Suncook River and Leighton Brook Basis of Design

Home , Avulsion (river), Little Suncook River

Inter-Fluve, Inc. Technical Memorandum

T O : Steve Landry, NH DES; Shane Csiki, NHGS F ROM: Nick Nelson, Inter-Fluve D ATE: April 26, 2013 R EGARDING: Basis of Design Memo for the Stabilization of the Suncook River - 90% complete designs

Contents Introduction ...... 2 Alternatives Analysis and Impact to Regulated Resource Areas ...... 2 Background ...... 9 Existing Studies ...... 11 Survey ...... 12 Hydrology ...... 14 Hydraulics ...... 15 Design ...... 19 Second Riffle Downstream of Rt 4 ...... 21 Avulsion Site: Lag Deposit ~1000 feet Upstream of the Avulsion ...... 30 Leighton Brook ...... 33 Channel Plan Form and Profile ...... 34 Cross Sectional Geometry...... 36 Bed and Bank Materials ...... 40 Buttress ...... 41 References ...... 47 Cost Estimate ...... 48

Appendices A-E follow the cost estimate:

Appendix A: Wetland delineation field maps and notes. Appendix B: Task 4 Technical Memo. Appendix C: USGS StreamStats information. Appendix D: Comparison of hydraulic cross sections between Inter-Fluve model and USGS model. Appendix E: 2011 Structural Assessment Technical Memo.

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 1 Introduction Inter-Fluve was contracted by the New Hampshire Department of Environmental Services (NH DES) to design and permit the restoration of the Suncook River in Epsom, NH. In 2006, a large flood event on the Suncook River resulted in a channel avulsion between the Huckins Mill Dam and the Rt 4 Bridge. This avulsion caused the abandonment of the primary and secondary channels and created a new channel to the east of Bear Island. The Suncook River is continuing to adjust to its shorter channel length by migrating laterally and incising. The incision is highlighted by a series of knickpoints that are migrating upstream on the Suncook River and its tributaries, primarily the Little Suncook River and Leighton Brook. The goal of this project is to limit further incision and excessive bank erosion in the vicinity of these knickpoints, in tributaries, and along the Suncook River upstream of the project site. Stabilizing vertical and lateral erosion will serve two primary purposes. First, stabilization will reduce the risk of the Rt 4 Bridge over the Suncook River and the Black Hall Rd culvert at Leighton Brook from being undercut by these migrating knickpoints. Second, stabilization will limit the further loss of wetland habitat, which would occur as incision moves upstream and changes the local water table elevations adjacent to the stream, and degradation of water quality, which could occur as a result of excessive erosion and downstream migration of sediment.

Alternatives Analysis and Impact to Regulated Resource Areas

A few alternatives to achieve the goals identified above were assessed during the design process. These alternatives include: do nothing, stabilize the channel bed and banks (chosen alternative as described in this report), replace the Rt. 4 Bridge and do no stabilization on the Suncook River, construct a valley-spanning grade control riffle downstream of the Rt. 4 Bridge, and place rock buttresses downstream of the Rt. 4 Bridge and the Black Hall Rd culvert.

Alternative 1: Do Nothing

Completing no work on the Suncook River or Leighton Brook could result in the undermining of bridge infrastructure (Rt. 4 Bridge on the Suncook River and the Black Hall Rd culvert on Leighton Brook), which could cause risk to human life and/or infrastructure. The Suncook River has eroded vertically and laterally since the avulsion occurred, and the vertical erosion is currently stalled at a series of cobble/boulder lag deposits that form riffles upstream of the avulsion site and downstream of the Rt. 4 Bridge. Because of the incision that has already

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 2 occurred, the abandoned railroad bridges (snowmobile bridges) over the Little Suncook River and Leighton Brook were undermined and have collapsed. A temporary bridge was built over Leighton Brook, but the bridge over the Little Suncook River has not been replaced. Site investigations revealed that the Suncook River is capable of eroding through the lag deposits and hydraulic modeling revealed shear stresses that could mobilize even the larger rocks in these lag deposits (see discussions of shear stresses below and the 2-Dimensional Hydraulic Modeling Report submitted in March, 2012). Further incision and bank erosion would result in further loss of agricultural land and likely damage to utility poles (west side of channel downstream of the Rt. 4 Bridge), roads, and private buildings.

Continued incision and bank erosion along Leighton Brook could cause similar types of damage and risk to human life and infrastructure. A temporary snowmobile bridge over Leighton Brook is currently showing signs of erosion and this would only likely continue if no stabilization measures were taken. Buildings on either side of Leighton Brook are within 5-15 ft of the channel and could be undermined and damaged if erosion continues. Black Hall Rd could be undermined if the knickpoints that are throughout Leighton Brook continue to migrate upstream.

In addition to potential damage to human life and infrastructure, doing nothing to stabilize the Suncook River and Leighton Brook would result in further loss of wetland habitat and degradation of in-channel habitat and water quality. Prior to the avulsion, Leighton Brook was a low-gradient stream that flowed through an extensive wetland before joining the Suncook River. When the avulsion occurred, nearly 1,000 ft of Leighton Brook channel and multiple acres of wetland were lost because of the migration of the Suncook River and the lowering of the water table. While portions of the wetland remain classified as a wetland adjacent to the existing mouth of Leighton Brook, the land surface here is approximately 20 ft above the channel elevation. Over time, the lowering of the water table at this location will likely result in a change in wetland classification. This type of ecosystem change could occur elsewhere along the Suncook River and tributaries without vertical and lateral stabilization of the Suncook River. Of particular concern is the large wetland complex on either side of the Little Suncook River upstream of the collapsed railroad bridge. While currently a functioning wetland, incision of an additional 6-10 ft, which could occur if the Suncook River continues to incise, could cause loss of wetland function and habitat in this area.

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 3 Not stabilizing the river channel may also result in the continued excessive erosion and release of sediments into the tributaries and mainstem Suncook River. About a month after the avulsion occurred, research showed extremely high Total Suspended Solids (TSS) levels of nearly 280 mg/L in some locations (Daley, 2006). Estimates of the volume of material that eroded downstream as a result of the avulsion range from 118,000 cubic yards (Perignon, 2007) to a NHGS estimate of 150,000 cubic yards (Wittkop et al., 2007 in Perignon, 2007). This material was deposited in the Suncook River channel and along the floodplains. While channel migration and movement of sediment within channels and floodplains is a natural and necessary process to the form and function of rivers and floodplains as well as the robustness of in-channel and riparian habitat, excessive amounts of sediment mobilization and deposition as a result of catastrophic changes may have significant impacts on aquatic organism survival. Completing no channel or bank stabilization along the Suncook River may continue the excessive erosion occurring along the Suncook River until the knickpoints dissipate miles upstream. The volume of sediment added to the river system as a result of this is unknown but would likely be well above the volumes typically experienced along the Suncook River.

Alternative 2: Stabilize the Channel Bed and Banks (alternative chosen by Project Partners)

The alternative preferred and chosen by the Project Partners is described in detail in this report. This alternative includes a few components that are summarized in this section and described in detail in sections later in this report: 1) The placement of stone within a portion of the Suncook River channel downstream of the Rt. 4 Bridge will improve vertical stability and reduce the likelihood of incision at this location; 2) stone and fabric lifts along the Suncook River banks between the Rt. 4 Bridge and the vertical grade control will improve lateral stability and reduce the likelihood of risk of loss to agriculture land, utilities, roads, and buildings; 3) sheet pile will be installed adjacent to the grade-control riffle (component 1 above) and across the floodplain to provide some protection in the event that the Suncook River changes course and moves away from its existing alignment; 4) stone and fabric lifts will be placed along the left bank of the Suncook River just upstream of the avulsion site to reduce the potential for the channel to erode around an existing grade-control riffle and cause continued incision that could migrate upstream to the Rt. 4 Bridge, up the Little Suncook River, and cause erosion that could result in loss of

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 4 agriculture land; 5) restoration of the Leighton Brook channel bed and banks to improve both vertical and lateral stability.

The measures identified above, if implemented properly, will reduce the risk of impacts to the Rt. 4 Bridge and adjacent infrastructure, loss of agriculture land, and impacts to Black Hall Rd. and adjacent infrastructure. They will also help maintain the existing conditions of the extensive wetlands within the lower portions of the Little Suncook River and limit the amount of excessive sedimentation in the Suncook River due to continued bank erosion.

The designs for the bank stabilization along the Suncook River involve the grading of banks and placing of stone along the channel banks above the existing low-water elevation. This method allows for construction to occur without the need for expensive and disruptive dewatering systems or major excavation of the channel bed. Two other alternatives for bank stabilization include trenching in rock along the bank or further widening the channel. Rock trenching would involve excavating a cavity in the river bed for the riprap treatments. This differs from the proposed option as riprap protection down to the scour depth would be placed pre-emptively by excavation rather than mounding extra rock to be launched following river bed scour. The benefit to this alternative is that there would be greater certainty in the location of the final installed treatment compared to launchable material. However, it is difficult to excavate trenches in sand in active river beds below the water line. Dewatering would be difficult and cost- prohibitive. There would also be more impacts to resource areas as larger areas of the existing channel bed would be disturbed to construct cofferdams and place riprap.

Widening the river was also considered as it would provide more flow conveyance area to dissipate flood energy and reduce the potential for further incision and bank erosion. This option would also reduce water surface elevations during floods. Negative aspects to this option include more impacts to resource areas, and additional loss of land for adjacent property owners. Resource areas would be impacted as excavation would require removing additional trees and disturbing larger areas. Residential and agricultural land also abut the bank tops in many locations. Excavating a wider floodplain area would require the loss of land usable for agricultural and residential purposes.

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 5 Impacted Wetlands and Upland Forests - Wetlands will be impacted during the implementation of the chosen designs. The eastern portion of the sheet pile will be driven into an overflow channel of the Suncook River which is a functioning Palustrine emergent wetland (Appendix A). The sheet pile will be driven in three feet below the existing ground surface in this wetland and the wetland soils removed to place the sheet pile will be replaced back on top of the sheet pile so that wetland functions can continue. Wetland trees and shrubs will be planted to match the species within the existing wetland and replace the trees and shrubs removed as part of the construction. Much of the work area within the Avulsion Site bank stabilization is within a Palustrine scrub/shrub wetland on top of the channel banks. This area will be cleared for vehicle access and for completing the bank stabilization. Following the construction, the entire area will be replanted with native seed, shrubs, and trees to match the existing wetland and replace the trees and shrubs disturbed.

Clearing and grubbing downstream of the Rt. 4 Bridge will be necessary to build the bank stabilization measures as designed. Many of the trees along these banks have become destabilized due to the incision and bank erosion since the avulsion. However, additional trees will need to be removed to access the banks as well. Following construction, all disturbed areas will be reseeded and replanted with trees and shrubs of the same species removed and in greater numbers than were removed.

Alternative 3: Replace the Rt. 4 Bridge

During the design and design review process, we assessed the feasibility of replacing the Rt. 4 Bridge with a valley-spanning bridge that would allow the Suncook River to freely migrate underneath across the alluvial valley. To span the valley of approximately 900 ft, this bridge would need to have approximately 6 spans, each 150 ft long. This option would result in less direct impacts to the Suncook River than active vertical and lateral channel stabilization. It would also allow the channel to migrate naturally, and adjust naturally, to its new conditions since the avulsion took place. A few factors resulted in this option being eliminated as a viable option:

• Cost - the cost of designing a new bridge of this magnitude would be $1.2-$1.5 million and the cost of building this bridge would be approximately $10-$14 million. This would

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 6 add approximately $8.6-$12.9 million to the project cost (Leighton Brook would still need to be stabilized). • Adjacent infrastructure - replacing the bridge and not stabilizing the river could result in loss of agriculture land, utilities, driveways, and houses upstream and downstream of the Rt. 4 Bridge. Residents have built on, and farmed, land adjacent to the river in these locations because of the recent stability of the channel as a result of the bridge and the impoundments caused by the dams downstream. • Impact to tributaries - no stabilization along the Suncook River could result in continued migration of knickpoints on the Suncook River that could continue up the Little Suncook River. A large knickpoint migrating up the Little Suncook River could result in loss of wetland habitat as well as impact the Black Hall Rd bridge further upstream.

NHDOT officials and other project partners did not feel they would have the resources to pursue this option and also compensate landowners for loss of land or homes, so this option was not approved.

Alternative 4: Construct Valley-Spanning Grade-Control Riffle

Because we are very hesitant to use sheet pile in, or near, river channels and wetlands, we assessed the possibility of replacing the sheet pile on the floodplains with a valley-spanning grade-control rock riffle. This type of rock grade control across the alluvial valley (visible within the channel but buried under a vegetated floodplain) has worked well on other projects where we want to allow the channel to migrate naturally, but we still need to provide vertical grade control. As the channel migrates laterally, the buried grade control becomes the new riffle in the channel and provides the same grade-controlling function as the original stone placed within the channel.

For the Suncook River, however, the valley is wide and the amount of grade loss to protect against is high, resulting in a large amount of rock. We assumed a valley width of about 650 ft, spanning from the railroad grade on the east to the race track on the west and spanning the channel where the existing second riffle is located downstream of the Rt. 4 Bridge (Figure 1). We assume a down-valley length of stone placement to be 200 ft - if the downstream knickpoint migrated to this point, the 10-ft knickpoint spread over 200 ft would result in a 5% slope. A steeper slope could cause challenges for fish and other aquatic organism passage. We also assumed a thickness of 7 ft of rock, approximately two grain size diameters in thickness of 3 to

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 7 4-ft rocks. Outside of the channel, the rock would be placed approximately 13-15 ft below the ground surface. If exposed, the rock would launch into the scour hole below and form a new channel alignment.

While not as costly as Alternative 3, the volume of rock required for this option was cost- prohibitive as well. While the downstream bank stabilization and floodplain sheet pile would have been removed from the project costs, the cost for this alternative would have added approximately $4 million to the total project cost, with the bank stabilization upstream as well as the work at Leighton Brook still needing to be completed.

2nd riffle

200

650

Figure 1: Approximate location of valley-spanning grade control riffle as proposed in Alternative 4.

Alternative 5: Buttresses Downstream of the Two Bridges

Another alternative discussed during the design process was placing rock in the channel and floodplains immediately downstream of the Rt. 4 Bridge and the Black Hall Rd culvert. Large volumes of rock could be buried in the form of a buttress in the channel and along the floodplain to connect with the road prism. The rock within this buttress, if exposed by upstream migrating

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 8 knickpoints, would fall into the scour hole and limit damage to the bridge and road infrastructure.

While this alternative results in less direct impact to the channel bed and banks along both rivers, and would likely be a lower-cost option, there are some challenges and risks that resulted in this alternative not being viable in achieving project goals. First, to create the buttress, a large hole would need to be dug in the channel and floodplain to fill with rock. Digging a large hole in the channel immediately downstream of a bridge or road crossing leaves open the possibility that a large flood during construction could result in the collapse of that hole and the resulting collapse of the bridge abutments or culvert. To reduce the volume of rock placed, and thus the size of the hole, another option would be to place sheet pile at the downstream end of the rock buttress across the channel. This sheet pile could be buried under the channel bed with rock on top so that it is only exposed if the knickpoints migrate upstream. Risk remains in this option, however, that if the knickpoint does migrate up to the sheet pile, a steep waterfall could form at the sheet pile. The sheet pile could fail because of the upstream pressure and lack of toe protection leading to the failure of the infrastructure immediately upstream.

This alternative also does not reduce the risk of impacts to agriculture fields, utilities, roads, buildings, and houses that are adjacent to the Suncook River and Leighton Brook. Bank erosion would likely worsen with the upstream migration of knickpoints, resulting in damage to private property and infrastructure and potentially putting lives at risk. If the Suncook River were to erode around the large knickpoint near the avulsion site, this knickpoint could rapidly migrate upstream through the Little Suncook River and result in a loss of wetland habitat and potential impact to the Black Hall Rd Bridge.

Background

In December 2011, Inter-Fluve submitted the 30% complete design drawings, basis of design memo, and opinion of probable cost. Following the review of these materials, NH DES requested the completion of the 2-dimensional hydraulic model for existing and proposed conditions downstream from the Rt 4 Bridge. The results of this and further design and hydraulic analysis suggested the need for bank stabilization measures between the Rt 4 Bridge and the second riffle downstream from the bridge. A preliminary opinion of probable cost was developed for rock bank stabilization in this area. Inter-Fluve also developed preliminary opinion of probable costs 2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 9 for constructing a valley-spanning rock grade control structure rather than the sheet pile as presented in the 30% designs (see Alternatives Analysis section above). The cost of the valley- spanning grade control, once presented to the Project Partners, was found to be too cost prohibitive and the original designs of using sheet pile were retained.

Due to the construction costs estimated for the Suncook River and Leighton Brook, the Project Partners decided not to pursue restoration work on the Little Suncook River. NH DES staff will secure letters of permission from abutters to the project area to conduct surveys to monitor the river and address the need for stabilization at a later time if necessary. The three remaining study areas, as presented in the design plans, are the Second Riffle (SR), Avulsion Site (AV), and Leighton Brook (Figure 2). This 90% Basis of Design Memo updates the 75% Basis of Design Memo based on project partner comments. We have retained some of the general background information for reference, but for additional details regarding various aspects of the data collection, hydraulic modeling and other tasks, please see the following reports:

• Suncook River Design Survey - December, 2011 • Site Assessment - partially completed task - December 2011 (to be completed in Feb 2013) • 30% Basis of Design Memo and Planset - December, 2011 • Structural Assessment and Utility Location Information - February, 2012 • Pre-Application Permit Meeting - February, 2012 • Suncook River 2-Dimensional Hydraulic Model - March, 2012 • Suncook River Existing Data Collection and Fluvial Geomorphic Analysis - March, 2012

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 10 Second Riffle (SR)

Lag Deposit

Avulsion Site (AV)

Leighton Brook

Figure 2. Areas of restoration design on the Suncook River and Leighton Brook. The three lag deposits resulting in riffles on the Suncook River are identified.

Existing Studies Since the avulsion in 2006, multiple studies have been completed on the Suncook River. Water quality was described in 2006 immediately after the avulsion (Daley, 2006). The geology and geomorphology have been well described by Perignon (2007) in an undergraduate thesis and the "Geomorphology-based Restoration Alternatives Study" completed by VHB (2008). The USGS completed a flood study of the entire Suncook River mainstem to update the FEMA maps and

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 11 flood lines (Flynn, 2009). Prior to the avulsion, the subsurface water resources were described in a geohydrology and water quality report completed by the USGS (Stekl and Flanagan, 1997).

In this report, we do not describe the geology, geomorphology, or water quality in detail as this has already been done, but we do refer to the reports mentioned above. Here, we describe the field work and modeling completed to guide the designs and the reasoning behind the designs.

Survey In September 2011, we completed topographic and bathymetric surveys in potential stabilization areas as well as along cross sections from upstream of the Rt 4 Bridge to downstream of Round Pond, including the Little Suncook River and Leighton Brook. Inter-Fluve completed this survey to collect the necessary information to complete a hydraulic model and design drawings sufficient for construction.

Inter-Fluve used a survey-grade RTK GPS (Top Con) as well as Sokkia total stations to complete the topographic surveys. We used the RTK in areas of minimal canopy cover and to establish control points throughout the study area. The base station for the RTK unit was set on the sand bar downstream of the avulsion site on river right as this provided a clear horizon and view of the sky as well as some protection from the public. We downloaded and post-processed all RTK survey data to produce results with root mean square (RMS) errors of less than 0.05 feet. Where the RTK was not efficient or beneficial, we used the total station. All total station data was tied to the RTK data by surveying multiple control points set with the RTK throughout the survey area. All combined survey data was brought into a common coordinate system: NAD83 New Hampshire State Plane Zone 2800, units in feet. We used NGVD29 as the vertical datum to maintain consistency with the USGS surveys and area control points established by Eastern Topographics in 2007 (Eastern Topographics, 2007). All survey data was aligned with control point HV0735 on Rt 4 established by Eastern Topographics. Comparison with a second control point (HV0738) collected by Eastern Topographics showed a difference of less than 0.06 feet horizontal and 0.01 feet vertical.

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 12 VHB and the USGS conducted previous cross sectional surveys in 2007 and 2009, respectively. Our survey included the re-survey of 27 VHB cross sections (see Appendix B: Task 4 Technical Memo) as well as the resurvey of USGS cross sections from 2500 feet upstream of the Rt 4 Bridge (USGS section 'DO') to downstream of Round Pond (USGS section 'CJ'). We identified the VHB and USGS cross sections by loading the cross sections, air photos, control points and other information onto a Trimble GeoXT GPS. When found, we resurveyed control points common to the VHB and USGS surveys so that all survey information could be combined into a common coordinate system and vertical datum to allow for accurate comparisons between surveys.

Detailed survey data was collected from the Rte 4 Bridge to the mouth of the Little Suncook River, at the failed railroad crossing over the Little Suncook River, along the banks and bed of the steep riffle upstream of the avulsion site, and between Black Hall Rd and the Suncook River along Leighton Brook. These were all locations where construction was expected, and detailed topographic information was necessary for accurate designs and construction details. The survey data reflects the topography at the time of the survey; the Suncook River continues to adjust and alter the channel bed and bank topography.

Inter-Fluve set permanent and temporary control points throughout the study area. Temporary control points were typically wooden stakes. Permanent control points were Figure 3: Types of control capped rebar, spikes in the ground, PK nails in the pavement points set for the topographic of roads or sidewalks, and chiseled marks on rocks and survey: (top to bottom) capped rebar, spike, and PK concrete (Figure 3). These control points will be used nail. throughout the construction process and for any future surveys.

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 13 Hydrology The USGS Flood Study provides recurrence interval flows for the 1.8-, 2-, 5-, 10-, 25-, 50-, 100-, and 500-year flood quantiles, as well as the April 2007 storm (Flynn, 2009). The USGS Flood Study used flow data from USGS gage 01089500 on the Suncook River at North Chichester, NH. For a complete description of the methods used by the USGS to develop flood quantiles for the Suncook River, refer to the USGS scientific investigations report #2010-5127 (Flynn, 2009). We used these flows in the steady state, 1-dimensional, HEC-RAS hydraulic model we developed for the main stem Suncook River (Table 1).

No flow data has been published, or could be found, for the tributaries in the study area, Little Suncook River and Leighton Brook. While the USGS model accounted for a flow change at the Little Suncook River, it did not account for Leighton Brook. We used the USGS StreamStats program to estimate peak flood magnitudes for the recurrence intervals listed above for both the Little Suncook River and Leighton Brook (Appendix C). The StreamStats program uses regional regression equations, drainage area, regional precipitation, and land use statistics to approximate flows in watersheds with no gage information. We compared the StreamStats output for the Little Suncook River with the difference in Suncook River flows in the USGS model based on the flow change locations. The StreamStats data ranged from 25% higher at the 2-yr flood to 7% higher at the 100-yr flood and 6% lower at the 500-yr flood (Table 1). These differences are likely due to the differences in timing of the peak flows between the Little Suncook River and the Suncook River. Suncook River peak flows are likely to occur after the Little Suncook River discharge has already peaked and is on the falling limb of the hydrograph. We used the StreamStats hydrology data for the HEC-RAS model as it provides more conservative data and is more accurate for peak flows specific to the tributaries.

Table 1: Water discharge at various recurrence intervals for the Suncook River and tributaries using data from the USGS Flood Study (Flynn, 2009) and the USGS StreamStats program. Flows (cfs) at Given Recurrence Interval (yrs) Stream and Source of April 1.8 2 5 10 25 50 100 500 Location Data 2007 Suncook River at USGS, 2009 2145 2,320 3,700 4,870 6,690 8,330 10,200 15,900 11,000 Rt 4 Little Suncook USGS, 2009 459 534 851 1,121 1,539 1,914 2,351 3,661 2,538 River at mouth Little Suncook USGS 650 713 1,100 1,420 1,820 2,150 2,530 3,440 2,700 River at mouth StreamStats Leighton Brook at USGS 35 39.7 68.3 93.3 128 158 194 283 200 mouth StreamStats 2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 14

Hydraulics The HEC-RAS hydraulic model developed recently by the USGS (Flynn, 2009) was modified for this project to build existing and proposed models. Channel geometry has continued to change on the Suncook River since the USGS survey was completed, so we resurveyed all USGS cross sections and added additional cross sections where necessary. The USGS model extends sufficiently upstream and downstream of the project reach to make the effects of boundary condition assumptions insignificant within the project reach.

Inter-Fluve performed a site survey for design in September 2011 as described above. The USGS provided a shape file showing the geographical locations of the cross sections it used for the 2009 FEMA model report. Our channel data surveyed in 2011 was combined with the USGS overbank data used in its hydraulic model. During the process of combining the data sets, we discovered that not all USGS survey data were referenced to a horizontal datum. Instead, the USGS surveyed its sections to a relative vertical datum, and one survey point at the end of each section was surveyed to a horizontal and vertical datum using an RTK GPS or a level-loop survey. Thus, we converted the elevations for each USGS section to the NGVD29 vertical datum, but spatial orientation in the horizontal plane was hand drawn relative to the endpoint of each section. The USGS method is a reasonable approximation for hydraulic modeling, but it was a challenging data set to combine with our 2011 data.

We developed HEC-RAS cross sections from our survey data and fit them within the USGS HEC-RAS sections by adding a constant to cross section stationing and adjusting the constant until the riverbanks aligned. USGS HEC-RAS cross sections were then updated with our survey data. In some cases, our survey provided overbank survey data and that was used instead of USGS data. A comparison of the sections used in our model and those used in the USGS model is provided in Appendix D.

We also added cross sections to our HEC-RAS model that are not present in the USGS model to attain a better resolution of project site hydraulics. The added cross sections represent pool-riffle sequences located within 1000 feet downstream of the Route 4 Bridge. Cross section locations are identified by their HEC-RAS River Station (RS) (Figure 3).

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 15 We also surveyed cross sections at Leighton Brook. Separate HEC-RAS hydraulic models were created for Leighton Brook and the main stem Suncook River. We modeled the tributary separately because the HEC-RAS model utilizes peak flows and differences in the timing of peak flows may exaggerate backwater conditions near the confluences with tributaries. By separating the tributaries and using a normal depth boundary condition (with bed slope), hydraulics in the tributaries are intensified to result in a more conservative design condition. Being conservative is considered a more desirable approach than underestimating hydraulic intensity, which could result from modeling a river network and overestimating backwater conditions near tributary confluences. Cross sections were surveyed between the Suncook River and Black Hall Rd on Leighton Brook and used in the HEC-RAS model (Figure 4).

2013 Inter-Fluve, Inc. Suncook River, Epsom, NH 16 4:

2011 Inter-Fluve, Inc. Suncook River Restoration 17 4:

2011 Inter-Fluve, Inc. Suncook River Restoration 18 Design This project is focused on the protection of the Rt. 4 Bridge over the Suncook River and the Black Hall Rd Bridge over Leighton Brook. The Rt. 4 Bridge has footings that are generally at or above the existing water surface and, therefore, are at risk of being undermined if any incision occurs at the bridge (2011 Structural Assessment Technical Memo, Appendix E). The overflow channel to the east passes under a bridge that is on deeper piles and is therefore more protected against scour. Two riffles, consisting of cobbles, boulders, and consolidated till, exist about 500 and 1000 feet downstream of the Rt 4 Bridge (Figure 5).

Downstream of each of these are deep scour pools. The Figure 5: Looking immediate concern for the bridge is the near-term migration downstream at lag deposits (top) about 500 feet and of the knickpoint that is currently stalled at these riffles. (bottom) about 1000 feet downstream from the Rt 4 A large riffle lag deposit upstream of the avulsion site, and Bridge. downstream of the two riffles described above, is also an area of great concern. A 'lag deposit' is defined for the purposes of this study as the glacially- formed deposits of cobbles and boulders found in distinct areas of the Suncook River. The cobbles and boulders are eroding out of a consolidated fine-grained matrix. While this lag deposit currently provides grade control through a steep reach (the channel bed drops in elevation approximately 10 feet over 600 feet of channel length at this riffle), the river is beginning to erode around the left side of the lag deposit through unconsolidated sand and is scouring the channel bed (Figure 6). If the channel migrates completely around this grade control and no large material is available to slow the knickpoint, an approximately 10-foot knickpoint could migrate upstream to the Rt 4 Bridge as well as up the Little Suncook River to the Black Hall Rd crossing. Our designs, as discussed below, address these concerns.

Channel dimensions, incipient grain size, and installed grain size distributions were based on the proposed 500-year flood elevations, velocities, and shear stresses. The 500-yr flood quantile was chosen as it provides a more conservative design criteria to reduce risk, and the larger discharges

2013 Inter-Fluve, Inc. Suncook River Restoration 19 account for some uncertainty in the 100-yr flood quantile as climate change alters hydrological processes.

Hard vs Soft Engineering Practices As a river and wetland restoration firm, we have built our reputation on sound scientific research and engineering standards. We developed and perfected many of the bioengineering practices that have become standard today in many regions of the country. Our goal on many projects is that within 5-10 yrs visitors to the restoration site will not be able to tell that any work was done; the site will look like a natural stream or wetland with little to no evidence of designs or construction. We use logs, root wads, natural river rock, fabric soil lifts and other techniques to achieve this natural look and to provide stability while also allowing Figure 6: Instability at lag for long-term channel mobility where possible. We restore deposit upstream of avulsion the natural riverine processes and allow these processes to site. (Top to bottom) Looking upstream at lag deposit; restore natural habitat. looking downstream at fallen trees along left bank; looking Some of Inter-Fluve's projects have challenges and at sand bank. restrictions that limit the full restoration of some natural processes. These include projects in urban areas where channel migration is unacceptable or where infrastructure is involved. The Suncook River restoration project is one of these types of projects, as a goal of this project is to protect bridge infrastructure. Because of the catastrophic changes that occurred to the river in 2006 and the location of the bridge infrastructure and adjacent houses and farms, channel Figure 7: Driveway and restoration techniques that would allow natural channel potential access on west side of channel looking north processes to recover are more costly. Buying out properties towards Rt 4. and replacing the Rt 4 Bridge and road prism with a larger

2013 Inter-Fluve, Inc. Suncook River Restoration 20 bridge span was too costly and determined to not be a viable option by the Project Partners. In this design submittal, we present and justify our designs that achieve the project goals.

Second Riffle Downstream of Rt 4 Within 1000 feet downstream of Rt 4, two riffles comprised of lag deposits of consolidated soils and boulders have slowed the upstream migration of the knickpoint (Figure 5). Shear stresses at these riffles and downstream site conditions necessitate the stabilization of this area to reduce the potential for scour and erosion at the Rt 4 Bridge. Our designs increase the vertical and lateral stability through a grade-controlling riffle in the channel, extension of the grade control across the floodplain, and rock bank protection between the bridge and the grade-controlling riffle.

Access, Staging, Stockpiling, and Clearing & Grubbing

Access into the areas of construction is available on both the east and west sides of the river (Drawing Sheet 2). On the east side of the river, an existing entrance to the farm field off of Rt 4 will be used to bring vehicles and materials into the site. A temporary access road will follow the edge of the river bank with a staging and stockpiling area immediately adjacent to the east and within the farm field. On the west side of the river, an existing dirt driveway will be used for access (Figure 7). This driveway extends south to an abandoned horse race track, part of which will be used for staging and stockpiling.

Access roads to the river to build the grade-controlling riffle will be built by excavating bank material at a 15% grade from the overbank areas down to the channel bed (Drawing Sheets 5 and 8). The access roads will serve two purposes: (1) to provide access to the river for rock placement along the channel banks and bed, and (2) to provide a cavity to fill with rock that will slow bank erosion and channel migration away from the second riffle’s existing location. The roads will have a 15-foot bottom width with side slopes grading up to the existing ground at a 1.5:1 (horizontal:vertical) slope. Following their use as access, the roads will be filled with rock. The finished grade that comprises the new channel banks will have a maximum slope of 2:1.

Because bank protection will be installed between the Rt 4 Bridge and the grade control riffle, all of the trees and vegetation will need to be removed along both sides of the river within this work area (Drawing Sheet 2). Following construction, trees and shrubs will be replanted on top of these banks to provide additional root stability.

2013 Inter-Fluve, Inc. Suncook River Restoration 21 Channel Bed Designs and Vertical Grade Control

Vertical grade control has been designed at the second lag deposit, or riffle, downstream from the Rt 4 Bridge. Here, rock will be installed just downstream of the riffle, filling in a portion of the existing scour pool. We chose to place the rock downstream of the riffle, rather than raising the riffle elevation, to limit additional rise in the water surface profile and increased frequency of flooding of adjacent infrastructure. The thickness of the proposed riffle rock varies longitudinally and laterally. Rock placement will begin at the upstream end at the existing riffle crest elevation. The slope along the length of the constructed riffle is designed to be at a 6% grade for about 85 feet downstream from the riffle crest and then transition to a 25% grade to the existing pool elevation (Figure 8). These slopes are more gradual than the existing channel bed slopes at this location. While the existing riffle is, and the constructed riffle will be, backwatered during all but low flows, the constructed riffle slope is designed to reduce the potential for supercritical flow conditions and a hydraulic jump at this location in the event that further incision occurs downstream and exposes more of the face of the riffle. These types of hydraulic conditions would require much larger rock sizes to remain stable.

Along the proposed riffle profile, the rock thickness will vary between 2 and 10 feet. At the grade break, however, we have specified a minimum 5 foot thickness laterally along the channel bed and banks.

To determine the required rock sizes to be used in the constructed riffle, we applied the methods of Isbash (1936) and Shields (Julien, 2010). We assumed that the Suncook River downstream of this point will continue to adjust vertically and horizontally in the future. While we designed bank stabilization downstream to prevent the rapid drop in stream elevation (described in more detail below), we assumed that the channel could continue to incise through that downstream lag deposit, or in some other way lower the bed elevation, at approximately a rate of 0.5 feet every 10 years. This treatment assumes that the downstream hydraulic control will be monitored for vertical erosion by the NH Geological Survey. It further assumes that if vertical migration is identified at the downstream hydraulic control through monitoring, that future action will be performed by the State to arrest lateral migration of the right bank. We applied the Isbash method and Shields parameter at the second riffle assuming that up to 2.5 feet of incision would occur at the downstream lag deposit over the next 50 years, which would result in the lowering

2013 Inter-Fluve, Inc. Suncook River Restoration 22 of the water surface elevation just downstream of the second riffle by a corresponding 2.5 feet. This lowering of the water surface elevation would result in larger velocities and shear stresses along the face of the constructed riffle. With a predicted velocity of 10.3 feet/second (Figure 9), we predicted an incipient grain size of 1.35 feet using the Isbash method. The Isbash method was applied since it is assumed that riffle rock will be placed in flowing water and the Isbash method was developed for rock placement in flowing water. The corresponding shear stress is 4.45 pounds per square foot (lbs/ft2) (Figure 10) which results in a 1.05-foot grain size at incipient motion with Shields parameter. Applying the larger of the two results as the 30th

percentile grain size (D30) and assuming a factor of safety of 1.5 produces a maximum rock diameter of 4 feet (Table 2).

Table 2: Grain size distribution for the riprap (Type IV on the designs) placed in the Suncook River at the second riffle downstream from the Rt 4 bridge. Diameter (in) Weight (lbs) % Finer max min max min 100 48 35 5546 2218 90 34 50 32 28 1631 1087 30 24 15 25 19 816 340

SuncookRiver Legend Flow Direction Rt 4 Bridge WS 500-Year 340 WS 100-Year

WS 25-Year

WS 10-Year

330 WS 2-Year Ground

Lag Deposits 320 Elevation (ft)

Channel Bed 310

Constructed Riffle

300 68600 68800 69000 69200 69400 69600 Main Channel Distance (ft) Figure 8: Proposed channel bed and water surface elevations for five flood frequency events under proposed conditions on the Suncook River downstream from the Rt 4 Bridge.

2013 Inter-Fluve, Inc. Suncook River Restoration 23 SuncookRiver 20 Legend

Flow Direction Rt 4 Bridge Vel Chnl 500-Year Vel Chnl 100-Year Vel Chnl 25-Year 15 Vel Chnl 10-Year Vel Chnl 2-Year

10 Lag Deposits Vel Chnl (ft/s)

Constructed Riffle

0 68600 68800 69000 69200 69400 69600 Main Channel Distance (ft) Figure 9: Channel velocity for five flood frequency events under proposed conditions on the Suncook River downstream from the Rt 4 Bridge.

SuncookRiver 8 Legend Flow Direction Rt 4 Bridge Shear Chan 500-Year Shear Chan 100-Year

Shear Chan 25-Year 6 Shear Chan 10-Year Constructed Riffle Shear Chan 2-Year

4 Lag Deposits Shear Chan (lb/sq ft)

0 68600 68800 69000 69200 69400 69600 Main Channel Distance (ft) Figure 10: Average channel shear stress for five flood frequency events under proposed conditions on the Suncook River downstream from the Rt 4 Bridge.

As a precaution against the channel migrating around the constructed riffle, rock will be placed within the voids created by the access ramps built through the banks to place the stone required to build the riffle. Filling the access ramps with stone will extend the rock protection laterally away from the channel approximately 43 feet on the east side and 52 feet on the west side. If the

2013 Inter-Fluve, Inc. Suncook River Restoration 24 channel begins to migrate to the east or west, this rock will reduce the likelihood that the river will erode around the constructed riffle and bank stabilization.

Because the meander belt width for the Suncook River is between 600 and 700 feet (Fluvial Geomorphic Analysis Tech Memo, Inter-Fluve, March 2012), the river has the potential to migrate well beyond the extent of this rock treatment. Therefore, extending grade control protection laterally eastward to the abandoned railroad bed and westward to the abandoned horse track was proposed. The high costs of constructing a valley-wide rock grade control resulted in a preferred alternative of placing sheet pile instead. The sheet pile designs are not yet completed, but preliminary discussions suggest that the sheet pile will likely be driven down to a potential scour elevation of approximately 306 feet and extend to the outer limits mentioned above. The sheet pile will likely begin close to the river immediately upstream of the rock-filled access ramps and just beyond the extent of the bank stabilization treatment. This will result in a lateral overlap of rock and sheet pile, so that 20 to 30 feet of the rock-filled access ramp will be just downstream of the sheet pile. The rock will provide added stability to the sheet pile in the event that the channel begins to migrate and scours out the sand around the riprap and sheet pile.

While the sheet pile alternative is the least expensive option for extending the grade control, it is still costly. If resources are not available to install the sheet pile at the same time as the rest of the project, rapid channel migration during a flood may flank all river bed and bank treatments and cause the Route 4 Bridge to fail, jeopardizing public safety and private and public property damage.

Bank Treatments

Two different bank treatments are proposed between the Route 4 Bridge and the constructed riffle to account for varying shear stresses, geomorphic conditions, and the location of infrastructure. Bank Treatment I extends from the constructed riffle at approximately Station 98+80 upstream to approximately Station 102+50 on the right bank and 103+50 on the left bank with Bank Treatment II extending from these points upstream to the existing riprap near the wingwalls of the Rt 4 Bridge. Both treatments have riprap along the channel banks where the shear stresses are greater than 1 lb/ft2 during the 500-yr flood (flood with the highest shear stresses). Prior to stone placement, geotextile fabric will be placed on the channel banks. The stone will hold the fabric in place, and the fabric will be keyed into the top bank surface. This

2013 Inter-Fluve, Inc. Suncook River Restoration 25 fabric will help prevent piping that could potentially destabilize the bank treatment. Above the rock, fabric encapsulated soil (FES) lifts will be built up to the existing top of bank elevation (note that the 'top of bank' is a higher elevation than the typical 'bankfull' elevation, as these two terms are usually defined, because of the incision that has occurred). These FES lifts provide bank stability under lower shear stresses but also provide a soil medium for plants to grow and provide root stabilization on the top of the banks. Both bank treatments will be sloped at a 2:1 angle for the riprap and FES lifts. The primary differences between the two bank treatments are the grain size distribution (Tables 3 and 4) and thickness of the riprap, which depends on the expectant shear stresses, and the number of FES lifts that will be built on top of each other.

Bank Treatment I involves excavating from the toe of the slope at the low flow water surface elevation (determined by an 80 cfs discharge in the HEC-RAS model) up to a height of 10 feet at a 2:1 slope. The Type I riprap will be placed along this excavated slope at a thickness of 3.4 feet. Varying numbers of FES lifts will be constructed on top of this rock to reach the top of the channel banks. The lifts will be offset from the top of the riprap away from the channel between 4 and 4.5 feet. This offset allows for the launching of riprap due to erosion without resulting in the failure of the FES lifts. The FES lifts will be staked and anchored into the ground surface until the roots from the planted vegetation can provide stability. To size the riprap, we applied the U.S. Army Corps of Engineers (USACE, 1994) method coupled with the HEC-RAS results for the proposed conditions. With a maximum velocity of 7.70 feet/second between stations 98+00 and 104+00 and a 1.25 factor of safety, we estimated an incipient grain size of 0.37 feet during the 500-year flood. Setting this size as the D30 in the rock gradation resulted in a maximum grain size of 1.5 feet (Table 3). The thickness of the riprap revetment is designed to be 1.5 feet since this was the larger of the maximum grain size or 1.5 times the median grain size.

Table 3: Grain size distribution for Bank Treatment I (riprap Type I). Percent Diameter (in) Weight (lbs) passing Max Min Max Min 100 18.0 13.2 290 116 90 12.8 50 11.9 10.4 85 57 30 8.8 15 9.5 7.1 43 18

Additional rock was included in the revetment to account for potential scour in the channel bed. With existing pool depths of up to 10 feet, we assumed that this amount of scour potential 2013 Inter-Fluve, Inc. Suncook River Restoration 26 existed throughout the Treatment I area. The theory behind adding rock to the banks is that, upon channel scour at the base of the banks, the rock will fall into the scour hole and limit further erosion. Assuming that 25% of the riprap will be lost upon eroding into the channel, as recommended by the USACE (1994), we added an additional 1.9 feet of rock thickness to the 1.5 feet for a total thickness of 3.4 feet. The additional thickness was included from the base of the riprap at the water surface up the bank 10 feet. Beyond 10 feet above the low-flow water surface, shear stresses drop below 1 lb/ft2 and FES lifts can be constructed.

Bank Treatment II includes larger riprap with a greater thickness due to higher shear stresses and velocities. This treatment area is between the Route 4 Bridge and station 102+50 on the river right and 103+50 on the river left. The USACE (1994) riprap design method was also applied to this area using a meander bend radius of 380 feet. For this analysis, we assumed the maximum velocity of 8.34 feet/s (which occurs during the 500-year flood) will be applicable throughout the treatment area and the factor of safety is 1.25. A 1.22-foot grain size was predicted to be at incipient motion given these conditions. Similarly to Treatment I, this rock diameter was applied to the D30 in the distribution with an additional factor of safety of 1.1 to obtain a standard rock gradation provided by the USACE. The resulting maximum grain size is 2.5 feet.

Table 4: Grain size distribution for Bank Treatment II (riprap Type II). Percent Diameter (in) Weight (lbs) passing Max Min Max Min 100 33 24.3 1805 722 90 23.6 50 22 19.2 531 354 30 16.3 15 17.4 13 265 111

Potential scour depths were estimated using the U.S. Bureau of Reclamation’s (USBR, 1984) suggested method for bends during peak floods. The method averages the Neill (1973), Blench (1959), and Lacey (1930) regime equations. Application to the Bank Treatment II area results in 10.73 feet of scour below the existing channel bed. This estimate appears reasonable as the scour pool between the first and second riffles downstream from Route 4 is about 10 feet deep.

To account for the additional scour depth, we included additional riprap along the bank designed to launch into the channel when scouring occurs. Adding riprap to the channel bank was preferred over excavating below the water surface because of the challenges, and resources 2013 Inter-Fluve, Inc. Suncook River Restoration 27 required, with keeping the water out of the work area. We increased the riprap volume by 25% to account for loss with the launched rock, resulting in a 6-foot thick rock treatment along the entire 12 feet of bank height.

The existing utility poles and road at the top of the west channel bank provided an additional constraint on placement. To reduce the risk of disturbance to utility poles during construction, we included a 3 foot buffer around the utility poles. Given that the 6 foot thick riprap layer on a 2:1 side slope results in a 12.6 foot width, we placed the bank top about 15 feet from the utility poles. To prevent a reduction in channel hydraulic capacity and increases in shear stresses due to placing rock along the right side of the river, we propose excavation of the left bank to be pulled back approximately 25 feet away from the channel and rock placed along this bank as well (Treatment II).

Flood Implications: Existing vs Proposed

The proposed designs increase the vertical and lateral stability of the Suncook River downstream from the Rt 4 Bridge while minimizing changes to the flood elevations. The 500-year flood elevation stays the same or decreases slightly through most of the impacted area downstream from Rt 4 (Figure 11) as the overall channel conveyance area is increased. Nevertheless, at one location, the 500-year flood rises 0.03 ft over a distance of about 200 ft. The minimal change in the 500-year flood elevation means the adjacent houses and other infrastructure will not see any change in the frequency of flooding following the completion of this project.

During more frequent floods, water surface elevations rise more significantly as the toe of the banks in some locations are being filled in. Just below Route 4 there is a maximum rise of 0.54 ft during the 2-year flood. The rise in water surface during these smaller discharges, however, remains within the bank tops and has no impact on adjacent infrastructure.

2013 Inter-Fluve, Inc. Suncook River Restoration 28 SuncookRiver Flow Direction Overbank Ground Legend 340 Surfaces WS 500-Year - PR 500-year Flood WS 500-Year - IFI Ext WS 2-Year - IFI Ext

WS 2-Year - PR 330 Lag Deposits Ground 2-year Flood LOB

ROB

320 Ground Elevation (ft) Rt 4 Bridge 310

Proposed Constructed Riffle 300

68200 68400 68600 68800 69000 69200 69400 69600 69800 Main Channel Distance (ft) Figure 11. Water surface profiles during the 2- and 500-yr flood events under existing and proposed conditions. Water surfaces drop in most areas during the 500-year flood. 'LOB' and 'ROB' are the left overbank and right overbank elevations respectively.

2013 Inter-Fluve, Inc. Suncook River Restoration 29 Avulsion Site: Lag Deposit ~1000 feet Upstream of the Avulsion To reduce and slow the lateral southeast migration of the channel just upstream of the avulsion site, we recommend Primary stabilizing the left (looking downstream) bank with Access appropriately-sized stone (see design sheets 13-16). This Railroad treatment assumes that the left bank is more susceptible to Grade lateral migration and the right bank will be monitored for erosion. It further assumes that if lateral migration is identified at the right bank through monitoring, that future action will be performed to arrest lateral migration of the right bank. Prior to stone placement, geotextile fabric will Primary be placed on the channel banks. The stone will hold the Access fabric in place, and the fabric will be keyed into the top bank surface. This fabric will help prevent piping that Figure 12: Access to avulsion could potentially destabilize the bank treatment. site. (Top) Looking towards Black Hall Rd along primary In a similar manner as described for the work area access through overgrown path; (bottom) looking towards river downstream of Rt 4, access roads necessary to reach the at access through field from channel bed will be built on the upstream and downstream railroad grade. ends of the construction area. These access roads will be filled with stone as the trucks retreat from the site. This lateral bank treatment will help prevent lateral migration of the channel around the left bank treatment at both ends of the site.

The recommended access for this site is through private land. A direct route from Black Hall Rd includes a previously existing path, which requires some minimal clearing (Figure 12). An alternative access route is further south along Black Hall Rd, where a dirt driveway is actively used to access the field adjacent to this project site for farming. Access through the field and staging in the field would need to be approved and done in cooperation with the landowner.

Hydraulics

All flood elevations up to the 500-year recurrence interval remain within the channel banks at the lag deposit near the avulsion site (Figure 13). Velocities and shear stresses are very high through the avulsion site riffle (Figure 14) with critical and super-critical flows occurring in the riffle

2013 Inter-Fluve, Inc. Suncook River Restoration 30 during most flood events. Large rock will need to be placed along the left bank to counteract the large forces exerted on this channel boundary and reduce further erosion. We applied the results of the hydraulic model to the USACE riprap design method (1994). The method is based on velocity and depth estimates with adjustments for side slopes, bend curvature, and rock angularity and thickness. We applied the USACE method to the cross section from the hydraulic model that produced the highest velocity and required the largest grain size. The method predicted an incipient grain size of 2.55 feet at the 500-year flood quantile. Applying this diameter as the D30 in the rock distribution, rather than the D50, is recommended for this type of treatment by the USACE as a more conservative approach to provide a higher level of confidence in the bank revetments. This approach resulted in a maximum grain size of 5 feet. The thickness of the riprap treatment will be 5 feet, or the thickness of the maximum grain size, as recommended by the USACE (1994). The rock gradation was used for the entire bank including the access road fill. A 5-foot thick launchable rock toe is also included in the design to account for up to 2.5 feet of potential channel incision.

SuncookRiver Legend Flow Direction WS 500-Year 330 WS 25-Year

WS 5-Year

WS 2-Year

Ground

LOB 320 Lag Deposit ROB Elevation (ft)

310

'Avulsion Site' 300 Design Area

66000 66500 67000 67500 68000 Main Channel Distance (ft) Figure 13. Water surface profiles at the avulsion site under existing conditions during the 2-, 5-, 25-, and 500-year floods. 'LOB' and 'ROB' are the left overbank and right overbank elevations respectively.

2013 Inter-Fluve, Inc. Suncook River Restoration 31 SuncookRiver 20 Legend Flow Direction Vel Chnl 500-Year Vel Chnl 25-Year Vel Chnl 5-Year 15 Vel Chnl 2-Year 'Avulsion Site' Design Area 10 Vel Chnl (ft/s)

0 66000 66500 67000 67500 68000 Main Channel Distance (ft) SuncookRiver 16 Legend

Shear Chan 500-Year 14 Flow Direction Shear Chan 25-Year

Shear Chan 5-Year 12 Shear Chan 2-Year

8 'Avulsion Site' Design Area

Shear Chan (lb/sq ft) 6

0 66000 66500 67000 67500 68000 Main Channel Distance (ft) Figure 14. (Top) Average channel velocity and (bottom) shear stresses at the avulsion site under existing conditions during the 2-, 5-, 25-, and 500-year floods.

Grading

The proposed grading along the avulsion site bank will include a 2:1 slope with a bench near the toe for additional launchable rock. To construct the new bank, an access road will be graded at the upstream and downstream ends of the site. The access roads will be sloped at a 15% grade from the top of the overbank area to the toe of the bank. The purpose of the access roads will be

2013 Inter-Fluve, Inc. Suncook River Restoration 32 twofold: (1) provide access to the edge of the river for grading the bank, and (2) create a void space to fill with rock to provide additional protection against lateral, left bank, erosion.

Access to the river is critical as the steep bank is nearly 20 feet tall. Working with an excavator from the top of the bank would require the excavator to be very close to the bank edge, which could possibly fail. Grading from the toe of the bank is therefore necessary for safety reasons. Access to the edge of water is also preferred because the distance from the bank top to the water surface with the proposed grading is approximately 50 feet, which is beyond the reach and loading capacity for long-arm excavators.

The void space left by the access roads will be filled with riprap once the bank grading is completed. In many instances, the transition from riprap to the existing bank is a point of failure with revetment projects (USACE, 1994). By increasing the volume of the rock at these transitions, we will reinforce stability at the transitions to the existing banks. Additional rock in the access roads will also reduce the likelihood of the channel eroding around the bank treatment to the east.

Leighton Brook

Multiple concerns exist at Leighton Brook. The final designs and constructed product need to protect the Black Hall Rd Bridge against the upstream migration of existing headcuts (Figure 15). We also needed to consider the possibility that the Suncook River will continue to migrate eastward thus continuing to shorten the length of Leighton Brook. As the mouth of Leighton Brook continues to fall into the Suncook River, the base elevation of Leighton Brook will lower, which could cause additional headcuts to migrate upstream. Because Figure 15: Looking of the steepness of the Leighton Brook channel, shear downstream at incision along stresses are relatively high compared to what is expected Leighton Brook (top) from Black Hall Rd and (bottom) for most channels of this size, adding to the potential for near the confluence with the instability. Suncook River.

2013 Inter-Fluve, Inc. Suncook River Restoration 33 Access and Staging

Access for the restoration of Leighton Brook will be along Rhodora Drive off of Black Hall Rd (Figure 16). Access into Leighton Brook will be at the location of the existing snowmobile bridge. The bridge will be removed and an access road will be graded at a 4:1 slope to the channel. Construction vehicles will use the existing Figure 16: Leighton Brook channel alignment as access for all remaining work in access along Rhodora Drive Leighton Brook. Because the dirt road at the end of from Black Hall Rd. Rhodora Drive is an active road for nearby residents, vehicular passage will need to remain open from Black Hall Rd to the end of Rhodora Drive and south along the dirt road. The staging and stockpiling area for this work is proposed to be in a forested area on NH State property. The trees cleared for this staging area will be incorporated into the channel bed downstream of the existing snowmobile bridge.

Channel Plan Form and Profile

The existing thalweg alignment of Leighton Brook is generally straight. The proposed conditions reflect the same thalweg location in the reach below the snowmobile bridge, but the reach between the snowmobile bridge and Black Hall Road was moved to prevent further erosion of the failing banks. The shear stresses at the snowmobile bridge could not be reduced sufficiently under proposed conditions with the bridge in place and there are concerns for hydraulics jeopardizing the integrity of the bridge, so the bridge will be removed as part of this project. With the bridge no longer present, we re-located the stream slightly to the north to eliminate a small meander. This planform change moves the stream away from the failing south bank and into the middle of the incised valley. Upstream of the snowmobile bridge, we moved the alignment to the south because the north banks are failing and there is a building about 5 to 8 feet from the top of the bank. The resulting alignment required cutting into the south bank approximately 10 feet.

2013 Inter-Fluve, Inc. Suncook River Restoration 34 In the 30% designs, we proposed building a series of steps throughout the channel to reduce shear stresses while trying to minimize costs. This configuration, however, would not have prevented scour of sand and gravels around and under the steps. That scour could have compromised the steps, and the channel may have eroded around them and continued incising and eroding the channel banks.

The roughened channel design proposed in these designs requires the placement of rock along the bed of the entire length of Leighton Brook from Black Hall Rd to the Suncook River (Figure 17). As described below, the thickness of this layer of rock will be 3 feet and will provide sufficient protection against scour and erosion. The roughened channel design is also a more natural design, with water flowing down and around cobbles and boulders rather than over rigidly designed steps. The cobbles and boulders will protrude from the channel bed to varying heights, which will result in variable flow velocities with slack-water behind rocks and eddies interspersed throughout the channel. This roughness and variable flow provides refuge and habitat for aquatic organisms and will allow easier passage up and down Leighton Brook. In the downstream half of the channel, below the existing snowmobile bridge, the logs that have fallen into the channel, and any logs cut for access and staging, will be incorporated into the channel bed and floodplain to provide additional roughness and habitat.

2013 Inter-Fluve, Inc. Suncook River Restoration 35 Black Hall Rd Flow Direction

Confluence with Suncook River

Figure 17. Thalweg and water surface profile of Leighton Brook during existing (EG) and proposed (PR) conditions. Q2 is the 2-year flood quantile and Q500 is the 500-year flood quantile discharge.

Cross Sectional Geometry The following criteria were considered when designing the cross sectional shape of the proposed channel:

• Minimize the top width of the proposed channel • Contain up to the 500-year flood in the portion of the stream that is lined with rock • Reduce shear stresses • Minimize excavation

Minimizing the top width of the channel was necessary as buildings are located close to the bank tops on the left and right sides of the stream. Creating a channel with a well-developed floodplain would require a wider corridor and may jeopardize the structural integrity of the buildings. We chose to have the 500-year flood quantile contained within the proposed channel because the existing channel substrate contains a large portion of sand. If some flows were allowed to spill over the bank tops, the shear stress required to initiate motion of sand grains is small and the proposed channel rock could be undermined resulting in instability. 2013 Inter-Fluve, Inc. Suncook River Restoration 36 Secondary criteria included minimizing shear stresses and excavation. Reducing shear stresses allows us to reduce the rock size which will be stable, and thus, the cost of the project. Nevertheless, minimization of shear stresses requires spreading the flow out over a wide area to dissipate flood energy. As we attempted to contain the floods in a small area to reduce impacts to adjacent buildings, this criteria had to be considered secondarily. To further reduce project costs, we attempted to minimize excavation costs by filling the majority of the new channel with rock rather than excavating a new sub-grade and replacing the void with rock. This was not possible in all areas, however, as a relatively continuous longitudinal profile was desired.

The resulting channel design has a 10 foot bottom width with 2:1 side slopes and 3.5 foot bank heights. The bank heights correspond approximately with the water surface of the 500-year flood in the upper reach (2.77% slope) and the bank heights are about 0.3 feet higher than the 500-year flood water surface in the lower reach (5.54% slope) (Figure 18). The additional channel capacity in the lower reach is necessary as large woody debris will be incorporated to roughen the channel. The wood occluding the channel will slightly increase water surfaces.

2013 Inter-Fluve, Inc. Suncook River Restoration 37 342 340 338

336 334 332

Elevation (feet) 330 Existing ground 328 Proposed ground 326 500-yr flood 324 0 10 20 30 40 50 60 70 80 90 100 Distance (feet)

352 350 348

346 344 342 Existing ground

Elevation (feet) 340 Proposed ground 338 500-yr flood 336 334 0 10 20 30 40 50 60 70 80 90 100 Distance (feet)

Figure 18. Cross sections at station 3+59 (top) and 6+38 (bottom) in the lower and upper reaches of Leighton Brook respectively. The existing ground (black line) and proposed ground (red line) is shown. While the water surface increases during all discharges due to the smaller channel dimensions, the water surfaces remain well below the top of the valley slopes.

2013 Inter-Fluve, Inc. Suncook River Restoration 38

14 EG Flow Direction PR 12 Black Hall Rd 10 Confluence with Suncook River 8

4 Shear stress (pounds per square foot) square per (pounds stress Shear 2

0 0 100 200 300 400 500 600 700 800 900 Distance (feet)

16 Flow Direction Black Hall Rd 14 EG PR 12 Confluence with Suncook River 10

4 Shear stress (pounds per square foot) square per (pounds stress Shear 2

0 0 100 200 300 400 500 600 700 800 900 Distance (feet)

Figure 19. Leighton Brook shear stress at the (top) 10-year and (bottom) 500-year flood quantile during existing (EG) and proposed (PR) conditions. The increase in shear stress between Station 400 and 500 is due to the increase in slope in the downstream portion of the proposed channel.

2013 Inter-Fluve, Inc. Suncook River Restoration 39

Bed and Bank Materials The new channel will be lined with rock as described in the previous section. The rock gradation

was developed based on sizing the rock size at the D30 as the incipient motion grain size (during the 500-year flood). The larger material in the gradation was developed around requirements of USACE (1994) while the smaller sizes were specified for maximum density of the substrate using the Fuller-Thompson (1907) equation.

Determination of the incipient grain size during the 500-year flood was based on the USACE (1994) steep slope channel design method which was developed for slopes between 2% and

20%. The estimated incipient motion of grain sizes of 12.3 inches and 8.2 inches for the lower

and upper reaches, respectively. The corresponding D100 is 22 to 30 inches in the lower reach and 15 to 20 inches in the upper reach. We chose to apply the gradation developed for the lower reach to the upper reach because (1) it provided an additional factor of safety, (2) the specification of one gradation may lead to lower costs, and (3) the incipient grain sizes for the two reaches were not significantly different. To re-build Leighton Brook with a more natural condition, we also sought to use rounded rock rather than angular quarried rock. Following calls to local quarries, however, we found that rounded rock was only available in sufficient quantities up to about 1.5 foot grain sizes. Therefore, the proposed rock gradation (Table 5) utilizes angular

rock at D30 through D100 and rounded stones below D30.

Table 5: Grain size distribution for Leighton Brook. % Diameter (in.) Weight (lbs) Passing Max. Min. Max. Min. 100 30.0 22.1 1349 540 90 21.4 492 50 24.9 22 775 517 30 19 161 15 1.5 1 - - 5 0.08 0.06 - -

Although the USACE (1994) proposes a thickness of rock equal to the thickness of the D100 or

1.5 times the D50, whichever is larger, we increased the thickness as extra insurance to minimize hyporheic flow under the channel and the undermining of the rock foundation.

2013 Inter-Fluve, Inc. Suncook River Restoration 40 The new channel was also designed to maximize roughness and dissipate flood energy. Large boulders approximately 4 feet in diameter will be incorporated into the channel bed with about ½ the diameter exposed. The increased thickness of the rock along the channel beds will ensure that the boulders are placed on a sub-grade of rock rather than directly on sand.

Buttress At the downstream end of Leighton Brook, we propose installing a rock buttress as described in the 30% designs and basis of design memo. The rock buttress is designed to protect against future Suncook River channel migration (Figures 20 and 21) and the undermining of Leighton Brook. The volume of rock within this buttress is 114 cubic yards and was designed to reduce vertical instability to Leighton Brook if the Suncook River were to continue migrating along its current path. If the Suncook River were to erode the outlet of Leighton Brook, the stone in the buttress would fall into this eroded channel and provide stability against an upstream-migrating headcut on Leighton Brook.

The bottom elevation of the rock buttress was set at the low flow water surface elevation in the Suncook River. Excavation lower than the water surface may be difficult as water may transport sand into the excavated hole. From the bottom elevation, rock will extend vertically approximately 5 feet to the proposed channel bed. Laterally, the buttress will terminate at the toe of the valley walls. The buttress will be buried and will not present a barrier to aquatic organisms moving into Leighton Brook.

2013 Inter-Fluve, Inc. Suncook River Restoration 41 Figure 20: Circles drawn to match the delineated outer top of bank for each year of data.

2013 Inter-Fluve, Inc. Suncook River Restoration 42 Figure 21: Meander belt width at the confluence with Leighton Brook.

2013 Inter-Fluve, Inc. Suncook River Restoration 43 Assumptions, Risk, Levels of Protection, Maintenance

The designs described above, if implemented properly, are meant to reduce the instability of the Suncook River and Leighton Brook that could lead to damage to the Rt 4 Bridge over the Suncook River and the Black Hall Road over Leighton Brook. The Rt 4 Bridge over the Suncook River is undersized and resting on footings inappropriately placed for the current river conditions. The Suncook River is a meandering river that historically migrated laterally hundreds of feet over time. The construction of the Rt 4 road prism eliminated this migration and locked the channel in place at that location. The main channel was locked in its current position with an overflow channel to the east. Full restoration of this site would include the replacement of the Rt 4 bridges and road prism with a structure that allows the Suncook River to migrate hundreds of feet across the valley as it needs to in adjusting to the slope changes downstream as a result of the avulsion. With the recent repairs to the Rt 4 Bridge and the cost of building a more sustainable road structure, this option was not deemed viable. The designs described here are meant to provide stability until the road prism and bridges can be replaced, sometime within the next 50 years.

Placing rock in a channel bed and along channel banks does not provide permanent stability on steep, dynamic rivers in sand-dominated alluvial valleys such as the Suncook River. Eventually, the Suncook River will continue its historic migrational patterns as it adjusts to downstream planform changes and changing hydrologic conditions. Our designs increase channel stability in a few different locations to minimize movement in the near future. The designs include multiple levels of stability so that additional protection is in place should the river erode through the first line of constructed defense.

The bank protection upstream of the avulsion site is meant to slow the upstream migration of an approximately 10-foot tall headcut by reducing the likelihood that the channel will erode around the left side of an existing lag deposit. If allowed to migrate, the headcut could place substantial stress on the constructed riffle upstream and cause instability, avulsions, or both. While the designs include stabilization of the left bank, no work is planned in the main portion of the channel or along the right bank. The channel bed is relatively stable at this time, but in the long term, the Suncook River could continue to erode through the lag deposit as the shear stresses are very high in this reach. In addition, the channel could begin to erode the right bank and, rather

2013 Inter-Fluve, Inc. Suncook River Restoration 44 than wrapping around the lag deposit to the left, could wrap around the lag deposit on the right. The lag deposit appears to extend further into the right bank and further upstream on the right side of the channel than on the left, but over time, the headcut could migrate upstream in this manner. To reduce the risk of this happening, the channel changes in this area should be monitored annually and if large changes occur, stabilization of the right bank may be advisable.

The constructed riffle downstream of the Rt 4 Bridge provides immediate reduction of incision rates at the two riffles, thereby reducing the risk of scour at the bridge. The rock placed at the riffle is also designed to provide channel stability in the event that the Suncook River does erode down 2.5 feet through the lag deposit downstream. The second riffle and the downstream bank stabilization at the lag deposit provide two measures to reduce the risk of continued incision along the existing channel bed.

The bank stabilization proposed between the bridge and the constructed riffle reduces the ability of the river to erode the channel banks and migrate laterally at this location. This will reduce the likelihood of damage to utility poles and infrastructure on the west side of the river and will also reduce the likelihood of the Suncook River eroding around the constructed riffle downstream.

To further reduce the risk of the Suncook River eroding around the constructed riffle, we have included additional lateral stability. At the constructed riffle, the access roads cut into the banks to access the channel will be filled with rock. Additionally, sheet pile will be driven into the ground from these access roads to the railroad berm to the east and the abandoned horse track to the west. If the Suncook River begins to erode around the constructed riffle, or if the river avulses to the overflow channel or another location, these designed features will slow the vertical incision, providing time for maintenance and restoring the channel to its current location. The sheet pile is a less costly alternative to other forms of valley-wide lateral stability, but it will require annual monitoring and, if exposed by river flows, maintenance. If the sheet pile is exposed and scour occurs along the face of the sheet pile, rock will need to be dumped into the scour hole to protect the toe of the sheet pile and prevent the sheet pile from tipping over.

Because of the infrastructure that is at risk from channel instability, the areas of construction described here need to be monitored regularly until the road prisms and bridges are updated appropriately, the houses and other buildings are moved outside of the meander wavelength, and the channel once again has the ability to migrate across its alluvial valley without posing great

2013 Inter-Fluve, Inc. Suncook River Restoration 45 risks to infrastructure. Any necessary monitoring should be completed by the NH Geological Survey.

2013 Inter-Fluve, Inc. Suncook River Restoration 46 References Blench, T. 1969. Mobile-bed fluviology. University of Alberta Press. Daley, M.L. 2006. Initial Water Quality Results for the Suncook River after avulsion on May 15, 2006. NH Water Resources Research Center, Department of Natural Resources, University of NH. Eastern Topographics, 2007. Photogrammetric Ground Control Report for Town of Epsom, NH: Suncook River Area, Epsom, NH. Eastern Topographics, Wolfeboro, NH. Flynn, R.H., 2009. Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009. Scientific Investigations Report 2010-5127. Fuller, W.B., and S.E. Thompson. 1907. The laws of proportioning concrete. Journal of the Transportation Division. American Society of Civil Engineers, 59: 67-143. Inter-Fluve, Inc., 2011. Task 4 Technical Memorandum: Suncook River Design Survey Lacey, G. 1930. Stable channels in alluvium. In: Proceedings of the Institute of Civil Engineers, vol. 229. Neill, C.R. 1973. Guide to bridge hydraulics. Roads and Transportation Association of Canada, University of Toronto Press, Toronto, Canada. Perignon, M.C. 2007. Mechanisms governing avulsions in transient landscapes: Analysis of the May 2006 Suncook River avulsion in Epsom, New Hampshire. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology. Stekl, P.J. and Flanagan, S.M., 1997. Geohydrology and Water Quality of Stratified-Drift Aquifers in the Upper Merrimack River Basin, South-Central New Hampshire. USGS Water-Resources Investigations Report 95-4123. Pembroke, NH. U.S. Army Corps of Engineers (USACE). 1994. Hydraulic design of flood control channels; engineer manual 1110-2-1601. Washington, D.C. U.S. Bureau of Reclamation (USBR). 1984. Computing degradation and local scour. Technical Guideline for Bureau of Reclamation, Denver, CO. VHB, 2008. Suncook River Avulsion, Geomorphology-based Alternatives Analysis: Final Technical Report. Bedford, NH. Wittkop, C., D. Bennett, R. Chormann, and D. Wunsch, 2007. Geology of the May 2006 Suncook River Avulsion, in: Guidebook to Field Trips in New Hampshire, Adjacent Maine & Massachusetts, 42nd Annual Meeting Northeastern Section, Geological Society of America, March 11, 2007. University of New Hampshire, Durham, NH, p. 45-55.

2013 Inter-Fluve, Inc. Suncook River Restoration 47 Cost Estimate The following cost estimate provides costs to the 90% design level for the phases of construction, 15% contingency, and construction observation. The sheet pile installation cost are separated out. The total cost with contingency and construction observation is approximately $2,929,306.

Combined Construction Cost Estimate 90% Design Submittal Construction Suncook River $1,597,187 Suncook River sheetpile $572,250 Leighton Brook $273,438 Subtotal $2,442,875 Contingency (15%) $366,431 TOTAL $2,809,306

Construction Observation Construction Observation $120,000

GRAND TOTAL $2,929,306

2013 Inter-Fluve, Inc. Suncook River Restoration 48 Suncook River Construction Cost Estimate 90% Design Submittal No. Bid Item Unit Unit Price Quantity Subtotal S1 Mobilization & Demobilization LS $197,222 1 $197,222 S2 Access & Traffic Control LS $15,000 1 $15,000 S3 Erosion & Pollution Control LS $50,000 1 $50,000 S4 Excavation (cut) CY $6 12,800 $76,800 S5 Excavation (fill) CY $6 1,000 $6,000 S6 Rock TON $65 17,500 $1,137,500 S8 Fabric encapsulated lifts FF $15 5,451 $81,765 S9 Seed LB $100 25 $2,500 S10 7-Gallon Container Plants EA $160 140 $22,400 S11 5-Gallon Container Plants EA $100 80 $8,000 Construction Subtotal $1,597,187

Alternate Items

S12 Suncook Floodplain Sheetpile SF $35 16,350 $572,250

15% contingency $325,415

Construction Total $2,494,852

Construction Observation

Construction Observation LS $80,000 1 $80,000

GRAND TOTAL $2,574,852

2013 Inter-Fluve, Inc. Suncook River Restoration 49 Leighton Brook Construction Cost Estimate 90% Design Submittal No. Bid Item Unit Unit Price Quantity Subtotal L1 Mobilization & Demobilization LS $24,858 1 $24,858 L2 Access & Traffic Control LS $5,000 1 $5,000 L3 Erosion & Pollution Control LS $10,000 1 $10,000 L4 Excavation (cut) CY $6 1,080 $6,480 L5 Excavation (fill) CY $6 700 $4,200 L6 Framework Rock TON $65 1,520 $98,800 L7 Matrix Rock TON $65 1,520 $98,800 L8 Existing wood salvage EA $200 30 $6,000 L9 Seed LB $100 25 $2,500 L10 7-Gallon Container Plants EA $160 80 $12,800 L11 5-Gallon Container Plants EA $100 40 $4,000 Construction Subtotal $273,438

15% contingency $41,016

Construction Total $314,454

Construction Observation LS $40,000 1 $40,000

GRAND TOTAL $354,454

2013 Inter-Fluve, Inc. Suncook River Restoration 50