Fort Folly Habitat Recovery

Geomorphic Assessment

published 30 Jan 2018 by FFHR in News category with 0 comments

Geomorphic Assessment

The Pollett River was divided into 3 main reaches made up of a total of 197 sub‐reaches. The assessment of the Pollett River started at Rte 114 where it crosses Mechanic Lake Brook and ended at the confluence with the Petitcodiac River. The majority of sub‐reaches were in a transitional or stressed state (57%), 21% of the sub‐reaches were classified as in adjustment and 22% were identified as in regime (Figure 3-13). The stable (in regime) sub‐reaches could be utilized as reference reaches; channel cross sections and thalweg profiles could be obtained for comparisons with the less stable sub‐reaches. Reference reaches could also be used to provide

Figure 3-13. Pollett River stability rankings

baseline data where cross sections and thalweg could be measured on a recurring basis to allow for identification of changes to the watercourse over time and provide insight into changes that may occur elsewhere in the system.

Aggradation was identified as the most common primary geomorphic process, with degradation being the second most common primary process. Widening and planform adjustment were relatively uncommon as primary geomorphic processes (Figure 3-14).

Figure 3-14: Pollett River primary geomorphic processes

Secondary geomorphic processes were more varied, with degradation and aggradation being identified as the most and second most common processes respectively. Widening and planform adjustment also made up a significant portion of secondary processes (Figure 3-15).

Figure 3-15: Pollett River secondary geomorphic processes

Assessment Results by Reach

The following sections provide a more detailed look into the assessed sub‐reaches. For the ease of data presentation, the three assessed reaches are presented in six sections. The first and second reaches (starting upstream moving downstream) are presented in their own in sections, while the third (lower) reach of the Pollett River is presented in 4 parts. In addition to providing discussion on the dominant geomorphic processes in each section, data analysis focuses on sub‐reaches classified as in‐adjustment. While this provides a more focused discussion, the importance of identifying and addressing problems in other sub‐reaches experiencing instability (i.e. sub‐reaches in a transitional or stressed state) should not be overlooked.

Reach 1: Rt 114 to where Mechanic Lake Brook joins the Pollett  (Sub-reaches MLB 2 1 to 2 28)

The assessment of the upper reach started at Rte 114 where it crosses Mechanic Lake Brook and ended at the confluence with the Pollett River. This included approximately 7 km of stream that was divided into 28 sub‐reaches. This section of river is experiencing a wide range of stability classes. Transitional or stressed was the most common condition. Sub‐reaches MLB 2 1, MLB 2 5, MLB 2 8, MLB 2 17, MLB 2 19, MLB 2 21, MLB 2 23, MLB 2 25, and MLB 2 27 were in a state of adjustment; MLB 2 3, MLB 2 4, and MLB 2 22 were classified as in regime (Figure 3-16).

Figure 3-16: Stability rankings for Reach 1 (sub-reaches MLB 2 1 to MLB 2 28)

The dominant primary geomorphic process for the sub‐reaches MBL 2 1 – MBL 2 28 was aggradation followed by planform adjustment (Figure 3-17). The dominant primary geomorphic process associated with the unstable (in adjustment) sub‐reaches was planform adjustment followed by aggradation.

Figure 3-17: Primary geomorphic processes for Reach 1 (sub-reaches MLB 2 1 to MLB 2 28)

Secondary processes for sub‐reaches MLB 2 1 to MLB 2 28 are outlined in (Figure 3-18), with aggradation being identified as the most common secondary process. A pattern in the data was noted where aggradation and planform adjustment appeared in conjunction with each other in the unstable reaches. Where one appeared as the primary process, the other was always the secondary process. This suggests that the unstable sites are in a state of adjustment due to an increase in bedload materials, bringing on a subsequent change in channel planform.

Figure 3-18: Secondary geomorphic processes for Reach 1 (sub-reaches MLB 2 1 to MLB 2 28)

Identification and control of the sediment sources in this reach should precede any instream restoration efforts. Otherwise, the excessive amount of sediments in the system could end up burying installed structures. Possible sources of sediment could be from eroding banks or mobile bed materials, poorly vegetated upslope areas, poorly constructed roads, poorly managed timber harvest and agricultural areas etc. Once sediment sources have been identified, some possible restoration options include: re‐establishment of vegetation via changing land use practises, seeding, planting, or slope regrading if banks are severely eroded.

It has been noted that a large amount of crown timberland and woodland as well as private woodlots exist in the surrounding watershed (GeoNB mapping service). It is important that harvesting activities adhere to existing legislation so that buffer zones maintain their function. The lack of good buffer or riparian zones adjacent to watercourses could lead to increased runoff and larger peak flows. More intense flows have more erosive energy and thus greater potential to carry more sediment. Another potential anthropogenic source of sediment would be at poorly installed stream crossings. Improperly sized or misaligned woods road culverts can lead to erosion of road berms or even road washouts in extreme cases. It would be prudent to ensure that buffer zones and stream crossings on the assessed length of stream and its associated tributaries are in good condition.

As evidenced by photographs, the area contains a number of beaver dams (Figure 3-19 provides an example). These could be a potential natural source of sediment. Large amounts of sediment can become mobile and move through the system as sediment accumulated upstream of beaver dams becomes free when the dam is breached.

Figure 3-19: Beaver dam along Reach 1 at sub-reach MLB 2 19

Reach 2: Start of Pollett to Churchs Corner (Sub-reaches PS 3 1 to PS 3 44 & PS 2 1 to PS 2 5)

This section of sub‐reaches carries on from the previous sub‐reaches on Mechanic Lake Brook, starting at the confluence of the Pollett River and Mechanic Lake Brook and ending just upstream of Church Hill Road. This includes approximately 13.7 km of river that is divided into 49 sub‐reaches. These sub‐reaches make up a large bend in the river, where the flow path changes from a westward direction to a primarily northward direction. Once again, transitional or stressed was the most common condition with 24 sub‐reaches and 16  sub‐reaches were in a state of adjustment. This section of sub‐reaches, when compared to the other sections of river presented, had the highest percentage of sub‐reaches in adjustment, making this section of river a particular concern due to its high instability. 8 of the 49 sub‐reaches outlined here were classified as in regime (Figure 3-20).

Figure 3-20: Stability rankings for Reach 2 (sub-reaches PS 3 1 to PS 3 44 & PS 2 1 to PS 2 5)

The dominant primary geomorphic process for these sub‐reaches was aggradation followed by degradation (Figure 3-21). Widening was never observed as the primary geomorphic process and planform adjustment was identified as the primary process in 2 sub‐reaches. The primary geomorphic process most associated with the unstable (in adjustment) sub‐reaches, by far, was aggradation; degradation and planform adjustment each were the primary geomorphic process for two unstable sub‐reaches. A pattern in the data was noted where sub‐reaches with aggradation and degradation identified as primary geomorphic processes often occurred in sequence with each other. This could be a result of materials eroding from degraded portions of channel and depositing further downstream in areas with lower sediment carrying capacity.

Figure 3-21: Primary geomorphic processes for Reach 2 (sub-reaches PS 3 1 to PS 3 44 & PS 2 1 to PS 2 5)

Degradation was identified as the most common secondary process; with the other geomorphic processes each also being identified as secondary processes in some sub-reaches (Figure 3-22).

Figure 3-22: Secondary geomorphic processes for Reach 2 (sub-reaches PS 3 1 to PS 3 44 & PS 2 1 to PS 2 5)

As with the previous reach, identification and control of the sediment sources in this reach should be the first step towards any restoration efforts. The presence of crown timberland and woodland as well as private woodlots in the surrounding watershed, like the upstream reach, remains prevalent in this section as well. Once again, ensuring that buffer zones and stream crossings on the assessed length of stream and its associated tributaries are in good condition would be beneficial. The occurrence of exposed bedrock and large areas of deposition was also noted in photographs taken in this section of the Pollett River (see Figure 3-23 and Figure 3-24).

Figure 3-23: Exposed bedrock on left bank in Reach 2 (sub-reach PS 3 10)

Figure 3-24: Large depositional feature in Reach 2 (sub-reach PS 3 36)

Reach 3 Part 1: Churchs Corner to below Elgin near Rt 895 (Sub-reaches  PS 2 6 to PS 2 38)

This section of sub‐reaches starts at Church Hill Road and ends approximately 400 metres upstream of where route 895 crosses over the river, just north of Elgin. This includes approximately 8.8 km of river that is divided into 33 sub‐reaches. The underlying geology changes in this section (Figure 3-25), which corresponds to a series of rapids and waterfalls that make up a large portion of this reach (Figure 3-26 and Figure 3-27). The waterfalls can be thought of as large headcuts caused by knickpoints in the less resistant sedimentary bedrock.

Figure 3-25: Major geological units, waterfalls and rapids in the Pollett River watershed

 

Figure 3-26: Rapids in Reach 2: sub-reach PS 2 14 (left); and sub-reach PS 2 17 (right)

Figure 3-27: Rapids and waterfalls in Reach 2: sub-reach PS 2 21 (bottom right); sub-reach PS 2 23 (top right ); and sub-reach PS 2 24 (left)

The majority of sub‐reaches (19 of 33) in this section were in a transitional or stressed state. There were fewer sub‐reaches in adjustment (6), and more sub‐reaches in regime (8) compared to the upper sections (Figure 3-28).

Figure 3-28: Stability rankings for Reach 3 part 1 (sub-reaches PS 2 6 to PS 2 38)

The dominant primary geomorphic process for these sub‐reaches was degradation followed by aggradation (Figure 3-29). Widening and planform adjustment were never observed as primary geomorphic processes. The primary geomorphic process often associated with the unstable (in adjustment) sub‐reaches, again, was aggradation; degradation was the primary geomorphic process for 3 unstable sub‐reaches. The pattern where sub‐reaches with aggradation and degradation identified as primary geomorphic processes occurred in sequence with each other was again noted.

Figure 3-29: Primary geomorphic processes for Reach 3 part 1 (sub-reaches PS 2 6 to PS 2 38)

Widening was identified as the most common secondary process; with aggradation, degradation, and planform adjustment also being identified as secondary processes in some of the sub‐reaches (Figure 3-30).

Figure 3-30: Secondary geomorphic processes for Reach 3 part 1 (sub-reaches PS 2 6 to PS 2 38)

Any restoration efforts attempted in the upper sub‐reaches of this section should take consideration the very shallow depth to bedrock, where the bedrock often makes up the channel bed and/or banks. In this case, anchoring in‐stream or bank stabilization structures will be more difficult compared to areas with a well‐established soil layer. Common restoration techniques applied to degraded streams include: raising the stream bed and/or lowering the floodplain to provide floodplain access and allow for energy dissipation in high flows (cut and fill areas from the floodplain and channel bed should be designed to ensure proper stream dimensions); and using grade control structures to prevent knickpoint/headcut migration, and stabilize the stream grade (grade control structures can also be designed to provide access to the floodplain).

Another consideration is the presence of aggradation causing instability in the first subreach and immediately upstream of the first sub‐reach. This would have to be addressed prior to commencing any in stream works downstream; otherwise the effectiveness of the work could be compromised if the sediments in the upstream sections migrate downstream.

The lower sub‐reaches of this section, downstream of the waterfalls, are mostly experiencing instability due to an increase in the bedload (refer to Figure 3-31 and Figure 3-32 for examples). Restoration efforts should focus on locating the source of the sediment and implementing strategies discussed in section on Reach 1 (sub-reaches MLB 2 1 to MLB 2 28). The presence of farmland, croplands and pastures as well as woodlots and residences adjacent to the river was noted in the area surrounding the aggraded reaches (GeoNB mapping service and Landuse datasets). Figure 3-33 outlines the aggraded areas on aerial maps with landuse. Ensuring proper buffer zones in this section of river as well as in adjoining tributaries using previously discussed techniques would help fulfill the requirement of identification and control of the sediment sources.

Figure 3-31: Aggradation in Reach 3 (sub-reach 3 31)

Figure 3-32: Aggradation in Reach 3 (subreach PS 3 34)

Figure 3-33: Land use and primary geomorphic processes for Reach 3 sub-reaches PS2 20 to PS 2 37

Reach 3, Part 2: Below Elgin near Rt 895 to Parkindale Bridge(Sub-reaches  PS 2 39 to PS 2 67)

This section of sub‐reaches starts north of Elgin and ends approximately 400 metres upstream of the Parkindale Road Bridge near the community of Pollett River. This includes approximately 8.7 km of river that was divided into 29 sub‐reaches. This section of river is experiencing all ranges of stability and is relatively more stable compared to sections upstream. Transitional or stressed was the most common condition. Sub‐reaches PS 2 59, PS 2 60, and PS 2 61 were in a state of adjustment; PS 2 46, PS 2 49, PS 2 53, PS 2 63, PS 2 66, and PS 2 67 were classified as in regime (Figure 3-34).

Figure 3-34: Stability rankings for Reach 3 part 2 (sub-reaches PS 2 39 to PS 2 67)

The dominant primary geomorphic process for these sub‐reaches was aggradation followed by degradation (Figure 3-35). Widening was the primary geomorphic process for 4 sub‐reaches and planform adjustment was never observed as the primary geomorphic process in these sub‐reaches. A common primary geomorphic process associated with the most unstable sub‐reaches was widening; aggradation was the primary geomorphic process for 1 sub‐reach in adjustment. Sub‐reaches with aggradation and degradation identified as primary geomorphic processes occurred in sequence with each other once again. Intermittent sections of widening as a primary process, especially in unstable subreaches, occurred between aggraded and degraded sub‐reaches.

Figure 3-35: Primary geomorphic processes for Reach 3 part 2 (sub-reaches PS 39 to PS 2 67)

Degradation was the most common secondary process; with the other geomorphic processes also being identified as secondary processes in some sub‐reaches (Figure 3-36).

Figure 3-36: Secondary geomorphic processes for Reach 3 part 2 (sub-reaches PS 2 39 to PS 2 67)

The prevalence of degrading sub‐reaches, as a primary and secondary geomorphic process, was partially evidenced by a significant amount of exposed bedrock in these sub‐sections (see examples in Figure 3-37).

Figure 3-37: Exposed sedimentary bedrock in Reach 3 part 2: sub-reaches PS 2 54

Unstable sub‐reaches should be handled appropriately as outlined in previous sections, whether by mitigating erosive forces with stabilization of the grade and/or providing floodplain access for degrading sections or identifying and controlling sediment sources for aggrading reaches. Channel restoration in widening sub‐reaches should be designed to narrow the channel by accumulating sediments towards the banks. Any in‐stream structures designed for improving stream habitats should also be designed to assist in narrowing channel width and not create scour along banks, particularly where the channel is also experiencing degradation.

Reach 3, Part 3: Above Parkindale Bridge to The Glades (Sub-reaches  PS 2 68 to PS 2 95)

This section of sub‐reaches starts near the community of Pollett River at ends just downstream of The Glades. This includes approximately 10 km of river that was divided into 29 sub‐reaches. Transitional or stressed was the most common geomorphic condition. 4 sub‐reaches were in a state of adjustment; 8 sub‐reaches were classified as in regime (Figure 3-38).

Figure 3-38: Stability rankings for Reach 3 part 3 (sub-reaches PS 2 68 to PS 2 95)

The dominant primary geomorphic process for these sub‐reaches was aggradation followed by degradation (Figure 3-39). Widening was the primary geomorphic process for 1 sub‐reach and planform adjustment was never observed as a primary geomorphic process in this section of sub‐reaches. Aggradation was always the primary geomorphic process associated with the most unstable sub‐reaches. Sub‐reaches alternated between aggradation and degradation as primary geomorphic processes, reflecting alternation between strong erosive forces and subsequent deposition downstream. One sub‐reach was widening as a primary process, likely brought on originally as degradation given its location between two degrading sub‐reaches and degradation being identified as the secondary geomorphic process within the widening sub‐reach itself.

Figure 3-39: Primary geomorphic processes for Reach 3 part 3 (sub-reaches PS 2 68 to PS 2 95)

Degradation was again identified as the most common secondary process; with the other geomorphic processes each being identified as secondary processes in some fraction of the sub‐reaches as well (Figure 3-40).  Once again, sediment sources should be identified and controlled prior to installation of any in‐stream structures so as to not jeopardize the work.

Figure 3-40: Secondary geomorphic processes for Reach 3 part 3 (sub-reaches PS 2 68 to PS 2 95)

Reach 3, Part 4: The Glades to confluence with Petitcodiac (Sub-reaches  PS 2 96 to PS 2 125)

This final section of sub‐reaches starts just north of The Glades and ends at the confluence between the Pollett River and the Petitcodiac River. This includes approximately 9.1 km of river that was divided into 30 sub‐reaches. Transitional or stressed was the most common geomorphic condition. 3 sub‐reaches were in a state of adjustment; 10 sub‐reaches were classified as in regime (Figure 3-41). In terms of the percentage of sub‐reaches in adjustment to the total number of sub‐reaches, this section of river is the most stable of all sections discussed in this report.

Figure 3-41: Stability rankings for Reach 3 part 4 (sub-reaches PS 2 96 to PS 2 125)

The majority of sub‐reaches for this section (20 of 30) had aggradation as a primary geomorphic process (Figure 3-42). Degradation was also identified as a primary process in some sub‐reaches; unique to this section was the absence of widening and planform adjustment as primary geomorphic process. Once again, aggradation was always the primary geomorphic process associated with the most unstable sub‐reaches and sub- reaches alternated between aggradation and degradation as primary geomorphic processes.

Figure 3-42: Primary geomorphic processes for Reach 3 part 4 (sub-reaches PS 2 96 to PS 2 125)

Degradation was again identified as the most common secondary process; with the other geomorphic processes each being identified as secondary processes in some sub‐reaches (Figure 3-43).

Figure 3-43: Secondary geomorphic processes for Reach 3 part 4 (sub-reaches PS 2 96 to PS 2 125)

Being at or near the mouth of the river, these lower sub‐reaches are wider in channel widths and have lower grades. As such, this makes them natural areas of deposition for sediments being carried down from upper reaches. Keeping this in mind, any channel or floodplain modifications should be designed with care. Determining bankfull discharge rates and appropriate channel dimensions will be especially crucial for any restoration efforts in these lower sub‐reaches.2


Recent Comments

0 comments

 
Leave Comment

Partners & Supporters