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Painted HIlls FEIS Appendix E - MAY 2023APPENDIX E: Flood Conveyance System Element Failure Risk and Impact Summary This page intentionally left blank. Appendix E – Flood Management System Elements Failure Risk and Impact Summary Appendix E - 1 Item System Element Description Potential Point of Failure Potential Failure Impacts Mitigation #1A Headwall and Trash Rack at the inlets of the two 48” pipes that collect floodwater at the south end of the site adjacent to Thorpe Road. (See Figure 3-9 for location) Seepage of water around headwall structures Soils surrounding the headwall structure could erode and undermine the stability of the structure As a result of an unlikely extreme flood event that overpowers the headwall designed to limit inflow, the pipes could fail, or due to excessive volumes beyond the pipes’ capacity floodwater could flow around or over the top of the pipe, resulting in an uncontrolled release of floodwaters at the south end of the site and south of Thorpe Road. Adherence to maintenance requirements of the O&M manual, including annual maintenance report to City; maintaining perpetual maintenance bond for system. Pipe openings clogged by debris during a flood event Floodwaters could back up south of Thorpe Road,flood adjacent and downstream property and/or undermine the roadway. Trash racks will be installed to prevent debris from entering and clogging pipes. #1B Two 48-inch pipes that convey overflows from Chester Creek (See Figure 3-9 for location) Pipe openings clogged by debris during a flood event Floodwater could flow around or over the top of the pipe, resulting in an uncontrolled release of floodwaters potentially causing property damage. Trash racks will be installed to prevent debris from entering and clogging pipes. Pipes damaged by aboveground forces (crushed by weight of heavy equipment or structures placed on top of them). Broken pipes could result is an uncontrolled release of floodwater, potentially causing property damage. Pipes will be located within an easement west of Madison Road where additional structure construction would be prohibited. Debris is intentionally dumped into pipes by vandals.Due to a blocked pipe floodwater could flow around or over the top of the pipe, resulting in floodwaters collecting at the south end of the site and flood adjacent and downstream property. Manholes would have locking lids that would prevent intentional dumping of debris. Pipes damaged due to excessive flow volumes Excessive volumes beyond the pipes’ capacity floodwater could flow around or over the top of the pipe, resulting in floodwaters collecting at the south end of the site and flood adjacent and downstream property Failure potential is mitigated through system design. Pipe capacity (154 cfs) is 1.5 times larger than the capacity needed to convey the design storm(106 cfs), making it unlikely that floodwaters would damage the pipes. #2 Pipe that captures flows from un- named tributary to east of project and direct them to the Triangle Pond. (See Figure 3-9 for location) Pipe openings clogged by debris during a flood event Excessive volumes beyond the pipes’ capacity could result in floodwaters collecting east of Highway 27. Vegetation in and near the ditch that enters the pipe is non- woody and would not contribute a measurable amount of debris to the ditch. Pipes west of Highway 27 damaged by aboveground forces (crushed by weight of heavy equipment or structures placed on top of them). Floodwaters escape from broken pipes, resulting in an uncontrolled release of floodwaters water would flow along the current FEMA floodplain to the south and would bypass the infiltration facility (Triangle Pond) and could eventually reach Painted Hills. Pipes are located within an easement where additional structure construction would be prohibited. Debris is intentionally dumped into pipes by vandals. Due to a blocked pipe floodwater could overtop the right channel bank and flood areas to the east of 27, potentially causing property damage. Manholes would have locking lids that would prevent intentional dumping of debris. Pipes damaged due to excessive flow volumes Due to excessive volumes beyond the pipes’ capacity,floodwater could overtop the right channel bank and flood areas to the east of 27, potentially causing property damage. Pipe capacity 3 times larger than 100-year design (47.16 cfs/16 cfs), minimizing the potential for pipes to be damaged due to excessive flows. #3 Off-site infiltration pond (Triangle Pond) and drywells (See Figure 3-9 for location) Fine sediment builds up on the pond bottom reducing permeability and limiting infiltration, causing water to collect and remain in the pond. The drywells become clogged with fine sediment, limiting permeability and infiltration capacity, causing water to collect and remain in the pond. The pond is unable to drain, the pond overflows and floodwaters flow downstream along the current floodplain, possibly reaching Painted Hills, potentially causing property damage. Routine maintenance will include periodic removal of collected sediment using a vactor truck. Appendix E – Flood Management System Elements Failure Risk and Impact Summary Appendix E - 2 Item System Element Description Potential Point of Failure Potential Failure Impacts Mitigation #4a Dry wells (See Figure 3-9 for location) The drywells become clogged with fine sediment, limiting permeability and infiltration capacity, causing water to collect and remain in the pond, or to overtop under extreme circumstances if infiltration were significantly reduced. The pond is unable to drain and floodwaters inundate portions of the PRD site, potentially causing property damage. Floodwaters could potentially overtop the infiltration facility and flood adjacent off-site properties and properties to the east of Madison. Routine maintenance will include periodic removal of collected sediment using a vactor truck. #4b Infiltration Pond (See Figure 3-9 for location) Fine sediment builds up on the pond bottom reducing permeability and limiting infiltration, causing water to collect and remain in the pond. The pond is unable to drain and floodwaters inundate portions of the PRD site, potentially causing property damage. Floodwaters could potentially overtop the infiltration facility and flood adjacent off-site properties and properties to the east of Madison. Routine maintenance will include periodic removal of collected sediment using a vactor truck. #4c Settling Pond (See Figure 3-9 for location) Excessive amounts of fine sediment collect in the pond, reducing holding capacity of the pond. Pond is not effective at settling out sediment due to lack of capacity and sediment enters the infiltration pond, leading to ineffective functioning of infiltration pond. Under extreme circumstances this could potentially lead to the pond overflowing and floodwaters inundating portions of the PRD site and adjacent off-site properties, potentially causing property damage. Routine maintenance will include periodic removal of collected sediment using a vactor truck. #4d Bioswale (See Figure 3-9 for location) The bioswale vegetation intended to filter water fails.Swale is not effective at filtering out sediment and excess sediment enters the settling pond and possibly the infiltration pond, leading to ineffective functioning of infiltration pond. Under extreme circumstances this could potentially lead to the pond overflowing and floodwaters inundating portions of the PRD site and adjacent off-site properties, potentially causing property damage. Routine maintenance will include inspection of bioswale vegetation and replacement of the vegetation if it dies or shows evidence of ineffectively filtering sediments. Figure 3-9Painted Hills Flood ManagementSystem Element LocationPainted Hills Residential Development DEIS Source: GIS data provided by Spokane County and USGS, NOAA, ESRI, and NPS.