about south river federation

After discussion at the public meeting on February 1, 2017 and careful review of the TMDL for sediment in the South River, the South River Federation (SRF) submits the following comments:

1. Edge of Stream Calculations - SRF questions the ability of the CBP P5.3.2 watershed model (CBP Model) to accurately capture watershed characteristics in the South River watershed. For example, the TMDL calculates sediment loading as edge-of-stream (EOS) loads, which re defines as the "loads that enter the modeled river reached... [and] represent not only the erosion from the land but all of the intervening processes of deposition on hillsides and sediment transport through smaller rivers and streams." (TMDL 13). While this definition appears to account for sediment loading from in-stream channel erosion, the equation used to calculate EOS loads does not. 

The EOS equation 2.1 reads:  EQ21 2 

This determines EOS load by adding acreage of a given land use, plus an edge of field erosion rate for that land use, plus a sediment delivery factor for that land use. However, in-stream area, or stream channel area, is not an included land use in the TMDL. So, while the narrative definition means, "sediment transport through smaller rivers and streams" the actual equation used to arrive at the EOS loading rates does not seem able to account for those loadings. Moreover, the narrative definition does not mention sediment delivery in the modeled river reaches, which are likely significant, given the 100cfs or greater flow found in these reaches. Clarification of how the CBP model accounts for in-stream erosion and sediment delivery is needed before an accurate baseline loading can be determines, and before the effect of and BMPs can be applied to the modeled EOS loads. 

2. Reference Watershed Approach - The Department arrived at the TMDL target loading of 1,546 tons/year by utilizing the CBP model and conditions in seven reference watersheds to determine what level of sediment the South River's non-tidal reaches can accommodate without adverse impact to benthic and fish communities. (TMDL 22-25). In order to account for variation between the reference watershed and the conditions in the South River, the Department developed a forest normalized sediment load, which is a representation of "how many times greater the current watershed sediment load is than the all forested sediment load" (TMDL 22). The all forested sediment load is a modeled load of sediment that would reach the South River if its entire watershed was forested. (This is hypothetical all-forested watershed has its own modeling challenges, as will be discussed infra). The "forested normalized sediment load for this TMDL is calculated as the current watershed sediment load divided by the all forested sediment load" (id). When considering the reference watersheds, "[t]he median and 27th percentile of the reference watershed forested normalized sediment loads [are] (also referred to as the sediment loading threshold)1. So, a forested normalized sediment load requires an accurate current watershed sediment load, which is problematic considering the failure to account for in-stream erosional sediment loading as discussed supra.This computational shortcoming extends to the basic TMDL equation, expressed as: 

EQTMDL2Thus, even assuming the accuracy of the CBP Model's mathematics, if the data inputs are flawed, those flaws will affect all calculations made under the model and may artificially lower or raise TMDL targets. SRF urges the Department to supplement the modeling figures expressed in the TMDL with actual measurements int he field as soon as possible to determine how well the CBP model serves the South River watershed, and make any modifications such measurements indicate are appropriate.

3. Anecdotal Sediment Loading Accounts - In addition to the challenges to the CBP Model describes above, SRF is aware of several specific items we hope the Department will consider:

  • The upper Patuxent Watershed (56,446 acres) is immediately adjacent to the South River watershed (36,200 acres) and shares similar land use patterns and soil conditions. Each is almost entirely located in the Coastal Plain geologic province, and each is composed primarily of Group B soils (Upper Patuxent - 47%; South - 65.4%). However, the Upper Patuxent has a TMDL of 66,421.1 tons/year, while the South River TMDL is 1,982 tons/year. Considering only area of the watershed, a reasonable inference is that the Upper Patuxent would have a sediment load a bit less than double that of the South River, not 33 times as much.
  • Anne Arundel County's Site Assessment Report for its Glebe Branch Tributary project (submitted last August) estimates bank erosion rates at 0.66 tons/yr/ft based on Rosgen BANCS methodology and the North Carolina BEHI erosion rate curves, with a total of 547 tons/yr of predicted erosion from the entire tributary. This is but one of fifteen major tributaries on the River, but would account for over 1/4 of the total sediment load predicted by the TMDL.
  • The CBP model inputs for forested land cover across the entire Chesapeake watersheds is unable to account for heavy historical agricultural land use has left highly erodible soil at the top of our watershed. Bacon Ridge Branch is so named because of intensive hog farming in the area for almost a century. The animals ground the soil into dust, and while there is a 50 year old forest there now, the soil underlying it is much finer than typical forest soil substrate. 

 

SRF offers these comments to help the Department make any refinements possible when finalizing this TMDL to more accurately reflect the facts on the ground and ensure that BMPs implemented to reduce sediment loading below 1,546 tons/year are successful.