Consequences of changes to the NRCS rainfall-runoff relations on hydrologic design

Authors: Moglen, Glenn; MCCUEN, R.H. - University Of Maryland; MOGLEN, R.L. - University Of Maryland

Submitted to: Journal Hydrologic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2018
Publication Date: 5/30/2018
Citation: Moglen, G.E., McCuen, R., Moglen, R. 2018. Consequences of changes to the NRCS rainfall-runoff relations on hydrologic design. Journal Hydrologic Engineering. https://doi.org/10.1061/(ASCE)HE.1943-5584.0001681.

Interpretive Summary: The Natural Resources Conservation Service (NRCS) rainfall-runoff model has existed in its present form for decades. The NRCS is considering adoption of proposed changes to how this model quantifies storage and partitions rainfall into runoff. This paper explores and quantifies the differences between the long existing and proposed models over a range of rainfall depths, storm types, and surface runoff conditions. Findings indicate that, depending on the conditions simulated, the proposed model may both under- or over-estimate runoff depths, peak discharges, and volumes that would be needed for stormwater management. The proposed model will tend to lead to smaller future designs for the larger, more rare, storms and to larger future designs for more commonplace storm events. Thus the risk profile of the range of design conditions this model may be applied to will shift in varying ways should the proposed changes to this model be adopted. Several examples illustrating design differences over a range of conditions are presented and an analysis of the trade-offs between potential cost-savings in engineering design and changes to failure risk and public safety are enumerated and discussed.

Technical Abstract: A proposed quantification of the fundamental concepts in the Natural Resources Conservation Service (NRCS) rainfall-runoff relation is examined to determine changes relevant to peak discharge estimation and drainage design. Changes to the NRCS curve number, storage, and initial abstraction relations result in different estimates of the runoff volume, timing of runoff, and peak discharge produced by long-established empirical equations that quantify event-based runoff. The ratio of existing-to-proposed runoff depth estimates is determined across a range of curve numbers and storm depths. Similarly the ratio of existing-to-proposed peak discharge is determined as a function of curve number, storm depth, and design storm. Using an infrastructure design based on the existing equations as the baseline, that infrastructure is under-designed (over-designed) if the proposed NRCS equations produce larger (smaller) estimates of volumes or discharges than the existing equations for the same watershed conditions. This study shows that the proposed NRCS equations more likely identify existing infrastructure as being under-designed for smaller storm events and lower curve numbers. Conditions and design storm produce large variation in the design ratios. Storms with return periods on the order of 2- through 10-years generally result in peak discharge under-design ratios that span from 1 to as high as 2 or 3 depending on the watershed conditions, storm size, curve number, and design element in question. Existing infrastructure over-design is more likely the case when return periods approach 25-, 50-, or 100-years with typical over-design ratios ranging downwards from 1.0 to as small as 0.8. In most cases, required storage volumes, as prescribed by the NRCS TR-55 approach, are found to be smaller using the proposed equations, leading to both cost savings and reduced pond residence times. Three common design problems are provided only to illustrate typical under- and over-design findings. The central focus is on the potentially negative consequences of proposed changes to a limited aspect of curve number hydrology.