The Trouble with Terrain Part 2: Stormwater Solutions for Karst Terrain
The karst terrain in the Bay watershed is very ancient and, in many areas, is deeply buried by residual soils. Consequently, many sinkholes form due to collapse of surface sediments caused by the intrusion of stormwater from the surface into deep, underlying voids. Consequently, karst terrain complicates the land development process and requires a unique approach to stormwater design. Some of the important considerations include:
- Increased risk of new sinkhole formation by stormwater runoff
- Failure of stormwater practices and infrastructure due to sinkholes
- Greatly increased post-development runoff rates when land is paved
- Underground karst features are hard to detect and vary greatly over just a few feet
- Strong and often mysterious runoff/groundwater interaction make it hard to understand flow paths and drainage patterns
- Watersheds have lower stream density, losing streams and karst swales
- Polluted runoff greatly increases risk of groundwater contamination
- Need for special groundwater injection permits
- Changes in recharge or runoff quality can harm endangered species
CSN has just released a stormwater design supplement for karst to help engineers, plan reviewers, and public works officials make better stormwater decisions in karst terrain. Until now, the limited guidance available on this topic has been uneven, sometimes conflicting and certainly not comprehensive. An informal working group of nearly 20 karst experts has spent the last year developing a 30-page Technical Bulletin. The karst working group has gone through five drafts, and CSN invites comment and feedback on this Version 2.0. For more information on karst terrain and stormwater, view the CSN slideshow.
So what are the tricks needed to minimizing risk when developing land and managing stormwater in karst terrain? It begins with careful geotechnical assessment, site layout, stormwater planning for the site that follows the general principles below:
- Designers should perform the preliminary and detailed site investigations prior to beginning site and stormwater design to fully understand subsurface conditions, assess karst vulnerability and define the actual drainage pattern present at the site.
- Any existing sinkholes and karst swales should be surveyed and permanently recorded on the property deed or plat. In addition, an easement, buffer or reserve area should be identified on the development plat for the project so that all future landowners are aware of their presence.
- Minimize site disturbance and changes to soil profile, including cuts, fills, excavation and drainage alteration, near karst features.
- Sediment basins should only be used as a last resort after all other erosion and sediment control options have been considered and rejected. In the rare instance they are employed they should serve small drainage areas (2 acres or less) and be located away from known karst features.
- Minimize the amount of impervious cover created at the site so as to reduce the volume and velocity of stormwater runoff generated.
- Take advantage of subsurface conditions when locating building pads and place foundations on sound bedrock.
- The location of new or replacement septic systems near improved sinkholes may be regulated by the local public health authority. Many recommend that septic systems should be located at least 100 feet away of the base of an existing or remediated sinkhole.
- Designers should place a high priority on preserving as much of the length of natural karst swales present on the site to increase infiltration and accommodate flows from extreme storms.
- Treat runoff as sheetflow in a series of small runoff reduction practices before it becomes concentrated. Practices should be designed to disperse flows over the broadest area possible to avoid ponding, concentration or soil saturation.
- Small-scale low impact design (LID) practices work well in karst areas, although they should be shallow and sometimes use perforated under drains to prevent groundwater interaction. For example, micro-bioretention and infiltration practices are a key part of the treatment train.
- Distributed treatment is recommended over centralized stormwater facilities, which are defined as any practice that treats runoff from a contributing drainage area greater than 20,000 square feet IC, and/or has a surface ponding depth greater than three feet. Examples include wet ponds, dry extended detention (ED) ponds, and infiltration basins.
- The use of centralized stormwater practices with large drainage areas is strongly discouraged even when liners are used. Centralized treatment practices require more costly geotechnical investigations and design features than smaller, shallower distributed LID practices. In addition, distributed LID practices generally eliminate the need to obtain an underground injection permit.
- Designers must address both the flooding and water quality aspects of post development stormwater runoff. In most localities, the sequence of stormwater practices should have the capacity to safely handle or bypass the 2 and 10 year design storm.
- Designers should maintain both the quality and quantity of runoff to predevelopment levels and minimize rerouting of stormwater from existing drainage.
- As a general rule, the stormwater system should avoid large contributing areas, deep excavation or pools of standing water.
- The potential hotspot status of the proposed use of the development should be evaluated prior to design. If the site is defined as a stormwater hotspot, full water quality treatment shall be provided prior to any discharge to groundwater.
- When existing or new sinkholes are determined to require remediation, the repair will use appropriate techniques outlined in the Bulletin.
Under some circumstances, post-development stormwater must be discharged into an existing sinkhole or other karst feature. In these cases, special rules pertain:
- The sinkhole or karst feature receiving post development stormwater runoff shall be considered a Class V Injection Well.
- The designer should conduct a survey for public or private drinking water wells with a ¼ mile of their improved sinkhole, and submit data on any wells found to the UIC permit authority.
- As such, the designer must notify the appropriate agency that regulates groundwater and administers the UIC permit. An underground injection permit will be extremely difficult to obtain if the proposed land use or operation at the site is designated as a severe stormwater hotspot.
- It is strongly advised that a dye trace be performed to understand how additional stormwater flows will move through groundwater, particularly if wells are located nearby.
- The design goals are to prevent increased runoff volumes from discharging to the sinkhole, but to maintain the discharge of the predevelopment runoff volumes so as to maintain groundwater recharge.
- Designers should maintain both the quality and quantity of runoff to predevelopment levels prior to discharge into an existing sinkhole. Operationally, this means that designers must treat the full water quality volume in an acceptable runoff reduction practice before discharging to a sinkhole (i.e., full runoff reduction volume for runoff produced by one inch of rainfall over contributing impervious surfaces.