Identifying, Understanding and Addressing Flood-Related Hazards

Transcription

1 Identifying, Understanding and Addressing Flood-Related Hazards Julie Moore, P.E. July 9, 2012 Agenda Traditional approach to river management Understanding flood-related hazards Common flood and flood recovery myths Addressing flood-related hazards Role of river corridor protection 2 1

2 We like to live next to the water Photo credit: Vermont DEC And we like our rivers straight and simple 2

3 Traditional Approach to River Management = CONFINEMENT Confined rivers result in: Increased flooding Altered sediment regimes Increased channel instability Reduced habitat quality Excessive nutrient export 3

4 Understanding Flood-Related Hazard Traditional approaches to flood recovery often create future flood hazards Need to overcome urge to simply put things back 7 We need to move away from the concept that rivers are static systems Photo credit: Vermont DEC 4

5 Common Flood Myths It is possible to ensure against flood damage by straightening and armoring stream channels. There is no way of completely avoiding future flood damages other than removing all permanent infrastructure from predicted flood inundation and erosion hazard areas Armoring banks and straightening channels provide shortterm fixes that will be effective until the next large flood Can have the unintended consequence of increasing downstream flooding and erosion risks 9 Common Flood Myths This was the 100-year flood so we will never see another flood like this during our lifetime. The 100-year flood has a 1% chance of occurring in any given year The northeast is seeing larger and more frequent flooding Over the past several years more large storms have taken place than the gage statistics suggest should be happening. 10 5

6 Common Flood Recovery Myths The replacement of failed culverts and bridges with structures of the same size is suitable. Culverts and bridges fail due to high flows, plugging with sediment and trees, and water flowing around the structure Many failed structures were improperly designed or installed Paying once for a larger structure that fits the stream channel is more economical than replacing smaller structures that repeatedly fail Photo credit: Lars Gange & Mansfield Heliflight 11 Common Flood Recovery Myths It is okay to fill in widened channels and floodplains. Many channels expanded two to ten times pre-flood widths, effectively forming floodplains during the flood The post-flood channel shows the space the river needs to convey the water, sediment, and debris during a large flood Filling should be as limited as possible to minimize future flood and erosion risks Photo credit: Lars Gange & Mansfield Heliflight 12 6

7 Common Flood Recovery Myths Cutting trees down in the floodplain will prevent debris jams during the next flood. Large numbers of trees can be carried down river during extreme events clogging bridges and culverts, depositing on islands, and making local flooding worse in some areas Post-flood surveys in Vermont indicate that: Wide forested floodplains stayed intact and the trees captured and retained flood debris Tree loss was most abundant along narrow buffers and thin stands in floodplains Design and planning is needed to consider the expected load of trees and other debris that may be moved down river during future floods. Photo credit: Roy Schiff, Milone & MacBroom 13 Common Flood Recovery Myths You can dig yourself out of a flood. Rivers move water, sediment, and woody debris. The shape of a natural river channel reflects a balance between the flow and the amount of sediment and woody debris that the stream carries. The wider and deeper a channel is, the slower the water moves, making it more likely that excessively large sediment bars will form. In channels in a narrow valley, digging deeper usually leads to more down-cutting and collapse of the banks. 14 7

8 15 Breaking the Cycle Flood amnesia often sets in within a period of months following a flood event Slowly and steadily we revert to old practices, which leaves us little better prepared for the next event River corridor protection can be key to breaking the cycle of armor encroach damage & loss 16 8

9 Functioning floodplains and river corridors create an intersection for the protection of public values Public Safety and Property Protection Floodplain Protection & Stream Equilibrium Ecological Integrity and Fish & Wildlife Resource Water Quality and Quantity Approaches to River Corridor Protection Technical and financial assistance to municipalities and individual landowners Most land use decisions are made at the local level Map and protect fluvial erosion hazard (FEH) zones Important to distinguish between erosion and inundation hazards Link Clean Water Act objectives with flood hazard avoidance/mitigation Municipalities and landowners are often more motivated to reduce exposure to flood impacts 9

10 Key Strategies: Zoning Avoid future conflicts Using FEMA driven flood insurance mapping to educate municipal officials about the importance of river corridors Vermont has developed model river corridor protection ordinances Work with state and federal programs to create/identify incentives financial and programmatic for river corridor protection Municipal zoning 10

11 Key Strategies: Easements Easements can be used to augment zoning Restrict channelization practices and unnecessary efforts to stop all erosion Passive restoration relies on river (and time) to reestablish meanders and floodplain Efforts need to be coordinated closely with other land conservation programs such as: Federal farm service agencies Local land trusts River Corridor Easements Map credit: Vermont DEC 11

12 Key Strategies: Restoration Active use of big yellow machines Needs to be strategic, as actively reconnecting rivers with historic floodplains is extremely expensive Allows for immediate gain in floodplain storage of sediment and debris Can help alleviate localized flooding Passive let nature run its course Unpredictable Things may get worse before they get better Active Restoration AFTER BEFORE 12

13 BEFORE: Existing railroad embankment Photo credits: Roy Schiff, Milone & MacBroom AFTER: Railroad embankment removed to floodplain elevation Approaches to River Corridor Protection Successful river corridor protection: Engages municipalities and landowners Recognizes existing encroachments Seeks opportunities and partnerships Takes the long view 13

14 Acknowledgments Mike Kline, Vermont DEC Roy Schiff, Milone and MacBroom Elise Annes, Vermont Land Trust Questions? 14