Minimum Control Measure #5 Post-Construction Stormwater Management

KEARNEY STORMWATER MANAGEMENT PLAN

MCM #5 Post-Construction Stormwater Management

Construction Bulletin April 2014

Goals for Construction Site Runoff Management

Ahh, Spring— warmer weather, thawing ground, and afternoon rain showers.   While welcoming the change of season,  it’s also time to consider Best Management Practices (BMPs) for keeping the soil on site and preventing stormwater run off and sediment pollution.

The ultimate goal for construction site runoff management is to prevent the pollution of stormwater runoff.  Best Management Practices (BMPs) aim to slow the velocity, control the volume, and/or  filter  site run– off.  Stormwater permits require BMPs to address erosion and sediment run-off, soil exposure, ground disturbance, compaction,  buffers,  outlet protection, and stabilization.  Below are some BMPs to consider for construction sites.

Grass

Stabilization, or planting ground cover,  allows run off to infiltrate the ground providing nutrients to the plants and replenishing ground water. Stabilization is required by the Nebraska general construction permit as soon as practicable on sites and no more than 14 days after construction activities have ceased.

 

Slope Drain

When the slope is steep channeling the runoff through a slope drain can be an effective erosion control.  Drains may be made of pipe,  s shown, or a constructed channel lined with rock,  turf replacement mats,  and wattles to slow the flow of water.

 

Flexible Gutter

Flexible rain gutters can direct roof water away from exposed soil.  The gutters can channel water to impermeable areas (e.g. concrete driveways) where clean stormwater can run to the gutter or to vegetated areas where plants and soil can absorb the water

InspectionRegular inspection,  every 14 days and within 24 hours of a 1/2” rain event,  is the best way to insure construction site BMPs are working effectively.  Look for evidence, or  potential,  of pollutants entering the drainage system.  An inspection report must be identify any incidents of non-compliance with permit conditions and actions taken to correct the issue.  If no incidents of non-compliance were found, the report must contain a certification that the site is in compliance with the Storm Water Pollution Prevention Plan (SWPPP). The  reports should be retained with the SWPPP for up to three years after the permit expires or is terminated.

 

 

Construction BMPs: Hazardous Materials Storage

Many materials used on construction sites can be classified as hazardous materials.  Some of the most common include fuel, oil, paint, concrete curing compounds, asphalt products, pesticides, herbicides, and septic wastes. The proper storage and handling of these materials is essential to good stormwater management. Every SWPPP should include procedures for hazardous material handling and storage as well as procedures for spill response and reporting.  For more information about both these topics, see below: Continue reading Construction BMPs: Hazardous Materials Storage

Construction BMPs: Temporary Stabilization

This straw mulch has been crimped into the ground for temporary stabilization.

Stabilization, or establishing ground cover to protect disturbed soils from erosion, is not only a good practice, it is required by law.  The Nebraska state Construction General Permit states that, with a few exceptions such as snow cover or frozen ground conditions, “stabilization measures  must be initiated as soon as practicable in portions of the site where construction activities have temporarily or permanently ceased, but in no case more than 14 days after the construction activity in that portion of the site has temporarily or permanently ceased.” Since it is unlikely that suitable vegetation can be established within 14 days, other methods of stabilization must be considered.  Covering exposed or disturbed areas protects the soil from raindrop impact, slows the flow of and infiltrates stormwater, and protects newly seeded areas.  It also helps retain soil moisture, which will help your vegetation become established more quickly.

One of the simplest methods of erosion control is mulching.  Straw or hay mulch should be applied at a rate of 2-3 tons per acre.  To provide good ground cover, at least 50% of the mulch should be ten inches in length or longer.  The mulch should be crimped using a weighted roller that anchors the mulch into the soil.  If the area is seeded before the mulch is applied, this can be a very simple and effective method of preventing erosion while vegetation is established.

Another method of erosion control is a rolled erosion control blanket.  These can be made from natural or synthetic materials and can be effective in protecting steeper slopes from erosion.  They are designed to be rolled onto the area and stapled into place.  Always follow installation specifications, as poor installation can cause these products to fail. 

This erosion contol blanket temporarily stabilizes the slope while vegetation is established.

If the blanket is not properly anchored and stapled, it can either be washed away or stormwater can wash out soil underneath the blanket, causing small gullies that are difficult to seed.  If the area is seeded and the erosion control blanket is installed correctly, grass will grow up through the blanket, and over time, the blanket will degrade.

For extremely steep slopes or areas with limited access, hydraulic mulching should be considered.  In this process, a slurry made up of mulch, seed, and a tackifying agent is sprayed onto the disturbed area.  There are also many other products and stabilization methods available.  Each site should be carefully evaluated to determine which product or combination of products is the most effective and economical way to achieve stabilization. 

 

 

 

 

Construction BMPs: Concrete Washout

This is an effective, inexpensive concrete washout.

For obvious reasons concrete trucks should never be washed out into the street or into the storm drain.  Solids that are washed out of the concrete trucks can clog storm drains, causing flooding and expensive clean-up.  However, it is also important to contain the concrete wash water as well.  The wash water is very alkaline, which means it has a very high pH.  Water must have a pH in the range of 6.5-9.0 in order for aquatic life to survive.  Concrete wash water typically has a pH of 12 or above.  Furthermore, the pH scale is logarithmic, meaning that a pH of 12 is 10 times greater than a pH of 11, 100 times greater than a pH of 10, and 1000 times greater than a pH of 9.0, which is the highest level of alkalinity that aquatic life can tolerate.  If allowed to escape the site, concrete wash water can have a severe effect on our streams and rivers.

Other significant pollutants in concrete wash water are heavy metals such as chromium.  If wash water that is high in heavy metals leaches through the soil to the water table, it can contaminate our groundwater, which is where most cities in Nebraska get all of their drinking water.

Concrete washout is also high in suspended and dissolved solids.  The average stream or river in the United States typically has a suspended solid count of no more than 60 parts per million (ppm).  The average suspended solids in concrete wash water is 27,000 ppm, well above the range at which aquatic life can survive.

Luckily, it is not very difficult to control concrete washout on your construction site.  There are several different ways to build a concrete washout facility, the most simple being to dig a pit and line it with plastic sheeting that is at least 10-mil thick.  If you don’t have a good location on your site for a facility like this, you can also build a portable facility by building a box with a liner to contain the washout.  There are also several products available for purchase that are effective, portable concrete washout containers.  

Always locate concrete washouts as far away from storm drains as possible.  Make sure they are clearly labeled and that drivers know where they are supposed to wash out.  By doing this, we can make sure that one of the biggest pollutants from construction sites does not reach our waterways.