Sediment traps, two-basin configurations, PAMs, and other strategies

By Richard A. McLaughlin

The Balsam Mountain Preserve development is a luxury residential community on a 4,400-acre tract in the Little Tennessee River Basin in Jackson County, NC. The tract had been previously owned by Champion Paper Co. and had been managed for wood production. The development plan includes 350 home sites of 2 acres each, an 18-hole golf course, a practice range, stables with several pastures, and several other amenities. More than 3,000 acres were set aside as the Balsam Mountain Trust, which will be preserved as a conservation easement for recreational use by residents. The preserve has 38 miles of streams, 15 miles of which are considered permanent. The development occurs on the western side of the tract with most potential impacts occurring on Sugarloaf Creek and Cashie Branch.

The purpose of this project was to work closely with the preserve developer to install and evaluate a variety of erosion and sediment control systems to reduce the movement of sediment into surface waters. Workshops to introduce these practices to developers, contractors, and public agency staff were held periodically as the systems were tested and evaluated. Three sites were monitored for portions of the project time period, and the results are discussed by site.

Sports Garden Site
This 6-acre area was cleared and grading was initiated in early 2003. Steep slopes had to be graded to provide level areas for tennis and other amenities. Two sediment basins were installed in series to detain runoff. The first basin had several configurations (Figure 1) before the final construction with a perforated riser and stone outlet (Figure 2 ). A sampler was first installed in the riser barrel, but it was discovered that the infiltration rate in the basin exceeded the runoff inflow rate in most storms so the water never rose to the barrel elevation. The sampler was then moved to the deepest point in the basin to provide the greatest chance of obtaining samples (Figure 3 ). However, this sampler proved very unreliable and tended to obtain samples during periods when no flow was occurring. As a result, many of the samples had very low turbidities because the sediment on Balsam Mountain tended to settle fairly quickly after each storm.

The site was stabilized with grass in May 2003 as further grading was postponed, although the channels that carried runoff to the basin were not stabilized so erosion in them continued.

The water exiting the upper basin flowed down a steep, rock-lined ditch to a culvert under a gravel road and into a second, much smaller basin (Figure 4 ). This basin had a permanent pool of 2 to 3 feet and a rock spillway. Some of the water entering this basin came from the roadside ditch above the culvert. We installed a V-notch weir in the spillway crest in order to measure flows out of the basin. Two samplers were also installed, one at the culvert and one at the outlet. A logging rain gauge was installed adjacent to the basin.

This two-basin design, with most of the heavy sediment being removed in the first basin, facilitates the use of polyacrylamide (PAM) in the second basin. This was attempted initially by inserting PAM logs into the culvert pipe under the road; however, this proved problematic in that the reduced flow capacity in the pipe backed water up over the road. The two logs were removed from the pipe and placed on the rock splash pad below the pipe outlet.

The sampler in the upper basin obtained samples from four storms in the May through July 2003 period. The first three resulted in very low turbidities and probably reflect low runoff amounts (Figure 4). The last storm sampled had very high turbidities, occasionally exceeding 8,000 nephelometric turbidity units (NTU). Rainfall records from nearby Cullowhee indicate nearly 3 inches of rain over the three-day period of July 1 through 3. The average turbidity was 5,600 NTU for this last storm, compared to around 400 NTU for the previous two storms. It is possible that the intake of the sampler was too close to the basin bottom and was sampling sediment settling. When these samples were allowed to settle overnight in the lab, the average turbidity was less than 30 NTU.

Erosion was evident on the site until grass was established in the late summer of 2003 (Figure 5 ). In addition, water conveyances continued to erode even after the site was fully vegetated (Figure 5 and Figure 6 ). This is common on active construction sites, and one recommendation would be to encourage the use of inexpensive materials in water conveyances to reduce erosion.

Average turbidity for the May 5 through 9, 2003, event at the entrance to the sports garden was 911 NTU, while for the exit it was 186 NTU (Figure 15). This indicates that there was some treatment during this period, possibly as a result of the PAM logs, because flow was relatively low. Higher turbidities in later storms with higher flows indicate that the PAM logs were ineffective and that the second basin had sufficient turbulence to prevent significant settling (example in Figure 16). Turbidity in these samples usually dropped two orders of magnitude when left on the lab bench overnight, indicating that velocity and turbulence were too high in the small basin to settle material efficiently.

Overall, we would recommend that frequent, heavy use of mulches and vegetation be used on sites similar to the Sports Garden site because the steep slopes can erode quickly as the grading progresses. Ditches should be stabilized using check dams and lining with geotextiles. Sediment control at the Sports Garden site was problematic due to the difficulty in creating enough sediment basin area on this steep slope. The installation of the two-basin system worked well for smaller storms, but turbidities were still high during high-flow rates.

Stables Site
The stables and associated pasture encompassed 5 acres of clearing adjacent to Cashie Branch. All of the site was graded to drain away from the creek and into a 4,000-cubic-foot sediment basin. We recommended the use of a flashboard riser outlet, which was installed in March 2003 (Figure 7 ). This was installed in a manner similar to a perforated riser, placed away from the dam wall and with a stone collar due to the contractor’s unfamiliarity with this type of outlet. More specific instructions to install it closer to the dam wall and anchored to the bottom with stakes or cement were needed. The purpose of the flashboard riser is to allow the formation of a permanent pool while retaining the ability to drain the basin if needed to remove sediment.

Most of the area was well stabilized with grass and mulch when grading was complete, but a fill area for the stables continued to contribute sediment to runoff for several months (Figure 8 ). The first attempt to reduce this impact was to install a small sediment trap below the disturbed area (Figure 9 ). While this did retain a large amount of the sediment, the water was then released on the pasture, creating more erosion (see Figure 8 ). The sediment trap was reconfigured as a skimmer basin with the outlet connected to the stabilized, rock-lined ditch that led to the sediment basin (Figure 10).

(Figure 11)

The first recorded rain event (February 22, 2003) had the highest turbidities, all greater than 1,000 NTU. This event occurred prior to the installation of baffles in the basin and before seeded grass had time to emerge. Jute/coir baffles spread the flow of water across the whole basin decreasing the turbulence and velocity of the water, and optimizing the settling potential of the basin (Figure 12). All subsequent runoff events had turbidities less than 1,000 NTU. Sediment loads leaving the site were calculated using total suspended solids (TSS) measurements from the collected samples. The February 22 rain event had a sediment load of 1,940 kilograms in approximately five hours, higher than a later rain event (May 5 through 9, 2003) of longer duration and higher peak exit flows (2 to 3 cubic feet per second, versus 1 to 2 cubic feet per second for February 22), which lost 354 kilograms. Turbidities fluctuated with flow, averaging 220 NTU.

This site went through a typical series of runoff events. The first events after clearing and grading had high sediment loads, but as the site became more stabilized the runoff water quality improved greatly. The primary sediment basin appeared to function much better with the rock checks and porous baffles that were added. The attempts to reduce turbidity by introducing PAM logs in the ditches were not successful because the logs tended to either get buried under sediment or dry out and become inactive. There is evidence that PAM logs were somewhat effective in the flashboard riser barrel, where they remained moist and did not accumulate sediment.

Practice Range Site
The Practice Range site is approximately 20 acres, which were cleared in the summer of 2002 (Figure 13). Runoff coming on to the site was redirected into drainage pipes to a point below the cleared area. A number of temporary sediment traps were placed around the site during the initial grading. These tended to have very high infiltration rates. During one storm, we witnessed water running into these traps, which were essentially excavated pits, with no water leaving them and no visible rise in water level. The site is apparently underlain by a landslide debris field and as a result has very high infiltration rates.

The final design consisted of three basins (B1, B2, and B3), one emptying into the next before water exits the site near Sugarloaf Creek. Automatic samplers were placed at the entrance to B1, at the exit of B1 (entrance to B2), and at the exit of B3, along with several single-stage samplers.

The first challenge was the inlet channel into B1, which collected runoff from the disturbed area above the basins. It emptied into the basin near the outlet, allowing the runoff to directly exit the basin with no settling time for the sediment. Silt fence and coir log diversions were installed to guide the water toward the front of the basin to allow full use of the basin and two jute/coir baffles. The outlet for B1 was a flashboard riser leading into B2, which also contained two jute/coir baffles. The final basin, B3, had one baffle. Two manufactured channel check dams, GeoRidge (Nilex Inc.) and Triangular Silt Dike (Triangular Silt Dike Co. Inc.), were placed in the channel leading into B1, while geotextile material was used under the rocks in the spillway between B1 and B2. PAM logs (APS 706) were placed in the flashboard riser to treat the turbid water as it passed into B2.

On July 25, 2004, the area received more than 6 inches of rain in approximately 8 hours. As the channel leading into B1 eroded, the sampler and rain gauge were washed out, not collecting any entrance samples. The first basin filled with sediment, burying the baffles (Figure 14 and Figure 17), and B2 also filled with sediment to the top of the baffles (approximately 4 feet high). Much of the rock in the spillway between B1 and B2 was displaced, but the underlying geotextile material prevented additional erosion from occurring. The sampler at the exit of the site below B3 recorded turbidities of 80 to 260 NTU, suggesting the majority of the eroded sediment was kept onsite. Following this event, site maintenance included digging out the deposited sediment; constructing a new channel, which was re-routed to enter B1 at the front end (farthest away from the outlet); and replacing baffles.

September had three hurricanes pass through the area, dumping more than 12 inches of rain on the site for the month. Turbidity data collected from the September 7 through 9 event shows an increase in turbidity from the entrance of B1 (range 380 to 5,940, average 1,900 NTU) to the exit of B1 (range 130 to 10,700, average 3,390 NTU). This probably does not accurately represent the entire event, as the entrance sampler collected only eight samples before the intake line was buried beneath sediment. Turbidities leaving the site ranged from 180 to 1,800 NTU and averaged 420 NTU with a calculated sediment load of 1,006 kilograms from the TSS measurements.

Through the fall of 2004 the site continued to have grading on the middle to upper slopes. Most of the runoff was still being directed into the first basin using the diversion ditch. However, this ditch had eroded badly due to the heavy rains in September and is probably the source of much of the sediment now filling the first basin (Figure 18). We recommended that this ditch be reworked and lined with geotextile because it appeared it would be in place for some time to come.

A ground cover demonstration was completed, including pine needles and an excelsior erosion control blanket (Curlex from American Excelsior Co.) in comparison to the tub grinder mulch and hydraulically applied wood fiber used onsite (Figure 19 ). Although we did not collect extensive data from these plots, we did observe that runoff collected in the tubs below plots treated with PAM at 20 pounds per acre was noticeably clearer.

Richard A. McLaughlin, Ph.D., is an associate professor in the Soil Science Department at North Carolina State University.

EC - November/December 2005