April 7, 2006

What Major Changes in MSMAM that Affect Design Engineers?

There are many changes in MSMAM (DID, 2000) compared to the Planning and Design Procedure (DID, 1975).

I have listed here some of the major changes that are important to a design engineer:

a. Computation of design storm:

PDP1 uses IFD (Intensity-Frequency-Duration) curves derived for 11
cities in West Malaysia, and for elsewhere it uses a set of 1/2,
2 and 24 hour isopleths of 2 and 20 year A.R.I. for Peninsular
Malaysia in HP1.

On the other hand, MSMAM uses a set of polynomial equations (Equation 13.2
in MSMAM) for 35 major urban centres in Malaysia. The application of this
formula is limited to 6 hr or less.

b. Computation of peak discharge:

PDP1 uses the Modified Rational Method for areas less than 52 km2, whereas
MSMAM recommends the use of the Rational Method for area less than
0.8 km2.

For area more than 0.8 km2, MSMAM recommends the use of the
Time-Area Method or the Runoff-Routing Computer Models to consider
the effect of catchment storage.

c. Reservoir routing:

PDP1 uses a graphical procedure to determine the maximum
differential storage based on the cumulative inflow and outflow.

MSMAM, on the other hand, uses a level-pool routing method.

d. Design of detention basin:

PDP1 uses the Modified Rational Method for inflow and a graphical
method of determining storage. Therefore it cannot directly route
a complete hydrograph through a basin like the level-pool method

On the other hand, MSMAM uses a hydrograph method to compute the
inflow and a level-pool method for routing through the basin. It
can route a complete inflow hydrograph directly through a basin
to get the outflow hydrograph.

MSMAM requires the Post-Development Peak to be less than or equal
to the Pre-Development Peak for both major & minor storms.

e. Design of sediment basin:

PDP1 uses empirical formula to determine the storage volume of
sediment basin.

MSMAM uses 3 or 6 month A.R.I for design storm < or > 2 year
project duration.

It differentiates between a wet or a dry sediment basin. The storage
volume is related to storm characteristics namely, time of
concentration and storm depth.

f. Water surface profile computation:

MSMAM recommends the use of HEC-RAS model for computing backwater
surface profile using a steady state one dimensional open channel

g. Culvert design procedure:

PDP1 uses imperial units and nomographs, while MSMAM uses
SI units and different nomographs with additional entrance
loss coefficients for outlet control.

Filed under Backwater Curve Analy, Culverts, DESIGN COMP/ANALYSIS, Design Rainfall, Design Runoff, Detention Basin, On Site Detention, RUNOFF CONVEYANCE, RUNOFF QUALITY CONTROL, RUNOFF QUANTITY CONTRL, Rational Mthd, Reservoir Routing, Retention Basin, Runoff Routing, Sediment Basins, Time-Area Mthd by

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