FLOOD DAMAGE REDUCTION

CHAPTER 9 - ROBERSON ET AL., WITH ADDITIONS


    THE ATTRACTION OF THE FLOOD PLAIN

  • THE LAND ALONG RIVERS OR STREAMS THAT IS PERIODICALLY FLOODED IS NATURAL.


    Local man showing level reached by flood near Kanakumbe, Karnataka, India (January 1992).

  • YEAR AFTER YEAR, THE RIVERS HAVE BROUGHT BOTH WATER AND SEDIMENT DOWNSTREAM.


    Landscape erosion in the Pirai river basin, Santa Cruz department, Bolivia.


    Pirai river near Comunidad San Jose, Santa Cruz department, Bolivia.

  • ACCUMULATION OF FERTILE SOIL, DUE TO FLOODS GOING OVERBANK.

  • ECONOMIC INVESTMENT IN FLOOD PLAINS HAS GROWN THROUGHOUT THE WORLD.

  • ANNUAL DAMAGES PRODUCED BY FLOODS HAVE CONTINUALLY INCREASED.


    The Mohave river at flood stage at Indian Trail, near Helendale, California, March 2005.

  • FLOOD CONTROL MEASURES BECOMES A ROLE OF GOVERNMENT.


    Downstream side of White River Bridge, California.
    The top of the pile on the bridge is level with the upstream side of the bridge,
    meaning about 15-20 ft of fill (061107).



Stream in the Lower Chaco, Paraguay.
  • U.S. ARMY CORPS OF ENGINEERS HAS RESPONSIBILITY FOR FLOOD CONTROL.

  • MORE PEOPLE MOVE INTO THE FLOOD PLAIN AFTER PROTECTION HAS BEEN PROVIDED.

  • RECORD FLOODS OCCUR AS A RESULT OF RAINFALL, OR RAINFALL WITH SNOWMELT.

  • RAINFALL PRODUCES THE HIGHEST PEAK FLOWS.

  • COMBINATION OF RAINFALL AND SNOWMELT PRODUCES THE LARGEST FLOOD VOLUMES.

  • FLOODS DEPEND ON SCALE OF BASIN.

  • A SINGLE RAINSTORM MAY COMPLETELY COVER A SMALL BASIN. FLOODS ON LARGER BASINS ARE DUE TO STEADY RAINFALL.

  • CREAGER FORMULA:

    Qp = 46CA0.894A-0.048

    qp = 46CA0.894A-0.048 - 1


  • C IS AN EMPIRICAL COEFFICIENT VARYING BETWEEN 30 AND 100 FOR THE RECORDED DATA.

  • CREAGER PLOTTED RECORDED PEAK FLOWS, WITH NO INDICATION OF FREQUENCY.

  • NOTE DECREASE OF PEAK DISCHARGE PER UNIT OF AREA WITH DRAINAGE AREA: A SIGN OF DIFFUSION ACTING TO REDUCE FLOOD FLOWS.

  • IS RECORDED MAXIMUM FLOW REPRESENTATIVE OF THE POSSIBLE MAXIMUM? ANSWER: NO.

  • OUR DATA COMPRISES ONLY A SMALL WINDOW.



Flooding on the Itajai river, Santa Catarina, Brazil, January 8, 1983.


Flooding on the Chane river, Beni, Bolivia, January 19, 1990.


Flooding on the Upper Paraguay river at Isla Margarita, Paraguay, January, 1992.


Flooding on the Cuiaba river, Mato Grosso, Brazil, January 15, 1995.


Flooding at La Mojana, Colombia.


    CONTROL OF FLOODING

  • FLOOD DAMAGE IS CREATED WHEN RIVER FLOWS ARE LARGE ENOUGH TO CAUSE FLOODING IN AREAS LESS OFTEN COVERED BY WATER.

  • MAIN CHANNEL IS FORMED BY FREQUENTLY OCCURRING FLOOD FLOWS.

  • THE BANKFULL CAPACITY OF THE RIVER HAS A RETURN PERIOD OF 2.33 YEARS!



    • FLOODWAY IS DEFINED BY FEMA AS THAT PART OF THE STREAM WHICH COULD CONTAIN THE 100-YR PEAK FLOW WITH NOT MORE THAN 1 FT INCREASE IN DEPTH ABOVE THAT WHICH WOULD OCCUR IF THE ENTIRE CROSS SECTION CARRIED TO 100-YR PEAK FLOW.



    • THE OBJECTIVE OF FLOOD-CONTROL ENGINEERING IS TO PREVENT OR LIMIT FLOOD DAMAGES BY CONTROLLING OR MANAGING THE FLOOD.

    • CONTROL CAN BE ACCOMPLISHED IN TWO WAYS:

      -- THE PEAK FLOW RATE MUST BE REDUCED.

      -- THE CAPACITY OF THE FLOOD CHANNEL MUST BE ABLE TO CONTAIN THE STAGE ASSOCIATED WITH THE PEAK FLOW RATE.



    Levee in the Lower Mississippi river.
    • PEAK FLOW RATE IS REDUCED BY:

      -- STORAGE OF AT LEAST PART OF THE FLOOD IN UPSTREAM RESERVOIRS.



      -- LAND MANAGEMENT UPSTREAM TO INCREASE INFILTRATION, INTERCEPTION, AND DETENTION LOSSES, AND THUS, BOTH DELAY AND REDUCE THE RATE AND VOLUME OF RUNOFF.



      Forest service crew constructing watershed retention, Wasatch Mountains, Utah, 1933


      Check dams used to restore and revegetate a gully in Alkali Creek, Colorado.


      Sign at Trout Creek, Western Colorado, showing effects of watershed improvements.


      Large beaver dam in Trout Creek, Colorado.


      Restored Clark's Creek, in Plumas County, California.
      RED CLOVER CREEK, CALIFORNIA

      -- INCREASING CHANNEL STORAGE THROUGH DIVERSION.

      -- DIVERTING FLOOD FLOWS INTO ANOTHER BASIN.

      MACHIAVELLI AND LEONARDO

      -- IMPROVING THE HYDRAULIC CAPACITY OF THE CHANNEL (QUESTIONABLE)

    • UPSTREAM IMPROVEMENTS OF LAND-USE MANAGEMENT HAVE BEEN HANDLED BY SCS.

    • SCS HAS PURSUED THE DESIGN AND CONSTRUCTION OF SMALL RESERVOIRS, AND THE IMPLEMENTATION OF SOUND MANAGEMENT PRACTICES FOR PREVENTING FLOODS AND CONSERVING SOIL RESOURCES.


    • ADVANCES HAVE BEEN MADE IN THE HYDRAULIC AND HYDROLOGIC METHODOLOGY TO ANALYZE RAINFALL AND FLOODS FOR DESIGN.


      FLOOD PLAIN MANAGEMENT

    • CONSTRUCTION OF RESERVOIRS TO STORE FLOODWATERS, OR THE IMPROVEMENT OF RIVER CHANNELS TO LOWER FLOOD STAGES HAS TENDED TO INCREASE DEVELOPMENT OF THE PROTECTED FLOOD PLAINS.

    • WHEN THE DESIGN FLOOD FOR THE CONTROL MEASURE IS EXCEEDED, DAMAGES ARE MORE THAT WOULD HAVE OCCURRED BEFORE CONSTRUCTION OF THE FLOOD CONTROL MEASURE.

    • FLOOD DISASTER PROTECTION ACT 1973: WAS TO KEEP PEOPLE AWAY FROM FLOODS RATHER THAN FLOODS AWAY FROM PEOPLE.

    • IN THE PRACTICE OF FLOOD PLAIN MANAGEMENT, THE EXTENT OF THE FLOOD PLAIN FOR A PARTICULAR DESIGN FLOOD (50 OR 100 YR) IS DEFINED.

    • THE FLOOD PLAIN IS DIVIDED INTO

      -- THE FLOODWAY, WHERE VELOCITIES ARE LARGE ENOUGH TO CAUSE DAMAGE DURING FLOODING, AND

      -- THE PERIPHERAL AREA, WHICH IS AN AREA OF OVERBANK FLOOD STORAGE WHICH EXPERIENCES ONLY SMALL VELOCITIES.



    • THE FLOODWAY IS SUBJECT TO STRONG BUILDING RESTRICTIONS.

    • LESSER RESTRICTIONS APPLY TO THE PERIPHERAL AREA.

    • A FLOOD PLAIN MANAGEMENT PROGRAM IS REQUIRED BEFORE FEDERALLY SUBSIDIZED FLOOD INSURANCE CAN BE MADE AVAILABLE TO PROPERTY OWNERS.



    Debris flow near Wasatch Mountains, Utah.


      DELINEATION OF THE FLOOD PLAIN

    • HYDRAULIC ANALYSIS OF FLOOD CONTROL STRUCTURES AND CHANNELS IS BASED ON PRINCIPLES PRESENTED IN CHAPTER 4.

    • THE FLOOD PLAIN IN THE U.S. IS USUALLY THE 100-YR FLOOD PLAIN.

    • THE CALCULATION OF THE 100-YR PEAK FLOW OR 100-YR FLOOD HYDROGRAPH IS BASED ON PRINCIPLES PRESENTED IN CHAPTER 2.

      ONLINE TR-55

    • IF THE LENGTH IS SHORT, GRADUALLY VARIED FLOW CAN BE USED (HEC-RAS).

      STEADY VS UNSTEADY FLOW WITH HEC-RAS

    • CROSS-SECTION MUST BE SURVEYED ALONG THE RIVER AT SMALL ENOUGH INTERVALS TO ADEQUATELY DEFINE THE VALLEY GEOMETRY.

    • THEN, VALUES OF THE RESISTANCE COEFFICIENT MUST BE ESTIMATED FOR BOTH THE MANNING CHANNEL AND THE OVERBANK AREAS.

    • TABLE 9-4 (PAGE 483) PROVIDES VALUES OF MANNING N THAT ARE TYPICAL FOR OVERBANK AREAS.




    Chattahoochee River near Leaf, Ga.



    Pea Creek near Louisville, Alabama.

    • VELOCITIES IN THE MAIN CHANNEL ARE ALWAYS LARGER THAN THOSE IN THE OVERBANK.

    • TO DELINEATE THE EXTENT OF OVERBANK FLOODING, IT IS NECESSARY TO COMPUTE THE WATER SURFACE PROFILE IN THE CHANNEL.

    • FOR FLOOD FLOW IN CHANNELS WITH OVERBANK FLOW, IT IS NECESSARY TO USE THE KINETIC ENERGY CORRECTION FACTOR α.

    • THIS FACTOR IS SIGNIFICANTLY LARGER THAN UNITY BECAUSE THE AVERAGE VELOCITY IN THE OVERBANK AREA IS GENERALLY MUCH SMALLER THAN THE AVERAGE VELOCITY IN THE CHANNEL.

    • ENERGY BALANCE LEADS TO:
    • ya + αVa2 / (2g) + SoΔx = yb + αVb2 / (2g) + Sf Δx

    • Ea - Eb = (Sf - So) Δx

    • THE EVALUATION OF α IS:

    • α = Σ (Vi3Ai ) / (V3A)

    • α = Σ (Qi3/Ai2 ) / (Q3/A2)

    • FOR COMPOUND FLOOD PLAIN CHANNELS, CONVEYANCE IN EACH SEGMENT IS DEFINED AS FOLLOWS:

    • K = (1.49 / n) A R2/3             [U.S. CUSTOMARY UNITS]

    • THE VELOCITY COEFFICIENT IN COMPOUND CHANNELS IS:

    • α = [(∑ Ai )2 / (∑ Ki )3] [ ∑ (Ki3/ Ai2) ]

    • MANNING, CHEZY, OR DARCY-WEISBACH ARE USED TO CALCULATE FRICTION SLOPE.

    • MANNING EQUATION IS USED MORE FREQUENTLY BECAUSE THE BOTTOM ROUGHNESS, SHAPE, HEIGHT, AND TEXTURE VARY GREATLY ALONG A NATURAL CHANNEL, AND THE GREATER ACCURACY AFFORDED BY USING THE RESISTANCE COEFFICIENT f IS USUALLY NOT WARRANTED.

    • MANNING'S N IS NOT A CONSTANT.

    • IT VARIES WITH STAGE (FLOW DEPTH).

    • MEAN VELOCITIES VARY WITH STAGE, LEADING TO THE S-CURVE FOR TIME OF TRAVEL THROUGH THE REACH.


    Boomerang curve for peak flood stage vs time of occurrence,
    Upper Paraguay river at Ladario, Mato Grosso do Sul, Brazil.

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