Temporal scales of concern for impacts from sedimentation.

ERDC TN-DOER-E19

March 2005

It appears that the consensus is that a combination of mass and some form of bulk density may

be the most widely useful characterizations, but that grain size and permeability may have

important implications. Even the apparently simplest variable, thickness, may be quite difficult

to measure in situ (see below) at the lower end of the ambient range (0.1-0.3 cm).

Temporal scales of concern for impacts from sedimentation. Temporal scales of concern can

be seen at several levels: duration of external event (days), timing of external events (seasons),

persistence of the effects of external events (weeks). Taking each in turn, we can provide some

boundaries for field and laboratory investigations. Duration of external event: assuming that

sedimentation results from dredging or disposal events, dredging operations would likely move

past an area of potential impact in 1-5 days, disposal operations would potentially last longer

with episodic plumes or density currents depositing fresh layers of sediment. The former would

approximate the duration of storm-induced disturbance, whereas the latter might approach

chronic, recurring frequency during one or more seasons. Timing of external events: the scale of

concern for timing of external events is closely related to the key seasonal events of reproduction

and critical life stages or presence of organisms in the vicinity of the event(s). Persistence of

effects of external events: in most sediment systems, newly settled sediment is subjected to

biological and physical mixing to a degree immediately after placement. How quickly bedded

sediments are incorporated into ambient sediments or re-transported is highly dependent on

hydrodynamics and the ambient biological community. In some habitats, the introduction of fine

sediments (for example, into coarse sands or gravel) might induce settlement and colonization of

new populations altering the community and potentially further affecting species of concern or

mediating the effects of the sedimentation events. Regardless, the time constant for assimilation

of bedded sediments into some level of equilibrium is likely to be on the order of weeks. If silts

become "embedded" into coarse sands or gravels, they may become resistant to erosion (it is

harder to resuspend a fine cohesive particle [consolidated] than a larger grain size noncohesive

particle).

Minimum length scale for time of sediment accumulation. The intent of this question was to

understand what might be the shortest (most ephemeral) sedimentation event of concern. Based

on responses received, events of hours or days could create impacts, but assessment of impacts

should consider periods longer than one day (suggested time is 3-5 days).

Most sensitive biological resources to evaluate. It is important to note that eggs and larvae of

estuarine and marine species typically suffer very high natural mortalities. While preventable

sources of mortality are not welcome or meaningless, they must be of sufficient scale to have a

measurable effect on population size or locally significant recruitment to overcome costs or

effects of prevention. Assessing effects on population size will likely require modeling rather

than observation. The following refined list focuses on direct effects of bedded sediments,

recognizing that water column effects are also significant but outside the scope of this report.

Eggs of benthic fishes that fail to attach, grow, or hatch. This may be the most sensitive

resource, and it would be a very high priority if sedimentation at a site had the potential to

cause the loss of an entire year class.