GUIDELINE FOR COMPLETING THE STORMTECH CHAMBER SYSTEM REPORT USING HYDROCAD

This document aims to provide a comprehensive step-by-step guide for designing, calculating, and preparing the StormTech Chamber System report, ensuring compliance with project approval requirements.

As we know, the StormTech Chamber System is one of the Low Impact Development (LID) solutions categorized under Best Management Practices (BMPs). It is widely applied in projects such as stormwater treatment for parking lots, commercial, and industrial facilities. This system reduces the peak flow and volume of stormwater runoff, helping to prevent flooding, erosion, and water quality degradation downstream.
The system typically requires a retention time of 40–72 hours.
So, how do we design and accurately calculate the stormwater storage volume, develop the infiltration chart for the StormTech Chamber System, and generate the final report for submission to the relevant authorities for project approval?
Below, I will guide you through each step using HydroCAD software to perform the calculations and produce the correct report format.

Contents

Step 1. Determine Precipitation Frequency Data

Based on the information provided by the client and the project location (latitude and longitude), obtain the Precipitation Frequency Data using the NOAA Atlas 14 database.

Image 1. This is a report of precipitation frequency data from NOAA.

Step 2. Define Post-Development Watershed Area Map

Using the proposed site plan, identify:

Watershed area
Time of concentration (Tc)
Surface material types

Simultaneously, design the stormwater collection system leading to the StormTech Chamber System. Collaborate with the client to determine the most appropriate location for chamber installation.

Image 2. This is a post development watershed area map

Step 3. Define Design Parameters for the Proposed Watershed in HydroCAD

Input the following into HydroCAD:

Watershed Area (sq-ft)
Curve Number (CN)
Soil Group (e.g., Soil Group B – Sandy Loam)
Time of Concentration (Tc), considering:
- Sheet Flow
- Shallow Concentrated Flow
- Pipe Flow

Note: Regularly review groundwater level reports to evaluate the soil’s drainage capability.

Image 3. Watershed area and Tc are defined in HydroCAD

Step 4. Declare the Cultec 330XLD Chamber

Utilize HydroCAD’s built-in chamber models and ensure you update the system with the latest manufacturer product catalogs for accurate design inputs.

Step 5. Specify the Soil Infiltration Rate

From Step 3, identify the soil group distribution and accurately determine the infiltration rate.

Sites within Soil Groups A or B (infiltration rate > 0.5 in/hr) are suitable for the StormTech Chamber design.
Sites within Soil Groups C or D require detailed geotechnical surveys to validate the design’s applicability.

Image 5. Exfiltration out rate in HydroCAD

Table: Typical Soil Infiltration Rate.

Texture Class	Hydrologic Soil Group	Infiltration Rate ( inch/hour)
Sand	A	8.00
Loamy Sand	A	2.00
Sandy Loam	B	1.00
Loam	B	0.50
Silt Loam	C	0.25
Sandy Clay Loam	C	0.15
Clay Loam	D	0.09
Silt Clay Loam	D	<0.09
Clay	D	<0.05

Source: ASCE, 1998

Step 6. Verify Peak Elevation at the 100-Year Storm Event

Check the peak water surface elevation during the 100-year storm event.
Ensure the elevation remains below the top elevation of the chamber system.
Confirm that the system completely drains within 72 hours.

Image 6. Verify Peak Elevation at the 100-Year Storm Event of Stormtech Chamber System

Step 7. Finalize the Report

Prepare the final Stormwater Report according to the required submission format.
Update input data promptly if there are any changes to the site plan or additional client requests.

Image 7. The Report of Pond CH1 by HydroCAD.

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