Loading...
HomeMy WebLinkAbout09042018 Planning & Zoning Work Session Packet \¥ « ^© / JAPANESE CREEK . | ■ ■ ; Dieckgraeff ' e , ■ Flood Mitigation & compilation of studies, information, and recommendations on < Japanese Creek and flooding of D@ck me9 Road. Japanese Creek Japanese Creek Dieckgraeff Road Flood Mitigation STREAM OVERVIEW Japanese Creek is a stream which drains a large watershed to the northwest of Seward. It is a large catchment, approximately 7.5 square kilometers,which contains the remnants of cirque glaciers and debris laden grounded ice in the valley. The outlet for the stream is confined to one pinch point between Mount Benson to the north and Mount Marathon to the south. The stream then veers sharply to the north due to a levee system which was constructed to protect infrastructure and the Gateway,Afognak and Forest Acres neighborhoods, which were built upon the alluvial fan the stream has made over many thousands of years.The gradient of the stream,once it passes the toe of the mountain slopes, is typical of one found on alluvial fans. The average slope is between 5 and 9 degrees with the lower slope located to the north and south of Dieckgraeff Road. ' Japanese Creek has an abundant supply of sediment which it erodes, transports, and deposits within its reaches. The retreating glaciers have left behind vast deposits of till in the valleys upstream and the unstable slopes deliver a constant supply of sediment through debris flows, rock and snow avalanches,and rock fall.When large magnitude storms occur, the unconsolidated sediment is easily mobilized. During the floods of 2006 and 2012, the corridor experienced massive sedimentation, with some areas experiencing roughly 3 meters of deposition.' r�. The community of Seward has experienced major flood events in 1986, 1995,2006,and 2012. During these events,and otherp less significant events, Japanese Creek has overwhelmed the Y existing culverts and overtopped Dieckgraeff Road, the only access to the community's solid waste transfer station, As ti sediment has built up in the stream corridor, the active channel has migrated across the lower reaches of the *� floodplain, filling the culverts and causing damages to the road, requiring closure and restoration on a more frequent basis. In recent years, the active channel has moved to the east, passing through drainage ditches and culverts, causing flooding in the Forest Acres subdivision. Japanese Creek outlet between Mt. Benson& Mt. 1 Excerpts from Mass Wasting&Sedimentation Trends in SBCFSA 2006-2615 Marathon. Note the unstable avalanche chutes on Mt. Benson. Page 1 Japanese Creek PROPERTY OWNERSHIP Japanese Creek flows from the alpine reaches on Alaska State Department of Natural Resources (DNR) lands, onto the City of Seward parcel west of the Gateway subdivision, onto Afognak Construction private property, to Alaska DNR and University of Alaska (UA) parcels west of Dieckgraeff Road, through culverts in the Kenai Peninsula Borough (KPB) road right-of-way, onto UA land northeast of Dieckgraeff Road and Forest Acres subdivision, to City of Seward property on the west side of the Seward Highway and Alaska Railroad bridges, onto private property on the east side of the highway to join Resurrection River. S Unhmrsft of Alaska State DNR ME fn dTA iOWS P Mile ., 1 l� f� S Imo+?a� �jjII ■�%litl� ,� � -A 7. Page 2 _. Japanese Creek IDENTIFIED PROBLEMS Existing Armored Embankment There is the potential for significant flood damages within the Japanese Creek alluvial fan, The City of Seward currently maintains an uncertified armored embankment approximately 5,200 feet in length that constrains the creek to its channel between the embankment on the east and a mountain ridge on the west. This embankment is inspected after heavy rain events and has required periodic maintenance. The city states that this structure has the capacity to divert flows associated with a 10- to 20-year flood event. However, the structure is not a US Army Corps of Engineers certified levee.The existing armored embankment is inadequate to provide protection against flaws with a greater than 10-20 year return interval. - f'. .4� `�i,7Ak 1 • Y. Y r i The area is characterized by development on the lower part of an alluvial fan with Japanese Creek running along the upper boundary. The embankment has been constructed in 3 different phases over a 20-year time frame. There is no data currently available pertaining to the material used during construction. The embankment was originally constructed to redirect the creek and prevent flood damages to personal property. Subsequent development in the alluvial fan fallout area has increased the amount of potential damages related to a potential failure of the embankment. Potential flood damages will likely continue to increase as the subdivisions are built to capacity. Page 3 Japanese Creek In 2013,there were 80 residential structures and five public buildings in the floodplain including:three schools, a senior care housing and medical facility, and the Seward Military Resort. This area has historically been susceptible to damages from break out flooding when the creek migrated due to water flow levels or when sediment filled the stream bed. Historical aerial photography identifies several historical flow paths of the creek where break outs have occurredz. Sediment Transport & Deposition A recent study for the Seward/ Bear Creek Flood Service Area (SBCFSA) mapped the volumes of sediment change between flood events for local creeks, including the Japanese Creek drainage. During the floods of 2006 and 2012, the corridor experienced massive sedimentation, with some areas experiencing roughly 3 meters of deposition. A past study conducted by Northwest Hydraulic Consultants Inc. measured the amount of sediment delivered in the October 2006 rainstorm and found that nearly 1SO,000 cubic meters of sediment was deposited within the reserved stream corridor in the matter of days.This addition of sediment reduces the capacity of the corridor which increases the risk of flood waters overtopping the levee system and , impacting Dieckgraeff Road and the neighborhood to the east. While it was concluded that the total volume available in the stream corridor is approximately 700,000 cubic meters, or 45 times the amount of sediment that was deposited in the storm of 2006, it was also 2 recommended to remove sediment from the corridor, concentrating efforts on the upper reaches(NHC, 2007)3. Change in Volum High : 4 m^3 Low : -4 This map captures the flood of October 2006, where massive sedimentation occurred. The red area in the middle is where the bridge was washed out during the event. The green areas experienced up to 4 meters of grovel deposition. Excerpts from USACE Japanese Creek CAP Section 205 Preliminary Fact Sheet,May 2013 3 Excerpts from Mass Wasting&Sedimentation Trends in S8CFSA 2006-2015 Page 4 Japanese Creek Channel & Culvert Blockage Dieckgraeff Road has one 12-foot diameter culvert,sided by two 4-foot diameter culverts for the main channel of Japanese Creek. There is a four-foot diameter culvert north of the main channel that captures waters from the wetlands west of the road. There are two additional 2-foot diameter culverts north of the main channel and a designed low area for sheet flows to cross the road. In 2018, two additional culverts were installed on the south side of the main channel,to capture the channel migration to the east. The floods of 2006 and 2012 scoured the upper reaches of the stream corridor and redistributed sediment to the lower reaches, upstream and downstream of Dieckgraeff Road. During these floods and other high water events, the culverts are filled with sediment and the road overtopped, closing access to the KPB Solid Waste Transfer facility, and requiring restoration work to be completed. 2-2 ft culverts ft culverts ZA Page 5 �AI r �M�. in � 4/ P .A $ k�i��,,V►� ,y� h �3 w r 1 / / / • / 1 • /I f • I ' I • t ' 1 x S 1 r. .00 Jay' F - .�: . :.: .. . s.:.-:. .�-�..:�:•"f �3.,.�'. _.�r, r ... -. :. r r rr 'r r •• r • r � r• • r • 'r r- 1 r 'r • • r / • r r r rr err -r Japanese Creek RECOMMENDATIONS In the Northwest Hydraulic Consultant's report on Japanese Creek,following the October 2006 flood,they had the following conclusions and recommendations which still apply twelve years later: NHC Conclusions • The October 2006 flood deposited nearly 200,000 cubic yards of material in the corridor, leaving depots that averaged 2 to 5 feet deep. In many places deposits were 5 to 10 feet thick or more. • It would only take about 900,000 cubic yards of material to completely fill the corridor to the top of the existing levee. This is less than 5 times the volume of sediment that was deposited during the October 2006 flood. • The corridor will continue to fill with sediment as future floods deposit material. At some point, long before sediment fills the entire corridor,Japp Creek will overtop the existing levee and find a new and likely destructive path down its fan. Therefore, sediment must be periodically removed from the corridor. • We do not know if the existing corridor can safely contain a 100-year flood. However, based upon reports that the levee was nearly overtopped in the 1986 flood and the fact that several large, sediment carrying floods have occurred since then, we suspect that it cannot. Since the NHC report, nearly 200,000 cubic yards of material have been deposited in the stream corridor, further reducing the carrying capacity. The US Army Corps of Engineers stated in the 2013 Japanese Creek preliminary fact sheet that the existing armored embankment is inadequate to provide protection against flows with a greater than 10-20 year return interval. NHC Recommendations • Begin the sediment removal process this fall [2007). If time and money will allow, remove roughly 100,000 cubic yards beginning at the upstream end of the existing levee and continuing downstream into and possibly through reach 4. We recommend lowering the floor of the corridor an average of about four feet in this area. A buffer of sediment should be left along the toe of the exiting levee and a low flow channel should be established. If 100,000 cubic yards is too costly,we recommend removing as much as the community can afford at this time. • Develop a concise and clear flood and sediment management plan for Japp Creek based upon scientific and engineering data. The plan should address monitoring and maintenance activities that need to take place over at least the next 50 years. It also should include a mechanism to fund the periodic removal of sediment and future maintenance to the levees. • Prepare a scope of work that identifies the tasks that need to be completed to develop the management plan and seek funding to complete the work. We recommend that the City target completing the plan by December 31, 2008. Page 7 h Japanese Creek Sediment Management There are several challenges involved in removing sediment from local streams, including site control, the costs to remove and haul material to another location, and finding an upland or permitted location to receive the fill. Site Control & Deposition Locations A Japanese Creek Sediment Management Plan would require cooperation from KPB, SBCFSA, the City of Seward, Great Northern Construction, as the lease holder of the Afognak property, and land use permits from Alaska DNR and possibly the University of Alaska. Recent conversations indicate that all entities would be willing to enter into an agreement for the management of streambed materials. Currently, there are at least two likely locations for the deposition of a large quantity of material: • The Afognak properties just south of Dieckgraeff Road and east of the lower end of the armored embankment (parcel numbers 14503104, 14503105, 14503106, 14503202, 14503203). Great Northern Construction has expressed interest in finding a market for the sale of materials. • The Alaska Railroad holds permits to fill in the property east of the Seward Highway(parcel number 14502517). In the past, the Railroad has offered to pay for haul fees to this location. • Other properties that could be considered for stockpiling material or receiving fill include the KPB landfill and City of Seward properties adjacent to Dieckgraeff Road. Permitting The following permits may be required for work within the Japanese Creek floodplain. Most of these permits can be written within 30 days of a completed permit application. • KPB Temporary Use permit for work on the Afognak property • City of Seward Floodplain Development permit • State of Alaska Fish & Game habitat permit • US Army Corps of Engineers US waters permit • University of Alaska land use permit • State of Alaska Dept. of Natural Resources land use permit o Can take up to 90 days. DNR also requires a site specific sediment management plan approved by the Commissioner in order for agencies to remove material from State lands for flood control purposes without paying a material sales fee. In the past, this fee has been $0.50 — $3.25 per cubic yard. Funding The SBCFSA has applied for, and been denied, FEMA funding for a watershed-wide sediment management plan through the Cooperative Technical Partnership program. There may be an opportunity for KPB, on behalf of the SBCFSA, to apply for a State of Alaska Department of Homeland Security & Emergency Management Page 8 Japanese Creek (DHS&EM) planning grant in the near future.A planning grant would allow for the development of a sediment management plan, but would most likely not fund the implementation of a plan, In recent years, the SBCFSA has received quotes for the removal of sediment at $6 — 11 per cubic yard. The shorter the distance to transport the material, the less the expense will be for the "harvest and haul" of streambed materials. Short-term Mitigation A project to train the upstream channel, cleanout the drainage ditches, culverts, and downstream channels would provide short-term relief until a longer-term mitigation project can be developed. Implementing one of these option without the others would negate the effectiveness of any mitigation work. Channel Training - Upstream To train the main upstream channel to the 12-foot diameter culvert, a 700 — 1,600-foot-long embankment could be pushed from river run materials. The channel would be dredged from current elevations 6— 12 feet deep, approximately 75 feet wide. The length would be determined by either following the current active channel, or building the berm to cut off the eastern corner of the floodplain where waters have recently migrated.This project may minimize the risk of Dieckgraeff Road washout and decrease the chance of flooding in the Forest Acres subdivision.This work may require a sole source contract to the lease holder of the Afognak Property, and would require all but the University of Alaska permits listed on the previous page. Culvert & Drainage Ditch Maintenance A vacuum truck and water pumps could be used to clean out material from the six previously existing culverts. An excavator and dump truck could remove the sediment and vegetation from 1,500 feet of the west side drainage ditches, allowing migrating waters to flow into the cleaned out culverts. This work could be completed in the Roads right-of-way and would require permits from the City and Alaska Fish & Game. Channel Restoration - Downstream At a minimum, the main downstream channel would need to be restored to 12 feet deep from the current elevations, approximately 25-50 feet wide for a length of 200 feet. Heavy equipment and dump trucks would need to be used for the removal of material. This work would require all except the Alaska DNR permits listed on the previous page. Estimated Budget Channel Training Upstream $8,000— 12,000 Culvert & Drainage Ditch Maintenance $16,000 Channel Restoration Downstream $21,000 $45,000—49,000 -----� _� Page 9 Japanese Creek Long-term Mitigation The KPB is working with DHS&EM on a disaster recovery (DR4161) project worksheet for the study, design and implementation of a longer-term mitigation project for Dieckgraeff Road. In 2013, the US Army Corps of Engineers completed a Continuing Authorities Program Section 205 Project fact sheet for flood risk management on Japanese Creek. Based on federal interest, the USACE Pacific Ocean Division was authorized to continue the project into the feasibility study phase. Section 205 of the 1948 Flood Control Act allows the USACE to study, plan, and construct small flood risk management projects, up to $10,000,000 without specific authorization by Congress. There is a 50/50 match from the local sponsor for the feasibility study and a 5-35% match for the construction phase. It is highly recommended the City and/or KPB contact the USACE to seek re-authorization of this Section 205 project. REFERENCES Available at the SBCFSA Administrative office Page 10