A simple hydrograph development technique, developed by H. Rooney Malcom in 1980, is often used to design detention facilities serving relatively small watersheds (less than 640 acre or 1.0 sq mi). Two hydrographs are to be developed using the Malcom Hydrograph Method for pre- and post-project conditions and the required detention volume is equal to the maximum cumulative difference between the two hydrographs (Figure 1).
Once a small watershed peak discharge and runoff volume under a design storm are determined, Malcom Small Watershed Hydrograph Method provides a set of equations (Figure 2) to develop a runoff hydrograph which preserves the originally calculated peak discharge and runoff volume (Figure 3).
The small watershed peak discharge Qp should be calculated sing methods approved by the regulating agencies such as Rational method or site runoff curve method.
The runoff volume V of the design storm is the product of rainfall excess (direct runoff) and drainage area. SCS curve number method can be utilized to calculated rainfall excess (direct runoff) as explained in Figure 4. In practice, a 24-hr storm is usually selected as the design storm and therefore the rainfall depth P is the 24-hr precipitation depth of the selected design storm (100-yr, 50-yr, 25-yr, or 10-yr, 5-yr, 2-yr).
Harris County Flood Control District (HCFCD) recommends a set of direct runoff values (inch) of 24-hr storms for different return periods (frequencies) and impervious area conditions for runoff volume calculation (HCFCD PCPM Section 3.6).
The Malcom Small Watershed Hydrograph Method does not need a specific rainfall distribution (hyetograph) except the runoff volume (rainfall excess) of the design storm. The Malcom Small Watershed Hydrograph can not be used in conjunction with other types of hydrographs including those generated from watershed HEC-HMS models or FEMA Flood Insurance Study. The time to peak of the Malcom Small Watershed Hydrograph Method is computed solely to match runoff volume and has no correlation with the rainfall durations and distributions used in other studies.
An example using Malcom Small Watershed Hydrograph Method is illustrated below for pre- and post-project conditions.
It should be noted that in the 1980 paper by H. Rooney Malcom, A Study of Detention in Urban Stormwater Management, the equations are somehow different as shown in Figure 6. It is unknown how the equations have evolved into today’s shape.