ND NP THRO 204 Hydraulics Report Addendum

Addendum
Subject:

ND NP THRO 204(1) Buck Hill Road –
Final Hydraulics Report Parking Lot Inlet

Date: March 2025

To:

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

From:

Luis Calderón, PE, Lead Hydraulic Engineer, CFLHD

Digitally signed by LUIS GERARDO

CALDERON

Date: 2025.03.18 09:09:26 -06'00'

LUIS GERARDO CALDERON

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)

PR05

NDDOT Type 2
double

Design
Storm

Peak Flow
(cfs)

10-year
50-Year

5.9
7.5

Flow Depth at
Grate
(50% clogging)
(in)
0.54
0.63

Flow Depth at
Grate
(0% Clogging)
(in)
0.34
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