be able to support a breeding trout population. Attention given to NPS and point sources (e.g. conservation tillage, integrated pest management, stream buffering, and innovative urban runoff BMPs) can contribute to restoring the ecosystem of the White Clay Creek.

Two underlying datasets are used in the GIS: a Soils Mapping Unit layer, and the 1990 Land Cover layer, both of which generated as part of a University of Pennsylvania Regional Planning Studio in 1993. The GIS dataset was encoded at a resolution of 8 meters, permitting the visibility of linear features such as hedgerows and utility corridors.

The soils mapping was based upon the Chester County and New Castle County Soil Surveys. Soil mapping units include information on slope, hydrologic soil group, erodability, texture and many other factors that affect the susceptibility of the landscape to NPS activities. For instance, soil erodability and slope class are aspects of the Soils dataset important in projecting erosion rates. Information in these datasets is manipulated in the GIS to project the inherent susceptibility of the landscape to human impacts.

The Land Cover Dataset was photointerpreted from 1990 400 scale aerial photographs so as to differentiate between land use activities that affect NPS impacts, as opposed to land use in the planning sense. In this manner, what is typically classified as an agricultural land use has been segregated into four different land covers according to their potential NPS impacts. Conservation tillage was differentiated from conventional tillage by the visual presence of contour cropping, strip cropping, and grassed waterways. Pastures and hayfields were separately classified as grassed areas, and the barn, home and outbuildings were classified as farmsteads. Wooded areas were classified as mature, emerging or conifer/nursery.

Where residential densities are low, they were segregated into the residential area and adjacent woods and/or grass areas according to the actual extent of the land cover delineated on the aerial photographs. Likewise, mushroom establishments were defined by the structure, pavement and wharf areas, with adjacent meadows and woods classified separately. Medium density and high density areas were similarly differentiated by the physical footprint of the built structures and associated impervious areas, as is the commercial/industrial categories.

In this manner, even though the boundaries of many tracts may be defined by fence lines, hedgerows and roads, land cover parameters are defined by the actual land uses in practice in 1990. Information in the land cover dataset is then used to display the spatial distribution of human activities throughout the watershed in terms of their potential for generating NPS impacts. By relating land cover to the underlying landscape susceptibility as discussed below, impacts of human activities are graphically displayed.

To project the relative amount of runoff to be expected in each watershed, the Soil Cover Complex Method of hydrological analysis (TR-55, U.S. Soil Conservation Service, 1979) is widely accepted. This method accounts for most of the factors involved in runoff hydrology such as storm intensity and characteristics of the land cover and underlying soils. Storm events involved in NPS impacts are the relatively frequent thunderstorms and frontal systems that generate from two to three inches of rainfall. Heavier rainfall events occur only once in several years or so, while lighter rainfall events generally result in minimal runoff due to the processes of interception, evaporation, infiltration and depression storage.

Land cover and soil hydrologic group effect infiltration and depression storage, factors that retain the first few inches of rainfall by a process known as "initial abstraction". Organic soils with a well-aerated surface horizon and a natural forest cover have infiltration rates so high that very little surface runoff occurs in the typical summer storm. Forest canopy cover further intercepts a substantial portion of initial precipitation. For the storm events of interest, this high initial abstraction results in negligible runoff when compared to a land cover disturbed by agricultural activities. On the other extreme, very little initial abstraction occurs on impervious surfaces, where runoff is almost equal to the precipitation amount, regardless of storm intensity.

Table 5 displays the amount of runoff to be expected from the typical storm event of 2.5 inches of rainfall using the CN values and equations set forth in TR-55. These results assume a uniform soil type with a hydrological soil group classification of "B", the dominant soil group in the watershed. Note that impervious runoff is segregated from that originating in the pervious coverage of each category to better reflect the processes involved in the runoff response. If impervious surfaces were accounted for by increasing the average CN value, impervious contributions to the total runoff from each category would be understated. This is particularly important in the more intense land cover categories.

The impervious percentages for the open space categories includes adjacent roads, which were not included as a separate category in the GIS due to their linear nature. The high value for farmsteads reflects the concentration of livestock which compact the soils, decreasing their permeability. The values allocated to other intense land uses fall within accepted ranges for such uses; it is likely that they should higher since some of the pervious area normally included in these land uses was separately classified in the open space categories. The high CN value for pervious cover in the mushroom category reflects the use of such areas for spreading compost.

Note that the total runoff from conventional tillage is six times greater than that for woods. This is largely due to the annual cycle of vegetation removal, plowing and compaction by machinery. Its pervious CN value is on the low end of the TR-55 tables, which were expressly designed to focus on agricultural uses. Conservation tillage reduces this runoff amount by half due to its continual vegetated cover and better filth. Low density residential uses generate roughly four times the runoff of woodlands. Note how the high intensity land covers generate runoff amounts in the range of ten to fifteen times that of woodlands.

To determine the annual runoff from each category, the GIS was used to multiply the percentage of each category by its event runoff to determine average event runoff throughout the watershed. This results in an average runoff amount of 0.499 inches, which happens to be one tenth the annual stormflow of 5.000 inches (or 0.127 meters), as determined by hydrograph separation of the White Clay Creek from 1963 to 1980 (US Geological Survey, unpublished data). Annual runoff represents the event runoff amount multiplied by ten and converted into meters. Annual runoff varies from 0.032 meters for mature woods to 0.660 meters for highways.

To project the relative amount of erosion to be expected in each watershed, the Universal Soil Loss Equation (USLE) is widely used. This equation follows the form:

Erosion (ton/ac/yr) = RKLSCP

where R is the rainfall impact coefficient (180 in this area), K is soil erodability (which varies from O.15 to 0.43 depending upon soil type), and LS is a tabular coefficient based upon length-slope, which is approximated for a 150 foot slope by the following equation in this analysis:

LS Factor = 67.65 x Slope^1.65

(A more involved equation exists, however, using the simplifying assumption that slope length is a constant 150 feet, this equation closely approximates the tabular coefficients within the slope ranges typical in the watershed.) C relates to the type of agricultural crop such as corn vs. alfalfa, while P is the value determined by erosion controlling practices such as contour or strip cropping.

The first four of these variables relate to the inherent erosion potential of the landscape according to the USLE: rainfall coefficient, field length, field slope and soil erodability. Assuming the slope length of 150 feet and a rainfall coefficient of 180, these factors are readily available from the soil dataset. The GIS first recodes each soil type according to its slope and erodability, calculates the LS factor from the slope coding as described above, and multiplies it by its erodability and the R value of 180 to obtain the inherent erosion potential of the landscape, which assumes a CP value of 1.00.

Even though the slope length factor is assumed a constant 150 feet, this approximation is adequate for the purposes of a relative comparison of inherent susceptibility. Figure V-3 "Erosion Potential" graphically displays how the steeper slopes found in the vicinity of the fall line in the southern part of the watershed have the highest potential for erosion. Since RKLS varies so widely, the relative erosion potential is displayed in qualitative terms. Values for RKLS in the GIS range from a low of 5.3 to 940 ton/ac./yr.

The USLE is primarily used for agricultural land uses, although some sources have used it to address the erosion potential of woodlands as well. For these purposes, the USLE relates erosion to Cover and Practice (CP) factors, which vary substantially according to the land cover and type of tillage practice. For completely bare plowed ground, the C-value is 1.0. As crops become established, it reduces to a value of 0.05 or less. Typical factors often used for corn vary from 0.63 at seedling stage to 0.26 at maximum growth. Practice values for conservation tillage vary from 0.10 for terraces on flat ground to 0.90 for contouring on steep ground. Strip cropping provides intermediate values in the range of 0.25 to 0.45, depending upon slope.

For agricultural crops under conventional tillage in the watershed, the CP factor was allocated at 0.100, reducing to a value of 0.024 for conservation tillage. This represents a P value of 0.24, reflecting the additional benefits of minimal tillage cropping (which is not documented in my sources). For other land covers for which there is no CP factor in the literature, annual sediment losses are based upon reported sediment Event Mean Concentrations (EMCs). Annual losses can be projected by multiplying the EMC by the annual runoff amount estimated in the previous section.

Table 6 lists the best estimates of EMC of suspended sediments in runoff from a variety of sources. Note that the values for EIMCs in the literature vary considerably; those values chosen reflect my best judgment as to the most appropriate values for the various land cover categories. Since this analysis estimates erosion losses as a function of EMC and projected runoff quantities, which have much less variation in TR-55, this method allows for some verification of estimated EMCs in land cover categories where annual erosion rates are well documented.

The estimate of EMCs displayed in Table 6 allocates a very high value of 6000 mg/l for conventional tillage. However, when multiplied by the annual runoff, it results in an annual erosion rate of nearly 6 ton/yr, which is on the low end for the literature values for agricultural crops. The reduction in EMC to 3000 mg/l allocated to conservation tillage accounts for the cleaner runoff anticipated from cover, crops and strip cropping. This could be higher than actually encountered, but it seems to generate appropriate values when multiplied by annual runoff to generate erosion losses of 1.4 ton/yr. Values for pasture/hayfield are on the high end of the literature for grasslands to account for the contribution from grazing. Likewise, to account for the disturbance of livestock activities, farmsteads are allocated a high EMC similar to conservation tillage. This allocation better adjusts for the effect of livestock in the pasture areas near to the farmsteads and helps to further differentiate pastures from hayfields, which were not separately coded in the GIS cover categories.

On the other hand, woodlands have very low values of EMC that reflect the absence of disturbance. These result in erosion rates similar to the values reported in the literature, resulting in negligible CP values. Other land uses also do not generate many sediments; residential and public facilities, for example, so their annual CP values would also be several orders of magnitude below that of tillage practices. The EMC of 200 mg/l allocated to mushroom facilities may be too low, considering the amount of compost manipulation that occurs. However, many facilities now use BMPs such as covering the wharves and detention ponds, which would greatly reduce sediment losses.

Brady, N.C. 1990. The Nature and Properties of Soils. MacMillan Publishing Co. New York.

Boyer, M.A. 1992. Aquatic Biology Investigation - UNT East Branch of White Clay Creek. Case No. 8-341-6130. PaDER.

Boyer, M.A. 1993. Aquatic Biology Investigation - UNT East Branch of White Clay Creek. Case No. 8-341-6130. PaDER.

Dunne, T., L.B. Leopold, 1978. Water in Environmental Planning. W.H. Freeman and Co. New York.

Shaver, E.J., J. Maxted, G. Curtis, D. Carter. 1995 Watershed Protection Using an Integrated Approach. In Proceedings from "Stormwater NPDES Related Monitoring Needs", Crested Butte, CO. August 12, 1994. Am. Society of Civil Engineers.

Maxted, J., E.J. Shaver, 1997. The Use of Retention Basins to Mitigate Stormwater Impacts in Aquatic Life. In Proceedings from "Effects of Watershed Development and Management on Aquatic Ecosystems", Snowbird, Utah. Engineering Foundation Conference.

Mill Creek Watershed Association 1997. 1997 Annual Report (need source)

Steele, C. 1986. Cause/Effect Survey on the East Branch White Clay Creek. PaDER

Wanielesta, M.P., Y.A. Yousef. 1992. Stormwater Management. John Wiley and Sons Inc., New York.

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