Analysis
Content
HYATT HIDDEN LAKES RESERVE MASTER PLAN
38
SEPTEMBER 2000
SITE SUITABILITY ANALYSIS
In addition to the slope analysis, a reserve suitability analysis was completed. This analysis (see site analysis map, page 40) identified site opportunities and constraints including surrounding land-use, possible access points, rights-of-way, setback limitations, existing water sources, and general site drainage. Coupled with the slope analysis, the suitability analysis helped guide the design and placement of reserve facilities, and development of the management recommendations.
Land Use
Part of what makes the Hyatt site so intriguing is that it is almost entirely surrounded by fully developed land with both residential and industrial uses. Two adjacent parcels owned by ACHD and the Meridian School District remain undeveloped. These would be valuable additions to the reserve and could increase its size by as much as 15 acres. At the time of this writing, negotiations are underway to integrate those properties into the reserve.
Site Access/Easements/ ROW’s
Access to the site at the north end of the property is limited to a specific area where the proposed Maple Grove extension will meet grade, allowing for a road into the site. A corner of ACHD property lies between the reserve property and the new Maple Grove road at this point. BP&R will seek an easement or other agreement with ACHD in order to use this parcel for reserve access and parking. Based on ACHD requirements, access at the south end of the site off of McMillan Road is limited to two curb cuts, situated directly opposite existing roads that intersect McMillan from the south. There are plans to widen McMillan Road concurrent with construction of the Maple Grove extension. The 98-foot right-of-way required to accommodate the proposed widening has been considered in this master plan. Settler’s Irrigation District requires a 50-foot canal easement (25 feet from centerline, each side) for maintenance access from its banks. Some uses are allowed within that easement, but no trees are to be planted or obstructions built within the easement. A crossing agreement will be required with the Irrigation District for the proposed reserve access bridge. The proposed Maple Grove extension requires a 20-foot construction easement into the reserve area. Per ACHD, construction of the retaining wall and road is currently planned to occur from the reserve property to the east, in order to reduce impacts to the wetland. The 20-foot easement will be required for exterior treatments and maintenance of the retaining wall. The Hyatt Wetland has been designated a jurisdictional wetland and has been delineated several times over the last 15 years. The most recent delineation, provided by ACHD in 1999, encompasses only the eastern edge of the site where the Maple Grove extension is proposed. Some changes in the wetland level and area have occurred since the initial delineation in the late 1980’s.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
SEPTEMBER 2000
39
HYATT HIDDEN LAKES RESERVE MASTER PLAN
40
SEPTEMBER 2000
Water Sources / Outlet
Various sources, including the results of a Level I Environmental Assessment of the site (1997), identify the main water source for the wetland to be seeps in the south cut bank of the wetland and a shallow groundwater table that feeds the wetland from underneath. One spring is visible in the southeastern-most corner of the wetland while two additional seeps are thought to exist along the southern edge. The flow of water in Settler’s Canal also has a significant effect on the wetland’s water flow, according to a study done over the last year by ACHD. A storm water inlet drains into the site at its eastern edge. This influx of untreated storm water has caused significant erosion to the steep eastern bank and should be addressed, to prevent future disturbance. Channels, originally constructed to drain the site during the gravel mining operation, are still somewhat visible in the wetland. These lead into the main outlet channel where one, possibly two, beaver dams exist that have backed up the water in the wetland to its current level. The single outlet for the wetland is at the northern edge. Outflow enters an 18” pipe that drains under Mt. View Road and the adjacent industrial property to Warm Springs Creek. ACHD has indicated that they will not modify this outlet, or resize this pipe, with the construction of the Maple Grove extension.
SUMMARY
The site analysis process determined that the Hyatt property, though enjoyed as a natural environment today by adjacent neighbors and others, has been subjected to invasive gravel mining, resulting in a compromised natural ecosystem. The site is currently healing itself through natural processes, but because it is surrounded by developed residential and industrial uses, its ability to become a healthy natural ecosystem without human intervention is limited. Therefore, some enhancement and management is necessary to produce a sustainable wildlife habitat reserve. Certain features within the site require augmentation in order to provide safe use of the property. Routinely breached by site visitors, the beaver dam has caused flooding of downstream businesses. Replacing it with a permanent water control structure will allow for safe and consistent management of the water level in the wetland. Other aspects of the site that require enhancement include the weed-infested upland grass areas, steep, eroding slopes, the sparse riparian edge of the wetland, and the cattail monoculture within the wetland.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
SEPTEMBER 2000
41
STORM WATER ANALYSIS
STORM WATER ANALYSIS
Ada County Highway District (ACHD) brought a proposal before the Boise Parks & Recreation Department to discharge storm water into the Hyatt Wetland. The district was looking to provide a location for detention and treatment of storm water run-off that currently drains into the Settler’s Canal from neighborhoods south of the site. The Hyatt Wetland offers a unique opportunity to accomplish these objectives. The ACHD proposal had merit in that it provided a storm water discharge alternative to the Settler’s Canal, which is nearing capacity. Additionally, the implementation of Best Management Practices would benefit ACHD and the downstream community by improving the quality of the storm water before it reaches the Boise River. The Boise Parks & Recreation Department has determined that this opportunity to partner with ACHD could be mutually beneficial for the following reasons: • Additional off-site storm water introduced into the wetland would help increase flow and reduce stagnation, which could improve water quality and the overall health of the wetland. This may also help discourage further outbreaks of Avian Botulism. • ACHD will compensate Boise Parks & Recreation for the quantity of storm water discharged into the wetland, which will help to offset the costs of reserve development, management and maintenance. • Treating storm water through the Hyatt Wetland offers public educational opportunities that could benefit both agencies and the community at large.
WETLAND FILTRATION
Despite the potential benefits of ACHD’s proposal, Boise Parks & Recreation was concerned about the potential for disruption and contamination of the wetland habitat through the introduction of off-site storm water. Wetlands, both natural and constructed, are known and proven to function as water filtration and pollution treatment mechanisms. According to an EPA bulletin on Constructed Wetlands Treatment, “constructed wetlands have been demonstrated to be effective at removing organic, metal and nutrient elements including nitrogen and phosphorous from municipal waste waters, mine drainage, industrial effluents and agricultural runoff.” The Hyatt Wetland is not classified as a constructed wetland because it was not built for the primary purpose of filtering water. However, it is not entirely accurate to classify it as a naturally occurring wetland since it resulted from an invasive gravel extraction process. Regardless of classifaction, the Hyatt Wetland has the necessary characteristics to filter pollutants from storm water. Long-term effects of storm water discharge into natural wetlands are hard to predict and can vary greatly, depending upon the quality and quantity of storm water introduced. Currently, the Army Corps of Engineers policy does not allow discharge of storm water directly into a naturally occurring wetland without some method of pre-filtration. In early discussions with the planning team, the Corps indicated they will allow discharge of storm water into the Hyatt Wetland, as long as some type of pre-treatment is provided.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
SEPTEMBER 2000
43
HYATT HIDDEN LAKES RESERVE MASTER PLAN
44
SEPTEMBER 2000
PRE-TREATMENT
The planning team explored various methods of storm water pre-treatment including both biological and mechanical pre-treatment options. To preserve the health of the wetland, it is critical that sediment, oils, and nutrients be removed from the storm water. It is anticipated that additional pollutants such as heavy metals and other chemicals would either settle out in the pre-treatment process or be transformed into inert substances through the biological processes within the wetland. Three of the four preliminary planing concepts included pre-treatment systems ranging from the use of mechanical filtration devices to the incorporation of a constructed wetland. Creating a wetland to filter storm water was an attractive option, largely because it offered a number environmental education opportunities. Further exploration determined that building a constructed wetland with enough capacity to handle the quantities that ACHD needed to discharge would reduce the already limited upland areas too greatly to be compatible with other desired uses, such as passive recreation. As a result, installation of a more compact, mechanical pre-treatment device combined with compost filtration emerged as the preferred alternative.
WATER LEVEL IMPACTS
Because the quantity of storm water identified for possible discharge into the wetland was significant (approximately 137 acre feet), it was necessary to assess what impact a raise in water level would have upon the wetland, specifically with regards to waterfowl nesting and roosting sites, and wetland edge vegetation. To this end, the team civil engineer performed calculations to estimate the detention and drawdown time for various water level increases ranging from one to seven feet, based on the size of the existing outflow pipe at the north end of the property. (Please refer to the appendix for storm water calculations). Additionally, a hydrologic model was produced to demonstrate how an increase in water level would impact the site. It was determined that if a berm were constructed at the north end of the site, the wetland could feasibly detain up to seven feet of additional water without significantly impacting the useable upland areas. It was also determined that the wetland edge vegetation could tolerate, and even benefit from, a temporary inundation as long as water levels returned to normal levels within four weeks. The nesting and roosting habitat would potentially be inundated as well, depending on the season, but the infrequency of the 100-year design storm lessens the severity of that impact.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
SEPTEMBER 2000
45
DESIGN STORM CAPACITY
After a thorough investigation of pre-treatment options, it was determined that filtering a 100-year storm event with a mechanical or constructed wetland pre-treatment system was not a feasible alternative. Storm water volumes are so large for a 100-year storm that pre-treatment facilities sized to handle these volumes would have a potentially negative impact on the site’s upland area. However, it is feasible to pre-treat a two-year storm event, which equates to approximately 80 percent of the storms experienced in the Boise area. The remaining 20 percent of storms would be pre-treated to the maximum capacity of the system, understanding that any additional storm water would then flush directly into the wetland. The risk of pollutants entering the wetland during peak flows is typically reduced since the accumulated pollutants have already been removed by the initial flush of run-off. In other words, storm water run-off in the early part of a storm event picks up the majority of sediments and pollutants that have accumulated on the roadways, and therefore tends to be ‘dirtier’ than run-off that occurs later in a storm event, after most of the ‘dirt’ has already been washed away. Though the higher flows from the extreme storm events would have minimal pollution impact on the wetland, they would require detention storage for the large volume of water flowing into the site for the duration of the storm. While the ultimate risk of pollution to the wetland is reduced by pre-treating two-year storms, the feasibility of installing a pre-treatment facility to treat larger design year storm events should still be considered.
STORM WATER SUMMARY
Results of the storm water analysis indicate that it would be feasible to allow ACHD to discharge up to a 100-year storm event volume into the wetland, raising the water level by approximately six feet. Provided that the water recedes back to its original level within four weeks, as estimated, the wetland and its habitat will not sustain any permanent damage. In order to gain acceptance by the Army Corps of Engineers, a mechanical pre-treatment device would need to be installed to filter water from up to a minimum two-year storm event, effectively removing the highest concentration of pollutants for the more frequent storm events. At the time of this writing, Boise Parks & Recreation and ACHD are negotiating to determine if it is economically viable to pre-treat and discharge the off-site storm water into the Hyatt Wetland. If a decision is made to accept the storm water, the next step will be to design the necessary water control structures and file for a dam permit with the Idaho Department of Water Resources.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
46
SEPTEMBER 2000
DESIGN EVOLUTION / PUBLIC INVOLVEMENT
38
SEPTEMBER 2000
SITE SUITABILITY ANALYSIS
In addition to the slope analysis, a reserve suitability analysis was completed. This analysis (see site analysis map, page 40) identified site opportunities and constraints including surrounding land-use, possible access points, rights-of-way, setback limitations, existing water sources, and general site drainage. Coupled with the slope analysis, the suitability analysis helped guide the design and placement of reserve facilities, and development of the management recommendations.
Land Use
Part of what makes the Hyatt site so intriguing is that it is almost entirely surrounded by fully developed land with both residential and industrial uses. Two adjacent parcels owned by ACHD and the Meridian School District remain undeveloped. These would be valuable additions to the reserve and could increase its size by as much as 15 acres. At the time of this writing, negotiations are underway to integrate those properties into the reserve.
Site Access/Easements/ ROW’s
Access to the site at the north end of the property is limited to a specific area where the proposed Maple Grove extension will meet grade, allowing for a road into the site. A corner of ACHD property lies between the reserve property and the new Maple Grove road at this point. BP&R will seek an easement or other agreement with ACHD in order to use this parcel for reserve access and parking. Based on ACHD requirements, access at the south end of the site off of McMillan Road is limited to two curb cuts, situated directly opposite existing roads that intersect McMillan from the south. There are plans to widen McMillan Road concurrent with construction of the Maple Grove extension. The 98-foot right-of-way required to accommodate the proposed widening has been considered in this master plan. Settler’s Irrigation District requires a 50-foot canal easement (25 feet from centerline, each side) for maintenance access from its banks. Some uses are allowed within that easement, but no trees are to be planted or obstructions built within the easement. A crossing agreement will be required with the Irrigation District for the proposed reserve access bridge. The proposed Maple Grove extension requires a 20-foot construction easement into the reserve area. Per ACHD, construction of the retaining wall and road is currently planned to occur from the reserve property to the east, in order to reduce impacts to the wetland. The 20-foot easement will be required for exterior treatments and maintenance of the retaining wall. The Hyatt Wetland has been designated a jurisdictional wetland and has been delineated several times over the last 15 years. The most recent delineation, provided by ACHD in 1999, encompasses only the eastern edge of the site where the Maple Grove extension is proposed. Some changes in the wetland level and area have occurred since the initial delineation in the late 1980’s.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
SEPTEMBER 2000
39
HYATT HIDDEN LAKES RESERVE MASTER PLAN
40
SEPTEMBER 2000
Water Sources / Outlet
Various sources, including the results of a Level I Environmental Assessment of the site (1997), identify the main water source for the wetland to be seeps in the south cut bank of the wetland and a shallow groundwater table that feeds the wetland from underneath. One spring is visible in the southeastern-most corner of the wetland while two additional seeps are thought to exist along the southern edge. The flow of water in Settler’s Canal also has a significant effect on the wetland’s water flow, according to a study done over the last year by ACHD. A storm water inlet drains into the site at its eastern edge. This influx of untreated storm water has caused significant erosion to the steep eastern bank and should be addressed, to prevent future disturbance. Channels, originally constructed to drain the site during the gravel mining operation, are still somewhat visible in the wetland. These lead into the main outlet channel where one, possibly two, beaver dams exist that have backed up the water in the wetland to its current level. The single outlet for the wetland is at the northern edge. Outflow enters an 18” pipe that drains under Mt. View Road and the adjacent industrial property to Warm Springs Creek. ACHD has indicated that they will not modify this outlet, or resize this pipe, with the construction of the Maple Grove extension.
SUMMARY
The site analysis process determined that the Hyatt property, though enjoyed as a natural environment today by adjacent neighbors and others, has been subjected to invasive gravel mining, resulting in a compromised natural ecosystem. The site is currently healing itself through natural processes, but because it is surrounded by developed residential and industrial uses, its ability to become a healthy natural ecosystem without human intervention is limited. Therefore, some enhancement and management is necessary to produce a sustainable wildlife habitat reserve. Certain features within the site require augmentation in order to provide safe use of the property. Routinely breached by site visitors, the beaver dam has caused flooding of downstream businesses. Replacing it with a permanent water control structure will allow for safe and consistent management of the water level in the wetland. Other aspects of the site that require enhancement include the weed-infested upland grass areas, steep, eroding slopes, the sparse riparian edge of the wetland, and the cattail monoculture within the wetland.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
SEPTEMBER 2000
41
STORM WATER ANALYSIS
STORM WATER ANALYSIS
Ada County Highway District (ACHD) brought a proposal before the Boise Parks & Recreation Department to discharge storm water into the Hyatt Wetland. The district was looking to provide a location for detention and treatment of storm water run-off that currently drains into the Settler’s Canal from neighborhoods south of the site. The Hyatt Wetland offers a unique opportunity to accomplish these objectives. The ACHD proposal had merit in that it provided a storm water discharge alternative to the Settler’s Canal, which is nearing capacity. Additionally, the implementation of Best Management Practices would benefit ACHD and the downstream community by improving the quality of the storm water before it reaches the Boise River. The Boise Parks & Recreation Department has determined that this opportunity to partner with ACHD could be mutually beneficial for the following reasons: • Additional off-site storm water introduced into the wetland would help increase flow and reduce stagnation, which could improve water quality and the overall health of the wetland. This may also help discourage further outbreaks of Avian Botulism. • ACHD will compensate Boise Parks & Recreation for the quantity of storm water discharged into the wetland, which will help to offset the costs of reserve development, management and maintenance. • Treating storm water through the Hyatt Wetland offers public educational opportunities that could benefit both agencies and the community at large.
WETLAND FILTRATION
Despite the potential benefits of ACHD’s proposal, Boise Parks & Recreation was concerned about the potential for disruption and contamination of the wetland habitat through the introduction of off-site storm water. Wetlands, both natural and constructed, are known and proven to function as water filtration and pollution treatment mechanisms. According to an EPA bulletin on Constructed Wetlands Treatment, “constructed wetlands have been demonstrated to be effective at removing organic, metal and nutrient elements including nitrogen and phosphorous from municipal waste waters, mine drainage, industrial effluents and agricultural runoff.” The Hyatt Wetland is not classified as a constructed wetland because it was not built for the primary purpose of filtering water. However, it is not entirely accurate to classify it as a naturally occurring wetland since it resulted from an invasive gravel extraction process. Regardless of classifaction, the Hyatt Wetland has the necessary characteristics to filter pollutants from storm water. Long-term effects of storm water discharge into natural wetlands are hard to predict and can vary greatly, depending upon the quality and quantity of storm water introduced. Currently, the Army Corps of Engineers policy does not allow discharge of storm water directly into a naturally occurring wetland without some method of pre-filtration. In early discussions with the planning team, the Corps indicated they will allow discharge of storm water into the Hyatt Wetland, as long as some type of pre-treatment is provided.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
SEPTEMBER 2000
43
HYATT HIDDEN LAKES RESERVE MASTER PLAN
44
SEPTEMBER 2000
PRE-TREATMENT
The planning team explored various methods of storm water pre-treatment including both biological and mechanical pre-treatment options. To preserve the health of the wetland, it is critical that sediment, oils, and nutrients be removed from the storm water. It is anticipated that additional pollutants such as heavy metals and other chemicals would either settle out in the pre-treatment process or be transformed into inert substances through the biological processes within the wetland. Three of the four preliminary planing concepts included pre-treatment systems ranging from the use of mechanical filtration devices to the incorporation of a constructed wetland. Creating a wetland to filter storm water was an attractive option, largely because it offered a number environmental education opportunities. Further exploration determined that building a constructed wetland with enough capacity to handle the quantities that ACHD needed to discharge would reduce the already limited upland areas too greatly to be compatible with other desired uses, such as passive recreation. As a result, installation of a more compact, mechanical pre-treatment device combined with compost filtration emerged as the preferred alternative.
WATER LEVEL IMPACTS
Because the quantity of storm water identified for possible discharge into the wetland was significant (approximately 137 acre feet), it was necessary to assess what impact a raise in water level would have upon the wetland, specifically with regards to waterfowl nesting and roosting sites, and wetland edge vegetation. To this end, the team civil engineer performed calculations to estimate the detention and drawdown time for various water level increases ranging from one to seven feet, based on the size of the existing outflow pipe at the north end of the property. (Please refer to the appendix for storm water calculations). Additionally, a hydrologic model was produced to demonstrate how an increase in water level would impact the site. It was determined that if a berm were constructed at the north end of the site, the wetland could feasibly detain up to seven feet of additional water without significantly impacting the useable upland areas. It was also determined that the wetland edge vegetation could tolerate, and even benefit from, a temporary inundation as long as water levels returned to normal levels within four weeks. The nesting and roosting habitat would potentially be inundated as well, depending on the season, but the infrequency of the 100-year design storm lessens the severity of that impact.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
SEPTEMBER 2000
45
DESIGN STORM CAPACITY
After a thorough investigation of pre-treatment options, it was determined that filtering a 100-year storm event with a mechanical or constructed wetland pre-treatment system was not a feasible alternative. Storm water volumes are so large for a 100-year storm that pre-treatment facilities sized to handle these volumes would have a potentially negative impact on the site’s upland area. However, it is feasible to pre-treat a two-year storm event, which equates to approximately 80 percent of the storms experienced in the Boise area. The remaining 20 percent of storms would be pre-treated to the maximum capacity of the system, understanding that any additional storm water would then flush directly into the wetland. The risk of pollutants entering the wetland during peak flows is typically reduced since the accumulated pollutants have already been removed by the initial flush of run-off. In other words, storm water run-off in the early part of a storm event picks up the majority of sediments and pollutants that have accumulated on the roadways, and therefore tends to be ‘dirtier’ than run-off that occurs later in a storm event, after most of the ‘dirt’ has already been washed away. Though the higher flows from the extreme storm events would have minimal pollution impact on the wetland, they would require detention storage for the large volume of water flowing into the site for the duration of the storm. While the ultimate risk of pollution to the wetland is reduced by pre-treating two-year storms, the feasibility of installing a pre-treatment facility to treat larger design year storm events should still be considered.
STORM WATER SUMMARY
Results of the storm water analysis indicate that it would be feasible to allow ACHD to discharge up to a 100-year storm event volume into the wetland, raising the water level by approximately six feet. Provided that the water recedes back to its original level within four weeks, as estimated, the wetland and its habitat will not sustain any permanent damage. In order to gain acceptance by the Army Corps of Engineers, a mechanical pre-treatment device would need to be installed to filter water from up to a minimum two-year storm event, effectively removing the highest concentration of pollutants for the more frequent storm events. At the time of this writing, Boise Parks & Recreation and ACHD are negotiating to determine if it is economically viable to pre-treat and discharge the off-site storm water into the Hyatt Wetland. If a decision is made to accept the storm water, the next step will be to design the necessary water control structures and file for a dam permit with the Idaho Department of Water Resources.
HYATT HIDDEN LAKES RESERVE MASTER PLAN
46
SEPTEMBER 2000
DESIGN EVOLUTION / PUBLIC INVOLVEMENT
