Steve Amann, P.E., CFM Baxter & Woodman, Inc. Introduction y Basement flooding from groundwater seepage y Out of the ordinary for floodplain managers y Two good things that don’t always go good together: y Stormwater infiltration y Basements in the Midwest y Potential issues to consider y Groundwater levels fluctuate slowly/hard to predict y Soils usually not homogeneous y Mass grading and other changes can impact groundwater flows y Building elevations unknown at due diligence stage y Names were changed A Tale of Two Subdivisions y Subdivision No. 1 –Phreatic Farms y 160 Acres; 205 single‐family lots y Preliminary plat filed in 1989 y No records of soil borings, field tile investigation y NRCS soils maps: y Well‐drained soils y More than 80” to groundwater y Not far from floodplain y 500 feet from edge of subdivision to SFHA y BFE: 2’ below ground surface Phreatic Farms y Property filled for grading and drainage y Detention ponds constructed in granular soils with normal gravity outlets y No groundwater issues observed in construction y Mass grading y Underground utilities y Basement construction y Sump pumps discharge to storm sewer system y Homes built and occupied from 1993 to last year Phreatic Farms y First concerns voiced in Fall 2004 y Low rainfall in 2005; concern abated y Adjacent development began due diligence y Concerns resurfaced in 2006 y Sump pump run times y Up to 12 cycles per hour for 10 days after rain y Continuously for 14 days after rain y Continuously year‐round y Never run y Loss of power y Lowering pond normal water level Phreatic Farms y So, what did we do? y Preliminary investigation Fall 2004 y Researched old plans, calculations and reports y Water surface observation/recording y Detention ponds y Natural water bodies y High groundwater level approximately 594 y Solution: Convey water away from subdivision by gravity y Extend conveyance to nearby River y Large‐diameter storm sewer: $780,000 y Swale with road culvert: $580,000 y Needed ROW/easements y Too expensive/public expense vs. private nuisance y Concerns abated with lower groundwater levels y Wait for downstream development Phreatic Farms y Renewed investigation in 2007 y Worked with adjacent development y Soil borings y 3‐4 feet of clay under topsoil y Loose to firm sand and gravel to end of borings y Groundwater levels at 590.5 to 592.5 y Drawdown test y Additional ground surveying y Basement elevations: inconsistent with sump pump run times y Exploratory excavation in specific areas y Looked for field tiles in subdivision y Found porous soils and rapid groundwater movement y Traced/repaired downstream field tiles y Balance against pond size/depth reduction y Resident survey y Required minimum basement elevations PhreaticHouse DrainFarms Tile Invert Pumping – Dry Pumping –Wet 1 590.35 12/hour Constant 2 590.63 Seldom Constant 3 591.76 Constant Constant 4 591.79 Constant Constant 5 591.82 Constant Constant 6 592.17 12/hour Constant 7 592.40 Constant Constant 8 592.56 Seldom Seldom 9 592.61 Seldom Seldom 10 592.74 Seldom 6/hour 11 592.81 Seldom 12/hour 12 592.88 Seldom 6/hour 13 592.89 Seldom Seldom 14 592.91 Seldom Seldom 15 592.99 Constant Constant 16 593.25 Seldom Constant 17 593.32 Seldom Constant Phreatic Farms y Developed alternative solutions y Public groundwater wells through subdivision y Private groundwater wells near homes y Infiltration pipe or trench through subdivision y Piped outlet at end of subdivision y Back‐up power y Convert basements to crawlspaces y Hired Hydrogeologist y Used data from adjacent development y Developed and ran Winflow groundwater model y Tested alternative solutions Phreatic Farms y Results y Public groundwater wells (11) through subdivision y Limited benefit at basements y Lowering of water level in existing ponds y Public responsibility for construction, operation and maintenance y $300,000 cost y Needs outfall: additional $780,000 y Infiltration pipe or trench (“French drain”) through subdivision y Needs gravity outfall ($780,000); would be below BFE y Lowering of water level in existing ponds y Public responsibility for construction and maintenance y Not considered further y Piped outlet at south (downstream) end of subdivision y Lowering of water level in existing south pond y Decreasing benefit at basements going north y Public responsibility for construction and maintenance y Not considered further Phreatic Farms y Private groundwater wells (external sump pumps) at affected homes y Highest benefit at basements y Re‐use existing drainage system within subdivision y Private construction, operation and maintenance cost y $12,000 per home y Village agreed to fund 13 individual pumps y Private improvements y Back‐up power y Basement conversion y Downstream field tiles y Repair/replace by Village y Ongoing monitoring and maintenance y Adjacent development y Changed housing product to eliminate basements y Clay liner proposed for detention pond y Utility installation not as difficult as expected Phreatic Farms y What’s happening now? y Most (internal) sump pump run times reduced significantly y Village maintaining field tiles y Residents monitoring adjacent development y Detention pond normal water level still an issue y One home still has sump pump issues Subdivision No. 2 ‐ Infiltration Acres y Preliminary Plat filed in 1992 y 60 Acres; 153 single‐family lots y Soil borings indicated y Granular soils at 1 to 8 feet below grade y Various groundwater levels throughout site y No evidence of field tile survey y NRCS maps y Well‐drained soils y More than 80” to groundwater y Upstream of floodplain y 2,000’ from edge of subdivision to SFHA y BFE 5’ below ground surface y Minimal mass grading y Sump pumps discharge to storm sewer system Infiltration Acres y Stormwater planned to infiltrate y Open‐bottom drainage structures y Detention ponds y Primary outlet through infiltration y Overflows above ground y Homes built and occupied from 1994 to 2000 y No concerns voiced by residences until Spring 2008 y Little sump pump usage until September 2007 y Rainfalls in March 2008 triggered significant sump pump run times y Rainfalls in May 2008 increased sump pump run times y Head in storm sewer system against sump pumps y Sump pumps ran continuously, but: y Sumps backed up into basements y Seepage from walls and floors Infiltration Acres y Residents: y Purchased and installed additional pumps y Disconnected pumps from storm sewer system y Contacted Village for assistance y Concerned about lowering pond level y Village: y Inspected storm sewers for blockages y Observed detention pond water levels y Bypass‐pumped storm sewers to reduce head against sump pumps y Concerned about downstream impacts of pumping Infiltration Acres y So, what did we do about it? y Met with residents to assess specific problems y Researched old plans and reports y Ground survey: y Top of foundation y Pond levels and storm sewer inverts y Hired soils consultant y Installed five groundwater observation wells y Installed one well logger y Checked wells monthly y Downloaded daily logger readings y Sealed bottoms of open drainage structures Infiltration Acres y Investigated field tiles y Records from adjacent work y Field tile installer/locator y Reviewed information and options with Village Staff y Groundwater levels vs. drain tile inverts y No gravity outfall readily available y Maintain existing pond normal water level y Pursued implementation of limited improvements y Surface conveyance improvements y Field tile extensions and repairs y Opinion of cost for groundwater pumping station: y Pump station and local drainage improvements: $230,000 y Downstream conveyance improvements: $380,000 y Total: $610,000 Infiltration Acres y So, what’s happening now? y Monitored groundwater through November 2009 y Groundwater responds quickly to rainfall events y Groundwater subsided over time y No concerns since May 2008 y Groundwater pumping station unfunded Lessons Learned y Ordinance Amendments y Reviewed other agencies’ requirements y Will County, Illinois: y Lowest floors above seasonal high groundwater y Reportedly, 2/3 of County becomes unbuildable y Ordinance not passed y Eau Claire County, Wisconsin y New ordinance y No enforcement experience y Waukesha County, Wisconsin y Relatively new ordinance y Limited enforcement experience y Staff concerned about veracity of reports y Field tile survey requirements y Clarified as requirements, not recommendations Lessons Learned y Considered local ordinance modifications y Resident protection vs. development time/cost y Met with development community y New regulations considered too harsh y Time and cost required to complete studies y Historical data difficult to obtain; monitoring periods excessive y Data inconclusive: y Local vs. global y Perched groundwater tables y Variables hard to mandate y Ground elevations (existing vs. proposed) y Rainfall patterns/groundwater levels y Building styles/elevations y Soil types/locations y Most developers perform adequate due diligence Lessons Learned y Ordinance amendments currently being considered y Formalize soils report requirements y Minimum number of borings y Appropriate locations for borings y Groundwater information y Development plan: y Ground/groundwater/building elevations y Soils strata y Soils consultant requirement y During concept plan, ideally y Prior to preliminary plat, minimally y Consultation during final plat and plan preparation y Basements are not a requirement y Option between builder and homeowner y Entail risk y Whose problem is it? Lessons Learned y Due Diligence: There is no substitute.