ModRat 3

[RWF7437]

The term "modified Rational Method" is widely used, and even more widely misunderstood. Any definitions of this term you have encountered would be interesting and helpful to us all. Those who practice in California, Colorado, Washington and other states are especially invited to respond to this query.

What does this term mean in your part of the world ?

 

[bltseattle]

I practice mainly in Washington and have not seen the modified Rational Method used. I've seen some out-of-state agencies that use what I would call the modified rational method. The method uses local storm conditions to take a Rational Method peak flow and develop a triangular hydrograph by establishing ascending- and descending- times to the peak runoff rate. These times may be based on travel time/time of concentration. Whereas Rational Method does not provide any runoff volumes, just a flow rate, modified Rational would. The hydrograph can then be used to size an R/D facility. Since storm durations vary by climatic region, the times assigned to the limbs of the hydrograph are computed based on local agency direction.

But here in Washington, curve number methods (mainly Santa Barbara Urban Hydrograph) have been used since about 1990. That is changing; our state established continuous modeling as the new standard method in 2001, so the state and several agencies (WSDOT, King County) have modified HSPF to use in their jurisdictions. It is being further implemented as local agencies update their NPDES status, and have to provide "equivalent" stringency to the state standards, which requires continuous modeling to size R/D facilities based on matching predeveloped peak rates and flow durations from 1/2 of the 2yr to the 50 or 100yr recurrence.

 

[RWF7437]

Thank you BLT,

I'm hoping more people respond, as you have. In California, many local agencies use something similar, I believe but I'm trying to understand the basis for these methods.

 

[LHA]

I am from soutcentral PA, USA. Here is the current status of modrat here:

Rational does not give a volume of runoff (cf), only a peak rate (cfs). The Modified Rational allows for construction of a volumetric hydrograph using the Rational peak rate, Tc, assuming a rate of ascension to, and descension from Qp at an assumed rate over a critical storm duration...rate as a function of time, volume as the integral.

Hydrology purists usually reject this method as a rough guess based on incorrect data, and technically, they are right. Engineers, at least in my experience, largely accept this method as valid and conservative enough for small, largely homogenous (parking lots, grass fields) watersheds. I have used Mod. Rational, I actually prefer it in those situations. I have reviewed and approved plans based on calculations from it.

I hope that answer was not TOO evasive...


Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[RWF7437]

Thanks Steve,

The method you describe is one I'm familiar with too and have used....but only because I was required to. There are other definitions of the term Modified Rational Method. Most notably, the Counties of Riverside, Los Angeles and San Diego have developed a procedure using a "C" value which varies with rainfall intesity and allows one to plot a hydrograph based on historical rainfall patterns. Whether or not these "homegrowm" methods are an improvement on other recognized methods such as TR-55 or the Santa Barbara Urban Hydrograph is the question I'm trying to find some answers to.

Your response is appreciated and not at all evasive. It is your experience and judgement which is of value here.

Thanks again,

Russ

 

[LHA]

More relevant to design in PA, currently modrat is soon-to-be-out-of-favor here:

Per our PADEP Stormwater Manual (Draft), here:

http://www.dep.state.pa.us/dep/subject/advcoun/Stormwater/Manual_DraftJan05/Section09-jan-rev.pdf

9.3 Existing Methodologies for Peak Rate/Hydrograph Estimations and their Limitations
9.3.1 The Rational Method
The Rational Method has been used for over 100 years to estimate peak runoff rates from relatively small, highly developed drainage areas. The peak runoff rate from a given drainage area s given by: Qy= C x I x A where: Qy = peak runoff rate (cubic feet per second) C = the runoff coefficient of the area (assumed to dimensionless) I = the average rainfall intensity (in./hr) for a storm with a duration equal to the time of concentration of the area A = the size of the drainage area (acres) The runoff coefficient is usually assumed to be dimensionless because one acre-inch per hour is very close to one cubic foot per second (1 ac-in./hr = 1.008 cfs). Although it is a simple and straightforward method, estimating both the time of concentration and the runoff coefficient introduce considerable uncertainty in the calculated peak runoff rate. In addition, the method was developed for relatively frequent events so the peak rate as calculated above should be increased for more extreme events. (Viessman and Lewis, 2003) Because of these and other serious deficiencies, the Rational Method should only be used to predict the peak runoff rate for very small, highly impervious areas. (Linsley et. al, 1992) Although the method has been adapted to include estimations of runoff hydrographs and volumes through the Modified Rational Method, it is further compromised by assumptions about the total storm duration and therefore should not be used to calculate water quality, infiltration, or capture volumes.

9.3.2 Modified Rational Method
The Rational Method, discussed in detail below, has been adapted to include estimations of runoff hydrographs and volumes through the Modified Rational Method. Due to the limitations of the Rational Method itself (see below) as well as assumptions in the Modified Rational Method about the total storm duration, this method should not be used to calculate water quality, infiltration, or capture volumes.


Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[RWF7437]

bltSeattle,

Am I the only person on this forum who doesn't know what "hspf" means, or is ? Can't find it in the WSDOT Hydraulics Manual.

Russ

 

[gbam]

Here in the southwest we have unit hydrographs that can be applied to the Rational Method. These unitless hydrographs reference the TR-55, 1986, based on time of concentration.
I must say that until I joined this forum I have not heard of the "mod" RAT METHOD. An additional note, our three gov entities (City(s), County(s) & State) each have their own methods and acceptable procedures for hydrology & hydrograph creation. WE typically meet the requirements of the governing entity.

 

[bltseattle]

RWF

HSPF = Hydrologic Simulation Program Fortran

and is a continuous simulation hydrology model developed by EPA.

Background:
HSPF historically was not user-friendly; like many of the govt DOS Hydrology/hydraulics programs (HEC-2, SWMM, etc) it requires extensive formatting of the input data (e.g. lining up columns) so was mostly used for planning by agencies (such as county govts). HSPF uses long-term (say 40 years) rain and evap records to simulate hydrology over a long period, then flow statistics such as "10-year peak flow" or "2-year runoff volume" can be computed from the results. It is very data intensive to set up from scratch due to the climatic data needed and obtuse methods of encoding the site basin data.

It is being used in this region because evidence mounted that the Pacific Northwest long-duration storms were overwhelming detention ponds designed by single-event methods such as curve numbers or modified-rational; ponds were partially full at the start of the "design storm". The result was that ponds overtop more frequently than intended, and the uncontrolled discharges caused severe stream erosion in cases (I saw a 20-ft deep "knickpoint" in a channel that was advancing upstream several feet per year). For a while, a "volume correction factor" was required to increase the pond volumes arbitrarily (as much as 50%) from what the modeling showed was needed. Using a continuous simulation results in the pond volume being tracked continuously, so that the net effect of "back-to-back" storms in incorporated into the pond sizing, and no further sizing factors are needed.

Eventually, to incorporate better science and reflect advances in PC technology, agencies decided to build customized "front-end" GUI for the un-friendly HSPF program, which had shown its ability to be calibrated to the regions soils and conditions through basin-planning in the 1980s. The user-friendly front-end enables less-sophisticated users to use the HSPF routines that are embedded in the various software versions, and reduces the need for a site designer to muddle through reams of climatic data, because now local data is provided with the models: King County released the King County Runoff Time Series program in about 1996, the state has issued Western Washington hydrology model in about 2001, and WSDOT uses MGS-FLOOD as of about 2004. The main differences are the interfaces and how the prepitation time-series used for the analyses are determined.

HSPF is also the hydrologic "engine" within the EPA BASINS software suite; for that package a Win-HSPF version was developed. While HSPF can be difficult to use, it can be verified/calibrated against real flow data to greater accuracy than simpler methods such as Rational or CNs.

Hope this background helps

BLT

 

[RWF7437]

Thanks BltSeattle,

Much useful information in your reply. I have downloaded the public domain version of MGS-Flood which appears to be very useful in Washington. Because most of my work is in Oregon, however, I'm looking for an equally useful approach with more general geographic applicability. Some version of a modified rational method seems to offer that promise if it can be calibrated to local conditions. Unlike WA, the Oregon Department of Environmental Quality (DEQ) has yet to offer similar guidance. That may change soon as Phase II of NPDES kicks in. Maybe all those mud puddle local detention basins I've designed over the past 20 years will finally have a purpose !

Thanks again,

Russ

 

[bltseattle]

The Portland Bureau of Environmental Services provides methods for Portland, that could be applied throughout at least Western Oregon/Willamette valley. If you don't use their "simple" approach, they indicate Santa Barbara Urban Hydrograph (SBUH) curve number method should be used. Of course, what applies in Portland might not apply in Bend or at Crater Lake, etc.

SBUH was the established method in Washington from 1992 to 2001 at which point Ecology decided to start the switchover to continuous modeling. It can be implemented in a spreadsheet fairly easily, or in a number of software packages. This is the most widely used method in Washington still, since most local agencies are in the process of adopting the more rigourous state standard. We still use the old Waterworks software because of its stability and ease of use.

 

[RWF7437]

Thanks again BLTSeattle,

I have used both the SBUH method and the so called "SCS 484" method. The problem with both of these are the storm types generally available ( only 4 ). Using the type IIA storm yields a very unrealistic looking hydrograph, as you probably know. In our part of the country flooding generally occurs after prolonged rainfall events greater than 24 hours in duration. It sounds from your earlier posts that the methods used in Washington get around this by devising local rainfall patterns which more realistically reflect the kinds of storm actually experienced.

Something similar for Oregon, and many other parts of the country, seems to be needed. The so called ModRat method seems to offer a better way and allow fine tuning to a particular geographic area through "calibration". We can't know with certainty what rainfall pattern will occur but the methods you have explained are better at predicting the response of the watershed to any given rainfall event.

Thanks again. Sure wish some California Engineers would weigh in on this discussion. Also Florida and Colorado seem to be leading in this direction.

 

[RWF7437]

All of this is interesting and I thank all who have replied. The central question remains unanswered, however: Ae any of these models "accurate" in the sense that they predict the runoff peak flow and volume from a known storm event within an acceptable margin of error ?" In short, can they, or have they been, calibrated ?

The rainfall will always be a probability and subject to large errors ( +/- 30% to 60%). But the runoff models might be made more accurate through calibration. At least that's my hope.