Rational Vs SCS for small urban detentionless Basins

[tkvail]

I am in the process of evaluating storm sewer drainage designs for small area urban basins with no detention(typically less than 10 acres and subdivided into basins of less than 0.5 acres in some instances). Is the SCS method still applicable for this type of analysis? Or should I say which would be more applicable, the SCS method or the Rational method. (understanding that the SCS is based on a 24 hour storm with a peak and that the rational method is a one hour constant rate)

When comparing results from both SCS (TR-55)method and the Rational method the results are quite different. I.E. for a 0.46 acre area that is 87% impervious the SCS 25 yr storm using a minimum Tc of 0.1 hrs, resulted in a developed flowrate of 0.8cfs, while the rational method using a minimum Tc of 5min (i=4.3) resulted in a flowrate of 1.7cfs.

After summing all the small basins obviously the differences become greater and ultimately drives different storm sewer and inlet sizes.

Does anyone recommend one over the other? Is SCS more accurate and the Rational just more conservative even on these smaller basins? Or is the rational more accurate and the SCS underestimate the flowrats?




Realizing the Rational method is not generally valis for large areas, say

 

[LHA]

For subcatchments to storm conveyance, I always use rational. It is the peak you want, and rational will give you a more conservative peak. For areas under one acre (and definitely for those under 0.5 acres), I also use rational, and most Ordinances here in PA, USA require this.

SCS is generally accepted to produce a better hydrograph for sizing and routing detention facilities.

 

[bltseattle]

Like lha says,

You can use Rational for conveyance design (gives you bigger pipes & less flooding potential) and hydrograph method (SCS, SBUH) for detention design. Rational is better at estimating peak flows from intense downpours, with no flood routing for flow to the collection point. Hydrographs are better for estimating runoff volumes, and typically use a routing coefficient that is based on basin characteristics to estimate flood routing attentuation. Pond sizing depends on accurate assessment of runoff volume, otherwise the pond may be too small.

Calibration of Rational method to yield runoff volumes for specific storms is sometimes undertaken by local agencies to simplify analyses and design, but the specific approach to be used depends on local climate conditions, eg. relationship of peak rainfall intensity to storm volumes. Using rational method for pond sizing is more common where large intense storms (e.g. thunderstorms) drive the detention design, as opposed to a more drizzly climate where longer duration, lower intensity storms drive the design.

 

[cvg]

using rational to size ponds is not allowed in most areas

 

[queque]

Calibration of Rational method to yield runoff volumes for specific storms is sometimes undertaken by local agencies to simplify analyses and design, but the specific approach to be used depends on local climate conditions, eg. relationship of peak rainfall intensity to storm volumes. Using rational method for pond sizing is more common where large intense storms (e.g. thunderstorms) drive the detention design, as opposed to a more drizzly climate where longer duration, lower intensity storms drive the design.

Can you elaborate on this calibration procedure to yield runoff volume?

 

[bltseattle]

The calibration procedure would consist of monitoring the runoff generated in a catchment of known characteristics for a known storm.

Using a "rational" approach to computing runoff volumes was the method adopted here in King County WA in 1979, but has since been superseded by Santa Barbara Urban Hydrograph, then an HSPF based method, because single event methods are not adequate for addressing multi-day storm events.

I have not used the old method that the County had codified, but I've done a lot of water quality modeling where loading coefficients were applied. Scheuler's Simple Method is an example of this approach; see the Center for Watershed Protection website for details of the Simple Method.

http://www.cwp.org

A summary of a method to find a C factor for runoff volume would be as follows: If you know the rainfall record for the storm you can compute the total storm volume, Pr (inches), for the catchment area (A) that is monitored. By measuring runoff rates (with a flow gauge/meter) over the course of the storm you can compute the total runoff volume, which can be expressed as average depth (inches) over the catchment, Qd. Then you can solve for loading factor C such that Qd = Pr * A * C . The challenge is that C varies depending on the size/intensity of the storm and the antecedent moisture conditions. Curve number methods use an initial abstraction computation that takes into account some of these factors, so CNs don't vary with storm size as a C-loading factor computed in this fashion would.