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Ground Seepage

Is ground seepage being addressed?

The developed areas along the river within the valley are built on alluvial silt, sand and gravel deposits. The Drumheller valley has a relatively shallow groundwater table which is recharged by infiltration of precipitation, snow-melt and stormwater run-off water that falls on or drains into the valley.  This groundwater moves across our townsites to discharge into the river through these granular soils which are hydraulically connected to the river.

During normal flow periods, the groundwater table is usually found at an elevation just above  the river surface.  However, during periods of flood when river surface rises quickly for short periods of time, the water from the river flows back into the ground along the river.  In areas with flood berms, when river levels rise the groundwater behind the flood protection berms will also rise.  The groundwater response within these deposits is dependent on the permeability of the subgrade . The placement of flood berms will moderate groundwater impacts during a flood, because the speed of water flow through the most permeable of gravel soils will still be hundreds of times slower than destructive overland flow.  In areas with gravelly deposits, the response will be quicker and the aerial extent will be larger than areas with silty subgrades.  In areas with these unfavourable subgrades, the houses nearest the flood berms will likely be impacted by rising groundwater levels caused by flood waters. 

The proposed flood mitigation measures are not being designed to mitigate groundwater seepage using methods such as vertical cut-offs below the berms.  As groundwater flows are 3 dimensional, subsurface cut-off walls or trenches would need to be installed to bedrock and would need to encircle the communities and be tied back into the valley walls to inhibit groundwater levels. In most areas of Drumheller, construction of vertical cut-off trenches would not possible through the gravels above the bedrock.  So the most practical cut-off option would be driven sheet pile walls which are prohibitively expensive. 

Even if a wall were practical or cost effective, it should be understood that cut-offs of this kind would restrict the movement of water both out of and into the river, including the precipitation and snow melt runoff infiltration which needs to be conveyed to the river through groundwater flow.  Cut off trenches would act as a barrier for this natural groundwater flow and would result in seasonally elevated groundwater levels behind the cut off trenches several times a year.  This could cause a much greater and frequent problem than the groundwater impacts of periodic floods which may happen years or decades apart.  To alleviate this seasonal problem would require many kilometres of groundwater collection pipes tied into numerous permanent dewatering pump stations to manage groundwater levels within low-lying developed areas of the Town.

If your property experienced groundwater seepage during past flood events, you can expect to experience similar impacts for future events.  To manage and reduce impacts it is recommended that residences develop onsite pumping strategy, and ensure critical infrastructure (ie electrical panels, HVAC) reinstalled above local flood levels.

What is going to be done about the seeping ground water from under the dikes and underground?

The overland flow from floods is much more damaging as it carries debris, silt and gravel. Dikes are built to mitigate the risk of overland flooding. It will also moderate the amount of groundwater fluctuation due to the flooding waters. The water table will still increase, but not as quickly as it would without a dike in place.

In the past, mine shafts that run under the river and throughout the Drumheller valley have been inundated by flood / storm water, what is being done to protect residential properties, i.e. from underground flooding?

Mine shafts are outside the scope of the current flood mitigation program.

What are the different levels of basement protection from groundwater and downspout discharge?

Assuming any roof discharge and yard grade issues have been addressed, the different levels of basement protection are, in order:

  1. A high-quality crushed gravel base layer below the basement slab (i.e., a minimum building code requirement).
  2. Damp-proofing of exterior basement wall surface prior to backfilling.
  3. Perimeter weeping tile tied into an interior collection sump with positive drainage/discharge away from the site.
  4. Add lateral drains connected into the collection sump for a site where seasonal groundwater might rise to within 1 m of the basement floor.  
  5. Upgrade damp-proofing to waterproofing.
  6. Add water stops between footings, basement walls and floor slabs.
  7. Add a free-draining gravel drainage mat across the basement area tied into 1 or 2 collection sumps.
  8. Add a water-proof membrane with the drainage mat.

How many levels of basement protection from groundwater and downspout discharge should our home have in place?

In terms of conventional basement protection, most houses in Alberta should at least have these three levels of protection. 

  1. A high-quality crushed gravel base layer below the basement slab (i.e., a minimum building code requirement).
  2. Damp-proofing of exterior basement wall surface prior to backfilling.
  3. Perimeter weeping tile tied into an interior collection sump with positive drainage/discharge away from the site.

Then, levels of protection should be added as the risk of high groundwater increases. Older development areas and houses in Drumheller may not have been built to provide any of these measures. Even if the first three levels of protection were provided, they may not be sufficient to eliminate seepage in houses close to dikes in areas with gravelly subgrades.  

The additional levels of protection are:

  1. Add lateral drains connected into the collection sump for a site where seasonal groundwater might rise to within 1 m of the basement floor.  
  2. Upgrade damp-proofing to waterproofing.
  3. Add water stops between footings, basement walls and floor slabs.
  4. Add a free-draining gravel drainage mat across the basement area tied into 1 or 2 collection sumps.
  5. Add a water-proof membrane with the drainage mat.