1 / 20

Watershed Assessment (ENSC 202)

Watershed Assessment (ENSC 202). Phase 2 – Step 7 The Rapid Geomorphic Assessment (RGA) Extracted from Vermont ANR Stream Geomorphic Assessment Protocols Handbook - Introduction. Objectives of the RGA. Adjustment Categories Degradation Aggradation Widening Planform adjustment.

sakura
Download Presentation

Watershed Assessment (ENSC 202)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Watershed Assessment (ENSC 202) Phase 2 – Step 7 The Rapid Geomorphic Assessment (RGA) Extracted from Vermont ANR Stream Geomorphic Assessment Protocols Handbook - Introduction ENSC 202 – 2004 Phase 2 Step 7 RGA

  2. Objectives of the RGA Adjustment Categories • Degradation • Aggradation • Widening • Planform adjustment Focus: Departure of a stream’s existing condition from an presumed reference condition (Phase 1) ENSC 202 – 2004 Phase 2 Step 7 RGA

  3. Reference Stream Types Phase 1, Step 2.11, Pages 23-25 ENSC 202 – 2004 Phase 2 Step 7 RGA

  4. Confined vs Unconfined Streams • Confined streams: confinement ratio <4 • Unconfined streams: confinement ratio > 4 Use the correct form! ENSC 202 – 2004 Phase 2 Step 7 RGA

  5. ‘Modified’ Reference Stream Types • Streams with dams • Urban streams Streams where “modifications may prohibit the evolution of the channel back to the natural reference stream type” ENSC 202 – 2004 Phase 2 Step 7 RGA

  6. Departure Not significant Minor Major Extreme Condition Reference Good Fair Poor Basic terminology of the RGA Departure from a reference condition Description • In regime • In adjustment • Active adjustment and departure ENSC 202 – 2004 Phase 2 Step 7 RGA

  7. Natural Exceptions • Streams controlled by bedrock (gorges) • Stream impounded by beaver • Natural alluvial fans and braided rivers ENSC 202 – 2004 Phase 2 Step 7 RGA

  8. Simple overview of the RGA • Four ‘adjustment categories’ • The key adjustment processes • Four ‘condition categories’ • The conditions • Four to six ‘evaluation criteria’ per adjustment category • The means to assess each adjustment category • Note that some of these are used repeatedly • Five ‘scores’ per condition category • Allows fine-tuning of the observed conditions ENSC 202 – 2004 Phase 2 Step 7 RGA

  9. Final computations • Total score is the sum of the individual scores • Condition rating is total score/80, the maximum score • Stream condition is… ENSC 202 – 2004 Phase 2 Step 7 RGA

  10. Some further notes • Note the general diagnostics for adjustment processes on the left margin • Do not bother with ‘stream departure type’ (STD) • Do not attempt to assess the ‘channel evolution stage’ unless this is something that interests you (see Appendix C) • Attempt, but don’t stress over, whether the adjustments you observe are ‘historic’ • Note that bolded boxes require a ‘poor’ score ENSC 202 – 2004 Phase 2 Step 7 RGA

  11. Key Definitions and Concepts • Headcuts or nickpoints • Bankfull & Floodprone widths • Entrenchment ratio • Incision ratio • Confinement ratio ENSC 202 – 2004 Phase 2 Step 7 RGA

  12. Headcuts or Nickpoints Ph2/5.3 Page 50 Suspiciously steep ‘riffles’ or steps where the stream is eating back upstream ENSC 202 – 2004 Phase 2 Step 7 RGA

  13. Bankfullthe point at which flow just begins to enter the active floodplain (See Appendix K) • Flat top of point or lateral bars • Point of ‘slope break’ on the bank • Lower extent of persistent woody vegetation • Upper extent of evident erosion ENSC 202 – 2004 Phase 2 Step 7 RGA

  14. Floodprone widthThe width of the stream at twice the bankfull depth ENSC 202 – 2004 Phase 2 Step 7 RGA

  15. Entrenchment RatioER = FPW/BFW ENSC 202 – 2004 Phase 2 Step 7 RGA

  16. Entrenchment Ratio estimated • Ideally you would measure BFW, BFD, FPW, and FPD to get ER • The RGA only distinguishes between ER greater or less than 1.2, 1.4 or 2.0 depending on stream type (confined or unconfined) • As you ‘eyeball’ the reach,… • is FPW more than 20-40% wider than BFW for confined streams (ER<> 1.2 to 1.4), or • is FPW more than double BFW for unconfined streams or not? (ER<>2) ENSC 202 – 2004 Phase 2 Step 7 RGA

  17. Incision Ratio high bank low bank depth (LBD) bank full depth (BFD) IR = LBD/BFD • Estimate if LBD is greater than BFD by… • 0-20% (some, but not much) • 20-40% (a little less than half) • 40-100% (double) • >100% (more than double) ENSC 202 – 2004 Phase 2 Step 7 RGA

  18. Confinement Ratio • CR = valley width/channel width • Don’t really need to measure • Has it been changed by humans or not? ENSC 202 – 2004 Phase 2 Step 7 RGA

  19. Stream ‘sensitivity’ to future change ENSC 202 – 2004 Phase 2 Step 7 RGA

  20. Stream type codeLetter/number ENSC 202 – 2004 Phase 2 Step 7 RGA

More Related