Ray Bergeron Responds to EPA Hudson River Cleanup Project Peer Review
On August 16, The Environmental Protection Agency Region 2 posted a draft peer review of the Hudson River Superfund cleanup project in its Web site.
A consultant connected with the project asked Ray Bergeron, president of Cable Arm Clamshell, to comment on several paragraphs in the review. Bergeron has released the comments to International Dredging Review. (See Hudson River Trustees Urge Continued Dredging by David Murray, IDR, July/August 2010)
From the consultant: “I would welcome your professional opinion as an experienced dredger.”
Sequence Dredging Bank to Bank and from Upstream to Downstream
Peer review: The Phase 1 report shows cases where a single dredge lane was advanced downstream as far as 600 to 800 feet in the direction of flow. This longitudinal approach creates a thalweg effect that can increase local river flow velocities, and can contribute to re-suspension and release by eroding the cut’s side slopes and bed. Vessels passing through such a cut also have the potential to accelerate re-suspension via slipstream and prop wash velocities.
Erosion and sloughing in the cut area increase re-suspension of PCBs into the water column and downriver. This condition is exacerbated during higher river flow conditions. Modifying the dredging sequence and monitoring the effects will optimize productivity while reducing generated residuals and re-suspension by erosion of residuals within the cut area, and encouraging deposition within the CU instead of downstream of the CU.
Bergeron: I agree with the above.
Peer review: Instead of dredging long downstream longitudinal lanes, dredging should be short cross-stream lanes dredged from bank-to-bank, then upstream to downstream. Dredging should target one acre CU sub-units (or another appropriately sized sub-unit) that are designed to limit creation of a thalweg-like channel. The sub-unit should be dredged from bank to bank or in predefined areas not necessarily aligned with the direction of water flow. To accelerate closure, and to the extent reasonably practicable, each dredged sub-unit should be surveyed, sampled, and covered while dredging on adjacent or downstream subunits proceeds. Final backfilling or capping should occur strictly from upstream to downstream.
Bergeron: There are two types of residuals in clamshell dredging - residuals created from the settling of sediment re-suspended by the dredging process, and sediment that was left in place by either failing to overlap the bucket footprint adequately or cut slope failure allowing material to fall back into a previously excavated area. Dredging from upstream to downstream at least partially solves the problem of residuals from re-suspension but does nothing about the problem of not picking up the material in the first place. Equipment selection and dredging techniques have a significant impact on the problem. Buckets with long, narrow footprints make it difficult to maintain the proper overlap and ensure complete coverage of the dredge area. Digging deep cuts to full depth then moving the dredge forward while digging deep cuts causes slope failures, stirs the bottom, and makes it possible for material to escape capture by the bucket. Taking material down in layers across the full dredging area is slower and more difficult but does a much better job of ensuring complete removal. If it is necessary to dig a deep face for productivity reasons, then two passes should be made over the area. The first pass should take the material down to within one bucket cut of the target depth, then a cleanup pass of one bucket thickness should be made over the area.
Peer review: The Panel recognizes that dredging will have to occur simultaneously at multiple locations along the river, making strict adherence to an upstream to downstream requirement impossible. For this reason, the BMP may involve dredging in multiple CUs located upstream and downstream of each other, as long as the final backfilling or capping is completed sequentially from upstream to downstream. Because Phase 1 indicates that residuals likely contribute to re-suspension and release from upstream to downstream, the immediate placement of a three to six-inch coarse sand layer will control near-term releases, buying time within a single dredge season to complete the final backfill and capping from upstream to downstream. All backfilling and capping must be completed before dredging terminates at the end of each year.
Bergeron: Digging at multiple locations usually causes problems with achieving cleanup goals at the downstream sites. If digging at multiple locations is necessary, then a two pass approach can be beneficial. All the sites can work initially, removing material down to a depth that leaves one bucket pass in place. Then the final pass achieving target depth can be made beginning with the most upstream site and progressing in a downstream direction. This approach allows the bulk excavation to proceed to minimize schedule delays but allows the final cleanup pass to move from upstream to downstream, minimizing the possibility of recontaminating an area with resuspended material.