ND_NP_THRO_204(1)_Final Hydraulics Report_Addendum_Mar2025.pdf

Addendum

Subject: ND NP THRO 204(1) Buck Hill Road – Date: March 2025 Final Hydraulics Report Parking Lot Inlet

To: Toni Pankau, Project Manager, CFLHD Wilmari Valentín, Highway Design, CFLHD

From: Luis Calderón, PE, Lead Hydraulic Engineer, CFLHD Signature: ____________________

Summary

The purpose of this addendum is to update the proposed inlet recommendation (Identifier PR05) for the Buck Hill Parking Lot reconstruction. Further hydraulic analysis of the site has determined that an NDDOT Inlet Type 2 – Double, with an approximate grate length of 6 feet, is adequate to collect runoff from the parking lot. Initially, this inlet was sized to handle runoff from a 50-year design storm. However, per FLH PDDM Section 7.3.3.2.1, parking areas can be designed for a 10-year design storm. Additionally, the clogging factor was originally set at 50% based on PDDM 7.3.3.3.3 guidance for grate inlets. Since the proposed inlet is a combination curb-grate type a clogging factor is not assumed unless it is a documented problem. For this revision, both 0% and 50% clogging factors are considered, as outlined below.

The inlet details are shown on Sheet T7 of the plans and specify the use of the double casting option.

Table 1 below provides the updated recommendations, including results for both the 10-year and 50-year design storms.

Table 1: Revised Inlet Recommendations

Identifier Inlet Type Minimum

Grate Length (Ft)

Design Storm

Peak Flow (cfs)

Flow Depth at Grate

(50% clogging) (in)

Flow Depth at Grate

(0% Clogging) (in)

PR05 NDDOT Type 2 double 6 10-year 5.9 0.54 0.34

50-Year 7.5 0.63 0.39

Figure 1: Inlet Design 10-Year No Clogging

Figure 2: Inlet Design 10-Year 50% Clogging

Figure 3: Inlet Design 50-Year No Clogging

Figure 4: Inlet Design 50-Year 50% Clogging