10.07.2015 Views

BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

oth the beach immediately fronting the bulkhead and downdrift beaches.This effect should normally be mitigated through beach nourishment.3. Passive erosion – If a bulkhead or revetment is built to protect backshoreassets fronted by an eroding beach, the beach will continue to erode and thehigh water line will migrate landward towards the structure. The effect ofthis migration will be the gradual narrowing of the beach in front of thebulkhead or revetment. Generally, use bulkheads or revetments only oneroding shorelines where protection of the backshore is determined to be ofgreater value than beach preservation. Mitigation of passive erosion throughbeach nourishment is possible but is often costly because of the ongoingneed for sediment.4. Groin effect – If a bulkhead or revetment is built to protect backshoreassets fronted by an eroding beach, while adjacent areas are allowed to erodenaturally, the structure will eventually extend into the lake beyond the highwater line. At this stage, the structure may act as a groin, preventingalongshore transport of sediment and increasing erosion at downdriftbeaches. The structure is also vulnerable to flanking at this stage – waveenergy can reach the backshore around the ends of the structure if it wasnot designed with return walls. The structure can become disconnectedfrom the backshore and fail.5. Active Erosion – In some cases, wave reflections from a vertical bulkheadcan increase the wave energy on the beach and actually increase the rate oferosion. This is site specific and is more likely to occur if the bulkheadextends into the lake beyond the high water line, so that there is deep waterat the bulkhead.Active erosion should not be confused with toe scour. Toe scour is the shorttermand localized loss of sediment at the toe of a structure that occurs duringstorms. This can lead to structural failure of a bulkhead that does not haveadequate toe protection. However, the beach normally recovers rapidly fromthis localized loss.Table 8-7 below summarizes the different mechanisms.Table 8-7: Potential Erosional Mechanisms Concerns when using a Bulkhead orLake WallPROCESS ANDSYMPTOMPlacement lossLoss of sandpreviously suppliedby eroding bluffsPassive erosionGroin effectActive erosionCOMMENTNormally small for bulkheads and lake walls. Leads to an immediate loss ofbeach fronting the structure.Geotechnical analysis required to determine whether this is an issue. Canlead to long-term beach erosion. Can be mitigated through beachnourishmentOccurs with chronically eroding beaches. In such cases, placement of abulkhead or lake wall values the backshore more than the beach. Difficultto mitigate.Can occur with chronically eroding beaches. Impacts adjacent properties.Can increase the rate of beach erosion in some cases. Most likely to occur ifthere is open water fronting the structure.TRPA <strong>BMP</strong> HandbookCHAPTER 8: Shorezone Protective Structures and <strong>BMP</strong>s2012 8.2 Bulkheads and Lake WallsPage 8-23

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!