§ 17.60.190 APPENDIX 5: HYDROLOGIC AND HYDRAULIC MODELS AND TECHNIQUES APPROVED BY MCSC.

Hydrologic Models or Techniques Approved by MCSC
Hydrologic Model or Technique	Less than 10 ac 5.c.(1)	Less than 20 ac 5.c.(2)(a)	Between 20 and 100 ac 5.c.(2)(b)	Greater than 100 ac 5.c.(2)(c)
Detention Volume vs. Percent Impervious Chart	X
Rational Method	Runoff but not Storage	Runoff but not Storage
TR-55	X	X	X
USGS Regression Equations			X10	X10
StreamStats			X10	X10
HEC-1	X	X	X	X
TR-20	X	X	X	X
HEC-HMS	X	X	X	X
10   Only allowed where all habitable buildings are above the BFE.

 

   1.   The hydrologic models or techniques approved by MCSC include, but are not limited to:

      a.   HEC-HMS, TR-20, HEC-1, TR-55, and the Rational Method for tributary areas less than 20 acres;

      b.   HEC-HMS, TR-20, HEC-1, TR-55, USGS Regression Equations, and StreamStats for tributary areas less than 100 acres but greater than or equal to 20 acres; or

      c.   HEC-HMS, TR-20, HEC-1, USGS Regression Equations, and StreamStats for tributary areas of 100 acres or more.

   2.   The Rational Method may be used to determine the peak stormwater runoff rate, but neither the Rational Method nor the Modified Rational Method shall be used to determine the required stormwater storage volume.

   3.   The Rainfall Depth Duration Frequency Tables in Appendix 4 shall be used for all hydrologic models.

   4.   Peak stormwater runoff rates determined using hydrograph producing models shall be based on the critical duration storm event using the appropriate Huff Rainfall Distributions in Appendix 4.

   5.   Peak stormwater runoff rates determined using TR-55 shall be based on the 24 hour NRCS (SCS) Type II distribution.

   6.   BFE determinations using USGS Regression Equations and StreamStats shall only be allowed where all habitable buildings are above the BFE.

(Ord. O-2020-09-15, passed 9-15-2020)