Fort Folly Habitat Recovery

Geomorphic Assessment

published 1 Feb 2018 by FFHR in News category with 0 comments

Geomorphic Assessment

The North River was divided into 6 main reaches with 138 sub‐reaches and covered approximately 46 km of channel from near Indian Mountain at Route 126 on downstream to the Village of Petitcodiac where it merges with the Anagance River, below which point the channel becomes recognized as the Petitcodiac River.  The fieldwork was conducted in a discontinuous manner, and the named sub-reaches do not reflect this more intuitive, flow based order. To make the collective analysis easier to read and understand they have been reassembled into reaches that follow flow based order.  However the names of the original sub-reaches were maintained in order to minimize further confusion by making it possible to readily access site specific data without needing to go back and forth constantly translating between two different numbering systems.

Reach 1: Route 126 to Pacific Junction Road (sub-reaches 6-0 to 6-8)

Reach 1 starts on the east side of Route 126, just south of Indian Mountain and ends at Pacific Junction Road. The assessed reach is approximately 2.4 km and is divided into 8 sub‐reaches. The first four sub‐reaches, 6‐0 to 6‐3 cover approximately 900 metres of the North River. Sub-reaches 6.0 and 6.1 head east from Route 126 while sub‐reaches 6.2 and 6.3 move west from Route 126. There is approximately a 2.0 km gap on the river that was not assessed between sub‐reaches 6‐3 and sub‐reach 6‐40. From comments on the RGA/RSAT forms, the stretch of the North River that was not assessed was due to the density of the alders within that reach. Figure 4-8 illustrates sub‐reaches 6‐0 to 6‐5.

With the exception of sub‐reach 6‐5, the remaining sub‐reaches are in a state of transition. Sub-reach 6‐5 is in a phase of adjustment and is experiencing channel degradation. The dominant geomorphic process occurring in sub‐reaches 6‐0 and 6‐1 is aggradation. Sub‐reaches 6‐2 and 6‐ 3 are undergoing channel degradation while the dominant geomorphic process occurring in sub‐reach 6‐4 is channel widening.

The plunge pool on the downstream side of the culvert passing under Route 126 likely explains some of the degradation occurring in sub‐reaches 6‐2 and 6‐3 (Figure 4-7).

Figure 4-7: North River, plunge pool at Route 126.

Figure 4-8: North River Reach 1 (Sub‐reaches 6‐0 to 6‐5).

Figure 4-9 illustrates that sub‐reaches 6‐6, 6‐7, and 6‐8 are in a state of transition with the dominant geomorphic processes being aggradation, degradation, and aggradation, respectively

Figure 4-9: North River Reach 1 (Sub‐reaches 6‐6 to 6‐8).

With the dense alder cover in sub‐reaches 6‐0 to 6‐3 (Figure 4-10), restoration efforts in these reaches would be difficult.

 

Figure 4-10: North River, alder cover.

 

Figure 4-11: North River, Culvert under Rte 126.

If fish passage is a priority, the culvert under Route 126 is an issue with the downstream invert hanging above the streambed. The culvert appears to be able to handle the flows as the oxidization line on the pipe is approximately a third of the way up from the bottom (Figure 4-11). It is also noted that looking though the culvert, based on the photograph, the pipe may be collapsing. However, this could simply be the angle the photograph was taken and further investigation may be warranted.

Reach 2: Pacific Junction Road to Taylor Road (Sub-reaches 5-1 to 5-40)

Reach 2 has 40 sub‐reaches and covers approximately 13 km of the North River. The reach begins on the west side of the Pacific Junction Road and goes to the east side of Taylor Road. The channel condition of sub‐reaches 5‐1 to 5‐15 is under stress. The dominant geomorphic process is aggradation. Sub‐reaches 5‐6, 5‐9, 5‐10, 5‐11, and 5‐14 are experiencing channel degradation. Channel widening is occurring in sub‐reach 5‐2. This is illustrated in Figure 4-12.

As aggradation is the most dominant geomorphic process occurring within the upper sub-reaches of Reach 5, finding the source or cause of the increase in bedload needs to be determined prior to instream or bank restoration work of this area. Any restoration efforts need to consider the natural bankfull conditions for sub‐reaches 5‐1 to 5‐15 to ensure that the channel is maintained by the natural deposition and erosion of material occurring in these sub-reaches.

Sub‐reaches 5‐16 to 5‐30 vary in channel conditions from undergoing adjustment to a state of transitional or stressed. One sub‐reach, 5‐20 is in a state of regime (stable). This sub‐reach may be used as a reference reach for gathering natural channel characteristics of Reach 5. As shown in Figure 4-13, the dominant geomorphic process is aggradation.

As with any increase in sediment load to a watercourse the source needs to be determined prior to implementing any instream restoration structures. Otherwise there is an inherent risk that the instream structures will be buried. Once the source or cause of the excess sediment in the system has been determined proper restoration or mitigation measures can be taken to assist with the natural movement of the material through the reach. By creating deposition or scour areas, equilibrium of sediment load through the system can be achieved ensuring that any instream habitat structures function as intended.

Half of the sub‐reaches between 5‐31 and 5‐40 are in adjustment and the other half are in a state of transition. Figure 4-14 demonstrates this but also shows that dominant geomorphic process includes aggradation, degradation, widening and planimetric adjustment. The RGA data for these sub‐reaches of Reach 5 indicate that a slight shift in a couple of identifying factors could result in any of the geomorphic process becoming the dominant process. Restoration efforts, whether for instream habitat or bank erosion, therefore need to consider what the end goal of the restoration is and the effects that will result from implementing such measures on upstream and downstream banks and instream features.

Figure 4-12: North River Reach 2 (Sub‐reaches 5‐1 to 5‐15).

Figure 4-14: North River Reach 2 (Sub‐reaches 5‐31 to 5‐40).

Reach 3 Taylor Road to Scott Road (Sub-reaches 1-1 to 1-36)

The length of Reach 3 is approximately 11.9 km and extends Taylor Road to Scott Road. Reach 3 was further divided into 36 sub‐reaches. Generally, the channel is in a state of transition or under stress with the exception of sites: 1‐2, 1‐22, 1‐28, 1‐30, and 1‐33 as seen in Figure 4-15, Figure 4-16, and Figure 4-17. These five sites are exhibiting characteristics that suggest the channel is in a state of adjustment due to an increase in bedload material (aggradation).

The dominant geomorphic process identified through Reach 3 is channel widening followed by aggradation. Sub‐reach 1‐29 was the only site to show degradation as the primary geomorphic process. As the data shows, sub‐reaches that are widening are followed by sub‐reaches that are accumulating sediment and aggrading in channel bed elevation. However, the second most dominant geomorphic process identified is channel degradation. Exposed cobble in the channel bed is one indication that the bed material has most likely been eroded away, revealing the parent channel material. The RGA data shows that the dominant geomorphic process in sub‐reach 1‐6 is widening with the secondary geomorphic process being degradation. Channel restoration efforts in Reach 3 should be designed to narrow the channel by accumulating sediments towards the banks. Accumulated sediments would develop lateral bars that would naturally narrow the channel.

It should be noted that any design structures intended for improving instream habitat should also be constructed to assist in narrowing channel width and not create scour along the banks. Bank restoration efforts in Reach 3 need to be designed to narrow the channel to a more natural width but able to handle varying discharges without creating erosion issues. The designs need to incorporate floodplain access, particularly where the channel has degraded or widened beyond the natural bankfull discharge width/depth.

Figure 4-15: North River Reach 3 (Sub‐Reaches 1‐1 to 1‐3)

Figure 4-16: North River, Reach 3 (Sub‐Reaches 1‐4 to 1‐26).

Figure 4-17: North River, Reach 3 (Sub‐Reaches 1‐27 to 1‐36).

Reach 4 Scott Road to Morton Road (Sub-reaches 4-1 to 4-8)

Reach 4 starts at on the west side of Scott’s Road, which is located south of Lewis Mountain. The RGA/RSAT assessments on this section of the North River covered approximately 4.7 km and ended on the east side of Morton Road, near Wheaton Settlement. The reach was divided into eight sub‐reaches depicted in Figure 4-18 and 4-19. The entire length of Reach 4 is in a transitional state with the exception of sub‐reaches 4‐6 and 4‐8. Sub‐reach 4‐6 is undergoing a channel adjustment while sub‐reach 4‐8 is in a state of stability. The dominant geomorphic process varies between aggradation and degradation; however, sub‐reach 4‐4 is slightly more towards channel widening than degradation.

Sub‐reach 4‐8 could be used as a reference reach for Reach 4 and the upper sub‐reaches of Reach 3. Bank or aquatic habitat restoration efforts should ensure that designs implemented do not increase sediment loads where there is already aggradation occurring or decrease sediment loads in areas where there is already degradation of the channel occurring.

 

 

Figure 4-18: North River Reach 4 (Sub‐reaches 4‐1 to 4‐5).

Figure 4-19: North River Reach 4 (Sub‐reaches 4‐6 to 4‐8).

Reach 5: Morton Road to Wheaton Settlement Road Access point (sub-reaches 3-1 to 3-8)

Reach 5 begins at Morton Road near Wheaton Settlement and goes southwest down the North River, ending just north of Fawcett at a point where Wheaton Settlement Road supplies ready access to the river from the east bank. There are eight sub‐reaches within Reach 5 covering 2.8 km of the North River. With the exception of sub‐reaches 3‐4 and 3‐5, Reach 5 is in a state of channel transition. Sub‐reaches 3‐4 and 3‐5 are going through a phase of channel adjustment.

Sub‐reaches 3‐1 to 3‐3 are experiencing channel degradation as shown in Figure 4-20. As sub-reach 3‐1 begins to the west of Morton Road (the downstream side of the bridge) further investigation should be given to determine the reason for channel degradation in these sub- reaches. One possible issue may be with the bridge constricting flow during higher discharge events. However, flow capacity through the bridge would need to be modeled to determine if there is any channel constriction occurring from this structure.

Aggradation of the channel bed is occurring in sub‐reaches 3‐4 and 3‐5. The bedload material is coming from the upper sub‐reaches but further field investigation would be needed to determine the source of this material.

The remaining sub‐reaches, 3‐6, 3‐7, and 3‐8, are dominated by the geomorphic processes, degradation, channel widening, and aggradation, respectively, as illustrated in Figure 4-21. However, the RGA data indicates that any one geomorphic process could easily become dominant within these three sub‐sections of Reach 3.

Before any restoration efforts begin in Reach 5, the source of the additional sedimentation to the streambed needs to be determined. Once the root source and cause of the sediment has been identified, channel restoration and/or bank restoration designs need to bring the channel into a state of stability through equalizing the sediment movement in this reach. In other words, the amount of sediment coming into Reach 5 should reflect the sediment load moving out of Reach 5.

Figure 4-20: North River Reach 5 (Sub‐reaches 3‐1 to 3‐5).

Figure 4-21: North River Reach 5 (Sub‐reaches 3‐6 to 3‐8).

Reach 6: Wheaton Settlement Road Access point to Anagance River (Subreaches 2-1 to 2-38)

Reach 6 extends from the Wheaton Settlement Road Access point to the confluence of the North and Anagance rivers, below which the channel is recognized as the Petitcodiac River. This reach covers approximately 11 km of the North River and was divided into 38 sub‐reaches. The geomorphic conditions identified through the RGA/RSAT assessment indicate that the channel of the North River in Reach 6 has a few stable sub‐reaches as well as sub‐reaches that are in a transitional state and in phases of adjustment. The stable sub‐reaches are: 2‐8, 2‐17, 2‐28, and 2‐33; these should not be considered for restoration but rather as reference reaches. Monitoring cross‐sections could be established in these reaches as well as thalweg profiles to provide a location to collect baseline data that would be measured yearly or after a large flow event. This would allow for identifying changes to the watercourse as a result of an event and provide some insights as to the changes that may occur elsewhere in the river system.

There are 24 sub‐reaches in Reach 6 that are in a geomorphic state of transition or stressed. The dominant geomorphic process identified by the RGA data indicates through sub‐reaches 2‐ 0 to 2‐12, eight of the sub‐reaches are experiencing aggradation. This is closely followed by degradation as the dominant geomorphic process occurring in the other five sub‐reaches as shown in Figure 4-22. Interestingly, each sub‐reach that was identified as undergoing degradation, the next reach or two downstream was experiencing aggradation. Restoration efforts through sub‐reaches 2‐0 to 2‐12 should keep this in mind when designing instream habitat structures.

Sub‐reaches 2‐13 to 2‐27 are dominated by a channel that is under transition. Four of the stub-reaches are in a state of adjustment and one sub‐reach is stable. The dominant geomorphic process is aggradation as seen in Figure 4-23.

Restoration efforts within these sub‐reaches should first focus on identifying where the additional sediment load is originating. Any instream habitat restoration completed prior to locating the source or reason for the aggradation of the streambed will jeopardize the longevity of the work. If efforts to control or at least identify the source of streambed aggradation are not part of the overall restoration plans for Reach 6, eventually structures to improve instream habitat will most likely be buried through these sub‐reaches.

In Reach 6, the sub‐reaches 2‐28 to 2‐38 are in the lower section of the North River as it joins with the Anagance River to become the Petitcodiac River. The North River through these lower sub‐reaches has a lower grade and wider channel width than the upper sub‐reaches of Reach 6.

This means it is a natural deposition area for sediments being carried by the water from the upper reaches of the North River. The dominant geomorphic process through these lower ten sub‐reaches varies between degradation and aggradation. Sub‐reach 2‐34 is undergoing widening as the primary geomorphic process but only by a slight margin over channel degradation. The other sub‐reaches alternate between accumulating sediment or channel degradation. Three sub‐reaches, 2‐31, 2‐37, and 2‐38 are in a state of adjustment. Sub‐reach 2‐ 33 is considered to be in regime, or in a state of relative stability. The remaining sub‐reaches are going through a transitional phase. Figure 4-24 illustrates these features.

Restoration efforts, whether it is instream habitat, bank stabilization, or floodplain creation, in sub‐reaches 2‐28 to 2‐38, should be attempted with caution. Determining bankfull discharge rates and depth/width ratios will be an important factor in any channel or floodplain restoration modifications through these lower sub‐reaches of Reach 6.

Figure 4-22: North River Reach 6 (Sub‐reaches 2‐0 to 2‐12).

Figure 4-23: North River Reach 6 (Sub‐reach 2‐13 to 2‐27).

Figure 4-24: North River Reach 6 (Sub‐reaches 2‐28 to 2‐38).

 


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