A Build-out Analysis of the Cohansey Watershed

Transcription

1 A Build-out Analysis of the Cohansey Watershed Richard G. Lathrop Tenley M. Conway November 2001

2 A Build-out Analysis of the Cohansey Watershed by Richard G. Lathrop Tenley M. Conway Grant F. Walton Center for Remote Sensing & Spatial Analysis Cook College - Rutgers University New Brunswick, NJ CRSSA Technical Report November 2001 ii

3 Table of Contents Introduction 1 Methods..1 Results 4 Summary and Recommendations...8 Acknowledgements 10 Literature Cited..10 iii

4 A Build-out Analysis of the Cohansey Watershed Introduction A build-out analysis maps potential future development across the landscape under specific sets of constraints. The scope and location of future development is identified in this type of analysis, although timing of development is not predicted. Information gained through a build-out analysis can be very useful in long-term planning efforts. The Cohansey Watershed is a largely rural watershed, with approximately half of the land used for agriculture. With easy accessibility to Philadelphia, there is growing potential for an increase in sprawling suburban development in the watershed. The likelihood for this type of development has lead to increased concern over the impacts of future development on the water resources in the area. The goal of this project is to understand the impacts of future development on the water resources in the Cohansey Watershed. Build-out scenarios were created based on regulations and environmental constraints. To identify the potential impacts of build-out, we used two resource indicators. Population size was used as a measure of residential water demand, and impervious surface cover was used as a measure of non-point source water pollution from urban run-off. Comparisons between the results of the indicators from 1995 and build-out can provide insight into the magnitude of future impacts on water resources. Figure 1. Land use in the Cohansey Watershed in Methods We created the spatially-explicit build-out model using a Geographic Information System. This is a computer based tool used to manage, manipulate, and analyze digital data. The approach allowed us to examine potential changes to specific areas within the watershed. To create the build-out scenarios, existing land use in the watershed needed to be identified. We used the New Jersey Department of Environmental Protection s (NJDEP) digital mapped land use/ land cover (LULC) data from 1995 (Figure 1; NJDEP 2000), the most recent data available. By using this data set, 1995 represents the baseline year of the study. 1

5 Next, we identified land that is available for development. Protected open space, already developed areas, and land that is undevelopable for environmental reasons (lands considered undevelopable for environmental reasons include wetlands and adjacent buffer lands) were identified as unavailable for development. In the Cohansey Watershed, permanently protected open space includes Wildlife Management Areas, state, county, and municipal land, privately managed open space, and preserved farmland. Land not excluded in this step was deemed available for development. The final step needed to create the build-out scenarios was to identify the type of development that could occur on the available land. Areas that were available for development were assigned a build-out land use based on municipal zoning. All other areas retained the land use assigned in the NJDEP's LULC data from Three build-out scenarios were created for the Cohansey Watershed. The first scenario is based on current regulations. The second scenario incorporates the 2001 Watershed Management Rules (NJDEP 2001a). Under these rules, proposed developments of more than five dwelling units located outside sewer service areas are reviewed by the NJDEP to ensure that development using septic tanks include large enough leach fields to protect local water resources. We assume residential lots smaller than three acres will not be approved under these rules, based on hydrological models and municipal regulations already in use in New Jersey. In the second scenario, future development is not allowed at intensities higher than one dwelling units per three acres outside sewer service areas. As 82 percent of the Cohansey Watershed land area does not currently fall within sewer service areas, implementation of the Watershed Management Rules could have a significant impact in this area. The third scenario combines zoning and coastal zone regulations. The 2001 updates of the Coastal Zone Management Rules (NJDEP 2001b) define the amount of impervious surface cover allowed in the coastal zone. The maximum impervious surface cover allowed on land being developed (Table 1) is determined by the Coastal Area Facilities Review Act (CAFRA) planning area designation. Table 1. Maximum impervious surface cover allowed in the CAFRA planning areas in the Cohansey Watershed. Planning Area Maximum Impervious Surface Cover Percent of Watershed Metropolitan 80 % 2.4 Suburban, Sewer Service 30 % 0.5 Suburban, No Sewer Service 5 % 0.4 Rural 5 % 21.0 Environmentally Sensitive 3 % 9.7 In the third scenario, the residential density allowed by zoning regulations was adjusted so the build-out impervious surface of each polygon was equal to or less than the maximum impervious surface allowed in that planning area. If the estimated build-out 2

6 impervious surface allowed by zoning was below the maximum value established by the Coastal Zone Management Rules then the intensity of development was not adjusted. The coastal zone only includes the southern half of the watershed, so build-out conditions in the northern half of the watershed are the same as the current regulations scenario. Dwelling Units and Population Increase The population living in the watershed will increase as more areas in the watershed are developed. A growing population can negatively impact the region s water resources, as a larger population requires a larger supply of freshwater. If the population continues to increase, the demand for potable freshwater could exceed the sustainable supply. Thus, it is important to understand the potential size of the population at build-out. To estimate the potential increase in population for the Cohansey Watershed, we began by estimating the number of new dwelling units. For each patch of land available for development, the number of new dwelling units was calculated by determining the number of units that could be built on that patch based on the minimum lot size requirements for the area as specified in the existing zoning maps. As new development requires public infrastructure (roads, new schools, etc.), we used the rule of thumb to calculate the number of new dwelling units. This rule of thumb states that generally 80 percent of the land will be used for residential homesites, while the remaining 20 percent is reserved for infrastructure (e.g., roads). Once the predicted number of potential new dwelling units was determined, we multiplied new dwelling units by the average number of people per dwelling unit based on 1995 census data. In 1995, Bridgeton City had an average of 3.35 people per dwelling unit; in Hopewell Township (the largest municipality in the watershed) the average number of people per dwelling unit is only The second number is very similar to other suburban areas in New Jersey. Because most of the future development is not predicted to reflect the high intensity of development found in Bridgeton City, the average for Hopewell Township was used to predict the future population. The resulting number is the predicted population growth at build-out. Adding the predicted new growth to the existing development, we then estimated the potential number of total dwelling units and population at build-out for the three scenarios. To better calibrate the methods used in the dwelling unit and population estimates, we compared predicted versus observed results for our baseline year of The predicted number of dwelling units was approximately nine percent greater than the observed number in The over prediction can be explained several ways. First, development does not always occur at maximum density as zoned, which is assumed in the build-out model. In many cases residential lots are larger than the zoned minimum lot size. The second major factor may be the 1995 digital mapped LULC data (NJDEP, 2000). The LULC data set was used to map existing development and these data are not mapped on an ownership parcel basis. Areas mapped as developable do not necessarily represent a complete parcel, or may be composed of several partial parcels that have been placed into one contiguous tract that meet the development criteria. Because of the data limitations and unpredictable nature of residential development, the number of new dwelling units is 3

7 given as a range. The initial model prediction of new dwelling units represents the high end and adjusted prediction (down-weighted by nine percent) represents the low end of the range of possible new dwelling units in the watershed. Impervious Surface Beginning with the 1970 Clean Water Act there has been concerted effort to improve biological, chemical, and physical characteristics of the nation s waterways. Most efforts have focused on improving water quality through regulation of point source pollution. The success of these programs has made non-point source pollution the leading threat to water quality. Non-point source water pollution includes pollution from surface run-off, percolation to groundwater, and atmospheric deposition or precipitation (Charbonneau and Kondolf 1993). Allan and Flecker (1993) identify land use as one of the most important factors determining non-point source pollution. When land is converted to urban land uses, there are physical, chemical, and biological impacts on water quality (Zandbergen 1998). While the percentage of urban land use in a watershed is one way to assess the water quality, the percent of impervious surface cover has been proposed as a more refined measure of non-point source pollution from urban run-off and a general indicator of watershed health (Soil Conservation Service 1975; Klein 1979; Arnolds and Gibbons 1996; Wang 2001). Impervious surface refers to streets, sidewalks, driveways, roofs, patios, and other impenetrable surfaces. Thus, there is a close connection between the forcing factors of human- mediated watershed disturbance and the resulting impacts to downstream freshwater and estuarine systems. Impervious surface is an important environmental indicator of the intensity of human land use and closely correlates with water quality degradation and altered runoff patterns in urban and urbanizing areas (Novotny and Chesters 1981; Brown 1988; Driver and Troutman 1989; Ferguson and Suckling 1990; Arnold and Gibbons 1996; Charbeneau and Barrett 1998). Areas that are more intensely developed tend to have a larger percentage of impervious surface cover, contributing more non-point source pollution to the water in the watershed. In compiling data from a number of watersheds, Arnold and Gibbons (1996) developed a set of impact thresholds: 1) < 10% impervious surface cover can be considered nonimpacted; 2) between 10-30% cover can be considered impacted; and 3) > 30% cover is generally considered degraded. While these thresholds should not be considered hard and fast breakpoints, they do provide a useful guide in evaluating the comparative risk of water degradation on a watershed scale. To estimate the percent impervious surface in the watershed at build-out, we used the NJDEP s impervious surface estimates from the 1995 digital mapped LULC data (NJDEP 2000). Build-out impervious surface values were determined by assigning the average value from areas with similar zoning classes that were already developed in Estimates were also calculated for 1986, by comparing land use codes, to understand recent changes. Impervious surface information was then summarized at the catchment level, the smallest hydrological unit identified in the Cohansey Watershed. Summarizing the data by catchments, which average 17 square kilometers, allowed us to 4

8 identify localized areas where the amount of non-point source pollution is potentially quite high. Results Figure 2. Land available for development. In 1995, 14 percent of the Cohansey Watershed was urban land and 53 percent available for development (Figure 2). Twenty percent of the remaining land in the watershed was wetlands and buffer zones around wetlands, seven percent was preserved farmland, and the remaining land is other protected open space. Although about half of the watershed is available for development, not all of this can be converted to urban land uses. In several cases, the land is zoned for agriculture. Dwelling Units and Population In 1995, 14,174 dwelling units are estimated to have existed in the Cohansey Watershed, with an associated population of 37,279 (Table 2). In the build-out scenario based on current regulations, 35,429 to 38,510 dwelling units could exist in the watershed, a potential increase of 150 to 172 percent. If down zoning to three acres is required outside sewer service areas, the total number of potential dwelling units would be 23,483 to 25,525, a 66 to 80 percent increase from Comparing the three build-out scenarios, the down zoning scenario predicts 44 percent fewer dwelling units and population and the coastal zone scenario predicts 11 percent fewer dwelling units and population than the baseline scenario. Although less growth is allowed than in the baseline scenarios, a considerable increase above 1995 levels can still occur even with the more restrictive regulations of the down zoning and coastal zone scenarios. The results of the dwelling unit and population calculations suggest that there is the potential for a tremendous increase in residential water demand. Table 2. Predicted number of dwelling units and population in the Cohansey Watershed. Percent Increase Dwelling Units Population from ,147 37,279 Build-out, current regulations Build-out, down zoning Build-out, coastal zone rules 35,429 38,510 93, , ,483 25,525 61,760 67, ,346 34,072 82,441 89,

9 One drawback of this approach is that water needed for industrial and agricultural uses is not considered. Although there is limited industry in the Cohansey Watershed, 35 percent of the land was used for agriculture in If all uses of water are considered, the per capita water demand in 1990 was 327 gallons per day in the Regional Water Resource Planning Area that includes the Cohansey Watershed, while the per capita water demand is only 194 gallons per day throughout New Jersey. The large per capita water demand in the Cohansey is mostly likely to support agriculture uses. As more development occurs in the watershed, less land will be used for agricultural purposes, because 77 percent of agricultural land was available for development in Thus, there is the potential for the population to increase while the water demand decreases, if less water is needed for residential use than agriculture. Impervious Surface In 1986, the impervious surface cover in the watershed was 3.3 percent and in 1995 the impervious surface cover was 3.8 percent (Figure 3). While impervious surface is low throughout the watershed, the two catchments that include Bridgeton City had 10 percent and 14 percent impervious surface cover in 1995, suggesting that water quality in these catchments may already be impacted by urban runoff. Figure 3. Estimated percent impervious surface cover. 6

10 In the current regulations build-out scenario, impervious surface basinwide is estimated as 9.5 percent, suggesting that water quality is not yet impacted. However, the Cohansey is nearing the 10 percent threshold where many watersheds have demonstrated significant negative impacts. Although the estimated impervious surface is below the 10 percent threshold, these results indicate the potential for impervious surface to more than double between 1995 and build-out. While basinwide the impervious surface is below 10 percent, several catchments are predicted to be significantly above this threshold. Most of the predicted change from 1995 to build-out is located in Hopewell Township. Hopewell is primarily zoned for 1 acre lots, although areas close to Bridgeton City are zoned for half acre lots. In 1995, most of this land was crop or pasture land, with little impervious surface cover. Due to the density of zoning, the catchment in southern Hopewell Township is estimated to increased from 4 percent impervious surface cover to 14 percent impervious surface cover at build-out. The catchment in northern Hopewell Township was 1 percent impervious surface in 1995 and could potentially increase to 13 percent by build-out. While these catchments represent the areas predicted to have the highest change, all of the northern catchments in the watershed are predicted to cover at least an additional five percent of the land with impervious surface between 1995 and build-out. The location of the predicted change indicates the potential for significant development to occur in the headwaters of the watershed, which could have impacts on water throughout the watershed. If down zoning to at least three acre lots is required outside sewer service areas, then the build-out impervious surface is predicted to be 7.5 percent. The differences between the current regulation scenario and the down zoning scenario are primarily located in Hopewell Township. The land currently zoned for 1 acre lots mostly occurs in areas that do not have sewer service. As a result, the impervious surface in the southern catchment in Hopewell Township is predicted to be 36 percent less, covering only 9 percent of the land. In the northern Hopewell Township catchment, the impervious surface is predicted to be 46 percent less, covering 7 percent of the land. The coastal zone scenario predicts 8.4 percent impervious surface cover at build-out. Most of the differences between the current regulations scenario and coastal zone scenario is located in southern catchment in Hopewell Township. The catchment is primarily part of a rural planning area, limiting impervious surface cover to only 5 percent. The northern catchments are the same as in the current regulations scenario, since the Coastal Zone only includes the southern half of the watershed. The coastal zone scenario results are based on CAFRA planning area designations and do not consider coastal centers. The 2001 Coastal Zone Management Rules also identify maximum impervious surface allowed in the coastal centers (NJDEP 2001b). The values allowed in the centers are generally much higher than allowed in the planning areas. Land within a coastal center has both a center designation and a CAFRA planning area 7

11 designation. The overlap of coast centers and CAFRA planning areas make it unclear which maximum impervious surface value should be followed. Within the Cohansey Watershed there are 7 coastal centers. Fifty-two percent of the land in the coastal centers is designated metropolitan planning area and allowed to have 80 percent impervious surface cover, which is similar to the coastal center maximums of 50 to 80 percent. However, 29 percent of the center land is designated a suburban planning area, 26 percent is designated a rural planning area, and 3 percent is designated an environmentally sensitive planning area. These three planning areas have a maximum impervious surface value well below the centers maximums. If the maximum impervious surface value is based on the center type, not the planning area designation, then the impervious surface within the coastal centers may be higher at build-out. During the conversion from agriculture to urban land use, there will mostly be an increase in soil erosion (Klien 1979). Once the new development is established, the level of soil erosion should reduce, but there will then be an increase in the flashiness of streams due to the increase in impervious surface cover (Leopold 1968; Klien 1979; Charbonneau and Kondolf 1993). The increase in urban run-off may be partially balanced by a reduction in non-point source pollution from agriculture. Changes in run-off patterns are also difficult to predict. One limitation with impervious surface as an indicator is that it does not include non-point source pollution from agriculture. As a large portion of the watershed was agriculture in 1986 and 1995, only examining urban run-off is ignoring a potentially large part of the non-point source pollution in the watershed. Through conversion of agriculture to urban land, there may a reduction in non-point source pollution from agriculture, but an increase in urban run-off. Thus, the actual change in total non-point source pollution may not be as great as predicted when measuring impervious surface cover alone. However, the results strongly indicate an increase in non-point source pollution from urban run-off. Summary and Recommendations The results of the build-out analysis suggest that the population in the watershed and impervious surface cover could more than double from 1995 to build-out. Changes of this magnitude will have significant impacts on the water resources in the watershed. Although most of the watershed is not currently served by sewer services, even if the proposed down zoning outside sewer service areas occurs, there is still the potential for significant growth. In this situation, if sewer service areas are increased in the future, the additional development could have far reaching impacts on the waters resources. Additionally, compliance with the Coastal Zone Management Rules will limit growth in a few catchments, but does not address the northern half of the watershed, outside the Coastal Zone. Due to changes in zoning, environmental regulations, and socioeconomic factors, the actual build-out landscape may deviate from the scenarios predicted in this analysis. However, based on the current situation, the analysis clearly identifies the potential for tremendous development to occur by build-out. 8

12 While the Coastal Zone Management and Watershed Management Rules are presented as two separate scenarios, realistically both sets of regulations will be functioning at the same time. The Coastal Zone Management Rules are more limiting than the Watershed Management Rules, based on our interpretations of the regulations, so if both sets of rules are fully implemented build-out in the Coastal Zone portion of the watershed should be the same as the coastal zone scenario. The remainder of the watershed would be equivalent to the down zoning scenario. In this situation, the predicted build-out population is 34 percent lower than without the additional regulations and the estimated build-out impervious surface is predicted to be only 7.1 percent. Although the implementation of the both sets of regulations will limit growth, the amount of development at build-out is still predicted to be well above 1995 levels. Indicator results highlight the need for polices designed to mitigate the impacts of impervious surface, with one possible mitigation activity being restoration and protection of vegetation corridors along riparian zones. Educational activities should apprise Municipal officials and the public of the problems associated with impervious surface, as well as ways to reduce non-point source pollution. If development in the watershed is further limited, through aggressive land preservation efforts, the build-out populations and impervious surface cover could be reduced. It may be feasible to accomplish this through widespread participation in New Jersey s farmland preservation program, as most future development is predicted to occur on agricultural land. However, a similar study in the Barnegat Bay Watershed indicates that land preservation alone will not necessarily be sufficient to protect the water resources (Lathrop and Conway 2001). The 2001 Coastal Zone Management Rules limits impervious surface cover on most of the land, while allowing high levels within coastal centers (NJDEP 2001b). By aggregating impervious surface to a few chosen areas, protecting the watershed s water resources may be possible, as most places should not be subject to negative impacts from impervious surface. Mitigation efforts can then focus on the few coastal centers. Supporting the Coastal Zone Management Rules may be one way to allow growth, while limiting impacts on water quality. In the Cohansey Watershed, 50 percent of the land in coastal centers was urban in 1995, but 46 percent was available for development, indicating there is room to accommodate future growth. Based on the results of the build-out scenario, current regulations do not take advantage of the high levels of impervious surface allowed in the coastal centers. The build-out impervious surface in the coastal centers is predicted to be 22 percent impervious surface, on current regulations, well below the 50 to 80 percent allowed. If local zoning laws were amended to align with the rules for the coastal centers, then more intense development could occur within the centers. In this situation, development outside the centers could be further limited, yet the same number of people could be accommodated in the watershed. Future work should focus on developing a water budget for the Cohansey Watershed. In the 1996 Statewide Water Supply Plan, the Cohansey Watershed is lumped into the Maurice Planning Area, making it unclear what the estimated water supply is for the watershed. If the amount of available water was estimated for the Cohansey Watershed, 9

13 then we could gain a better understanding of the likelihood that there will be sufficient water available to meet build-out water demand. Additionally, the impacts on water resources from agriculture versus urban development should be further considered, as there is great potential within the Cohansey Watershed for agricultural land to be converted to residential development in the future. Acknowledgements Bob Brewer, Kim Brown (Cumberland County Planning Department), and David Dumont provided necessary data for the build-out model. Jay Springer (New Jersey Department of Environmental Protection) offered useful feedback on the model design. Scott Haag and Steve Lennartz provided valuable assistance during the digitizing and analysis phases. Cooperation and assistance from the Partnership for the Delaware Estuary, in particular Kathy Klein and Joe Matassino, was essential to the project. This project was supported by the Partnership for the Delaware Estuary, with the original funding provided by the U.S. Environmental Protection Agency. Literature Cited Allan, J.D. and A.S. Flecker Biodiversity conservation in running waters. Bioscience 43: Arnold, C.L., Jr. and C.J. Gibbons Impervious surface coverage: The emergence of a key environmental indicator. Journal of the American Planning Association 62(2): Brown, R.G Effects of precipitation and land use on storm runoff. Water Resources Bulletin 24: Charbeneau, R.J. and M.E. Barrett Evaluation of methods for estimating stormwater pollutant loads. Water Environment Research 70: Charbonneau, R. and G.M. Kondolf Land use change in California, USA: nonpoint source water quality impacts. Environmental Management 17: Driver, N.E. and B.M. Troutman Regression models for estimating urban stormrunoff quality and quantity in the United States. Journal of Hydrology 109: Ferguson, B.K. and P.W. Suckling Changing rainfall-runoff relationships in the urbanizing Peachtree Creek watershed, Atlanta, Georgia. Water Resources Bulletin 26: Klein, R.D Urbanization and stream quality impairment. Water Resources Bulletin 15(4):

14 Lathrop, R.G. and T.M. Conway A Build-out Analysis of the Barnegat Bay Watershed. CRSSA Technical Report Leopold, L.B Hydrology for urban land planning a guidebook on the hydrological effects of urban land use. U.S. Geological Survey Circular 554, Washington DC. NJDEP Land Use/Land Cover New Jersey Department of Environmental Protection, Trenton, New Jersey. NJDEP. 2001a. Watershed Management Rules. New Jersey Department of Environmental Protection, Trenton, NJ. NJDEP. 2001b. Coastal Zone Management Rules. N.J.A.C. 7:7E. Novotny, V. and G. Chesters Handbook of Nonpoint Pollution. Van Rostrand Reinhold, NY, NY. 555p. Soil Conservation Service Urban Hydrology for Small Watersheds. U.S. Department of Agriculture. Technical Release No. 55. Wang, X Integrating water-quality management and land-use planning in a watershed context. Journal of Environmental Management 61: Zandbergen PA Urban watershed ecological risk assessment using GIS: a case study of the Brunette River watershed in British Columbia, Canada. Journal of Hazardous Materials 61 (1-3):