# Tau Critical-Erosion value

Hi everyoneThe EFDC used m^2/s^2 unit for tau critical-erosion valueBut I think, N/m^2 unit also popular unit of tau.So How can I convert m^2/s^2 to N/m^2;As I look into dimension, m^2/s^2=(m^3/kg) * (N/m^2). then should I multiply density at m^2/s^2??Thank you

Hi Jongwi Chang, Thank you for this question. Which input file / document or code section are you referring to regarding the critical-erosion value. Mostly you can see that the units for critical shear stress is dynes/cm2 throughout EE and EFDC+. Please post the link / screenshot of document that you are referring to. However, multiplying my density would give you required units as you said. Best, Janesh

Hi- Thank you for your respond, Janesh!The unit (m2/s2) is in EFDC.inp file (TAUR in C40)C40 COHESIVE SEDIMENT PARAMETER SET 2 REPEAT DATA LINE NSED TIMES* DATA REQUIRED IF NSED>0, EVEN IF ISTRAN(6) = 0** IWRSP: 0 USE RESUSPENSION RATE AND CRITICAL STRESS BASED ON PARAMETERS* ON THIS DATA LINE* >0 USE BED PROPERTIES DEPENDEDNT RESUSPENSION RATE AND CRITICAL* STRESS GIVEN BY EFDC FUNCTIONS CSEDRESS,CSEDTAUS,CSEDTAUB* FUNCTION ARGUMENSTS ARE (BDENBED,IWRSP)* 1 HWANG AND MEHTA - LAKE OKEECHOBEE* 2 HAMRICK’S MODIFICATION OF SANFORD AND MAA* 3 SAME AS 2 EXCEPT VOID RATIO OF COHESIVE SEDIMENT FRACTION IS USED* 4 SEDFLUME WITHOUT CRITICAL STRESS* 5 SEDFLUME WITH CRITICAL STRESS* >= 99 SITE SPECIFIC* IWRSPB:0 NO BULK EROSION* 1 USE BULK EROSION CRITICAL STRESS AND RATE IN FUNCTIONS* CSEDTAUB AND CSEDRESSB* WRSPO: REF SURFACE EROSION RATE IN FORMULA* WRSP=WRSP0*( ((TAU-TAUR)/TAUN)TEXP ) (gm/m2/sec)* TAUR: BOUNDARY STRESS ABOVE WHICH SURFACE EROSION OCCURS (m/s)2 TAUN: (NOT USED, TAUN=TAUR SET IN CODE) TEXP: EXPONENT OF WRSP=WRSP0*( ((TAU-TAUR)/TAUN)TEXP ) VDRRSPO: REFERENCE VOID RATIO FOR CRITICAL STRESS AND RESUSPENSION RATE IWRSP=2,3* COSEDHID: COHESIVE SEDIMENT RESUSPENSION HIDING FACTOR TO REDUCE COHESIVE* RESUSPENSION BY FACTOR = (COHESIVE FRACTION OF SEDIMENT)**COSEDHID*C40 IWRSP IWRSPB WRSPO TAUR TAUN TEXP VDRRSPO COSEDHID 0 0 .03 .4 .4 1 1 0 0 0 .003 .4 .4 2 1 0thank you!!

Hi Jongwi, TAUR in Card C40 represents TAUR = WATER DENSITY NORMALIZED CRITICAL SHIELDS STRESS Let’s break down the unit of Shear stress a bit here : Rho = N/m2 = (kg m s^-2)/m3 = (kg/m3) * (m2/s2) So if you normalize that value by density of water then the unit you will be left with is m2/s2. I hope it is clear now. Best,Janesh