Better Construction Management Practices Could Reduce Silt-Laden Runoff
12/29/24 – Several rounds of intense rain in the past week triggered a slew of emails from concerned residents about the need for better construction management practices to control sediment-laden runoff.
Chris Summers, a retired commercial photographer, included pictures of the runoff from the Foster Elementary site in Kingwood’s Trailwood Village. Contractors recently cleared the site for reconstruction of the school. Summer’s photos illustrate the dangers from sediment pollution and the need for better construction management practices.
A Major Source of Sediment Pollution
Construction site runoff is a major source of sediment pollution. Construction typically involves large-scale disturbance of soil, which can significantly increase the amount of sediment carried away by stormwater runoff. This sediment-laden runoff can have serious environmental impacts, especially when it enters local water bodies, such as Lake Houston..
Clearing, excavating and grading disturb soil. Stripping away vegetation makes soil more prone to erosion. So, when it rains, the exposed soil can easily wash away into nearby water systems.
In many cases, vegetation or other stabilization measures (e.g., erosion control blankets, mulching) may not be installed until later stages of construction, leaving the site vulnerable to sediment loss in the meantime.
Construction sites are most vulnerable during heavy rainfalls, which can lead to significant runoff, especially from areas without adequate sediment-control measures.
In many cases, construction schedules are not aligned with seasonal weather patterns, so sites may be left exposed during periods of heavy rain. This can lead to water-quality degradation, increased costs for water treatment, higher water bills, and infrastructure damage.
Excessive sediment can clog stormwater systems, leading to flooding and costly repairs.
Regulations and Best Management Practices
Due to the significant environmental impact of construction-site runoff, many countries and regions have strict regulations and Best Management Practices (BMPs) to control it. In the U.S., for example, the Clean Water Act requires construction sites to control stormwater discharges through permits such as the National Pollutant Discharge Elimination System (NPDES) permit. This often includes implementing sediment control measures like silt fences, sediment basins, and erosion control methods.
Rainfall and Soil Saturation in Last Week
The HCFCD gage at West Lake Houston Parkway and the San Jacinto West Fork shows that the area near Foster Elementary received about 2 inches of rain in the last few days.
That made the soil go from “wet” to “saturated.”
And that radically increased runoff.
Photos of Foster Elementary Construction Site After Heavy Rains
Summers’ photos show the need for better construction management practices to control sediment-laden runoff.
Silty water streamed out of the site after an intense rain.
The flow was steady and strong.
Water exited the site in multiple places and filled half the street.
Inlets had no protection. (See #8 below).
The site had large areas of disturbed and unprotected soil.
Runoff left inches of soil in the street. Imagine what the storm sewer looked like…
…especially since storm-sewer inlets were unprotected by any of the usual methods. (Again, see #8 below.)
Ways to Reduce Sediment-Laden Runoff
Here are several common and effective methods used to minimize silt-laden runoff. They represent best construction management practices.
1. Silt Fencing intercepts and slows down water flow, allowing sediment to settle before water flows off the construction site. It’s normally installed around the perimeter.
2. Sediment Basins & Traps capture sediment from runoff until it settles out of suspension.
3. Erosion Control Mats or Blankets stabilize disturbed soil and prevent erosion. They are most often used during the early stages of construction.
4. Mulching with organic materials – such as straw, hay, or wood chips – helps to protect soil while promoting vegetation growth. They are most effective when applied immediately after soil is disturbed.
5. Planting Vegetative Cover, such as fast-growing grass, can also help stabilize soil and reduce runoff. Plant roots bind the soil, making it less susceptible to erosion. Vegetation also slows down the flow of water, reducing the energy available to carry away sediment.
6. Sediment Control Logs (Wattles) are tubular barriers filled with straw or other materials. When placed along the contours of the construction site, they capture sediment before it leaves the site.
7. Stormwater Diversion Channels direct runoff to sediment-control ponds or a controlled discharge areas.
8. Stormwater-Inlet Protection, such as sandbags, can be placed around stormwater inlets to capture sediment before it enters the storm drainage system.
9. Shallow temporary or Permanent Vegetative Swales collect runoff and allow it to slowly infiltrate into the soil while trapping sediment in the vegetation and soil.
10. Construction Entrance/Exit Stabilization with gravel minimizes the tracking of mud onto roads from construction vehicles. But the gravel must be regularly maintained.
11. Grading can help control water flow and minimize erosion by directing water flow away from disturbed areas towards sediment control features.
12. Geotextiles can also provide erosion control and soil stabilization.
Need for Regular Inspections
Regular inspections and maintenance ensure that all erosion and sediment control measures are installed functioning properly.
Compare the measures in this list to the photos above. With the exception of gravel at the entrance, Summers’ photos show that few of these measures appeared to be implemented at the Foster construction site.
Posted by Bob Rehak on 12/29/24 with photos by Chris Summers
2679 Days since Hurricane Harvey
The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.
