DESIGNING BENDWAY WEIRS

ARE BENDWAYS THE RIGHT CHOICE? LOOK AT I&E DATA ----WIDTH/DEPTH RATIO > 10 ----CEM STAGE IV OR V ----Unvegetated Pt. Bar @ < 50% bankfull depth ----Radius Curvature/Bankfull Width >6 ----Rc/Bankfull Width >4<6

ARE BENDWAYS RIGHT? CAN POINT BAR MATERIAL BE MOVED BY FLOW? IS SOME SCALLOPING BETWEEN WEIRS ACCEPTABLE? IS CHANNEL BED AT LEAST 20 FT WIDE? ARE ENTRANCE AND EXIT CHANNELS RELATIVELY STABLE?

START THE LAYOUT PLACE FIRST WEIR AT VERY START OF ERODING BANK ANGLE UPSTREAM SO THAT FLOW IS DIRECTED TOWARD POINT BAR AND INSIDE EDGE OF BEND ANGLE SHOULD BE 10 TO 20 DEGREES UPSTREAM OF PERPENDICULAR

RECORD DATA LEFT OR RIGHT BANK STATION LENGTH ANGLE AZIMUTH (COMPASS READING)

LAYING OUT WEIR 2 STAND IN STREAM AT KEY OF WEIR NO. 1 TURN 90 DEGREE ANGLE DOWNSTREAM FROM WEIR PLACE FLAG AT INTERSECTION OF SIGHTLINE WITH TOE OF BANK MEASURE DISTANCE TO FLAG

LAYING OUT WEIR 2 IF DISTANCE TO INTERSECT IS LESS THAN 4 TIMES THE LENGTH OF WEIR 1 ---PUT WEIR #2 JUST ABOVE TH EINTERSECT POINT. IF DISTANCE IS MORE THAN 4 TIMES LENGTH OF WEIR 1 ---MOVE UPSTREAM TO POINT WHERE SPACING IS 4 TIMES LENGTH OF W-1

REPEAT PROCESS UNTIL LAST WEIR IS LESS THAN 4 WEIR LENGTHS FROM STABLE POINT ON BANK THEN SET ANGLE ON LAST WEIR TO DIRECT FLOW AT 90 DEGREES INTO THE CENTER OF THE EXIT CHANNEL (COULD BE 0 DEGREES OR EVEN ANGLED DOWNSTREAM)

ADJUST WEIR LENGTHS GO BACK AND ADJUST LENGTHS TO MAKE ENDS OF WEIR FORM A SMOOTH ALIGNMENT THRU WEIR TIPS.

RE-CHECK ANGLES GO BACK THRU THE LAYOUT AGAIN TO CHECK WHERE EACH WEIR IS DIRECTING FLOW. PERDENDICULAR FROM MIDPOINT SHOULD NORMALLY DIRECT FLOW NEAR THE TIP OF THE NEXT WEIR DOWNSTREAM.

WEIR HEIGHT GO DOWNSTREAM TO RIFFLE LOCATION SHOOT ELEVATION OF RIFFLE AND MEASURE AVERAGE FLOW DEPTH. ADD 1.5 FT. TO RIFFLE ELEVATION AND THAT SHOULD BE YOUR “CREST ELEVATION” FOR ALL WEIRS IN THIS SET.

QUANTITIES USE WATER SURFACE AS LEVEL AND MEASURE WATER DEPTH ALONG WEIR EVERY 10 FT. AND COMPUTE AVERAGE DEPTH. SUBTRACT AVERAGE DEPTH OF WATER AT RIFFLE FROM 1.5 FT. AND ADD TO AVERAGE WATER DEPTH TO GET RIFFLE HEIGHT.

QUANTITIES (ALL WEIRS IN A SET SHOULD BE SAME HEIGHT ABOVE WATER LINE) FIND END AREA FOR DESIGN HEIGHT ON CHART AND MULTIPLY BY WEIR LENGTH TO GET VOLUME MULTIPLY VOLUME (CU. FT.) BY 105 LBS. DIVIDE BY 2000 LBS TO GET TONS.

QUANTITIES “KEYS” ARE A MINIMUM OF 5 FT. DEEP AND 4 FT. WIDE. HEIGHT OF KEY SHOULD BE APPROX. HEIGHT OF “LOW BANK” OR 2 TIMES MAX. BANKFULL DEPTH. COMPUTE REQUIRED STONE FOR “KEYS” AND ADD TOTAL

DETERMINING STONE SIZE USE VELOCITY CALCULATED DURING I&E MULTIPLY VELOCITY BY 2 TO APPROX. MAX. VELOCITY (PROVIDES A SMALL MARGIN OF SAFETY) SELECT D100 STONE SIZE FROM CHART OF VELOCITY REQUIRED TO MOVE STONE ( I&E SECTION)