Sand-Mine Structural Control BMPs Could Make Difference in Next Flood

In an effort to reduce sediment escaping into the San Jacinto River from sand mines, the Texas Commission on Environmental Quality (TCEQ) has issued Draft Guidelines on Best Management Practices (BMPs). Yesterday, I discussed vegetative controls for erosion. Today, I will discuss structural control. The two types often work together or in sequence.

At the very end of this post, I make some recommendations to strengthen BMPs, and describe how to submit public comments.

Structural Controls

Structural controls do several things:

Divert runoff away from disturbed areas
Reduce runoff velocities
Filter sediment
Remove sediment by ponding.

They include the following.

Temporary Structures (Section 2.2.1)

Installed before and during construction. After removing temporary stormwater controls the areas disturbed by the temporary structures must be revegetated.

Permanent Structures (Section 2.2.2)

Permanent structures remaining after construction. Once construction of areas outside of the sand-mining pit has ceased, permanent structural control BMPs must be implemented and operational.

Diversion Ridges, Berms or Channels of Stabilized Soil (Section 2.2.3)

These divert runoff into “sediment basins.” If they remain in place more than 30 days, they must be covered with temporary or permanent vegetation. Maximum allowable drainage area is five acres.

Silt Fences (Section 2.2.4)

Silt fences capture sediment from sheet flow. Six to eight inches of the fence material must be buried in a trench about four inches deep and four inches wide. Silt fences that are not buried have no useful function. They must never be installed across streams. Fencing must be removed when sediment deposits reach one-half the fence height.

Straw Bales (Section 2.2.4 Continued)

Can also be used as sediment barriers in small areas. Maximum grade: 3:1. Water depth must not exceed one foot at any point. Bales with bindings must be entrenched a minimum of four inches and anchored with stakes. Straw bales that are not buried are improperly installed.

Sediment Basins (Section 2.2.5)

Allow retention of sediment “prior to discharge” or recycling. Side slopes must be 2:1 or less. Sediment must be removed when the volume has been reduced to 27 cubic yards per acre of drainage area. Dikes must be well compacted and vegetated. Installed prior to construction but not in flowing streams. Use diversions to direct drainage to basins.

*Better structural controls might have prevented a sand mine upstream from discharging 56,000 million gallons of white sludge into the West Fork in 2019.*

Riprap Outlet Protection (Section 2.2.6)

Riprap outlet protection must be placed at the outlet end of culverts or channels to reduce the depth, velocity, and energy of water so that the flow will not erode the receiving stream.

Check Dams (Section 2.2.7)

Small dams across swales or drainage ditches that reduce flow velocity and erosion. Not used in flowing streams. Maximum height: two feet. Center must be at least six inches lower than the outer edges to prevent erosion around the edges. The maximum spacing between dams must be such that the toe of the upstream dam is at the same elevation as the top of the downstream dam.

Accumulated sediment must be removed from behind the check dams when it reaches one half the dam height. Erosion around dam edges must be corrected immediately, ensuring that the dam center is six inches lower than the edges.

Construction Entrance/Exits (Section 2.2.8)

Aggregate must stabilize entrances and exits to reduce sediment tracked onto public roads. Aggregate must be at least six inches thick and 50 feet long. Tire washing may also be needed.

Housekeeping Practices (Section 2.2.9)

Petroleum products, paints, solvents, litter, debris, sanitary waste, and sediment from unstabilized areas, TCEQ BMPs specify:

Designated areas for equipment maintenance and repair;
Waste receptacles at convenient locations;
Regular collection of waste;
Protected storage areas for chemicals, paints, solvents, fertilizers, and other potentially toxic or hazardous materials; and
Adequately maintained sanitary facilities.

Post-Construction/Stormwater-Management Measures (Section 2.2.10)

Control measures must be installed to control pollutants in stormwater after construction is complete. These controls include, but are not limited to:

Retention ponds. Minimum volume is the first inch or half inch of stormwater runoff containing the first flush of pollutants.
Vegetated Swales and Natural Depressions. There are grass-lined areas that filter sediments from runoff, thus helping to prevent erosion. Vegetated swales must have side slopes of 4:1 or less.

General

As with vegetative controls, operators must inspect structural controls once every seven (7) calendar days. That includes controls in areas used for storage of materials; maintenance areas; plus site entrances and exits.

Operators must replace or modify ineffective or damaged structural controls “in a timely manner, but no later than the next anticipated storm event.”

Recommendations for Public Comment Structural Controls

As we saw in yesterday’s post on vegetative controls, I have often seen gaps between real and ideal. However, for this post, I realized in looking back through thousands of aerial photos today, that I have never photographed one:

Stabilized channel
Silt fence
Straw bale
Check dam
Outlet stabilized by riprap or
Vegetated swale

…on a sand mine site. Period. Let alone one that met these requirements.

I’m not saying they don’t exist. I’m just saying that I’ve never seen them on the days I flew over.

Maybe operators feel they don’t need them. Or maybe they’re just not using them for other reasons.

So once again, I recommend that you write the TCEQ and ask them to put teeth into their BMP requirements.

In addition, I recommend you request:

Stronger wording on the general requirement to fix damaged or ineffective structural controls in a “timely manner.” That’s just too subjective. It lets operators defer maintenance way past the point it may be needed.
Clarification on “prior to discharging” in section 2.2.5. Are operators capturing sediment only to discharge it into the river at a later time? What do they mean by discharge? Where?
Detention ponds big enough to catch an inch of rain in an area where Atlas 14 requirements specify 16.9 inches of rain in 24 hours? (Section 2.2.10) That seems wholly inadequate. Harris County Flood Control District recommends minimum detention volumes for developments at .65 acre feet per acre for areas up to 640 acres. That’s about 8 inches of rainfall.

Please submit your thoughts on structural control and other BMPs to the TCEQ by emailing Macayla.Coleman@Tceq.Texas.gov with the subject line “BMPs Guidance Document” before August 19, 2021.

The house you save could be your own.

Posted by Bob Rehak on 8/13/2021

1445 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.